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THE 
 
 PANORAMA 
 
 OP 
 
 SCIENCE AND ART; 
 
 EMBRACTNG 
 
 C!)e §)c(ences 
 
 OF 
 
 AEROSTATION, AGRICULTURE AND GARDENING, ARCHITECTURE, ASTRONOMY, CHEMISTRY, 
 
 ELEcraicirv, galvanism, hydrostatics and hydraulics, magnetism, 
 mechanics, optics, and PNEU>UTICS ; 
 
 THE AMTS 
 
 OF 
 
 Building, Brewing, Bleaching, Clockwork, Distillation, Dyeing, Drawing, Engraving, 
 Gilding and Silvering, Ink-making, Japanning, Lacquering, Willwork, 
 Moulding and Casting in Plaster, Painting, Staining Glass, 
 
 Staining Wood, and Varnishing: 
 
 THE METHODS OF 
 
 W ORKING IN WOOD AND METAL, 
 
 applicable in 
 
 annealing, boring and drilling, filing, grinding, tempering STEEL, MAKING SCREWS, 
 SOLDERING, COMMON AND ELLIPTIC TURNING, &c. 
 
 And 
 
 a iHistellaneaus g^election 
 
 OF 
 
 INTERESTING AND USEFUL PROCESSES AND EXPERIMENTS. 
 
 BY JAMES SMITH. 
 
 With Forty-nine illustrative Engravings, by eminent Artists, 
 
 JN TWO VOLUMES. VOL. L 
 
 LI VERPOOL, 
 
 PRINTED FOR NUTTALL, FISHER, AND CO. 
 
 1815 . 
 

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PREFACE. 
 
 Among the earliest indications of a mind sus« 
 ceptible of attaining general knowledge, appears that 
 which consists in the delight afforded by mechanical 
 performances. With this disposition, opportunities 
 of acquiring the means of imitative execution, and of 
 perceiving the advantages derivable from the posses.- 
 sion of scientific principles, eminently tend to establish 
 that activity of mind, and that predilection for the 
 acquisition of useful knowledge, which beneficially 
 influence the character through life, and present 
 unfailing sources of enjoyment for the hour of soli- 
 tude and leisure. 
 
 If those who are ingenious and skilful in the per- 
 ception of scientific truth, were practically conversant 
 with the manufacture of every instrument they use, 
 the progression of the sciences would be materially 
 accelerated. The theorist, however excellent and 
 original his views, inevitably suffers many of his plans 
 to perish, when mechanical dexterity is requisite to 
 63. VoL. I. b 
 

 iv. PREFACE. 
 
 verify them, because lie cannot contend with the 
 expense or the difficulty of having them executed by 
 others, or has no adequate knowledge of the degree 
 in which they may be realized. The science of 
 chemistry, in the course of a few years, has risen to 
 a pre-eminent station of utility and splendour; and 
 among the leading causes of its advancement it is 
 observable, that the mechanics who have performed 
 the operations of the laboratory, and contrived and 
 executed the novel part of the apparatus, w^ere the 
 very individuals who, as men of science, w^ere 
 actuated by expanded views and the zeal of research. 
 Dr. Herschels observations and discoveries are the 
 most valuable which a single individual has ever 
 made in the departments of astronomy requiring the 
 aid of instruments; and we find upon investigation, 
 that the telescopes constructed by himself excel all 
 others: even his telescope of forty-feet, which is one 
 of the most splendid trophies of mechanical skill 
 existing, was wholly executed under his own direc- 
 tion, by common ai tizans or labourers. 
 
 In adverting to these considerations, the Editor 
 concluded to allot the first portion of these volumes, 
 to the most general and radically important opera- 
 tions in working wood and metal. Architecture and 
 Building follow, after which, to the page w hich ter- 
 minates the essay on gardening, the sciences and their 
 correlative arts, are taken up according to the order 
 of succession in which they may be conveniently 
 studied. By this arrangement, Drawing and its 
 dependents are stationed near the conclusion of the 
 
PREFACE. 
 
 .V 
 
 work, which is closed by a selection of miscellanies, 
 comprising subjects adapted rather for reference than 
 regular perusal. 
 
 Historical sketches of the rise and progress of the 
 sciences have been introduced, when they appeared 
 calculated to interest the general reader, to inculcate 
 useful truths, or to facilitate the apprehension of sub- 
 sequent facts and principles. 
 
 On subjects within the scope of this publication, 
 where minuteness of investigation could not be 
 entered upon, an attempt has been made to delineate 
 the most important features: of those which had 
 yet received no popular illustration, the Editor has 
 been especially solicitous to seek new information; 
 and in aid of this object he has been favoured by a 
 friend with the communication of an entire essay. 
 He alludes to the treatise on Architecture, which is 
 the production of Thomas Rickman, of Liverpool, 
 whose system of English architecture he cannot doubt 
 will be acceptable to those who are interested in the 
 the subject, or who have pleasure in contemplating 
 the monuments of vernacular skill. 
 
 The Editor has occasionally availed himself of 
 the language of the ablest writers, where it appeared 
 most conducive to his purpose. Of entire extracts, 
 he has had the pleasure of specifying the source: of 
 those which have been assimilated to his views by 
 alterations, or taken in a detached and limited man- 
 ner; where the information was of general notoriety, 
 or its origin uncertain, acknowledgment becaipe 
 superfluous or impracticable. 
 
VI. 
 
 PREFACE. 
 
 Convinced that no permanent advantages accrue 
 from attempts to deprecate censure, or to angle 
 for praise, the Editor waves all digression from 
 the general exposition of his- plan; but aware that 
 a work like the present — a first publication, compiled 
 in the scanty intervals of leisure afforded by profes- 
 sional avocations, cannot be free from imperfections, 
 he terminates these prefatory remarks with assuming 
 the liberty to observe, that the communication of hints 
 for emendations will receive his thanks. 
 
CONTENTS OF VOL. I 
 
 
 Page, 
 
 MECHANICAL EXERCISES. 
 
 
 
 Of Iron . , 
 
 
 1 
 
 Forging and welding 
 
 • • • 
 
 10 
 
 Tools used in forging iron, and the working of 
 
 
 metals generally 
 
 • . . 
 
 14 
 
 Boring and drilling 
 
 . • 
 
 18 
 
 Filing 
 
 . . • 
 
 22 
 
 Grinding • 
 
 . . 
 
 32 
 
 Annealing . 
 
 . • • 
 
 36 
 
 The straight-edge and square 
 
 37 
 
 Screws 
 
 ... 
 
 39 
 
 Of the most common metals.- 
 
 — Copper, 41 — Tin, 44 
 
 
 — Lead, 46 — Zinc 
 
 • • • 
 
 49 
 
 Soldering . 
 
 • • t 
 
 50 
 
 Of TURNING . 
 
 # * • 
 
 53 
 
 Of Chucks, 60 — The boring and mandrel collars, 61 — 
 
 The hand lathe or turning bench, 62 — The pole 
 lathe, 65 — The tools used in turning, 67 — The 
 parallel rest, 69 — Cutting screws in the lathe, 72 — 
 Elliptic turning, 76 — Miscellaneous remarks on 
 turning . , . . • 79 
 
 Of TIMBER . . , . . 85 
 
 Of Saw.mills, 98 — Curving wood, 99 — An advanta- 
 geous method of converting the trunks of trees 
 into square timber, 102 — The tools used in work- 
 ing wood, 104 — Saws, 105 — Planes, 107 — Chisels, 
 
 112 — Boring tools, 113 — The Hammer, Mallet, 
 Square, Bevel, Mitre-square, Gauge, Straight- 
 edge, Winding-sticks, 116 — Qlue, 120 — The 
 mortise an4 tenon , * <. 122 
 
 ARCHITECTURE. 
 
 English Architecture • . • 129 
 
 The Jirst^ or Norman Style, 
 
 Of Norman doors, 134 — Windows, 135 — Arches, 
 136— iPiers, 136 — Buttresses, 137 — Tablets, 137 
 —Niches, 137 — Ornaments, 137 — Steeples, 138 
 
 The second^ or early English Style. 
 
 Of early English doors, 139 — Windows, 140 — Arches 
 141 — Piers, 141 — Buttresses, 142 — Tablets, 143 
 = — Niches, 143 — Ornaments, 144 — Steeples 144 
 
via 
 
 COxXTENTS, 
 
 Page, 
 
 ARCHITECTURE, confmued. 
 
 English Architecture. 
 
 The third, or Decorated Stj/le, 
 
 Decorated English doors, 145 — Windows, 147 — 
 Arches, 149^ — Piers, 149 — Buttresses, 149 — Ta- 
 blets, 150 — Niches, 1.50 — Ornaments, 151 — 
 
 Steeples . , . . . 151 
 
 The fourth, or Perpendicular Style. 
 Perpendicular English doors, 153 — Windows, 153 
 — Arches, 155 — Piers, 155 — Buttresses 156 — 
 Tablets, 157 — Niches, 157 — Ornaments, 158 — 
 
 Steeples, 159 — Miscellaneous remarks on perpen- 
 dicular buildings . . . 159 
 
 Of Battlements . . . . 160 
 
 Roofs , . , . . 161 
 
 Miscellaneous remarks . . . 165 
 
 Description of plates I. and II. . . 167 
 
 - Of Grecian Architecture, 170 — Tuscan order, 173 — 
 Doric order, 175 — Grecian Doric, 175 — Roman 
 Doric, 176 — Ionic order, 177 — Corinthian order, 
 
 179 — Composite order . . . 180 
 
 BUILDING . . . . . 182^ 
 
 Of Bricks, 182 — The tools used in bricklaying, 192 
 — Foundations, 196 — Cements or mortar, 198 — 
 
 Brick bond and walling . . . 209 
 
 Masonry ..... 216 
 
 Of the mason’s tools, 216 — Stones, 218 — Cements, 
 
 221 — Stone walls, 222 — Stone columns, 224 — 
 
 Stone bridges .... 227 
 
 Miscellaneous remarks relatiTe to building . 241 
 
 Of the situation and plan of houses, 241 — Rooms, 
 243 — Chimneys, 245 — Doors, 251 — Windows, 
 251 — Stairs, 253 — Roofs, 254 — Floors, 258 — 
 The proportions of timbers for small buildings, 
 260 — The proportions of timbers for large 
 
 buildings . . . • . 261 
 
 Building Act, 263 — External walls, 264 — Party walls, 
 
 265 — Chimneys, . . . 267 
 
 MECHANICS. 
 
 Of Matter ..... 269 
 
 Motion . . . . . 281 
 
 Laws of motion .... 289 
 
 Central forces .... 290 
 
 Centre of gravity . . . 290 
 
 The mechanical powers, 293 — The lever, 295 — 
 
 The balance, 300 — The steel-yard, 305 — The 
 pulley, 306 — The wheel and axle, 311 — The 
 inclined plane, 316 — The wedge, 317 — The 
 screw, 319 — The endless screw . 321 
 
CONTENTS. 
 
 ix. 
 
 Page. 
 
 MECHANICS, continued. 
 
 Of compound machines « . . 323 
 
 Fiy wheels .... 334 
 
 Friction ..... 337 
 
 The application of men and horses, as moving 
 
 powers in machinery . . . 343 
 
 MilFwork .... 345 
 
 Practical rules and observations relative to mills, 347 
 
 — The millwright’s table, 348 — Method of setting 
 out wheels, 353 — Bevel-geer, 354 — Hooke’s uni- 
 versal joint, 355 — The cycloid and epicycloid, 
 and the formation of the teeth of wheels, 355 — 
 Wipers for raising stampers and hammers, 3fi2 — 
 Description of a corn. mill . . 363 
 
 Of Wheel, carriages . . . . 368 
 
 Clock-work .... 374 
 
 Mode of dividing the circumference of circles, 383 — 
 Proportioning the diameters of wheels and pinions, 
 
 385 — The pendulum . . . 389 
 
 Abstract of mechanics . . . 396 
 
 OPTICS. 
 
 Historical remarks .... 407 
 
 Definitions . . . . . 411 
 
 Of Light, 412 — The refrangibility of light, 420 — 
 
 The reflexibility of light, 431 — The different 
 refrangibility of the rays of light, 439 — The 
 rainbow, and the natural phenomena dependent 
 upon the refrangibility of light, 443 — The inflec- 
 tion and deflection of light . . 447 
 
 The eye and the general phenomena of vision, 451 
 
 Optical Instruments: 
 
 Of spectacles, 460 — Burning lenses and mirrors, 463 
 — Camera obscura, 466 — Magic lantern and phan- 
 tasmagoria, 469 — The single microscope, 470— 
 Ellis’s single aquatic microscope, 475 — Hand 
 megalscope, 476 — Double or compound micros- 
 cope, 477 — The solar microscope, 478 — The 
 opaque microscope, 479 — Microscopic objects, 479 
 Of telescopes, 483 — The astronomical telescope, 484 
 — The night-glass, 486 — The terrestrial telescope, 
 or perspective glass, 487 — The Galilean teles- 
 cope, 487 — Achromatic and aplanatic object- 
 glasses, 488 — The reflecting telescope, 491 — 
 Gregorian telescope, 492 — Newtonian telescope, 
 
 493 — Herschel’s telescope, 494 — Cassegrainian 
 telescope, 495 — Of the magnifying power of teles- 
 copes, 495 — Miscellaneous remarks on telescopes, 496 
 Micrometers .... 500 
 
 Abstract of optics . . . 602 
 
X. 
 
 CONTENTS. 
 
 ASTRONOMY. 
 
 Historical remark^ 
 
 Definitions .... 
 
 Of the apparent motions of the heavenly bodies. 
 
 The figure, motion, and magnitude of the earth — 
 Magnitude and distance of the sun and moon, 
 and of planetary motion 
 
 Of the solar system, 531 — The sun, 532 — Mercury, 
 538 — Venus, 539 — The earth, 541 — The moon, 
 546 — Mars, 552 — Vesta, 554 — Juno, 554 — 
 Ceres Ferdinandea, 554 — Pallas, 555 — Jupiter, 
 556 — Saturn, 558 — Herschel 
 
 Synopsis of the solar system. — Distances of the 
 planets from the earth, 563 — Diameter of the 
 sun and planets, 563 — Density of the sun and 
 planets, 564 — Quantity of matter in the planets, 
 564 — Number of feet through which a heavy body 
 would fall at the surfaces of the planets, 564 — 
 Rotation of the planets round their axes, 564 — 
 
 Page. 
 
 505 
 
 510 
 
 513 
 
 519 
 
 562 
 
 Tropical revolutions of the planets. 
 
 , 
 
 565 
 
 Comets .... 
 
 . 
 
 565 
 
 Fixed stars 
 
 . 571 
 
 —585 
 
 Constellations of the zodiac 
 
 
 575 
 
 Constellations of the north side of the 
 
 zodiac. 
 
 575 
 
 Constellations of the south side of the zodiac. 
 
 576 
 
 The tides- .... 
 
 * 
 
 586 
 
 The harvest moon and hunter’s moon 
 
 
 595 
 
 The horizontal moon 
 
 
 600 
 
 The aberration of light 
 
 
 602 
 
 Nutation of the earth’s axis 
 
 
 605 
 
 Precession of the equinoxes 
 
 
 605 
 
 Eclipses .... 
 
 • 
 
 607 
 
 Eclipses of the moon, 607 — Eclipses of the sun, 
 
 609 — Description of a total eclipse of the sun, 613 
 Division of time, . . . . 617 
 
 Of the day, 617 — The week, 619 — The month, 619 
 — The year, 620 — The lunar and solar cycles 
 and indiction, 622 — Epochs and eras . 623 
 
 Abstract of astronomy . . . 624 
 
JDirections for placing the Plates in VoL /. 
 BIECHANICAL EXE.HCISES, Plate I. to face page 
 
 
 II 
 
 
 
 III. ... 
 
 
 
 IV. ... 
 
 
 ARCHITECTURE 
 
 I 
 
 
 
 II 
 
 
 ’ 
 
 III. ... 
 
 
 
 IV. ... 
 
 
 
 V. ..... 
 
 
 BUILDING 
 
 
 
 
 II 
 
 
 MECHANICS 
 
 I 
 
 
 
 II 
 
 
 
 Ill 
 
 
 
 IV. ... 
 
 
 
 V 
 
 
 
 VI. ... 
 
 
 OPTICS 
 
 
 
 
 II 
 
 
 
 III. ... 
 
 
 
 IV. ... 
 
 
 
 V 
 
 
 
 VI. ... 
 
 
 
 VJI, .. 
 
 
 ASTRONOMY (solar system).... 
 
 I 
 
 
 
 II 
 
 
 
 Ill 
 
 
 
 IV. ... 
 
 
 
 V 
 
 
 96 
 96 
 96 
 102 
 168 
 168 
 172 
 180 
 J80 
 254 
 254 
 306 
 312 
 328 
 360 
 374 
 394 
 432 
 432 
 438 
 464 
 476 ^ 
 488 
 500 
 532 
 530 
 560 
 604 
 612 
 
Directions for placing the Plates in Vol. II, 
 
 PNEUMATICS Plate 
 
 I. 
 
 to face page 
 
 24 
 
 
 II. 
 
 
 26 
 
 
 III. 
 
 
 40 
 
 HYDROSTATICS AND HYDRAULICS .... 
 
 I. .. 
 
 
 88 
 
 
 II. 
 
 
 116 
 
 
 III. 
 
 ...... 5j .... 
 
 128 
 
 STEAM ENGINE 
 
 
 
 140 
 
 AEROSTATION— MAGNETISM 
 
 
 
 186 
 
 ELECTRICITY 
 
 I. . 
 
 
 218 
 
 
 II. 
 
 
 230 
 
 
 III. 
 
 5) •••• 
 
 268 
 
 GALVANISM 
 
 
 
 290 
 
 CIIExVilSTRY 
 
 I. . 
 
 
 310 
 
 
 II. 
 
 
 322 
 
 BREWING— DISTILLING 
 
 
 
 582 
 
 DRAWING 
 
 I. 
 
 
 712 
 
 
 II. 
 
 
 716 
 
 
 III. 
 
 .*..... 55 .... 
 
 720 
 
 
 IV. 
 
 5J •••• 
 
 720 
 
 MISCELLANIES 
 
 
 
 816 
 
 ERRATA. 
 
 VOL. I. 
 
 faee. line. 
 
 aso, 11, for "fig:. 13” read " liR. 17.” 
 
 255, 4, for " fig. IS ” read “ fio. 16.” 
 
 272, 13, for “ yards” read " miles.” 
 
 286 , 22, after "ABC” insert “ fig. 2, plate I.” 
 
 288 , 4, from the bottom, for “ark ” read " A r A.’> 
 307, 25, for “ fig. 2 ” read " fig. 3.” 
 
 312, 5j from Hie bottom, for “R” read “S,” and in 
 next line for "S” read “R.” 
 
 315, 20, for " A ’ read “ it.” 
 
 331, 3, for "hand” read "band.” 
 
 332, 1, for "was ” read " has.” 
 
 300, 12, from the bottom, after "turned ” insert 
 ' " above.” 
 
 354 , 9, for "a J ” read " a p.'' 
 
 357, 24, for " TCHO ” read “ TCHQ.” 
 
 368, 17, from the bottom, for “ah'"' read “ a pj' 
 
 376, 8, for " ST” read " SS ; ” and in line 35, for 
 
 " IK ” read " IR.” 
 
 377, 3, 4, and 7, for " K ” read " R.” 
 
 382, 14, from the bottom, for “ g” read “/i.” 
 
 390, 3, for “ RS ” read “ BG.” 
 
 444, 20, for " S/’A” read “Sfg,” and injine 23, 
 for “ S c A ” read "Sc g.” 
 
 435, IS, for " eye ” read " retina.” 
 
 519, 6, for " assigns” read " signs.” 
 
 665, 10, for " 29 17 44 3 ” read " 27 7 43 5.” 
 
 *89, 7, for " farther ” read " faster.” 
 iC07, 1, for “ y T ” read “ VT.” 
 
 VOL. II. 
 
 Page, line, 
 
 4, 24, for " B ” read “ P.” 
 
 26 , 3, for " A” read " F.” 
 
 210 , 9 , for “ e ” read " c.” 
 
 230, 19, for " A” read " E.” 
 
 301, 17, for " nitio-muriatic” read “ nitro-muriatic.” 
 307, -9, for “ crucible ” read “ retort.” 
 
 322, 12, for " fig. 5 ” read " fig. 10.” 
 
 .346, 13, for “ burn ” read " burns. ’ 
 
 398, 26, for “ air ” read " hair.” 
 
 406, n, for “ no” read " not.” 
 
 661, 3, for " where” read " were.” 
 
 709, 3 and 7, from the bottom, for " a: v R ” read 
 "ar Ry,” and in line 2 from the bottom, for 
 “vlK ”read " c R t.” 
 
 714, 20, for " Va: ” read “ V ar/.” 
 
 714, 21, for " w VP ” read “/w VP,” and strike out 
 " through the point t.” 
 
 714, 22, insert "/” after "to.” 
 
 605, 7, from the bottom, for “ mercury ” read 
 " alum,” 
 
 ' (Most of the errata for both vols. occur only ia part 
 of the copies,) 
 
THE 
 
 PANORAMA 
 
 OF 
 
 SCIENCE AND ART. 
 
 MECHJNICJL EXERCISES. 
 
 Of Iron. 
 
 Of all metallic substances, iron is the most abundantly diffused, 
 and the most intrinsically valuable. It may be detected in 
 plants, and in animal fluids it is the chief cause of colour 
 in earths and stones ; sands, clays, the waters of rivers, springs, 
 rain, and snow, are seldom perfectly free from it ; and in several 
 parts of the w^orld, whole mountains are composed of iron ore. 
 if the use of this metal were lost to mankind, the arts and sciences 
 would dwindle into insignificance, and civilization itself 'become 
 rapidly retrogressive. An inquiry, therefore, into the properties 
 and means which render it subservient to such important purposes, 
 will not, it is presumed, prove uninteresting to the general reader; 
 while the prudent artizan, whose first care is generally to provide 
 himself with tools adapted to his labours, will attentively review 
 every hint which may improve his knowledge of that material, the 
 proper choice and management of which constitute the first step 
 towards success in mechanical pursuits. 
 
 Iron is employed in three states, viz. that of cast iron, wrought 
 iron, and steel. Cast iron is the metal in its first state, rendered 
 fusible by its combination with those tw o substances which 
 chemists distinguish by the names of carbon and oxygen. In 
 the great iron works, the ore, broken into small pieces, and mixed 
 with a portion of limestone to promote its fusion, is thrown into 
 the furnaces, which are from sixteen to thirty feet high. Baskets 
 
 r * * — ■■.■■■ ■ - 
 
 • Metallic grains of iron have been found in strawberries; and one-twelfth 
 part of the weight of dried oak wood is said to consist of this metal. — The blood 
 contains much iron, to which it owes its red colour. 
 
 1.— VoL. I. B 
 
MECHANICAL EXERCISES. 
 
 Manufacture of cast and wrought iron. 
 
 of charcoal or coke, in due proportion, are thrown in along with 
 it. A part of the bottom of the furiiace is tilled with fuel only. 
 This being kindled, the whole is raised by the blast of the 
 great bellow s, to a most intense heat. The metal, as it is reduced, 
 sinks down tlirough the fuel, and collects at the bottom of the 
 furnace. More ore and fuel are supplied above, and the oper- 
 ation goes on till tlie melted metal, increasing in quantity, rises 
 almost to the aperture of the blast ; a passage is then made for it 
 at tlie side of the furnace, and it is run into what are called pigs of 
 cast iron. A furnace will furnish daily from two to five tons of 
 iron, according to the richness of the ore, and the skill with w hich 
 the operation is conducted. Ores of iron combined with mag- 
 nesia, are very refractory, and, as well as those which contain sul- 
 phur and arsenic, require to be roasted before they are cast into 
 the smelting furnace. 
 
 Pig iron is of very different qualities; that which is called 
 No. 1, and the fracture of which is of a dark colour, runs so 
 fluid as to be admirably suited for grates, and ornamental w ork. 
 Cast iron cutlery is manufactured from it, as no other would run 
 fine enough for the purposes to which it is applied, such as forks 
 and small scissars, fish hooks and needles. These articles obtain 
 by annealing a considerable degree of malleability, and are even 
 rendered capable of being welded. When great strength is 
 required, as for large wheels, beams, pillars, or railways, the 
 iron w hich contains a smaller proportion of carbon is preferable, 
 as that called No. 2. Tlie proportion of carbon in cast iron 
 varies in the different sorts, from one-fifteenth to one-twenty-fifth. 
 Cast iron also frequently contains a portion of the phosphuret 
 of iron, in which case it breaks of a white colour, and must, from 
 its excessive hardness, be rejected for purposes which require it 
 to be filed, or turned, or cut with the chissel. It may be ob- 
 aerved, that the whiter the metal is, tlie harder it is also ; whether 
 these properties are owing to its quality, or the mode of its ma- 
 nagement. Cast iron expands in passing from the fiuid to the 
 solid state ; it consequently assumes the exact figure of the mould 
 into which it is poured, a circumstance which adds greatly to its 
 value for casting. 
 
 Crude or cast iron is converted into wrought iron, by keeping 
 it in a state of fusion for a considerable time, and repeatedly 
 stirring it in the furnace ; the oxygen and cai bon which it contains 
 unite, and fly off in the state of carbonic acid gas, and as this 
 takes place the iron becomes more infusible ; it gets thick or stiff 
 in the furnace ; and the workmen know, by this appearance, that 
 it is time to submit it to the repeated action of the hammer, or 
 die regular pressuie of large steel rollers, by which the parts 
 
MECHANICAL EXERCISES. 
 
 5 
 
 Qualities of wrought iron. — Manufacture of blistered steel. 
 
 which still partake of the nature of crude iron so much as to retain 
 the fluid state, are forced out, and the metal is rendered malleable, 
 ductile, more closely compacted, of a fibrous texture, and totally 
 infusible. In this state it is known in commerce by the name of 
 bar iron. The loss of weight sustained by iron, in the process of 
 refining, is considerable, generally amounting to one-fourth, and 
 sometimes to one-half. 
 
 Forged, like cast iron, varies greatly in quality. Thus 
 some of it is tough and malleable both when it is hot and 
 when it is cold. This is the iron in common use, and it 
 is the best and most useful. It may be known generally by 
 the equable surface of the forged bar, which is free from trans- 
 verse fissures, or cracks in the edges ; and by a clear, white, 
 small grained, or rather fibrous texture. The best and toughest 
 iron is that which has the most fibrous texture, and is of a 
 clear grayish colour. This fibrous appearance is given by the 
 resistance which its particles make to separation. The texture 
 of the next best iron, which is also malleable in all temperatures, 
 consists of clear, whitish, small grains, intermixed with fibres. 
 Another kind is tough when it is heated, but brittle when cold. 
 This is called cold-short iron, and is distinguished by a texture 
 consisting of large shining plates, without any fibres. It is less 
 • liable to rust than any other description of forged iron. A fourth 
 kind of iron, called red-short, or hot-short, is extremely brittle 
 when hot, and malleable when cold. On the surface and edges of 
 the bars of this kind of iron, transverse cracks or fissures may" 
 be seen, and its internal colour is dull and dark. 
 
 The quality of iron may be much improved by violent compres- 
 sion, as by forging and rolling, especially when it is not long ex- 
 posed to violent heat, which injuries and at length destroys its 
 metallic properties. But though iron is rendered malleable by 
 hammering, this operation may be continued so long, as to deprive 
 it of its malleability. 
 
 Steel is made of the purest malleable iron, by a process called 
 cementation. In this operation, layers of bars of malleable iron, 
 and layers of charcoal, are placed one upon another, in a proper 
 furnace, the air is excluded, the fire raised to a considerable 
 degree of intensity, and kept up for eight or ten days. If upon 
 the trial of a bar, the whole substance is converted into steel, the 
 fire is extinguished, and the whole is left to cool for six or eight 
 days longer. Iron thus prepared is called blistered steel, from 
 the blisters which appear on its surface. In England, charcoal 
 alone is used for this purpose ; but Duhamel found an advantage 
 in using from one-fourth to one-third of w ood ashes, especially 
 when the iron was not of so good a quality as to afford stew. 
 
4 
 
 MECHANICAL EXERCISES. 
 
 Tempering steel by colour. 
 
 possessing tenacit}? of body as well as hardness. These ashes 
 prevent the steel-making process from being effected so rapidly 
 as it would otherwise be, and give the steel pliability without 
 diminishing its hardness. The blisters on the surface of the steely 
 under this management, are smaller and more numerous. He also 
 found that if the bais, when they are put into the furnace, be 
 sprinkled with sea salt, this ingredient contributes to give body to 
 the steel. If the cementation be continued too long, the steel be- 
 comes porous, brittle, of a darker fracture, more fusible, and in* 
 capable of being welded. On the contraiy, steel cemented with 
 earthy infusible powders, is gradually reduced to the state of forged 
 iron again. Excessive or repeated heating in the forge is attended 
 with the same effect. 
 
 The properties of iron are remarkably changed by cementa- 
 tion, and it acquires a small addition to its weight, which con- 
 sists of the carbon it has absorbed from the charcoal, and amounts 
 to about the hundred and fiftieth or two hundredth part. It is 
 much more brittle and fusible than before ; though it may still be 
 welded like bar iron, if it has not been fused, or over cemented ; 
 but by far the most important alteration in its propeities, is, 
 that it can be hardened or softened at pleasure. If it be made 
 red hot, and instantly cooled, it attains a degree of hardness, 
 which is sufficient to cut almost every other substance; but if 
 heated and cooled gradually, it becomes nearly as soft as pure 
 iron, and may with much the same facility be manufactured into 
 any determinate form. A rod of good steel, in its hardest state, 
 possesses so little tenacity that it may be broken almost as easily 
 as a rod of glass of the same dimensions. This brittleness can 
 only be diminished by diminishing its hardness, and in the pro- 
 per management of this point, for different purposes, consists the 
 art of tempering. The colours which successively appear on the 
 surface of steel slowly heated, are, yellowish white, yellow or straw 
 colour, gold colour, brown, purple, violet, and deep blue. These 
 signs direct the artist in reducing the hardness of steel to any par- 
 ticular standard. If steel be too hard, it will not be proper for 
 tools which are intended to have a fine edge, because it will be so 
 brittle that the edge will soon become notched ; if, on the con- 
 trary, it be too soft, it is evident that the edge will turn or bend. 
 Some artists inclose the tools to be hardened, in an iron case or 
 box, and slowly heat them to ignition ; they then take the case 
 out of the fire, and drop the pieces into water, in such a man- 
 ner as will allow them to come as little as possible into contact 
 Xvith the air. This method answers two good purposes ; it causes 
 the heat to be more equally applied, and prevents the scaling 
 occasioned by the contact of the air. When the work has been 
 
MECHANICAL EXERCISES. 
 
 5 
 
 Manufacture of cast steel. 
 
 polished, and well defended from the air, it is, when hardened, 
 nearly as clean as before. If the tool be unpolished, they brighten 
 its surface upon a stone ; it is then laid upon burning charcoal, 
 or upon the surface of melted lead, or upon an ignited bar or 
 plate of iron, till it acquires the desired colour ; at which instant 
 they plunge it into cold water. The yellowish white indicates a 
 temper so little reduced as to be used for few edge tools; the yellow 
 or straw colour, the gold colour, and the brown, are used for pen- 
 knives, razors, and gravers ; the purple for tools used in working 
 upon metals, especially iron; the violet for springs, and for instru- 
 ments used in cutting soft substances, such as cork, leather, and 
 the like ; but if the last blue be waited for, the hardness of the 
 steel will scarcely exceed that of iron. When soft steel is heated 
 to any one of these colours, and then plunged into water, it does 
 not acquire nearly so great a degree of hardness, as if previously 
 made quite hard, and then reduced by tempering. The degree 
 of ignition required to harden steel is different in the different 
 kinds. The best kinds require only a low red heat. It has been 
 ingeniously supposed, that the hardness of steel depends on tlie 
 intimate combination of its carbon ; and, on this supposition, it 
 follow's, that the heat which effects this is the best, and tl^at a 
 higher degree will be injurious. 
 
 The texture of steel is rendered more uniform by fusion. 
 When it has undergone this operation it is called cast steel ; which 
 is wrought with more difficulty than common steel, because it is 
 more fusible, and is dispersed under the hammer, if heated to a 
 white heat. The cast steel of England is made from the frag- 
 ments of the crude steel of the manufactories and steel works. A 
 crucible, about ten inches high, and seven inches in diameter, i& 
 filled with these fragments, and placed in a wind furnace, like that 
 of the founders, but smaller, because intended to contain one pot 
 only. It is likewise furnished with a cover and chimney, to in- 
 crease the draught of air. The furnace is entirely filled with 
 coke, and five hours are required for the perfect ffision of the 
 steel. It is then cast into ingots, and afterwards forged in the 
 same manner as other steel, but with less heat and more precau- 
 tion, as it is more liable to break. Cast steel is about thirty per 
 cent, dearer than other good steel. Its uniformity of texture is 
 for many works an invaluable advantage. It is dady more and 
 more used in this country, but must necessarily be excluded from 
 many works of considerable size, on account of the difficulty of 
 welding it, and the facility witli which it is degraded in the fire. 
 Cast steel takes a fine firm edge, and receiving an exquisite polish, 
 of which no other sort of steel is, in so high a degree, susceptible. 
 It is made use of for all the ffuest cutlery ; but though it may be 
 
6 
 
 MECHANICAL EXERCISES. 
 
 Tempering steel with tallow, oil, &c. 
 
 cast into ingots, it is too imperfectly fluid to be cast into small 
 wares. The tenacity of steel hammered at a low heat, or even 
 when cold, is considerably increased ; but the effect of this ham- 
 mering is taken off by strong ignition. Tools, therefore, made of 
 cast steel, and intended to sustain a good edge, for cutting iron and 
 other metals, are not afterwards softened, but the ignition is care- 
 fully regulated at first, as the most useful hardness is produced by 
 that degree of heat which is just sufficient to effect the purpose. 
 Cast steel, annealed to a straw colour, is softened nearly as much 
 as other kinds to a purple or blue. 
 
 A convenient mode of tempering a great number of article* 
 at once, and of heating them uniformly, however irregular their 
 shape, is to put them into a proper vessel with as much oil or 
 tallow as will cover them, and then to place them over the fire, 
 or the flame of a lamp, until a sufficient heat is given. Clock 
 and watch pinions, watch verges, &c. are tempered in this man- 
 ner, sometimes many dozens at once, as expeditiously as a single 
 article. The requisite temper may be known by the following 
 circumstances : when the tallow is first observed to smoke, it 
 indicates the same temper as that called a straw' colour. This 
 will reduce the hardness but little ; but if the heat be continued 
 till the smoke becomes more abundant, and of a darker colour, 
 it w'ill be equal to a browm, and indicates a temper that may be 
 wTought, that is, turned or filed, though with difficulty, and 
 only when a mild sort of steel is employed. If the tallow^ be 
 heated so as to yield a black smoke, and still more abundant, 
 this w ill denote a purple temper ; and if the steel be good, it 
 will now' work more pleasantly, though still hard enough to wear 
 well in machinery. The next degree of heat may be known by 
 the tallow' taking fire, if a lighted body be presented to it, but 
 yet not so hot as to continue to burn when the light is withdraw n ; 
 and this will denote a blue. If the whole of the tallow be allow- 
 ed to burn away, or burn dry, as it is termed by the w'orkmen, 
 it imparts the temper which clockmakers mostly use for their 
 work. Further tallow’ is useless; a small degree of heat more 
 W’ould just be seen in a dark place, or the lowest degree of red 
 heat. Any single article, to spare the trouble of brightening its 
 surface, may be smeared with oil or tallow', and its temper, when 
 heated, ascertained in a similar manner. Small articles, such 
 •s pendulum and other springs, need not be dropped into water, 
 but only made to pass through the air, by tossing them out, 
 and letting them fall to the ground, which will make them hard 
 enough for most purposes. Small drills may be hardened by 
 holding their points in the flame of a candle, and, when suffi- 
 •ieiitlv hot, suddenly plucking tliem outj^ the air will harden 
 
MECHANICAL EXERCISES. 
 
 7 
 
 Effects of saline liquids and mercury, in hardening steel. 
 
 them ; or they may be laid upon a plate of cold iron or lead, and 
 another plate upon them. They may be tempered, if found too 
 hard, by taking a little of the tallow upon their point, then passing 
 them through the flame at about half an inch above the point, and 
 holding them there till the tallow begins to smoke. 
 
 Solid tallow is an excellent material for hardening drills, and 
 other articles, which require considerable hardness, but must not 
 be made brittle ; tallow differs from oil, in the absorption of heat 
 for its fusion. Oil is found to harden the surface of steel much 
 more than its internal part, so that it resists the file, but is much 
 less easily broken by the hammer. This effect is owing to its 
 imperfectly conducting quality, and the elevated temperature it 
 demands to be converted into the vaporous state ; a covering of 
 coal is also formed round the steel by the burned oil, wliich 
 greatly retards the transmission of the heat. Any other fluid 
 which covers the steel in like manner, for instance water holding 
 soap in solution, produces a similar effect. Hence the vehicle in 
 which ignited steel is plunged, is of great consequence. The 
 colder it is, the more effectually it hardens the metal. Various 
 artists avail themselves of different substances for this purpose. 
 Some use urine, others water charged with common salt, nitre, 
 or sal ammoniac. Saline liquids produce rather more hardness 
 than common water, and aquafortis, in particular, possesses this 
 property in an eminent degree. Files are covered with the grounds 
 of beer and common salt, and dipped while wet in a powder made 
 of burned or parched horn, leather, or other coally animal matter. 
 By this means they are not only defended from scaling, by the 
 fused salt and animal coal, which covers them on all sides, but even 
 rendered rather more steelly on the surface, by the absorption of 
 carbon. They are taken out as soon as they have acquired the 
 low red heat called cherry red, and instantly plunged into pure 
 cold water. 
 
 When steel is required to possess the greatest possible degree of 
 hardness, it may be quenched in mercury, which will render it so 
 hard as to cut glass like the diamond; but this fluid, it is obvious, 
 can only be used on a small scale. 
 
 Wrought iron may tye hardened, in a small degree, by ignition 
 and plunging in water, but the effect is confined to the surface ; 
 except, as very often happens, the iron contain veins of steel. 
 These are no small impediments to the filling and working of this 
 material. 
 
 The general method of chusing steel for edge tools, is to 
 break a bar, observe its fracture, and select the closest grained. 
 But this mode is not always certain, as a variation in the fracture 
 ■will be occasioned by the. difference of its temper, and the 
 
8 
 
 MECHANICAL EXERCISES. 
 
 Choice of steel. — Case hardening. 
 
 greater or less heat at which it has been hammered ; and some steel 
 breaks of a very close gram, though of indifferent quality. The 
 surest method is to have one end of the bar drawn out under a low 
 heat, such as an obscure red, and then to plunge it suddenly, at 
 this heat, into pure cold water. If it prove hard, for instance, if 
 it will easily cut glass, and requires a great force to break it, what- 
 ever its fracture may be, it is good, the excellence of steel being 
 always proportionate to the degree of its tenacity in its hard state. 
 In general, a neat curved line fracture, and even grey texture, 
 denotes good steel, and the appearance of threads, cracks, or 
 brilliant specks, is a proof of the contrary. 
 
 If diluted nitrous acid (aqua fortis) be applied to the surface 
 of steel previously brightened, it immediately produces a black 
 spot ; but if applied to iron in like manner, the metal remains 
 clean. By this test it will be easy to select such pieces of iron or 
 steel as possess the greate.st degree of uniformity ; as the smallest 
 vein of either upon the surface, will be distinguished by its pecu- 
 liar sign. 
 
 The hardness and polish of steel may be united, in a certain 
 degree, with the ffrmness and cheapness of malleable iron, by 
 what is called case-hardeningy an operation much practised, and of 
 considerable use. It is a superficial conversion of iron into steel, 
 and only differs from cementation in being carried on for a sliorter 
 time. Some artists pretend to great secrets in the practice 
 of this art, using saltpetre, sal ammoniac, and other fanciful 
 ingredients, to which they attribute their success. But it is now 
 an established fact, that the greatest effect may be produced by 
 a perfectly tight box, and animal carbon alone. The goods 
 intended to be case-hardened, being previously finished w ith the 
 exception of polishing, are stratified with animal carbon, and 
 the box containing them luted with equal parts of sand and clay. 
 They are then placed in the fire, and kept at a light red heat for 
 half an hour, when the contents of the box are emptied into 
 water. Delicate articles may be presei^ed, like files, by a saturat- 
 ed solution of common salt, with any vegetable mucilage to 
 give it a pulpy consistence. The carbon here spoken of, is 
 nothing more than any animal matter, such as homs, hoofs, 
 skins, or leather, just sufficiently burnt to admit of being re- 
 duced to powder. The box is commonly made of iron, but the 
 use of it, for occasional case-hardening upon a small scale, may 
 be easily dispensed with ; as it will answer the same end to en^ 
 lope the articles with the composition above directed to be used 
 as a lute, drying it gradually, before it is exposed to a red heat, 
 otherwise it will probably crack. It is easy to infer, that the 
 depth of the steel induced by case-hardening, will vary with the 
 
MECHANICAL EXERCISES. 
 
 -9 
 
 Bluing of steel. — Expansion of steel in hardening. 
 
 time the operation is continued. In half an hour it will scarcely 
 be the thickness of a sixpence, and therefore will be removed by 
 violent abrasion, though sufficient to answ'er well for fire irons, and 
 a multitude of other utensils, ill the common usage of which its 
 hardness prevents its being easily scratched, and its polish is pre- 
 served by friction with so soft a material as leather 
 
 The bluing of steel has a remarkable influence on its elas- 
 ticity. This operation consists in exposing steel, tlie surface of 
 wliich has been brightened, to the regulated heat of a plate of 
 metal, or of a fire or lamp, till the surface has acquired a blue 
 colour. If this blue coal, so commonly considered rather as 
 ornamental than useful, be partially or wholly removed by 
 grinding, or in any other manner, the elasticity is proportion- 
 ately impaired, and the original excellence of this property can 
 only be restored by bluing the steel again. Saw-makers first 
 harden their plates in the usual way, in which state they are 
 brittle and warped; they then soften ffiem by blazing, which 
 consists in smearing the plate with oil or grease, and heating it 
 till tliick vapours are emitted, and burn off with a blaze. They 
 then hammer them flat, and afterwards blue them on a hot iron, 
 which renders them stiff and elastic, without altering their flat- 
 ness. 
 
 It may be useful to apprize the inexperienced, that tlie harden- 
 iug of steel increases its dimensions ; so that such pieces of work 
 as are finished with nicety in their soft state, will not fit their places 
 when hardened. The amount of this expansion cannot be accu- 
 rately stated, as it varies in different sorts of steel, and even in the 
 same steel, operated upon at different degrees of heat, Rinman 
 found that bars of steel six inches long, six lines wide, and half 
 an inch thick, were lengthened at least one line, after hardening 
 at a w hitish red heat, which is about one-seventieth part of the 
 linear dimensions ; but, according to the experiments made by 
 others, the expansion is not so considerable. 
 
 It is also a curious fact, that intense cold has an unfavourable 
 effect upon steel ; so that, in severe frosts, workmen often find 
 their tools incapable of receiving the temper they wish. 
 
 A slender rod of wrought iron may be expeditiously con- 
 verted into steel, by plunging it into cast iron in fusion ; — a sa- 
 tisfactory proof that cast iron contains the steel-making principle, 
 w hich, we need not repeat, is carbon. In fact, as it is princi- 
 pally in the superabundance of its carbon that it differs from 
 steel, many attempts (and not wholly without success) have been 
 made to convert it into the latter, without the intermeffiate opeia- 
 tion of rendering it malleable. But the best steel made pursuant 
 to this idea, is very imperfect. It is, hovyever, not unimportant 
 
io 
 
 MECHANICAL EXERCISES. 
 
 Hardening cast iron. — Forging and welding. 
 
 to observe, that all cast iron so far resembles steel, as to be hard- 
 ened, in a high degree, by sv.^den cooling, which imparts to it, at 
 the same time, whiteness of colour, brittleness, and closeness of 
 texture. This property of crude iron may be advantageously 
 employed on many occasions ; for instance, in the fabrication of 
 axles and collars for wheels, which are easily turned or filed in 
 their soft state, and may afterwards be hardened, so as to wear ad- 
 mirably well. 
 
 The heat applied to cast iron, previously to^its being plunged 
 into w ater to harden, is greater than that to w hich steel is subjected 
 for tlie same purpose ; it should be little short of a white heat. 
 Cast iron, also, when once hardened, admits not, like steel, of that 
 hardness being reduced, by various gradations, to any specific de- 
 gree ; to soften it materially, it must be submitted, for some time, 
 to complete ignition, and very gradually cooled. 
 
 The smaller ramifications of cast iron work, and those por- 
 tions of metal which have the furthest to run from the git, are 
 often found so extremely hard, tliat the best file w ill make no im- 
 pression upon them,' while the remainder of the casting is sufii- 
 ciently soft and manageable. This effect is owing to the heat 
 carried off by evaporation from the moistened sand of the mould, 
 by which the portions of metal, under the circumstances alluded 
 to, are suddenly cooled. To increase the number of gits, and to 
 use the sand as dry as possible, are the obvious means of prevent- 
 ing this defect ; but when it has taken place, annealing is the only 
 remedy. 
 
 The chemical properties of iron, and the best metliods of pre- 
 serving it from rust, to which it is so liable, we shall speak of 
 hereafter. The mechanical management of it, which constitutes 
 our present subject, now requires us to proceed with the opera- 
 tions of 
 
 Forging and Welding, 
 
 In forging, the fire must be regulated by the size of the work ; 
 and in heating the iron, the workmen, when the flame begins to 
 break out, beat the coals, round the outside of the fire, close 
 together with the slice, to prevent the heat from escaping. To 
 save fuel, they damp their coals, and throw water on the fire, if 
 it extend beyond its proper limits. To ascertain the state of the 
 work, they draw it partly out of the fire, and thrust it quickly 
 in again, if not hot enough. The heat the iron receives in forg- 
 ing, is judged of by the eye, and is not commonly distinguished 
 into more Uian these three degrees, viz. the blood or cherry red 
 heat, the white flame heat, and the sparkling or welding heat. 
 The cherry red heat is us^ when it is oifly intended to smooth 
 
MECHANICAL EXERCISES. 
 
 11 
 
 White flame heat. — Welding heat. — Use of sand in forging. — ‘Upsetting. 
 
 ; 1 
 
 the surface of the iron, or stiffen it in a small degree, operations 
 which are performed by striking evenly with the hand hammer ; 
 with light bloAvs, when smoothness of surface only is wanted, and 
 using considerable force, when it is desirable, at the same timej 
 rather to harden the work. When stiffness alone is required, the 
 iron is usually hammered cold, by which means it may be rendered 
 considerably elastic. Bell-springs are rarely made of any thing 
 else than sheet iron thus managed. 
 
 In changing the form of iron, the white flame heat is used, and, 
 according to the size of the work, it is battered by one, two, or 
 more men, with sledge hammers, the largest size of which, called 
 About Sledges, are slung entirely round, with both hands nearly at 
 the extremity. When the iron is nearly reduced to the proposed 
 f©rm and size, it is finished with the hand hammer, the dexterous 
 use of which will save much trouble in filing. 
 
 When the iron is required to be doubled, or two or mora 
 pieces consolidated, the sparkling or welding heat is used, by 
 which the metal is brought nearly to a state of fusion, and ap* 
 pears to be covered with a strong glaze or varnish. This varnish 
 is still more abundant in steel. As soon as the two pieces of iron 
 to be united, have attained the w'elding heat, they are taken out 
 of the fire with the utmost dispatch, the scales or dirt, which 
 would hinder their incorporation, scraped off*, placed in contact 
 at the heated part, and hammered until no seam or fissure re- 
 main. If they have not been sufficiently incorporated, the 
 beating and hammering ought to be repeated, until the work is 
 perfectly sound. Workmen differ very considerably in the care 
 they bestow in this respect ; and when axletrees and other parts 
 of machinery give way, a defective forging is generally vei’y 
 apparent. To make the iron come sooner to a welding heat, 
 stir the fire with the hearth staff, and throw out the clinkers, as 
 well as the cinders upon which the iron may have run, as they 
 will prevent the coals from burning. The fire for welding should 
 be free from sulphur ; and the rods may, in part, be prevented 
 from wasting, by taking care to supply them, at the heated part, 
 with powdered glass, or sand ; or a mixture of sand and the scales 
 wliich fly from ignited iron in hammering. Care must also bo 
 taken to prevent the iron from running, which w ill make it so brittle 
 as to prevent its forging, and so hard as to resist the file. If this 
 accident occur, the whole of the iron supposed to be injured by 
 the extreme heat to w'hich it has been exposed, must be cut off 
 and rejected. 
 
 When it is required to thicken any part of a bar of iron with- 
 out welding, the operation called upsetting must be resorted to^ 
 This consists in giving it th« whit« flame heat at the part to bo 
 
12 
 
 MECHANICAL EXERCISES. 
 
 Necessity of blending steel and iron. — Damascus steel. 
 
 thickened, and, while one end rests upon the anvil, hammering at 
 the other till the required size is produced. 
 
 In forging steel, great care must be taken not to use a higher 
 degree of heat than is absolutely necessary to effect the desired 
 purpose, as well as to use the fewest heats possible. To unite 
 steel to iron completely, without injuring the former, is an oper- 
 ation that demands a nicety of management which workmen are 
 hot often very anxious to display. Those, therefore, whose 
 purposes require it to be well performed, will only employ men 
 on whom they can depend. It is not always merely for economy, 
 that steel is welded to iron, but often principally with the view 
 of uniting the opposite qualities of the metal in each state. If 
 jthe mandrel of a lathe were made of the best steel, sufficiently 
 hard to wear well in the collar, it would be snapped by a sudden 
 check : and an axe, wholly of steel, if soft, would be useless ; and 
 if hard, would probably neither bear the shock of a violent blow, 
 nor the twisting to which such tools are subjected. But by unit- 
 ing a proper quantity of iron with the steel, the inconvenience, 
 and even danger, resulting from such accidents, are avoided. 
 In applying them to each other, regard must be paid to the 
 manner in which the tool will be used. For an axe, the edge 
 of w^hich is formed by grinding both sides, the steel is placed in 
 the middle, between two plates of iron; the blade of a plane, 
 which is ground only on one side, requires the iron also to be only 
 on one side, namely, the back of it ; and for that part of a man- 
 drel which works in the collar of a lathe, the steel must enciicle 
 the iron. 
 
 Damascus was anciently famed for the excellence of the steel 
 goods manufactured there, especially its swords, which are said 
 to possses all the advantages of flexibility, elasticity, and hard- 
 ness. These united distinctions are supposed to have been ef- 
 fected by blending alternate portions of iron and steel ; the latter, 
 by repeatedly drawing out, doubling, and welding the work, 
 feeing diffiised throughout the former, almost as completely as 
 a drop of ink is diffused, by intermixture, with a glass of w ater. 
 But the best attempt wdrich we are at present aware of having 
 been made in England, to imitate Damascus steel, according 
 to the plan here pointed out, did not perfectly succeed, the 
 mass produced having cracked in tempering. It appears pro- 
 bable, that the desired imitation may be effected with muck 
 greater advantage by the use of steel alone, the iron Irom which 
 it is made being judiciously selected, and afterwards very care- 
 ftiily cemented and forged. It is Swedish iron that is mostly 
 converted into steel ; but that kind called old sable (which, we 
 believe, conies from Russia,) possesses, in point of tenacity, ac- 
 
MECHANICAL EXERCISES. 
 
 15 
 
 Walby’s forge hammer. — Welding cast steel. 
 
 cording to the experiments of an ingenious philosopher, a 
 very decided superiority over every other kind. It would, 
 doubtless, therefore, be suitable for the purpose ; the properties of 
 steel being influenced, as will easily be supposed, by the proper- 
 ties of the iron from which it is manufactured : and, in confir- 
 mation of what has been said of the advantages of good forging, 
 we may here take notice of the forge hammer,* invented by 
 George Walby, of London, for which invention, the Society 
 for the Encouragement of Arts, See. rewarded him with their 
 silver medal and forty guineas. Although it weighs seventy 
 pounds, it may be wrought by one man, with the greatest accu- 
 racy and ease, at the rate of three hundred blows per minute, 
 and performs the work of two or three men. The inventor 
 states to the Society, that the steel is kept in better temper by 
 this hammer, and fewer heats are required for the same work, 
 than in the common way ; that the trowels made with it by him,, 
 will bear any pressure of bending, and return by their elasti- 
 city to their original shape, and they will even cut a chip from 
 a bar of iron, without hurting their edge ; they also are lighter 
 and more handy than common trowels, and serve much longer 
 in use. 
 
 The steel which contains the smallest proportion of carbon, 
 as, for example, shear steel, is the most easily welded ; but it is 
 by fusion, which entirely destroys its fibrous texture, that it is 
 rendered incapable of being welded to itself, and some maintain 
 that genuine cast steel has never been united even to iron by 
 welding. Yet others have stated, that the means by which this 
 may be accomplished, consist in placing between the iron and 
 the steel another kind of steel, in the form of filings, or a thin 
 plate, the iron being brought to a wielding heat, and the cast 
 steel made as hot as can be done with safety. Such, however, 
 are the difficulties of the operation, and so frequently imperfect 
 the work when finished, that other means of effecting the union 
 have been resorted to. One of them, for which a patent has 
 been granted, has been brought into common use, for the blades 
 of joiners’ planes, and many other purposes ; it consists in unit- 
 ing the steel to the iron with soft solder or'tin. In this process, the 
 cast steel, not being exposed to much heat, loses none of its good 
 properties ; but the union is not so substantial as that afforded by 
 welding. 
 
 O 
 
 * For the description of this hammer, illustrated by an engraving, see the 
 Transactions of the Society for the Encouragement of Arts, Manufactures, and 
 Commerce, or the Repertory of Arts, vol. 7, second series, I80d. 
 
14 
 
 MECHANICAL EXERCISES. 
 
 The forge bellows. — The anvil. — The vice. 
 
 Of the Tools used in forging Iron^ and in the working of MetaU 
 
 gejierally. 
 
 Minutely to describe the vaiious tools made use of in forging 
 iron, and in the working of metals generally, would be more 
 likely to tire than to please our readers, to whose information on 
 such points, a volume of the most elaborate description, would 
 not add so much, as a few moments’ inspection of workshops 
 which may be seen in every village of the kingdom. We 
 shall, therefore, on this, as on other occasions of the kind, 
 confine our remarks to particulars M'hich are, for the most part, 
 cither not generally practised, or not often communicated by 
 workmen, or not the most likely to catch the eye of the 
 looker-on. 
 
 The best position for the bellows is on a level with the fire- 
 place, but they are frequently placed higher, and the blast com- 
 municated through a bent tube, for the purpose of gaining room 
 near the floor. The small end of the pipe of the bellows passes 
 through the back of the forge, where it is fixed in a strong iron 
 plate, called a tue iron, or patent back, in order to preserve the 
 bellows from injury, and the back of the forge from requiring fre- 
 quent repair. 
 
 The a?ivil is a substantial mass of iron, to the surface of 
 which a plate of steel is firmly welded, and made sufficiently 
 hard to withstand the file, or the blow of a hammer. It is usu- 
 ally made, for forging iron upon, with one or tw’o projecting 
 arms, and is then called a beak iron. These arms are useful in 
 giving the requisite form to various sorts of work : when there is 
 only one, it is preferred of a conical shape ; when there are tw'O, 
 one of them is pyramidical. They are affixed lengthways, a 
 little below the surface of the body of the anvil, and rather in- 
 clining upwards towards the point. In Birmingham, where 
 attic rooms are frequently converted into workshops, the block 
 upon which the anvil is fixed, is placed upon a stratum of sand, 
 which prevents the vibrations that would otheiwvise be commu- 
 nicated to the floor, and much of the noise which w^ould incom- 
 mode the inhabitants of the room below. The contrivance is 
 simple, and susceptible of other applications. Clock-makers 
 use very small anvils or beak irons, w hich they fix in the vice when 
 in use. The anvils of tin-plate workers are of various sizes, and 
 ye often made w ith concavities and projections upon them, by tb« 
 help of which they can readily communicate different shapes to 
 their work. 
 
 The large vice must be firmly fixed to the side of the work- 
 bench, to the edge of which ite chaps must b« parallel, their 
 
MECHANICAL EXERCISES. 
 
 15 
 
 Hand vice. — Hammers. 
 
 upper surface being at the same time exactly horizontal. The 
 best elevation for a vice, is that of the workman’s elbow, when 
 the upper ann is held vertically against the side ; and the lower 
 arm, for the sake of trying tlie height, is held at right angles 
 thereto. In riling, if the vice or the work be above this position, 
 which is seldom heeded, or even thought of, the stroke will not 
 be so pow erful as the same exertion would otherw ise make it ; and, 
 whether higher or lower, it will be found exceedingly difficult to 
 carry the rile in a horizontal direction. As the teeth on the inner 
 surface of the chaps would mar rine work, if pressed against it 
 sufficiently hai d to keep it steady, they are, as often as the occasion 
 requires, covered with plates of lead, about the eighth of an inch 
 thick. These plates must be large enough to extend about half 
 an inch on each side beyond, and an inch above the chaps, to each 
 of which, w hen screw ed tight, one of them is secured by hammer- 
 ing down the projecting parts. 
 
 The hand vice is used to hold small articles in the act of 
 filing ; it is held in the left hand, and the parts of the iron, 
 while pressed upon the end of the bench, or upon a bit of 
 W'ood or bone in the large vice, is successively turned to the 
 file, which is held in the right hand. A nick is made in the 
 wood or bone, to keep the work from being carried aside by the 
 file. 
 
 Hammers, like anvils, are faced with steel, in a state of 
 considerable hardness. Tlieir handles are almost always made 
 of nearly a uniform thickness in every part, or if they differ from 
 such figure, it is not for any specific purpose. Hence the vibra- 
 tions of the hammer head are communicated to the hand, to 
 which they occasion very unpleasant sensations, and the workman 
 is tired before he has much exerted his strength. If the handle 
 of the hammer, at a little distance from its upper end, be made 
 considerably smaller, for a short space, than in any other part, the 
 alteration will be found a decisive improvement. Such a ham- 
 mer will, as it is technically termed, fall well ; diminishing, at 
 the same time, the workman’s fatigue, and convincing him that 
 his blows 'are solid and effectual. Fig. 1. pi. III. will clearly 
 designate this construction : it represents a hammer for chipping 
 iron ; for which purpose, the head need not be more than sixteen 
 ounces in w'eight, and the handle about tsvelve inches long. In 
 a hammer of any given shape, calculated to give the hardest 
 blows with the least weight, and, consequently, with the least 
 fatigue, the quantity of iron in the head should be equal on the 
 opposite sides of a line supposed to be drawn perpendicular to 
 the centre of the face. Hammers, therefore, made for the pur- 
 |)Oie of drawing nails, with claws, which lean backwards from 
 
16 
 
 MECHANICAL EXERCISES. 
 
 Rivetting — Cutting metals with shears — chisels — saws. 
 
 this line, are not calculated to produce the best effect in ^riking. 
 Clockmakers, tin-plate workers, and braziers, polish the face of 
 their planishing hammers, by rubbing them upon a soft board, 
 covered with a mixture of oil and finely washed emery. Watch- 
 makers and silversmiths lake still more pains with theirs, select- 
 ing them free from every flaw, removing every scratch, and 
 giving them an exquisite lustre with colcothar or putty. These 
 various artists, also, for their respective purposes, require them to 
 be made of a numberless variety of shapes, convex, concave, cylin- 
 drical, &c. 
 
 In rivetting two pieces of metal together, if tlie head of the 
 rivet is not intended to project, the hole must be widened a little 
 at the top and bottom. One of the heads of a rivet should be 
 made before it is put into its place, in which it is secured, by strik- 
 ing the edge of the other end of the shank (previously filed flat, 
 with light blows, till it is evenly spread all round, when heavier 
 blow s may be used, till it is sufficiently firm. When the head of 
 a rivet or screw is on a level with the surface of tlie w ork, it is said 
 to be countersunk. 
 
 In cutting sheet iron or brass, and even bars of the same 
 metals, shears are used. They are frequently made three or four 
 feet long ; one handle is screwed fast in the vice, or secured to 
 the bench, and the uppennost only is moveable. The harder 
 the work they have to do, the more obtuse the angle by which 
 the edge is formed. A chisel is often used instead of a pair of 
 shears, and though it does not cut with so much rapidity, it is, 
 on many occasions, more convenient, as it can be made of dif- 
 ferent figures, guided in various directions, and stopped at any 
 given point. Plates of metal to be cut with a chisel, are laid, 
 during the operation, upon a mass of lead, or upon an anvil ; 
 if the latter be used, they are not cut quite through, to prevent in- 
 juring the chisel, yet tliey are so nearly divided that the separation 
 can be effected by striking them with the hammer while held on 
 the edge of the anvil, or by wriggling them with the hand or in 
 the vice. 
 
 Saxes for cutting metals, are made very narrow, (see fig. 2. 
 pi. Ill :) and stretched by a screw at one end ; they are, in ge- 
 neral, rather thicker on the edge than at the back ; the teeth are 
 small, and are not bent like those for joiners’ use. Clock and 
 watch makers often make their saws of broken watch springs, the 
 temper of which is suitable for the purpose, and the metal com- 
 monly excellent. In sawing malleable iron and steel, oil must 
 be used ; crude iron and brass require no oil, but for the latter, a 
 very sharp saw is necessary, and it may also be rather harder tlian 
 for iron. 
 
MECHANICAL EXERCISES. 
 
 17 
 
 The chipping chisel. — The punch. 
 
 Metals are sometimes wrought by chipping. This operation 
 not only often produces the intended effect in an expeditious 
 manner, but saves much expence in the files which would other- 
 wise be required. It is most frequently applied to cast iron, the 
 dark rind or outside of which, taken as it comes from the mould, 
 is always harder than the rest, and frequently so very hard, that 
 it w ould spoil tlie best file in a few minutes, w hile, at no greater 
 depth than the twentieth part of an inch, or even less, it is 
 nearly as soft as brass. The chisel wdll penetrate this hard crust, 
 and afterwards, as may be easily understood, its edge need 
 only be made to act upon the soft part. The chisel, for this de- 
 scription of work, need not be more than seven inches long, but 
 it ought to be made of the best cast steel. Fig. 3. pi. III. re- 
 presents such a chisel. — No. 1, showing the front, and No. 2, the 
 side of it, to point out the nature of its edge. The hammer to 
 be used wdth it has been already mentioned. It is held in an 
 angle of about forty-five degrees, and the blow's of the hammer 
 are given in quick succession. Some dexterity, certainly, which 
 can only be acquired by practice, is requisite, to preserve a 
 tolerably equable surface, but the art is not of difficult acquire- 
 ment. A pellicle of iron may, by the chisel, be taken from a 
 surface of a hundred square inches, in four or five hours, and 
 when it has been well done, the file very speedily levels the ine- 
 qualities which it leaves. When much exactness is required, it is 
 advisable to examine the work, before the chipping is commenced, 
 and if improper protuberances or hollows appear in it, the chisel 
 must be struck deeper, or not so deep, at such places, as the cir- 
 cumstance dictates. 
 
 Malleable iron, in a state of ignition, is easily perforated 
 with a steel punch, which is made of the size and shape of 
 the hole required, except that it must always be tapered more 
 or less towards the lower end, to facilitate drawing it out. 
 It is seldom pointed at the extremity, which is hardened 
 without tempering, ns the heat of the iron will soften it suffi- 
 ciently, and sometimes too much; to check the latter effect, 
 it is plunged into water, as often as it is supposed to be con- 
 siderably heated. The hole may be finished with a file, or 
 by hammering it at a low heat upon a smooth mandrel or pin, 
 or by a well tempered triangular, square, or octagonal bar, 
 fixed to a handle, and wrought the same way as a carpenter's 
 auger. A tool of this description is called a rimer, and is made to 
 taper a little from the handle to the lower end. In using it the 
 motion must be slow% The triangular and square form answer 
 well for brass, and the softer metals ; but the octagonal one is 
 much more suitable for iron; as the other would take hold lo 
 l.—VoL. L D 
 
18 
 
 MECHANICAL EXERCISES. 
 
 Old method of boring cylinders — new method. 
 
 deeply as to break with the force requisite to turn them round. 
 A sharp-pointed punch will penetrate a piece of cold iron, not 
 exceeding the tenth of an inch in thickness, sufficiently deep to 
 cause a projection on the under side ; when this projection is tiled 
 off, if the hole does not appear, a repetition of the punching will 
 immediately produce it ; and it may be widened by the octagonal 
 tool above-mentioned. Brass may be managed in the same way, 
 with still more facility : the plate of metal to be pierced, should 
 be laid upon lead ; or the under surface, opposite the point of the 
 punch, should be placed over a hole in an anvil. 
 
 As punching is not applicable to cast iron, nor to small and 
 deep, or very large holes in any metal, and is, besides, apt to 
 throw the piece out of shape, mechanics have recourse, accord- 
 ing to the nature of the work they have in hand, to the different 
 methods of 
 
 Boring a?id Drilling, 
 
 The steam engines of the present day are not more indebt- 
 ed for their excellence to modern improvements in their con- 
 struction, than to the new methods which have been adopted 
 to render them faultless in point of workmanship. In the 
 latter respect, the boring of the cylinder presents one of the 
 most remarkable features of difference from the old plans. 
 The way usually was, at some of the first founderies, to put 
 it upon a carriage, insert the cutter block, set the mill to 
 work, hang a cloth at the open end to keep in the dust, and 
 let it bore away, which it would be doing, on a large cylin- 
 der, for three weeks or a month ; and if it was tolerably 
 smooth, it was said to be well done. As the cylinder is cast 
 hollow, though the molder pursues the most correct method 
 his art is capable of, yet it is impossible to be certain that, 
 when the mould has received the metal from the furnace, it 
 shall come out quite straight ; and if it come out crooked, it 
 must remain so, for this despicable mode of boring will never 
 remedy that imperfection. It is not like boring a solid piece 
 of metal, as in boring ordnance, &c. All that this old bor- 
 ing can do to a cylinder, is to make it round and smooth, for 
 there is nothing to conduct the boring bit in its progress, but 
 the form given it by the moulder, whose best exertions cannot 
 ensure success: it complies, therefore, with the twdstings of its 
 road, and the cylinder is inaccurate. If the metal be harder on 
 one side than another, it produces an additional source of imper- 
 fection. 
 
 The new method of boring originated with John Wilkin- 
 son, iron master, and the cylinders were executed in a man- 
 
MECHANICAL EXERCISES. 
 
 19 
 
 New method of boring cylinders. 
 
 ner which has not since admitted of improvement. When 
 the process is conducted by an intelligent workman, if the 
 cylinder should be cast ever so crooked, or ever so thick on 
 one side more than another, he can take out the redundancy 
 from that side, and scarcely touch the other. This will rea- 
 dily be admitted, when it is understood, that the cutting ap- 
 paratus is conducted along a tool (called by the workmen a 
 boring bar) which is itself a masterpiece of workmanship, 
 a perfect cylinder. Hence, whatever is carried along this bar, 
 parallel to its axis, must move in a right line. When, there- 
 fore, it has been turned with the utmost care and precision, 
 it is to have two grooves cut opposite each other in this direc- 
 tion. A cast iron socket is then bored and ground upon the 
 bar, so as to fit it in the most exact manner. The external 
 part of this socket is made conical, with four or six studs 
 upon the base of it, to receive the cutter block ; and fillets 
 fastened upon the inside of it, and falling into the grooves, 
 while they allow it to slide along the bar, prevent its being 
 carried round, unless the bar be carried round at the same 
 time. To give a progressive motion to the socket and cutter 
 block, while the bar is turning on its own axis, a collar of 
 metal is fitted on the socket, and that collar is connected with 
 two racks, long enough to reach through the cylinder, and 
 communicate with a pair of pinions, which being acted upon 
 by two levers, carrying a sufficient weight to overcome all 
 resistance in the operation, the socket is drawn along the 
 boring bar, and the cutters fastened in it effectually perform 
 their work. The manner in which the lever is applied, will 
 be understood by an inspection of fig. 4, pi. III. by which 
 it is obvious, that if the weight at its upper end be sufficient to 
 overcome the resistance, it will move the pinion till it reaches 
 the earth ; at this moment, or a little before, the workman who 
 attends the machinery, lifts it up, and the catch affixed to it, 
 which takes hold of the ratchet wheel, prevents its return with- 
 out giving motion to the pinion. 
 
 In fitting up the boring apparatus, some diversity of prac- 
 tice prevails. By some, a hole, to admit a single rod, is drilled 
 through the whole length of the bar, and a groove is sunk 
 entirely through one side of it, so as to come into the hole 
 thus drilled. A branch from the internal part of the socket 
 is fitted into the groove, with an eye to receive the end of the 
 rod, to which it is secured by a screw, so that when the rod 
 is drawn along, the socket moves at the same time in the same 
 direction. A weight with a rope over a pulley, is applied to 
 give the progressive motion to the socket upon the bar. This 
 
20 
 
 MECHANICAL EXERCISES. 
 
 Boring of ordnance. — Drilling with the lathe — the bow. 
 
 mode of constructing the bar is the best way for the boring of 
 small cybnders, as there is no incumbrance upon the socket; 
 and if the bar is sufficiently strong, it will move with great steadi- 
 ness. 
 
 Ordnance were formerly cast hollow; they are now always 
 cast solid, and afterwards bored by machinery. The gun to 
 be bored lies with its axis parallel to the horizon, and in that 
 position, moving in a collar fixed at each end, it is turned 
 round its axis. The borer is laid truly horizontal, in the di- 
 rection of the axis of the gun, and is incapable of motion in 
 any direction except that of its length ; and in this direction 
 it is constantly moved, so as to pierce and cut the gun, by 
 means of rack-work, a lever and weight, as above described for 
 cylinder boring. The outside of the gun is smoothed at the 
 same time by men, with instruments lit for the purpose, while it 
 revulves, so tliat the bore may be exactly in the centre of the 
 metal. 
 
 Boring differs fmm drilling only in being commonly applied 
 to larger works. Drilling may be effected in a lathe with great 
 facility. The drill is screwed, or otherwise fastened, upon the 
 spindle, so that its point shall turn exactly opposite the point of 
 the screw in the right hand puppet. The piece to be drilled is 
 then slightly pierced with a punch, where the drilling is to com- 
 mence, and also where it is intended to come out. Against the 
 latter puncture, the point of the screw in the right hand puppet 
 is directed, and gradually pressed forward as the dr’dl, on turning 
 the wheel, is found to cut. The motion of the wheel must be 
 slow', especially for iron. The rest, or any temporary support, 
 may be used to keep the work steady, which may then be 
 perforated with expedition and accuracy. A short lever, with 
 a weight at the end of it, may be applied to advance the screw, 
 60 as to leave both hands of the workman at liberty for otlier 
 matters. 
 
 Small drills, used by clock-makers and others, are usually 
 made of a single piece of steel wire, upon which, about the 
 middle, a pulley or drill barrel is driven, (see fig. 5 . pi. III.) 
 Sometimes, a shank or small mandrel is used, with a square 
 hole, about half an inch deep, at the end of it, into which drill 
 bits of various sizes can be alternately inserted. The disadvan- 
 tage of this construction is, that the chill bit is seldom held true, 
 which causes it to perform indiflerently. It is, therefore, but 
 little used by those workmen who can readily furnish themselves 
 with the other kind as they want them. When these small drills 
 are used, they are held horizontally, and pressed against the 
 work by a breast piece, which is sometimes made of w'ooc^ 
 
MECHANICAL EXERCISES. 
 
 21 
 
 The hand-drill. — Precaution in tempering drills. 
 
 and sometimes of sheet iron ; but, in either case, is rather con- 
 cave on its inside, to rest more steadily upon the breast, and in 
 the centre of the outside is fixed a bit of steel, for the blunt end 
 of the drill to work in. The drill is turned by drawing backwards 
 and forwards an elastic bow, the string of which is coiled once 
 round its pulley. The best bows are made of steel, and the 
 strings of catgut ; the strength of them must be proportioned to 
 the size of the drill. A piece of stout cane makes no bad substi- 
 tute for a steel bow. 
 
 To make large holes, more force is required than can be 
 given by the bow and string, instead of which a brace, not very 
 unlike that used by joiners, is employed, and the drill itself is 
 fitted as a bit ; but instead of the stock M'hich, in the joiner’s tool, 
 remains stationary, while the rest is turning, we have here a long 
 tapering spindle, which being nothing more than a continuation of 
 the brace, is necessarily carried round at the same time. The 
 upper end of the spindle works in an iron or steel plate, which 
 is fixed on the under side of a beam, called the drill beam. One 
 end of the beam turns upon a transverse pin between two uprights, 
 pierced with various holes, to fix it at dilferent elevations; the 
 other end, which is pressed down by a weight, passes, when great 
 steadiness is wanted, between tw'o other uprights. The point of 
 the bit being then placed upon the part of the metal to be drilled, 
 the brace is revolved by the hand, and a hole to any required depth 
 may be made. The bit should be well fitted to the brace, though, 
 as very small holes are not made with this apparatus, the disad- 
 vantage of its shaking a little is not of so much moment as in the 
 breast drill. The drill is commonly fitted up so that the w^ork to 
 which it is applied can be fixed in the vice. Fig. 6. pi. III. re- 
 presents the manner of fitting up the hand drill, and fig. 7- one of 
 the bits separately. 
 
 The vertical part of the crank, by which the hand revolves 
 the drill, ought to be very smooth, or, what is still better, it 
 may be covered with a loose handle. If this handle be made of 
 iron, it may be bent round and soldered ; if of wood, it may be 
 made out of a hollow cylinder, cut in two pieces, between which 
 the vertical part of the crank may be enclosed, and it may then 
 be fastened by glue, or by a hoop at either end, the diameter of 
 the hoop being made large enough to pass over any part of the 
 brace. 
 
 Drills ought to be made of the best steel, and the cutting part 
 only should be hard; they are therefore tempered by keeping 
 the lower end out of the fire, but heating the rest considerably, 
 till the point attains the desired colour, when it is instantly 
 cooled in the usual manner. By this means, the cutting part of 
 
23 
 
 MECHANICAL EXERCISES 
 
 Use of a fly in drilling. — Filing. 
 
 the bit may be tempered to a straw colour, while the rest is not 
 higher than blue, so that its liability to break when in use, is 
 greatly diminished. We may observe, in passing, that this mode 
 of tempering from the back of the tool, so as to have the edge 
 only in a state of great hardness, is observed as a general principle 
 in the art. 
 
 The application of a fly wheel to the upper part of the large 
 hand drill, would be a considerable improvement ; not merely on 
 account of its weight, but because its centrifugal force would tend 
 greatly to keep the drill exactly vertical. 
 
 In drilling, as in sawing, forged iron and steel require oil, but to 
 brass and cast iron none must be used. For brass, also, the drill 
 bit is made thinner, harder, and the cutting edge formed by a more 
 acute angle than for iron. 
 
 Filing. 
 
 In the working of metals, there is no operation more common 
 than that of Jilingy and perhaps there is none so little understood. 
 A file is an instrument too familiar to every one to require descrip- 
 tion. To use it well, generally proves one of the most difficult 
 tasks w'hich the practical mechanic has to encounter, and this 
 difficulty is owdng more to the want of a proper plan in setting 
 about the work, than to any other cause. Plane surfaces, for 
 instance, for the plates of air-pumps, and a thousand other pur- 
 poses, are of indispensable use ; but a knowledge of the manner 
 in which they may be readily and completely executed, is con- 
 fined to veiT few ; and a workman, aware of the exactness required 
 from him, can rarely be found who will undertake to execute 
 them. Grinding is the common and dernier resort of those 
 who w ish to produce, on such occasions, the last degree of accu- 
 racy ; but tw o surfaces of metal may be ground together for ever 
 without being made plane, unless, by some previous operation, 
 all their cross-zci tidings are completely removed. In the execu- 
 tion, however, of this previous operation, nearly the whole 
 difficulty of the business lies. In what must it consist f Giind- 
 ing has a tendency to perpetuate any regular convexity or con- 
 cavity wdiich either surface may have, and even to produce one 
 or other of these forms on each piece, although both were plain 
 to begin with. The application of turning to the production 
 of plane surfaces, (for which see the section on turning,) ia 
 not an easy undertaking, and requires an expensive apparatiis; 
 and often the mere fixing (upon the chuck) the metd to be 
 turned, takes as much time as ought to be required for the 
 completion of the work. We would incite, therefore, the in- 
 genious artist to place confidence in the Jile, with which, we 
 
MECHANICAL EXERCISES. 
 
 23 
 
 Value of the rt of filing. — General reflections. 
 
 hesitate not to assure him, that more beautiful and accurate 
 workmanship may be ei uted, than most of those who are, in 
 other respects, very resp table mechanics, are either apprized of 
 or disposed to consider possible. In this line of exertion, we 
 have witnessed, with admiration, the performances of one who 
 now holds a distinguished situation in the Royal Mint. With 
 the file alone, as his cutting and polishing tool, he has not only 
 produced specimens of workmanship which challenge all com- 
 petition, and the severest scrutiny, but effected his purpose with 
 a degree of expedition, and consequent economy, of which no 
 other method would admit. The work (the appearance of 
 which, though remarkably fine, was only a secondary considera- 
 tion,) required the exact parallelism of its several sides, some of 
 which presented a surface of not less than fifty or sixty square 
 inches ; and in his hands the file did all this, in such a manner as 
 to set at defiance the elegant art of turning, and to render the 
 dirty and tedious process of grinding wholly unnecessary. How 
 often, in provincial towns especially, have embrjo inventions been 
 kept back, for the want of workmen of sufficient skill to execute 
 the proposed contrivance ; and how often w'ould inventors them- 
 selves carry into effect their designs, if they were not filled with 
 the apprehension that the acquirement of a competent share of 
 manual dexterity was too difficult a task to be attempted! 
 Those who have had the most ample opportunities of observation, 
 will not consider these idle surmises ; they cannot but be sensible, 
 that the inventions which become publicly known, are few in com- 
 parison with those which spring up in the minds of ingenioui 
 men, and perish from such obstacles as have been just stated, often, 
 perhaps, with the hour wffiich gave them birth. What one man 
 has accomplished, let not another despair of accomplishing also. 
 Superior opportunities of experience, are often vanquished by 
 superior exertions ; and if these remarks on the excellence attain- 
 able in an art of the first importance to the practical mechanic, 
 should stimulate one person to the improvement of his skill, they 
 will not be useless. 
 
 The practical directions belonging to this subject now claim 
 our attention. Here the general principle, upon the proper 
 application of which success depends, may in the first place 
 be noticed ; it is simply this, that if a plane surface, already 
 known to be true, could be made use of so as to show, with 
 perfect facility and correctness, the errors of another upon 
 which the artist may be employed, as often' as he wishes to as- 
 certain the state of his work, a file, or any tool by which all the 
 projections may be removed without reducing the other parts, 
 w'ill enable him at length to bring the latter surface to an exact 
 
24 
 
 MECHANICAL EXERCISES. 
 
 The different forms and names of files. — Choice of files. 
 
 correspondence with the former. Such a surface is, therefore, 
 indispensably necessary in the art of flat filing ; and we may add 
 to it another implement of almost equal utility, though very little 
 used, namely, a perfectly straight steel ruler, for which we shall 
 adopt the technical term, by calling it a straight edge. On the 
 production or procuring of these two things, we shall speak in a 
 future section ; at present we shall suppose them to be obtained ; 
 then an assortment of files follow^s of course, as also a vice, or some 
 other method of steadily supporting the metal upon which the file 
 is intended to operate. 
 
 Files are differently formed, and of various sizes for differ- 
 ent purposes, their sections being either square, oblong, trian- 
 gular or segmental ; the files of these sections are respectively de- 
 nominated square, flat, three square, or half round. That sort of 
 file called the safe-edge, (on account of its not being cut on one 
 edge,) which is flat on both sides, and of equal or nearly equal 
 breadth in every part, is the best for every purpose to which its 
 form admits of its being applied, and is particularly to be recom- 
 mended for flat filing. 
 
 In chusing files, some degree of attention is requisite, and 
 will save much subsequent trouble ; a file, the surface of which 
 is twisted in various directions, (a circumstance which very often 
 happens in hardening,) will constantly deceive the workman, as 
 it will produce nothing but false strokes. They must, therefore, 
 be chosen free from such imperfection, but a small degree of re- 
 gular convexity is not detrimental. The goodness of a file, so far 
 as its shape is concerned, may be readily determined by the eye, 
 in the same way as the joiner examines the straightness of a piece 
 of wood. 
 
 It is perhaps too obvious to require remark, that the scratches 
 made by a file will be proportionate to the size of its teeth ; and 
 that the larger these are, the greater will be the effect which an 
 adequate force wdll produce at one stroke : hence the very evident 
 propriety of commencing the work with the coarsest file intended 
 to be used ; and afterwards, in regular gradation, employing finer 
 and finer ones, as it approaches to the finished state. — Files may 
 be obtained, the teeth of which are so extremely fine, that they 
 w'ill leave the surface of metal, especially if it be brass, almost as 
 smooth as an oil-stone. These are, however, seldom necessary ; 
 and for most purposes, files of tliree or four degress of fineness, 
 are quite sufficient. 
 
 As most of the articles of manufacture, to which the file 
 can be applied, are composed of flat surfaces ; as he who can 
 file a flat surface w^ell, will find no difficulty in executing what- 
 ever the file will enable him to do j we shall detail the progress 
 
MECHANICAL EXERCISES. 
 
 25 
 
 Particular directions for filing. 
 
 of a block of metal taken rough from the foundery, till it is 
 brought to a tiiiished state ; and supposing a rectangular figure 
 to be aimed at, its surfaces Avill then be truly flat, and, accord- 
 ing to their situation, either exactly parallel, or exactly at 
 right angles to each other. As somewhat greater difficulties 
 occur in filing iron than brass, and as cast iron is not in ge- 
 neral so easy to manage as the other descriptions of the same 
 metal, we shall suppose it to be a block of cast iron. Merely 
 for the sake of having definite ideas of our subject, as we go 
 along, let us suppose it to be nine inches in length, seven in 
 breadth, and one in thickness. On receiving it, the first step is 
 to examine the state of the metal, whether it be hard or soft, 
 warped or tolerably straight, perfectly solid, or interspersed 
 with cavities. If it prove very hard, which may be known by 
 trying it with a file, it will be adviseable to anneal it; which 
 w'ill greatly facilitate our w ork ; but the outside \vill still be 
 somewhat harder than the internal part, owing principally to 
 some of the sand of the mould closely adhering to it ; this out- 
 side, or rind, some workmen remove by chipping, in the man- 
 ner already spoken of; others, who have the convenience, take 
 it off with a large grindstone turned by machiner}*; and othei*s, 
 again, use the file immediately, taking the precaution only 
 of using, in the first instance, a file that is already rather worn, 
 as a new one would quickly be spoiled. Chipping is upon 
 tlie whole the most economical and convenient process, and 
 when, for the removal of imperfections, or any other purpose, 
 it is requisite to reduce the block materially, it is decisively to 
 be preferred. If after the outside has been removed, there 
 
 appear any cavities or other imperfections, which are not 
 likely to be removed by the file, and wffiich w ill unfit the piece 
 for its destination, they may be drilled out, and the holes made 
 by the drill filled with rivets. Small imperfections may be 
 removed by drilling to the depth of alx)ut half an inch, 
 aed then driving in a plug made of wire, which may be 
 
 fitted sufficiently tight to bear any degree of hardship, and 
 sufficiently correct to avoid the slightest appearance of a flaw', 
 without the trouble, as in ri vetting, ‘of making the top of the 
 hole wider than the rest. With a view, however, to complete 
 security, some tap the hole they have drilled, and then screza 
 in a pin which exactly fits it ; but when this is done, and the 
 screw has a fine thread, in filing the surface level, that part of the 
 thread which is nearest to the surface, is apt to break off, to the 
 extent of a semicircle, and thus leave the work imperfect ; where- 
 as, when the plug or the rivet is well fitted in, the place cannot 
 
 afterwards be distinguished from the other parts of ffie block, by 
 
 2,— VoL.I. E 
 
20* 
 
 MECHANICAL EXERCISES. 
 
 Particular directions for filing. 
 
 any other circumstance than the superior brightness of the mal- 
 leable iron. 
 
 As the holes in a piece of cast iron, which are occasioned 
 either by stagnated air, or the falling in of part of the mould, 
 have mostly not only very rough surfaces, but are wider internally 
 than at the outside, they may be filled with melted lead, pewter, 
 or some other soft metal, which they will retain : type-metal will 
 answer extremely well, as, from the antimony it contains, it ex- 
 pands in passing from the fluid to the solid state. This mode is 
 applicable when levelness of surface is the principal object in 
 view, and it is not necessary to regard the uniformity of its appear- 
 ance, the equal hardness of its several parts, or its being able to 
 bear a strong heat. If we were speaking of a piece of metal, 
 eventually to be subjected to considerable stress, we might here 
 observe, that thus to fill up the hollows it contains, will greatly 
 increase its capability of resistance. 
 
 Let us now suppose that the block we have in hand, is com- 
 pletely freed from its hard black scurf, and, as far as may be 
 thought necessary, from every imperfection which the subsequent 
 operations with the file are incapable of removing. We now select 
 the file we intend to use first, and in doing this we pitch upon a 
 safe-edge one, about fourteen inches long, an inch and a half 
 broad, and containing about fourteen rows of teeth in each inch 
 of its length. In the act of filing, the file is held by the handle 
 and pushed forward by the right hand ; while the left hand, near 
 the waist, pressing upon its lower end, gives effect to the stroke, 
 which must be directed as nearly horizontal as possible. By 
 the occasional application of the straight-edge to the surface we 
 are filing, in various directions, but in particular, diagonally, we 
 easily ascertain the state of our work, and remove in succession the 
 elevated parts. The inequalities at length become so small, that 
 it would be tedious to apply the straight-edge to discover them ; 
 but being provided with a surface which we know to be true, 
 (and which w e shall designate by calling it a table, as it ought 
 always to be larger than the work we are filing, and for general 
 purposes, may with much advantage contain several square 
 feet,) w'e now make use of it, for the detection of the remaining 
 imperfections, in the following manner : w e mix finely washed 
 red chalk or ochre, with olive or any other oil which is not vis- 
 cid, and we rub this mixture upon it w ith a piece of cloth, so as 
 to cover the whole of it very thinly and evenly. If the surface 
 we are filing be then turned down upon it, and moved a few 
 times backwards and forwards, it will be every-where equally 
 covered with ochre fiom the table, provided it be equally level. 
 But as this will never happen at the first trial, those parts which 
 
MECHANICAL EXERCISES. 
 
 27 
 
 Particular directions for filing. 
 
 are highest will alone be reddened, and they must be reduced 
 by the re-application of the tile. As soon as the marks left by 
 the ochre have disappeared, and we think we have removed the 
 inequalities they pointed out, we again try the state of our work 
 as before, and continue to repeat the same process till it is fin- 
 ished. When it approaches nearly to a perfect plane, the 
 ochre will redden a great number of places in small spots or 
 strips, and then we not only, agreeably to the remark already 
 made, use a fine file, but hold it rather differently. Instead of 
 pressing it down, as when we began, with tlie broad part of the 
 hand, we now merely press upon it with two or three of our fin- 
 gers, by which means we are enabled to observe more distinctly 
 the spot upon which we bear, and to move with more expedition 
 from one part to another. 
 
 Before we begin to finish our work with much nicety, we 
 carefully attend to one thing : turning that side of the block w'e 
 have been filing down upon the table, w'e strike the back of it, 
 at the corners, centre, and various other parts at pleasure, w ith 
 a mallet, or the end of the handle of a hammer held perpen- 
 dicularly. If a dead sound, such as would be heard on strik- 
 ing the table itself in a similar manner, be produced, we feel 
 gratified by the assurance thus afforded, that w'e have none of 
 those twistings of the surface wliicli are technically termed cross- 
 wiridingSy to remove ; but if a sharp chinking sound be pro- 
 duced, it is evident that the surfaces of the table and the block 
 do not coincide, for the blow of the hammer has pressed one 
 part of the block lower down than it was before, and raised ano- 
 ther part ; and to the action of the surfaces upon each other thus 
 occasioned, the ringing sound is attributable. If the corner of 
 the block, to the extent of a square inch, or even much less, be 
 lower than the remainder of the surface, in no greater degree than 
 the common thickness of a sheet of writing paper, this mode of 
 trial will make the imperfection very distinctly perceptible. It^ 
 therefore, the block will not stand the test of this examination, w e 
 immediately proceed, by the use of the ochre, to detect the extent 
 of the elevated parts ; and in moving the block upon the table for 
 this purpose, we are careful to press only on those parts under 
 which we know, by our previous trial w ith the hammer, they are 
 comprized. Having obtained the marks we desire, we file away, 
 to the best of our judgment, the convexities they indicate, and re- 
 peat the experiment and filing, till the block wall lie perfectly solid 
 upon the table. This object, so essential to good work, being ob- 
 t^ed, and it ought always to be obtained as early as possible in 
 our progress, we shall approach with surer steps, to the successful 
 accomplishment of our task. 
 
38 
 
 MECHANICAL EXERCISES. 
 
 Use of the callipers and guage in filing. 
 
 The practitioner, however, will soon discover, that although 
 the test by the hammer answers an important purpose, in prov- 
 ing the existence or non-existence of cross-windings, yet its appli- 
 cation extends but little further ; the depression of any particular 
 part, before it can point it out, must not only extend to the edge 
 of the block, but must embrace a small portion at least of two 
 sides. Without, therefore, expecting from it what it cannot 
 afford us, we use it merely as a collateral help ; the use of the 
 ochre simply is our universal test ; but if we w ish to know the 
 measure of any particular imperfection, we resort to a good 
 straight-edge, the application of the arris of w'hich, to any 
 part we chuse to try, gives us, with the utmost precision, the 
 information we are seeking. If the surface tried be perfectly 
 true, no light w ill pass between it and the straight-edge ; but if 
 any hollow be present, the breadth and depth of the line of light 
 which appears, betrays its extent . — Arris is a common term in the 
 arts, applied to signify the line of concourse or meeting of two 
 surfaces. 
 
 Let us now suppose, that one surface of the block will bear 
 examining in the different ways above mentioned; it will then 
 coincide with the table so exactly, that when laid upon it, the 
 finest hair could not be drawn out, or even moved, at whatever 
 part betw een the tw'O planes a portion of it w ere placed. Not- 
 withstanding this, the surface, though very smooth, has not 
 been nicely polished ; the polishing we leave, if not to the last, 
 at least till the opposite side, to which we now proceed, is 
 equally advanced. Here w e have an additional object to attend 
 to ; w e have not only to make the second side as level as the first, 
 but also to make it parallel with it at the same time. The 
 flatness is obtained by a repetition of the means adopted to bring 
 the first surface to that state, and the parallelism of the two 
 sides is a necessary consequence of making the block every-where 
 equally thick. Having, therefore, set a pair of callipers to the 
 thickness intended, or adopted some other equivalent mode of 
 measurement, we frequently examine it w ith respect to this par- 
 ticular. Callipers, in experienced hands, may be made to 
 answer for this purpose very well, but they are apt to mislead 
 the unwary, as they afford different indications with slight 
 differences in the manner of holding them. In using them, 
 therefore, we ahvays hold the centre of the head in such a man- 
 ner that a line passing through it, and exactly midway between 
 the points, shall be parallel with the surfaces they inclose. Cal- 
 lipers are often superseded by what is called a guage, which is 
 nothing more than a piece of sheet iron, steel, or brass, cut 
 in the manner show'n by fig, 8. pi. III. so that the distance be- 
 
MECHAmCAL EXERCISES. 
 
 29 
 
 Advantage of using two guages. — Effect of heat produced in filing. 
 
 tween the legs A B, which ought to be exactly parallel with 
 each other, whll exactly take in the proposed thickness of 
 the block. It is much easier to file correctly with the assistance 
 of a guage than a pair of callipers ; and as the width of the 
 former always remains the same, we have an additional reason 
 for preferring it, when it is probable we shall often have occasion 
 to measure like dimensions ; callipers, even if we wish to keep 
 them to one extent, being easily deranged by a fall cr other com- 
 mon accident, and the frequent resetting of them frittering away 
 our time. 
 
 Those who wish to avail themselves of the utmost refinement 
 of artificial help, will not be displeased by the mention of 
 another expedient belonging to this subject. Two guages may 
 be made, one of them of the true width, and the other a very 
 little wider ; the block may then be filed down to the latter with 
 rather a coarse file, and afterwards to the former with a fine one. 
 Those who think fit to take this pains, can scarcely fail to succeed 
 to their wish. Another hint deserves a place : we are attentive to 
 make the block fit the guage tightly ; for of the degree of tightness 
 we can correctly judge ; but if we make them slack to each other, 
 we can hardly determine the degree of that slackness with even 
 tolerable accuracy. 
 
 When we discontinued filing the first side, we have re- 
 marked, that we left it unfinished or unpolished. Tlie reason 
 for this requires explanation; labour bestowed in polishing, at 
 the time alluded to, would have been thrown away. The heat 
 produced by the strokes of a large coarse file, expands the sur- 
 face upon which they act, renders it convex, and the opposite 
 one necessarily concave. These effects remain in part, after the 
 equilibrium of temperature is restored. While we are employ- 
 ed upon the first side, they are overlooked, but when, after 
 having nearly finished the second side, we find upon trial with 
 the ochre, that the other no longer affords the same indications 
 of correctness which it did before, w^e are convinced of the 
 propriety of having postponed the finishing of it. In a block 
 eight or ten inches long, the error seldom exceeds the five 
 hundredth part of an inch, and therefore, not having begun to 
 polish when it occurs, we can use a file, by which it will quickly 
 be removed. 
 
 Having now so far accomplished our purpose as to have ren- 
 dered the two principal surfaces of our block correctly plane 
 and parallel with each other, w e immediately direct our atten- 
 tion to the four which yet remain in the rough state ; these, 
 for the sake of distinction, we may call the edges. We begin 
 upon one of the two longest of them, and file it fiue, in the 
 
30 
 
 MECHANICAL EXERCISES. 
 
 Precaution in using the square. — Manner of fastening the block to be filed. 
 
 same manner as we did in the first example, except that we make 
 use of a square, applied alternately from the two sides already 
 filed, in order to assist us in keeping it exactly at right angles 
 with them. As soon as this edge is true, we make the opposite 
 one parallel with it by a suitable guage, checking the chance of 
 error by applying the square, which can quickly be run along the 
 whole length of the edge ; and ascertaining, as usual, the general 
 flatness of the whole surface by the use of the ochre and table. 
 As soon as tw^o of the edges have been made true, the remaining 
 two are brought to the same state, by a repetition of exactly the 
 same means. 
 
 If we are provided with a rectangular bar of iron, or any 
 hard metal, the sides of which are very smooth, and exactly 
 perpendicular w^hen it is placed upon the table, we may make 
 use of it, in the filing of these edges, as follows : cover one side 
 with the ochre and oil, place upon the table either of the sides 
 at right angles with the one thus coated, opposite wdiich place 
 tliat edge of the block which is to be tried ; press the block and 
 the bar down upon the table and against each other at the same 
 time, moving one of them, while they are in contact, back- 
 wards and forwards two or three times. By the matks left upon 
 tlie block, we detect at once all its deficiencies This mode of 
 trial would also completely succeed in other cases ; for example, 
 if we had to file the inside of a frame such as printers use to fasten 
 their types in, to which no other method would be so advanta- 
 geously applicable. 
 
 We pass one of our smoothest files along the arris of the 
 two surfaces upon which we are going to apply the square, in 
 order to take off that extreme sharpness, and those overhanging 
 particles of iron, produced by filing, and which would prevent 
 that instrument from affording a correct indication of the angle 
 examined. 
 
 As our block is too broad to be held between the chaps of 
 the vice, we placed it, before we began to file the principal 
 surface, upon a piece of stout board, in breadth about an inch 
 each way larger than itself. Close to the edge of the block, 
 we drove a strong nail here and there into the board, so as to 
 prevent its horizontal motion, but not its being lifted up and taken 
 off perpendicularly. By a square piece of wood, about two 
 inches broad, being firmly screw^ed to the under side of the board, 
 and fastened in the vice, a steady and convenient support is ob- 
 tained for our work. But as soon as the filing of the edges 
 was commenced, this board was discarded, and the vice alone, 
 its teeth only being covered with lead, was used to hold the 
 block. 
 
MECHANICAL EXERCISES. 
 
 31 
 
 Economical way of using files. 
 
 Experienced plane-makers and others, who use files to 
 smooth their wood-work, select those, the teeth of which are not 
 jagged by cross cutting, and we find that upon iron, files of this 
 sort answer better for polishing than any other. e accordingly 
 use them of such a degree of fineness as will efiect our purpose, 
 if that can be ettected by a file : the last degree of smoothness can 
 only be obtained by grinding. We always take care, when using 
 a fine file, to spread the ochre so tliin as hardly to colour the table, 
 otherwise we should presently choke up its teeth, and not finish 
 our work with so much exactness. Ochre is too soft a material to 
 injure a file, and when it does choke up the teeth, it may be re- 
 moved with a brush. 
 
 We have now' aetailed the means by which a rectangular 
 prism of cast iron, possessing a remarkable degree of correct- 
 ness of figure, may be produced. On the uses for which such a 
 block may be required, on the application of the general plan 
 to other sorts of work, and on the importance and multipHed 
 advantages of conect filing in general, we presume it is un- 
 necessary- for us to enlarge. If there be those who are more 
 attentive to authority than reason, and w'ho inquire by whom a 
 process is used, rather than what is its merit, w-e assure them 
 that the method of filing here pointed out, is adopted in the 
 far-famed manufactory of Bolton and Watt, at the Soho, near Bir- 
 mingham. 
 
 Workmen who have various sorts of metal to woik, have an 
 economical mode of management in the use of files, which deserves 
 to be noted. They use all their new files to brass in the first in- 
 stance ; when the original keenness of the teeth has been dimi- 
 nished by this metal, they lay them aside to be ready for filing cast 
 iron ; and when they cease to be sharp enough for cast iron, they 
 use them to malleable iron, for which they will serve tolerably 
 well aw bile longer. Let this order be reversed ; let a new file be 
 used first to malleable iron, then to cast iron, and lastly to brass, 
 it will hardly do more than half the service ; the teeth strike into 
 the malleable iron, and the best portion of them is broken ofi* ; the 
 file is then of little value for cast iron ; and glides over brass, which 
 requires a keen edge, almost without effect, unless seconded by a 
 great exertion of strength. 
 
 Tlie last uses of a file may be to smooth wood or metal revolv- 
 ing in the lathe ; some keep them for a short time red hot in the 
 open fire, and then retemper them before they use them in this 
 way ; the scale which they cast leaving them somewhat sharper 
 than they were prenously : others make such of their old files as 
 have stood well, into ^crew-plates and chisels, on the presumption 
 that they cannot have better steel. 
 
a2 
 
 MECHANICAL EXERCISES. 
 
 General remarks. — Method of grinding a true plane. 
 
 Grinding. 
 
 We restrict the signihcatioii of this term to the abrading of 
 metals and other substances, by rubbing them against each 
 other. In general, some hard body, such as emery, in a pul- 
 verulent state, and mixed with water, oil, &c. is interposed 
 between the pieces ground together; but when one or both of 
 them, as in the example of grit-stone, is composed of particles 
 which will cut, and motion alone separate, the powder,- and 
 even the fluid, are often dispensed with. When grinding is only 
 employed to produce a smooth shining surface, it is called 
 polishing. 
 
 An observation or two has already been thrown out on the sub- 
 ject of grinding, but this method of working metals, requires a 
 little further elucidation. If the artist be not fully aware of the 
 general principle upon which he must proceed, and of what grind- 
 ing will really enable him to do, he will often incur considerable 
 expence and disappointment. 
 
 In grinding two surfaces together, the usual sources of error are, 
 that the cutting pow der is unequally spread between them, that 
 they are not every where equally hard, and that some parts receive 
 a greater number of strokes than others. 
 
 Suppose we have two similar blocks of iron, and wish to 
 make one of them, at least, perfectly flat, from which state, as 
 far as the eye can judge, let it be granted that they are already 
 not very remote. Let us grind them together with emery. 
 Having done this for some time, they assume a very smooth 
 appearance; but w^e find, upon examining them, that we have 
 not attained our object — they are not plane. If we have been 
 grinding long enough, we shall generally find, that one of 
 them is pretty regularly concave, and the other correspondently 
 convex ; and we may be assured that, w'ithout some further 
 device, no variety of rectilinear, circular, or elliptical strokes, 
 will ever bring either of them to the state w^e desire. But 
 suppose we are provided with a third block of a similar size, 
 and concave in about the same degree as one of the two in ques- 
 tion; if we grind these tw^o concave surfaces together, they will 
 mutually correct the defects of each other. Here then we are 
 furnished with the key to success in this art : — to grind one 
 surface perfectly flat, it is indispensably necessary to grind three 
 at the same time. In practice, we do not ascertain the state of 
 each surface by experiment, with the view of bringing similar 
 defects together, (as just stated in order to render the explanation 
 more evident,) but proceed by grinding them interchangeably, 
 often reversing their position, pressing upon every part with 
 
MECHANICAL EXERCISES. 
 
 33 
 
 JNlethods of grinding, 
 
 an equal weight, spreading the ernerv, or whatever may be used as 
 the cutting powder, as evenly as possible, sometimes moving dia- 
 gonally and sometimes from side to side, with an intermixture of 
 rectilinear, circular, and elliptical strokes. To execute good work 
 in this art, so far at least as plane surfaces are concerned, manual 
 labour can alone be depended upon. Though glass is much more 
 easily ground true than most of the metals, yet the plate glass, 
 which is ground by machinery, is perhaps never so perfect as it 
 might be made by the hand, and sometimes parts ol looking glasses 
 called plane, are demonstrably possessed of the properties of con- 
 cave or convex mirrors. 
 
 Emery is the substance most commonly employed as the cutting 
 powder in the grinding of metals. The artist should be provided 
 with it of various degrees of fineness ; and as it is necessary that 
 the finest sort should be entirely free from the admixture of 
 coarse grains, it is adviseable, on this account, to keep it at a dis- 
 tance from the other kinds. It is the practice of some, not to 
 polish in tlie same room, or even in the same clothes, they used 
 for the rougher part of their operations. As soon as the emery 
 thrown upon the work, is found to have little or no effect, being 
 converted into mud, it must be entirely washed away, and the sup- 
 ply renewed. When iron or brass have been ground with emery, 
 it is difficult to file them ; the file w ears very fast, and produces 
 but a slight effect, till the emery which has entered into the pores 
 of the metal is removed. 
 
 Water, having the stioiig recomineudation of cheapness, is the 
 fluid most commonly used in grinding ; but with oil, metals are cut 
 more evenly, expeditiously, and fewer scratches are produced. It 
 is therefore most used to finish and polish with. 
 
 The softer the three bodies intended to be ground, the greater 
 may be their inequalities at the commpncement of this labour, 
 with an equal chance of making them perfect in the end. Sile- 
 cious substances, such as emery and sand, take such powerful 
 hold upon calcareous fossils, to which class of substances belong 
 almost all the hardest stones in common use, that they may be 
 ground, from rather a rough state, till they become very true. 
 The stone and marble masons care nothing about that high degree 
 of accuracy which is here contended for ; but if any one who may 
 be disposed to follow the directions given in the last section for 
 filing, be supplied by one of these artists with three slabs of mar- 
 ble, or some close-grained homogeneous stone, made true in their 
 best manner, he may afterwards, with proper care, finish them, ac- 
 cording to the principle here laid down, so as to possess a table, 
 with which he will probably have little reason to be dissatisfied. 
 This must be understood to be the resort, when it is impossible or 
 
 2. — VoL. I. F 
 
34 
 
 MECHANICAL EXERCISES. 
 
 Methods of grinding. 
 
 inconvenient to procure an iron table, which is always to be pre- 
 ferred, and may in fact be manufactured with ease, especially if 
 no extraordinary size is required, by the help of one of stone, ob- 
 tained as above recited. 
 
 The metals are of a much more unyielding nature than 
 irtone ; and when they are the subjects of our labour, a different 
 line of policy must be pursued. The grinding of them should 
 always be comparatively a short operation. In the removal of 
 considerable inequalities, the file will often do more in a quarter 
 of an hour, than grinding in a whole day ; but when the imper- 
 fections extended over a surface become exceedingly numerous 
 and minute, such, in short as a file or any tool which acts only on 
 a small spot at once, is necessarily disposed to leave, and which 
 scarcely at all affect the general level of the whole, the case is 
 reversed, and the file will not produce so finished an effect in a 
 day, as grinding in a quarter of an hour. If any one of three 
 plates or slabs, which have been ground together, will adhere so 
 closely to either of the other two, as to exclude the atmosphere 
 from the surfaces in contact, and allow both to be lifted up by 
 taking hold of one of them, a strong presumption is afforded that 
 all of them are true. 
 
 Tlie common grindstoney upon which tools are sharpened, 
 requires no description ; yet it may be useful to observe, that it 
 will be much easier to grind plane-irons, chisels, See. upon it, if 
 the circumference in the direction of the axis be kept a little con- 
 vex. When these and similar tools are ground, the stone should 
 also turn towards the person holding them, so as to run against 
 their edge. Grindstones are often turned by machinery. When 
 the velocity is very great, they do not cut well, and will also some- 
 times break, a circumstance which has occasioned the most serious 
 accidents. It was formerly the invariable practice to hang them 
 on an axis passing through a piece of wood which occupied a 
 square hole cut through the centre of them ; w hen this wood be- 
 came wet, the swelling of it powerfully seconded the tendency 
 of rapid motion to break them. The practice of securing them 
 by a circular plate, screwed firmly against each side, was therefore 
 adopted, and is becoming more general. The stones upon 
 which cutlery is ground, are carried at the rate of about six hun- 
 dred feet per second. At Wickersley, near Sheffield, stones are 
 obtained w'hich heat so little that they admit of being used dry. 
 When clogged, they are cleared with a bar of soft iron. This 
 process of dry grinding is of the most destructive nature to the 
 men employed ; the sharp particles of iron, constantly flying about, 
 find their way to the lungs, and ultimately produce incurable com- 
 plaints. 
 
MECHANICAL EXERCISES. 
 
 35 
 
 The glazor. — The polisher — The brush. — Grinding copper-plates. 
 
 In the manufacture of cutlery, the use of the stone is followed 
 by that of the lap or glazor. This is formed of a number of 
 pieces of wood, in such a maimer that the edge or face may always 
 present the end way of the wood, or it would change its figure. 
 Some glazors are covered with strong leather, others with an alloy 
 of lead and tin. After the face of them is turned to the proper 
 form and size, it is covered with rakes or notches, which are filled 
 up with emery and tallow. The glazor is carried at the rate of 
 fifteen hundred feet per second. 
 
 The polisher, in the same branch of art, is a circular piece of 
 wood, running upon an axis like the stone and glazor. It is co- 
 vered with butf leather, and its surface is from time to time reple- 
 nished with colcothar. The polisher is not allowed to move quicker 
 than about seventy or eighty feet per second. 
 
 A brush, consisting of a circular piece of wood fitted upon 
 an axis, and set upon the face with strong bristles, is used to polish 
 those parts which have been filed, and which the lap and the polisher 
 cannot touch. 
 
 Copper-plates are prepared for the engraver, by placing 
 them upon a board forming an inclined plane, and rubbing 
 them with a piece of sharp grit stone, first in the direction of 
 their length, and then in that of their breadth, with rectilinear 
 strokes, till the marks of the planishing hammer and other de- 
 fects are taken out. The low'er edge of the plate may lie in a 
 trough of w^ater, so that the necessary supply of this fluid will 
 be carried over the plate without stopping to throw' it on. The 
 scratches left by the grit stone are then removed by rubbing in 
 the same way w ith a piece of pumice stone. Charcoal is next 
 used, with water, to remove the scratches left by the pumice 
 stone, and the operation is finished with the same substance and 
 a little oil. If the piece of charcoal, when it is tried, glide over 
 the surface with little or no effect, another piece must be select- 
 ed. Its fitness for the purpose may be distinguished by its 
 making no scratches, yet, when wet and rubbed upon the copper, 
 seeming rough, and making a low' murmuring noise. Clock- 
 makers finish the plates betw'een w’hich the w heels of clocks are 
 enclosed, in a similar manner, except that they do not take so 
 much pains to remove every scratch, but obtain a high gloss by 
 rubbing w ith colcothar or putty laid upon leather. A piece of old 
 hat makes a good polisher, as does also paper rolled up till it 
 forms a cylinder of sufficient size, when the end, being first cut 
 straight, must be used. If the hat, in the dying of which iron is 
 employed, be immersed a few minutes in sulphuric acid, the iron 
 will pass to the state of red oxide, and it then answers the purpose 
 still better than before. 
 
36 
 
 MECHANICAL F,XERCl!>ES. 
 
 Tripoli. — Polishing rouge. — Burnisher. — Annealing. 
 
 A species of the clay genus, called tripoli, or rotten stone, is 
 much used to prepare metals, marble, and glass, for receiving, after 
 the use of emery or sand, the highest polish of which they are 
 capable. It is of a yellow colour, tastes like common chalk, and 
 is rough or sandy between the teeth, although no sand can be se- 
 parated from it ; and its particles are in fact so tine and soft, as to 
 leave no perceptible scratches. 
 
 Goldsmiths, to give the last polish to their work, which they 
 commonly call colouring it, employ what is termed polishing 
 rouge. This powder is said to be a very pure native red oxide 
 of iron. Sometimes it is of a red inclining to purple, and has the 
 appearance of very fine colcothar; but this sort is of inferior 
 quality. 
 
 Burnishing is too nearly allied to the above methods of polish- 
 ing, to require a separate section. The burnisher used by the 
 makers of spurs and bits, 8cc. is partly iron, partly steel, and partly 
 wood. It consists of an iron bar, wdth a wooden handle at 
 one end, and a hook at the other, to fasten it to another piece of 
 wood held in the vice, while the operator is at w'ork. In the mid- 
 dle of the bow', wuthin side, is w^hat is properly called the burnisher, 
 being a triangular piece of steel with a tail whereby it is rivetted 
 to the bow'. 
 
 The iron ore called red haematites, or blood-stone, is much used 
 for burnishing metals, but steel burnishers are more common for 
 this purpose than any other sort. They are much varied in shape 
 according to the fancy of the user, or the w ork for w'hich they 
 are intended. The form shown at fig. 9- ph HI« or some near 
 resemblance of it, is most frequently adopted. The steel of which 
 a burnisher is made should be in a very hard state, entirely free 
 from flaw s, and exquisitely polished 
 
 Annealing, 
 
 In a considerable number of instances, bodies which are 
 capable of undergoing ignition, are rendered hard and brittle by 
 sudden cooling. Glass, cast iron, and steel, are the most 
 remarkably afliected by this circiunstance ; the inconveniences 
 arising from which are obviated by cooling them very gradu- 
 ally, and this process is called annealing. Glass vessels are 
 carried into an oven over the great furnace, called the leer, 
 where they are permitted to cool, in a greater or less time, ac- 
 cording to their thickness and bulk. Steel is most effectually 
 annealed by making it red hot in a charcoal fire, which must 
 completely cover it, and be allowed to go out of its own accord. 
 Cast iron, which may require to be annealed in too large a 
 
MECHANICAL EXERCISES. 
 
 37 
 
 Annealing. — The straight-edge. — Square. 
 
 quantity, to render the expence of charcoal very agreeable, may 
 be heated in a turf or cinder tire, which must completely envelope 
 and defend the pieces from the air till they are cold. The fire 
 need not be urged so as to produce more than a red heat ; a little 
 beyond this, bars and thin pieces would bend, if destitute of a 
 solid support ; and w ould even be melted without any vehement 
 degree of heat. If it be required to anneal a number of pieces 
 expeditiously, and the fire is not large enough to take more than 
 one or two of them at once; or if it be thought hazardous to 
 leave the fire to itself, from an apprehension that the heat might 
 increase too much, the following scheme may be adopted : heat 
 as many of the pieces at once as may be convenient, and as soon 
 as they are red hot, bury them in dry saw dust. Cast iron, when 
 annealed, is less liable to warp by a subsequent partial exposure 
 to moderate degrees of heat, than that which has not undergone 
 this operation. 
 
 The above methods of annealing render cast iron easy to 
 work, but do not deprive it of its natural character. Cast iron 
 cutlery is therefore stratified with some substance containing 
 oxygen, such as poor iron ores, free from sulphur, and kept in a 
 state little short of fusion for twenty-four hours. It is then found ' 
 to possess a considerable degree of malleability, and is not unfit 
 for several sorts of nails and edge-tools. 
 
 Copper forms a remarkable exception to the general rule of 
 annealing. This metal is actually made softer and more flexible 
 by plunging it when red hot into cold water, than by any other 
 means. Gradual cooling produces a contrary effect. 
 
 The Straight-edge and Square. 
 
 A steel ruler, made, by filing and grinding, perfectly true, is 
 called, by mechanics, a straight-edge, and is an instrument of 
 great use and value to the workman. A straight-edge is not made 
 to any fixed length, which must be varied according to the work 
 to which it is intended to be applied. Unless very short, its 
 breadth is commonly from one to two inches, and its thickness 
 should in all cases be sufficient to support its own weight. To 
 have this property, it must be thicker than would be generally 
 supposed If it be made thirty inches long, it should not be less 
 than half an inch thick, and if forty inches long, its thickness 
 should be five-eights of an inch. If made materially thinner of 
 these lengths, it will be found to sink in the middle, when sup- 
 ported at the ends, a fact easily ascertained by trying it with the 
 arris of another straight-edge. 
 
 This instrument is not even known by name, to a great 
 number of proviiidai artists, who might be benefited by its use. 
 
38 
 
 MECHANICAL EXERCISES. 
 
 Straight-edge. — Square. 
 
 It should be made of the. best cast steel, and filed, ^vitll all pos- 
 sible exactness, agreeably to the directions given on that subject. 
 Three must be made at the same time, and they must be ground 
 A<ith strict attention to the general directions for that operation. 
 If they be true, in whatever manner they be laid flat upon each 
 other, no light can pass between them. Straight-edges are com- 
 monly bevelled on one side, so as to make one edge considerably 
 thinner than the other, in the same manner as common wooden 
 rulers are generally made. 
 
 The use of a straight-edge in filing has already been shown. 
 In turning it is equally useful, where great accuracy is wanted ; 
 for example, in turning a cylinder, or a cone, the application of it 
 will instantly show the irregularities of the figure. 
 
 Two rectangular prisms, joined at one of their extremities, 
 so as to form a right angle both internally and externally, consti- 
 tute the instrument called a square. See pi. III. fig. 10. A 
 good square is not often met with ; those pieces of metal sold 
 under that name, by the clock and w atch tool makers, are gene- 
 rally worthless things, no two of them corresponding with each 
 other. A square, therefore, fit to serve as a guide in the filing of 
 metals, must be the work of the individual artist by whom it is 
 required ; or at least he must finish it, — any common w orkman 
 will make it true to the fiftieth part of an inch. It should be 
 made of good steel, and as it is not to be tempered, it should be 
 well hammered, so as to be rendered stiff and elastic. Like the 
 straight-edge, also, it should be filed w ith the greatest accuracy, 
 and ground and polished on every side. 
 
 To try whether the square is accurate or not, lay it on a 
 level plate of metal at tl»e least as broad as itself, and twice its 
 length, provided also with a ledge perpendicular to it, and 
 known to be truly fiat. Against this ledge, so as to reach to 
 about the middle of it, press the external edge of one limb of 
 the square. Hold it steadily there, and with a graver, or 
 some finely pointed steel instrument, draw a line upon the plate, 
 as close to the external edge of the other limb as possible. 
 Now' turn over the square, so that it shall lie on its other side, 
 and press the same edge against the other half of the ledge. 
 While thus pressed against the ledge, bring it up to the line just 
 drawm, and accurately examine if another line can be made exactly 
 coincident with it. If this can be done, the square is true; if it 
 cannot, as soon as any part of the square touches the line, draw 
 another line ; an acute angle will be produced, and half the breadth 
 of this angle, at any given distance from the ledge, shows the de- 
 viation of the instrument from a right angle, at the same distance 
 from the same point. 
 
MECHANICAL EXERCISES. 
 
 39 
 
 Ditferent methods of cutting screws. 
 
 Screzos. 
 
 Screws are cut prhicijjally by means of screw-plates, by 
 stocks or dies, aud in the lathe. The part left standing 
 between the spiral groove of a screw, is called the thread. The 
 pin by which the spirals of a screws nut are formed, is called a 
 tap. When it is cylindrical, or of the same diameter in every 
 part, it is called a plug tap ; and when it forms the portion of 
 a cone or a pyramid, it is aptly enough called a taper tap, or as 
 it is the most frequent form, it is simply denominated a tap. The 
 screw cut' by a tap is called an inside or concave screw’ ; the tap 
 itself, and all such screws as are or can be formed by plates or 
 dies, are, for necessary contradistinction, called outside or convex 
 screws. The screws plates on sale at the ironmongers’ shops, 
 are, in effect, generally worn out before they are used. In a 
 good tap, the groove is sharp at the bottom, as the thread is at 
 the top, and m proportion as they vary from this configuration 
 they may be considered bad. The longitudinal section of ordi- 
 nary taps and plates, instead of being bounded by a zigzag line, 
 and the thread appearing like a series of triangles touching each 
 other at the base, is bounded by an undulating line, and the 
 groove, though seldom perhaps so bad as to correspond with the 
 segment of a circle, very often comes no nearer a tj iaiigle than a 
 portion of the smaller end of an oval. Hence those who require 
 perfect screws for any piece of mechanism, w ill perceive the ne- 
 cessity of examining the taps and plates intended to be used in 
 maki»g them. 
 
 A screw’-plate is a cheap and handy instrument for making 
 screws ; but as it cuts at once the whole depth of the groove 
 which it will make, it is apt, from the force necessarily em- 
 ployed in forcing it forward, to twist the pin upon which it 
 operates. This is a serious inconvenience, when the pin has 
 been tumed, and is required to be quite straight. The best 
 outside screws are therefore cut with what are called stocks or 
 dies, of which we have given a plan in pi. III. fig. 11. 
 although, like screw’-plates, they are almost too well known to 
 require notice, unless it be for the purpose of laying before 
 those who are unaccustomed to such things, such hints as may 
 enable them to prevent their work from being spoiled, by the 
 ignorance or carelessness of those whom they employ for its 
 execution. A square frame, A B, is welded to two handles, 
 C D, and on the inside of the opening, next each handle, is an 
 angular projection to the extent shown by the dotted lines. 
 This projection takes hold in a corresponding groove cut in two 
 pieces of steel bbj. the edge of one of which is shown at fig. 12. 
 
40 
 
 MECHANICAL EXERCISES. 
 
 Screw stocks. — Taps. 
 
 These pieces are put in at the part c, ^^ here there is no angular 
 projection ; they are then slipped down to their proper place, 
 and a piece of iron put over them, so as to occupy a great part 
 of the space c. Upon this piece of iron is pressed the end of 
 the screw f, by which the pieces, bby are brought and kept as 
 near to each other as may be required. They aie then tapped, so 
 that one-half of the concave screw formed is upon each of them. 
 Before they are tempered, they are often gTOund or filed so as to 
 make the aperture, when they meet, less than a circle ; by which 
 means they can be used to cut a smaller screw than they were ori- 
 ginally tapped with. In using them, the screw J] can be turned 
 back, so that the aperture will easily admit the pin intended to be 
 cut ; thus is obtained the advantage of making the groove very 
 shallow in the first instance, and of deepening it gradually by turn- 
 ing furtlier in the screwy. 
 
 The bolt or pin intended to be tapped, either with a screw- 
 plate or stocks, is tapered in a small degree at the extremity, 
 previous to the operation. If a screw-plate be employed, it is 
 then pressed downwards upon it, with a force proportionate to its 
 size, and turned, at the same time, with a progressive and retro- 
 gressive motion, always, in advancing, (to efi'ect the necessary revo- 
 lution,) going over a greater space than in returning. Stocks are 
 used with a similar motion, but they are not pressed downwards 
 except at the commencement, before a thread of one revolution 
 has been formed upon the pin. Screw-plates and stocks are often 
 ruined by being used to iron rough fiom the forge, and covered 
 with scales, which, from their hard, gritty nature, grind away the 
 tlireads. 
 
 As in making outside screws, the pin is tapered at the extre- 
 mity, that the operation may be more readily commenced ; so, in 
 making inside screws, (which, when made for screwing upon bolts, 
 are called ?iutSy) the taper tap is, for the same reason, used first, 
 and afterwards the plug tap to finish w ith. 
 
 The tap-wrench is simply a lever, with a hole in or about the 
 middle of it, to admit the rectangular head of the tap, for the pur- 
 pose of turning it round, in the same manner as the plate and 
 stocks are turned. 
 
 The taps used to cast iron are made square ; for brass they 
 are sometimes square, and sometimes triangular. As with these 
 shapes the thread can only be on each arris, they clear their w ay 
 better ; and there is room in the hole for the particles which are 
 rapidly cut from these metals ; and to clear away which, if 
 this provision were not made, the tap must very frequently be 
 drawn out ; or it w ill be broken by the force applied to overcome 
 the accumulated obstruction to its progress. Taper taps for 
 
MECHANICAL EXERCISES. 
 
 41 
 
 Properties of copper. 
 
 malleable iron are frequently squared ; but plug taps for the same 
 metal are only channelled, in the direction of their length, in two 
 or three places, so as to take out the thread where the grooves are 
 made. 
 
 The method of cutting screws in the lathe, will be found in 
 the section on Turning. 
 
 Copper. 
 
 We refer to the article of Chemistry, for a minute enumera- 
 tion of the whole of the known metals ; but in this place we 
 shall, with the exception of iron, which has already been noticed 
 rather at length, introduce a general practical view of the 
 properties, applications, and combinations with each other, of 
 those most frequently occurring in common arts and common 
 life, flaking this our plan, the first object claiming our attention 
 is copper. 
 
 Copper is a very brilliant, sonorous metal, of a fine red 
 colour, possessing a considerable degree of hardness and elasti- 
 city. It is extremely malleable, and may be reduced to leaves 
 so fine, that they may be carried about by the wind. Its 
 tenacity is very great. A ware of one-tenth of an inch in 
 diameter, will support a weight equal to SOOlbs. avoirdupoise 
 w ithout breaking. It does not melt till the temperature is ele- 
 vated to about 27° of Wedgw^ood, or (by estimation) 1450° of 
 Fahrenheit. When rapidly cooled, it exhibits a granulated and 
 porous texture. When the temperature is raised beyond what 
 is necessary for its fusion, it is sublimed in the form of visible 
 fumes. None of the malleable metals is so difficult to file or turn 
 smooth as copper ; but it is cut by the graver, or ground by gritty 
 substances, with great ease. 
 
 When miners wish to know whether an ore contains copper, 
 they drop a little nitric acid upon it; after a little time, they 
 dip a feather into the acid, and wipe it over the polished blade 
 of a knife; if there be the smallest quantity of copper in it, 
 this metal will be precipitated upon the knife, to which it will 
 impart its peculiar colour. — Roman vitriol, much used by 
 dyers, and in many of the arts, is a sulphate of copper. A 
 
 solution of this salt is used for browning fowling-pieces and 
 tea-urns. 
 
 In domestic economy, the necessity of keeping copper 
 vessels perfectly clean, cannot be too strongly inculcated ; but 
 it is worthy of remark, that fat and oily substances, and 
 vegetable acids, do not attack copper w'hile hot; and therefore 
 copper vessels may be used, for culinary purposes, with perfect 
 safety, if no liquor be ever suffered to grow cold in them. The 
 2.--^Vol.I. G 
 
42 
 
 MECHANICAL EXERCISES. 
 
 Manufacture of brass. 
 
 mere tinning of copper and brass vessels does not afford com- 
 plete security. The tinning is never so perfect as to cover every 
 part of them. 
 
 Compounds formed by the mixture of two or more different 
 metals, are called alloys. The alloys of copper, especially those 
 in which this metal predominates, are more numerous and im- 
 portant in the arts than those of any other metal. Many of them 
 are perfectly well known, and have been immemorially in use. 
 The exact composition, and particularly the mode of preparing 
 several, are kept as secret as possible. By the aid of chemistry, 
 we may detect the precise composition of an alloy ; yet we may 
 not always be able, by common methods, to produce a mixture 
 having all the excellencies, which, perhaps, mere accident has 
 taught the possessor of the secret to combine. 
 
 Brass is the most important of all the alloys of copper. It is 
 more fusible than copper, less liable to tarnish from exposure to 
 the atmosphere, and its fine yellow colour is more agreeable to 
 the eye. It is much more malleable than copper when cold. 
 Sieves of extreme fineness are woven with brass wire, after the 
 manner of cambric w^eaving, which could not possibly be made 
 with copper wire. Three parts of copper and one of calamine, 
 or native carbonate of zinc, constitute brass. The calamine is 
 first pounded in a stamping mill, and then washed and sifted, in 
 order to separate the lead with which it is mixed. It is then 
 calcined on a broad, shallow^, brick hearth, over an oven 
 heated to redness, and frequently stirred for some hours. In 
 some places, it is calcined in a kind of kiln, filled with alternate 
 layers of calamine and charcoal, and kindled from the bottom, 
 where a sufficient quantity of wood has been deposited for the 
 purpose. When the calamine has been thoroughly calcined, it 
 is ground in a mill, and mixed at the same time with a third or a 
 fourth part of charcoal, and is then ready for the brass furnace. 
 Being put into crucibles with the requisite proportion of grain cop- 
 per, copper clippings, or refuse bits of various kinds, the whole 
 is covered with charcoal, and the crucibles luted up with a mix- 
 ture of clay or loam and horse clung. The heat employed, is, 
 for a considerable time, not sufficient to melt the copper, w'hich it 
 is at length raised so as to fuse, and the compound metal is then 
 run into ingots. 
 
 in general, the extremes of the highest and lowest proportion 
 of zi.ic ciie from tw'eive to twenty-five per cent, of the brass. 
 Even with so nmch as twenty-five per cent, of zinc, brass is per- 
 fcciiy miiileabie, if well manufactured, though zinc itself scarcely 
 5 ! s to ihe hammer at common temperatures. 
 
 Cood brass, when received from the foundery, is nearly 
 
MECHANICAL EXERCISES. 
 
 43 
 
 Pinchbeck. — Tombac. — Prince’s metal. — Bronze. — Bell-metal. 
 
 melastic, but exceedingly flexible, and \vhen polished, the naked 
 eye cannot discover any pores, which are frequently observable in 
 the brass made in the country. The liberal use of the hammer 
 imparts a considerable portion of elasticity to brass, and renders 
 it at the same time less flexible. Clock-makers, watch-makers, 
 and all artists w’ho employ this metal, in forms that admit of the 
 operation, hammer it well before they turn or file it, otherwise 
 their work would wear indifierently, and a trifling cause injure its 
 figure. Brass is not malleable when igmted. 
 
 Hammering is found to give a magnetic property to brass, 
 perhaps occasioned by the minute particles of iron separated 
 from the hammer and the anvil during the process, and forced 
 into its surface; but this circumstance makes it necessary to 
 employ unhammered brass for compass boxes and similar ap- 
 paratus. 
 
 Five or six parts of copper and one of zinc, form pinchbeck. 
 Tombac has still more copper, and is of a deeper red than pinch- 
 beck. Princes’ metal is a similar compound, excepting that it 
 contains more zinc than either of the former. 
 
 The alloys of copper with different proportions of tin, are 
 of great importance in the arts. They form compounds which 
 have distinct and appropriate uses. Tin renders copper more 
 fusible, less liable to rust, harder, denser, and more sonorous. 
 Copper and tin separately, are not more remarkable for their duc- 
 tility, than, when united, the compounds they form are for their 
 brittleness. 
 
 Eight to twelve parts of tin, combined with one hundred 
 parts of copper, form bronze, w'hich is of a greyish yellow 
 colour, harder than copper, and the usual composition for 
 statues. The customary proportions for bell-metal are, three 
 parts of copper and one of tin. The greater part of the tin 
 may be separated by melting the alloy, and then throwing a 
 little water upon it. The tin decomposes the water, is oxidiz- 
 ed, and thrown upon the surface. The proportion of tin in 
 bell-metal is varied a little at different founderies, and for 
 different sorts of bells. Less tin is used for church bells than 
 clock bells ; and in very small bells, a trifling quantity of zinc is 
 used, which renders the composition more sonorous, and it is 
 still further improved, in this respect, by the addition of a 
 little silver. A small quantity of antimony is occasionally 
 found in bell metal. When copper, brass, and tin, are used to 
 form bell metal, the copper is from seventy to eighty per cent, 
 including the proportion contained in the brass, and the re- 
 mainder is tin and zinc. When tin is nearly one-third of the 
 alloy^ it is then beautifully white, with a lustre almost like 
 
44 
 
 MECHANICAL EXERCISES. 
 
 Properties and uses of tin and its alloys. 
 
 mercury, extremely hard, close grained, and brittle ; but when the 
 proportion of tin is one-half, it possesses these properties in a 
 still more remarkable degree, and is susceptible of so exquisite a 
 polish, as to be admirably adapted for the speculums of telescopes. 
 If more tin be added than amounts to half the weight of the 
 copper, the alloy begins to lose that splendid whiteness for which 
 it is so valuable as a mirror, and becomes of a blue grey. As the 
 quantity of tin is increased, the texture becomes rough grained, 
 and totally unfit for manufacture. 
 
 Of Tin. 
 
 Tin is a metal of considerable importance in the arts. It is 
 of a silver white colour, very ductile, malleable, and gives 
 out, while bending, a peculiar crackling noise. Its specific 
 gravity is 7-291 ; a cubic foot weighs about 5l6lbs. avoirdupoise. 
 Its purity is in proportion to its levity. It melts at the 400th 
 degree of Fahrenheit’s thermometer, and promotes the fusibi- 
 lity of the metals with which it is mixed. Two parts lead and 
 one of tin form plumbers’ solder, which melts sooner than 
 either of the metals separately. Eight parts of bismuth, five of 
 lead, and three of tin, form a metal which melts at a heat not 
 exceeding that of boiling water. Tea-spoons are made of this 
 alloy, to surprise those unacquainted with their nature. They 
 have the appearance of common tea-spoons, but are melted in 
 hot tea. 
 
 Tin is used to form boilers for dyers, and worms for recti- 
 fiers’ stills. The common mixture for pewter, is 112 pounds of 
 tin, 15 pounds of lead, and six pounds of brass. But the 
 name of pewter is given to any malleable white alloy, into which 
 tin largely enters, and perhaps no two manufacturers employ 
 the same ingredients in the same proportions. The finest kinds of 
 pewter contain no lead whatever, but consist of tin with a small 
 quantity of antimony, and sometimes a little copper. Pewter 
 may be used for vessels containing wine, and even vinegar, pro- 
 vided the tin constitutes about three-fifths of the alloy. 
 
 The consumption of tin, in the operation called tinning, is 
 very considerable. The principal secret in tinning, is to pre- 
 serve the tin and the surface of the metal to which it is intended 
 to be applied, perfectly clean, and in a pure metallic state. 
 Thin plates or sheets of iron, which when coated with tin are so 
 well known under the name of tin-plates, white iron, or latten, 
 are prepared by scouring them with sand. They are then 
 immersed in water acidulated with sulphuric acid, in which they 
 are kept for twenty-four hours, being occasionally turned 
 during that time, so that they may rust equally in every part. 
 
MECHANICAL EXERCISES. 
 
 45 
 
 Tinning sheet jron — copper and iron vessels. 
 
 When taken out, they are again scoured and made perfectly 
 clean. They are then dipped in pure water, and kept there 
 till wanted for tinning. The tin is melted in an iron crucible, 
 narrow, but deeper than the length of the iron plates, which are 
 plunged in downright, so that the tin swims over them. The 
 surface of the tin, to prevent its oxidation, is covered with some 
 oily or resinous matter. Reaumur states, that the Germans 
 cover it v\'ith suet, previously prepared by frying and burning, 
 which surprisingly puts the iron in a condition to receive the 
 tin. The melted tin must also have a certain degree of heat; 
 if not hot enough, it will not adhere to the iron ; and if it be too 
 hot, the coat will be very thin, and the plates discoloured. 
 Plates intended to have a very thick coat, are first dipped into 
 the crucible when the tin is very hot, and afterwards when it is 
 cooler. For the second dipping, the suet must not be prepared, 
 but used in its common state. The tin not only adheres to the 
 surface of iron plates,' but penetrates and intimately combines 
 with them. 
 
 Copper is tinned after it has been formed into utensils. If the 
 copper be new, its surface is first scoured with salt and diluted 
 sulphuric acid. Pulverized resin is then strewed over the in- 
 terior of the vessel, into which, after heating it to a considerable 
 degree, a sufficient quantity of melted tin is poured, and spread 
 upon it by means of a roll of hard twisted flax, which renders the 
 coating uniform. Pure tin is rarely used for this purpose ; it is 
 generally, though injuriously, alloyed with a small proportion 
 of lead. 
 
 The use of the resin is important; for the heat given to the 
 copper is sufficient to oxidize its surface in some degree, and an 
 alteration of this sort, however slight, would prevent the perfect 
 adhesion of the tin. The resin is equally useful, in preventing the 
 partial oxidation of the tin, or in reviving the small particles of 
 oxide which may be formed during the operation. 
 
 For tinning old vessels a second time, the surface is first 
 scraped clean and bright with a steel instrument, or scoured with 
 iron scales, then pulverized sal ammoniac is strewed over it, and 
 the melted tin is rubbed on the surface with a solid piece of sal 
 ammoniac. 
 
 The process for covering iron vessels with tin, corresponds 
 with that last described ; but they ought to be previously cleaned 
 with the muriatic acid, instead of being scraped or scoured. 
 Iron nails, which cannot be conveniently tinned in a bath, are 
 easily covered with tin by including them, with a due proportion 
 of tin and sal ammoniac, in a stone bottle, and agitating them 
 while heating and cooling. 
 
46 
 
 MECHANICAL EXERCISES. 
 
 Tinning of pins. — Properties of lead. 
 
 The follo\4*ing method of tinning is highly esteemed for its 
 permanency and beauty : the utensd is cleaned in the usual 
 manner ; its inner surface is beaten on a rough anvil, or scratch- 
 ed with a ^ire brush, that the tinning may adhere more closely to 
 the copper ; and one coat of pure tin is then laid on with sal am- 
 moniac, as above directed for tinning old copper. A second coat, 
 consisting of tsvo parts of tin and three of zinc, must next be uni- 
 formly apphed with sal ammoniac, in a similar manner : the sur- 
 face is now to be beaten ; scoured with chalk and water ; smoothed 
 with a proper hammer ; exposed to a moderate heat ; and lastly 
 dipped in melted tin. This sort of tinning eflectually prevents 
 the utensils from rusting. 
 
 Pins are whitened by filling a pan with alternate layers of 
 them and grain tin ; a solution of super-tartrate of potass (cream 
 of tartar) is then poured upon them, and they are boiled for four 
 or five hours. The tartaric acid first dissolves the tin, and then 
 graduaUy deposits it on the surface of the pins, in consequence 
 its greater affinity for the zinc which enters into the composition of 
 the brass wire. 
 
 There are two kinds of tin known in commerce, viz. block tin 
 and grain tin. Block tin is procured from the common tin ore ; 
 grain tin is found in small particles, in what is called stream tin 
 ore. It owes its superiority, not only to the purity of the ore, but 
 to the care with which it is washed and refined. 
 
 Of Lead. 
 
 Lead unites with most of the metals. It has little elasticity, 
 and is the softest of them all. Gold and silver are dissolved by it 
 in a slight red heat, but when the heat is much increased, the lead 
 separates, and rises to the surface of the gold, combined with all 
 heterogeneous matters. This property of lead is made use of in 
 the art of refining the precious metals. 
 
 If lead be heated so as to boil and smoke, it soon dissolves 
 pieces of copper thrown into it ; the rmxture, w hen cold, is brittle. 
 The union of these two metals is remarkably slight, for upon ex- 
 posing the mass to a heat no greater than that in which lead 
 melts, the lead almost entirely runs off by itself. This process, 
 which is peculiar to lead ^vith copper, is called eliquation. It has 
 lately been discovered, that a certain proportion of lead may be 
 mixed with the metal formerly used for white metal buttons, 
 ■without injuring the appearance ; thus affording a considerable 
 addition of profit to the manufacturer. 
 
 The consumption of lead for water pipes, cisterns, and to 
 cover buildings, is veiy extensive. Sheet lead is made by suf- 
 fering the melted metal to run out of a box through a long 
 
MECHANICAL EXERCISES. 
 
 47 
 
 Type metal. — Manufacture of small shot. 
 
 horizontal slit, upon a table prepared for the purpose. The table 
 is generally covered with sand, and the box is drawn over it by 
 appropriate ropes and pulleys, leaving the melted lead behind to 
 congeal in the desired form. The requisite uniformity and thin- 
 ness are given to these sheets, by rolling them between two cylin- 
 deis of iron acting upon the same principle as the copper-plate 
 printing press. 
 
 The alloy of lead and antimony is used for printers’ types. 
 Chaptal made a great variety of experiments to ascertain 
 the best proportions of these metals to each other for this 
 use. He always found four parts of lead to one of antimony 
 form the most perfect composition. But if the antimony be 
 pure, one part of it, to seven or eight of lead, form an alloy 
 too brittle to be extended under the hammer, and as hard as the 
 generality of types. To give hardness to the lead, is not the 
 only use of antimony in this composition. It renders the lead 
 more fusible, more fluid when melted, and as it expands in 
 passing to a solid state, it is calculated to produce a sharper im- 
 pression of the mould, than could be easily obtained by lead alone. 
 Antimony (which in trade is commonly called regulus of anti- 
 mony, or regulus only,) requires, when alone, much more heat for 
 its fusion than lead, in combining with which metal, as it is little 
 more than half its weight, it rises to the surface, and requires to be 
 well stirred before it will incorporate. Different parts of the 
 same block of type-metal, often possess very different degrees of 
 hardness. Stereotype plates are almost always harder on the face 
 than on the back. 
 
 The method of granulating lead in the making of small shot, 
 is curious. In melting the lead, a small quantity of arsenic is 
 added, which disposes it to urn into spherical drops. When 
 melted, it is poured into a cylinder, whose circumference is 
 pierced with holes. The lead streaming through the holes, soon 
 divides into drops, which fall into water, where they congeal. 
 They are not all spherical ; therefore those that are so, must be 
 separated ; which is done by an ingenious contrivance. I'he 
 whole are sifted on the upper end of a long, smooth, inclined 
 plane, and the grains roll down to the lowxr end. But the pear- 
 like shape of the bad grains makes them roll down irregularly, 
 and they waddle as it were to a side, while the spherical ones 
 roll straight on, and are afterwards sorted into sizes by sieves. 
 The manufacturers of the patent shot have fixed their furnace, 
 for melting the metal, at the top of a tower one hundred feet 
 high, and obtain a much greater quantity of spherical grains, 
 by letting the lead fall into the water from this height, as the 
 shot is gradually cooled before it reaches the w^ater. The arsenic 
 
48 
 
 MECHANICAL EXERCISES. 
 
 Methods of reviving the oxides of lead. 
 
 is generally added in excess to a small quantity of lead, which 
 is covered and closely luted till the incorporation is complete. 
 The compound is called slag or poisoned metal. Ingots of this 
 slag are then added to soft pig lead, in such proportion as is 
 found upon trial to cause it to drop in a globular form. The 
 smaller the shot, the less the height from which it requires to be 
 dropped. The smallest kind need not fall from an elevation of 
 more than eight or ten feet. If the lead, at the time of casting, 
 be too hot, the shot wdll be apt to crack ; and if it be too cold, 
 it will stop up the holes in the cylinder. Instead of the cylinder, 
 a plate of copper, about the size of a trencher, is used in the mak- 
 ing of small quantities, with a hollowness in the middle, about 
 three inches compass, pierced with thirty or forty holes, according 
 to the size of the shot wanted. The part containing the holes 
 may be thin, but the thicker the brim the better it will retain 
 the heat. 
 
 The surface of melted lead, as every one knows, becomes 
 quickly covered with a skin or pellicle, often assuming different 
 lively hues at first, and subsequently increasing in quantity and 
 darkness of colour. This effect, termed by chemists oxida- 
 tion, as it is occasioned by the action of the oxygen of the 
 atmosphere, the activity of which is greater in proportion to the 
 heat of the lead, w^astes the metal so fast, that it becomes an 
 object of importance to those who melt much lead, to check its 
 formation, or to convert it, when formed, by the cheapest 
 process, into the metallic state again. A thick coating of ashes 
 of any kind, will check the formation of the oxide, and may 
 be easily pushed back, when a quantity of lead must be taken 
 out of the crucible or melting pan. Charcoal, which is also a 
 good covering for lead in the pan, will convert dross into metal, 
 when assisted by a sufficient heat ; fat, oily, and bituminous 
 substances in general, have a similar effect. Common resin 
 answers exceedingly well ; thrown in powder upon melted lead, 
 and stirred about, it immediately converts the oxide into metal, 
 causes the surface to shine like mercury, and if any thing 
 remains, it is only a black dirt, containing little or no lead. 
 But in taking off this dirt, small globules of pure lead, 
 skimmed off at the same time, get mixed wdth it ; by throwing 
 it into water, stirring it thoroughly, and pouring off all that 
 does not immediately sink, these grains may be separated. 
 If part of what has appeared to be dirt, is found to be so heavy 
 as instantly to sink to the bottom of w ater, it may be suspected 
 to be true dross or oxide, and may be revived by mixing it with 
 charcoal, and exposing it to a considerable heat. It is always, 
 however, more prudent and economical, to use means of 
 
MECHANICAL EXERCISES. 
 
 49 
 
 Observations on lead. — Zinc. 
 
 preventing the formation of oxide, than to bestow much time upon 
 its revival. 
 
 Lead becomes less fluid every time it is melted, and by much 
 or frequent exposure to a high temperature, a state in which it is 
 said to be rotten, is superinduced. To use new lea»l, and not to 
 melt it oftener, or expose it to a greater heat than is indispensable, 
 are necessary precautions to preserve this metal in its best state. 
 Plumbers, when they cast it into sheets, strew common salt upon 
 the table, to facilitate its spreading, when they are not using new 
 lead, and are for that, or any other reason, apprehensive that it 
 will not run well. 
 
 The observations above recited on the management of lead, 
 apply, with equal propriety, to tin, antimony, zinc, bismuth, &c. 
 and all the alloys of these metals with lead or each other. In 
 fact, as lead is so much cheaper than the other metals just enu- 
 merated, the object of saving it from destruction is proportion- 
 ately of less consequence. 
 
 Of Zinc. 
 
 Zinc is a very combustible metal, of a bluish, brilliant white 
 colour. It seems to form the link between the brittle and malle- 
 able metals. It is a modern discovery, that at a temperature of 
 from 210° to 300° of Fahrenheit, it yields to the hammer, may 
 be draw n into wire, or extended into sheets. After having been * 
 thus annealed, it continues soft, flexible, and extensible, and does 
 not return to its partial brittleness ; thus admitting of being ap- 
 plied to many uses for which zinc was formerly deemed unfit. 
 Hobson and Sylvester, of Sheffield, have taken out a patent, se- 
 curing to themselves the benefits accruing from the application of 
 this discovery to the arts. 
 
 There can now be no difficulty in forming zinc into sheathing 
 for the bottoms of ships, into vessels of capacity, w'ater-pipes, 
 and utensils for various manufactories. As an internal lining for 
 culinary vessels, instead of tin, it has already been applied with 
 success. It is much harder and cheaper than tin, and may be 
 spread very uniformly. 
 
 Zinc, at a very elevated temperature, may be pulverized. 
 It may also, like several other metals, be minutely divided, by 
 pouring it, when in fusion, into water. These are the most con- 
 venient means of reducing it into small particles. Files have no 
 considerable action upon it ; besides, it wears and chokes them 
 up in a short time. Zinc, in filings or small particles, is used to 
 produce those brilliant stars and spangles which are seen in the 
 best artificial fire-works ; but the tilings of cast iron produce, at a 
 cheaper rate, an effect scarcely inferior. 
 
 S.-^VoL. I. H 
 
50 
 
 MECHANICAL EXERCISES. 
 Solder for platina — gold — silver. 
 
 ■hi 
 
 Calamine, or lapis calaminaris, used in converting copper into 
 brass, is found both in masses and in a crystallized state, and is 
 generally combined with a large portion of silex. It is a native 
 oxide of zinc, combined with carbonic acid. Zinc is also found 
 in an ore called bhmh, or, as the miners term it. Black Jack. It 
 is a sulphuret of zinc : in Wales it was employed till lately for 
 mending the roads. 
 
 Soldering,. 
 
 To unite two pieces o^ the same or of different metals, by 
 fusing some metallic substance upon them, is called soldering. 
 
 It is a general rule, that the solder should be easier of fusion 
 than the metal to be soldered by it. It is, in the next place, de- 
 sirable, though seldom absolutely necessaiy, nor always attempted, 
 that the solder and the metal to which it is intended to be applied, 
 should be of the same colour, aixl of the same degree of hardness 
 and malleability. 
 
 Solders are distinguished into tw o principal classes, viz. the 
 hard and the soft solders. For the hard solders, which are 
 ductile, and admit of being hammered, some of the same sort 
 of metal as that to be soldered, is, in the greater number of 
 instances, alloyed with some other which increases its fusibility. 
 Some of the facts already detailed, respecting the metals, prove 
 diat the addition made with this view need not always be itself 
 easier of fusion. 
 
 The solder for platina is gold, and the expence of it will, 
 therefore, contribute to hinder the general use of platina vessels, 
 even in chemical experiments. 
 
 The hard solder for gold, is composed of gold and silver ; 
 gold and copper; or gold, silver, and copper. Goldsmiths usu- 
 ally make four kinds; viz. solder of eight, in which, to seven parts 
 of silver, there is one of brass or copper ; solder of six, where 
 only a sixth part is copper ; solder of four, and solder of three. 
 But many who may have occasionally to solder gold, camiol 
 encumber themselves with these varieties. For general purposes, 
 therefore, the following composition may be provided : melt two 
 parts of gold, with one of silver and one of copper ; stir the mass 
 w ell to make it uniform, add a little borax in powder, and pour it 
 out immediately. If cast into very thin narrow sbps, it will be 
 the more handy for subsequent use. To cleanse gold w'hich has- 
 been soldered, heat it almost to ignition, let it cool, and then boil 
 it in urine and sal ammoniac. 
 
 The hard solder for silver may be prepared by melting two 
 parts of silver with one of brass. It must not be kept long ia 
 fusion, lest the zinc of the brass fly off in fumes. If the silver 
 
MECHANICAL EXERCISES. 
 
 51 
 
 Solder for copper — brass — iron. 
 
 to be soldered, be alloyed with much copper, the proportion of 
 brass may be increased : for example, the following composition 
 may be used ; four parts of silver and three of brass, rendered 
 easier of fusion by the addition of a sixteenth part of zinc. 
 
 Silver which has been soldered, may be cleaned by heating it^ 
 
 and letting it cool, as directed for gold, but it must be boiled in 
 alum water. 
 
 The hard solder for copper, is a soft fusible sort of granu- 
 lated brass, well known to artists under the name of spelter. 
 It consists of brass mixed with an eighth, a sixth, or even 
 
 one-half of zinc. The brasier’s use no other kind of hard 
 
 solder, and it is commoniy sold by them. As spelter melts 
 sooner than common brass, it serves for the solder of the latter 
 as well as for copper. Standard silver makes an excellent solder 
 for brass. It is more fusible than speller, proportionately easier 
 to manage, and equally as durable. A slight demand for silver 
 solder may, to many, be supplied at an easy rate, in consequence 
 of the number of small silver articles in common use, ami w hich 
 are frequently wearing out ; no one need to be at a loss for it, 
 while they can provide themselves with a worn-out silver thimble 
 or toothpick. 
 
 Iron may be soldered with copper, brass, gold, or silver. 
 Brass or spelter is most commonly used, and the operation is then 
 called brazing ; but a carburet of the same metal, viz. the dark 
 grey or most fusible sort of pig iron, called No. 1. is the most 
 durable solder that can be used. The pig iron loses some of its 
 brittleness, and the malleable metal becomes harder in the proxi- 
 mity of the parts soldered. 
 
 The parts upon which hard solder is intended to operate, 
 are touched with finely powdered borax moistened with water. 
 They must also, as in all soldering and tinning operations, be 
 perfectly clean. The borax, quickly running into a kind of 
 glass, promotes the fusion of the solder, and preserves from oxi- 
 dation the surfaces to which it is applied. The pieces intended 
 to be soldered, are fastened together with iron wire, or secured 
 by some contrivance having the same effect. Spelter being com- 
 posed of so many grains, is apt to spread when the borax boils up ; 
 but just as it becomes fused, the workmen bring it to the place 
 where it is wanted, by a slender iron rod. The flame of a lamp, 
 * directed by the blow-pipe against the solder covering the intended 
 joint, which must be laid upon charcoal, is sufficient for small 
 things. For large work, a common culinary fire may be made to 
 effect the desired fusion, though a forge is still more convenient. 
 The fire should not touch the work, nor the ashes be allowed to 
 fall upon it. 
 
52 
 
 MECHANICAI. EXERCISES. 
 
 Solder for lead — tin — fine brass work. 
 
 The soft solders melt easily, but are partly brittle, and there- 
 fore cannot be hammered. The solder for lead is usually com- 
 posed of two parts lead and one of tin. Its goodness is tried by 
 melting it, and pouring about the size of a crown piece upon a 
 table ; little shining stars m ill arise upon it if it be good. By 
 diminishing the proportion of lead, we form what is called strong 
 solder ; we may also increase the proportion, which is adviseable 
 when w e wish to solder vessels for containing acids ; because lead 
 is not so easily corroded or dissolved as tin. 
 
 The metal with which tea-chests are lined, familiarly called 
 tea-leadf is an alloy principally composed of lead and tin. It 
 is generally sold at so low a rate, that it is bought by manufac- 
 turers w ho require alloys of lead, as a cheap method of obtain- 
 ing that metal already somewhat enriched. Jn some of it, the 
 proportion of tin is very small : other parcels contain so much 
 tin that they make excellent solder for lead w ithout any furtlier 
 admixture. I^^o solder can be obtained at so low a price as this. 
 These valuable portions of tea-lead may be distinguished by their 
 brilliancy, having sulfered little from oxidation ; also, when they 
 principally consist of tin, by that crackling noise while bending, 
 which is peculiar to tliis metal and some of the alloys into which 
 it largely enters. We may observe, in passing, tliat tea-lead, 
 when niixed with antimony, generally produces a compound of 
 less hrmness when cold, and when melted, of less fluidity, than 
 if pure new lead had been used. It is not improbable, that when 
 manufactuied into sheets, it is occasionally spoiled by too much 
 heat. 
 
 The solder for tin may consist of four parts pew ter, one of 
 tin, and one of bismuth. 
 
 The soldering iron of the tin-plate workers, is an ingot of cop- 
 per, flattened at the point, in a pyramidical form ; it is screwed or 
 rivetted to an iron stalk driven into a wooden handle. The cop- 
 per is seldom more than four or five inches long, and when it is 
 worn away, the same stalk and handle are used for another piece. 
 The copper, when sufliciently heated, will melt and take up the 
 solder, so as to afford a ready means of applying it to the intended 
 juncture. Powdered resin, and sometimes pitch, is used along 
 with the soft solders, to preserve the metals employed from oxi- 
 dation. 
 
 Tin-foil, applied between the joints of fine brass work, first 
 wetted with a strong solution of sal ammoniac, and held firmly 
 together while heated, makes an excellent juncture, care being 
 taken to avoid too much heat. 
 
MECHANICAL EXERCISES. 
 
 53 
 
 Description of the foot lathe. 
 
 TURNING, 
 
 Turning is an art universally admired; the simplicity of the 
 operation, the facility with which precision in performing it is 
 attained, the agreeable exercise it affords to the mind, the 
 beauty and utility of its products, have drawn, for the amuse- 
 ment of a leisure hour, as w ell as for objects of real importance, 
 men of all ranks into the number of its practisers. It is an art of 
 great antiquity, but when or by whom it was first adopted, we 
 must leave the antiquarian to decide. In this place it claims no- 
 tice, on account of its contributing so essentially to the perfection 
 of several other arts. 
 
 The machine in which turning is perfonned, is called a Lathe. 
 Lathes differ very considerably in their general form, their size, 
 and the materials of which they are made. . They are commonly 
 classed according to the manner in which they receive their mo- 
 tion. Hence we have the Wheel Lathe, the Foot Lathe, the 
 Hand Lathe or Turnmg Bench, and the Pole Lathe. For very 
 large work, there are other lathes used, w'hich are wrought by 
 horses, water wheels, or steam engines. 
 
 We shall, in the first instance, describe the foot lathe, which 
 may be converted into a wheel lathe, as often as occasion de- 
 mands ; as, for this end, it is only necessary to fix it in such a situ- 
 ation as to admit of the requisite addition of a large wheel, turned 
 by one or more men. 
 
 Fig. 1. pi. I. is the perspective view of a foot lathe made 
 entirely of iron, excepting the wheel. The shears or cheeks, 
 which constitute the bed of the lathe, and one of which, BB, 
 is seen in front, are fastened, at one end, by two bolts, c d, to 
 the upright O ; and at the other end by the bolts a h, to the 
 upright Q. These bolts pass entirely through the shears and 
 the uprights, and are each of them screwed at the end, so as to 
 be drawn perfectly tight by a nut at the back. The heads are 
 countersunk, so that they lie level in front, and can occasion no 
 inconvenience. The feet PR are xast in the same piece with 
 their respective uprights O Q. They are fastened to the floor by 
 screws passing through them, and are large enough to afford a 
 solid bearing for the lathe. The shears are parallel, and enclose a 
 space for the puppets, or, as they are sometimes called, the head- 
 stocks, to slide in. 
 
 On the rim of the great wheel or fly, K, are three annular 
 grooves, gradually narrowing to the bottom, the section of each 
 exhibiting an angular indentation, which form is the most suitable 
 to take effectual hold of the band, and give it more power to turn 
 the mandrel E, the pully F of which has three corresponding 
 
54 
 
 MECHANICAL EXERCISES. 
 
 The foot lathe. 
 
 grooves, of different diameters, to give it different velocities. 
 The dy ^vheel, the grooves of which are also of different diame- 
 ters, has its least diameter upon the same side as the greater dia- 
 meter of the pulley. The band is made of strong catgut ; in order 
 to make it suit, when applied to all the different grooves of the 
 pulley, the wheel K can be elevated or depressed by means of the 
 screws p p, each of which acts upon a sliding piece in the up- 
 rights. The axle of the wheel, at one end, works in the sliding 
 piece of the upright Q ; but at the other in the end of the screw 
 fy which passes through the slider of the upright O, and affords 
 the convenience, not only of making the motion of the wheel 
 steady, but of taking it out, or putting it in, without disturbing the 
 other parts of the machine. The axle of the wheel is of iron, 
 except at each extremity, which is steeled, left very hard, and of 
 a conical figure. At L it is bent so as to form a crank, receiving 
 one I^ook of the connecting rod ^I, the other hook of which is 
 attached to the treadle frame N. The hooks of the connecting 
 rod being screwed in, can be lengthened or shortened as the wheel 
 is elevated or depressed. The treadle frame is made of wood ; to 
 each end of the back part q q, is affixed a conical piece of steel, 
 one of which works in a corresponding hole in the foot P, and the 
 other in R, upon the same plan as the axle of the fly, 
 
 The puppets C D are cast in one piece. On the under side 
 they have a projecting part, pointed out by dotted lines near B, 
 exactly filling in breadth the space between the shears, upon 
 which they are drawn down by the screws 7U m, which enter 
 this projecting part, and render tliem immoveable at pleasure. 
 The puppet G is fastened in a similar manner, by the screw /. 
 The spindle or mandrel E, runs in a brass or bell-metal collai* 
 in each of the puppets C D. The collar in the puppet C is of 
 a single piece, very carefully drilled ; but that in the puppet D 
 is in two pieces, which are fltted so as to slide down an angular 
 groove, and fmther secured by a plate r on the top of it, which 
 plate is fastened by two screws. The part E is called the 
 nose of tlie spindle, and the screw upon it is intended to receive 
 the chucks. The spindle, where it works in the collars, must be 
 steel, welded round the iron part, and turned cylindrical in the 
 most accurate manner. It is supplied with oil by small holes, 
 one of which is drilled from the top of each puppet through the 
 collars. It is intended to be used occasionally as a traversing 
 mandrel, but when not used for this purpose, it has a groove, 
 for a piece of steel on the puppet C to fall into, so as to 
 prevent its horizontal motion. By slackening the screws I m my 
 it is evident that the puppets are at liberty to slide horizontally, 
 and that D and G can be fixed at various distances from each 
 
MECHANICAL EXERCISES. 
 
 
 The foot lathe. 
 
 Other. G is the puppet moved to suit the length of different work, 
 which requires the use of the point of the screw H as a centre. 
 An accommodation of a few inches is obtained by screwing H 
 further through or out of the headstock. When the screw H has 
 been brought to its proper place, it is fastened by turning the 
 screw s. To prevent the screw s from damaging the thread of H, 
 a small piece of brass intercepts the end of it, and is pressed by it 
 upon the Screw. 
 
 The rest, I, can be affixed at any necessary distance from the 
 axis of the work, by the bolt which is drawn tight by the 
 nut k, the peculiar shape of which is very convenient in prac- 
 tice, as it admits, in turning it, of the ready application either 
 of the hand or any sort of a lever. As the opening which admits 
 the bolt i is not stopped off at one end, the rest can be drawn 
 from the lathe without taking off the nut k from the bolt z, 
 which, when the rest is withdrawn, immediately falls from 
 between the cheeks. The cross part, g, of the rest, having a 
 cylindrical stem, which exactly fits a hole passing perpendicu- 
 larly through the pillar in the fore part of the rest, admits of 
 being moved entirely round, or of being raised or lowered, and 
 it may be fixed in any situation which can be necessary, by the 
 screw h. 
 
 The pulley F is commonly made of mahogany, or some 
 hard wood which will stand well. On the face, it is covered 
 with a brass plate, upon which are a number of concentric cir- 
 cles, each divided into a different number of equal parts by small 
 holes. There is a stop t, consisting of a stout cylindrical piece 
 that can be moved round on a screw on the puppet D ; to tho 
 end of it is affixed a thin piece of steel, so that when turned up, 
 and a short point, near the top of it, inserted in any of the 
 divisions of the pulley, it has sufficient spring to keep it there. 
 This contrivance is used to divide the circumference of any 
 thing turned into equal parts. It may also be applied to the 
 cutting of the teeth of wffieels, so as to make the lathe not inferior, 
 for this purpose, to the clockmaker’s engine, which is often 
 more costly than a complete lathe. To use it in this w^ay, let the 
 part g of the rest be drawn out, and a strong plate of iron, w ith a 
 stem of the same kind, substituted. Upon this plate, let there be 
 a sliding piece, to be pushed forward by a spring, and containing 
 a small circular cutter, with teeth on its circuinferenee like those 
 of a file. The cutter runs horizontally ; on its axis must be a 
 pinion, turned by a wheel, of some considerable diameter, and 
 working in the same sliding piece. This wheel has a small winch, 
 by the turning of which the teeth are cut, the stop t being first 
 fixed in one of the holes of the brass plate, the circle used being 
 
56 
 
 MECHANICAL EXERCISES. 
 
 The foot latlie used as a wheel lathe. 
 
 determined by the number of teeth wanted, and the cutter brought 
 to the circumference of the wheel in the lathe, upon which it is 
 to operate. When one tooth is cut, the stop is slipped into ano- 
 ther hole, and the process repeated till all are completed. 
 
 The wheel, and consequently the pulley with which the band 
 connects it, is put in motion by pushing down the treadle ; the 
 crank, previously to the first push, being brought nearly to its 
 highest elevation. By the momentum which the wheel has thus 
 acquired, the motion is continued, when, as the treadle and crank 
 rise, the workman can exert no power on the machine. As soon 
 as the crank begins to descend, the treadle receives another im- 
 pulse from the foot, by the repetition of which, at each revolution 
 of the wheel, the force destroyed by friction, and by the tool in 
 the act of turning, is constantly renewed. The wheel is fre- 
 quently made of iron ; it is not then solid, but the rim, which is 
 of considerable weight, is connected by four or five spokes or 
 arms with a centre-piece, through which there is a square hole, 
 for the axis to pass, and it is fastened by thin wedges. When an 
 iron wheel is used, the arms and centre-piece should be light, that 
 nearly its whole weight may be accumulated in its rim or circum- 
 ference. The arms, also, should present a thin edge in that direc- 
 tion in which they revolve, to lessen the resistance of the air. 
 
 The foot lathe has not power enough for the turning of very 
 heavy work. It is therefore converted into the wheel lathe by 
 using a large fly wheel, (five, six, or seven feet in diameter, for 
 instance,) standing in a separate frame, at the distance of several 
 feet, or even yards, from the pulley, and turned by one or two 
 men, who use both hands at once to the winch. The band, to 
 give it greater power over the pulley, is crossed ; that part of it 
 which came from the top of the wheel consequently goes directly 
 to the bottom of the pulley, and the men who turn can face the 
 lathe, to assist them in knowing when it is proper to stop, which 
 is necessary when no contrivance is resorted to for stopping the 
 mandrel, without stopping the great wheel. The following de- 
 scription of an ingenious, though more complex, method, adopted 
 by Maudslay, an eminent turner in London, in applying the power 
 of the large wheel, is given in Gregory’s able treatise on Me- 
 chanics : The large fly w heel which the men turn, w orks by a strap 
 on another wheel fixed to tlie ceiling, directly over it ; on the axis 
 of this wheel is a larger one, which turns another small wheel or 
 pulley fixed to the ceiling directly over the mandrel of the latlie ; 
 and this last has on its axis a larger one which works the pulley F 
 by a band of catgut. These latter wheels are fixed in a frame of 
 cast iron, moveable on a joint ; and this frame has alw ays a strong 
 tendency to rise up, in consequence of the action of a heavy weight; 
 
MECHANICAL EXERCISES. 
 
 57 
 
 Manner of using the foot lathe as a wheel lathe, 
 
 ■; the rope from which, after passing over a pulley, is fastened 
 
 I to the frame : this weight not only operates to keep the mandrel- 
 
 I band tight, when applied to any of the grooves therein, but 
 
 i always makes the strap between the two wheels on the ceiling 
 
 { fit. As it is necessary that the workman should be able to stop 
 
 1 his lathe, without the men stopping who are turning the great 
 
 ' wheel, there are two pulleys or rollers, (on the axis of the 
 
 » wheel over the lathe,) for the strap coming from the other wheel 
 
 • on the ceiling; one of these pulleys, called the dead pidhy, is 
 
 i fixed to the axis, and turns with it, and the other, which slips 
 
 round it, is called the live pulley : these pulleys are put close to 
 each other, so that by slipping the strap upon the live pulley y it 
 will not turn the axis; but if it is slipped on the other it will 
 turn with it : this is effected by a horizontal bar, with two 
 upright pins in it, between which the stiap passes. This bar is 
 moved in such a direction as will throw the strap upon the live 
 pulley, by means of a strong bell-spring ; and in a contrary 
 direction it is moved by a cord fastened to it, w'hich passes over 
 a pulley, and hangs down w'ithin reach of the workman's hand : 
 to this cord is fastened a weight, heavy enough to counteract 
 the bell-spring, and bring the strap up to the dead pulley, to 
 turn the lathe ; but when the weight is laid upon a little shelf, 
 prepared for tlie purpose, it will be stopped by the action of the 
 spring. 
 
 ^ It may be considered a difficult and expensive undertaking 
 to make an iron lathe upon the plan represented by fig. 1 ; but if 
 the cheeks be cast in such a manner, that the transverse section 
 of them, when placed as they are to lie in the lathe, shall be 
 like fig. 2, the projecting parts opposite each other, and on the 
 top, which are all that need be filed flat, will be so narrow that 
 the making of them true will be very materially expedited. 
 In metallic lathes, it must not be concealed, there is an elastic 
 tremor which is disadvantageous; but they admit of and retain 
 so much greater exactness in their workmanship, they are so 
 much more compact and durable than w^ood, that they merit a 
 decided preference, and the use of them is becoming so general, 
 that they will probably, in a short time, supersede every other 
 kind. In its first cost, if the business be set about in a proper 
 manner, an iron lathe need not be made to exceed the price of 
 a really good wooden one ; a small quantity of the cheapest sort 
 of wood will make the patterns for a large lathe ; one pattern, 
 for example, it is clear, will serve for both the uprigfedts, and 
 the same may be observed of the cheeks, which are also both, 
 alike. When a wooden lathe is intended for nice work, it 
 S.— VoL. L I 
 
58 
 
 MECHANICAL EXERCISES. 
 
 Requisites of a good lathe. 
 
 ought to be made of mahogany; but whether wood or iron be 
 the material, it is recommendable to make the cheeks of consider- 
 able depth. 
 
 The stronger and the more correct the workmanship of a 
 lathe, the easier it is to execute work in it, with expedition and 
 truth ; but good work may be performed with an indifferent lathe, 
 by taking care to cut so little at a time, that the parts of the 
 engine may never be shaken out of contact. 
 
 It is essential to a good lathe, that the centres of the collars 
 of the mandrel, and the centre of the screw in the moveable or 
 right hand puppet head, be in one line, parallel to the bed or 
 shears. If the collars and the mandrel be truly formed, the 
 latter, when the rotation of it is slowly made by the hand, 
 will be equally stiff in every stage of its progress, and the 
 wearing parts, when examined, will have, every-where, the 
 same appearance. When the mandrel is fitted with accuracy, 
 the head of the moveable puppet, which must previously be 
 finished in that part which slides between the shears, may be 
 drilled in the following manner : screw firmly upon the mandrel 
 a piece of steel ; turn it true ; let the right hand end of it be 
 made conical like the extremities of the axle of the wheel K, 
 fig. 1 ; from the base of this conical end, to at least the same 
 extent as the thickness of the moveable puppet, at the part to be 
 drilled, let it be turned somewhat smaller than the broadest part 
 of the cone. File the cone, so as to leave two or three edges un- 
 touched by the file, but which will cut like those of a drill. Now 
 bring up the moveable puppet, and bore it with this tool ; — -the 
 axis of the hole produced, will be in the same line as the axis of 
 the mandrel. While the puppet is drilling, the screw which 
 fastens it to the cheeks, should be tight enough to prevent its 
 shaking, but not so tight as to prevent its being impelled forward 
 as the tool cuts. 
 
 It is not customary, in foot lathes for general purposes, to 
 make the centre of the collars more than eight or nine inches 
 above the bed. i\s one means of ensuring perfect steadiness, it 
 is always proper to make them as low as the work intended to 
 be done will admit ; but in the lathe above described, an 
 arrangement may be made for the occasional turning of work of 
 extraordinary diameters. That part of the puppets C D which 
 slides between the cheeks, does not extend to the front of D ; 
 so that when these puppets are turned round, and D stands 
 where C now is, the nose of the mandrel is even with the 
 outside of the upright Q. Hence the body to be turned will 
 be relieved from the interference of the cheeks or the upright, 
 by tlie chuck, c-r whatever else is used to effect the rotation. 
 
MECHAISICAL EXERCISES. 
 
 59 
 
 Method of stopping the foot latlie. — The standards. — The mandreU 
 
 When this plan is adopted, the method of fixing the rest, and 
 also the right hand centre, when two centres are wanted, must be 
 left to the judgment of those who may require its assistance, as 
 different workmen will have different conveniences for accom- 
 plishing the same end; the possession of a substantial bench 
 will, in this case, relieve one person from embarrassment, while 
 the proximity of a wall will afford stability to the fixtures of 
 another. 
 
 The method of stopping the motion of the foot lathe, is 
 extremely simple ; a considerable piece of thick canvass or 
 leather, is suspended near the circumference of the fly, on that 
 side next the workman, who presses his knee upon the rim, with 
 the canvass or leather interposed, and the momentum of the 
 wheel is too inconsiderable not to be instantly overcome without 
 difficulty. 
 
 Fig. 3, is an elevation of one of the standards and feet of 
 the foot lathe, shown endways. The ribs a b are merely intended 
 to afford additional strength to the standard, which, if made a 
 little stronger than the proportion here represented, will not re- 
 quire their aid. The screw by which the sliding piece (for receiv- 
 ing the extremity of the axle of the fly,) is elevated or depressed, 
 is attached by a collar, in which the upper end easily turns round. 
 That part of the standard itself, in which the screw works, is 
 cast with a hole in the centre, in which a brass nut is afterw^ards 
 inserted. 
 
 Fig. 4, is the mandrel separately. When the mandrel is not 
 used as a traversing one, it has generally, besides the shoulder 
 against which the chucks are screwed, another shoulder which 
 is pressed against the collar of the middle puppet by a screw ; 
 for an example of this arrangement see fig. 13. When the 
 mandrel is not provided with a shoulder of this sort, it is, at 
 that part which w orks in the collar of the middle headstock, of a 
 conical figure, so that pressing it forward with the screw at the 
 back, will at all times produce the effect of making it fit the collar. 
 But w'hen this form is given to the mandrel, the friction in the 
 collar is prodigiously increased by the slightest pressure of the 
 back screws, from its operating like a wedge ; and it is difficult to 
 regulate it in such a way, that the mandrel is not so tight as use- 
 lessly to destroy any part of the power applied to the machine, or 
 so slack as to be unfit for use. The nose of a mandrel is almost 
 always screwed externally, for the reception of chucks, and is be- 
 sides often furnished with a square hole for the ready insertion of 
 small arbors, boring tools, &c. An inside screw is much better 
 than a square hole, and, in lathes of the best construction, is also 
 more common. 
 
60 
 
 •MECHANICAL EXERCISES. 
 
 Means of fixing the work in taming. 
 
 Of Chucks. 
 
 The chucks of a lathe are pieces of wood or metal, screwed 
 or otherwise fastened to the nose of the mandrel, and used to 
 sustain the work in its rotation. Their construction is varied 
 very considerably, to suit different purposes. When made of 
 metal, brass is most commonly selected. The work, as the 
 nature of it renders most convenient, is fastened to a wooden 
 chuck by- cement, or by glue, or screwed into it, or gently 
 driven into a cell prepared to receive and hold it fast. To 
 sustain the latter operation, or any other ^iolence, wooden 
 chucks are hooped with brass or iron. As it would be almost 
 impossible to screw a wooden chuck upon the convex nose of a 
 mandrel, and take it off as occasion required during the pro- 
 cess, w ithout altering the position, it is found much the best to 
 make the medium of fastening it to the mandrel of brass. Tliis 
 may be done in two ways : one-half of a solid cylinder of 
 brass, somewhat thicker than the nose of the mandrel, mav be 
 reduced so as to leave a shoidder, and properly screwed to fit 
 the concave screw of the mandrel ; the remaining part maythen 
 be screwed with a coarse thread, for the reception of the chuck, 
 w hich is to be tightly screw ed upon it ; the screw' and the chuck 
 may then be removed together from the mandrel, as often as 
 may be necessary, and again screwed on exactly to the same 
 bearing. It is proper to use a little oil upon tlie thread of 
 the screw' v\hich enters tlie mandrel, lest it be so fast that the 
 position of tlie wooden chuck may alter in a small degree when 
 it is taken out. The other way of adopting the intervention of 
 metal, in securing a wooden chuck, is still more secure, and 
 generally used for large w ork : it consists in using a short hollow' 
 cylinder of brass, internally screw ed w ith the same thread as the 
 external screw* of the mandrel, upon which it can consequently 
 be screwed with facility at any time, and as easily taken off. 
 Upon one end of it is soldered a circular plate of iron or brass, 
 so as to form upon it what is called a flanch, the diameter and 
 strength of which must be proportioned to the magnitude of the 
 chuck, and the weight of the work. When the chuck is six inches 
 in diameter, the diameter of the flanch ought not to be less than 
 two and a half or three inches. Four or five holes are made near 
 the circumference of the flanch, for screws to pass through, in 
 order to fasten the brass to the wood. — A wooden chuck is so 
 liable to warp, from changes of temperature, that even after they 
 have once been made true, it is customary to turn them a little 
 every lime they are used. 
 
MECHANICAL EXERCISES. 
 
 61 
 
 Means of fixing the work in turning. 
 
 Besides the chucks above described, there are several other 
 kinds, considerably dilferent in their form ; some of the most 
 usefid we shall describe. Fig. 5, is a universal chuck. It con- 
 sists of a short, hollow cylinder, one end of which can be screwed 
 upon the nose of the spindle, and on the circumference, at equal 
 distances from each other, and from the edge of the other end, 
 are hiserted four screws. As these screws may be altered to any 
 distance within the cylinder, they can be screwed upon work of 
 any size which the cylinder will admit, so as to hold it while it is 
 turned. 
 
 Fig. f), is one part of another contrivance for holding work 
 of various sizes ; it is used in conjunction with the centre, fig 
 7, and the chuck fig. 8. Suppose we have a bar of iron to 
 turn into a cylinder ; let the chuck fig. 8, be screwed upon the 
 nose of the mandrel, and the centre, fig. 7, into it ; then put the 
 bar a little w^ay through the tool, fig. 6, which must be fastened 
 thereon by its screw, so that if it be carried round, the bar must 
 revolve at the same time. Suspend the bar, by punch holes, 
 at one end on the centre fig. 7, and at the other on the point of 
 the screw in the right hand puppet, the arm a of the tool fig. 
 6, being first put into the nick of the chuck fig. 8 ; the bar 
 may then be revolved upon the two centres, and turned with the 
 greatest facility, in every part except that which is immediately 
 under and on the left hand of the tool fig. 6. It can often be so 
 contnved, that the part thus left unturned may be cut off either in 
 the lathe or afterwards ; but if this be inconvenient, nothing more 
 is necessary, when this part is all that remains undone, than to 
 reverse the posuion of the bar, and fasten the tool fig. 6, upon 
 the finished end ; we are then at liberty to complete our work. 
 When there is an inside screw at the end of the mandrel, the 
 centre fig. 7, and the chuck fig. 9, are screwed to fit it ; but when 
 there is only a square hole, they must necessarily possess a cor- 
 respondent form. 
 
 The chuck, fig. 9? is most commonly used for revolving long 
 pieces of wood. Its prongs, which are sometimes made to stand 
 in a line, and sometimes at equal distances from each other close 
 to the circumference, are driven into the wood ; it is then screwed 
 into the nose of the mandrel, and the other end of the wood being 
 supported, by its centre, on the screw, as in the last instance, it is 
 ready for turning. 
 
 Of the Boring and Mandrel Collars. 
 
 Fig. 10 is a boring collar : the holes are conical, and their 
 centres are all precisely at the same distance from the axis of the 
 collar. This collar is used to support the end of any long 
 
62 
 
 MECHANICAL EXERCISES. 
 
 The boring collar. — The hand lathe. 
 
 body which must be turned hollow, and which could not be 
 kept steady by a chuck. It is usually made of iron or brass. 
 In using it, remove the common right hand puppet, and provide 
 another considerably lower. Through this low headstock, drill 
 a hole of the same size as that in the centre of the boring collar. 
 The centre of this hole must be in the same perpendicular line 
 as the centre of the mandrel. The collar is attached to it, so as 
 to face the end of the mandrel, by a screw w ith a button head, 
 tightened at the back with a nut. When properly fixed, the centre 
 of the hole in the boring collar is exactly opposite the axis of the 
 mandrel, and when the largest hole is used, and is therefore upper- 
 most, it entirely clears the top of the headstock to which it is 
 affixed. The end of the cylinder to be bored, being placed in the 
 hole w hich fits it, the boring tool is held upon a rest against its 
 centre, and the boring may then be performed, with great accu- 
 racy, to any required depth. 
 
 Fig. 11, is a boring collar with one aperture, capable of ad- 
 justment by turning the screw at the top. As for w^ork of dif- 
 ferent sizes, it requires to be fixed at different heights ; the opening 
 for the screw which fastens it, is not circular, but of sufficient 
 length to admit of the required variation. A cylinder revolving 
 in this boring collar, will always touch one point of each side of 
 the triangular aperture, which is sufficient to produce the requisite 
 steadiness while the boring is executed. 
 
 Fig. 12, is the collar of the puppet D. Before it is divided, 
 which is done with a fine saw, the hole for the mandrel is very 
 carefully turned. It is very common to make the collars of lathes 
 with steel; but when formed of an alloy containing thirteen or 
 fourteen parts of copper, to two or three of tin, they have much 
 less friction, and will last a very long time, several years perhaps, 
 in constant use, without requiring to be renewed. To make the 
 mandrel and its collar perfectly fit, they may be ground together 
 a little in their places, with finely powdered Turkey hone : emery 
 must not be used, as it would enter the pores of the metals, w hich 
 could never be freed from it by cleaning. 
 
 The Hand Lathe, or Turning Bench. 
 
 The wheel of the hand lathe is so situated, that it can be 
 turned by the left hand, while the right is employed in holding 
 the chisel. It is seldom more than eighteen or twenty inches in 
 diameter, and is placed just behind the pulley of the mandrel, so 
 that the winch, at its greatest distance, is within an easy reach to 
 the workman. But the wheel is not an inseparable part of the 
 hand lathe ; in which the rotary motion of the work is sometimes 
 produced by the drill bow. 
 
MECHANICAL EXERCISES. 
 
 63 
 
 The hand lathe. 
 
 Fig. 13, is an elevation of a hand lathe of the most approv- 
 ed cousUuction. CC are two metallic pillars, firmly screwed 
 to a board or table at the bottom, and at the top to the triangular 
 bar BB. One of the flat sides of this bar is parallel to the table, 
 consequently an angular edge is uppermost. The puppets 
 D E F have angular openings, which exactly fit the bar ; two 
 of them, DF, are exactly alike, and an elevation, frontwise, 
 of one of these, is given m fig. 14. These puppets can 
 be taken ofl' the bar, w ithout unscrewing the pillars C C ; 
 there is an aperture on both sides of each of them, a little 
 below the under side of the bar; a plate of iron being placed 
 across each puppet, through these apertures, a screw' is then 
 passed perpendicularly through the centre of it from below. By 
 pressing this screw against the bar, the puppet is rendered im- 
 moveable. The shoulder of the mandrel is pressed against the 
 collar in the puppet E by the screw’ g, the end of which, like that 
 of the mandrel, is convex, and tliey therefore touch each other 
 only in a point. 
 
 The support, H, for the rest, G, is flat on the top ; it is 
 moveable along the bar, and fixed exactly in the same manner as 
 the puppets. The pillar and upper part of the rest, resemble 
 the same parts in the foot lathe ; but the plate, I, to the end of 
 which the pillar is fixed, has no aperture through it for the ad- 
 mission of a bolt. The under side of it is, how ever, grooved in 
 the direction of its length, so that the section of it, the narrow' 
 way, resembles fig. 16. Hence it admits the button head of a 
 bolt c, fig. 15, which passes dow n the back part of the support 
 H, at the bottom of which it is tightened by a nut. The plate 
 of the rest, from its peculiar formation, can not only, like the 
 plate of the common rest, be drawn out, in a direction at right 
 angles to the axis of the mandrel, but can be turned round upon 
 its support H, a motion which is occasionally advantageous. It 
 may also be observed, that by slackening only the screw which 
 fastens the support H, the rest may be slidden to another part of 
 the bar, without the liability, as in the usual construction, of 
 altering its position otherwise than in the direction of the length 
 of the bar. 
 
 The pin h, of the puppet F, has a convex centre at one 
 end, and a concave centre at the other. It is perfectly cylin- 
 drical, and tits equally well either end foremost. It is not itself 
 screwed, but is fixed at the place required, by a screw' from the 
 top of the puppet. It is not unusud to force forw ard this pin 
 by a screw, which acts upon it exactly in the same way as the 
 screw a that prevents the mandrel from receding. ’^This mode 
 of using an unscrewed pin, in the right hand puppet, is used in. 
 
64 
 
 MECHANICAL EXERCISES. 
 
 The hand lathe. 
 
 lathes of all sizes, particularly in those of the very best, or, at 
 least, the most expensive sort. It is, however, not approved 
 by all workmen, who consider a screw, upon the whole, more 
 handy. 
 
 All the screws, in the lathe fig. 13, by which the puppets 
 and collars are tightened, it will be noticed, have rings to turn 
 them by. This form is more convenient than the common 
 thumb screw, which has no perforation, as either the hand or any 
 small lever thfft happens to be within reach, can with equal faci- 
 lity be applied, according to the tightness with which they must 
 be fixed. 
 
 When a wheel is used to this lathe, it is placed between two 
 standards fixed to a board six or eight inches broad, which, in the 
 direction of its length, lies at right angles to the bar, and slides in 
 an angular groove at each side, in which it has a range of a few’ 
 inches, for the purpose of tightening the band. When the drill 
 bow is used to effect the rotation of the w’ork, the mandrel is com- 
 monly withdrawn, and a centre pin substituted in the puppet D, 
 similar to that in tlie puppet F. The method of turning upon 
 two centres, whether the bow or the wheel produce the revolution, 
 admits of great truth and precision. 
 
 The vibrations of a mandrel, of a short one especially, are 
 much to be (beaded ; and cannot be too carefully guarded 
 against by accuracy of workmanship. In proportion as the 
 length of a mandrel is increased, the consequence of a given 
 quantity of vibration is diminished. It is therefore a general 
 rule to make the mandrel as long as will be convenient. In a 
 foot lathe, its length should never be less than foui teen or fifteen 
 inches. 
 
 The turning benches of the clock and watchmakers, are almost 
 always constructed with a rectangular bar for the puppets to slide 
 upon ; but a bar of this sort is much inferior to a triangular one. 
 These artists, also, have frequently no screw upon the nose of 
 the mandrel, but merely a square hole in the end of it. They 
 have arbors of various sizes, one of the ends of which fits this 
 hole, and the other extremity runs upon a centre. These arbors 
 are turned true, but not polished, and they are slightly conical 
 in their figure. To turn w’heels and other work which is per- 
 forated through the axis, it is only necessary to drive them, with 
 a few light blows, upon these arbors, which, though driven into 
 a cylindrical hole, become sufficiently tight upon them for the 
 operation. 
 
 For small lathes, which are exposed to no very extraordmary 
 strain, the mandrel may be made of cast steel, which wears longer 
 and more imifonnly than any other kind of steel. 
 
MECHANICAL EXERCISES. 
 
 65 
 
 Use and construction of the pole lathe. 
 
 I The Pole Lathe. 
 
 I This lathe is commonly remarkable for the extreme simpli- 
 I city of its construction ; but the manner in Avhich the body to be 
 turned in it receives motion, forms its essential characteristic. It 
 i can be easily made by any common artist, at a small expence ; it 
 
 I can be wrought by one man, and possesses considerable power; 
 
 ! but notwithstanding these important recommendations, it is gra- 
 ! dually sinking into disrepute and disuse. As in turning, the tool 
 can only be applied to cut, while the body upon which it is in- 
 tended to operate revolves in one direction, the pole lathe, which 
 produces a regular alternation of contrary revolutions, wastes the 
 time of the workman. This sort of motion, which forms the 
 principal disadvantage inseparable from the nature of the n»achine, 
 when used to enforce the action of edge-tools, is a very useful 
 one in some kinds of grinding and polishing. Stop-cocks and 
 valves may more quickly and effectually be made water and air- 
 tight in a pole lathe than in any other kind of a lathe. This hint 
 may be of considerable use to experimentalists, who may require 
 the temporary use of a lathe motion, for purposes of this kind, 
 when they may not have an opportunity of procuring it without 
 more trouble and expence than is convenient. Let a short cylin- 
 drical pin be driven into a wall, or fixed piece of upright timber, 
 such as the cheek of a door; place upon it a pulley which is 
 thick enough to hang a little over the end of it. The pole, the 
 treadle, and a temporary rest, are easily arranged. Let the pulley 
 be used as a chuck ; they may then give their work the best mo- 
 tion that can be contrived for it. It is not even necessary that 
 the pulley they use should be turned. Three circular pieces of 
 thin board, one of them about an inch in diameter less than the 
 other tw'o, may be united by nails or glue, the small one being 
 placed in the middle, and they will form a very tolerable pulley, 
 if bored in the centre so as exactly to fit the pin. If not suffi- 
 ciently near true on the face, the pulley may either be turned 
 with almost any tool that can be obtained, or the work may be 
 accommodated to it by applying a greater thickness of cement on 
 one side than on another. To prevent the pulley from grazing 
 llie wall, a small circular piece should be put on the pin before it, 
 to serve as a washer. 
 
 The pole lathe, see fig. 17, pi. L consists of two wooden, 
 horizontal parallel cheeks, with a space between them for the 
 insertion of the lower part or tenon of the puppets, and sup- 
 ported by legs or standards upon the same plan as the foot lathe. 
 The puppets, of which there are only two, reach some inches 
 below the chucks, and are fastened at any part of the bed, by 
 3.— VoL. L K 
 
66 
 
 MECHANICAL EXERCISbO. 
 
 Description of the pole lathe. 
 
 W'edges, n n, passing through mortises near the lower ends of 
 them. The mortises extend a little within the cheeks, otherwise 
 the wedges would not draw the puppets tight. The puppet L 
 contains a fixed point or centre, and the puppet M a moveable 
 centre or regulating screw. Between these centres the work is 
 supported. K is a long elastic pole, fixed at one extremity to the 
 ceiling, and at the other attached to a cord which is coiled once 
 or twice round the body to be turned, or round a pulley which 
 carries that body along with it, and then passes on to the treadle, 
 to which the lower end of it is fastened. The cord passes to the 
 treadle on the same side that it came from the pole. It is of such 
 a length, that when coiled round the work, the pole, if left to it- 
 self, will keep the treadle as elevated as the workman finds con- 
 venient. When the treadle is pressed down by the foot, the re- 
 action of the pole elevates it again, the moment that pressure is 
 forborne, and the backward and forward revolutions of the piece, 
 round which the cord is coiled, necessarily follow. 
 
 As it is evident that the workman, in pressing down the 
 treadle, has to overcome the resistance of the pole, the strength 
 of the latter should not be more than sufficient to produce the im- 
 mediate elevation of the treadle, otherwise he will uselessly waste 
 his strength. 
 
 When the pole lathe is used, a groove for the cord must be 
 turned at one end of the work as early as possible ; or a perfo- 
 rated pulley may be placed upon the fixed centre, and the 
 work connected with it in the manner already described for 
 connecting the tool, fig. 6, with the chuck fig. 8. As simplicity 
 of workmanship so generally distinguishes this lathe, a nail, of 
 sufficient size, bent in the middle to a right angle, one end of it 
 passing into a nick in the pulley, and the other driven into the 
 piece to be turned, may be mentioned as a contrivance still more 
 in character, and not unfrequently used by the humble provincial 
 artist. 
 
 Though the pole lathe has been described, as it is generally 
 found, made of the cheapest materials, and in the simplest 
 manner, yet it must be obvious to the reader, that there is no 
 impediment to its being made of metal, with all the excellence 
 of workmanship so often bestowed upon other lathes, but that 
 of incurring too much expence for a machine of circumscribed 
 utility. 
 
 The rest mostly used to this lathe, is similar to that for the 
 foot lathe, fig. 1. 
 
MECHANICAL EXERCISES. 
 
 67 
 
 Handles for tools. — The gouge. — The chisel. 
 
 Of the Tools used in Turning. 
 
 To describe the various tools used among the different descrip- 
 tions of turners, would be impossible. They are so infinitely di- 
 versified in form and size, according to the necessities, the inge- 
 nuity, and often, no doubt, even the stupidity, of those who use 
 them, that a volume would not suffice to do them justice. Me- 
 chanics are apt to consider the tools of their own invention the 
 best of any, and this attachment, not to say bigotry, is often ac- 
 companied with a silly attempt to conceal from their neighbours 
 the benefits of their amazing discoveries as to the best form of a 
 chisel. But though we may not exhibit many of those peculiar 
 forms of tools, upon which particular aitists are disposed to pride 
 themselves, we shall not omit those most essential and adequate 
 to the execution of every description of work* 
 
 The handles of turning tools, it may be premised, must be 
 varied in size according to the manner in which they are 
 intended to be held. For heavy work, when the lathe is turned 
 by machinery, or by men at the great wheel, they must be long 
 enough to reach to the shoulder, upon which one end of them 
 must rest in the act of turning, besides being held by both hands 
 at the same time. In using the foot lathe, the tools are held by 
 the two hands only, and their handles are seldom more than half 
 the length necessary in the former instance. In using the hand 
 lathe, as one hand is employed in turning the wheel, the handles 
 of the tools need not be longer than what can be comprised in 
 the other. 
 
 In turning wood, the gouge, fig. 13. pi. III. is the first tool 
 used, as no other will so quickly reduce its irregularities. Its 
 cutting edge is rounded ; in using it, the rest is generally on a level 
 with the axis of the work, and its handle is inclined downwards, 
 so that its cutting edge is considerably above the axis. i\s gouges 
 are made of all sizes, they are useful in making a variety of con- 
 cave mouldings. 
 
 The chisel follows the use of the gouge ; its cutting edge is 
 oblique to its sides, and formed by bevelling both the upper and 
 under surface, so as to make it in the middle of the thickness of 
 the tool. In using it, the rest is elevated considerably above the 
 axis of the work, so that, though held with a less inclination than 
 the gouge, its edge operates on a higher part of the surface. The 
 cutting edge of broad chisels is commonly made a little convex. 
 The gouge and the chisel are the only tools held above the level 
 of the axis; and the chisel is the only tool, the edge of which is 
 formed by its being bevelled on both sides. Fig. 14, No. 1, is 
 the front, and No. 2, the profile of the chisel. 
 
68 
 
 MECHANICAL EXERCISES. 
 
 Tools used in turning. 
 
 Fig. 15, is a right side toolj with two cutting edges, viz. a side 
 edge and an end edge, so as to cut at once both the bottom and 
 the side of a cavity. hen it is used, the bevel which forms the 
 edge is dow nwards. 
 
 A left side tool, the side-edge of which is opposite to that last 
 mentioned, is useful in smoothing the left external surface of 
 spheres, and other rotund figures. 
 
 Fig. l6, the round tool. Though the gouge is generally used 
 to form and sometimes to finish concave mouldings, yet, as it is 
 veiy difficult, in whetting, to give it a regular convexity, the round 
 tool, w hich may be formed w ith considerable exactness, is a useful 
 assistant for the same purpose. 
 
 Some workmen chuse to have point tools, see fig. 17, for 
 making nicks or small mouldings, as w ell as finishing the shoulders 
 and fiat ends of work ; while others, instead of them, use the 
 sharp comers and arrises of the chisels. 
 
 The tools included under tlie general term, inside tools, for 
 turning out hollows and cups of all descriptions, exhibit the 
 greatest variety of shapes, and exercise most the judgment of the 
 artist, in suiting them to his wants. The forms exhibited in fig. 
 18, 19, 20, 21, and 22, are some of the most useful. 
 
 The parting tool, fig. 23, is used for making deep incisions, 
 and cutting off any part of the w ork. 
 
 The drill, for making holes, is variously applied. It may be 
 used along wiili the boring collar; or it may be screwed upon 
 the mandrel, and the work held against it; or the latter arrange- 
 ment iiidy be reversed, and the work being fastened to a chuck, 
 the drill may be held against it, while the tool is steadied upon 
 the rest. The drill is sometimes formed so as to resemble half 
 a hollow cylinder, but its section is a crescent, and its edges are 
 moderately sharp. Its end is nearly like the mouth of a tea-spoon, 
 ami its hollow is greater than that of a gouge of the same breadth. 
 This sort of a drill is useei for small holes, but it is apt to turn so 
 ill upon its point as to bore awi*y* d he drill, fig. 24, is therefore 
 used in preference, especially for large holes. 
 
 The use of the outside sereze-tool, fig. 25, and the inside screrc- 
 tool, fig 26, are explained in the section on turning screw s. It 
 is obvious that the w ork ought to be turned very true, before they 
 are applied. 
 
 Turning graters, triangular and square tools, with various 
 other nameless sorts, the contrivance of individual skill, are 
 used in turning hard bodies, such as bone, ivory, and metals. 
 When the turning graver, fig 27, is used, it is the first tool 
 taken to iron and steel. The tool, fig. 28, of which No. 1 is 
 the front and No. 2 the profile, is often used instead of the 
 
MECHANICAL EXERCISES. 
 
 69 
 
 Tools used in turning. — Parallel rest. 
 
 graver, and cuts extremely well, but is not so easily managed 
 with one hand. Triangular and square tools, are denominated 
 ( from their respective sections being of these figures. They are 
 
 1 flat at the end. The former have three cutting edges, viz. each 
 
 an is in the diiection of their length ; the latter, which are 
 : mostly used for turning brass, have four, viz. each arris at the 
 
 extremity. In a powerful lathe, the tool, fig. 29, is a very 
 useful one for iron. It is flat at the end, and has four cutting 
 
 edges, all alike. It cuts too keenly for a small lathe, and 
 
 I requires, besides, two hands to hold it ; but with two hands, as 
 
 > one edge can be pressed upon the rest, it may be held remarkably 
 
 steady. 
 
 To describe the use of the tool, fig. 30, may afford a fami- 
 liar illustration of the advantage attending a proper adaptation 
 ! of the figure of the tool to the work. It is intended to turn the 
 
 I flat heads of the small brass nails used in fastening the sheathing 
 
 upon ships. These nails are little more than an inch long, and 
 as a very trifling charge is made for turning the heads, the oper- 
 ation requires to be performed with proportionate expedition. 
 A square hole, which just admits the shank of the nail, is made 
 in the end of the mandrel ; the nail is placed in this hole, the fly 
 w heel set agoing, and the tool, fig. 30, applied so that the notch 
 shall touch the rim, and the low er part of the tool the face of 
 the head ; then as the parts of the tool thus applied, have both 
 cutting edges, the rim and face of the head are both instantly 
 turned, while the nail is prevented from flying out of the lathe. 
 — When one nail is done, another is inserted without stopping 
 the fly wheel, and thus several hundreds may be done in an 
 hour. 
 
 Of the Farallel Rest. 
 
 If a tool, opposed to any body revolving in a lathe, be 
 drawn along, parallel to the axis of the mandrel, a cylinder 
 will be produced ; if it move in a line forming an angle with the 
 axis of the mandrel, a conical figure will be obtained ; and if it 
 operate at right angles to the same axis, a flat surface will be 
 the result. The machine in which a turning tool is fixed, to 
 produce these effects, as it is frequently applied to the turning 
 of parallel surfaces, has occasionally obtained the name of the 
 Farallel Rest 
 
 Fig. 31, pi. III. is a perspective view of the parallel rest, 
 which must be wholly made of iron. It consists of two parallel 
 cheeks, like a lathe, but the standards B B, are very short, as the 
 whole height of the machine to the chisel F, must not be more 
 than that of the axis of the mandrel from the bed of the lathe in 
 
70 
 
 MECHANICAL EXERCISES. 
 
 The parallel rest. 
 
 which it is intended to be used. The standards, B B, are firmly 
 screwed to a strong plate CC, or, what is still better, cast in the 
 same piece with that plate. The cheeks are united to the stan- 
 dards by four screws, in the same manner as those of the foot 
 lathe already described. The lower part of the headstock D fits 
 the space between them, in the most accurate manner, so that it 
 cannot be thrown askew, but yet is not so tight as to prevent its 
 free horizontal motion. If at any time it is found too slack, it 
 may be tightened by a screw a, at the bottom of it. 
 
 The lower part of the interior side of each cheek is chamfered^ 
 and the plate through which the screw a passes, to act upon the 
 headstock, slides between the chamfered portion of the cheeks. 
 This arrangement is shown in the section, fig. 32 ; a is the head- 
 stock ; b b the cheeks ; c the plate through which the screw passes, 
 and in which there is no screw thread required ; d the tightening 
 screw. On the top of the headstock D, fig. 31, there is a rec- 
 tangular groove for the chisel F, which is fixed in any part of it 
 by a plate or cap on the top of the headstock. This plate or cap 
 is fastened down by screws, passing perpendicularly through itf 
 two screws, in this situation, are sufficient for a small machine, 
 but four will be required for a large one. 
 
 To effect the horizontal motion of the headstock D, a screw 
 E, the nut of which is in the standard B, is connected with it by 
 means of a collar, and by turning the winch b of this screw, the 
 motion required is produced. The screw must be long enough to 
 push the headstock to the further end of the space between the 
 cheeks, and it is adviseable to make its thread a fine one, particu- 
 larly when iron is intended to be turned with the machine, in order 
 that the motion communicated to the headstock in 2 Ly with greater 
 facility be made very slow and uniform 
 
 In adjusting the machine to the lathe, the strong iron plate 
 HI, %. 33, is made use of. There are two wide grooves in it, 
 fi g, by which, with the help of two bolts, it is fastened upon the 
 lathe, in the manner of a rest. The heads of the bolts must not 
 project above the surface of the plate ; therefore the grooves must 
 be considerably wider at the top than the bottom, and the heads 
 may then be countersunk. On the plate H I are also three im- 
 moveable cylindrical pins, h i A:, all of them screwed at the top 
 and fitted with nuts. In using the machine, the plate C C, fig. 
 31, is placed on the plate HI, fig. 33, so that the pins of the 
 latter enter the apt itures Imn of the former. The middle pin 
 exactly fills the middle aperture, and upon it as a centre, from the 
 width of the other apertures, the machine has,, without moving 
 the lowermost plate, fig. 33, a short range back and forward, until 
 the nuts in it are screwed down upon the upper plate CC, fig. 31. 
 
MECHANICAL EXERCISES. 
 
 71 
 
 The parallel rest. 
 
 Now suppose a bar of iron, intended to be formed into a cylinder, 
 be put into the lathe, let the machine be fixed so that its cheeks 
 may be, as nearly as can be ascertained, parallel to the axis of the 
 intended cylinder, at the left hand end of which place the tool so 
 that it shall take off a slender shaving. Set the lathe to work, and 
 slowly turn the winch b, till the tool has completely traversed the 
 bar. Repeat the operation, if necessary, till the irregularities in 
 the figure of the bar are removed, and the tool has touched every 
 part of the surface; then, with a pair of callipers or a guage, exa- 
 mine whether the bar, at both ends, is of equal diameter ; if any 
 inequality appear, the rest has not been set parallel to its axis, 
 and it, consequently, is not a cylinder. The matter is rectified 
 by slackening the nuts of the pins h i k, and pushing in that side 
 of the rest, which is opposite the thick end of the bar, just half 
 the extent of the error. The nuts being then screwed down, and 
 the tool made to traverse the surface again, the cylinder will be 
 completed. 
 
 As it has been supposed that the bar was, in the first instance, 
 by some oversight or other, turned rather conical, the method of 
 making a regular cone with the parallel rest, when occasion re- 
 quires, needs no explanation. When a flat surface is to be turned, 
 it must be well secured to a chuck, the machine fixed across the 
 bed of the lathe, and the cutting edge of the chisel F precisely on 
 a level with the axis of the mandrel, or some part of the centre 
 will remain unfinished. The tool may be made to cut with so 
 much exactness, that if the face of a rectangular block of cast 
 iron were attempted to be turned flat with it, the edges will not 
 be jagged, when the circle of revolution, extending beyond the 
 shorter diameter of the piece, is not complete. 
 
 W hen the chisel F, in the parallel rest, requires to be moved 
 a little further in, some chuse to alter it by percussion, and keep 
 it only so tight that the blow of a moderately sized hammer will 
 drive it in ; others think it better to regulate it by a screw, and 
 provide a frame for the back of it, similar to that at the back of 
 the puppet D, fig. 13, pi. I. in which the screw a acts upon the 
 end of the mandrel. 
 
 The section of the cheeks of the parallel rest, to abridge the 
 labour of filing, may be made to resemble that proposed for the 
 cheeks of the foot lathe, fig. 2, pi. I. 
 
72 
 
 MECHANICAL EXERCISES. 
 
 Use of tlie screw-tools. — Traversing mandrel. 
 
 Of cutting Screzcs in the Lathe 
 
 The art of cutting screws in the lathe, constitutes one of 
 the most curious and useful branches in the art of Turning. 
 Accordingly, it generally proves one of the most interesting 
 exercises to the young practitioner, who is further stimulated by 
 the celebrity of those who can cut every description of screw 
 with facility, — an attainment commonly considered, among 
 turners, one of the most decisive proofs of skill that can be ex- 
 hibited. 
 
 In proportion as the art of cutting screws has been cultivated, 
 the methods by which the object might be accomplished, have 
 been diversified. We shall notice some of those contrivances 
 which are least expensive, most easily reducible to practice, and 
 most suitable for general use. 
 
 If the screw-tool, fig. 25, be opposed to a cylinder revolv- 
 ing in a lathe, and at the same time be moved along the rest, 
 with a regular horizontal motion, it will cut a screw on that 
 cylinder, the threads of which will fill the angular spaces 
 between the teeth of the tool. Fig. 25, is an outmde screw^- 
 tdol ; if the cylinder had been hollow, and intended to be 
 screwed internally, the inside screw-tool, fig 26, must have 
 been employed, which, when pressed against the side of the 
 cavity, while drawn out horizontally as the cylinder revolved, 
 would have produced the desired effect. There is some diffi- 
 culty in acquiring the art of cutting screws in this manner, though 
 the process is in very general use among experienced turners. To 
 obviate every disadvantage which attends it, and ensure per- 
 fect precision in the operation, was the object of the invention 
 of the traversing mandrel. Of this ingenious contrivance, we 
 shall next, therefore, endeavour to give the reader a description. 
 At the end of the mandrel E, at e, fig. 1, pi. I. there is a screw 
 about two inches long, the thread of which is like that intended 
 to be made. Upon this screw, called the guide, is fitted a 
 piece of wood, the motion of which is entirely prevented by 
 any mode of fastening which may be found convenient. The 
 piece of steel on the headstock C, which falls into the groove 
 of the mandrel, and hinders its horizontal movement, being 
 then withdrawn, and the great wheel turned, the mandrel 
 assumes at once a rotary and rectilinear motion, which is con- 
 tinued till it has gone so far, that the screw' e can no longer 
 turn in the piece of wood. If, as soon as this circumstance 
 occurs, or a little sooner, the great wheel be turned the contrary 
 way, the rotary and rectilinear motion of the mandrel immedi- 
 ately takes place again, but in a reversed direction. Thy 
 
MECHANICAL EXERCISES.' 
 
 73 
 
 Traversing mandrel. — Traversing chuck. 
 
 compound motion of the mandrel, is precisely what is wanted 
 to facilitate the use of the screw-tool, which, while it is going 
 on, only requires to be held steadily upon the rest, against the 
 revolving body, and the sciew will be produced. The teeth of 
 the screw-tool must correspond with the screw upon the mandrel, 
 as if it had been made by holding it against that screw revolving 
 in a lathe. 
 
 It is customar}" to cut three or four screws, of different 
 threads, one behind another, upon a traversing mandrel, as a 
 single one would only be of limited use. But even as three or 
 four screws are often insufficient to meet the wants of the artist, 
 and the length of so many together is awkward and inconvenient, 
 it is better to make a concave screw in the end of the mandrel, 
 to which any variety of convex or guide screws may then be 
 alternately attached. — The revolution of the guide screw, without 
 the mandrel, may be prevented by a screw z, near the end of the 
 latter. 
 
 In cutting screws, the proper motion cannot be communicated 
 from a fly wheel to the mandrel by means of the foot acting upon 
 a treadle. If a fly wheel be used, it must be turned backwards 
 and forwards by a winch, through a space proportionate to w^hat 
 the guide screw will allow, so that two persons will be required 
 for the operation. But to cut screws in a foot lathe, the fly of 
 which is unprovided with a winch, and to render one person ade- 
 quate to the performance, a cord descending from a spring, as in 
 the pole lathe, is coiled round the pulley of the mandrel, and 
 attached to the treadle, the range of which may be suited to the 
 occasion. 
 
 With respect to the mode of fastening the wood in which the 
 guide screw turns, a word may be expected. Let a stock or 
 horizontal piece, w, be screwed to or cast along with the head- 
 stock C ; let the end of it be tapped to receive the screw x, which 
 must be taken out previously to fixing the wood upon the guide 
 screw e. When the w ood is in its proper place upon the guide, 
 it must hang down over the end of the stock zc\ and there must 
 be a hole in it just large enough to admit the screw Xy by which 
 it can then be made perfectly secure. 
 
 The use of the traversing mandrel will probably in a little 
 time give way to that of the traversing chuck, which was 
 invented by Robert Healy, A. B of Lublin, and a description 
 of it, communicated by him, inserted in the Philosophical 
 Magazine. On the common mandrel A, fig. 1, pi. IV. is 
 screwed the chuck B, to which may be screwed the chucks of 
 the lathe, aS R. On the outside of this chuck B, is turned a 
 screw, which is fitted to an inside screw worked in the ’’oular 
 4.— VoL.I. L 
 
74 
 
 MECHANICAL EXERCISES. 
 
 Traversing chuck. 
 
 block C, from which block extends an arm D, as long as may 
 be thought fit for the purpose of permitting another arm to 
 slide up and down it ; a piece of iron should be screwed to the 
 circular block C, of such a length as to be capable of moving 
 in a groove that may be cut in the collar, or adapted to it. This 
 piece of iron should be regular in its shape, and well fitted to the 
 groove ; it is intended to prevent the block C, from being turned 
 round, and to allow it only a steady rectilinear motion. The rest, 
 G F O, must not stand as usual parallel to the work, in cutting an 
 outside screw' ; but at right angles, as when an inside screw is 
 to be cut, in order that the furthef arm of the rest F, may be 
 joined to the end of the second or intermediate arm E. It is 
 necessary that this second or intermediate arm E, be capable of 
 fastening firmly the hi st arm D, to any part of the rest, G F ; it 
 must also have a joint at each end to admit, in a horizontal plane, 
 its free play. Thus, as the lathe turns to us or from us, the arms 
 must traverse forwards or backwards ; which gives a similar 
 motion to the tool H, that is held steadily or hxed wdth a screw 
 on the further arm F, of the rest ; and thus a screw is cut with a 
 tool of a single point. Is is unnecessary to mention, that no 
 joggling should arise from the motion of the arms, as that would 
 cause a failure in cutting a perfect screw. If the centre of the 
 rest should be drawn nearer to us, and by that means bring the 
 tool closer to the intermediate arm E, then a screw of a much 
 larger size will be cut ; for as the rest, turning within its socket, 
 (the thumb-screw for fixing it in the pillar, being in this operation 
 always withdrawn,) moves on a centre, the further the tool is 
 moved from this centre, the greater wdll be the radius of the circle 
 described, and consequently the coarser will be the screw ; and, 
 vice versa, the nearer the tool is brought to the centre, the smaller 
 will be the radius of the circle, and thus the screw will be finer. 
 Should the intermediate arm E, be connected with the nearer 
 arm of the rest G, and the tool held on the further one F, then a 
 left-handed screw will be cut, of a thread the distance between 
 the turns of which w ill vary according to the distance of the point 
 at which the tool is held betw een the centre and extreme end ; 
 for, as the lathe turns to us, the arms receive a forw'ard motion, 
 except the further arm F, of the rest, which receives a backward 
 motion ; but when the lathe turns from us, then the further arm 
 receives a forward motion ; and as the tool meets the w'ood, so it 
 cuts a left-handed screw'. 
 
 It may be apprehended that a piece of wood so far removed 
 from the collar K, might spring its motion; but this may be 
 obviated by not making use of the traversing chuck B, till the 
 screw is to be turned; for as the cutting of it is light work. 
 
MECHANICAL EXERCISES. 
 
 75 
 
 Traversing chuck. 
 
 there will be little resistance, and, of course, but little spring; 
 or the traversing screw, B, may be turned on the mandrel A. 
 Another disadvantage would seem to arise from the impossibility 
 of cutting screws when the puppet head is made use of, to pre- 
 vent the springing of a long piece of wood. But this may be ob- 
 viated by lengthening the intermediate arm E, to the part where 
 we intend cutting the screw, and thus we have the same screw as 
 that of the traversing one : if a finer or coarser screw should be 
 required, then, by having an arm of the rest to slide in and out, 
 and the intermediate arm to be connected with the centre of the 
 rest, we have just the same power of turning screws as in the 
 former case. A socket S, is represented, the lower part of which 
 slides on the rest, and may be fastened firmly to it by a screw : 
 the upper part, that turns on a pivot, admits the intermediate arm 
 to slide through it, which arm is held stationary in it by a screw. 
 
 If the rest were to make a right angle with the piece of wood 
 on which the screw was to be turned, at the commencement of 
 the process, and to become parallel “to it when the screw was 
 finished, an approximation would take place from a larger thread 
 to a smaller, or me versd ; but it is impossible for the rest to 
 become parallel to the work, from the connection of the arms. 
 Now let the traversing arm D, lie in the centre of the screw B, 
 on which it plays, and let the rest make a right angle with the 
 wood on which we intend to cut the screw. The rest may tra- 
 verse thirty degrees on either side of the right angle ; which will 
 not cause any sensible approximation in the thread, and will admit 
 a motion sufficiently extensive for turning the common length of 
 Screws. But as the method answers for a short screw of a few 
 turns, that is sufficient for every purpose ; for, in order to make a 
 long screw, there may be three different ways of accomplishing our 
 object : 
 
 1st, At the commencement, the rest stands at right angles 
 with the w'ood on which the screw is to be cut ; by its describing 
 an arch of a few degrees, a short screw is cut ; then by bringing 
 back the rest to its original angle, the right one, and sliding for- 
 ward the single pointed tool to the last thread of the screw that 
 was just cut, w^e proceed to any length by repeating the same 
 process. 
 
 Sndly, When one or two threads of a screw are cut, by mak-- 
 ing use of a common screw tool, the most unskilful hand will be 
 able to continue the screw to any length. 
 
 3dly, Should a side tool with many teeth, instead of the single 
 pointed one, be made use of, a screw of any length may be cut^ 
 the rest describing its usual arch. 
 
76 
 
 MECHANICAL EXERCISES. 
 
 Apparatus tor elliptic turniii(>. 
 
 Elliptic Turning, 
 
 PI. II, exhibits perspective views and elevations of the appa- 
 ratus for elliptic, or (as it is vulgarly called) oval turniug. An 
 oval is smaller at one end than the other, like an egg, and there- 
 fore differs essentially from an ellipse. In all the different figures, 
 1, % 5j 6, 7, and 8, the same letters refer either to the same things, 
 or to corresponding parts of the machine. 
 
 Fig. l,ds~a front view of the machine. IK is the principal 
 iron plate, to^whicli all the subordinate parts of it, except the ring, 
 (afterwards described,) are affixed. A short screw W, is rivetted 
 or soldered to tiffs plate, upon which the material to be turned 
 must be fastened, either with or without the intervention of a 
 chuck, as the nature or form of it dictates. This screw, to raise 
 the back or lower part of it above the level of the plate I K, has a 
 shoulder, which -affords a useful bearing to whatever is screw^ed 
 upon it, and prevents the inconvenience which would otherwise 
 arise from the projecting heads of the screws x x x x. 
 
 Fig. exhibits the various parts of the machine on the back 
 of the plate I K, at each of the four corners of which there is a 
 short square pillar, on the summits of which are placed the letters 
 of refei ence, d d dd. Within these pillars, are placed too narrow 
 side ribs or pieces of steel, ffj which reach the whole length of 
 the plate 1 K. Each of these pieces, on the side opposite the 
 other, is bevelled, so that when placed on the plate it forms an 
 angular groove in the direction of its length. The two angular 
 grooves formed by the pieces ff^ are filled by the chamfered sides 
 of the slider EF, which is capable of a free longitudinal motion 
 between them. Wlieii the slider has been put in its place, tw'O 
 end ribs or pieces of steel m m, are placed w iilffn the pillars, 
 ddddj parallel to each other ; they bear upon the side pieces ff^ 
 to which, and to the plate I K, they are firmly held by the screws 
 XXX X. The nut L, is cast in the same piece with the slider EF; 
 in using the apparatus, it is screw^ed upon the nose of the mandrel, 
 and its size must accordingly be proportioned to the mandrel for 
 which it is intended. When the end pieces, m m, are fixed, the 
 slider cannot be throwm out of its situation in the grooves, as they 
 limit its only motion, the longitudinal one, to the space between 
 them, because the nut L acts as a stop, at either end to which 
 the slider may be impelled. This effect, however, though a 
 necessary consequence of the construction, is not esssential to 
 the excellence of the machine ; the principal use of the end 
 pieces, m m, is of a different nature. The space between them 
 is exactly equal to the; diameter. of the ring. OP, fig. 3, upon 
 
MECHA'I^ICAL EXERCISES. 
 
 77 
 
 Apparatus for elliptic turning. 
 
 the outside of which they revolve, when the nut L is screwed 
 upon the mandrel. Two arms, RS, are connected with this 
 ring, and in each of them there is a groove, nearly their whole 
 length. These grooves are exactly straight and opposite each 
 other, and a line carried from one to the other, along the mid- 
 dle of them, \\ ould intersect the centre of the ring. The pro- 
 jection of the ring above the arms, is cleaidy shown by the eleva- 
 tion, fig. 4. 
 
 The elliptic machine is connected with the lathe, and its pecu- 
 liar motion obtained in the following manner : let E F G H, fig. 
 9, represent the upper part of a headstock, through which two 
 holes have been drilled, at a little distance from the collar, the 
 centres of which holes are precisely in a line with the centre of 
 the mandrel M ; let the ring be fastened to the headstock by 
 means of two screws 1 1, with button heads, the shanks of which 
 pass through its grooves, and through the two holes made in the 
 headstock, at the back of which they can be drawn tight by nuts. 
 When the ring is in this situation, it \vill be perceived, that it can 
 only be moved from side to side, and its centre, from what has 
 been said of the position of its grooves and that of the holes 
 through which the screws that fasten it pass, must always be in 
 the same horizontal line with that of the mandrel. Tighten the 
 nuts of the screws tt. No\v let the apparatus A B C D, fig. 2, 
 be united to the mandrel, by screwing upon it the nut L; the 
 inner surface of the end ribs or pieces m m, will fall at the same 
 time upon the outside of the ring. The plate I K, if the ring have 
 been set so that its centre exactly coincides with that of the man- 
 drel, will, when motion is communicated to it, revolve in a circle ; 
 but if the centre of the ring be in the smallest degree on one side 
 of that of the mandrel, it will revolve in an ellipse, the difference 
 between the conjugate and the transverse, or Ipng and short, dia- 
 meters of w hich, will be double ,the distance between the centre 
 of the ring and that of the mandrel. When, therefore, the work 
 is fastened to the screw W, as in common turning it is to the nose 
 of the mandrel, it becomes as easy to turn an ellipse as in other 
 cases it is to turn a cylinder. . 
 
 The slider EF ought to move wdth great steadiness, and at 
 the same tiine w ith freedom ; — requisites which cannot be com- 
 bined without considerable accuracy of workmanship. To obtain 
 an easy mode of making the w^earing parts of the machine fit, and 
 also to lessen the friction in some degree, several little arrange- 
 ments are made, to some of which it may not be, improper to ad- 
 vert. The slider is made of bell-metal, or a composition similar 
 to that already recommended for the collar of a lathe, and only a 
 Jiarrow’ strip on each side of it touches the back of the plate I K j 
 
78 
 
 MECHANICAL EXERCISES. 
 
 Apparatns for elliptic tnrnine. 
 
 these narrow surfaces of friction are sHo'smi by a lighter shade of 
 the engraving, on the right and left of E, fig. 1 ; the rest of the 
 surface is cast about the twentieth part of an inch lower than these 
 strips, and it therefore requires but little labour to make the slider 
 true. The other or upper side of the slider E F, fig. II, is not so 
 thick as to touch the end ribs, m rrij when sliding under them. 
 
 In each of the four pillars, dddd, are tvvo screws. Four of 
 these screws, w n ri n, press upon the end ribs m m, which, by 
 screwing them in, can at any time be brought nearer to each other, 
 for the purpose of correctly fitting the ring. The side ribs, in 
 like manner, to embrace the slider tightly, may be brought nearer 
 to each other, by the four other screws, only two of which, r r, 
 can be seen in fig. 9.. The four screws, 1 1 1 Xj the square heads 
 of which are seen in fig. 1, and their ends in fig. 2, are screwed 
 only into the end ribs m m ; they are rather smaller than the holes 
 in the plate I K, and those in the side ribs, f /*, through which they 
 pass, otherwise the screws in the pillars would not enable us to 
 drive the ribs fiuther in. 
 
 Fig. 5, and 6, are two elevations of the machine, which ex- 
 hibit the form and relation of some parts of it to each other, 
 more clearly than the perspective views. Pig. o, is the side, and 
 fig. 6, the end of the machine. The same letters of reference 
 being, as previously observed, placed upon the same things, the 
 position of which only is varied in the different figures, much 
 further description would be superfluous. It may be observed, 
 however, that fig. 6, shows distinctly the bevel given to the sides 
 of the slider E F, as well as that given in a contrary direction, 
 to the side ribs ff] in order to form the grooves which receive 
 the slider. 
 
 Fig. 7, represents one of the side ribs, and fig. 8, one of the 
 end ribs, separately. 
 
 The form given to the ring, fig. 3, though eligible for a 
 small machine, would be unsuitable for a large one. The limit of 
 its proprierty is determined by the breadth of the puppet for which 
 it is intended; h cannot be wrong, when the puppet is broad 
 enough to admit the holes for the screws 1 1, fig. 9, to be placed 
 so as to allow the ring its full lateral range, in which case it can 
 be brought close up to the mandrel. But when the shape here 
 delineate is inadmissible, the grooves or openings for the screws, 
 may be formed within the ring, by two stout ribs on each side of 
 its centre, and it may then bO fa^ened to a puppet of the custo- 
 mary dimensions. 
 
MECHANICAL EXERCISES. 
 
 79 
 
 Earl Stanhope’s turning rest. — Smart's rest. 
 
 Miscellaneous Remarks on Turning. 
 
 The parallel rest, already described, is similar in principle to 
 a machine invented by Earl Stanhope, for turning flat surfaces. 
 This patriotic Nobleman found, that tlie method of stereotyping 
 invented by him, as well as that construction of the printing-press 
 which is distinguished by his name, could not be carried to the 
 perfection he had in view, without devising some method of mak- 
 ing large surfaces of cast iron accurately flat. He has always had 
 in view the important object of rendering the successful appli- 
 cation of his inventions as independent as possible of manual dex- 
 terity ; and, accordingly, the rest or tool which he has invented 
 to perfect his stereotype labours, is, if we judge by its effects, and 
 the manner in which it is w rought, commensurate at once to his 
 genius and his wants. Massive blocks of iron, presenting a sur- 
 face of eight or nine square feet, are turned by it with wonderful 
 facility, and with a degree of exactness which w ill probably never 
 be exceeded. It is, like the paiallel rest, peculiarly adapted to 
 the turning of metals, and doubtless exceeds, in the excellence 
 of its performance, every thing of the kind before attempted. — 
 Of the contrivances which have been adopted to facilitate, upon 
 similar principles, the expeditious and correct turning of wood, 
 that by Smart, of Ordnance Wharf, Westminster, deserves par- 
 ticular notice and approbation. The apparatus which this 
 ingenious mechanic has invented, is used in his manufactory, 
 and when applied to a common lathe, enables one man, with 
 the assistance of two labourers at the great wheel, to turn six 
 hundred poles, each of them a very accurate cylinder, and five 
 and a half feet long, in the course of twelve hours. His man- 
 drel revolves twelve hundred times in a minute, in which short 
 space of time one pole is finished. The means by which he 
 attains his object, have the merit of not only being efficacious, 
 but of possessing great simplicity. The gouge for roughing 
 out the work is fastened in a block, or cutter-frame, which is 
 nothing more than a piece of wood, containing a cylindrical 
 hole, large enough to be shoved over, without touching, the 
 work to be turned. The gouge passes through the block into 
 this cavity, where its edge projects, just as the blade of a 
 joiner’s plane projects from the bottom of the block in which it 
 is fixed. The chisel, which succeeds the use of the gouge, is 
 fastened in another frame of a similar description. The gouge 
 and chisel are held in their respective places by screws. The 
 remaining part of the apparatus consists of two strong wooden 
 cheeks ; and as these must alw ays be as long as the w ork, it is 
 
80 
 
 MECHANICAL EXERCISES. 
 
 Smart’s turning rest. 
 
 best to make them, at once, the full length of the bed of the 
 lathe, measured from the middle puppet. They may form a , 
 separate frame, or they may be fastened one on each side of the 
 middle and right hand puppets, parallel to one another, and 
 their upper surfaces in the same horizontal plane. Their posi- 
 tion will then be similar to that of the cheeks of the lathe itself. 
 To give them additional steadiness, if found necessary, they 
 may be supported, in one or two places, by feet resting upon the 
 lathe, or by short puppets of sufficient breadth to reach under 
 them. On the bottom of the gouge and chisel frames, there 
 are two grooves, the same size and distance from each other as 
 these additional cheeks, which they are intended to admit into 
 them. The cheeks must be of such a height, that the cutter- 
 frames can be slidden along upon them, with the axis of the 
 holes, into w'hich the gouge and chisel project, coincident with 
 the axis of the mandrel, and consequently coincident with that 
 of the work. The projection of the mandrel, and that of the 
 screw in the right hand puppet, is always rather more than the 
 breadth of both the cutter-frames together, in order that the latter, 
 when in use, may be made entirely to clear the ends of the pole. 
 At the commencement of the process, after the cheeks upon 
 which the cutter-frames slide have been fixed, let the cutter- 
 frames themselves, (that containing the gouge being outermost,) 
 be placed against the right hand puppet, the screw' of which, if 
 far enough out, w ill then extend tlirough the centre, and a little 
 beyond them. The pole to be turned, which we will suppose 
 already prepared, by hewing it octagonally, or somewhat rounding 
 it, is fixed to the lathe in the customary manner, and the gouge 
 frame, the men having begun to turn the great wheel, is pushed 
 along its whole length ; and the tool being previously adjusted 
 so as to take off a shaving of sufficient thickness, as soon as it 
 has cleared the end of the pole, it is left over the mandrel. The 
 chisel frame is next pushed along in like manner, and thus it is 
 that one minute suffices to complete a very smooth and accurate 
 cylinder. It is obvious, that the best position for the gouge and 
 chisel in their frames, will be that which gives each of them the 
 same inclination to the surface of the work, that is found most 
 advantageous in turning by hand. This mode of turning may be 
 classed, perhaps, among those inventions, which every one requir- 
 ing their aid is apt to wonder he has not thought of, or to believe 
 that he could have thought of ; and the very simplicity of which, 
 while, instead of disparaging, it enhances their value, and the debt 
 which the public owe to their authors, is one of the main obsta- 
 cles to the discovery of them. 
 
 A numberless variety of figures may be produced in the 
 
MECHANICAL EXERCISES. 
 
 81 
 
 MiscellaDeons rt-marks on tnrninff. 
 
 latbe, by regulating the action of the tool, in its advance to, or 
 recess from, the face of the piece exposed to its action. Medal- 
 lions, and other similar pieces, have been executed by enthusiasts 
 in the art, who have spared neither expence in procuring the iks 
 cessar>’ apparatus, nor patient perseverance in the use of it. In 
 the British Museum, there is a profile in bass-relief, of Sir Isaac 
 Newton, wholly made by turning ; the resemblance is considered 
 very correct, and the place in which it is deposited may be con- 
 sidered a sufficient proof of the difficulty with which it has been 
 executed. 
 
 Watch-cases, snuff-boxes, and various sorts of trinkets, are 
 sometimes formed by w hat is called rose-w ork. Plates of iron or 
 brass, indented or waved on the edge, in any curve or form which 
 may be desired, are screwed upon the mancheL These plates, 
 which are called roves, ser\e as guides to regulate the action of 
 the tool, in producing a correspondent fonn on the work. 
 
 The copper cylinders used at Manchester in printing calicoes 
 by machinery, afford specimens of turning, or rather of engraving 
 in the lathe, of tlie most curious and interesting kind. No pro- 
 duction of the art can be more beautiful than the workmanship 
 of many of them, nor more admirable than the effect produced 
 by their use. A whole web or piece of calico is printed by them 
 in three minutes. They are, in the first place, turned accu- 
 rately cylindrical, and polished with as much care as the copper- 
 plates for common engraving ; the pattern is then cut upon 
 them, and it is tliis part of the process, in preparing them for 
 use, which most remarkably exercises tlie genius of the artist. 
 Two methods of executing it present themselves — the graver 
 and the lathe. As the latter, for every pattern to which it can 
 be applied, is so much more expeditious and acciuate than the 
 former, a desire to make use of it in preference naturally 
 follows ; and accordingly it is made use of in cutting a vast 
 variety’ of beautiful patterns, to the production of which few 
 would consider any tool but the graver, directed by the most 
 complete manual dexterity, in any degree adequate. The 
 identical methods pursued by different artists, in this branch of 
 turning, are but little known, but a geueral idea of the nature 
 and possibility of the thing is not of difficult comprehension. 
 Let the pattern intended for the copper be cut upon the circum- 
 ference of a small steel wheel, which must be made to revolve 
 upon an axis. L<et this wheel be held against the copper 
 cyhnder, which, when revolving in a lathe, will carry it round, 
 and receive from it an impression of the pattern on its circum- 
 ference. When one ring of the pattern impressed by the 
 wheel has been obtained, it is plain, that by repeating the 
 4. — VoL. I. 
 
I 
 
 82 MECHANICAL EXERCISES. 
 
 Miscellaneons remarks on turning. 
 
 operation, others may be made at any required distance from each 
 other. As the wheel operates almost like a punch, the marks 
 made by it are a little raised at the edge ; but these inequalities 
 are soon levelled by a hie, or the common process of turning and 
 polishing. It may also be observed, that the cavities made by 
 the wheel must be left rough or notched at the bottom, in order 
 that they may better retain the ink till they meet the calico. To 
 cut the wheel so as to produce this effect, facilitates its entrance 
 into the cylinder. 
 
 Practical directions for turning might easily be multiplied, 
 but the necessity for much further minuteness of detail will be 
 removed by a little observation and experience ; yet we are 
 unwilling to refuse a place to a few remarks which may be ac- 
 ceptable to the novice. In turning a hollow sphere, the convex 
 surface is first completed, and then perforated with a centre-bit 
 to admit of the tool, fig. 20, pi. Ill, by which the interior may 
 be cut away. But as it may not be desirable to make a very 
 large hole for the admission of the tool, it is customary to per- 
 forate the sphere in six places, each hole being made in a direct 
 line to the centre, to which they must all approach within half 
 fheir diameter. They are also bored at equal distances from each 
 other, and each hole is at right angles with all the rest except one, 
 to which it is exactly opposite. Hence the points at which these 
 holes ought to be made, may be obtained by drawing circles upon 
 the sphere which divide it into quarters ; the points of intersection 
 of these circles, are the places sought. Place the sphere in a 
 chuck, with the axis or middle of any two of the holes in the same 
 line with that of the mandrel. Turn out a portion of the interior 
 from the hole in front ; then in succession bring every other hole 
 to the front, and proceed in like manner. The partitions divid- 
 ing the several excavations will at length be cut through, or will 
 be made so thin that they may be cut tVom the interior surface of 
 the sphere by a bent saw. 
 
 To turn one sphere within another, cylindrical holes as in the 
 last case are required, but the thickness left between each pair of 
 opposite ones, must at least be equal to the diameter of the inner 
 sphere intended to be left. The tool, fig. 21, or that fig. 22, is 
 used to make the excavation or space between the concentric sur- 
 faces, by entering in rotation, as before, the six holes. By using 
 the tool represented by tig. 18, or that of fig. IQ, a cube might 
 have been turned instead of tlie inner sphere. 
 
 To turn a series of spheres within each other, the depth of 
 the cylindrical holes must be such as to leave the thickness 
 between each pair no more, or but very little 'more, than the 
 diameter of the smallest sphere intended to be left. With form- 
 
MECHANICAL EXERCISES.' 
 
 83 
 
 Miscellaneous remarks on turning. 
 
 ing this innermost sphere, the operation may be most properly 
 begun, and afterwards continued in regular progression to the 
 larger, till at last the one next the exterior sphere is completed. 
 If it be thought desirable, the spheres may have their cylindrical 
 holes proportioned to their respective sizes, by boring at first a 
 little way with a large centre-bit, and afterwards using a smaller 
 and a smaller one, in approaching the centre. 
 
 When the band of a lathe is crossed, and the wheel is turned 
 with great velocity, it is apt to wear by rubbing against itself. 
 This effect may be prevented by interposing a pulley at the cross- 
 ing place. The pulley may turn upon a pin at the end of a piece 
 of wood rising aslant, and attached to the cheek of the lathe, or 
 fastened to the floor. 
 
 The number of turns M’hich the mandrel ought to make in a 
 given time, must be varied according to the nature of the material 
 in the lathe. The velocity of rotation, for wood, can scarcely be 
 too swift ; it must be rather slow for lead, pewter, brass, and bell- 
 metal; still slower for cast iron, and slowest of all for forged 
 iron and steel. The reason for these limits appears to be, that 
 a certain time, varying with the material, is requisite for the act 
 of cutting to take place, and that the tool itself, if much heated, 
 will instantly become soft, and cease to cut. In a lathe turned 
 by the foot, three turns of the pulley of the mandrel, for one of 
 the fly wheel, will be found sufficiently quick for iron ; four or 
 five turns of the pulley, for one of the fly, may be allowed to 
 brass ; and ten or twelve will not be too much for wood. As, 
 how'ever, in turning a large fly wheel very slowly, to produce the 
 required slow motion of the mandrel, would occasion the loss of 
 the power accumulated by its increasing momentum when swiftly 
 revolved, a small wheel may be placed on the same axis. This 
 small wheel may receive the band, and the size of it may be so 
 proportioned, that it may be turned with the large one as swiftly 
 as w'e please, without making the velocity of the mandrel too 
 great. 
 
 The temper of the tools employed in turning iron and steel, 
 when they are annealed, is not commonly higher than purple. 
 But when the inequalities of steel have been reduced* and it is re- 
 quired to cut it extremely clean, the use of a sharp hard tool will 
 be advantageous. Steel and cast iron at a high temper also require 
 a very hard tool, and the angle forming the edge of the tool must 
 be considerable ; if equal to seventy or eighty degrees, it will not 
 be too obtuse. Steel and iron are cut more freely, if kept con- 
 stantly wetted with water, or thinly covered with oil or tallow. 
 
 When iron and steel are to be turned in a small lathe of 
 little power, as soon as the work is fixed, it is with many a prac- 
 
84 * 
 
 BIECHAMCAL EXERCISES. 
 
 MisceilaneoiiR remarks on turning. 
 
 tice to Dick it all round with a graver, in one or two places; if the 
 nick be deeper on one side than another, a file is employed to 
 make it every-where alike ; and when this has been done, the 
 power employed to finish it may be very inconsiderable, without 
 much loss of time. 
 
 The bank, or upper part of the rest, upon which the tool is 
 held, is short, broad, unpolished, and very strong, for metallic 
 w ork, in order that the tool may be steadied upon it in the most 
 effectual manner ; but in turning wood, it is long, narrow on the 
 top, and polished, so that the tool can be swiftly glided from end 
 to end. To a foot lathe, the bank of the rest, for metal, is sel- 
 dom made more than three inches ; while for wood its length is 
 twelve or fourteen inches. In turning bed-posts and other things 
 of great length, a piece of wood, the whole length of the w'ork, 
 is made use of as a rest, to prevent the trouble of those frequent 
 removals which the common rest would require. With a like 
 view^ of saving time, the rest, for globular work, is often made of 
 a semi-circular form. 
 
 When the work is too long and slender to bear the action of 
 the tool without yielding, it is supported in its proper place, from 
 the back, by a frame, which, like the rest, can be fixed in any situ- 
 ation, and carries an arm with a semi-circular excavation in front. 
 This opening admits the w ork, to which it is brought close up, 
 and which it prevents from being bent or broken by the tool. 
 
 When the material which has been turned is wood, and it 
 has been made as smooth as possible w ith the chisel, or other 
 edge-tool w'hich its peculiar form may have required, it may be 
 polished with shark^s skin, or Dutch rushes. I’he shark’s skin, 
 being obtained from different species of that fish, is sometimes 
 of a reddish and sometimes of a dark grey hue, without any 
 material difference of quality ; it is too rough for polishing at 
 first, and therefore must be worn a little by using it to the 
 rougher states of work, before it is applied to this purpose. The 
 Dutch rush is the equiselum hyemale of Linnaeus; it grows in 
 moist places among the mountains, and is remarkable for having 
 flinty particles in the substance of its leaves, which render it so 
 useful in polishing. It has a naked, simple, and round stem, 
 about the thickness of a goose quill. The oldest plants are the 
 best. Before they are used, they should be moistened a little 
 with water, otherwise they presently break, and become almost 
 useless. The use of the rush is particularly proper for hard 
 woods, such as box, lignum-vitae, or ebony. After having well 
 smoothed up the work, it should be rubbed gently either with wax 
 or olive oil, then wiped clean, and finished by rubbing it with a 
 cloth or its own shavings. 
 
MECHANICAL EXERCISES. 
 
 85 
 
 Oak timber — beech. 
 
 Ivory, horn, silver, and brass, may be smoothed with files, 
 or with pulverized pumice-stone, laid upon leather or a cloth a 
 little moistened. When a very fine surface is required, tripoli, 
 and afterw'ards oxide of tin, may succeed the use of the pumice* 
 stone. To polish iron and steel, after they have been made as 
 smooth as possible with files, very fine flour of emery is mixed 
 with oil, and laid upon two pieces of soft wood, between the 
 surfaces of which, thus coated, the w ork is pressed and made to 
 revolve. 
 
 Of TIMBER. 
 
 From a view of the properties, uses, and principal modes of 
 working the metals in most common use, we now proceed to de- 
 tails of a similar kind respecting timber. One description of the 
 mechanical operations belonging to this subject, have, indeed, 
 already been anticipated by the sections devoted to the mixed art 
 of Turning, to which both wood and metals are alike extensively 
 subjected ; but much yet remains to be adverted to, which forms 
 a proper part of the knowledge of the turner, though more parti- 
 cularly subservient to the purposes of the carpenter, the joiner, 
 the millwright, and the cabinet-maker. We shall commence* 
 with an enumeration of the most useful sorts of wood, with a 
 few remarks on their properties, their uses, and the best methods 
 of seasoning timber in general. 
 
 Of all the different kinds of timber produced in this country, 
 oak is the best for building, and for almost every purpose of 
 rural and domestic economy, particularly for staves, laths, and 
 spokes of wheels. Even when it lies exposed to air and water, 
 it is preferable to almost all other woods ; and as it is, besides, 
 hard, tough, tolerably flexible, and not very apt to splinter, its ex- 
 cellence for ship-building is unequalled. Its quality is improved, 
 if the tree be suffered to stand three or four years after it has 
 been barked, as it thus becomes perfectly dry; the inspissated 
 sap renders it much stronger than the heart of those trees which 
 have not been stripped, and its hardness, weight, and durability, 
 are also increased. 
 
 Beech is also a wood of great utility; it is very tough and 
 white ^hen young, and of great strength ; but liable to warp 
 very much when exposed to the weather, and to be w^orm-^aten 
 when used within doors. Its greatest use is for planks, bed- 
 steads, chairs, and other household goods ; and for these 
 purposes, it seems almost as necessary to the cabinet-makers 
 and turners of the metropolis, as oak is to the ship-builder. 
 
86 
 
 MECHANICAIj exercises. 
 
 Beech timber — elm — chesnut. 
 
 The worm by which it is so liable to be destroyed^ is supposed 
 principally to feed on the sap that remains in the wood, con- 
 sequently the best method of preserving it, is to extract the food 
 on which the worm subsists. For this purpose, scantlings of 
 beech, when large, should be laid to soak in a pond for several 
 weeks, according to the size of the timber, and the season of 
 the year. In the heat of summer the desired effect is more 
 speedily produced. Generally, beams and thick planks should 
 remain about twenty w eeks in w ater ; joists and rafters about 
 twelve w'eeks ; and the thinner boards about two months. As 
 the planks or boards are in danger of warping, when exposed 
 to dry, they should be sheltered from the sun and rain; laths 
 ought to be placed at intervals between the boards, to prevent 
 their contact, and the whole pressed by a considerable weight. 
 If they are large pieces, for beams, joists, §^c. they need only be 
 left to dry gradually under sheds. Beech, by these means, will 
 be rendered as good and durable as elm ; but w hen it is used for 
 building, it is advisable to prepare that part of the timber which 
 touches the brick-work with a thick coat of pitch, to guard it 
 against the effects of moisture. If the wood have been felled in 
 the heat of summer, the sap, of which it is then full, may be more 
 readily extracted than if it be felled in winter. When this wood 
 is intended for small work, such as chairs, turnery, the handles of 
 saws, and the blocks of joiner’s planes, it is recommended to boil 
 it in water for tw'o or three hours. This mode of preparing it 
 extracts all the sap ; makes it work more smoothly, and renders 
 it more beautiful and durable. 
 
 The consumption of elm is very considerable ; it is very 
 tough, pliable, and the best kinds of it are very hard; it does 
 not readily split, and bears the driving of bolts and nails into it 
 better than any other wood. It is used for making axletrees, 
 mill-wheels, keels of boats, water-pipes, chairs, and coffins. It 
 is frequently changed by art, so as to make an excellent resem- 
 blance of mahogany. For this purpose, planks of it are 
 stained with aquafortis, and rubbed over with a tincture, of 
 which alkanet root, aloes, and spirit of w ine, are the principal 
 ingredients. 
 
 Wild chesmit timber is very durable, and is by many 
 esteemed as good as oak. It seems to have been much used 
 formerly in building; it excels oak in two respects, namely, it 
 grows faster, and the sappy parts of it are more firm, and less 
 liable to corruption. At the age of eighteen or twenty months, 
 it may be cut for hop-poles, for which it is very serviceable. It 
 is superior to elm for jambs, and several other purposes of 
 house-carpentry; but on account of its possessing a precarioiu^ 
 
MECHANICAL EXERCISES. 
 
 87 
 
 Ash timber — waluut tree — pine. 
 
 brittleness, which renders it unsafe for beams, it ought not to 
 be employed in any situation where it will have to support an 
 uncertain weight or strain. Perhaps this is the principal reason 
 why it has rather fallen into disuse for building. It is generally 
 agreed, that it is peculiarly excellent for casks, as it neither 
 shrinks, nor changes the taste or colour of the liquor. It is often 
 converted into articles of furniture, and may be made to imitate 
 mahogany, by rubbing it over with alum water, then brushing it 
 with a hot decoction of logwood, and lastly with a decoction of 
 Brazil wood. 
 
 Ash is a very useful wood, which possesses the very remark- 
 able property of being almost equally good, whether cut 
 young, or at full maturity. It is tough, hard, and more 
 elastic than most woods. It answers well in buildings, and for 
 any other use, when screened from the weather. It is much used 
 for making implements of husbandry, particularly hop-poles, also 
 for handspikes, oars, and the handles of tools, such as the axe, 
 adze, Sec. 
 
 Walnut-tree is excellent for the joiner’s use, and was in 
 great request for all the best articles of furniture till superseded 
 by mahogany ; it is still in repute for the best grained "and 
 coloured wainscot ; with the gunsmith for stocks ; w ith the 
 coach-maker, for wheels and the bodies of coaches ; w ith the 
 cabinet-maker, for inlaymgs, especially the hrm and close timber 
 about the root. To render this wood the better coloured, joiners 
 put the boards into an oven, or lay them in a warm stable; 
 and w hen they work it, polish it over with its owm oil very hot, 
 which makes it look black and sleek. The oldest wood is the 
 most valued. 
 
 The rcild pine-tree, called m this country the Scotch fir, 
 from its growing naturally in the mountains of Scotland, fur- 
 nishes the best red or yellow deal, so much employed in the 
 making of masts, floors, wainscots, tables, boxes, and for num- 
 berless other purposes. The wood is very resinous, and the 
 most durable of any of the kinds of fir yet known. The w'ood 
 of the black and white spruce firs, is very light, and decays 
 when exposed to the air for a considerable length of time ; it is 
 chiefly employed for packing cases, musical instruments, and 
 the like. 
 
 Beals, or common fir boards, may be much hardened and 
 improved, by immersing them, as soon as they are sawn, in 
 salt water for three or four days, care being taken to turn them 
 frequently during that time. They should then be dried by 
 exposing them to the sun and air; but neither this, nor any 
 other inode of preparing them yet known, will prevent their 
 
88 
 
 MECHANICAL EXERCISES. 
 
 Larch timber — poplar — birch. 
 
 *hrinking. The shrinking is greatest transversely ; in the direction 
 of the length of their fibres, when once w ell seasoned, they shrink 
 80 little, that a piece of deal is highly esteemed for the rod of a 
 pendulum. 
 
 The timber of the larch-tree is possessed of many very 
 valuable properties. It is exempt from the depredations of 
 
 worms ; it is peculiarly calculated for ships’ n)asts and the 
 building of vessels, or for strengthening the wooden frame- 
 work of bridges ; for it is capable of supporting a much greater 
 w'eight than the oak itself, and almost petrifies under water. 
 It also resists the influence of our climate, and is excellent for 
 gates, pales, shingles, and other works which are exposed to 
 all the vicissitudes of the weather. Houses constructed with 
 larch timber, have a whitish cast for the first tw'O or three years ; 
 after which the outside becomes black, while all the joints and 
 crevices are firmly closed with the resin extracted from the pores 
 of the w ood by the heat of the sun ; and which, being hardened 
 by the air, forms a kind of varnish, not inelegant in its appearance. 
 Buildings constructed with it, have been observed to remain 
 sound two hundred years. No wood affords more durable staves 
 for casks, and the flavour of the wine is at the same time pre- 
 served. The charcoal made from the larch is not only much su- 
 perior in quality, but in quantity, to that made from a like mea- 
 sure of the fir tree. 
 
 The common cedar (of Lebanon) is a species of the pine-tree. 
 The character of its wood is well known, and firmly established. 
 Its uniformity and softness recommends it to the use of the ma- 
 nufacturer of black-lead pencils ; while its neat appearance, and 
 its not being liable to the depredations of worms, occasions 
 a great demand for it from the cabinet-maker, for the drawers 
 and interior divisions of desks, &c. where great strength is not 
 required. It is admirably calculated to withstand the effects of 
 moisture. 
 
 The different species of poplar treesy supply timber much 
 used instead of fir ; it looks better, and is tolerably tough and 
 hard. Poplar wood is not very subject to the ravages of worms, 
 nor to warping or shrinking. It is advantageously employed for 
 wainscotting and floors, as well as for water-pipes, packing boxes, 
 and turnery wares. For bedsteads it is deemed unsuitable, as it 
 is considered more liable than other woods to be infested with 
 bugs. It is a very incombustible wood, and therefore admirably 
 adapted for the floors of workshops where ignited bodies are apt 
 to be throwm down. 
 
 The wood of the birch or alder treey differs not vei^ mate- 
 rially from that of the poplar, cither in quality or use. The 
 
MECHANICAL EXERCISES. 
 
 89 
 
 Maple. — Pear-tree. — Cherry-tree. — Yew. — Holly. 
 
 soles of what are called wooden shoes, and women's shoe heels, 
 are made of it. It is also much used for hoops, and by the 
 turner, for bowls and dishes of all sorts. 
 
 The whitest and finest kinds of maphy are used by cabinet- 
 makers for inlaying. On account of the lightness of this wood, 
 it is frequently used for musical instruments. It is excellent for 
 the use of the turner ; cups made of it may be turned so thin as 
 to transmit light. These characteristics belong more particularly 
 to English maple, which is also well known under the name of 
 sycamore or plane tree, and when grown on favourable situa- 
 tions, is almost as hard, w^hite, and beautiful, as holly. The 
 American maple is mostly of a dull light browm colour ; it is a 
 softer wood, and its grain has some slight resemblance of that of 
 the cherry-tree. 
 
 Pear-tree zeood is smooth, light, and compact; very suitable 
 for the turner’s use, and when stained black, makes a good imi- 
 tation of ebony, for picture frames, and similar articles. It is 
 sometimes engraven upon, but for this purpose it is much inferior 
 to box, and even to holly. 
 
 The wood of the cherry-tree is smooth, tough, yellowish, with 
 a grain like the commoner kinds of mahogany, of which, with a 
 very little assistance from staining, it makes an excellent imitation. 
 It IS therefore used for the middling qualities of furniture, by the 
 cabinet, chair, and bedstead makers. 
 
 The yew-tree was formerly cultivated for the manufacture of 
 bows, but since the decline of the demand for it on this account, 
 it has been much neglected. The wood is, however, valuable, 
 though, like the two last kinds, not very abundant; it is hard 
 and smooth, beautifully veined with red streaks, admits of a 
 fine polish, and is almost incorruptible. It is employed by the 
 turner and cabinet-maker, the millwright and the engraver. It 
 may therefore be seen in tables, chairs, cups, spoons, toys, orna- 
 mental tea-caddies, urns, &c. Converted into axletrees, cogs 
 for mill wheels, and floodgates for lish-ponds, it is found very 
 proper and durable. By the engraver it is only used for large 
 and coarse work. 
 
 Of all hard w^oods, that of the holly is the whitest, and is 
 much used for inlaying, especially under thin plates of ivory. It 
 is excellent for the use of the turner, and is highly prized by the 
 millwright for the cogs of wheels ; it also makes the best handles 
 and stocks for tools, flails, the best riding rods and carters’ whips, 
 bowls, sheaves, and pins for blocks. Next to box, it is the most 
 suitable w ood for the use of the engraver. It is tougher, but not 
 so hard as box. 
 
 4.— VoL. I. 
 
 N 
 
90 
 
 MECHANICAL ENEKCISES. 
 
 Box-wood. — Ebony. — Lignnin- vitae. 
 
 Bor is the only European wood which will sink in water. Its 
 closeness of grain, hardness and toughness, are such, as to render 
 it admirably adapted to the purposes of the engraver on wood. 
 •The engraving is always made on the end of the wood, so that 
 the fibres stand perpendicularly. When cut in this manner, the 
 graver will make a clean stroke in ever}* direction, and the piece 
 is liable to warp but little. When cut plankwise, boxwood is ex- 
 tremely apt to warp, unless very well seasoned. No kind of 
 wood turns smoother than this, and its yellow* colour, when it is 
 weU polished, is very beautiful ; it is much used for pulleys ; for 
 shuttles ; for the bottoms of joiners’ planes, especially their mould- 
 ing planes ; and, in short, is valuable for every purpose requiring 
 wood which will bear friction well. When ivory would be too 
 expensive, or cannot be obtained of the requisite dimensions, box- 
 wood is commonly the substitute for it. Hence the consumption 
 of it for combs, button-moulds, knife handles, and particularly for 
 mathematical instruments, is very considerable. The bitter qua- 
 lity of boxwood secures it from the attacks of w orms. 
 
 Ebony is an exceedingly hard and heavy kind of foreign w ood, 
 of a very smooth even grain, susceptible of a remarkably fine 
 polish, and on that account used in mosaic and inlaid ^vorks, for 
 toys, &c. It is of various colours, most usually black, brown, red, 
 and green. The black is the kind most generally known, and 
 preferred to that of other colours. The best is a jet black, free 
 from veins and rind, vei*)* massive, astringent, and of an acrid 
 pungent taste. Ebony is not in so much demand as formerly, 
 from the improvements which have been made in giving other 
 hard woods, especially the holly, a black colour. It is used for 
 parallel rulers, and other mathematical instruments not requiring 
 to be marked with figures, which the darkness of its colour w ould 
 prevent from being distinctly seen. It is brought from Mada- 
 gascar, the Mauritius, and the West Indies. The tree of the 
 West India kind is seldom more than eighteen feet high, and the 
 trunk five or sLx inches in diameter. 
 
 Lignum-xita is another foreign wood, fii*m, solid, ponderous, 
 very resinous, of a blackish yellow colour in the middle, and a hot 
 aromatic taste. It is a native of the West Indies, and the warmer 
 parts of America. It is of considerable utility in the arts : by the 
 sugar planters it is manufactured into wheels and cogs for sugar 
 mills. The sheaves or pulleys in ship blocks are mostly made of 
 this w’ood ; which is also frequently formed into bowls, mortals, 
 and various utensils. It is much used by the turner, making ex- 
 cellent castors, handles for tools, and other small ware ; but as it 
 is expensive, hard to work, and not very remarkable for its beauty, 
 it is in little demand for the cabinet- niaker. 
 
MECHANICAL EXERCISES. 
 
 91 
 
 Mahogany. 
 
 Mahogany is one of the most valuable woods imported inta 
 this country, and the tree, growing in full maturity, is one of the ^ 
 noblest productions of nature, attaining often the majestic height 
 of one hundred feet. Mahogany balks are often three or four 
 feet in diameter, and the diameter of one lately imported into Lan- 
 caster measured five feet. Mahogany varies very much in qua- 
 lity ; that grown on rocks is the hardest, heaviest, closest in the 
 grain, and most beautifully veined; and Jamaica wood is prefer- 
 able to that obtained on the coast of Cuba and the Spanish Main, 
 on account of its being mostly found on rocky eminences, while 
 the latter is cut in swampy soils near the sea-coast, and is light, 
 porous, pale coloured, and open grained. On soils neither rocky 
 nor swampy, the wood is of a medium excellence. Hence a 
 good idea of the value of a parcel of mahogany may be formed, 
 if we know correctly the nature of the soil upon which it grew. 
 Different parts, however, of the trunk of the same tree, vary some- 
 what in quality ; and in felling the timber, the most beautiful por- 
 tion of it is commonly left behind. The negro workmen raise a 
 scaffolding of four or five feet elevation from the ground, and 
 hack up the trunk, which they cut into balks. The part below, 
 extending to the root, is not only of larger diameter, but of a closer 
 texture than the other parts, most elegantly diversified with shades 
 or clouds, or dotted like ermine with spots. This part is only to 
 be come at by digging below the spur, to the depth of two or 
 three feet, and cutting it through ; an operation too laborious to 
 be often attempted. — The remark just made, with respect to the 
 superiority of the wood of the mahogany-tree, near the earth, is 
 applicable to timber in general, and ought not to escape the ob- 
 servation of those who are desirous of selecting the choicest and 
 most ornamental portions for particular purposes. The exquisite 
 beauty of the finer kinds of mahogany, the incomparable lustre of 
 which it is susceptible, exempt also from the depredations of 
 worms, hard, durable, warping and shrinking very little, it is pre- 
 eminently calculated to suit the work of the cabinet-maker. Ac- 
 cordingly, these admirable properties, added to its abundance, and 
 the largeness of its dimensions, have occasioned it to be manufac- 
 tured into every description of furniture. From its being so little 
 subject to shrink and warp, it is particularly excellent and much 
 used for the patterns of iron and brass founders, especially for the 
 patterns of wheel-work and other tilings which require the greatest 
 nicety. It is the commoner sorts of mahogany which are gene- 
 rally wrought up in this way. The commoner kinds also are often 
 stained black, and made to look to great advantage, for small 
 turnery wares, such as picture frames. 
 
 The preceding catalogue of woods might easily be enlarged. 
 
92 
 
 MECHANICAL EXERCISES. 
 
 Felling of timber. 
 
 but not, perhaps, with advantage. Several woods of home 
 growth, which seldom come into consumption, and are alike, 
 from their quality and their scarcity, of limited utility, as well as 
 many of foreign production, such as dyewoods, which are rarely 
 unj>loyed in the mechanic arts, we pass over as unnecessary in 
 this enumeration. It will be more useful to subjoin a few 
 remarks on the mode of seasoning and properties of timber in 
 general. The goodness of timber not only depends on the soil 
 and situation in which it stands, but likewise on the season in 
 which it is felled. With respect to the latter point, considerable 
 disagreement of opinion prevails ; some are for having it felled 
 as soon as its fruit is ripe, others in spring, and many in 
 autumn. But as the sap and moisture of timber is certainly the 
 cause that it perishes much sooner than it otherwise would do, 
 it evidently seems to be a good general rule, that timber should 
 be felled when there is the least sap in it, viz. from the time that 
 the leaves begin to fall, till the trees begin to bud. The only 
 plausible objection to a practice founded upon this idea, is, that 
 tlie sap in winter is thicker than at any other season, and therefore 
 may be of more difficult extraction, in the subsequent seasoning, 
 than if the tree had been cut when it w as more abundant and 
 more fiuid. In England, the work of felling timber usually 
 commences about the end of April, because the bark then rises 
 more freely ; and w here a quantity of oak timber is to be felled, 
 the statute requires it to be done at that time, for the advantage of 
 tanning. 
 
 The age at which timber is cut, is a matter of great import- 
 ance ; if cut too old or too young, it w ill not be so durable as 
 when cut at a proper age. It is said, that oak should not be 
 cut under sixty years old, nor above two hundred. It is easy 
 to offer as a general rule, that timber trees should be cut in their 
 prime, w hen almost fully grown, and before thf^y beiiin to decay ; 
 but when it is inquired, how these particulars are to be deter- 
 mined, w e are compelled to refer to judgment and experience as 
 the only guides which can be depended on. Difference of soil, 
 situation, and climate, hasten or retard the grow th of timber so 
 much, that the age at which any particular kind of tree arrives at 
 maturity, cannot be coirectly assigned. Marshall observes, that 
 poplars may stand from thirty to fifty years; ash and elm-trees, 
 from fifty to a hundred. 
 
 V\ ith respect to the best mode of seasoning timber after it 
 has been sawed, a variety of opinions are entertained ; but 
 practical men, who seldom regard the notions of the speculatist, 
 and who require a process not only effectual but convenient upon 
 a large scale, consider no method better than that of exposing 
 
MECHANICAL EXERCISES. 
 
 93 
 
 Seasoning of timber. 
 
 the timber they intend to season, for a sufficient length of time to 
 a free current of air, and every vicissitude of wind and weather. 
 Boards from one to two inches in thickness, which have under- 
 gone this exposure twelve or eigliteen months, they deem tit for 
 use. They observe, that the mere drying of timber is not the same 
 thing as seasoning it, and that unless it is frequently wet, while 
 exposed, it receives little benetit. This is only stating, in other 
 words, the necessity of extracting the sap. Some persons, how- 
 ever, advise the planks to be laid up in a dry airy place, out of 
 the wind and sun, or at least free from the extremes of either ; and 
 that they may not decay, but dry evenly, they recommend them 
 to be daubed over with cow-dung. Ihey must not be piled on 
 their ends, as in the common w ay, but one plank must be laid over 
 another, small pieces of wood being interposed, to prevent the 
 contact which w ould otherwise occasion an injurious mouldiness. 
 This mode, though not so convenient, nor probably so effectual as 
 the former, or common one, is certainly more rational than that 
 recommended by others, of burying the timber in the earth 
 
 To season timber in a short time, it may be laid in a pool, 
 or running stream, in order to extract the sap, and afterwards 
 dried in the sun or air. On a small scale, boiling water, as di- 
 rected for beech, may be employed, by which the seasoning of 
 green wood may be accomplished with the greatest expedi- 
 tion. The process will be tound useful to turners and cabinet- 
 makers. But whatever mode of seasoning be adopted, against 
 shrinking, more or less, there is no remedy. The principal disad- 
 vantage occasioned by the shrinking of timber, is from the dimi- 
 nution of its transverse dimensions. When this kind of shrinking 
 takes place in casks, for example, in any considerable degree, the 
 hoops drop off, and these vessels fall to pieces ; perhaps as great 
 a number of them are destroyed by this cause as by any other. 
 To obviate its effects, G. Smart has lately taken out a patent for 
 constructing casks on a new principle : before he puts them to- 
 gether, he presses the w ood into a smaller compass than it would 
 ever be reduced to by drying : 32,000 vessels made according to 
 this plan, have verified its utility in the most ample manner, not 
 one of them having leaked, under circumstances that rendered 
 those made in the common w ay useless. 
 
 The Venetians are supposed to be the first in modem times, 
 w ho adopted the method of seasoning timber by charring it, which 
 was done by exposing the piece to be seasoned to a strong fire, 
 in the flame of which it was continually turned by an engine, 
 till it was completely covered with a black coally crust, when 
 it was taken out and fit for use. By this means, it became so 
 hardened, as to resist the effects of earth, air, and water, for cen- 
 
MECHANICAL EXERCISES. 
 
 94 
 
 Preserving timber from decay. 
 
 turies. The beams of the theatre at Herculaneum, were con- 
 verted into charcoal by the lava which overliowed that city, and 
 after a lapse of more than seventeen hundred years, the charcoal 
 is as perfect as if it had been formed but yesterday. Casks 
 charred in the inside are used to preserve w ater uncorrupted, and 
 are particularly to be recommended for long voyages. Charring, 
 also, is the best preparation which piles, or any kind of stakes, 
 intended to be driven into the ground, can receive. 
 
 When boards or planks have been properly seasoned, without 
 charring, additional care becomes necessary to preserve them 
 against the depredations of worms, the effects of air, moisture, Scc. 
 For this purpose, Evelyn directs common sulphur to be put into a 
 glass retort, with as much nitrous acid as will cover it to the depth 
 of about two inches. The whole must be distilled to dryness, 
 and rectified two or three times. The remaining sulphur is then 
 to be exposed to the open air on a marble, or in a shallow glass 
 vessel, where it will liquefy into a kind of oil, with which the 
 timber must be rubbed over. This mixture, he asserts, will not 
 only infallibly prevent the attacks of worms, but also preser\e 
 every kind of wood from decay, either in air or water. Two or 
 three coats of linseed oil may also be used to defend timber from 
 the inliuence of air or moisture ; and some have recommended the 
 wood-work of buildings to be painted, but this ought always to 
 be deferred, till it is thoroughly dry. 
 
 If the w aiiiscotting or other timber of a building be used too 
 green, and has in consequence riven or cracked, it has been 
 strongly recommended to cover it immediately with a solution of 
 beef-suet, which will often close the crevices so effectually, that 
 the defect will be scarcely perceptible. Some carpenters close 
 the devices w ith a composition of grease and fine saw-dust. 
 
 The timber employed in building, without due precaution, 
 is extremely liable to destruction from the dry rot, which 
 appears, by some late communications to the Society of Arts, 
 &c. to be occasioned by a plant. It will destroy half-inch deal 
 wainscotting in a year. The plant is of the creeping kind, and 
 cannot rise above tw o inches ; so that w ood, in all cases, must 
 be in contact w ith the earth to support it. To preserve w ood, 
 then, from its effects, it must be charred, painted, or prevented 
 from touching the earth by bricks and mortar. It is never ob- 
 seiv'ed to commence in the middle of floors, so that it will pro- 
 bably be found sufficient to secure the ends of beams or joists. 
 The plant has no adhesive powers but in contact with wood. 
 Timber thoroughly impregnated with brine, or a solution of com- 
 mon salt, has been found by experiment to be secure from its 
 destructive effects. 
 
MECHANICAL EXERCISES. 
 
 95 
 
 General observations on timber. 
 
 The experiments which have been made to ascertain the 
 strength and quality of timber, under different circumstances, 
 have been attended with results widely different from each other ; 
 but as it would be unsuitable to the plan of this work to enter 
 upon the lengthened statements by which contradictory results 
 are supported, we shall merely collect a few remarks, which seem 
 to have obtained general assent. The wood next the bark of a 
 tree, called the white, or alburnum, is much weaker than the rest. 
 The wood of the north side of all trees which grow in Europe, 
 is the w eakest, and that of the south-east side is the strongest ; 
 this difference is most remarkable in hedge-row' trees, and such as 
 grow singly. The heart of a tree is never in its centre, but 
 always nearer to the north side, and the annual coats of wood are 
 thinner on that side. In conformity with this, it is a general 
 opinion of carpenters, that timber is stronger whose annual plates 
 are thicker. The trachea, or air-vessels, are weaker than the 
 simple ligneous fibres. These air vessels are the same in dia- 
 meter and number of rows in trees of the same species, and they 
 make the visible separation between the plates, or annual layers. 
 Therefore the thicker these plates are, the greater the proportion 
 they contain of the simple ligneous fibres, and the better the 
 timber. A contrary opinion is nevertheless prevalent, and w'ood 
 with a fine grain, or thin annual layers, is preferred. The tenacity 
 of wood is greatest when it is green, and diminishes with drying. 
 No person, perhaps, has made experiments on wood wdth so 
 much minuteness as Bufton, who observes, that he invariably found 
 the heaviest pieces to be the strongest, and he recommends an 
 attention to this circumstance as the surest guide in the choice of 
 timber. 
 
 Banks is of opinion, that beams should be strong enough to 
 bear twenty times the force they have to resist, or they will pro- 
 bably bend, and in time break. The same author also observes, 
 that one piece of wood is much stronger than another, not only 
 cut out of the same tree, but out of the same rod ; or a piece of 
 a given length, planed equally thick, and cut into several equal 
 parts, these pieces will be broken with different weights. From a 
 great number of experiments which he made on the strength of 
 wood, he found that the worst or weakest piece of dry heart of 
 oak, one inch square, and one foot long, bore six hundred and 
 sixty pounds, though much bent, and two pounds more broke it. 
 The strongest piece he tried of the same dimensions, broke wdth 
 nine hundred and seventy-four pounds. The worst piece of deal 
 he tried, bore four hundred and sixty pounds, but broke with four 
 pounds more ; the best piece bore six hundred and ninety pounds, 
 but broke with a little more. 
 
96 
 
 MF.CHASICAL EXERCISES. 
 
 General observations on timber. 
 
 The fibies of timber requiring so great a force to tear them 
 asunder in a vertical direction, and being easily broken by a 
 liansvcise strain, when compared to that of a rope carrying 
 nrarly an equal weight in all directions, opens a wide licld for 
 useful experiments. All timber trees have their annual circles 
 or growths, which vary greatly according to the soil and expo- 
 sure to the siin. The north-east side of the trees (being much 
 smaller in the grain than the other parts, which are more 
 exprrsed to the sun,) is strongest for any column that has a 
 weight to support in a vertical direction; because its hard cir- 
 cles, or tubes, are nearer each other, and the area contains a 
 greater quantity of them ; nor are they so liable to be compressed 
 by the weight, or to slide past each other, as when they are at 
 a greater distance. On the other hand, this part of the tree is 
 not fit for a transverse strain ; because the nearer the hard circles 
 are to each other, the easier tlie beam will break, there being so 
 little space between them, that one forms a fulcrum to break 
 the other upon ; but that part of a tree, tire tubes of w hich are 
 at a greater distance, or of larger grain, is more elastic, and 
 requires a greater force to break it ; because the outside fibre on 
 the convex side cannot snap till the next one is pressed upon it, 
 which forms the fulcrum to break it on. It is generally observed 
 in large timbers, such as masts, that the fracture is seldom on the 
 convex, but usually on the concave side ; which is owing to the 
 fibres on the concave side being more readily forced past each 
 other, and those on the concave side being so difticult to be torn 
 asunder, that they cannot snap, in consequence of the largeness 
 of the segment of the circle they describe when on the strain. 
 Tl’he curve described by the inner layers of the wood being so 
 large, and indeed little less than a straight line, cannot form a ful- 
 crum to break the outer ones upon ; and as the convex side, or 
 that on which the fibres are extended, ought to be always free 
 frtmi any mortise or incision on the outside,' the strength decreases 
 as it approaches the centre. 
 
 In early periods, the trunks of trees were split with wedges 
 into as many and as thin pieces as were required, in a manner 
 similar to that used by lath-cleavers at the present day. The 
 saw, though so convenient and beneficial, has not been able en- 
 tirely to banish the practice of splitting timber used in building, 
 or in making furniture and utensils. To be aware of the peculiar 
 advantages of splitting timber, may be useful to artisans in 
 wood generally. By its advantage.s, we do not so much allude 
 to its being more expeditiously performe<I than sawing, as to 
 the circumstance that split limber possesses greater strength and 
 elasticity than that which has been sawn; for the fissure follows 
 


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MECHANICAL EXERCISES. 
 
 97 
 
 Difference between metal anti wooden springs. — Measurement of round timber. 
 
 . the grain of the wood, and leaves it whole ; whereas the saw, 
 I which proceeds in the line chalked out for it, divides the fibres, 
 j| and therefore lessens its cohesion and solidity. Though split tim- 
 !j ber turns out often crooked or warped, this fault, which may sonie- 
 !l times be amended, is, on many occasions, not prejudicial. The 
 |j fibres retaining their natural length and direction, thin boards, par- 
 ! ticularly, can be bent much better. This is a great advantage in 
 i making staves, or sieve frames, and in forming various articles of 
 1 the like kind. 
 
 There is a cm ions point of difference between wood and metal, 
 j when employed as springs, wdiich deserves to be noted, as it is not 
 I very generally known, though the information may occasionally 
 prove very useful to the engineer. A metallic spring, if it has no- 
 thing to stop against, but is suffered to vibrate after performing the 
 requisite action, will, in a short space of time, if the action be fre- 
 quently repeated, either break or set. A wooden spring, when the 
 vibration cannot be avoided, is the best substitute w hich can be em- 
 ployed, as, in the property alluded to, it is the reverse of a me- 
 tallic one ; if stopped in its vibrations, it soon sets or breaks ; but 
 if permitted to vibrate, its temper or elasticity suffers not the 
 smallest diminution. The best wood for the purpose, is clean- 
 grained deal, perfeci*ly free from knots. 
 
 To measure round timber, let the mean circumference be found 
 in feet and decimals of a foot : square it, multiply this square by 
 the decimal, 0.79577, and the product by the length. For exam- 
 ple, suppose the mean circumference of a tree be 10.3 feet, and the 
 length 24 feet. Then 10.3 x 10.3 x 0.079577 X 24 = 202.615, the 
 number of cubical feet in the tree. The foundation of this rule 
 is, that when the circumference of a circle is 1, 'the area is 
 0.0795774715, and that the areas of circles are as the squares of 
 their circumferences. But the common way used by artificers, for 
 measuring round timber, differs widely from this in its result. 
 They measure the circumference or girth, and reckon one-fourth 
 of it equal to the side of a square, whose area is equal to the area 
 of the section of the tree. They therefore square this estimate of 
 one-fourth of the girth, and multiply the product by the length of 
 the tree. According to this method, the tree of the last example 
 would only exceed by a small remainder 139 cubical feet; for one- 
 fourth of 10.3, or 2.573X 2.575x24=159.135000. 
 
 In measuring hewn or square timber, the custom is to take the 
 breadth in the middle, by placing two rules against the sides of the 
 tree, and measuring the distance between them. In like manner 
 they measure the breadth the other way, and if the two measure- 
 ments are unequal, they are added together, and half their sum i# 
 taken for the true side of the square. 
 
 5.~Vol.I. O 
 
98 
 
 MECHANICAL EXERCISES 
 
 Mechanism of a saw-mill. — Smart’s saw-mill. 
 
 Of Saw-mills. 
 
 The sawing of timber is accomplished by manual labour 
 and by machinery. Sawing by manual labour is so familiar 
 to every one, as to require no particular description in this 
 place. With respect to sawing by machinery, the mills for the 
 purpose are not numerous in Great Britain, nor does the utility 
 of them appear to be so properly appreciated as in America, 
 ^Norway, and other countries. A general] description of the 
 objects to be attained by the mechanism of a saw-mill on the 
 largest scale, may be comprised in a few words ; the saw is 
 drawn up and down as long as is necessary, by a motion com- 
 municated (commonly by w ater) to the wheel ; the piece of tim- 
 ber to be cut into boards is advanced by a uniform motion to 
 receive the strokes of the saw ; for here the w ood is to meet the 
 saw, and not the saw to follow the w’ood, therefore the motion of 
 the wood and that of the saw ought immediately to depend the 
 one on the other ; and when the saw has cut through the whole 
 length of the piece, the machine should stop and remain immove- 
 able ; lest, having no obstacle to surmount, the force of the moving 
 power should turn the wheel with too great rapidity, and break 
 some part of the machine. 
 
 Circular saws, for ripping up boards or scantlings of moderate 
 thickness, are not so generally used by artists, as would be 
 found advantageous. We shall therefore particularly notice 
 the construction of a circular saw^-mill, invented by Smart, and 
 used in his manufactory.' Like his improvement in the art of 
 turning cylinders, already described, it is distinguished by its 
 simplicity and utility. A B, fig. 2, pi. IV. is a strong table, 
 made of planks firmly braced together in the form of a joiner's 
 bench. In the middle of this bench, a longitudinal opening, 
 r 0 , admits the circular saw, F, w hich is made of well-tempered 
 steel plate. G is a pulley on the same axis with the saw, and a 
 rapid motion is communicated to it by means of an endless strap 
 from a large fly wheel, turned by horse pow’er. The saw is 
 fixed on its spindle D, (fig. 3,) by a shoulder c?, against which 
 it is held by another moveable shoulder e, pressed against it by 
 a nut, ky screwed on the end of the spindle, which is tapped 
 for the purpose. The hole in the centre of the saw must fit 
 the spindle exactly, and may be either square or circular. If it 
 be circular, it must have a small notch in it, to fit a fillet on 
 the spindle, that the saw and the spindle may revolve together. 
 The ends of the spindle are turned off to cones, in the custom- 
 ary manner for working in centres. The cone or point nearest 
 the saw, works in the end of a screw, c, fig. 2, screwed into the 
 
MECHANICAL EXERCISES. 
 
 99 
 
 Curving of wood. 
 
 bench ; the other point works in a similar screw, screwed through 
 a cross beam H, morticed between two vertical beams, KK, ex- 
 tending from the floor to the ceiling. The cross beam, H, can be 
 raised or lowered in its mortises through the beams, KK, by 
 wedges, nn, above its tenons, and two others below them. A long 
 straight piece of wood, LL, called the guide, is connected with 
 the bench by joints similar to those of a parallel ruler. It can be 
 set at any distance from the saw, and fixed by screws passing 
 through circular grooves, d dj cut through the bench. The front 
 of the guide, LL, must be perpendicular to the plane of the bench"; 
 and it may then be made use of to set the plane of the saw also 
 perpendicular to the same plane. In using the machine, the work- 
 man slides the end of the piece of wood to be cut against the saw 
 as it turns round, and presses its edge against the guide, LL, at 
 the same time, so that it may be cut straight. 
 
 When the saw is blunted by use, the centre screw, c, or that 
 in the cross piece, H, must be turned back ; the spindle and 
 saw can then be removed ; and by taking off the nut A:, fig. 3, the 
 saw will be loose, and another may be put on, or it may be 
 sharpened, in the same manner as any other saw, while fixed in 
 a vice. The teeth of the saw are set, that is, bent out of the 
 plane of the saw, one tooth on one side, the next on the other, 
 and so on alternately all round ; the outsides of the teeth are not 
 filed to leave a surface perpendicular to the plane of the saw, 
 but inclined to it, and in the same direction that each tooth so 
 filed is bent in the setting. By this means the saw, when 
 cutting, first takes away the wood at the two sides of the kerf, or 
 passage which it makes, leaving an angular ridge in the middle 
 of it, the use of w'hich is to keep the saw steady in a right line, 
 that it may not have so much tendency to get out of the straight, 
 in any place where the wood is harder on one side than on the 
 other. 
 
 On giving a curved form to Wood, 
 
 Curved wood is frequently purchased by millwrights at a 
 very high price ; and such is its scarcity, that very imperfect 
 pieces are frequently made use of, not only from motives of 
 economy in the first cost, but from necessity. The inequali- 
 ties also of wood which is naturally curved, are often so consi- 
 derable as materially to impede the workmen; but when it is 
 intended to curve it artificially, it may be dressed in its straight 
 state, so as to require very little labour afterwards. The follow- 
 ing observations, therefore, on the curving of wood, from a 
 treatise on Carpentry, by J. H. Hassenfratz, will be interesting 
 
100 
 
 MECHANICAL EXERCISES. 
 
 Curving of wood. 
 
 to many, and will develope some principles with respect to the 
 management of wood, which are not very generally known. 
 
 \^’hen trees are young and tender, their stems may be bent 
 either by cords, or by poles, stakes, or frames. They are kept in 
 this situation so long that they retain the curvature intended to be 
 given them, even when disengaged from the obstacles by which 
 they are held. 
 
 Of all the methods of bending trees, that applied to young 
 and grow ing wood is the most easy and convenient ; their sup- 
 pleness and their elasticity admit of their assuming any form 
 that is desired. There are few’, to which, with proper care, the 
 most singular forms may not be given ; but, it is true, they are 
 often reduced to a state of constraint and disease prejudicial to 
 their growth. 
 
 The curving of w’cod after it has been cut, though more 
 difficult, is, however, more customary, because such pieces may 
 be selected as are best adapted to the objects for which they are 
 intended, and a suitable curvature may be given them imme- 
 diately. 
 
 The process generally employed, is founded on the property 
 possessed by caloric, of augmenting the elasticity of wood by pene- 
 tiating it, and of diminishing its elasticity when it retires. 
 
 Accordingly, to give a curvature to thin pieces of wood, such 
 as pipe staves, and the planks that cover the sides of boats, they 
 are heated in the part where the curve is required, and they are 
 gradually bent as they become hot. 
 
 But caloric, applied to a particular portion of the wood, while 
 the other is in contact with the air, heats it unequally, and only 
 partially increases its elasticity : in curving, some parts become 
 stiff, and others bend, w hich produces an inequality of curvature, 
 and sometimes cracks in the interior, and splinters on the surface 
 of the w ood. The only method of correcting this inequality, is to 
 heat the wood alike in every part. 
 
 Ovens and stoves, gradually heated, facilitate the curvature of 
 wood, by procuring an equal heat; but the risk of injuring the 
 wood in a dry heat is very considerable. The elasticity of w ood, 
 also, is in proportion not only to its temperature, but likwise its 
 humidity. At an equal temperature, the same pieces of wood 
 have different degrees of elasticity, according to the quantity of 
 svacer by w hich they are impregnated ; in the same manner as, 
 with an equal degree of humidity, they are the more elastic the 
 more they are heated. 
 
 We have an example of the two-fold intluence of humidity 
 and of caloric, in putting together two pieces of wood, as the 
 tenon and mortise, in w hich the mortise is only one-third of the 
 
MECHANICAL EXERCISES. 
 
 101 
 
 Curving of wood. 
 
 i size of the piece that is to go into it. This manner of joining, 
 ^ apparently so extraordinary, is considered such an important in- 
 > vention, that most of those by whom it is practised, keep the 
 [ method a secret. It was the process employed in producing 
 j this effect, that suggested the method now employed for curving 
 f with facility the thickest and most obstinate pieces of timber; the 
 i whole art consists in impregnating the pieces of wood with humi- 
 dity, by procuring them a uniform temperature, then to bend 
 them, and suffer them to cool, in the form they are intended to 
 assume. 
 
 To heat and to impart humidity to wood, three different pro- 
 cesses are employed ; the first is by boiling water, the second by 
 steam, and the third by wet heated sand. 
 
 The boiling of the wood in water, is attended with the 
 inconvenience of dissolving part of the substance of the wood ; 
 at least, on drying again, it shrinks both in thickness and in 
 length ; its strength and elasticity are considerably diminished. 
 The next method tried was the vapour stove, which was a chest 
 proportioned in size to the wood to be curved, formed of thick 
 planks, firmly joined together. The wood intended to be sub- 
 mitted to the action of the vapour, is placed upon supports. 
 For small chests, a boiler may be placed at one of the extremi- 
 ties, the wood being introduced by a door at the other. In 
 large chests, the boiler is placed in the middle, and the wood is 
 introduced at both ends. The boilers communicate in the inte- 
 rior of the chest by means of a pipe. The vapour formed by the 
 ebullition of the water, impregnates the wood with humidity, aug- 
 ments its elasticity, and renders it fit to be curved. Vapour stoves 
 require little care or expence ; but they can only be used for planks 
 of a certain thickness, because they cannot impart to wood a 
 higher temperature than that of boiling water, which is insuffi- 
 cient to give thick pieces the elasticity they require in order to be 
 curved. 
 
 These considerations led to the invention of the sand stove, 
 which is formed of four walls of stone or brick, having in the 
 middle two fire-places that communicate with several circular 
 flues to convey the caloric, the heated air, and smoke, t<j the two 
 chimnies at each end. Over these flues are plates of metal, 
 forming the bottom of the chest in wdiich the sand is put. The 
 flame and the smoke circulating in the flues, heat the plates, which 
 communicate heat to the sand. This stove is an imitation of the 
 sand baths, which have been immemorially employed in a great 
 number of chemical operations, and m various manufactories, for 
 aflording an equal heat. 
 
 As sand is capable of being heated to a much higher tern- 
 
102 
 
 MECHANICAL EXERCISES. 
 
 Smart’s method of converting the trunks of trees into square timber. 
 
 perature than boiling water, the wood placed in this kind of stove 
 may be subjected to a much more powerful heat; but if there 
 was nothing in the stove but the sand and the wood, the heat 
 might disengage from the latter, the gaseous substances which 
 compose it, and convert it into charcoal. To prevent this, one 
 or two boilers, filled water, are placed in the middle of the 
 stove. The steam created by their ebullition impregnates the sand 
 with humidity, which likewise penetrates the wood, and the heat 
 evaporates only the water which is continually supplied by that 
 which is disengaged ; by these means, the constituent parts of the 
 wood are preserved. 
 
 It may be supposed that in this operation, some portion of 
 the component parts of the wood are evaporated, and that it is 
 consequently liable to a commencement of deterioration : but if 
 care be taken to remove it as soon as it is sufficiently hot and hu- 
 mid, the injury is imperceptible. 
 
 The sand stove is covered throughout its whole length, to im- 
 pede evaporation and the loss of heat. 
 
 The wood is introduced lengthways into the stove, at the tw’o 
 ends, and being placed on gratings fixed for the purpose, it is 
 covered witli sand. 
 
 When the wood has been rendered sufficiently hot and humid, 
 it must be bent upon a surface making the desired curve. The 
 force which produces the curvature may act by means of cords, 
 pulleys, and even capstans. The piece having assumed the de- 
 sired form, it must be left, with the force still acting upon it, to 
 cool and dry. 
 
 When the piece of wood is not thick, the pressure of men, or 
 even of weights, will frequently afford sufficient force to produce 
 the necessary curvature. 
 
 Advantageous Method of converting the Trunks of Trees int» 
 square Timber^ 
 
 The method we are about to describe, of converting all tim- 
 ber that is straight, and intended for square beams, to great 
 advantage in general use, is included in Smart’s patent for 
 hollow masts; but the ingenious patentee, as far as relates to 
 lessening the consumption of English oak, and introducing the 
 larch and firs of our own growth into general use, has liberally 
 granted licences to all who chose to apply to him for them, 
 with some trifling exceptions with respect to masts, yards, bow- 
 sprits, &c. 
 
 Fig. 4, is a section of the but-end of a tree, two feet in 
 diameter, sawn or chopped diagonally. Fig. 5, is the other 
 

 ^ Li : ( 1 1 A> 1 1 ' A L c XL, 1 1 r j si: s . 
 
 PL.n. 
 
 Jfi . 
 
 

MtCHANICAL EXERCISES. 
 
 103 
 
 Smart’s method of converting the trunks of trees into sq.uare timber. 
 
 end, sawn square, one foot each side : cut it exactly through 
 the centre in two cross-cuts, aby de ; it will produce four pieces, 
 which are put together, as in fig. 6 and 7, with the centre turned 
 outwards, the but-end of one piece with the small end of the 
 other, and dowel and bolt them together as in fig. 8. A beam 
 will then be formed, whose section is shown in fig. 6 and 7, regu- 
 lar from one end to the other, with the advantage of having the 
 heart of the tree in the place where the hardness and strength are 
 most wanted, viz. in the corners, which form the abutments; 
 whereas the same tree squared into a parallel beam, would have 
 been much smaller, and the soft or sappy parts of the wood ex- 
 posed to the action of the air and moisture. In flush framing, it ’ 
 IS observeable, that the failure of all timber in old buildings has 
 commenced much sooner than it otherwise would have done, 
 owing to the sappy Wood being at the corners of the principal 
 beams. This sappy wood soon decays, as its spongy quality at- 
 tracts the moisture ; whereas the heart, especially of oak, will be 
 as sound as the first day it w'as used. 
 
 As beams, taking their weight horizontally or on any transverse 
 bearing, have their principal strain on the upper and lower sur- 
 face, every workman ought to guard against having sap in beams, 
 because if they do not immediately decay, they shrink, so as to 
 loosen all the framing, and soon cripple the building or machine ; 
 but on the above plan, the sappy or w’orst part of the wood is 
 excluded from what would cause its decay, and the timber in- 
 creased in quantity so as considerably to overbalance the extra 
 labour and expence. A tree of oak, forty feet long, two feet in 
 diameter at the but-end, and one foot at the top, when put toge- 
 gether on this plan, will have its sides each eighteen inches square, 
 and w ill contain ninety feet ; whereas on the old plan, forty feet 
 would be the contents of a square beam cut from the same tree ; 
 fifty cubic feet would have been cut off as slabs, or chopped up for 
 the fire. Estimating the expence of thus putting together a beam, 
 of the dimensions in question to cost c£3, (and it would probably 
 not amount to more where the price of labour is highest,) and the 
 fifty feet saved to be worth no more than <£6; the proprietor 
 W'ould save £S by each beam so converted. The dowels ought 
 not to go through, as that would weaken the timber. In an eigh- 
 teen-inch beam, the dowels should come within three inches 
 of the outside ; but where a mortise is cut in place of a dowel 
 It IS proper to have an iron screw bolt to prevent the joint open- 
 ing with the pressure of the tenon ; and the work ought to be put 
 together with screw clamps, for nails or hammers bruise the wood, 
 -ind weaken or destroy the cohesion of its fibres for a considerable 
 depth. 
 
104 
 
 MECHANICAL EXERCISES." 
 
 The axe. — The adze. 
 
 Of the Tools used in the working of Wood. 
 
 We do not here intend to speak of the tools used in turning 
 wood, they having been already noticed. We allude more pai- 
 ticularly to the tools required by the carpenter, joiner, and ca- 
 binet-maker, and which differ not from each other so much in 
 their general construction and name, as in size, and the varieties 
 necessary for coarse and large, or ornamental and small work. 
 We offer the enumeration of the tools used by these artisans, 
 not with the hope that the mere enumeration will convey any iu- 
 formation of importance, but to afford an opportunity of making 
 a few remarks on the mode of using and choice of them, and on 
 those pt^:uliarities in their form, upon which their excellence chiefly 
 depends. 
 
 The principal tools employed in the working of w ood, are, the 
 axe, the adze, various sorts and sizes of saws, planes, chisels, ham- 
 mers, and boring tools. 
 
 The construction of the axe and the adze, and their use in 
 chopping or hewing, scarcely require any remark. In grinding 
 these tools for use, the general rule observed in grinding all other 
 edge-tools must be attended to, viz. that of suiting their edge to 
 the work for which they are intended. When the wood they 
 are to cut is hard and knotty, the part ground off to form the 
 edge must be short, so as to leave the tool rather thick and 
 strong near the edge; on the contrary, for soft, clean-grained 
 stuff, the part alluded to may be brought to an acute angle, so 
 as to fomi a thin wedge. A w'orkman applied his axe to the 
 chopping of bones, and thought the metal it was made of very 
 bad, or badly tempered, because it became notched or gapped, 
 almost at every stroke, till the edge was gone. The fact was, 
 that it w as ground to so acute an angle as only to be fit for cut- 
 ting fir. Such mistakes, however gross, are not uncommon. The 
 tool in question should have been ground so as to have had an 
 edge almost like that of the chisel for chipping iron. The axe is 
 ground on both sides to form the edge : the adze is a much 
 thinner and lighter tool than the axe, it is ground only on one side, 
 namely the inner ; the part ground is at a right angle to the handle ; 
 and the blade is arched to the portion of a circle, the radius of 
 which is somew hat less than the length of the handle. The adze 
 is much used by the cooper, as well as the carpenter and joiner ; 
 it will pare away very thin slices or chips, and leave a much 
 smoother surface than the axe, which, besides, cannot, like it, be 
 applied to a surface in a horizontal position. That part of an axe, 
 adze, or any other tool, which is ground ofl to form the edge, is 
 called the basiL ' 
 
HECHA^IICAL EXERCISES. 
 
 105 “ 
 
 General remarks on saws. 
 
 Of Sarcs. 
 
 Saws are made of plates of steel; if they possess great 
 elasticity, and bend equally in a bow, they are judged to be well 
 tempered, and evenly ground. The edge in which the teeth are 
 cut, is thicker than the back, that the back may readily follow the 
 edge. The teeth are cut and sharpened with a triangular file, the 
 blade of the saw being first fixed in a whetting-block or vice. 
 After the teeth have been filed, they are set, that is, turned out 
 of the right line, that they may make the kerf or fissure wider 
 than the thickness of the saw-plate, and thus prevent the friction 
 which would otherwise impede the motion of the tool. If the 
 first tooth be bent to the left, then the next is turned to the right, 
 and so on ; and it may also be remarked, that the extremity of 
 each tooth, at the outside corner, is left higher than on the inside 
 corner, which tends to facilitate the cutting. Hence it will be ob- 
 served, that the teeth of Smart’s circular saw, which have been 
 particularly noticed, are formed in the same way as those of the 
 common saw, and for tlie same reasons, in addition to those stated 
 on page 99- 
 
 The instrument with wdiich the teeth of saws are bent, is usually 
 a piece of iron or steel, five or six inches long, with several nicks 
 in the edge, at right angles to its length, and of different sizes. 
 The tooth intended to be bent, being slipt into the nich which it 
 will exactly fill, and the saw in the mean time being held fast, th® 
 effect of bending the tooth may readily be produced by twisting 
 the instrument up or down. The teeth of a saw are made larger 
 for coarse cheap stuff, than for hard and fine, in cutting which 
 large teeth would make too great a resistance. The plate of a 
 saw should be quite straight, or it cannot be depended upon for 
 making a straight kerf. 
 
 In large towns, there are men who earn a portion of their 
 livelihood by sharpening saws, and those who perform the 
 business well, receive considerable praise for their ingenuity, 
 from journeymen, not always of the most clumsy and idle 
 class, who employ them without reflecting that the skill they 
 admire is easily attained by a little attention ; that the time em- 
 ployed in carrying, looking after, and fetching their tools, is 
 generally equal to what would be required for repairing them 
 at home; and that they can therefore seldom call themselves 
 gainers, even if they had nothing to pay for the work they have 
 had none, or if it were no inconvenience to be for a time de- 
 prived of a tool almost constantly required to be at hand. The 
 teeth of saws have a proper degree of acuteness, w hen comprise 
 ing an angle of about sixty degrees. In sharpeoitig them, the 
 p. VoL. I. P 
 
106 
 
 MECHANICAL EXERUSES. 
 
 The whip-saw. — Hand-saw, — Pannel-saw. — Frame-saw. — Tenon-saw. 
 
 whole of the outer arris of each tooth, should be made sharp ; this 
 can only be done by moving the file in a straight direction, 
 which will make the slanting sides of the teeth flat. Saws used 
 for dividing wood longitudinally, or in the direction of its 
 fibres, may have the front edge or apex of each tooth standing 
 almost as forward as the base of the tooth on that side next the 
 lower end of the plate. But this form, in transverse or cross- 
 cutting, would be inconvenient, as it would hinder the workman 
 from pushing forward the saw ; tenon-saws are therefore usually 
 made so that the apex of the tooth is not more forward than the 
 centre of its base. 
 
 The saws in most common use are the following, viz. the^izV- 
 saWy which is a large tw^o-handed saw, for sawing timber in pits, 
 chiefly used by the sawyers. 
 
 The whip-saw, which is also two-handed, is used in sawing 
 such large pieces of timber as the hand-saw will not easily 
 reach. 
 
 The hand-saw, which is made for a single man’s use; the 
 length of the plate is about twenty-six inches, and it is generally 
 made with about four teeth in an inch. It is used in cutting 
 wood across, as well as in the direction of its fibres. The teeth 
 toward the low^er end of it are rather smaller than those at the 
 upper end, or broadest part of the plate, which facilitates the 
 working of the saw in that part of its course, when the work- 
 man has the least power upon it, and the wood, on the surface 
 and at the sides of the kerf, particularly in cross-cutting, are 
 not so much torn as they would be if the teeth were all of equal 
 size. 
 
 The plate of the pannel-sazo is about the same length as that 
 of the hand saw, but it contains about half as many more teeth in 
 the same compass. It is used for cutting very thin boards in any 
 ♦direction which may be required. 
 
 The how OY frame-saw is furnished with cheeks ; by the twist- 
 ed cords from the upper parts of these cheeks, and the tongue in 
 the middle of them, the upper ends are drawn closer together, 
 and the lower set further apart; so as to tighten the plate, 
 -which is too long and narrow to be kept straight without a 
 frame. 
 
 The tenon-saw is used for cutting across the fibres of wood, 
 ?and derives its name from its use in forming the shoulders of 
 tenons. The smallest saw to which this name is given, is about 
 fourteen inches, and the largest about twenty inches long. The 
 • number of teeth in an inch are from eight to ten, according- to the 
 - length of the plate, the larger sizes of which have the fewest teeth 
 in the same compass. 
 
MECHANICAL EXERCISES. 
 
 107 
 
 
 The sash-saw. — Dove-tail-saw. — Compass-saw. — Key-hole-saw. — Planes. 
 
 The saw used in cutting the tenons of sashes, is called a saskr 
 saw ; the plate is about eleven inches in length, and the number of 
 teeth in the inch about fourteen. 
 
 The dove-tail-saw is used by joiners and cabinet-makers in 
 dove-tailing drawers, &c. ; the plate is about nine inches long, 
 and the number of teeth in the inch about fifteen. — ^The plates 
 of the tenon-saw, the sash-saw, and the dove-tail-saw, are so 
 thin, that the back of them is let into a stout piece of iron or 
 brass, to keep them from bending, and as they are not intended 
 to cut into wood their whole breadth, this addition is no disad- 
 vantage. 
 
 The compass-sazv is used for cutting a circular or any other 
 compass kerf : its formation is peculiar ; the teeth are not set, 
 as the setting of a saw has a tendency to keep it in a right 
 line ; the teeth are small, about five in an inch ; the plate 
 narrow, about an inch in the broadest part, and gradually di- 
 minishing to about a quarter of an inch at the lower end ; the 
 cutting edge is thick, and the back very thin, so that it may have 
 a compass to turn in. The sides of this saw should either be cor- 
 rectly flat, or a little concave like a razor, otherwise it will not 
 work well. 
 
 A small kind of compass-saw, called a key-hole-saw, is used 
 for quick curves, such as key-holes. The handle is long, and in 
 shape similar to that of a chisel, but perforated through its whole 
 length, in order that the saw, which is fixed by a screw, may 
 be set at any distance within the handle. Hence in cutting the 
 smallest curves to which it can be applied, or at the commence- 
 ment of the work, or when it is only wanted to saw through an 
 inconsiderable depth, but a small portion of the saw^ is allowed to 
 project from the handle ; by which means the springing or unstea- 
 diness of sawing with the end of a long narrow blade is avoided, 
 and more force can be applied, w ithout the hazard of breaking th^ 
 plate. 
 
 Of Planes. 
 
 Planes of different kinds form a very important part of the 
 tools of artizans in wood. A few remarks in explanation of 
 technical terms, will enable us subsequently to be more concise 
 and intelligible in noticing this class of tools. The block of 
 wood in which the blade or chisel of a plane is fixed, is called 
 the stock ; it is mostly made of beech or some other hard wood, 
 exceedingly w^ell seasoned. The blade or chisel is called the 
 iron ; it is composed of iron and steel welded together, the fore 
 part of the lower half of it, when in the stock, containing the 
 steel. The underside of the stock is called the sole. The height 
 
108 
 
 MECHANICAL EXERCISES. 
 
 General remarks on planes. 
 
 or depth of a plane are synonymous terms, signif}ing the dimen- 
 sion from the sole to the upper surface. The handle of a plane 
 is called the tote. That part of the aperture in the stock upon 
 which the iron is laid and secured by the wedge, is called the bed, 
 which is a plane surface, making a ddferent angle w ith a line per- 
 pendicular to the sole, according to the use for which the plane 
 IS intended. For the jack-plane, the trying and the smoothing- 
 planes, the angle of the bed is usually from forty-two to forty-fiye 
 degrees ; for moulding planes about thirty-five, and for those 
 planes which operate by scraping, it is almost perpendicular, not 
 making an angle of more than live or six degrees. The angle 
 W'hich the iron makes with the perpendicular alluded to, is called 
 its pitch, and the greater this angle, the low'er is said to be the 
 pitch of the iron. The basil of the iron forms an acute angle with 
 the steel side, which is not ground, but always kept level. In 
 grinding and whetting plane irons, the basil must be made as flat 
 as possible, or in a small degree concave, otherwise it will not 
 seem to be sharp when in use. 
 
 Plaires are generally about three inches and one-eighth deep ; 
 the jack-plane is sometimes rather more, and the smoothing- 
 plane is mostly rather less. The blades of planes are, in many 
 cases, made double, a simple expedient of remarkable utility 
 in planing cross-grained stuff. The addition made to the blade 
 for this purpose, consists of a piece of iron of the same breadth 
 as the blade, with its lower end very thin, and of the same shape 
 at the edge as the edge of the blade. This piece of iron, usually 
 called the top-iron, i« connected by a screw with the blade, at any 
 necessary distance from the edge of which it can be flxed The 
 top-iron, the edge of which should never extend below the sole, 
 is fastened upon the front or steel side of the blade, and the space 
 between its edge and that of the blade, determines the thickness 
 of the shaving. It is always necessary to make ibe top-iron fit 
 the blade so correctly that no shaving can get between them ; for 
 this end, it is arched a little towards the lower end, and the con- 
 cave side of this arch being turned inw ards, the screw' necessarily 
 makes the edge tit closely the level surface of the blade. The 
 top-iron is generally employed in the jack-plane, and alniost al- 
 ways to the trying-plane, the long-plane, and the jointer. To 
 the smoothing-plane, and the various sorts of moulding-planes, it 
 is not used. 
 
 If the iron of a plane project too far, the blow of a hammer, 
 on the fore end of the stock, will slacken the wedge and raise it 
 in a small degree. In this case, the wedge must be re-fastened 
 by driving it down with a light blow or tw'o before the plane is 
 used again. A smart blQW on the fore end of a plane wHi loosen 
 
MECHANICAL EXERCISES. 109 
 
 The jack-plane. — Trying-plane. — Long-plane. — Jointer. — Strike-block. 
 
 the wedge so much that the iron may easily be withdrawn by the- 
 hand. Instead of striking the fore end, for these purposes, some 
 w'orkmen strike the upper surface of the stock, near the orifice for 
 the shavings. 
 
 I’he jack-plane used by joiners, is generally about seventeen 
 inches in length. Its use is to take off the greater irregularities of 
 the stuff, left by the axe, the adze, or the saw, and it is therefore 
 the first plane employed. To suit the coarseness of its work, the 
 cutting edge of the iron rises with an arch of considerable con- 
 vexity in the middle, and the opening or mouth which admits the 
 shavings through the stock, is wider at the sole than that of any 
 other plane. The iron is often used without a cover ; the quantity 
 of Its projection must be regulated by the texture of the stuff, in 
 proportion to the hardness or knottiness of which it must be les- 
 sened ; the due degree of it is easily ascertained by trial : it must 
 be adjusted so as not, on one hand, to require hard pressing down, 
 or many strokes to be made in reducing the wood ; nor, on the 
 other, to fatigue the workman and tear the stuff, by taking hold too 
 keenly. The convexity of the cutting edge of the iron, prevents 
 the corners from entering the wood, which ought never to occur, 
 as the effect would be to spoil the work and impede the progress 
 of the artizan. 
 
 When a piece of stuff has been nearly reduced to the intended 
 form by the jack-plane, the trying-plane is made use of to pro- 
 duce a higher degree of regularity and smoothness. It is four or 
 five inches longer than the jack-plane, and its iron is broader, set 
 with a less projection, not so convex on the edge, and, like the 
 two following sorts of planes, always used double. This plane, in 
 taking off a shaving, is pushed along the whole length of the stuffj, 
 whereas the strokes given with the jack-plane are only within 
 arms’ length. 
 
 The third plane made use of in facing a piece of stuff with 
 the utmost exactness, is the long-plane, which is four or five 
 inches longer than the tr\ing-plane, and proportionately broader, 
 while the projection and convexity of the iron are somewhat 
 less. 
 
 The jointer is the longest plane of all ; its edge is very fine, 
 and scarcely stands out above a hair’s breadth ; it is chiefly 
 used for shooting the edges of boards perfectly straight, ^o that 
 when joined together their surfaces will exactly coincide, and the 
 juncture be hardly discernible. The jointer is made about thirty 
 inches long. 
 
 As the last-mentioned plane, from its extraordinary dimen- 
 sions, would be unhandy in shooting short blocks, a short kind 
 of jointer, called the strike-block is also in common usO. It- is 
 
110 
 
 MECHANICAL EXERCISES. 
 
 Smoothing-plane. — Tooth-plane. 
 
 much employed in planing the ends of boards, across the fibres, 
 and the inclined plane forming the bed, is lower than that of the 
 jointer. When employed for soft wood, the angle is only in- 
 creased two or three degrees ; but for fine cabinet work, when the 
 stuff is hard, it is often considerably more, so as to make an angle 
 with the perpendicular of fifty-five or even sixty degrees. In the 
 latter case, the position of the basil is reversed, so as to be in front, 
 or next the fore end of the stock. The usual length of the strike- 
 block is eleven br twelve inches. 
 
 The smoothing-plane is about seven inches in length, it has 
 no tote or handle, and otherwise differs in shape from any of 
 the planes yet mentioned. The sides of the stock are convex, 
 and its whole figure resembles that of a coffin. The inclination 
 of the bed is similar to that of the jointer, which it also resem- 
 bles in the set of the iron. It is the last plane used in finishing 
 off the surtace of w'ood, and from its smallness can easily be 
 applied to smooth any small part which the large planes cannot 
 touch; and the direction in which it is wrought can also be 
 varied with facility, so as to suit cross-grained stuff. To secure 
 these advantages more effectually, it is wTOught, like the jack- 
 plane, with short strokes. From this description of its use, it 
 will be obvious, that though it is used in finishing of wood, it is 
 smootJmess, and not straightness of surface, w hich it is calculated to 
 produce. But if the work be w ell managed, the inequalities w'hich 
 it leaves are not perceptible to the eye, and are therefore left with 
 impunity in tables, bureaus, desks, and other furniture, even of the 
 best kinds. • 
 
 Though the double iron is an excellent invention, and the 
 use of it is, in fact, the best general remedy known against the 
 curling or cross-grained stuff of ordinary quality; yet, without 
 some other assistance, the planing of many of the finest specimens 
 of mahogany, and many other w oods, among which fustic may 
 be particularly mentioned, would be to the last degree a difficult 
 and perplexing operation to the workman. Hence a plane, the 
 stock of which is usually made of the shape and size of the smootli- 
 ing-plane, is fitted up so as to act by scratching or scraping. 
 The blade, or iron, on the steel side of it, is covered with rakes or 
 small grooves close to each other, and all of them in the direction 
 of its length ; when therefore it is ground, and the basil formed, 
 its edge presents a series of teeth like those of a fine saw ; the 
 bed of the stock intended to receive it is inclined only about six 
 degrees, and consequently when the iron is fixed it is almost per- 
 pendicular. On account of these teeth in the iron, the plane ob- 
 tains the name of the tooth-plane. With this kind of a plane, 
 however haid the stuff may be, or however cross and twisted iU 
 
MECHANICAL EXERCISES. 
 
 Ill 
 
 Compass and Forkstaff planes. — Round-sole. — Rebating-plane. — Plough. 
 
 grain, the surface may be made every-where alike, and will not be 
 rougher than if it had been rubbed with a piece of new fish-skin. 
 This roughness may be effectually removed with the scraper, which 
 is a thin plate of steel, like part of a common case-knife, the back 
 of it being let into a piece of wood, as a handle. 
 
 To form the concave or convex surfaces of the rims of 
 carriage-wheels, or the top rails of a camp-bedstead, and work of 
 a similar nature, the sole of the plane must be curved in the same 
 degree as the concavity or convexity to be . produced. Planes 
 of this description are called compass-planes ; they resemble 
 the smoothing-plane in size, and also in shape, excepting so far 
 as regards the curve of the sole. A plane of the size and shape 
 in question, with a concave sole, is also distinguished by the name 
 of uforkstaff-plane ; and one which is convex, is sometimes called 
 a round-sole. 
 
 The rahhet or rehating plane j is employed in taking away 
 (by shavings) from the edge of a board, a piece in the form of a 
 square or rectangular prism, so as to leave a groove consisting of 
 two surfaces at right angles to each other. This mode of 
 reducing the stuff is required for some cornices, and various 
 sorts of ornamental work. The groove formed by the rebating- 
 plane, is also employed to receive the edge of another board 
 cut ill a similar manner, so that the two lap over each other to 
 the breadth of the rebate, and form one even surface. Rebating- 
 planes deliver their shavings at the side, and not at the top, 
 like the planes hitherto described. They are also of various 
 kinds ; some of them are provided with a fence which regu- 
 lates the horizontal breadth, and others with a stop, which de- 
 termines the vertical extent or depth of the rebate ; while some 
 have both stop and fence, and others neither. Rebating-planes 
 without a fence have the iron the w hole breadth of the sole ; some 
 of them have the cutting edge of the iron only on the side, and 
 others only on the bottom of the stock ; these are employed for 
 dressing and finishing w ith exactness, separately, either side of the 
 rebate. 
 
 The plane by which a square groove is taken out of the 
 edge of a board, so as to leave a ridge on each side, is called a 
 plough, and the operation of cutting with it is called plough- 
 ing. To prevent the necessity of having, for grooves of dif- 
 ferent sizes, a great number of ploughs, which w^ould be cum- 
 bersome and expensive, a tool of this description, called a univer- 
 «al plough, is manufactured. The stop and fence of the univer- 
 sal plough are moveable, and it admits alternately, according to 
 the extent of the groove desired, ten or a dozen different sizes of 
 irons. 
 
112 
 
 MECHANICAL EXERCISES. 
 
 Moulding-planes. — The gouge. — The firmer chisel. — Mortise chisel. 
 
 Moulding-planes admit of the greatest diversity of contour, 
 which is necessarily the reverse of the moulding produced. The 
 figure of the edge of the iron and that of the sole, should exactly 
 correspond ; in whetting the iron, great care must be taken not to 
 injure its form : the whole of the sole, or at least the ridges of the 
 moulding, especially if narrow at the base, should be made of 
 box-wood, which unites, in a greater degree than perhaps any 
 other wood, the valuable properties of hardness, toughness, smooth- 
 ness, and durability. 
 
 Of Chisels » 
 
 The very large chisels used by carpenters, millwrights, and 
 others, for heavy coarse work, are generally composed of iron 
 and steel, welded together, — the steel forming but a small por- 
 tion of the whole mass of metal, as it seldom extends higher than 
 the broad part of the tool, and often constitutes no more than a 
 third of the thickness. The small and middle-sized chisels of 
 the best kind, are alw^ays made of cast steel. As all chisels, not 
 exclusively employed in turning, are driven more or less by per- 
 cussive force, they are (except the socket chisel) provided with a 
 shoulder, which abuts against the end of the handle into which 
 the tang is driven, and prevents it from being split by blows. 
 The basil of chisels is on one side, and if well formed should be 
 quite tlat. 
 
 The gouge used by the joiners and cabinet-makers is similar 
 to that of the turner, though not always sharpened in the same 
 w^ay. The edge is generally, by joiners and cabinet-makers, for 
 small work, made straight across the end, and not convex like th« 
 turner’s gouge. Tlie millwrights, again, often make the basil on 
 the hollow or concave side of the gouge, in order to cut with it 
 perpendicularly. 
 
 The thin broad chisel, the sides of which are parallel for a 
 certain length, and which afterwards becomes narrower towrards 
 the shoulder, obtains the name of the firmer chisel when driven by 
 the mallet, and of the paring chisel^ when the hand only is em- 
 ployed in cutting w ith it. 
 
 The common mortise chisel, the section of which is a rec- 
 tangle approaching almost to a square, is, as its name implies, em- 
 ployed in making mortises ; the basil is made on one side of its 
 narrow sides. It is, from its form, very strong, which is necessary 
 not only on account of its having to sustain extremely heavy blows 
 with the mallet, but because it is partly used as a lever to get out 
 the pieces of wood as they arc severed, in the course of cutting the 
 mortise. 
 
MECHANICAL EXERCISES. 
 
 113 
 
 The socket-chisel. — The auger. 
 
 The socket-chisel is distinguished from other chisels by its 
 having a conical socket, instead of a tang and shoulder, to receive 
 the handle. It is used for the same purposes as the mortise-chisel, 
 but is not so thick in proportion to its breadth. It is much used 
 for very large work. 
 
 The upper end of the handles of chisels which are driven by 
 percussion, should be made convex, as they will then be least 
 liable to be split or injured by blows. 
 
 Boring Tools. 
 
 The largest of the boring tools for w ood, is the anger. The 
 oldest construction of the auger, which is yet in common use, 
 in various parts of the country, cannot be wrought till a small 
 excavation has been made, which is mostly done with a gouge, 
 at the place where the hole is to be ; and till the auger arrives at 
 a considerable depth, the motion of it is very unsteady. This 
 old auger is shaped like a gimblet, except at the point, which 
 is like that of a nose-bit. An improved construction of the 
 auger, by Phineas Cooke, appeared to possess so much merit, 
 that the Society for the Encouragement of Arts, presented thirty 
 guineas to the inventor. This is called the spiral auger, for it 
 consisted of a rectangular bar of steel, twisted in the shape of 
 a bottle-screw, terminating in a short taper screw’, with a double 
 worm like a gimblet. The upper part, like that of the com- 
 mon auger, is formed into a large ring, in which the handle is 
 inserted, at right angles to the length of the auger. That part 
 of the screw adjoining the spiral, presents an edge which cuts 
 the wood. This auger is not very commonly used, but it 
 pierces the wmod much truer than the common one ; no picking 
 is necessary before it can be wrought, nor does it require to be 
 drawn out to discharge the chip. It is, how'ever, better 
 adapted to the boring of soft wood than hard. Its use being 
 on this account more limited than w’orkmen like, besides its being 
 not cheap in its first purchase, and if not made of good metal and 
 very carefully tempered, easily changing its form, it w ill probably 
 not regain the character it once acquired. The latest construction 
 of the auger has been found to answ er so well, that it will proba- 
 bly, ere long, nearly supersede the use of the spiral and common 
 auger. Like the spiral one it terminates with a gimblet-screw, 
 which draws it dowm into the wood, while the workman turns it 
 round and presses upon it; and another peculiar advantage of 
 which is, that its point can be set precisely upon the centre marked 
 for the perfonttion, the proper direction of which there is then a 
 goodxhance of preserving, w hile the broad -ended auger is apt to 
 deviate considerably at its very commencement. Immediately 
 5. — VoL. L Q 
 
114 
 
 MECHANICAL EXERCISES. 
 
 Disadvantages of the anger. — Stock and bit. 
 
 above the spiral screw, it is, for a short length, rather of a pris- 
 moidal shape, taperiug a little upwards, like the socket chisel 
 below the conical part. The prismoidal part has one cutting 
 edge which cuts the sides of the hole, and another which cuts the 
 bottom. The core rises as the act of boring goes on, in the fonn 
 of a spiral shaving. Above the prismoidal part, the shaft may 
 be of any shape at pleasure, that possesses sufficient strength, tak- 
 ing the obvious precaution of making its diameter less than that of 
 the bore. The general disadvantage of augers with gimblet points, 
 is, that when they encounter knots or hard places in the wood, 
 they are apt to break. 
 
 Every one who makes use of an auger in the usual way by 
 hand, knows by experience that he never can so completely exert 
 his strength in tins operation, as wdien he bores down perpendicu- 
 larly, with his body leaning over his work ; and it is very evident 
 that by every degree of the auger’s elevation from this situation, 
 his power is of less effect, consequently his labour is increased, 
 and his work so much retarded, that in the former position he can 
 bore four holes for one in the latter. In hand boring, also, the 
 unsteady and irregular motion of the auger, (particularly when 
 the common old-shaped one is used,) at its first entrance into the 
 wood, occasions the holes to be bored very crooked, often larger 
 w itliout than w ithin, and very w ide of the direction aimed at, espe- 
 cially if tlie wood proves hard and knotty, and the holes are deep. 
 Regarding the prevention of these disadvantages as a matter of 
 considerable consequence to ship-builders, and a variety of other 
 artists, the Society for the Encouragement of Arts, &c. presented 
 the sum of fifty pounds to William Bailey, for his invention of a 
 machine for boring auger-holes, by the use of which the force of 
 the workman, and consequently the dispatch of his operations, 
 are equally exerted in all directions. It is unavoidable also, in 
 Uie usual way of boring, for the action of the auger to be discon- 
 tinued twice in every revolution; but with the machine the mo- 
 tion is continued with equal force and velocity, till the auger hai 
 bored to the depth required. A description of this machine, 
 illustrated by a plate, may be seen in Bailey’s Advancement of 
 Arts ; our limits will not allow us the further notice of it here, 
 but the fact of such a contrivance having been executed, being 
 mentioned, the ingenious mechanic will not perhaps find it very 
 difficult to contrive one for himself. 
 
 The contrivance for boring next entitled to notice, is the 
 stocky wliich is in effect a crank, not unlike the hand-drill, and 
 frequently made of iron, though generally of w^ood, defended 
 by brass, at the paits most subject to wear. Where the crank 
 terminates, two short limbs project from it, in a line with each 
 
MECHANICAL EXERCISES. 
 
 115 
 
 The gouge-bit. — Centre-bit. — Countersink. — Gimblet. 
 
 Other, and parallel with that part of it by which it is revolved. 
 In the end of one of these limbs, which is called the pad, the 
 piece of steel by which the boring is performed, is inserted ; the 
 other limb is connected with a broad head, rather convex exter- 
 nally, which head is placed against the breast, and is stationary 
 while all the other parts are revolved. 
 
 The piece of steel inserted in the stock is called the bit ; as 
 it can readily be taken out or put in, the same stock serves for 
 bits of all sizes. They are differently shaped, according to 
 their use. The gouge-bit is best adapted for boring small holes 
 in soft w^ood ; it is shaped nearly like the turner’s gouge, but is 
 1 ather more pointed like a spoon at the extremity ; the basil is 
 made in the inside, and the sides are brought to a cutting edge 
 like those of a gimblet. The centre-bit has a small conical 
 point projecting from the lower end ; this point entering the 
 w ood first, keeps the tooth of the bit from wandering out of its 
 proper course, and the hole is bored straight with great ease. 
 The taper shell-hit is used Cor widening holes; it diffeis from the 
 gouge-bit chiefly in tapering gradually from the pad to the lower 
 extremity. 
 
 The bit for widening the upper part of a hole, to admit the 
 head of a screw, is called a countersink. The head of the 
 countersink is conical, and the cutting edge is single when 
 made for wood alone, and stands out a little from the side of 
 the cone. Joiners and cabinet-makers, however, are generally 
 provided with countersinks for brass, and these, which have 
 ten or a dozen teeth on the surface running slantwise from the 
 base up the sides of the cone, they frequently make use of for 
 wood, especially when it is hard, and they are anxious to avoid 
 tearing it ; for the teeth of the brass countersink act like those 
 of a tile 
 
 The gimblet is a boring implement too well known to require 
 any explanation of its construction ; but with respect to its 
 management, it may not be wholly useless to remind the novice, 
 that like other boring tools of a similar conformation, it requires 
 to be withdrawn to remove the core as often as the cup or 
 groove is filled, and this will be sooner or later, not only in pro- 
 
 f ortion to the depth penetrated, but the density of the wood, 
 ndeed, in boring such wood as lignum-vitae, which clogs the 
 tool, it is adviseable to withdraw the gimblet, to clear away the 
 core, before the cup is full. The auger gives warning of the 
 time to stop, by the difficulty of turning it, when surcharged 
 with shavings, and is too strong a tool to be in danger of being 
 twisted ; but the smallness of the gimblet renders it liable to be 
 twisted and broken before the workman is aware, if not often 
 
lid 
 
 MECHANICAL EXERCISES. 
 
 Sprig-bit. — Hammers. 
 
 enough ^vitlldra\vn and emptied. Gimblets which are broken- 
 pointed, or blunted on the an is of the screw, are generally thrown 
 aside, it being tedious, and laborious also when they are large, 
 to work with them in such a state ; but we may observe, that 
 though the grindstone cannot be employed to sharpen the w'orm, 
 a tile may, so that a few minutes’ labour will render them tit for 
 use again. 
 
 The smallest sort of boring tool is a kind of bodkin, called 
 the hrad-azd, or spidg-bit, as it is chiefly used in making the per- 
 foration to admit those small slender nails, which have no head 
 except a tiifling projection on one side, and are called brads in 
 some parts of the country, and sprigs in other parts. The sprig- 
 bit is generally made with a shoulder where the tang terminates ; 
 below the shoulder it is cylindrical, to within a short distance of 
 the extremity, which is flattened, and thereby made rather broader 
 than the diameter of the cylindrical part ; but so thin at the same 
 time towards the end as to form an edge. Unlike other boring 
 tools, the sprig-bit takes away no part of the substance of the 
 w ood, nor is it turned entirely round in making a hole, but merely 
 wrought backwards and forwards about half round before the mo- 
 tion is reversed. 
 
 The Hammer — Mallet — Square --Bevel — Mitre-square — 
 Gauge — Straight-edge — IV hiding-sticks. 
 
 Though hammers, of various sizes, are indispensable in the 
 working of wood, yet we may pass over the consideration of them 
 in this place, with little more than referring to what has been 
 already said with respect to them, in treating of the w'orking of 
 metals. 'Fhe object of having the head of the hammer perfectly 
 weix secured to the handle is certainly well worth attention, from 
 the serious accidents which may attend the neglect of it ; but to 
 attain it, we recommend not the use of those hammers which have 
 plates of iron extending from the head, forming a kind of socket 
 for the reception of the handle. These plates, it is true, afford 
 ample means of uniting the head and the handle ; but they render 
 the latter inflexible at the very part w'here it is desirable there 
 should be some spring. Hence those hammers are best, in w’hich 
 the handle simply passes through a perforation in the head, 
 wooden wedges being driven in at the end of it, after it has been 
 tightly fitted, to make it fast. If the aperture in the head be 
 rather wider at the back than where the handle enters, and the 
 wedges be dipped in glue before they are driven in, the fastening 
 may be made complete. To admit the wedges, the end of the 
 handle is cut a little way down with a saw. 
 
MECHANICAL EXERCISES. 
 
 117 
 
 Mallet. — Square. 
 
 The mallet is in effect a hammer, but is made of wood ; it 
 consequently does not damage substances struck with it so 
 much as the hammer, while presenting a large surface with an 
 equal weight, it is more easy to hit the ends of the chisels, &c, 
 \\ ith it. Alallets are made of the soundest and toughest wood 
 which can be found; either ash or beech, or the hardest kind 
 of elm, is usually preferred. They are mostly made rather 
 concave on that side wdiich the handle enters, and convex on 
 the other ; this is done because it is customary, or because it is 
 supposed to look best : the diameter of the convex end, measured 
 at right angles to the handle, is greater than that of the concave 
 one, consequently the ends with which objects are struck are not 
 parallel with the handle, but inclined to it and to each other ; 
 this is done for convenience, for the mallet is generally used in 
 such a manner tliat the end with which the blow is made, notwith- 
 standing its obliquity with respect to the handle, is parallel with 
 the surface struck. 
 
 The squares used by joiners and cabinet-makers are fre- 
 quently manufactured by these artists for their own use, in 
 which case they are made of wood ; but these wooden squares 
 are always so much inferior in point of durability, and gene- 
 rally in point of correctness, to those sold by the ironmongers, 
 at very reasonable rates, and which are made partly of wood 
 and partly of metal, that we shall only notice the latter kind. 
 The blade is a thin plate of steel, at a spring temper, of equal 
 thickness in every part, and the opposite edges, or at least the 
 two edges in the direction of its length, are correctly parallel 
 with each other. The stock or wooden part is of considerable 
 thickness, seldom less than half an inch in the smallest squares, 
 such, for example, as are only three or four inches long, and the 
 blades of which aie not above a twelfth of that thickness. The 
 blade is let into the stock so as to form a right angle with it 
 both internally and externally. The mortise or kerf which 
 receives the blade, is made at one end, in the middle and 
 entirely across its breadth ; great care is taken to make it paral- 
 lel with the sides of the stock, but it is not cut so deep as to 
 take in the whole breadth of the blade, the part left out being 
 partly designed to admit of the outer edge being repaired, w'hen 
 worn. The inner edge of the stock, which forms one side 
 of the interior square, is faced with brass. The stock, from its 
 thickness, forms, on each side of the blade, a shoulder, which 
 being pressed against one edge of a piece of wood, acts as a 
 guide or stop to keep the blade (which is extended over the 
 adjoining surface) at right angles to the arris, while a line is 
 drawn along its outer edge. The interior square is mostly used 
 
118 
 
 MECHANICAL EXERCISES. 
 
 Bevel. — Mitre-square. — Gauge. 
 
 for examining the squareness of a piece of stuff, and not for 
 drawing lines. In this case, the sides of the square are not held 
 parallel with, but perpendicular to the piece examined. The 
 mode of ascertaining the correctness of the square used in the 
 working of metals, has been already detailed, and will probably 
 suggest the proper method of trying the square now described. 
 Here we have no occasion for a ledge, the shoulder is pressed 
 against the edge of a rectangular piece of stuff, and a line drawn 
 close to the blade ; the square is then turned over, and another 
 line drawn as in the former case, and consequently if the instru- 
 ment deviates from a correct figure, the error is detected upon the 
 same principle as before. 
 
 The hetel consists of a blade and stock similar to those of the 
 square, from which it differs only in one particular, viz. that the 
 blade is moveable, and can therefore be set at any angle which 
 may be required. The joint should be stiff, otherwise the bevel 
 cannot be depended on for remaining as it has been set. Though 
 rarely practised among the workers in wood, it certainly would be 
 easy in all cases to adapt a screw to the bevel, so as to hold the 
 blade firmly at any angle desired. The stone-masons generally 
 take this precaution. 
 
 The mitre-square is a bevel, the blade of which is immove- 
 ably fixed in the stock, and is commonly set for marking an 
 angle of forty-five degrees, this angle being more frequently 
 required in joinery than any other angle except the right angle. 
 Pieces of stuff bevelled at their extremities, and joined by placing 
 two of the bevelled surfaces together, are said to be mitred. 
 Mitring is often used in plane work; but when the pieces to be 
 joined at an angle, are moulded, and the continuation of the 
 mouldings is not to be broken, it is absolutely necessary. Hence 
 its frequent use ; instances of it occur at the corners of rooms, 
 where the skirting boards of two different sides meet, in the 
 surbase under the same circumstances, at the angles of picture- 
 frames, &c 
 
 The gauge is an instrument consisting of a stem, usually in the 
 form of a square prism, with a small steel poifit, nearly at the end 
 of one of the surfaces in the direction of its length, and just pro- 
 jecting enough to mark distinctly when pressed upon wood ; the 
 stem passes at right angles through a mortise in the middle of a 
 "piece of wood called the head, to which it is about equal in point 
 of thickness; but those surfaces of the head through which the 
 mortise passes, should not be less than three times the diameter 
 of the stem. The head can be set at any distance required from 
 the steel point, and there secured by a small w'edge, passing 
 through a mortise in one of its sides, and bearing upon the stem. 
 
MECHANICAL EXERCISES. 
 
 119 
 
 Mortise-gauge. — Straight-edge. — Winding-sticks. 
 
 The use of the gauge is to draw lines parallel to the arris of a 
 piece of stuff, to serve as a guide for the saw, the plane, or the 
 chisel. In drawing the line, it is necessary to keep that side of 
 the head which is next the steel point rather firmly pressed against 
 the edge of the stuff, otherwise the point will be apt to deviate 
 from its proper course, if it meet with knots or irregularities ia 
 the grain. 
 
 A gauge made \sdth two points projecting on the same side, 
 and one of which (being moveable in a groove or mortise) can be 
 placed at any distance from the other, is called a mortise-gauge ; 
 it is used alike in gauging mortises and tenons. 
 
 Though the steel straight-edge is hardly known even by name 
 to a great majority of the artists who work in metal, yet the 
 wooden straight-edge is familiar enough to most who work in 
 wood. Wooden straight-edges should be made of stuff exceed- 
 ingly well seasoned ; it is usual to make two at the same time ; 
 the sides are first made true, and each piece of equal thickness ; 
 they are then placed against each other, and fastened in the cheeks 
 of the bench-screw, in which situation their upper edges are 
 planed true with the assistance of a straight-edge which can be 
 depended on, or as nearly true as can be determined by the eye, 
 if w^e cannot readily obtain a straight-edge for the trial. When 
 the pieces are supposed to be true, they are taken out of their 
 situation, and the edges last planed are placed upon each other. 
 If the surfaces coincide so exactly that no light can pass between 
 them, the straight-edges are finished : but if any error be detected, 
 the planing with the jointer must be renewed in the same manner 
 as before, and the examination repeated till the result is satisfac- 
 tory. The use of the straight-edge, in ascertaining the straight- 
 ness of other surfaces, is not inconsiderable. 
 
 Winding-sticks are always used in pairs, and the use of them 
 constitutes another contrivance for determining the levelness of 
 any given surface, that the workman may reduce it more or less 
 ia any particular part, in order to make it true. Straight-edges are 
 customarily finished only on one edge; but if both edges were 
 finished, and they were made correctly rectangular, that is, of 
 equal breadth through their whole length, they would answer the 
 end of winding-sticks, which are used in the following manner ; 
 one of them is placed at each end of the surface to be examined, 
 the eye is then directed from the uppermost edge of the nearer 
 one to that perpendicular side of the further one which is next 
 the observer. If the eye be elevated or depressed, till one end of 
 the nearer winding-stick intercepts the view of the opposite perpen- 
 dicular side of the other; and the other two ends are observed to 
 have the same relative situation, the ends of the surface examined 
 
120 
 
 MECHANICAL EXERCISES. 
 
 Draw-bore pins. — Glue. 
 
 are already in the same plane. But if the nearer winding-stick 
 will not intercept an equal portion at each end of the further one, 
 the part found to be too high must be reduced till this is the case. 
 It is always proper to examine the surface by placing the wind- 
 ing-sticks in various situations, but especially across near the 
 corners, so that the eye may look at them diagonally. 
 
 Draw-bore pins are used in forcing a tenoned piece into its 
 proper place in the mortise. They are made with tangs and 
 shoulders, and fitted into handles like chisels. Below the shoulder 
 they are round, and taper slightly to within a short distance of 
 the point, where they are turned off* to cones, like the extremities 
 of an axis running in hollow centres. To use them, bore a hole 
 through the mortise-cheeks, at the place where the pin intended 
 to fasten the mortised and tenoned piece together will be required, 
 and which is generally made nearer the shoulders than the end of 
 the tenon. Insert the tenon, and when it is as nearly in its proper 
 place as it can b^e driven, mark it on both sides through the hole 
 in the mortise-cheeks. Take out the tenon, and bore it through, 
 a little nearer its shoulders than the centre of these marks ; insert 
 it again in the mortise, and use the draw -bore pins by entering 
 them at the holes, to draw' its shoulders against the abutments. 
 The use- of the draw'-bore pins, also, condenses or hardens the 
 wood on the sides of the holes, and w'hen the wooden peg is 
 driven in, it has, on this account, a better hold. This is, indeed, 
 their principal use ‘at present, as the cramping-frame, which acts 
 by means of a screw, is much more powerful in forcing the shoul- 
 ders of tenons against their abutments ; but the draw'-bore pins, or 
 some substitute for them, will obviously be convenient to those 
 who may occasionally require means to force tenons to their bear- 
 ings, without possessing a complete apparatus. 
 
 Of Glue, 
 
 To prepare glue, it must be steeped for a number of hours, 
 over night, for instance, in cold w'ater, by which means it will 
 become very considerably swelled and softened. It must then be 
 gently boiled, till it is entirely dissolved, and of a consistence not 
 too thick to be easily brushed over wood. About a quart of 
 water may be used to half a pound of glue. The heat employed 
 in melting glue should not be more than is required to make 
 water boil; and to avoid burning -it, the joiners, &c. as is well 
 known, suspend the vessel containing it in another vessel contain- 
 ing only w'ater, which latter vessel is generally made of copper, 
 in the form of a common tea-kettle without a spout, and alone, 
 receives the direct influence of the fire. 
 
MECHANICAL EXERCISES. 
 
 Ul 
 
 Directions for using glue. — Glue which resists water. 
 
 I'he circumstances most favourable to the best effect which 
 glue can produce, in uniting two pieces of wood, are the fol- 
 lowing : that the glue should be thoroughly dissolved, and used 
 boiling hot at the tirst or second melting ; that the wood should 
 be waim and perfectly dry; that a very thin covering of glue be 
 interposed at the juncture, and that the surfaces to be united, be 
 strongly pressed together, and left in that state, in a warm but not 
 hot situation, till the glue is completely hard. In veneering, 
 and lor all very delicate w^ork, the whole of these requisites, as 
 they not only ensure the strongest joint, but the glue sets the 
 soonest, should be combined in tlie operation ; but on some occa- 
 sions this is impossible, and therefore the most essential must be 
 regarded, such as the hotness of the glue, and the dryness of the 
 wood. When the faces of joints, particularly those that cannot 
 be much compressed, have been besmeared with glue, which 
 should always be done w ith the greatest expedition, they should be 
 rubbed lengthwise one upon another two or threQ times, to settle 
 them close. 
 
 When glue, by repeatedly melting it, has become of a dark 
 and almost black colour, its qualities are impaired ; wdien newly 
 melted, it is of a light ruddy brown colour, nearly like that of 
 the dry cake held up to the light ; and while this colour remains, 
 it may be considered fit for almost every purpose. 
 
 Though glue w'hich has been newly melted is the most suita- 
 ble for use,* other circumstances being the same, yet that which 
 has been the longest manufactured is the best. To try the good- 
 ness of glue, steep a piece three or four days in cold water ; if it 
 swell considerably without melting, and when taken out resumes, 
 in a short time, its former dryness, it is excellent. If it be soluble 
 in cold water, it is a proof that it wants strength. 
 
 x\ glue which does not dissolve in water, may be obtained by 
 melting common glue with the smallest possible quantity of w'ater, 
 and adding by degrees linseed oil rendered drying by boiling it 
 w ith litharge ; while the oil is added, the ingredients must be well 
 stirred to incorporate them thoroughly. 
 
 A glue which will resist water, in a considerable degree, is 
 made by dissolving common glue in skimmed milk. 
 
 Finely levigated chalk added to the common solution of glue 
 in water, constitutes an addition which strengthens it, and renders 
 it suitable for sign-boards, or other things which must stand the 
 weather. 
 
 A glue that will hold against fire or w^at^r, may be prepared 
 by mixing a handful of quick lime with four ounces of linseed 
 .oil : thoroughly levigate the. mixture, boil it to a good thickness, 
 and then spread it on tin plates in the shade; it will become 
 (j.— VuL. 1. R 
 
122 
 
 MECHANICAL EXERCISES. 
 
 Proportions of mortises and tenons. 
 
 exceedingly hard, but may be dissolved over a fire, as ordinary 
 glue, and is then fit for use. 
 
 Several glues, such as that of isinglass, which, for a variety 
 of reasons, are not used in the common course of business among 
 joiners and cabinet-makers, we shall speak of particularly under 
 the head Cements. 
 
 Of the Mortise and Tenon, 
 
 The proportion which mortises and tenons ought to bear to 
 the wood, under different circumstances, has never been demon- 
 strated by experiments made for that purpose. It is common 
 practice, therefore, alone, which dictates the rules to be observed. 
 In general the tenon is one-third of the thickness of the stuff ; but 
 when the mortise and tenon are intended to lie horizontally, and 
 the juncture will be unsupported, the tenon should not be more 
 than one-fifth of the thickness of the stuff, otherwise a strain or 
 w^eight on the upper surface of the tenoned piece would probably 
 split off the under cheek of the mortise, while the tenon itself re- 
 mained sound. 
 
 In joining two pieces of timber, so that the tenoned piece 
 shall not pass the end of the mortised piece, to prevent the 
 necessity of cutting open the side of the mortise, the tenon must 
 be reduced one-third or at least one fourth of its breadth. In 
 either case, the mortise will still be so near the end of the piece 
 in which it is made, as to be split by a small force in driving in 
 the tenon. To prevent this accident, it is customary, on such oc- 
 casions as admit of the precaution being taken, to make the end 
 beyond the mortise considerably longer than it is intended to re- 
 main ; the tenon may then be driven tightly in, and the super- 
 fluous wood afterwards cut off. 
 
 The above proportions for the mortise and tenon, refer to the 
 joining of timber, like dimensions of which are of the same 
 strength, and therefore it will be necessary to vary them, accord- 
 ing as one piece is weaker or stronger than the other. 
 
 In making deep mortises, especially in hard wood, it is custo- 
 mary to shorten the labour, by commencing it with boring a num- 
 ber of auger holes, the compartments between which are speedily 
 cut aw'ay with the chisel. 
 
 In neat work, before a saw is employed to cut the shoulder of 
 a tenon, nick the place with a paring chisel ; the saw will not 
 then tear the wood, and the line of its entrance may be correctly 
 determined. 
 
 When the mortise is to pass entirely through a piece of stuff, 
 the space allotted for it is gauged on both sides with the greatest 
 precision, and when it has been half cut from one side, the 
 
MECHANICAL EXERCISES. 
 
 123 
 
 Concluding reflections. 
 
 remaining half is cut from the opposite one. By this means, if 
 there be any error in the direction of the chisel, it is of little con- 
 sequence, the irregularity being confined to the middle of the mor- 
 tise. The sides of the mortise should, however, be made as even 
 as possible, that they may every-where come in contact with the 
 sides of the tenon. 
 
 The sides of a mortise that passes through the stuff, should be 
 inclined to each other a little towards the shoulders of the tenon, 
 because the latter, after it is driven in, is expanded by wedges. 
 
 We cannot close these details of the primary operations by 
 which metals and timber are fitted for mechanical purposes, with- 
 out offering a few sentiments to the consideration of the young 
 artist interested in them, whether he is one who is anxious to ex- 
 cel in a particular branch of art, as affording the means of honour- 
 able livelihood ; or claims merely the appellation of an amateur, 
 who studies mechanical operations from the love of knowledge, 
 the desire of amusement, or the hope of celebrity in making 
 discoveries or improvements. Let him not be discouraged by 
 the failure of first attempts. Instead of losing his time in 
 Hseler^’y regretting his disappointment, let him examine into 
 t’ ^ cause of it, and promptly repeat his ex'periments with more 
 precaution. It is a mistaken idea, that manual dexterity is 
 absolutely dependent upon length of practice; uncommon are 
 the cases in which it fails to be the early reward of those who 
 unite perseverance — patient and prompt, with an attention ever 
 alive to avail themselves of every new light which the liberality 
 of others, or the course of their own experience supplies. But 
 those who postpone ardent exertion, by satisfying themselves 
 with the hope, that length of practice will perfect them, will 
 in the end regret their delusion, and may ineffectually try to re- 
 cover their loss, when habitual languor, and other injurious habits, 
 have rendered the mind averse to observe, and the hand unable to 
 perform. 
 
 Much might be said on this subject, but we forbear a length- 
 ened disquisition ; yet we cannot omit observing, that those who 
 take an early delight, and attain an early proficiency in mecha- 
 nical arts, are preparing an excellent groundwork for investiga- 
 tions of a higher order. They are acquiring, in the disguise of 
 amusement, that dexterity of hand, and facility of contrivance, 
 that readiness in supplying their casual wants, and habit of 
 methodical arrangement, which will afterwards qualify them to 
 explore the paths of knowledge through the medium of philo- 
 
124 
 
 MECHANICAL E^ERCISE^. 
 Concluding reflections. 
 
 sophical experiment; and in proportion as these interest their 
 attention, they will be disposed to study in earnest the fundamental 
 principles of science. 
 
 It may be well for the young artists we are addressing, to 
 be apprized, that to make any useful proficiency in mechanical 
 pursuits, to be distinguished for skill and promptitude of execu- 
 tion, requires a degree of patient assiduity, of which few who 
 have been brought up at the desk or behind the counter, can form 
 any adequate idea ; and who, therefore, would be uneasy and un- 
 happy, under a degree of exertion which they must learn to dis- 
 play without entertaining a sentiment of its hardship. Let them 
 not, however, be disheartened at the prospect; the habits indis- 
 pensable to their full success, if acquired, allowed their due influ- 
 ence, and guided by* moral prudence, are of inestimable value : 
 they are extending the means of multiplying their comforts ; they 
 are increasing the pow er of the head to contrive as well as of the 
 hand to execute, and the steadiness of attention superinduced, will 
 be beneficial to them in every action of their lives. 
 
 iUl knowy but few^ act as if they believed, tiiat.. accident out 
 of the question, success in the general issue of om exertions, 
 is made up of success in little matters, or individual operations. 
 Let us then be allowed to impress this truism upon the reader's 
 mind, and to elucidate it by stating, that if two workmen have 
 each fifteen thousand motions to make in ten hours, he who shall 
 perform each motion half a second more quickly than the other, 
 will terminate his labour tw o hours sooner ! Tw'o hours of 
 spare time may be employed to great advantage ; yet what is 
 half a second considered by itself.^ and by how many may this 
 economy of time be practised, without a perceptible addition 
 of fatigue ? The quantity of labour is no criterion of the 
 fatigue it will occasion, even to those who are alike in corporal 
 power. The sentiments by which we are actuated in the per- 
 formance of a task, have a paramount influence ; languor of 
 mind prematurely exhausts the body, and the weariness experi- 
 enced is often disproportioned to the animal strength possessed. 
 But our sentiments are much under our control, and it surely 
 should be our study to cherish those, which by inspiring alacrity 
 of exertion, are essentially conducive to our interest. To the la- 
 bourer, there is the prospect of commanding respect, and of bet- 
 tering his condition ; to the amateur, besides the common, daily 
 advantages of knowledge, and the indeprivable gratification which 
 the acquirement of it affords, there is the animating hope, that if 
 discoveries crown his exertions, he may be considered a benefactor 
 to mankind. 
 
ARCHITECTURE. 
 
 125 
 
 Introductory remarks. — Ancient architecture. 
 
 ARCHITECTURE. 
 
 The science of Architecture may be considered, in its most 
 extended application, to comprehend building of every kind ; 
 but at present we must consider it in one much more restricted, 
 according to which, Architecture may be said to treat of the 
 planning and erection of edifices, which are composed and em- 
 bellished after certain long established rules, according to two 
 principal modes, 
 
 1st, the English or Gothic, 
 
 2nd, the Antique, or Grecian and Roman. 
 
 We shall treat of these modes in distinct dissertations, 
 because their principles are completely distinct, and indeed 
 mostly form direct contrasts. But before we proceed to treaty 
 of them, it will be proper to make a few remarks on the dis- 
 tinction between mere house-building, and that higher character 
 of composition in the Grecian and Roman orders, which is pro- 
 perly styled Architecture ; for though we have now very many 
 noble architectural houses, we are much in danger of having 
 our public edifices debased, by a consideration of what is conve- 
 nient as a house, rather than what is correct as an architectural 
 design. 
 
 In order properly to examine this subject, vve must consider a 
 little, what are the buildings regarded as oiu* models for working 
 the orders, and in what climate, for what purposes, and under 
 what circumstances, they w^ere erected. This may, perhaps, lead 
 to some conclusions, w’hich may serve to distinguish that descrip- 
 tion of work, which, how’ever rich or costly, is still mere house- 
 building, in point of its composition. 
 
 It is acknowledged, on all hands, that our best models, in 
 the three ancient unmixed orders, — the Doric, Ionic, and 
 Corinthian, are the remains of Grecian temples. Most of them 
 were erected in a climate, in wdiich a covering from rain was by 
 no means essential, and we shall find this circumstance very in- 
 fluential ; for as the open space within the walls was always 
 partially, and often wdiolly open, apertures in those walls for 
 light, were unnecessary; and we find, also, in Grecian struc- 
 tures, very few, sometimes only one door. The purpose for 
 which these buildings w'ere erected, was the occasional reception 
 of a large body of people, and not the settled residence of any. 
 But, perhaps, the circumstances under which they were erected, 
 have had more influence on the rules which have been handed 
 
126 
 
 ARCHITECTURE 
 
 Ancient architecture. — Difference of mere building and architectural design 
 
 down to us, as necessary to be observed in composing architec- 
 tural designs, than either the climate or their use. It is now 
 pretty generally agreed, that the Greeks, if they were acquainted 
 with the mathematical properties of the arch, did not use it till 
 it was introduced by the Romans. Here then we see at once a 
 limitation of the intercolumniation, which must be restrained by 
 the necessity of finding stones of sufficient length to form the 
 architrave. Hence the smaller comparative intercolumniations 
 of the Grecian buildings, and the constant use of columns ; and 
 hence the propriety of avoiding arches, in compositions of the 
 purer Grecian orders. 
 
 The Romans introduced the arch very extensively, into 
 buildings of almost every description, and made several altera- 
 tions in the mode of working the orders they found in Greece, 
 to which they added one order, by mixing the Corinthian and 
 Ionic, and another by stripping the Doric of its ornaments, 
 riieir climate, also, was so far different as to require more 
 general roofing, but still, from the greater necessity of providing a 
 screen from the heat of the sun, than apertures to admit the light, 
 it does not appear that large windows were in general use, and 
 hence an important difference in modern work. Although, by 
 roofs and arches, much more approximated to modern necessities 
 than the Grecian models, still those of Rome which can be re- 
 garded as models of composition, are temples, or other public 
 edifices, and not domestic buildings, which, whenever they have 
 been found, appear variously unadapted to modern wants, and 
 therefore unfit for imitation. 
 
 In a few words, we may sum up the grand distinctions be- 
 tween mere building and architectural design : — the former looks 
 for convenience, and though it will doubtless often use architec- 
 tural ornaments, and preserve their proportions, when used as 
 smaller parts, yet the general proportion may vary very widely 
 from the orders, and yet be pleasing, and perhaps not in- 
 correct ; — but all this is modem building, and not architecture 
 in its restricted sense ; in this the columns are essential parts, 
 and to them and their proportions all must be made subservient ; 
 and here we may seek, with care and minuteness, amongst the 
 many remains yet left in various parts, (and of which the best are 
 familiar to most, from the valuable delineations we possess of 
 those who have accurately examined them,) for models, and in 
 selecting and adopting these, the taste and abilities of the archi- 
 tect has ample space. 
 
 As an introduction to the dissertations, it may not be amiss to 
 take a hasty sketch of the progress of Architecture in England. 
 
 Of the British architecture, before the arrival of the Ro- 
 
ARCHITECTURE. 
 
 127 
 
 British architecture. 
 
 mans in the island, we have no clear account; but it is not 
 likely it differed much from the ordinary modes of uncivilized 
 nations ; the hut of wood with a variety of coverings, and some- 
 times the cavities of the rock, were doubtless the domestic 
 habitations of the aboriginal Britons ; and their stupendous pub- 
 lic edifices, such as Stonehenge and others, still remain to us. 
 The arrival of the Romans was a new era ; they introduced, at 
 least in some degree, their own architecture, of which a variety 
 of specimens have been found ; some few still remain, of which, 
 perhaps, the gate at Lincoln is the only one retaining its original 
 use. Although some fine specimens of workmanship have been 
 dug up in parts, yet by far the greatest part of the Roman work 
 was rude, and by no means comparable with the antiquities of 
 Greece and Italy, though executed by the Romans. When they 
 left the island, it was most likely that the execution of the Bri- 
 tons was still more rude, and endeavouring to imitate, but not 
 working on principle the Roman work, their architecture became 
 debased into the Saxon and early Norman, intermixed wdth orna- 
 ments perhaps brought in by the Danes. After the conquest, 
 the rich Norman barons, erecting very magnificent castles and 
 churches, the execution manifestly improved, though still with 
 much similarity to the Roman mode debased ; but the introduce 
 tion of shafts, instead of the massive pier, first began to approach 
 that lighter mode of building, which, by the introduction of the 
 pointed arch, and by an increased delicacy of execution, and 
 boldness of composition, ripened, at the close of the twelfth cen- 
 tury, into the simple, yet beautiful early English style. At the 
 close of another century, this style, from the alteration of its win- 
 dows, by throwing them into large ones, divided by mullions, 
 introducing tracery in the heads of windows, and the general 
 use of flowered ornaments, together with an important alteration 
 in the piers, became the decorated English style, \vhich may be 
 considered as the perfection of the English mode. This was 
 very difficult to execute, from its requiring flowing lines 
 where straight ones were easier combined ; and at the close 
 of the fourteenth century, we find these flowing lines giving 
 way to perpendicular and horizontal ones, the use of which con- 
 tinued to increase, till the arches were almost lost in a continued 
 series of pannels, which, at length, in one building, the chapel 
 of Henry the VII. covered completely both the outside and 
 inside, and the eye, fatigued by the constant repetition of small 
 parts, sought in vain for the bold grandeur of design which had 
 been so nobly conspicuous in the preceding style. The refor- 
 mation, occasioning the destruction of many of the buildings 
 tlie most celebrated, and mutilating others, or abstracting the 
 
128 
 
 ARCHITECTURE. 
 
 British architecture. 
 
 funds necessary for their repair, seems to have put an end to the 
 working of the English styles on principle. The square pan- 
 iielled mullioned windows, and wooden pannelled roofs and 
 halls, of the great houses of the time of queen Elizabeth, seem 
 rather a debased English than any thing else ; but during the 
 reign of her successor, the Italian architecture began to be 
 introduced, hist only in columns of doors, and other small 
 parts, and afterwards in larger portions, though still the general 
 style was this debased English. Of this introduction, the most 
 memorable was the celebrated portico of the schools at Oxford, 
 \ihere, into a building adorned with pinnacles, and having 
 mullioned windows, the architect has crowded all the hve 
 orders over each other. Some of the works of Inigo Jones are 
 little removed beyond this barbarism. Longleat, in Wiltshire, 
 is a little more advanced, and the banqueting-house, Whitehall, 
 seems to mark the complete introduction of Roman workman- 
 ship. The close of the seventeentli century produced Sir Chris- 
 topher Wren, a man whose powers, confessedly great, lead us 
 to regret he had not studied the architecture of his English an- 
 cestors, with the success which he did those of Rome ; for while 
 he has raised the most magnificent modern building we possess, 
 he seems to have been pleased to disfigure the English edifice 
 he had to complete. While his _works at St. Mary, Aldermary, 
 and St. Dunstan in the East, prove how well he could execute 
 imitated English buildings when he chose, though even in them 
 he has variously departed from the true English principles. By 
 the end of the seventeenth century, the Roman architecture seems 
 well established, and the works of Vitruvius and Palladio success- 
 fully studied ; but Sir John Vaubrugh and Nicholas Haw'ksmoor 
 seem to have endeavoured to introduce a massiveness of s4yle 
 which happily is peculiar to themselves. The works of Palladio, 
 as illustrated by some carpenters, seem to have been the model 
 for working the orders during the greatest part of the eighteenth 
 century, but in the early and middle part of it, a style of orna- 
 ment borrowed from the French was much introduced in interiors, 
 the principal distinctions of which were the absence of all straight 
 lines, and almost of any regular lines. 
 
 The examples of this are now nearly extinct, and seem to 
 have been driven out by the natural operation of the advance of 
 good w^orkmanship in the lower class of buildings. All orna- 
 mental carvings were difficultly executed in wood, and were very 
 expensive ; but tow’ards the latter end of the eighteenth century, 
 the Adams’s introduced a style of ornament directly contrary to 
 the heavy carving of their predecessors. This was so fiat, as to 
 be easily worked in plaster and other compositions, and orna- 
 
AECHITECTl'RE. 
 
 129 
 
 Four styles of English architecture. 
 
 ■ * 
 
 merit being sold very cheap, was profusely used in carpenters* 
 work. This flatness was more or less visible in many consi- 
 derable buildings ; but near the close of the century, the magni- 
 ficent works of Stuart and Revet, and the Ionian antiquities of 
 the Dilletante Society, began to excite the public attention, and 
 in a few years a great alteration was visible ; the massive Doric, 
 and the beautiful plain Grecian Ionic, began to be worked, and 
 our ordinary door cases, 8cc. soon began to take a better cha- 
 racter. The use of the simple, yet bold mouldings and orna- 
 ments of the Grecian models, is gradually spreading, and per- 
 liaps we may hope, from the present general investigation of the 
 principles of science, that this will continue without danger of 
 future debasement, and that a day may come when we shall have 
 Grecian, Roman, and English edifices, erected on the principles 
 of each. 
 
 As the earliest in point of execution in England, we shall 
 begin w ith the dissertation on 
 
 ENGLISH ARCHITECTURE, 
 
 In a w ork like the present, there will be little propriety in 
 lengthened disquisitions on. the origin of this mode of build- 
 ing ; but much service may be rendered to individuals, by a 
 clear detail of those distinctions which, being once laid dow’ii 
 with precision, will enable persons of common observation, to 
 distinguish the difference of age and style in these buildings, as 
 easily as the distinctions of the Grecian and Roman orders. 
 
 During the eighteenth century, various attempts, under the 
 name of Gothic, have arisen in repairs and rebuilding of eccle- 
 siastical edifices; but these have been little more than making 
 clustered columns and pointed windows, every real principle of 
 English architecture being, by the builders, either unkiiowm or 
 totally neglected. 
 
 English architecture, then, which has been too long called 
 Gothic, may be divided into four distinct periods, or styles, which 
 may be named, 
 
 1st, the Norman style, 
 
 2nd, the Early English style, 
 
 3rd, the Decorated English style, and 
 4th, the Perpendicular English style. 
 
 The dates of these styles we shall state hereafter, and it may 
 be proper to notice, that the clear distinctions are now almost 
 entirely confined to churches ; for the destruction and alteration 
 of castellated buildings has been so great, from the alterations of 
 a-VoL.I. S 
 
130 
 
 ARCHITECTURE. 
 
 Definitions. 
 
 the nTodes of warfare, &e. that, in them, we can hardly tell which 
 is original and which addition. 
 
 ' Before we enter on a description of the styles separately, it 
 will be necessary to explain a fe\v terms which are made use of 
 in describing churches, &c. and without understanding which, it 
 will be impossible to comprehend the subject clearly. 
 
 Most of the ancient ecclesiastical edifices, when considered 
 complete, were built in the form of a cross, with a tower, lantern, 
 or spire erected at the intersection. The interior space was usu- 
 ally thus divided : 
 
 The space westward of the cross is called the nave. 
 
 The divisions outw ard of the piers are called aisles. 
 
 The space eastward of the cross is generally the choir. 
 
 The part running north and south is called the cross or 
 transept. 
 
 The choir is generally enclosed by a screen, on the western 
 part of which is usually placed the organ. 
 
 The choir, in cathedrals, does not generally extend to the 
 eastern end of the building, but there is a space behind the altar, 
 usually called the lady chapel. 
 
 The choir is only between the piers, and does not include 
 the side aisles, which serve as passages to the lady chapel, 
 altar, &c. 
 
 The transept has sometimes side aisles, which are often sepa- 
 rated by screens for chapels. 
 
 Chapels are attached to all parts, and are frequently addi- 
 tions. 
 
 The aisles of the nave are mostly open to it, and in cathedrals 
 both are generally without pew s. 
 
 In churches not collegiate, the eastern space about the altar is 
 called the chancel. 
 
 To the sides are often attached small buildings over the 
 doors, called porches, which have sometimes vestries, schools, &c. 
 over them. 
 
 The font is generally placed in the western part of the nave, 
 but in small churches its situation is very various. 
 
 In large churches, the great doors are generally either at the 
 west end, or at the end of the transepts, or both ; but in small 
 churches, often at the sides. 
 
 To most cathedrals are attached a chopter-house and cloisters, 
 which are usually on the same side. 
 
 The chapter-house is often multangular. 
 
 The cloisters are generally a quadrangle, with an open space 
 in the centre ; the side to which is a series of arches, originally 
 glazed, now mostly open. The other wall is generally one side 
 
ARCHITECTURE. 
 
 131 
 
 Definitions. 
 
 of the church or other buildings, with which the cloisters commu- 
 nicate by various doors. The cloisters are usually arched over, 
 and formed the principal communication between the different 
 parts of the monastery. 
 
 The lady chapel is not always at the east end of the choir; 
 at Durham it is at the west end of the nave ; at Ely, on the 
 nortli side. 
 
 The choir sometimes advances westward of the cross, as at 
 Westminster. 
 
 The spaces in the interior, between the arches, are piers. 
 
 The windows above the arches, which appear on the out- 
 side, over the roof of the aisles, are called clerestory windows. 
 
 i\ny building above the roof may be called a steeple. If it be 
 square-topt, it is called a tower. 
 
 A tower may be round, square, or multangular. The tower 
 is often crowned wdth a spire, and sometimes with a short tower 
 of light work, which is called a lantern. An opening into the 
 tower, in the interior, above the roof, is also called a lantern. 
 
 Tow'ers of great height in proportion to their diameter, are 
 called turrets; these often contain staircases, and are sometimes 
 crowned with small spires. 
 
 Large towers have often turrets at their corners, and often one 
 larger than the others, containing a staircase ; sometimes they have 
 only that one. 
 
 The projections at the corners, and between the windows, are 
 called buttresses. 
 
 The walls are crowned by 2 l parapet y which is straight at the 
 top, or a battlement j which is indented ; both may be plain, or 
 sunk pannelled, or pierced. 
 
 Arches are round, pointed, or mixed ; 
 
 A semicircular arch has its centre in the same line with its 
 spring. 
 
 A segmental arch has its centre lower than the spring, 
 
 A horse-shoe arch has its centre above the spring. 
 
 Pointed arches are either, equilateral — described from two 
 centres, which are the whole breadth of the arch from each other, 
 and form the arch about an equilateral triangle ; or drop arches, 
 which have a radius shorter than the breadth of the arch, and are 
 described about an obtuse-angled triangle ; or lancet arches, which 
 have a radius longer than the breadth of the arch, and are de- 
 scribed about an acute-angled triangle. 
 
 All these pointed arches may be of the nature of segmental 
 arches, and have their centres bel6w their spring. 
 
 Mixed arches are of three centres, which look nearly like 
 ^iptical arches ; or of four centres, commonly called the Tudor 
 
132 
 
 ARCHITECTUKE. 
 
 Definitions. 
 
 arch ; this is flat for its span, and has two of its centres in or near 
 the spring, and the other two far below it. 
 
 The ogee or contrasted arch, has four centres ; two in or near 
 the span, and two above it, and reversed. 
 
 The spaces included between the arch and a square formed 
 at the outside of it, are called spandrellsj and are often orna- 
 mented. 
 
 Windows are divided into lights by mullions. 
 
 The ornaments of the divisions at the heads of windows, &c. 
 are called tracery. Tracery is either flowings where the lines 
 branch out into leaves, arches, &c.; or perpendicular j where the 
 mullions are continued through in straight lines. 
 
 The horizontal divisions of windows, &c. are called tran~ 
 soms. 
 
 The parts of tracery are ornamented with small arches and 
 points, which is called feathering or foliation^ and the small 
 arches cusps^ and according to the number in immediate connec- 
 tion, they are called trefoilj quatrefoilj cinquefoil, &c. for which 
 see the plate. 
 
 The cusps are sometimes again feathered, and this is called 
 double feathering. 
 
 Tablets are small projecting mouldings, or strings, mostly 
 horizontal. 
 
 The tablet at the top, under the battlement, is called a cor- 
 nice, and that at the bottom a basement, under which is generally 
 a thicker wall. 
 
 The tablet running round doors and windows, is called a drip^ 
 stone, and if ornamented, a canopy. 
 
 Bands are either small strings round shafts, or a horizontal 
 line of square, round, 8cc. pannels, used to ornament towers, 
 spires, 8cc. 
 
 Niches are small arches, mostly sunk in the wall, and orna- 
 mented often very richly with buttresses, canopies, &c. 
 
 A corbel is an ornamented projection from the wall, to sup- 
 port an arch, niche, ^^c. and is often a head or part of a figure or 
 animal. 
 
 A pinnacle is a small spire, generally square and ornamented, 
 which is usually placed on the tops of buttresses, both external 
 and internal. 
 
 The small bunches of foliage ornamenting canopies, pinna- 
 cles, &,c. are called crockets. 
 
 The larger bunches on the top are called and this term 
 
 is sometimes applied to the whole pinnacle. 
 
 The seats for the dean, canons, Sec. in the choirs of collegiate 
 churches, are called stalls. 
 
ARCHITECTURE, 
 
 133 
 
 j ^ 
 
 I Definitions. — Division of the subject. 
 
 f | The bishop’s seat is called his throne. 
 
 ^Die ornamental open work over the stalls, and in general any 
 minute ornamental open work, is called tabernacle work. 
 
 . " . In some churches, not collegiate, there yet remains a screen, 
 
 I with a large projection at the top, between the nave and chancel, 
 i on which w as anciently placed certain images ; this was called a 
 ' rood loft. 
 
 Near the entrance door is sometimes found a small niche, 
 
 I w ith a basin, which held, in catholic times, their holy water ; these 
 are called stoups. 
 
 Near the altar, or at least where some altar has once been 
 If placed, there is sometimes found another niche, distinguished 
 
 I I from the stoup by having a small hole at the bottom to carry off 
 the remains of the consecrated wane ; this is called a piscina ; it 
 
 i;i is often double, with a place for the bread. 
 
 On the south side, at the east end of some churches, are found 
 iij stone stalls, either one, two, or three ; of which the uses have 
 been much contested. 
 
 ’ Under several large churches, and some few small ones, are 
 
 I certain vaulted chapels, these are called crypts. 
 
 In order to render the comparison of the different styles easy, 
 we shall divide the description of each into the following sec- 
 tions : 
 
 Doors, 
 
 AVindow's, 
 
 jArches, 
 
 Piers, 
 
 Buttresses, 
 
 Tablets, 
 
 Niches, and ornamental arches, or pannels, 
 
 Ornamental carvings, 
 
 Steeples ; 
 
 And at the end of the styles will be noticed, in one series, 
 the sections of battlements and roofs. 
 
 We shall first give, at one view, the date of the styles, and 
 their most prominent distinctions, and then proceed to the parti- 
 cular sections as described above. 
 
 1st, the NormaTi Style, which prevailed to the end of the 
 reign of Henry II, in 1189; distinguished by its arches being 
 generally semi-circular, and not pointed, with bold and rude orna- 
 ments. This style seems to have commenced before the con- 
 quest, but we have no remains really known to be more than a 
 very few years older 
 
 2nd, the Early English Style, reaching to the end of the 
 reign of Edward I, in 1307 ; distinguished by pointed arches. 
 
134 
 
 ARCHITECTURE 
 
 Norman doors. 
 
 and long narrow windows, without mullions ; and a peculiar 
 toothed ornament, more fully described hereafter. 
 
 3d, Decorated English, reaching to the end of the reign of 
 Edward III, in 1377, and perhaps from ten to fifteen years 
 longer. This style is distinguished by its large windows, which 
 have pointed arches divided by mullions, and the tracery in flow- 
 ing lines of circles, arches, &c. and not running perpendicularly ; 
 its ornaments rich, and very delicately carved ; and ornament used 
 to a very great extent, yet seldom crowded. 
 
 Perpendicular English. This is the last style, and appears 
 to have been in use, though much debased, even perhaps as 
 far as to 1630 or 1640, but only in additions. Probably the 
 latest whole building is not later than Henry the VIII. The 
 name clearly designates this style, for the mullions of the win- 
 dows, the ornamental pannelling, &c. run in perpendicular lines, 
 and form a complete distinction from the last style, and the richer 
 buildings are often so crowded with ornament, as to destroy the 
 beauty of the design. The carvings are generally very delicately 
 executed. 
 
 It may be necessary to state, that though many writers speak 
 of Saxon buildings, those which they describe as such, are either 
 known to be Norman, or are so like them, that there is no real 
 distinction. But it is most likely, that in some obscure country 
 church tower, &c. some real Saxon w'ork of a much earlier date 
 may exist ; hitherto, however, none has been ascertained to be of 
 so great an age. 
 
 We shall now begin to trace 
 
 The First, or Norman Style. 
 
 Normati Doors. 
 
 There seems to have been a desire in the architects who suc- 
 ceeded the Normans, to preserve the doors of their predecessors, 
 whence we have so many of these noble, though, in most cases, 
 rude elforts of skill remaining. In many small churches, 
 where all has been sw ept away, to make room for even perpen~ 
 dicular alterations, the Norman door has been suffered to re- 
 main. They are varied, yet there is no prominent distinction 
 to make it necessary to subdivide them. » The arch is semi-circu- 
 lar, and the mode of increasing their richness, was by increasing 
 the number of bands of moulding, and of course the depth of 
 the arch. Shafts are often used, but not always, and we find 
 very frequently, in the same building, one door with shafts, 
 and one without. When shafts are used there is commonly 
 an impost moulding above them, before the arch mouldings 
 spring. These mouldings are generally much oruameAted, and 
 
AECHITFXTURE. 
 
 135 
 
 Norman windows. 
 
 the wave or zigzag ornament in some of its diversities, is almost 
 universal, as is a large round moulding, with the heads on the 
 outer edge projecting their beaks over this moulding. There 
 are also mouldings w ith a series of figures enclosed in a running 
 ornament; and at one church at York, these figures are the 
 zodiacal signs. The exterior moulding often goes dowm no lower 
 than the spring of the arch, thus forming an apparent dripstone, 
 though it does not project so as really to form one. The door 
 is often square, and the interval to the arch filled with mould- 
 ings. Amongst the great variety of these doors in excellent pre- 
 servation, it is difficult to point out particulars, but Iffley church, 
 near Oxford, is perhaps the best specimen, as it contains three 
 doors, all of which are different; and the south door is nearly 
 unique, from the flowers in its interior mouldings. South Ock- 
 enden church, in Essex, has also a door of uncommon beauty of 
 design and elegance of execution. Durham, Rochester, Wor- 
 cester, and Lincoln cathedrals, have also fine Norman doors. In 
 these doors, almost all the ornament is external, and the inside 
 often quite plain. 
 
 There does not appear to have been any double Norman 
 doors. 
 
 'Norman Windows, 
 
 The window’s, in this style, are diminutive doors as to their 
 ornaments, except that, in large buildings, shafts are more fre- 
 quent, and often with plain mouldings. The size of these win- 
 dows is generally small, seldom, except in very large buildings, 
 so large as even a small door ; there are no mullions ; the 
 arch is semi-circular, and if the window is quite plain, generally 
 sloped sides, either inside or out, or both ; the bottom often 
 nearly horizontal. The proportions of the Norman windows are 
 generally those of a door, and very rarely, if ever, exceed two 
 squares in height, of the exterior proportions, including the or- 
 naments. 
 
 The existing Norman window’s are mostly in buildings retain- 
 ing still the entire character of that style ; for in most they have 
 been taken out, and others of later styles put in, as at Durham 
 and many other cathedrals. 
 
 There are still remaining traces of a very few circular .win- 
 dows of this style ; the west w’indow at Iffley was circular, but 
 it is taken out ; there is one in Canterbury cathedral, which 
 seems to be Norman; and there is one undoubtedly Norman at 
 Barfreston, rendered additionally singular, by its being divided by 
 grotesque heads, and something like mullions, though very rude, 
 into eight parts. 
 
136 
 
 ARCHITECTURE. 
 
 Norman arches, — piers. 
 
 There seems to have been little if any attempt at feathering or 
 foliating the heads of Norman doors or windows : but there is a 
 singular door in the cloisters at Chester, with a semi-circular 
 head, that has an ornament of this kind, but from its situation, 
 and the alterations which have been made, it is uncertain whether 
 it is original or not. 
 
 Norman Arches. 
 
 The eai'ly Norman arches are semi-circular, and in many 
 instances this form of the arch seems to have continued to the 
 latest date, even when some of the parts were quite advanced 
 into the next style; of this the Temple church is a curious 
 instance ; here are piers with some of the features of the next 
 style, and also pointed arches with a range of intersecting arches, 
 and over this the old round-headed Norman window. But though 
 the round arch thus continued to the very end of the style, the 
 introduction of pointed arches must have been much earlier, for 
 we tind intersecting arches in buildings of the purest Norman, 
 and whoever constructed them, constructed pointed arches ; but 
 it appears as if the round and pointed arches were for nearly a 
 century used indiscriminately, as was most consonant to the ne- 
 cessities of the work, or the builders ideas. Kirkstall abbey has 
 all its work exteriorly round arches, but the nave has pointed 
 arches in the interior. There are some Norman arches so near 
 a semi-circle as to be only just perceptibly pointed, and still w ith 
 the rudely carved Norman ornaments. 
 
 There are a few Norman arches of very curious shape, 
 being more than a semi-circle, or what is called a horse-shoe, and 
 in a few instances a double arch. These arches have sometimes 
 plain faces, but are much oftener ornamented with the zigzag, and 
 other ornaments peculiar to this style. 
 
 Norman Piers. 
 
 Tliese are of four descriptions, 1st, Tlie round massive colum- 
 nar pier, which has sometimes a round, and sometimes a square 
 capital ; they are generally plain, but sometimes ornamented with 
 channels in various forms, some plain zigzag, some like network, 
 and some spiral. They are sometimes met with but little more 
 than two diameters high, and sometimes are six or seven, and 
 those with square-headed capitals are generally the tallest. 
 
 2d, A multangular pier, much less massive, is sometimes 
 used, generally octagonal, and commonly with an arch more or 
 less pointed. 
 
 3d, The common pier with shafts ; these have sometimes 
 plain capitals, but sometimes much ornamented with rude 
 
ARCHITECTURE. 
 
 137 
 
 ' Norman buttresses — tablets — niches — ornaments. 
 
 foliage, and occasionally animals. The shafts are mostly set in 
 square recesses. 
 
 4th, A plain pier, with perfectly plain round arches in two or 
 three divisions. 
 
 In some cases, the shafts are divided by bands, but the in- 
 stances are very few. 
 
 No?'?na?i Buttresses. 
 
 Tliese require little description ; they are plain broad faces, 
 with but small projection, often only a few inches, and often run- 
 ning up only to the cornice tablet, and there finishing under its 
 projection. Sometimes they are finished with a plain slope, and 
 in a few instances are composed of several shafts. Bands or 
 tablets running along the walls, often run round the buttresses. 
 
 Norma?i Tablets. 
 
 In treating of tablets, that which is usually called the 
 comice is of the first consideration ; this is frequently only a 
 plain face of parapet, of the same projection as the buttresses ; 
 but under it there is often placed a row of blocks, sometimes 
 plain, sometimes carved in grotesque heads, and in some in- 
 stances the grotesque heads support small arches, in w’hich case it 
 is called a corbel table. A plain string is also sometimes used as 
 a cornice. 
 
 The next most important tablet is the dripstone, or outer 
 moulding of windows and doors; this is sometimes undistin- 
 guished, but oftener a plain round or square string, frequently 
 continued horizontally from one window -to another round the 
 buttresses. 
 
 The other tablets under windows, &c. are generally plain 
 slopes above or below a flat string. In tlie interior, and in some 
 instances in the exterior, these are much carved in the various or- 
 naments described hereafter. 
 
 'Norman Niches, 
 
 These, if so they may be called, are a series of small arches 
 with round and often with intersecting arches, sometimes without, 
 but oftener with shafts. Some of these arches have their mould- 
 ings much ornamented ; but very few, if any, appear to have been 
 intended for statues. 
 
 Norman Ornaments. 
 
 The ornaments of this style consist principally of the different 
 kinds of carved mouldings surrounding doors and windows, and 
 used as tablets. The first and most frequent of these is the 
 e.—VoL.i. T 
 
138 
 
 ARCHITECTURE. 
 
 Norman steeples. 
 
 zigzag or chevron moulding, which is generally used in great 
 profusion. The next most common on door mouldings, is the 
 beak-head moulding, consisting of a hollow and a large round ; 
 in the hollow are placed heads of beasts or birds, whose tongues 
 or beaks encircle the round. After these come many varieties, 
 almost every specimen having some difference of composition; 
 a good collection of these may be seen in the Archaeologia, and 
 King’s Munimenta Antiqua. 
 
 The capitals of piers and shafts are often very rudely carved 
 jn various grotesque devices of animals and leaves ; but in all, the 
 design is rude, and the plants are unnatural, 
 
 Norman Steeples, 
 
 The Norman steeple was mostly a massive tower, seldom 
 rising more than a square in height above the roof, and often not 
 so much. They are sometimes plain, but often ornamented by 
 plain or intersecting arches, and have generally the flat buttress, 
 but that of St. Alban’s runs into a round turret at each corner of 
 the upper stage ; and at St. Peter’s, Northampton, there is a sin- 
 j»ular buttress of three parts of circles, but there is some doubt 
 if it is not an addition. It does not seem likely we have any Nor- 
 man spires, but there are some turrets crowned with large pinna- 
 cles, which may be Norman— such is one at Cleve in Gloucester- 
 shire, and one of the towers at the side of the west front of Ro- 
 chester cathedral. 
 
 Having gone through the parts, it remains to speak of the 
 general appearance of the Norman buildings, of which we have 
 very few, if any, remaining unaltered. Almost all the west 
 fronts, and many transept ends, have had new windows, but 
 some small churches remain nearly entire. These present ap» 
 pearances of great solidity, but not much beauty; the exterioi 
 doors being generally the best portion. But though heavy and 
 dark, from the smallness of the windows, some of the large Nor- 
 man edifices, when complete, were very magnificent. Amply to 
 show this, enough remains at Durham, Southwell, Gloucester, 
 Rochester, 8cc. In imitating or restoring buildings of this style, 
 the work should not be polished too highly, as all the Norman 
 work remaining, is, however difficult of execution, still rude, and 
 not finely polished. 
 
ARCHITECTURE. 
 
 139 
 
 Early English doors. 
 
 Of the Second, or Eaely English Style; 
 
 Early English Doors, 
 
 As the Norman doors may be said to be all of semi-circular 
 arches, these may be said to be all pointed, at least all the 
 exterior ornamented ones; for there are small interior doors of 
 this style with flat tops, and the sides of the top as it were 
 supported by a quarter circle from each side. The large doors 
 of this style are mostly double, the two being divided by either 
 one shaft or several clustered,, and a quatrefoil or other ornament 
 over them. These doors are often as finely recessed as the Nor- 
 man, but the bands and shafts are more numerous, being smaller; 
 and in the hollow mouldings they are frequently enriched W'ith 
 the peculiar ornament of this style,, a singular toothed projection, 
 which, when well executed, has a fine eifect. But although this 
 ornament is often used, (and sometimes a still higher enriched 
 moulding or band of open-work flowers,) there are many doors of 
 this style perfectly plain. Of this kind the door of Christchurch, 
 Hants, is a fine specimen.. 
 
 The dripstone is generally clearly marked, and often small, 
 and supported by a head. In many doors, a trefoil and even 
 cinquefoil feathering is used, the points of which generally finish 
 with balls, roses, or some projecting ornament. The principal 
 moulding of these doors has generally an equilateral arch, but 
 from the depth and number of the mouldings, the exterior 
 becomes often nearly a semi-circle. In interiors, and perhaps 
 sometimes too in the exterior, there are instances of doors with 
 a trefoil-headed arch. The shafts attached to these doors are 
 generally round, but sometimes filleted, and they generally, 
 but not always, stand quite free in a hollow moulding. They 
 have a variety of capitals, many plain, but many with delicate 
 leaves running up and curling round under the cap moulding, often 
 looking like Ionic volutes. The bases are various, but a plain 
 round and fillet is often used, and the reversed ogee sometimes 
 introduced. All these mouldings are cut with great boldness, 
 the hollows form fine deep shadows, and the rich bands of open- 
 work leaves are as beautiful as at any subsequent period, being 
 sometimes entirely hollow, and having no support but the attach- 
 ment at the sides, and the connexion of the leaves themselves. 
 Of these doors, though they are not so numerous as the Norman, 
 many still remain in perfect preservation; York, Lincoln, Chi- 
 chester, and Salisbury, have extremely fine ones ; and Beverley 
 minster one, of w'hich the mouldings are bolder than most of 
 them. The door of the transept at York, and those of the choix 
 
140 
 
 ARCHITECTURE. 
 
 Early English windows. 
 
 screen at Lincoln, have bands of tlie richest execution — and there 
 is a fine double door at St. Cross. 
 
 There are many wooden doors, both of this style and the Nor- 
 man, which seem to be of the same age as the stone work, and 
 some very curiously ornamented with ramifications of iron-work 
 from the hinges, &c. 
 
 Earli/ English Windows. 
 
 These are, almost universally, long, narrow, and lancet-headed, 
 generally w ithout feathering, but in some instances trefoiled. 
 
 From this single shape of windows, a variety of appearance 
 results from their combination. At Salisbury, one of the earliest 
 complete buildings remaining, there are combinations of two, 
 three, five, and seven. Where there are two, there is often a 
 trefoil, quatrefoil, &c. between the heads ; and in large buildings, 
 where there are three or more, they are often divided by so small 
 a division as to seem the lights of a large w indow, but they are 
 really separate windows, having their heads formed from indivi- 
 dual centres, and in general separate dripstones. This is the case 
 even at Westminster, where they approach nearer to a division by 
 mullions, from having a small triangle pierced beside the quatre- 
 foil, and a general dripstone over all. In small buildings, these 
 are generally plain, with the slope of the opening considerable, 
 and in some small chapels the windows are very narrow and long. 
 In larger buildings they are often ornamented with very long and 
 slender shafts, which are frequently banded. Most of our cathe- 
 drals contain traces of wdndow s of this character, but some, as at 
 Durham, have tracery added since their original erection. Salis- 
 bury, Chichester, Lincoln, Beverley, and York, still remain pure 
 and beautiful ; at York north transept are windows nearly fifty 
 feet high, and about six or eight w ide, which have a very fine 
 effect. Although the architects of this style worked their ordi- 
 nary w indows thus plain, they bestowed much care on their cir- 
 cles. Beverley minster, York, Durham, and Lincoln, have all 
 circles of this style peculiarly fine, and there may be many others ; 
 that of the south transept at York, usually called the marygold 
 window', is extremely rich, but the tracery of the circles at West- 
 minster is of a much later date. 
 
 There is in all the long windows of this style, one almost 
 universal distinction ; from the straight side of the window open- 
 ing, if a shaft is added, it is mostly insular, and has seldom any 
 connection with this side, so as to break it into faces, though the 
 shafts are inserted into the sides of the doors, so as to give great 
 variety to the opening. 
 
ARCHITECTURE 
 
 141 
 
 Early English arches — piers. 
 
 At Westminster abbey, there are a series of windows above 
 those of the aisles, which are formed in spherical equilateral 
 triangles. 
 
 Early English Arches, 
 
 The window arch of this style being generally a lancet arch, 
 and some persons having considered the shape of the arch to 
 be a very distinguishing feature of the differeiU styles, it may 
 be necessary in this place to say a few words on arches generally. 
 If we examine with care the various remains of the different 
 styles, we shall see no such constancy of arch as has been ap- 
 prehended; for there are composition lancet arches, used both 
 at Henry the Vll.’s chapel, Westminster, and at Bath; and there 
 are dat segmental arches in the early English part of York ; 
 and upon the whole it will appear, that the architect was not 
 confined to any particular description of arch. The only arch 
 precisely attached to one period, is the four-centered arch 
 which does not appear in windows, &c. if it does in the composi- 
 tion of groins, before the perpendicular style. In large buildings, 
 the nave arches of the early English style were often lancet, but 
 in some large and many small ones, they are flatter, some of 
 one-third drop, and perhaps even more, and sometimes pointed 
 segmental. 
 
 At Canterbury, in the choir, are some curious pointed horse- 
 shoe arches, and perhaps, though not common, they may be found 
 in other places. 
 
 The architraves of the large arches of rich buildings are now 
 beautifully moulded like the doors, with rich, deep, hollow^ mould- 
 ings, often enriched with the toothed ornament. Of this descrip- 
 tion, York transepts, and the nave and transepts of Lincoln, are 
 beautiful specimens ; Salisbury is worked plainer, but not less 
 really beautiful, and Westminster abbey is (the nave at least) 
 nearly plain, but with great boldness of moulding. 
 
 The arches of the gallery in this style, are often with tre- 
 foiled heads, and the mouldings running round the trefoil, even 
 to the dripstone; Chester choir is a fine specimen, and there 
 are some beautiful plain arches of this description in Winchester 
 cathedral. 
 
 Early English Piers, 
 
 Of the piers of large buildings of this style, there are two 
 distinguishing marks; first, the almost constant division of the 
 shafts which compose them, by one or more bands in their 
 length, and secondly their being ranged circularly round the 
 centre, In geiieral they are few, sometimes only four, some* 
 
142 
 
 ARCHITECTURE. 
 
 Early English buttresses. 
 
 times eight, set round a large circular one ; such are the piers of 
 Salisbury and of Westminster abbey; there aie sometimes so 
 many as nearly to hide the centre shaft, as at Lincoln and York ; 
 but the circular arrangement is still preserved, and there are some 
 few, as at the choir at Chester, which come so near the appear- 
 ance of decorated piers, as to be almost alone distinguishable by 
 this circular arrangement. 
 
 The capitals of these shafts are various; in many, perhaps 
 the greater number of buildings, they are plain, consisting of 
 a bell with a single or double annulet under it, and a sort of 
 coping, with more annulets above, and these mouldings are 
 continued round the centre pier, so as to form a general capital. 
 The dividing bands are also formed of annulets, and fillets, and 
 are often continued under windows, &c. as tablets, and are, like 
 the capitals, continued round the centre shaft. Another and 
 richer capital is sometimes used, which has leaves like those 
 in the capitals of the door shafts. This kind of capital is gene- 
 rally used where the shafts entirely encompass the centre one, as 
 at York and Lincoln, and has a very fine effect, the leaves being 
 generally extremely well executed. The bases used are fre- 
 quently near approaches in contour to the Grecian attic base, but 
 the reversed ogee is sometimes used. There is another sort of 
 pier, in buildings that appear to be of this style, w'hich is at 
 times very confusing, as the same kind of pier seems to be used 
 in small churches even to a very late date ; this is the plain mul- 
 tangular (generally octagonal) pier with a plain capital, of a few 
 very simple mouldings, and w'ith a plain sloped arch. Piers of* i; 
 this description are very frequent, and it requires great nicety of | 
 observation and discrimination to refer them to their proper date; ^ 
 but a minute examination will often, by some small matter, detect 
 theii' age, though it is im-possible to describe the minutiae without 
 many figures. 
 
 JLarly English Buttresses. 
 
 These are of four descriptions. 1st, The old Norman flat 
 buttress is often used, but it is not always as broad, and its 
 tablets, &c. are more delicate 
 
 2nd, A buttress not so broad as the flat one, but nearly of 
 the same projection as breadth, and carried up, sometimes wdth 
 only one set-off, and sometimes without any, and these have often 
 their edges chamfered from the window tablet. They sometimes 
 have a shaft at the corner, and in large rich buildings are occasion- 
 ally pannelled. 
 
 3d, A long slender buttress, of narrow face, and great projec- 
 tion in few stages, is used in some towers, but is not very common. 
 
ARCHITECTURE. 
 
 143 
 
 Early English tablets — niches. 
 
 4th, Towards the latter part of this style, the buttress in stages 
 was used, but it is not very common, and is sufficiently distin- 
 guished by its triangular head, the usual finish of this style, which 
 can hardly be called a piimacle, though sometimes it slopes off 
 from the front to a point. 
 
 Karly English Tablets^ 
 
 The cornice is now become sometimes rich in mouldings, and 
 often with an upper slope, making the face of the parapet per- 
 pendicular to the wall below ; there are cornices of this style still 
 resembling the Norman projecting parapet, but they consist of 
 several mouldings. The hollow moulding of the cornice is gene- 
 rally plain, seldom containing flowers or carvings, but under the 
 mouldings there is often a series of small arches resembling the 
 corbel table. 
 
 The dripstone of this style is various, sometimes of several 
 mouldings, sometimes only a round with a small hollow. It is, 
 in the interior, occasionally ornamented with the toothed orna- 
 ment, and in a few late instances, as the interior of the choir at 
 Westminster, with flowers. In a few buildings, the dripstone is 
 returned, and runs as a tablet along the walls. It is in general 
 narrow', and generally supported by a corbel, either of a head or 
 a flower, &c. There are frequently, in large buildings, in the or- 
 namented parts, bauds of trefoils, quatrefoils, &c. some of them 
 very rich. Although a sort of straight canopy is used over some of 
 the niches of this style, yet it does not appear to have been used 
 over w'indow's or doors. In some few' buildings where they are 
 found, they appear to be additions. The tablets forming the base 
 mouldings are sometimes a mere slope, at others, in large build- 
 ings, are of several sets of mouldings, each face projecting farther 
 than the one above it ; but the reversed ogee is very seldom used, 
 at least at large and singly. 
 
 Early English Niches, 
 
 The most important niches are those found in chancels, in 
 the walls of the south side, and of which the uses do not yet 
 appear to be decided. Of these there are many of all stages of 
 Early English ; there are sometimes tw^o, but oftener three, and 
 they are generally sunk in the wall, and adapted for a seat, the 
 easternmost one often higher in the seat than the others. They 
 are sometimes a plain trefoil head, and sometimes ornamented 
 W'ith shafts, Scc. ; they are generally straight-sided. The statuary 
 niches, and ornamental interior niches, mostly consist of a series 
 of arches, some of them slope-sided, and some with a small but 
 not very visible pedestal for ffie statue. They are often grouped 
 
144 
 
 ARCHITECTURE. 
 
 Early English ornaments — steeples. 
 
 two under one arch, with an ornamental opening between the 
 small arches, and the large one like the double doors ; a straight- 
 sided canopy is sometimes used, and a plain tinial. These niches, 
 except the chancel stalls, and the stoup and piscina, are seldom 
 single^ except in buttresses, but mostly in ranges. 
 
 Early English Ornaments. 
 
 The first ornament to be described, is that already noticed 
 as the peculiar distinction of this style, to which it seems nearly, 
 if not exclusively confined; it is the regular progression from 
 the Norman zigzag to the delicate four- leaved flowers so common 
 in Decorated English buildings. Like the zigzag it is generally 
 straight-sided, and not round like the leaves of a flower, though, 
 at a distance in front, it looks much like a small flower. It is 
 very difficult to describe it, and still more so to draw it accu- 
 rately; it may perhaps be understood by considering it a succes- 
 sion of small open pyramids of four legs, which are formed of 
 half a cube, and set on the edges of a hollow^ moulding. This 
 ornament is used very profusely in the buildings of this style, in 
 Yorkshire and Lincolnshire, and frequently ' in those of otlier 
 counties. 
 
 Another ornament, which, though not peculiar, in small 
 works, to this style, was seldom but during its continuance 
 practised to so large an extent ; this is the filling of the spaces 
 above the choir arches with squares, enclosing fom-leaved 
 flowers. This is done at Westminster and at Chichester, in both 
 of which the workmanship is extremely good, and it has a very 
 rich effect. 
 
 In many parts, as in the spandrells of door arches, and other 
 plain spaces, circles filled with tiefoils and quatrefoils, with 
 flowered points, are often introduced. In the early part of the 
 style, crockets were not used, and the finial was a plain bunch of 
 three or more leaves, or sometimes only a sort of knob ; but in 
 small rich works, towards t’he end of the style, the beautiful fiiiials 
 and crockets of the next sVyle were used. 
 
 Earh/ English Steeples. 
 
 The Norman towers were short and thick, the Early English 
 rose to a much greater height, and on the tower they placed that 
 beautiful addition the spire. 
 
 Some of our finest spires are of this age, and the proportions 
 observed between the tower and spiie, are generally very good. 
 Salisbuiw, which stands unrivalled in height and beauty, and 
 Chichester, are of this ag;e, as are the towers of Lincoln minster. 
 Wakefield has a fine steeple, as to proportion, though plain, and 
 
ARCHITECTURE. 
 
 145 
 
 Early English steeples. — Decorated English doors. 
 
 it is singular for its machicolations, in the top of the tower. 
 The towers are flanked by octagonal turrets, square flat buttresses, 
 or, in a few instances, with small long buttresses, and generally 
 there is one large octagonal pinnacle at the corners, or a collection 
 of smaller niches, &c. There often is no parapet, but the slope 
 of the spire runs down to the edge of the wall of the tower, and 
 flnishes there with a tablet ; and there is a double slope to con- 
 nect the corners with the intermediate faces. The spire is often 
 ornamented by ribs at the angles, sometimes with crockets on the 
 ribs, and bands of squares, filled with qiiatrefoils, 8cc. surrounding 
 the spire at different heights. There are many good spires of this 
 style in country churches. 
 
 Of this style we have the great advantage of one building, re- 
 maining, worked in its best manner, of great size and in excellent 
 preservation ; this is Salisbury, and it gives a very high idea of 
 the great improvement of this style on the Norman. Magnificent 
 without rudeness, and rich though simple, it is one uniform whole. 
 The west front is ornamented, but by no means loaded, and the 
 appearance of the north side is perhaps equal to the side of any 
 cathedral in England. The west front of Lincoln is fine, but the 
 old Norman space is too visible not to break it into parts. Peter- 
 borough and Ely have perhaps the most ornamented fronts of 
 this style. Westminster is spoiled by additions, but its north 
 transept end is fine, as are both the transepts of York. Interiorly, 
 after the simple Salisbury, the transepts of York are perhaps the 
 best specimens, though there are parts of many other buildings 
 deserving much attention. 
 
 Not much has been done in either restoring or imitating this 
 style ; it is certainly not easy to do either well, but it deserves 
 attention, as in many places it would be peculiarly appropriate, 
 and perhaps is better fitted than any for small country churches. 
 It may be worked almost entirely plain, yet if ornament is used, it 
 should be well executed ; for the ornaments of this style are in 
 general as well executed as any of later date, and the toothed 
 ornament and hollow bands equal, in difficulty of execution, the 
 most elaborate perpendicular ornaments. 
 
 Of the Third, or Decorated English Style. 
 
 Decorated English Doors. 
 
 The large doors of the last style are mostly double, and 
 there are some fine ones of this, but they are not so common, 
 there being more single doors, which are often nearly as large 
 as the early English double ones, and indeed but for the orna- 
 ments they are much alike, having shafts and fine hollo\v 
 
 7.— Vol.L U 
 
146 
 
 ARCHITECTURE. 
 
 Decorated English doors. 
 
 mouldings; in small doors there are often no shafts at all, but 
 the arch mouldings run down the side, and often almost to the 
 ground without a base. The shafts do not in this style generally 
 stand free, but are parts of the sweep of mouldings, and instead 
 of being cut and set up lengthways, all the mouldings and 
 shafts are cut on the arch stone, thus combining great strength 
 with all the appearance of lightness. The capitals of these 
 shafts differ from the early English, in being formed of a woven 
 foliage, and not upright leaves': this, in small shafts, generally 
 has an apparent neck, but in larger ones often appears like a 
 round ball of open foliage. The bases to these shafts mostly 
 consist of the reversed ogee, but other mouldings are often 
 added, and the ogee often made in faces. Although the doors 
 in general are not so deeply recessed, as the Norman and early 
 English, yet in many large buildings they are very deep. The 
 west doors of York, and the later west doors of Beverley, are of 
 the richest execution, and very deep. To the open work bands 
 of the last style, succeeds an ornament equally beautiful, and 
 not so fragile ; this is the flowery hollow moulding ; there are 
 often three or four in one door-way, and to the toothed orna- 
 ment succeeds a flower of four leaves, in a deep moulding, with 
 considerable intervals between. This flower, in some build- 
 ings, is used in great profusion to good effect, and a perforated 
 ball in other buildings in equal abundance. Over these doors, 
 there are several sorts of canopies ; the dripstone is generally 
 supported by a corbel, which is commonly a head ; in some 
 instances a plain return is used, but that return seldom runs 
 horizontally. The canopy is sometimes connected with the 
 dripstone, and sometimes distinct. The common canopy is a 
 triangle, the space betw'een it and the dripstone is filled with 
 tracery, and the exterior ornamented with crockets, and crowmed 
 with a finial. On the side of the doors, small buttresses or 
 niches are sometimes placed. The second canopy is the ogee, 
 which runs about half up the dripstone, and then is turned the 
 contrary way, and is finished in a straight line running up into 
 a finial. This has its intermediate space filled with tracery, &c. 
 and is sometimes crocketed, and sometimes not. Another sort 
 of canopy is an arch running over tlie door, and unconnected 
 w ith it, which is doubly foliated ; it has a good effect, but is not 
 common. 
 
 In small churches, there are often nearly plain doprs, 
 having only a dripstone and a round moulding on the interior 
 edge, and the rest of the wall a straight line or bold hollow, 
 and in some instances a straight slope side only. In some doors 
 of this style, a series of niches witli statues are carried up like 
 
ARCHITECTURE. 
 
 147 
 
 Decorated English windows, — Two descriptions of tracery. 
 
 a hollow moulding ; - and in others, doubly foliated tracery 
 hanging free from one of the outer mouldings, give a richness 
 superior to any other decoration. The south door of the choir 
 at Lincoln is perhaps hardly any where equalled of the first 
 kind ; and the west doors at Beverley are good illustrations of 
 the other. 
 
 Decorated English Windows, 
 
 In these, the clearest marks of the style are to be found, and 
 they are very various, yet all on one principle : an arch is divided 
 by one or more mullions, into two or more lights, and these 
 mullions branch into tracery of various figures, but do not run in 
 perpendicular lines through the head. In small churches, win- 
 dows of two or three lights are common, but in larger four and 
 five lights for the aisles and clerestory windows, five or six for 
 transepts and the end of aisles, and in the east and west windows 
 seven, eight, and even nine lights, are used. Nine lights seem to 
 be the extent, but there may be windows of this style containing 
 more. The west window of York, and the east window of Lin- 
 coln cathedrals, are of eight lights each; the west window of 
 Exeter cathedral is of nine, and these are nearly, if riot quite, the 
 largest windows remaining. 
 
 There may be observed two descriptions of tracery, and 
 although, in different parts, they may have been worked at the 
 same time, yet the first is generally the oldest. In this first 
 division, the figures, such as circles, trefoils, quatrefoils, &c. 
 are all worked with the same moulding, and sometimes do not 
 regularly join each other, but touch only at points. This may 
 be called geometrical tracery ; of this description are the windows 
 of the nave of York, the eastern choir of Lincoln, and some of 
 the tracery in the cloisters at Westminster abbey, as well as most 
 of the windows at Exeter, which contains, perhaps, the richest 
 variety of windows of any cathedral in England, and some of them 
 are of such admirable workmanship as to almost belong to the 
 second division. 
 
 The second division consists of what may be truly called 
 flowing tracery. Of this description, York minster, the 
 minster, and St. Mary’s, at Beverley, Newark church, and 
 many northern churches, as well as some southern churches, 
 contain most beautiful specimens. The one engraved is from 
 the west end of the south aisle of Newark, and is perhaps one 
 of the most beautiful in its composition. The great west 
 window at York is, perhaps, the most elaborate. In these 
 windows, various wheels are sometimes introduced. In the 
 richer windows of this style, and in both divisions, the princi- 
 
148 
 
 ARCHITECTURE. 
 
 Decorated English windows. 
 
 pal moulding of the mullion has sometimes a capital and base, 
 and thus becomes a shaft. One great cause of the beauty 
 of fine flowing tracery, is the intricacy and delicacy of the 
 mouldings ; the principal moulding often running up only one 
 or two mullions, and forming only a part of the larger design, 
 and all the small figures being formed in mouldings, which 
 spring from the sides of the principal. This is a distinction the 
 plate was too small to admit, which takes much from the 
 beauty of the window. The architraves of ^windows of this 
 style are now much ornamented with mouldings, which are 
 sometimes made into shafts. The dripstones and canopies of 
 windows are the same as in the doors, and have been described 
 under that head. Wherever windows of this st^le remain, an 
 artist should copy them ; the varieties are much greater than 
 might be supposed, for it is very difficult to find two alike in 
 difterent buildings. 
 
 It does not appear that the straight horizontal transom was 
 much if at all used in window's of this style ; wherever it is 
 found there is generally some mark of the window originating 
 after the introduction of the perpendicular style ; but it may have 
 been used in some places, and there are a very few instances 
 of a light being divided in height by a kind of canopy, or a qua- 
 trefoil breaking the mullion ; the church of Dorchester, in Ox- 
 fordshire, has some very curious windows of this kind. In some 
 counties, where flint and chalk are used, the dripstone is some- 
 times omitted. The heads of the windows of this style are most 
 commonly the equilateral arch ; though there are many examples 
 both of lancet and drop arches ; but the lancet arches are not very 
 sharp, perhaps never exceeding one-third of the equilateral. 
 There are a few windows of this style with square heads ; but 
 they are not very common. 
 
 The circles of this style are some of them very fine ; there are 
 some very good ones in composition at Exeter and Chichester, 
 and the east window of old St. Paul’s was a very fine one ; but 
 perhaps the richest remaining is that of the south transept at 
 Lincoln, which is completely flowing. ' 
 
 Tow'ards the end of this style, and perhaps after the com- 
 mencement of the next, we find windows of most beautiful com- 
 position, with parts like the perpendicular windows, and some- 
 times a building has one end decorated, the other perpendicular ; 
 such is Melrose abbey, whose windows have been extremely fine, 
 and, indeed, the great east window of York, which is the finest 
 perpendicular window in England, has still some traces of flowing 
 lines in its head. 
 
ARCHITECTURE. 
 
 149 
 
 Decorated English arches — piers — buttresses. 
 
 Decorated English Arches. 
 
 Though the arch most commonly used for general purposes 
 in this style is the equilateral one, yet this is by no means con- 
 stant. At York this arch is used, but at Ely a drop arch. The 
 architrave mouldings of interior arches do not differ much from 
 those of the last style, except that they are, perhaps, more fre- 
 quently continued down the pier w ithout being stopt at the line of 
 capitals. The dripstones are of delicate mouldings, generally 
 supported by heads. The arches of the galleries are often beau- 
 tifully ornamented with foliated heads, and often fine canopies ; 
 and in these arches the ogee arch is sometimes used, as it is freely 
 in composition in the heads of windows. 
 
 Of this style, or perhaps of the next, is that singular yet beau- 
 tiful reversed arch in the nave of Wells’ cathedral. 
 
 Decorated English Piers. 
 
 A new disposition of shafts marks very decidedly this style in 
 large buildings, they being arranged diainondwise, with straight 
 sides, often containing as many shafts as wull stand close to each 
 other at the capital, and only a fillet or small hollow between 
 them. The shaft which runs up to support the roof, often 
 springs from a rich corbel between the outer architrave mouldings 
 of the arches ; Exeter is a fine example. The capitals and bases 
 of these shafts are much the same as those described in the sec- 
 tion on doors. Another pier of the richest effect, but seldom exe- 
 cuted, is that at York minster, where the centre shaft is larger 
 than those 'on each side, and the three all run through the spring 
 of the roof. Three also support the side of the arch ; these shafts 
 are larger in proportion than those of Exeter, &c. and stand close 
 without any moulding betw^een. 
 
 Another pier, common towards the end of this style, and the 
 beginning of the next, is composed of four shafts, about two-fifths 
 engaged, and a fillet and bold hollow half as large as the shafts 
 between each ; this makes a very light and beautiful pier, and is 
 much used in smaller churches. All these kinds of piers have 
 their shafts sometimes filleted, as are also often some of the archi- 
 trave mouldings. In small country churches, the multangular 
 fiat-faced pier seems to have been used. 
 
 Decorated English Buttresses. 
 
 These, though very various, are all more or less worked in 
 stages, and the set-off’s variously ornamented, some plain, some 
 moulded slopes, some with triangular heads, and some with 
 pannels ; some with niches in them, and with all the various 
 
150 
 
 ARCHITECTURE. 
 
 Decorated English tablets — niches. 
 
 degrees of ornament. The comer buttresses of this style are 
 often set diagonally. In some few instances small turrets are 
 used as buttresses. The buttresses are variously finished, some 
 slope under the comice, some just through it; some run up 
 through the battlement, and are finished with pinnacles of va- 
 rious kinds. 
 
 Decorated English Tablets, 
 
 The comice is very regular, and though in some large 
 buildings it has several mouldings, it principally consists of a 
 slope above, and a deep sunk hollow, with an astragal under it ; 
 in these hollows, flowers at regular distances are often placed, and 
 in some large buildings, and in towers, &c. there are frequently 
 heads, and the comice almost filled with them. The dripstone 
 is of the same description of mouldings, but smaller, and this too 
 is sometimes enriched with flowers. The small tablet running 
 under the window has nearly the same mouldings, but mostly 
 without the astragal, and this sometimes runs round the buttress 
 also. The dripstone very seldom, if ever, runs horizontally, 
 though in a few instances a return is used instead of the more 
 common corbel head or shield. 
 
 The basement tablets are sometimes numerous, and often 
 have the reversed ogee repeated. 
 
 Decorated English Niches. 
 
 These form one of the greatest beauties of the style, and are 
 very various, but may be divided into tw'O grand divisions, which, 
 if necessary, might be again variously divided, such is their diver- 
 sity, but these tw’O may be sufficient. The first are pannelled 
 niches, the fronts of whose canopies are even with the face of the 
 wall or buttress they are set in. These have their interiors either 
 square with a sloping side, or are regular semi-hexagons, &c. In 
 the first case, if not very deep, the roof is a plain arch, but in the 
 latter case the roof is often most delicately groined, and some- 
 times a little shaft is set in the angles or the ribs of the roof, sup- 
 ported by small corbels. The pedestals are often high and much 
 ornamented. 
 
 The other division of niches have projecting canopies ; these 
 are of various shapes, some conical like a spire, some like 
 several triangular canopies joined at the edges, and some with 
 ogee heads ; and in some very rich buildings are niches with the 
 canopy bending forsvards in a slight ogee, as well as its contour 
 being ogee ; these are generally crowned with very large rich 
 finials, and very highly enriched. There were also, at the 
 latter part of this style, some instances of the niche with a flat- 
 
ARCHITECTURE. 
 
 151 
 
 Decorated English ornaments — steeples. 
 
 headed canopy, which became so common in the next style. 
 These projecting niches have all some projecting base, either a 
 large corbel, or a basement pedestal carried up from the next 
 nrojecting face below. All these niches are occasionally flanked 
 by small buttresses, and their pinnacles ; those of the first kind 
 have very often beautiful shafts. 
 
 The chancel stalls of this style, are many of them uncom- 
 monly rich, their whole faces being often covered with ornamental 
 carving. 
 
 Decorated English Ornaments, 
 
 As the word decorated is used to designate this style, and 
 particularly as the next is often called florid, as if it were richer 
 in ornament than this, it will be necessary to state, that though 
 ornament is often profusely used in this style, yet these orna- 
 ments are like Grecian enrichments, and may be left out without 
 destroying the grand design of the building, while the orna- 
 ments of the next are more often a minute division of parts of 
 the building, as pannels, buttresses, &c. rather than the carved 
 ornaments used in this style. In some of the more magnificent 
 works, a variety of flowered carvings are used all over, and yet 
 the building does not appear overloaded ; while some of the 
 later perpendicular buildings have much less flowered carvings, 
 yet look overloaded with ornaments, from the fatiguing recurrence 
 of minute parts, which prevent the general design being compre- 
 hended. 
 
 The tomb of tlie Percys at Beverley, and one or two at 
 York, are as rich as can well be conceived in ornamental carvings, 
 yet the general design is noble, and may be clearly understood, 
 while the design of Henry the VII.’s chapel can hardly be com- 
 prehended, from the constant repetition of the same ornaments, 
 which, if worked singly, are not very rich. 
 
 The flower of four leaves in a hollow moulding, has already 
 been spoken of, and in these hollow mouldings various other 
 flowers are introduced, as well as heads and figures, some of 
 them very grotesque ; and as to capitals there are very seldom 
 found two alike. The foliage forming the crockets and finials 
 is also extremely rich, and the pinnacle, in its various forms, is 
 almost constantly used. The spandrells of ornamental arches 
 are sometimes filled with beautiful foliage, perhaps few superior 
 to some in the church at Ely, which was the lady chapel of the 
 cathedral. 
 
 Decorated English Steeples. 
 
 Of this style are many of these beautiful ornaments of the 
 
152 
 
 ARCHITECTURE. 
 
 Decorated English steeples. 
 
 country ; at the commencement of it, several fine spires were 
 added to towers then existing, and in after times many very line 
 towers and spires were erected. Grantham and several other 
 Lincolnshire spires are very line, and there are many good towers. 
 These are generally Hanked with buttresses, many of which are 
 diagonal, and are generally crowned with fine pinnacles. Per- 
 haps the church of St. Michael, at Coventry, is as elegant a spire 
 and tower as any of this age, and is curious, from the spire stand- 
 ing on a lantern above the tower. In Lincolnshire and some of 
 * the adjoining counties, there are many village churches with line 
 spires, and some- of this style; of these, perhaps few, if any, ex- 
 ceed in beauty of proportion and delicacy of composition that of 
 Norton, a village in Leicestershire, a few miles to the left of the 
 road from Uppingham to Leicester. The singular crowned 
 steeple of St. Nicholas, at Newcastle upon Tyne, is either of 
 this style or early in the next. 
 
 There are many of the towers of this age whose windows, 
 or at least the mullions, seem to have been renewed in the per- 
 pendicular style, and indeed, in small churches, it is not always 
 easy precisely to fix the style of the tower because of these 
 alterations. 
 
 Although they have some appearances of the window s which 
 belong to the next style, yet to this age should be referred the 
 towers of York minster, which possess uncommon beauty. 
 
 Though we have not the advantage of any one large building 
 of this style in its pure state, like Salisbury, yet we have the ad- 
 vantage of four most beautiful models, which are in the highest 
 preservation, besides many detached parts. These are at Lin- 
 coln, Exeter, York, and Ely, and though differently w'orked, are 
 all of excellent execution. Of these, Exeter and York are far the 
 largest, and York, from the uncommon grandeur and simplicity 
 of the design, is certainly the finest ; ornament is no where spared, 
 yet there is a simplicity which is peculiarly pleasing. Amongst 
 the many smaller churches, Trinity church at Hull deserves pecu- 
 liar notice, as its decorated part is of a character which could 
 better than any be imitated in modern w'ork, from the great height 
 of its piers, and the smallness of their size. The remains of IM el- 
 rose abbey are extremely rich, and, though in ruins, its parts are 
 yet very distinguishable. In imitations of this style, great deli- 
 cacy is required to prevent its running into the next, which, from 
 its straight perpendicular and horizontal lines, is so much easier 
 wotked; whatever ornaments are used, should be very cleanly 
 executed, and highly finished. 
 
ARCHITECTURE. 
 
 153 
 
 Perpendicular English doors — windows. 
 
 Of the Fourth, or Perpendicular Style. 
 
 Perpendicular English Doors, 
 
 An impression from an engraving of a perpendicular door 
 having been given on the cover of several numbers of this work, 
 our readers must, by this time, be well acquainted with it. A 
 copy is annexed, for the purpose of permanent reference, with 
 the other plates. It has been drawn to convey as distinct an idea 
 as possible of the character of the generality of these doors, 
 the great distinction of w'hich, from those of the last style, is 
 the almost constant square head over the arch, which is sur- 
 rounded by the outer moulding of the architrave, and the span- 
 drell filled with some ornament, and over all a dripstone is gene- 
 rally placed. This ornamented spandrell in a square head, occurs 
 in the porch to Westminster Hall, one of the earliest perpendi- 
 cular buildings, and is continued to the latest period of good exe- 
 cution, and in a rough way much later. In large very rich doors, 
 a canopy is sometimes included in this square head, and some- 
 times niches are added at the sides, as at King’s college chapel, 
 Cambridge. This square head is not always used interiorly, for 
 an ogee canopy is sometimes used, or panne! s down to the arch, as 
 at St. George’s, Windsor; and there may be some small exterior 
 side doors, without the square head, but they are not common. 
 The shafts used in these doors, are small, and have plain capitals, 
 which are often octagonal, and the bases made so below the first 
 astragal. It is also very common for the architrave to consist 
 of ogee mouldings, as well as the rounds and hollows which have 
 been before used. 
 
 Perpendicular English Windozvs. 
 
 These are easily distinguished by their miillions running in 
 perpendicular lines, and the transoms, which are now general. 
 The varieties of the last style were in the disposition of the prin- 
 cipal lines of the tracery ; in this, they are rather in the disposi- 
 tion of the minute parts, a window of four or more lights is gene- 
 rally divided into two or three parts by stronger mullions running 
 quite up, and the portion of arch between them doubled, from 
 the centre of the side division. In large windows, the centre one 
 is again sometimes made an arch, and often in window's of seven 
 or nine lights, the arches spring across, making two of four of 
 fi\'e lights, and the centre belonging to each. The heads of win- 
 dows, instead of being filled with flowing ramifications, have 
 slender mullions running from the heads of the lights, between 
 each principal miillion, and -4hese have small transoms till tlie 
 7— VoL. I. X 
 
154 
 
 ARCHITECTURE. 
 
 Perpendicular English windows. 
 
 window is divided into a series of small pannels ; and the heads 
 being arched, are trefoil ed or cinquefoiled ; sometimes these small 
 mullions are crossed over each other in small arches, leaving 
 minute quatrefoils, and these are carried across in straight lines. 
 Under the transom is generally an arch of some kind, but in York- 
 shire, Lincolnshire, and Nottinghamshire, and perhaps in some 
 other parts, there is a different mode of foliating the straight line 
 without an arch, which has a singular appearance, (see plate I.) 
 In the later windows of this style, the transoms are often orna- 
 mented with small battlements, which, when well executed, have 
 a very fine effect. Amidst so great a variety of windows, (for 
 perhaps full half the windows in English edifices over the king- 
 dom are of this style,) it is difficult w hich to notice ; but Windsor, 
 St. George’s, for four lights, and the clerestory w indow s of Henry 
 the Vll.’s chapel for five, are some of the best executed; for a 
 large window, the east window of York has no equal, and by tak- 
 ing its parts, almost any sized window may be formed. There are 
 some good windows, of which the heads have the mullions alter- 
 nate, that is, the perpendicular line rises from the top of the arch 
 of the pannel below it. The windows of the Abbey church, at 
 Bath, are of this description. 
 
 It is necessary here to say a little of a wdndow' which may be 
 mistaken for a decorated window : this is one of three lights, used 
 in many country churches, the mullions simply cross each other, 
 and are cinquefoiled in the heads, and quatrefoiled in the three 
 upper spaces ; but to distinguish this from a decorated window, 
 it will generally be necessary to examine its arch, its mullion 
 mouldings, and its dripstone, as w'ell as its being (as it often is) 
 accompanied by a clearly perpendicular window at the end, or 
 connected with it so as to be evidently of that time. Its arch is 
 very often four-centred, which at once decides its date ; its mul- 
 lion mouldings are often small, and very delicately worked ; its 
 dripstone often has some clear mark, and when the decorated tra- 
 cery is become familiar, it will be distinguished by its being a mere 
 foliation of a space, and not a fiowing quatrefoil with the mould- 
 ings carried round it. 
 
 Large circular windows do not appear to have been in use 
 in this style; but the tracery of the circles in the transepts of 
 Westminster abbey appear to have been renewed during this 
 period. At Henry the VII.’s chapel, a wdndow is used in the 
 aisles, which seems to have led the w^ay to that wretched sub- 
 stitute for fine tracery, the square-headed windows of queen 
 Elizabeth and king James the First’s time. This window is a 
 series of small pannels forming a square head, and it is not fiat, 
 but in projections, and these, with the octagonal towers used 
 
ARCHITECTURE. 
 
 1.55 
 
 Perpendicular English arches — piers. 
 
 I] for buttresses, throw the exterior of the building into fritter, ill- 
 b; assorting with the richness of the clerestory windows. In most 
 ; of the later buildings of this style, the window and its architrave 
 completely tills up the space between the buttresses, and the east 
 1 and west windows are often very large ; the west window of St. 
 
 : George’s, Windsor, has fifteen lights in three divisions, and is 
 
 a grand series of pannels, from the floor to the roof ; the door is 
 amongst the lower ones, and all above the next to the door is 
 pierced for the window. The east window at Gloucester is also 
 f very large, but that is of three distinct parts, not in the same line 
 J of plan. 
 
 [ When canopies are used, which is not so often as in the 
 
 t last style, they are generally of the ogee character, beautifully 
 
 crocheted. 
 
 Perpendicular English Arches, 
 
 Although the four-centred arch is much used, particularly irr 
 I the latter part of the style, yet, as in all the other styles, we have 
 
 I in this also arches of almost all sorts amongst the ornamental parts 
 
 of niches, Scc. and in the composition lines of pannels, are arches 
 i from a very fine thin lancet to an almost flat segment. Yet, wkh 
 i all this variety, the four-centred arch is the one most used in large 
 ' buildings, and the arches of other characters, used in the division 
 of the aisles, begin to have what is one of the great distinctions of 
 this style, — the almost constant use of mouldings running from 
 the base all round the arch, w ithout any stop horizontally, by way 
 of capital, sometimes with one shaft and capital, and the rest of 
 the lines running. The shafts in front running up without stop 
 to the roof, and from their capitals springing the groins. ln« win- 
 dow arches, shafts are now very seldom used, the architrave run- 
 ning all round, and both w'indow arches and the arches of the in- 
 terior are often enclosed in squares, with ornamented spandrells, 
 either like the doors, or of pannelling. Interior arches have now 
 seldom any dripstone when the square is used, but at Bath there is 
 a clear dripstone distinct from the arch mouldings. 
 
 Another great distinction of these arches, in large buildings, is 
 the absence of the triforium or gallery, between the arches of the 
 nave and the clerestory window^s ; their place is now supplied by 
 pannels, as at St. George’s, Windsor, or statuary niches, as at 
 Henry the VII. ’s chapel ; or they are entirely removed, as at 
 Bath, and Manchester Old church, &c. 
 
 Perpendicular English Piers. 
 
 The massive Norman round pier, lessened in size and ex- 
 tended in length, with shafts set round it, became the early Ei> 
 
156 
 
 AECHITECTrRE. 
 
 Perpendicular English piers — buttresses. 
 
 glisli pier; the shafts were multiplied and set into the face of tlie 
 pier,'Mhich became, mils plan, lozenge, and formed the decorated 
 piei ; we now tind the pier again altering in shape, becoming 
 mucli tl inner between the arches, and its proportion the other 
 wa>, fi om the nave to the aisle, increased, having those shafts 
 which 1 un to the roof, to support the springings of the groins, 
 added in front, and not forming a part of the mouldings of the 
 arch, but having a bold hollow between them : this^ is particularly 
 apparent at King’s college chapel, Cambridge, St. George’s, 
 Windsor, and Henry the Vil.’s chapel, the three great models of 
 enriched peipeudicular style; but it is observable in a less degree 
 ill many otliers. In small churches, the pier mentioned in the 
 last st}ie, of four shafts and four hollows, is still much used; but 
 many small churches have humble imitations of the magnificent 
 arrangement of shafts and mouldings spoken of above. There 
 are still some plain octagonal, &c. piers, in small churches, which 
 may belong to this age. 
 
 Though filleted shafts are not so much used as in the last 
 style, the exterior moulding of the architrave of interior arches is 
 sometimes a filleted round, which has a good eifect ; and in gene^ 
 ral the mouldings and paits of piers, architraves, &c. are much 
 siiiaiier than those used in the last stjle. 
 
 Perpendicular English Buttresses. 
 
 These differ very little from those of the last style, except tliat 
 triangular heads to the stages are much less used, the set-ofi's 
 being much more often bold projections of plain slopes ; yet 
 many fine buildings have the triangular heads. In the upper 
 stor\, the buttresses are often very thin, and of diagonal faces. 
 'I here are few large buildings of this style without fl}ing but- 
 tresses, and these are often pierced; at Henry the VH.’s chapel 
 they are of rich tracery, and the buttresses are octagonal turrets. 
 At King’s college chapel, Cambiidge, which has only one height 
 within, the projection of the buttresses is so great as to allow 
 chapels between the wail of the nave and another level with the 
 front of the buttresses. At Gloucester, ,aiid perhaps at some 
 other places, an arch or half arch is pierced in the lower part of 
 the buttress. There are a few' buildings of this style without any 
 buttresses. All the kinds are occasionally ornamented with sta- 
 tuary niches, and canopies of various descriptions, and the dia- 
 gonal corner buttress is not so common as in the last style; but 
 the two buttresses often leave a square, which runs up, and some- 
 times, as at the tower of the Old church at Manchester, is 
 crowmed with a third pinnacle. 
 
 Although pinnacles are used very fireely in tliis style, yet 
 
ARCHITECTCRK.' 
 
 157 
 
 Pupendicular Eiialish rablets — niches 
 
 there are some buildings whose buttresses run up and finish square 
 witiioiit any ; erf this description is St. George’s, .Windsor. The 
 buttresses of the small eastern addition at Petei borough cathedral 
 are curious, having statues ot saints for pinnacles. 
 
 In interior oinaments, the buttresses used are sometimes 
 small octagons, sometimes panneiled, sometimes plain, and 
 tlien, as well as the small buttresses ot niches, are often banded 
 with a band different from the Early English, and much broader. 
 Such are the buttresses between the doors of Henry the Vll.’s 
 chapel. 
 
 Perpendicular English Tablets. ' 
 
 The cornice is now, in large buildings, often composed of 
 many small mouldings, sometimes divided by one or two consi- 
 derable hollow s, not very deep ; yet still, in plain buildings, 
 the old cornice mouldings are much adhered to ; but it is more 
 often ornamented in the hollow with fiowers, &c. and sometimes 
 with grotesque animals; of this the churches of Gresford and 
 Mold, in Flintshire, are curious examples, being a complete 
 chase of cats, rats, mice, dogs, and a variety of imaginary 
 figures, amongst which various grotesque monkeys are very con- 
 spicuous. In-the latter end of the style, something very analo- 
 gous to an ornamented frieze is perceived, of w hich the canopies 
 to the niches, in various w orks, are examples ; and the angels 
 so profusely introduced, in the later rich w orks, are a sort of cor- 
 nice ornaments. Ihese are very conspicuous at St. George's, 
 Windsor, and Henry the VI l.’s chapel. At Bath, is a cornice 
 of two hollows, and a round between with fillets, both upper 
 and under surface alike I'he dripstone of this style is, in the 
 heads of doors and some windows, much the same as in the last 
 style, and it most generally finishes b' a plain return ; though 
 corbels are sometimes used, this return is frequently continued 
 horizontally. Sometimes a much smaller dripstone is used, of 
 only a round and holiow^ 
 
 Tablets under the window's are like this last or other drip- 
 stone, and sometimes fine bands are carried round as tablets. Of 
 these there are some fine remains at the cathedral, and at the 
 tower of St. John’s, Chester. 
 
 The basement mouldings ordinarily used are not materially 
 different from the last style ; reversed ogees and hollows, vari- 
 ously disposed, being the principal mouldings. 
 
 Perpendicular English Niches. 
 
 These are very numerous, as amongst them w e must include 
 nearly all the stall, tabernacle, and screen w ork, in the English 
 
158 
 
 ARCHITECTURE. 
 
 Perpendicular English niches — ornaments. 
 
 churches ; for there appears little if any wood-work of an older 
 date, and it is probable that much screen work was defaced at 
 the reformation, and restored in queen Mary’s time, and not 
 again destroyed, at least the execution of much of it woidd lead 
 to such a supposition, being very full of minute tracery, and 
 much attempt at stiffly ornamented friezes. Many niches are 
 simple recesses, with rich ogee canopies, and others have over- 
 hanging square-headed canopies, with many minute buttresses and 
 pinnacles, crowned with battlements ; or, in the latter part of the 
 style, with what has been called the Tudor flower, an ornament 
 used instead of battlement, as an upper finish, and profusely 
 strewed over the roofs, &c. of the richer later buildings. Of 
 these niches, those in Henry the VII.’s chapel, bet\veen the 
 arches and clerestory windows, are perhaps as good a specimen as 
 any. Of the plain recesses, with ogee canopies, there are some 
 fine ones at Windsor. 
 
 The whole interior of the richer buildings of this style, is more 
 or less a series of pannels, and therefore, as every pannel may, on 
 occasion, become a niche, we find great variety of shape and size ; 
 but like those of the last style, they may generally be reduced to 
 one or other of these divisions. 
 
 Perpendicular English Ornaments. 
 
 The grand source of ornament, in this style, is pannelling ; 
 indeed, the interior of most rich buildings is only a general 
 series of it; for example. King’s college chapel, Cambridge, is 
 all pannel except the floor ; for the doors and windows are no- 
 thing but pierced pannels included in the general design, and the 
 very roof is a series of them of different shapes. The same may 
 be said of the interior of St. George’s, Windsor, and still fur- 
 ther, Henry the VII.’s chapel is so both within and withoiU, 
 there being no plain wall all over the chapel, except just the 
 exterior below the base moulding, all above is ornamental 
 pannel. All the small chapels of late erection, in this style, 
 such as those of Bishop Fox at Winchester, and several in 
 Windsor, are thus all pierced pannel. Exclusive of this general- 
 source of ornament, there are a few peculiar to it : one, the 
 battlement to transoms of windows, has already been mentioned ; 
 this, in works of late date, is very frequent, sometimes extend- 
 ing to small transoms in the head of the window’, as well as the 
 general division of the lights. Another, the Tudor flow^er, is, 
 in rich work, equally common, and forms a most beautiful 
 enriched battlement, and is also sometimes used on the transoms 
 of windows in small w'ork. Another peculiar ornament of this 
 style, is the angel cornice, used at Windsor fuid Henry the 
 
ARCHITECTURE. 
 
 159 
 
 Perpendicular English steeples. — Miscellaneous rei^iarks. 
 
 Vll.’s chapel; but though according with the character of those 
 buildings, it is by no means fit for general use. These angels 
 have been much diffused, as supporters of shields, and as corbels 
 to support roof beams, &.C. Plain as the Abbey-church at Bath 
 i. is in its general execution, it has a variety of angels as corbels, 
 for different purposes. 
 
 Flowers of various kinds continue to ornament cornices, &c. 
 
 ; and crockets were variously formed towards the end of the style, 
 those of pinnacles were often very much projected, which has a 
 disagreeable effect ; there are many of these pinnacles at Oxford, 
 principally worked in the decline of the style. 
 
 Perpendicular English Steeples. 
 
 Of these there remain specimens of almost every description, 
 from the plain short tower of a country church, to the elaborate 
 and gorgeous towers of Gloucester and Wrexham. There are 
 various fine spires of this style, which have little distinction from 
 those of the last, but their age may be generally known by their 
 ornaments, or the tow^ers supporting them. Alnjost every 
 conceiveable variation of buttress, battlement, and pinnacle, is 
 used, and the appearance of many of the towers combines, in 
 a very eminent degree, extraordinary richness of execution and 
 grandeur of design. Few counties in England are without some 
 good examples; besides the two already mentioned, Boston in 
 Lincolnshire, All-Saints in Derby, St. Mary’s at Taunton, St. 
 George’s, Doncaster, are celebrated ; and the plain, but excel- 
 lently proportioned, tower of Magdalen college, Oxford, deserves 
 much attention. 
 
 Amongst the smaller churches, there are many tow^ers of un- 
 common beauty, but few exceed Gresford, between Chester and 
 Wrexham ; indeed, the whole of this church, both interior and 
 exterior, is worth attentive examination. Paunton, near Grant- 
 ham, has also a tower curious for its excellent masonry. There 
 are of this style some small churches with fine octagonal lanterns, 
 of which description are two in the city of Y ork. 
 
 Miscellaneous Remarks on Perpendicular Buildings. 
 
 Of this style are so many buildings in the finest preserva- 
 tion, that it is dilficult to select ; but, on various accounts, 
 several claim particular mention. The choir at York is one of 
 the earliest buildings; indeed it is, in general arrangements, 
 like the nave, but its ornamental parts, the gallery under the 
 windows, the windows themselves, and much of its panneiilng 
 in ihe interior, are completely of perpendicular character, though 
 the simple nobility of the piers is the same as the nave. The 
 
160 
 
 ARCHITECTURE. 
 
 Battlements in "eneral. 
 
 choir of Gloucester, is also of this style, and most comphtely so, 
 for the wliole interior is one series of ojien-work panneis laid on 
 the Norman work, parts of which are cut away to receive them; 
 it forms a very ornamental whole, but by no means a model for 
 imitation. 
 
 Of the later character, are three most beau'iful specimens, 
 King’s college chapel, Cambridge, Henry the \ 11. ’s chapel, 
 and St. George’s, Windsor; in these, richness of ornament is 
 lavished on every part, and they are particularly valuable for 
 being extremely different from each other, though in many re- 
 spects alike. Of these, undoubtedly St. George’s, Windsor, is 
 the most valuable, from the great variety of composition arising 
 from its plan ; but the roof and single line of w all of King’s col- 
 lege chapel, Cambridge, deserves great attention, and the details 
 of Henry the VH.’s chapel will always command it, from the 
 great delicacy of their execution. 
 
 Of small churches, there are many excellent models for imita- 
 tion, so that in this style, with some care and examination, nothing 
 hardly need be executed but from absolute authority. The monu- 
 mental chapels of this style are peculiarly deserving attention, and 
 often of the most elaborate w orkmanship. 
 
 Of Battlements. 
 
 Having now gone through the styles in detail, w’e come to 
 those sections which, as before mentioned, it is necessary to give 
 in a connected series. 
 
 From exposure to w'eather and various accidents, we find 
 very few^ roofs in their original state, and from the vicinity of 
 the battlement, &c. we find these also are very often not origi- 
 nal. It seems difficult to ascertain what the Norman battlement 
 was, and there seems much reason to suppose it was only a plain 
 parapet; many Norman structures have either battlements evi- 
 dently of later date, or parapets as evidently mutilated, and 
 in the larger buildings of the early English style, tlie parapet 
 continues mostly to be used. Perliaps some of the earliest 
 battlement is that at the west end of Salisbury cathedral, 
 plain, of nearly equal intervals, and with a plain capping 
 moulding ; but it may be doubted if even tliis is original. An or- 
 namented parapet continued to be used through the next style, 
 but with the very frequent use of battlements of several sorts, 
 both plain and pierced ; and as these continued with less altera- 
 tion than many other parts, through the perpendicular style, it 
 will be better to describe them altogether, just observing that a 
 considerable degree of perpendicular pannelling prevails in the 
 battlements of tjie later edifices. The most frequent early 
 
AUCttlTFXTURfe. 
 
 161 
 
 English battlements — roofs. 
 
 pierced parapet, is a series of interchanged trefoils with a 
 fine serpentine , line separating them; this has a tine eftect, and 
 is mostly used in Decorated English buildings ; for in the Per- 
 pendicular, the dividing line is straight, making a series of 
 interchanged triangles. Of pierced battlements there are many 
 varieties, but the early ones have frequently quatrcfoils, either 
 for the lower compartments, or on the top of the pannels of the 
 lower, to form the higher ; the latter have often two heights of 
 pannels, one range for the lower, and another over them forming 
 the upper; and at Loughborough is a line battlement of rich 
 pierced quatrefoils, in two heights, forming an indented battle- 
 ment. These battlements have generally a running cap moulding 
 carried round, and generally following the line of battlement. 
 There are some few later buildings, which have pierced battle- 
 ments, not with straight tops, but variously ornamented; such 
 is the tomb house at Windsor, with pointed upper compart- 
 ments, and such is the battlement of the eastern addition at Peter- 
 borough, and the great battlement of King’s college chapel, Cam- 
 bridge, and also that most delicate battlement over the lower side 
 chapels ; this is perhaps the most elegant of the kind. Some- 
 times exteriorly, and often interiorly, the Tudor flower is used as 
 a battlement, and there are a few instances of the use of a battle- 
 ment .analogous to it in small works long before; such is that at 
 Waltham cross. 
 
 Of plain battlements there are several descriptions : 1st, 
 that of nearly equal intenals, with a plain round capping 
 running round with the outline. 2nd, The castellated battle- 
 ment, of nearly equal intervals, and sometimes with large bat- 
 tlements and small intervals, with the cap moulding running 
 only horizontal, and the sides cut plain ; this is perhaps 
 the best in point of effect of any. 3d, A battlement like 
 the last, with the addition of a moulding which runs round 
 the outline, and has the horizontal capping set upon it. 4th, The 
 most common later battlement, with the cap moulding broad, 
 of several mouldings, and running round the outline, and thus 
 often narrowing the intervals, and enlarging the battlement. To 
 one or other of these varieties, most battlements may be reduced ; 
 but they are never to be depended on alone, in determining the 
 age of a building, from the very frequent alteration they are 
 liable to. 
 
 Of Roofs. 
 
 Roofs may be conveniently divided into two principal divi- 
 sions ; the first including those in whic-h the sloped framing, carry- 
 7.~Vol. I. Y 
 
162 
 
 ARCHITECTURE. 
 
 English roofs. 
 
 ing the lead or other covering, is visible ; and the second those 
 which have an inner roof of various materials. 
 
 It is difficult to say what were the open Norman roofs, but 
 it seems most likely they exposed the rafters and other framing 
 of the roof, and probably had straight beams laid across the 
 walls of the nave over each pier. If any original roofs of this 
 kind remain, Rochester cathedral seems most likely to be one. 
 The first attempt to ornament these roofs, seems to have been 
 to make a timber arch over each pier, and to frame timbers in 
 squares diagonally, and these are sometimes made into quatre- 
 foils, and afterwards the arch framing became variously orna- 
 mented, till it came to the gorgeous hall roof, of w hich there are 
 many fine specimens, but perhaps few, if any, superior to that of 
 Westminster hall. The roof of St. Stephen’s chapel was of pecu- 
 liar beauty, and that of the chapter-house at Exeter, is much like 
 it. From the piers, springs an arch which is pierced in the span- 
 drells, and richly ornamented with pieroed featherings ; and the 
 sloping roof is constructed in small squares, beautifully orna- 
 mented with quatrefoils. 
 
 There are buildings in which, though the upper roof is 
 shown, there is a preparation for an inner roof ; such is Chester 
 cathedral, where only the lady chapel, and the aisles of the 
 choir, are groined, and the whole of the rest of the church is 
 open ; but on the top of the shafts are the commencement 
 springing of a stone groined roof. There is a chapel in a 
 church in Cambridgeshire, W illiugham, between Ely and Cam- 
 bridge, which has a very singular roof ; stone ribs rise like the 
 timber ones, the intervals are pierced, and the slope of the roof is 
 of stone ; it is high pitched, and the whole appears of decorated 
 character. 
 
 The second division, or inner roofs, are very various; from 
 history it seems as if the most early inner roof was fiat over 
 the beams, and these were planked and painted, as at St. Alban’s 
 and Peterborough. The latter is, indeed, a singularly beautiful 
 relic ; it has lately been repainted, as it w as originally, and now 
 presents an appearance of rich mozaic, like a carpet full of 
 stiff lines, and its general division is into lozenges, with fiowers of 
 Norman character, and the whole according in design wdth the 
 ornaments of tliat style. This kind of roof most likely contri- 
 buted much, especially when the exterior roof was covered with 
 shingles, to spread those destructive fires which we so frequently 
 read of in the history of the early churches. There are some 
 roofs of this construction, in country churches, but generally of 
 a very late date, and sometimes painted with wretched attempt* 
 at clouds. 
 
ARCHITECTURE. 
 
 English roofs. 
 
 There does not seem to be any wooden inner roofs, except 
 plaster groining, now remaining, till we come to the perpendi- 
 cular ordinary style of roofing, which was rich, though easily 
 constructed ; a rib crossed above the pier, with a small fiat arch, 
 and this M^as crossed by another in the centre of the nave, and 
 the spaces thus formed were again divided by cross ribs, till re- 
 duced to squares of two or three feet ; and at each intersection, a 
 flower, shield, or other ornament, was placed. This roof was 
 sometimes in the aisles made sloping, and occasionally coved. 
 In a few instances, the squares were filled with fans, &c. of small 
 tracery. 
 
 The next and most important mode of interior roofing, is 
 the regular groined roof ; and of this description w’e have a 
 regular and valuable series, from the plain Norman round 
 arched roof, to the elaborate pendanted roof of Henry the VII.^s 
 chapel. 
 
 The various Norman crypts, and some small churches, give 
 very good specimens of the Norman roof, which has simply 
 four cross springers, often without straight ribs from the opposite 
 piers. The cross springers w ere ornamented in the usual man- 
 ner with carvings of zigzag and other Norman ornaments. The 
 first pointed roof added nothing to the Norman, in ribs, except 
 the one from pier to pier. The ribs were often enriched by the 
 toothed ornament, and generally a boss or knot at the centre in- 
 tersections. Canterbury, some parts of Lincoln, but above all, 
 Salisbury cathedral, are admirable specimens of these roofs, w hich 
 were erected mostly in the time of the Early English style, or at- 
 tached to buildings of that date. 
 
 The next advance appears in the roof of the nave at Wells' 
 cathedral ; in this a plain rib runs longitudinally at the top, 
 crossing the rib from the piers, and also the intersection of the 
 cross springers, and another rib runs crossways at the top of the 
 window arches, crossing the centre intersection. To this soon 
 succeeded the multiplication of the ribs, which meet the longi- 
 tudinal straight cross rib, and at their intersections have bosses. 
 To this is added, in the richer roofs, short ribs running from one 
 of these bosses to another, and these are increased in the later 
 roofs, till the whole is one series of net work, of which the roof of 
 the choir at Gloucester, is one of the most complicated speci- 
 mens. The later monumental chapels and statuary niches, mostly 
 present in their roofs very complicated tracery.. 
 
 Of the ribbed roofs, which aie rich without being gorgeous, 
 perhaps York minster exhibits a specimen not inferior to any 
 other. 
 
 We now come to a new and most delicate description of 
 
164 
 
 Hr 
 
 ARCHITECTURE. 
 
 English roofs. 
 
 roof, that of fan tracery, of which probably the earliest, aud 
 certainly one of the most elegant, is that of the chnsters at Glo-u- 
 cester. In these roofs, from the top of the shaft springs a small 
 fan of ribs, which doubling out from the points of the pannels, 
 ramify on the roof, and a quarter or half circular rib forms the 
 fan, aud the lozenge interval is formed by some of the ribs of the 
 fan running through it, and dividing it into portions, which are 
 filled With ornament. King’s college chapel, Cambridge, Henry 
 the VIl.’s chapel, and the Abbey church at Bath, are the best 
 specimens, after the Gloucester cloisters; and to these may be 
 added the aisles of St. George’s, Windsor, and that of the eastern 
 addition to Peterborough. To some of these roofs are attached 
 pendants, which, in Henry the Vll.’s chapel, come down as low 
 as the springing line of the fans. 
 
 The roof of the nave and choir of St. George’s, Windsor, is 
 very singular, and perhaps unique. The ordinary proportion of 
 the arches and piers is half the breadth of the nave ; this makes 
 the roof compartments two squares, but at Windsor the breadth 
 of the nave is nearly three times that of the aisles, and this makes 
 a figure of about three squares. The two exterior parts are such 
 as, if joined, would make a very rich ribbed roof ; and the centre 
 compartment, which runs as a flat arch, is filled with tracery pan- 
 nels, of various shapes, ornamented with quatrefoils, and foiming 
 tw o halves of a star ; in the choir the centre of the star is a pen- 
 dant. d his roof is certainly the most singular, and perhaps the 
 richest in effect of any w e have ; it is profusely adorned with 
 bosses, containing shields, &c. 
 
 There still remains one more description of roof, which is 
 used in small chapels, but not common in large buildings; this is 
 the arch roof; in a few' instances it is found plain, with a simple 
 ornament at the spring and the point, and this is generally a hol- 
 low moulding with flowers, &c, but it is mostly pannelled. Of 
 this roof the nave of Bath is a most beautiful specimen. ’^I'he 
 arch is very flat, and is composed of a series of small rich pannels, 
 with a few large ones at the centre of the compartments formed 
 by the piers. Another beautiful roof of this kind is the porch to 
 Henry the VII.’s chapel ; but this is so hid, from the want of light, 
 as to be seldom noticed. 
 
 The ribbed roofs are often formed of timber and plaster, but 
 are generally coloured to represent stone work. 
 
 There may be some roofs of difterent arrangements from any 
 of these ; but in general they may be referred to one or other of 
 the above heads. 
 
ARCHITECTURE. 
 
 165 
 
 Ro.slyn chapel. — Alterations of ecclesiastical edifices. — Locality of style. 
 
 Miscellaneous Remarks on Buildings of English Architecture, 
 
 Having now given an outline of the details of the different 
 styles, it remains to speak of a fe^v matters which could not 
 so well be previously noticed. As one style passed gradually 
 into another, there will be here and there buildings partaking 
 of tM O, and there are many buildings of this description whose 
 dates are not at all authenticated. Litchfield cathedral is a line 
 insiance of the gradation from richer Early English into Deco- 
 rated, as are some of those delicate monuments, the crosses of 
 Edward the I. and many of the Lincolnshire spires. There are 
 also many beautiful gradations from Decorated to Perpendicu- 
 lar. Of these, the choir of York, and the upper part of the two 
 towers at the west end, and the remains of Melrose abbey, may 
 be mentioned. 
 
 There is one building which deserves especial mention, from 
 the singularity of its character, ornaments, and plan ; this 
 is Rosiyn chapel. It is certainly unclassable as a whole, being 
 unlike any other building in Great Britain of its age, (the 
 latter part of the fifteenth century,) but if its details are mi- 
 nutely examined, they will be found to accord most completely 
 in the ornamental work with the style then prevalent, though 
 debased by the clumsiness of the parts, and their w ant of propor- 
 tion to each other. I’here seems little doubt that the designer 
 was a foreigner, or at least. took some foreign buildings for his 
 model. 
 
 It will now be proper to add a few words on the alterations 
 and additions which most ecclesiastical edifices have received ; 
 and some practical remarks as to judging of their age. The 
 general alteration is that of window s, w hich is very frequent ; 
 very few' churches are without some Perpendicular window's. 
 B e 11] ay therefore pretty safely conclude that a building is at 
 least as old as its w indows, or at least that part is so which con- 
 tains the windows. But we can by no means say so with respect 
 to doors, which are often left much older than the rest of the 
 building. 
 
 A locality of style may be observed in almost every county, 
 and in the districts where flint abounds, it is sometimes almost 
 impossible to determine the date of the churches, from the absence 
 of batlleinents, architraves, and buttresses ; but wherever stone is 
 used, there is generally enough of indication to assign each part to 
 Its proper style, and with due regard to do the same with plates of 
 ordinary correctness, a little habitual attention would enable many 
 persons to judge at once, at the sight of a plate or drawing, of its 
 correctness, from its consistency, or the contrary, with the details 
 of its apparent style. 
 
ARCHITECTURE. 
 
 I6b 
 
 Grecian and English architecture contrasted. 
 
 1 
 
 In a sketch like the present, where the Author is confined to 
 a certain space, it is impossible to notice every variety ; hut at 
 least he now presents the world, for the first time, with a ra- 
 tional arrangement of the details of a mode of architecture on 
 many accounts valuable, and certainly the most proper for 
 ecclesiastical edifices. Still further to enable the reader to dis- 
 tinguish the principles of Grecian and English architecture, he 
 adds a few striking contrasts, which are formed by those princi- 
 ples in buildings of real purity, and which will at once convince 
 any unprejudiced mind of the impossibility of any thing like a 
 good mixture. 
 
 Grecian. 
 
 The general running lines 
 are horizontal. 
 
 English. 
 
 The general running lines ► 
 are perpendicular. 
 
 - 1 ^ 
 
 
 Arches not necessary. 
 
 An entablature absolutely 
 necessary, consisting always of 
 two, and mostly of three dis- 
 tinct parts, having a close re- 
 lation to, and its character and 
 ornaments determined by the 
 columns. 
 
 The columns can support 
 nothing but an entablature, and 
 no arch can spring directly from 
 a column. 
 
 Arches a really fundamental 
 principle, and no pure English 
 building or ornament can beHl 
 composed without them. JH 
 
 No such thing as an entab- 
 lature composed of parts, and 
 what is called a cornice, bears 
 no real relation to the shafts^ i 
 which may be in the same 
 building. \ 
 
 The shafts can only sup- | 
 port an arched moulding, and J 
 in no case a horizontal line. 1 
 
 A flat column may be called 
 a pilaster, which can be used 
 as a column 
 
 The arch must spring from 
 a horizontal line. 
 
 Columns the supporters of 
 the entablature 
 
 Nothing analagous to a pi- i 
 laster ; every flat ornamented 
 projecting surface, is either a j| 
 series of pannels, or a buttress. 
 
 No horizontal line necessa- | 
 ry, and nevf^r any but the small | 
 cap of a shaft. 
 
 Shaft bears nothing, and is 
 only ornamental, and the round 
 pier stili a pier. Hdt 
 
ARCHITECTURE.^ 
 
 167 
 
 Grecian and English architecture contrasted. 
 
 Grecian. 
 
 No projections like but- 
 tresses, and all projections stop- 
 ped by horizontal lines. 
 
 Arrangement of pediment 
 fixed. 
 
 Openings limited by the 
 proportions of the column. 
 
 Regularity of composition 
 on each side of a centre neces- 
 sary. 
 
 Cannot form good steeples, 
 because they must resemble un- 
 connected buildings piled on 
 each other. 
 
 JEnglish. 
 
 Buttresses essential parts, 
 and stop all horizontal lines. 
 
 Pediment only an orna- 
 mented end wall, and may be 
 of almost any pitch. 
 
 Openings almost unlimited. 
 
 Regularity of composition 
 seldom found, and variety of 
 ornament universal. 
 
 From its perpendicular lines, 
 may be carried to any practi- 
 cable height, with almost in- 
 creasing beauty. 
 
 In the foregoing details we have said nothing of castellated 
 or domestic architecture ; because there does not appear to be 
 any remains of domestic buildings, so old as the latest period of 
 the English style, which are unaltered; and because the cas- 
 tellated remains are so uncertain in their dates, and so much 
 dilapidated or altered, to adapt them to modern modes of life or 
 defence, that little clear arrangement could be made, and a care- 
 ful study of ecclesiastical architecture will lead any one, desirous 
 to form some judgment of the character of these buildings, to 
 the most accurate conclusions ou the subject which can well be 
 obtained in their present state. 
 
 Description of Plate I. 
 
 In order that the plate may be kept free from figures, which 
 would have taken olf greatly from its good effect, a description 
 of what it contains is annexed without letters of reference. And 
 It may be necessary to state, that in order to comprise as much as 
 possible of the different styles, in a small space, there has been an 
 unavoidable departure from the relative proportion of parts in 
 several instances, but not to any very great extent. 
 
168 
 
 ARCHITECTURE. 
 
 Description of Plate first. 
 
 The upper part of the plate contains sixteen detached speci- 
 mens, whicli, beginning at the left hand at the top, are as 
 follow : 
 
 1st, The plain semi-circular arch. 
 
 <2nd, The segmental arch. 
 
 3rd, The equilateral arch. 
 
 4th, The drop arch. 
 
 olh. The lancet arch. 
 
 6th, The horse-shoe arch. 
 
 7th, The ogee or contrasted arch. 
 
 8th, The four-centred or Tudor arch. 
 
 9th, A plain circular trefoil window, with ornamented ; 
 points. 
 
 10th, A plain trefoil window head. 
 
 lith, A triangular pannel feathered in three divisions, each of i 
 which is again feathered. 
 
 12th, The straight-headed cinquefoil, so much used in small 1 
 tracery of windows, in the eastern part of the kingdom. 
 
 13th, A plain circular quatrefoil window. mL 
 
 14th, The usual mode of cinquefoiling an ogee head, and it^K 
 insertion in a transom. ^ 
 
 15th, A very beautiful mode of double feathering a cinquefoil, ■ 
 from the east window' at York. 
 
 l6th, A plain circular cinquefoil w indow. 
 
 The lower design is divided by buttresses into three com- • 
 partments, of which the lovver part of the centre contains two 
 Norman windows, one with shafts, the other with the zigzag : 
 moulding, and having between them a Norman buttiess and a i 
 Norman cornice over them. The upper part of this compart- 
 ment is Early English, having three windows with shafts divided 
 by bands, and resting, on a tablet. The pediment has a pro- ] 
 jectiiig parapet with a cornice moulding under it, and^a cap \ 
 
 moulding above. This parapet is ornamented with sunk qua- i 
 trefoils. 
 
 ^riie left hand compartment, and its two buttresses, are De- 
 corated English. The window of flowing tracery is of the style 
 of that in the west end of Newark, but much of its delicate 
 small ornaments are obliged to be omitted, on account of its size. 
 The canopy is executed at Chester cathedral. The cornice is 
 tlie ordinary Decorated cornice with flowers. The battlement is 
 that of the nave of York minster. The pinnacles are of the de- 
 scription very common in the w'est of England, with square bases 
 and small battlements. 
 
 The right-hand compartment contains a perpendicular win- 
 dow of flve lights in a four-centred arch, with a crocketed ogee 
 
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ARCHITECTURE. 
 
 169 
 
 # 
 
 I 
 
 Plate second. — References to delineations of English architecture. 
 
 canopy. It is divided by a transom, and has the transom mould- 
 ing battlemented. This window is of simple construction, and is 
 executed with slight variations in various parts. 
 
 The cornice is tliat of the upper roof of the Abbey-church at 
 Bath, and the battlements are also taken frorh thence ; but frohi 
 the small size of the plate, both are obliged to be much simpli- 
 fied as to their mouldings. The pinnacles are the ordinaiy pin- 
 nacles of this style. 
 
 Plate II. affords an example of the square-headed door of 
 the Perpendicular style ; it has a shaft supporting the inner 
 moulding of the arch, and another supporting the exterior mould- 
 ings, which forms also the square, and both are included in a hol- 
 lovv moulding. The dripstone is nearly the most frequent plain 
 dripstone, and the cornice shows the effect of the introduction 
 of flowers. 
 
 Plates of almost all the buildings mentioned in this sketch, 
 may be found in one or other of the following works, which all 
 contain plates of good character, though of course perhaps not ail 
 of equal correctness in drawing the details : 
 
 The Cathedrals, &c. published by the Society of Antiquaries, 
 Carter’s Ancient Architecture, 
 
 King’s Munimenta Antiqua, 
 
 Halfpenny’s Engravings of York Minster, 
 
 Buckler’s Views of the Cathedrals, 
 
 Britton’s Architectural Antiquities, 
 
 Storer and Greig’s Illustrations of Pennant’s London, 
 
 Antiquarian and Topographical Cabinet, 
 
 Ancient Relics, 
 
 Lyson’s Magna Britannica, 
 
 Britton and Bra\ ley’s Beauties of England, 
 
 Archaeologia, 
 
 Chalmer’s History of Oxford. 
 
 8.— VoL. I. 
 
 Z 
 
170 
 
 ARCHITEGTURE. 
 
 Five orders of Grecian architecture distinguished. 
 
 GRECIAN ARCHITECTURE. 
 
 The many valuable treatises and excellent delineations of 
 the Grecian and Roman buildings, and the details of their 
 parts, will render unnecessary that minuteness in this dissertation 
 which, from the total absence of a previous system, it became 
 necessary to adopt in the description of the English styles. 
 But in this sketch something of a similar plan’ will be followed, 
 first giving the name and grand distinctions of the orders, then 
 describing the terms and names of parts necessary for those who 
 have not paid attention to the subject to understand, and a con- 
 cise description of each order will follow ; but from the diver- 
 sity of judgment prevailing, it will be most proper to leave the 
 reader to select his own examples, as in this country we have 
 not, as in the English architecture, the originals to study, but a 
 variety of copies, adapted to the climate and the convenience of 
 modern times. 
 
 In dividing the Grecian and Roman architecture, the word 
 order is used, and much more properly than style ; the English 
 styles regarded not a few parts, but the composition of the 
 whole building, but a Grecian building is denominated Doric or 
 Ionic, merely from its ornaments ; and the number of columns, 
 windows, &c. may be the same in either order, only varied in 
 proportion. 
 
 The orders are generally considered to be five, and are usually 
 enumerated as follows : 
 
 Tuscan, of which the usual height of the column is 7 diameters. 
 
 Doric, - - - 8 
 
 Ionic, - - - 9 
 
 Corinthian, - - - 10 
 
 Composite, - - - 10 
 
 Their origin will be treated of hereafter. Their prominent 
 distinctions are as follow : 
 
 The Tuscan is quite plain, without any ornament whatever. 
 
 The Doric is distinguished by the channels and projecting in- 
 tervals in the frieze called triglyphs. 
 
 The Ionic by the ornaments of its capital, which are spiral, 
 and are called volutes. 
 
 The Corinthian by the superior height of its capital, and its 
 being ornamented with leaves, which support very small 
 volutes. 
 
 The Composite has also a tall capital with leaves, but is dis- 
 tinguished from the Corinthian by having the large volutes of 
 the Ionic capital. 
 
ARCHITECTURE. 
 
 171 
 
 Division of a complete order. 
 
 A complete order is divisible into three grand divisions, which 
 are occasionally executed separately, viz. 
 
 The column, including its base and capital, 
 
 The pedestal, which supports the column. 
 
 The entablature, or part above and supported by the column. 
 
 These are again each subdivided into three parts. 
 
 The pedestal into base or lower mouldings ; dado or die, the 
 plain central space ; and surbase, or upper mouldings. 
 
 The column into base or lower mouldings, shaft or central 
 plain space, and capital or upper mouldings. 
 
 The entablature into architrave, or part immediately above 
 the column ; frieze or central flat space ; and cornice or upper 
 projecting mouldings. 
 
 These parts may be again divided thus : the lower portions, 
 viz. the base of pedestal, base of column, and architrave, divide 
 each into two parts ; the first and second into plinth and mould- 
 ings, the third into face or faces, and upper moulding or tenia. 
 
 Each central portion, as dado of pedestal, shaft of column, 
 and frieze, is undivided. 
 
 Each upper portion, as surbase of pedestal, capital of column, 
 cornice of entablature, divides into three parts : the first into bed- 
 mould, or the part under the corona ; corona, or plain face j and 
 cymatium, or upper moulding. 
 
 The capital, into neck, or part below the ovolo ; ovolo or 
 projecting round moulding ; and abacus or tile, the flat upper 
 moulding, mostly nearly square. These divisions of the 
 capital, however, are less distinct than those of the other 
 parts. 
 
 The cornice into bedmould, or part below the corona ; 
 corona, or flat projecting face; cymatium, or moulding above 
 the corona. 
 
 Besides these general divisions, it will be proper to notice a 
 few terms often made use of. The ornamental moulding run- 
 ring round an arch, or round doors and windows, is called an 
 architrave. 
 
 An ornamental moulding for an arch to spring from, ig 
 called an impost. 
 
 The stone at the top of an arch, which often projects, is called 
 a key-stone. 
 
 The small brackets under the corona in the cornices, are 
 called mutules or modiUions ; if they are square, or longer in 
 front than in depth, they are called mutules, and are used in the 
 Doric order. If they are less in front than their depth, they are 
 called modiUions, and in the Corinthian order have carved leaves 
 •pread under them. 
 
172 
 
 ARCHITECTURE. 
 
 Architectural mouldings. 
 
 A truss is a modillion enlarged, and placed flat against a wall, 
 often used to support the cornice of doors and w'indow'^s. 
 
 A console is an ornament like a truss carved on a key-stone. 
 
 Trusses, when used under modillions in the frieze, are called 
 cantaiivers. 
 
 1 he space under the corona of the cornice, is called a soffit, 
 as is also the under side of an arch. 
 
 Dentils are ornaments used in the bed mould of cornices; 
 they are parts of a small flat face, which is cut perpendicularly, 
 and small intervals left between each. 
 
 A flat column is called a pilaster ; and those w'hich are used 
 with columns, and have a diflerent capital, are called ant&. 
 
 A small height of panne! ling above the cornice, is called an 
 attic, and in these pannels, and sometimes in other parts, are in- 
 troduced small pillars, swelling towards the bottom, called balus- 
 tres, and a series of them a balustrade. 
 
 If the joints of the masonry are channelled, the w ork is called 
 rustic, w hich is often used as a basement for an order. 
 
 Columns are sometimes ornamented by channels, which are 
 called Jhites. These channels are sometimes partly filled by a 
 lesser round moulding ; this is called cabling the flutes. 
 
 For the better understanding the description to be given of the 
 orders, it will be proper first to notice the different moulding: 
 which by different combinations form their parts. 
 
 I he most simple mouldings are, 1st, the Ovolo, or quarter 
 round, see pi. III. fig. 1. 
 
 2nd, The caveito, or hollow, fig. 2. 
 
 3rd, The torus, or round, fig. 3. 
 
 From the composition of these are formed divers others, and 
 from the arrangenieut of them, with plain flat spaces between, 
 are formed cornices and other ornaments. A large flat space is 
 called a corona if in the cornice ; a fascia in the architrave ; and 
 the frieze itself is only a flat space. A small flat face is called 
 a fillet, or listel, and is interposed between mouldings to divide 
 them. 
 
 A fillet is, in the bases of columns and some other parts, 
 joined to a face, or to the column itself by a small hollow^, then 
 called apophijges, fig. 4. 
 
 The torus, when very small, becomes an astragal, w'hich 
 projects, fig. 5 ; or a bead, which does not project, fig. 6, 
 No. 1, 2. 
 
 Compound mouldings are, the cyma recta, which has the 
 hollow uppermost and projecting, fig. 7. 
 
 The cyma reversa, or ogee, which has the round uppermost 
 and projecting, fig. 8. 
 
ARc mTECTVltK . 
 
 £<rrl(*u 
 
 Puf-hth 'J hv yutfitf/ Fi^ftfr k Dhv.'n JSlz. 
 
ARCHITECTURE/ 
 
 173 
 
 Tuscan order. 
 
 The scotia ; which is formed of two hollows, one over the 
 other, and of different centres, fig. Q* 
 
 In the Roman works, the mouldings are generally worked 
 of equal projection to the height, and not bolder than the above 
 regular forms; but the Grecian mouldings are often bolder, and 
 worked with a small return, technically called a quirky and these 
 are of various proportions. 
 
 7 he ogee and ovolo are most generally used as represented by 
 fig. 10 and 11. 
 
 Several beads placed together, or sunk in a flat face, are 
 called reedings, see fig. 12. 
 
 All these mouldings, except the fillet, may be occasionally 
 carved, and they are then called enriched mouldings. 
 
 From these few simple forms, by adding astragals and fillets, 
 and combining differently ornamented mouldings, faces, and soffits, 
 are all the cornices, pannels, &c. formed, and ihe modern com- 
 positions in joiners’ work, &c. are very numerous, and too well 
 known to need describing. 
 
 Tuscan Order. 
 
 Though this is not, perhaps, the most ancient of the orders, 
 yet, from its plainness and simplicity, it is usually first noticed. 
 Its origin is evidently Italian ; for the Grecian work, however 
 plain, has still some of the distinctive marks of massive Doric, 
 whilst the Tuscan always bears clear marks of its analogy to the 
 Roman Doric. 
 
 The pedestal, when used, is very plain, but it is more often 
 set on a plain square block plinth, which suits the character of 
 the order better than the higher pedestal. This block projects 
 about half the height of the plinth of the base beyond its 
 face. 
 
 The column, including the base and capital, is about seven 
 diameters high. The column, in this, and indeed in all the 
 orders, is diminished the upper two-thirds of its height, so 
 that the diameter under the neck of the capital is from one-sixth 
 to one-fourth less than that just above the base. This diminu- 
 tion is not conical, but bounded by a curved line, which is 
 variously determined, but does not differ much from what an 
 even spring would assume, if one part of it were bound, in the 
 direction of the axis of the shaft, to the cylindrical third, and 
 then, by pressure at the top only, brought to the diminishing 
 point. 
 
 The Tuscan base is half a diameter in height, and consists 
 cf a plain torus with a fillet and apophyges, which last is part 
 
174 
 
 ARCHITECTURE. 
 
 Tuscan order. 
 
 of the shaft, and not of the base, as indeed all apophygae are 
 considered to be ; and also all the astragals underneath the capi- 
 tals, as well as the upper fillet of the base in all the richer orders, 
 and in masonry should be executed on the shaft stones. 
 
 The capital of the Tuscan order is (exclusive of the astra- 
 gal) half a diameter in height, and consists of a neck on which 
 is an ovolo end fillet, joined to the neck by an apophyges, and 
 over the ovolo a square tile, which is ornamented by a projecting 
 fillet. 
 
 The shaft of this order is never fluted, but some architects 
 have given to this order, and some have even added to the richer 
 orders, an ornament (if so it may be called) of large square 
 blocks as parts of the shaft, which are called rustications, and 
 are sometimes roughened. 
 
 The Tuscan entablature should be quite plain, having 
 neither^ mutules nor modillions. The architrave has one or some- 
 times tw'O faces, and a fillet; the frieze quite plain, and the 
 cornice consisting of a plain cyma recta for cymatium, and the 
 corona with a plain fillet, and small channel for drip in the 
 soffit. The bedmould should consist of an ovolo fillet and 
 cavetto. 
 
 This Tuscan is that of Palladio ; some other Italian architects 
 have varied in parts, and some have given a sort of block modil- 
 lions like those used in Coveiit-Garden church ; but these are of 
 wood, and ought not to l)e imitated in stone. 
 
 This order is little used, and will most likely, in future, be 
 still less so, as the massive Grecian Doric is an order equally ma- 
 nageable, and far more elegant. 
 
 Having explained the parts of one order, it will be necessary 
 to make a few remarks, which could not so well be previously 
 introduced. — If pilasters and columns are used together, and 
 they are of the same character, and not antae, the pilasters should 
 be diminished like the columns ; but where pilasters are used 
 ?lone, they may be undiminished. 
 
 The moulffing under the cymatium, which, in rich orders, 
 is often an ogee, is part of the corona, and as such is continued 
 over the corona in the horizontal line of pediments, where the 
 cymatium is omitted ; and is also continued with the corona in 
 interior work, where the cymatium is often with propriety 
 omitted. 
 
 In pediments, whose cornices contain mutules, modillions, 
 or dentils, those in the raking cornice must be placed perpen- 
 dicularly over those in the horizontal cornice, and their sides 
 must be perpendicular, though their under parts have the rake of 
 the cornice. 
 
ARCHITECTURE. 
 
 175 
 
 Grecian Doric. 
 
 Doric Order. 
 
 The ancient Grecian Doric appears to have been an order of 
 peculiar nobility, simple and bold, its ornaments were the 
 remains of real utility, and perhaps originally it was worked 
 with no moulding but the cymatium, to cover the ends of the 
 tiles, its triglyphs being the ends of the beams, ami its mutules 
 those of the rafters. In after times its proportions were made 
 rather less massive, and its mouldings and ornaments, though not 
 numerous, were very beautiful. The Romans considerably 
 altered this order, and by the regulations they introduced, ren- 
 dered it peculiarly difficult to execute on large buildings. As the 
 examples of the tw o countries are very different, we shall treat 
 of them separately, and first therefore of the 
 
 Grecian Doric. 
 
 The columns of this order were, in Greece, generally placed 
 on the floor, without pedestal and without base; the capital, 
 which occupied a height of about half a diameter, had no as- 
 tragal, but a few plain fillets, with channels betw^een them, under 
 the ovolo, and sometimes a small channel under the fillets. The 
 ovolo is generally flat, and of great projection, with a quirk or 
 return. On this was laid the abacus, which was only a plain tile, 
 without fillet or ornament. 
 
 In the division of the entablature, the architrave and frieze 
 have each more than a third in height, and the cornice less. 
 The architrave has only a plain broad fillet, under which are 
 placed the -drops or guttae, which appear to hang from the tri- 
 glyphs. 
 
 The triglyph, in Greece, appears to have been generally 
 placed at the angle, thus bringing the interior edge of the tri- 
 glyph over the centre of the angular column. The metope, or 
 space between the triglyphs, being the square of the height of 
 the frieze, and a mutule was placed not only over each triglyph, 
 but also over each metope, and it appears probable that the 
 triglyphs were often omitted, except over the centre of each 
 column, and at the angle, and hence the order would be easily 
 worked at any desirable intercolumniation. The cornice of this 
 order, in Greece, consisted of a plain face, under the mutule, 
 which w^as measured as part of the frieze, and then the mutule, 
 which projected sloping forward under the corona ; so that the 
 bottom of the mutule in front was considerably lower than at 
 the back, where, in some examples, an ogee is placed under the 
 mutule. Over the corona was commonly a small ovolo and fillet, 
 and then a larger ovolo and fillet for the cymatium. 
 
176 
 
 ARCHITECTURE. 
 
 Roman Doric. 
 
 The ornaments of this order, in Greece, were, 1st, the flut- 
 ings of the column, which are peculiar to the order, and are 
 twenty in number, shallow, and not with fillets between them, but 
 sharp edges. These flutes are much less than a semicircle, and 
 should be elliptic^ 
 
 2d, At the corner, in the space formed in the soffit of the 
 corona, by the interval beween the two angular mutules, was 
 sometimes placed a flower, and the cymatium of the cornice had 
 lions’ heads, which appear to have been real spouts. 
 
 3d, In addition to the drops under the triglyph, the mutules 
 also have several rows of drops of the same shape and size. 
 
 This order appears in general to have been worked very 
 massive, the best examples are about six diameters high, which 
 is lower than the Italians usually worked the Tuscan ; but this 
 gave peculiar nobility to the temples in which it is thus employ- 
 ed. It does not appear that a pedestal was used in Greece with 
 this order. 
 
 Roman Doric, 
 
 This differs from the Grecian in several important particu- 
 lars, which will appear from the following rules, from the' 
 strictness of which follows that extreme difficulty of execution 
 which has been so often complained of in this order: 1st, The 
 triglyphs must be precisely over the centre of the columns. 
 2d, the metopes must be exact squares; 3d, the mutules also 
 must be exact squares. 
 
 As, therefore, the intercolumniation must be of a certain num- 
 ber of triglyphs, it will be easily conceived how difficult it will 
 be, in large buildings, where a triglyph is several feet, to accom- 
 modate this order to the internal arrangements. 
 
 The Roman Doric is sometimes set on a plinth, and some- 
 times on a pedestal, which should be of few and plain mould- 
 ings. The bases usually employed, are either the attic base of 
 a plinth, lower torus, scotia, and upper torus, with fillets 
 between them, or the proper base of one torus and an astragal ; 
 or, in some instances, of 'a plinth and simple fillet. The shaft, 
 including the base and capital, each of which is half a diame- 
 ter, is generally eight diameters high, and is fluted like the 
 Grecian. The capital has an astragal and neck under the ovolo, 
 which has sometimes three small fillets projecting over each 
 other, and sometimes another astragal and fillet. The ovolo 
 should be a true quarter round. The abacus has a small ogee 
 and fillet on its upper edge. 
 
 I'he architrave has less height than the Grecian, being only 
 two-thirds of the frieze, which is equal in height to the cornice^ 
 
ARCHITECTURE. 
 
 177 
 
 Roman Doric. — Ionic order. 
 
 Ill a few instances the architiave has two faces, but mostly only 
 one. 
 
 The frieze has notliiug peculiar to this mode ; if plain, its 
 metopes being, as before observed, square. 
 
 The cornice diders much from the Grecian, having its soffit 
 flat, and the mutules square, with a square interval between them. 
 The Grecian drops in the mutules generally appear in front, 
 below' the mutules ; but the Roman do not, and are sometimes 
 omitted. 
 
 The cymatium is often a cavetto, with an ogee under it, 
 and sometimes a cyma recta. The mutules have a small ogee, 
 which runs round them, and also round the face they are 
 formed of ; and under the mutules are an ovolo and small fillet, 
 and the flat fillet which runs round the top of the triglyphs here 
 belongs to the cornice, and not, as in the Grecian, to the 
 frieze. 
 
 In some instances, dentils appear to have been used in the 
 Doric cornice at Rome, but there are not many examples of this 
 practice. 
 
 The Roman Doric is susceptible of much ornament, for in 
 addition to the flutes, the guttse of the triglyphs, and the roses 
 ill the soffit of the corona, the neck of the capital has sometimes 
 eight flowers or husks placed round it, the ovolo carved, and the 
 metopes in the frieze filled w ith alternate ox skulls and pateree, or 
 other ornaments. In interior decorations sometimes one or two 
 of the mouldings of the cornice are enriched ; but w'ith all this 
 ornament, the Roman Doric is far inferior, in real beauty, to the 
 Grecian. 
 
 Ionic Order, 
 
 .As the Greeks and Romans differed much in their modes of 
 working the Doric order, so there was considerable difference in 
 their execution of the Ionic, though by no means so great as in 
 the former. 
 
 The distinguishing feature of this order is the capital, which 
 has four spiral projections called volutes. These in Greece 
 were placed flat on the front and back of the column, leaving 
 the tw'o sides of a different character, and forming a balustre; 
 but this at the external angle producing a disagreeable effect, 
 an angular volute was sometimes placed there, showing two 
 volutes to each exterior face, and a balustre to each interior ; 
 but this not forming a good combination, a capital w'as invented 
 with four angular volutes, and Uie abacus with its sides hol- 
 lowed out. This is called the modern Ionic capital. In the 
 ancient^ the list or spiral line of the volute runs along the face 
 8. — VoL, I. 2 A 
 
178 
 
 ARCHITECTURE. 
 
 Ionic order. 
 
 of the abacus, straight under the ogee ; but in the modern, this list 
 springs from behind the ovolo, and in the hollow of the abacus, 
 which is an ovolo, fillet, and cavetto, is generally placed a flower. 
 
 The Ionic shaft, including the base, which is half a diameter, 
 and the capital to the bottom of the volute generally a little more, 
 is nine diameters high. 
 
 The pedestal is a little taller and more ornamented than the 
 Doric. 
 
 The bases used to this order are very various ; some of the 
 Grecian examples are of one torus and two scotia, with astragals 
 and fillets ; others of two large tori and a scotia of small projec- 
 tion ; but the attic base is very often used, and with an astragal 
 added above the upper torus, makes a beautiful and appropriate 
 base for the Ionic. 
 
 The cornices of this order may be divided into three divisions ; 
 1st, the plain Grecian cornice; 2nd, the dentil cornice; 3d, the 
 modillion cornice. 
 
 In the first, the architrave is often of one face, the frieze 
 plain, and the cornice composed of a corona with a deep soffit, 
 and the bedmould moulding hidden by the drip of the soffit, or 
 coming very little below it, and sometimes with a -plain dentil set 
 close under the corona. The cymatium generally a cyma recta, 
 and ogee under it. 
 
 The second, which was mostly used with the ancient capital 
 at Rome, has generally two faces in the architrave, and the cor- 
 nice, which is rather more than one-third of the height of the en- 
 tablature, has a corona with a cyma recta and ogee for cymatium, 
 and for bedmould a dentil face between an ovolo and ogee. The 
 soffit of the corona sometimes ornamented. 
 
 The third, or modillion entablature, has the same architrave, 
 frieze, and cymatium of its cornice, as the last, but under the soffit 
 of the corona are placed modillions, which are plain and sur- 
 rounded by a small ogee ; one must be placed over the centre of 
 each column, and one being close to the return makes a square 
 pannel in the soffit at the corner, and between each modillion, 
 which is often filled with a flower. The bedmould below is gene- 
 rally an ovolo fillet and cavetto. 
 
 It was once the custom to work the Ionic frieze projecting 
 like a torus, thus giving an awkward weight to an order which 
 ought to be light. The introduction of good Grecian models 
 has driven out this impropriety, and much improved the 
 present execution of the order, which is very beautiful, if well 
 executed. 
 
 The Ionic shaft may be fluted in twenty-four flutes, with 
 filets between them ; these flutes are semi-circular. This order 
 
ARCHITECTURE. 
 
 179 
 
 Corinthian order. 
 
 may be much ornamented if necessary, by carving the ovolo of 
 the capital, the ogee of the abacus, and one or two mouldings of 
 both architrave and cornice ; but the ancient Ionic will look ex- 
 tremely well without any ornament whatever. 
 
 Corinthian Order. 
 
 This order originated in Greece, and the capital is said to 
 have been suggested by observing a tile placed on a basket left in 
 a garden, and round which sprung up an acanthus. All the other 
 orders have, in various countries and situations, much variety; 
 but the Corinthian, though not without slight variations even in 
 the antique, is much more settled in its proportions, and its greater 
 or less enrichment is the principal source of variety. 
 
 The capital is the great distinction of this order, its height 
 is more than a diameter, and consists of an astragal, fillet, and 
 apophyges, all which are measured with the shaft, then a bell 
 and horned abacus. The bell is set round with two rows 
 of leaves,, eight in each row, and a third row of leaves sup- 
 port eight small open volutes, four of which are under the 
 four horns of the abacus, and the other four, which are 
 often interwoven, are under the central recessed part of the 
 abacus, and have over them a flower or other ornament. These 
 volutes spring out of small twisted husks placed between the 
 leaves of the second row, and which are called caulicoles. The 
 abacus consists of an ovolo, fillet, and cavetto, like the modern 
 Ionic. There are various modes of indenting the leaves, which 
 are called, from these variations, acanthus, olive, &c. The co- 
 lumn, including the base of half a diameter, and the capital, is ten 
 diameters high. 
 
 If a pedestal is used, it should have several’ mouldings, some of 
 which may, if necessary, be enriched. The base may be either an 
 attic base, or with the addition of three astragals, one over each 
 torus, and one between the scotia and upper torus ; or a base of 
 two tori and two scotia', which are divided by two astragals : one 
 or two other varieties are sometimes, but not often, used. 
 
 The entablature of this order is very fine. The architrave 
 has mostly two or three faces, which have generally small ogees 
 or beads between them. 
 
 The frieze is flat, but is often joined to the upper fillet of the 
 architrave by an apophyges. 
 
 The cornice has both modillions and dentils, and is usually 
 thus composed: above the corona is a cymatium, and small ogee* 
 under it the modillions, whose disposition, like the Ionic, must be 
 one over the centre of the column, and one eloic to the retuin o| 
 the cornice. 
 
180 
 
 ARCHITECTURE. 
 
 Composite order. 
 
 These inodillions are carved with a small balustre front, and 
 a leaf under them ; they are surrounded at the upper part by a 
 small ogee and lillet, which also runs round the face they spring 
 from. Under the inodillions is placed an ovolo, and then a fillet 
 and the dentil face, which is often left uncut in exterior work. 
 Under the dentils are a fillet and ogee. In some cases this order 
 is properly worked with a plain cornice, omitting the inodillions, 
 and leaving; the dentil face uncut. 
 
 O ^ • • 
 
 The enrichments of this order may be very considerable ; 
 some of the mouldings of the pedestal and base may be enriched. 
 The shaft may be fluted as the Ionic, in twenty-four flutes, which 
 may be fdled one-third high by staves, which is called cabling the 
 flutes. The small mouldings of the -architrave, and even some 
 of its faces, and several mouldings of the cornice, may be en- 
 riched ; the squares in the soffit of the corona pannelled and 
 flowered, and the frieze may be adorned with carvings. But 
 though the order will bear all this ornament without overloading 
 it, yet, for exteriors, it seldom looks better than when the capitals 
 and the modillions are the only carvings. 
 
 The Composite Order. 
 
 The Romans formed this order by mixing the Corinthian 
 and Ionic capitals ; like the Corinthian, the capital is its prin- 
 cipal distinction. This is of the same height as the Corinthian, 
 and it is formed by setting, on the two lower rows of leaves of 
 the Corinthian capital, the modern Ionic volutes, ovolo, and 
 abacus. The small space left of the bell is filled by caulicoles, 
 with flowers, and the upper list of the volute is often flowered. 
 
 The column is of the same height as the Corinthian, and the 
 pedestal and base differ very little from those of that order, the 
 f)edestal being sometimes a little plainer, and the base having an 
 astragal or two less. 
 
 The entablature mostly used with this order is plainer than 
 the Corinthian, having commonly only two faces to the archi- 
 trave, the upper mouldings being rather bolder ; and the cor- 
 nice is different, in having, instead of the modillion and dentil, 
 a sort of plain double modillion, consisting of two faces, the 
 upper projecting farthest, and separated from the lower by a 
 small ogee. Under this modillion is commonly a large ogee 
 astragal and fillet. 
 
 A plain cornice, nearly like that used to the Corinthian order, 
 is sometimes used to this order, and also a cornice with the modil- 
 lions bolder, and cantalivers under them in the frieze. 
 
 This order may be enrich-ed in the same manner as the 
 Corinthian. 
 

 
 /:ii',>r,u\'i/ />;/ /)>x^ i.t. 
 
 I'n .VnthiH iWilti’rX-Dixt'ii, /ci, 
 
 
ARCHITECTURE. 
 
 181 
 
 Compos^^d orders. — Emlyn’s attempt to form a new order. 
 
 Having gone through the most usual forms and distinctions 
 of the orders, it is proper to say, that, even in Greece and 
 Rome, we meet with specimens whose proportions and compo- 
 sition do not agree with eitiier of them. These are comprised 
 under the general name of composed orders, and though some 
 are beautiful as small works, scarcely any of the ancient ones 
 are worthy of imitation in large buildings, and modern compo- 
 sition has run very wild, and produced scarcely any thing 
 worth prolonging by description. There was, however, one 
 attempt of a singular kind, made some years since by an 
 architect at Windsor, who published a magnificent treatise, and 
 executed one portico and a few door-cases in and near Windsor. 
 This was H. Emlyn, who conducted the restoration of St. 
 George’s chapel. His order, he says, was first brought into his 
 mind by the twin trees in Windsor forest. He makes an oval 
 shaft rise about one-fourth of its height, and then tw o round shafts 
 spring from it close to each other, and the diminution affords 
 space for tw'O capitals, wdiich have volutes, and, instead of leaves, 
 feathers like the caps of the knights of the garter. His entabla- 
 ture has triglyphs, and his cornice mutules. The triglyphs are 
 ostrich feathers, the guttae acorns, and the metopes are filled with 
 the star of the garter. 
 
 To conceal tire awkward junction of the tw^o columns to the 
 lower part, an ornament is placed there, which is a trophy with 
 the star of the garter in the centre. 
 
 It is obvious that this order must be extremely unmanage- 
 able, as it is difficult, and indeed almost impossible, to make a 
 good angle column, and if its entablature is proportioned to 
 the diameter of one column, it will be too small ; if to the whole 
 diameter, it will be too heavy, and a mean will give the capitals 
 wrong ; so that in any shape some error arises. In the portico 
 above mentioned, the entablature is so light as to appear pre- 
 posterous. This attempt is not generally known, as the book 
 was very expensive, and the portico at a distance from a public 
 road ; but it deserves consideration, because, though the idea 
 was new, its execution seems completely to have failed; and, 
 indeed, in large designs, no composed order has ever yet appeared 
 that can come into competition with a scrupulous attention to 
 those excellent models of Greece and Rome, now, through the 
 effects of graphic art, happily so familiar to alnaost every English 
 architect. 
 
182 
 
 BUILDING. 
 
 Durability of the bricks of the ancients. — Means of equalling them. 
 
 BUILDING, 
 
 Bricklaying is the art of building with bricks. 
 
 Of Bricks. 
 
 Previous to entering upon details belonging exclusively to the^ 
 art of bricklaying, we shall introduce an account of the manufac-^ 
 ture and dift'erent sorts of bricks. 
 
 On comparing the strength and durability of modern bricks> 
 with those of the ancients, it is evident that the former are in 
 every respect inferior. The ancients appear not only to have 
 selected the best sort of clay, but to have combined it with 
 other ingredients well adapted to improve its properties. Pro- 
 fessor Pallas, on his last journey through the southern provinces 
 of Russia, discovered, in the stupendous Tartar monuments, 
 bricks which would scarcely yield to the hammer. i\nother cir- 
 cumstance contributing materially to the excellence of the bricks 
 and tiles manufactured by the ancients, arose from their burning 
 them uniformly, after they had been thoroughly dried. No doubt 
 can be entertained, that if modern brickmakers were to pay more 
 attention to their art, by digging the clay at projjer seasons, ex- 
 posing it much longer to the air than is done at present, working 
 it sufficiently, bestowing more care upon the burning of the 
 bricks, and that tJie latter operation may be done uniformly, mak- 
 ing them much thinner than is prescribed by the standard form, 
 we should be provided with bricks equal in point of strength and 
 durability to the best of former times. In a variety of instances, 
 persons may have it in their power to obviate some of these 
 causes of defect, and it is therefore proper to mention them ; but 
 the state of society is not favourable to any general change in the 
 system which brickmakers pmsue; the expedition wuth w'hich a 
 given number of bricks can be furnished, and the cheapness of the 
 rate at which they can be manufactured, are to them the primary 
 objects of consideration ; and the speculative builder cares little 
 about the real value and durability of his edifice, provided the dif- 
 feience between the cost and sale price is sufficiently ample to 
 recompense him to his satisfaction. 
 
 It is an erroneous notion that bricks may be made of any 
 earth that is not stony, or even of sea ouse; too much sand 
 
BUILDING. 
 
 185 
 
 Nature of the earth fit for bricks — method of preparing it. 
 
 entering into their composition, renders them heavy and brittle, 
 and too much fat argillaceous matter causes them to crack in 
 drying : those only Avill burn red which contain iron par- 
 ticles. In England they are chiefly made of a motley, yellow- 
 ish, or somewhat reddish fat clayey earth, commonly called 
 loam. Those of Stourbridge clay, and Windsor loam, are 
 esteemed the most proper and durable bricks, and they will 
 stand very high degrees of heat without melting. The earth for 
 this manufacture should not contain too many calcareous and 
 fenuginous ingredients ; as the former prevent the mass from 
 becoming Arm in burning, and occasion the bricks to crumble 
 when exposed to the air ; while a superabundance of iron particles 
 retards the preparation of bricks so much as often to occasion 
 considerable difficulty in giving them due consistence. This 
 inconvenience may, however, be remedied, by allowing the 
 clay to lie a considerable time under the influence of the atmo- 
 sphere, then soaking it in pits, and afterwards working it well 
 in the usual manner. The common potter’s clay, which is also 
 employed in the manufacture of bricks, is opake, impaits a 
 slight colour, sometimes yellowish, bluish, greenish, but more 
 frequently of different shades of light grey ; when kneaded and 
 spread, it becomes smooth and glossy ; it is soft, fat, and cold, 
 somewhat agreeable to the touch, slightly adheres to the tongue, 
 has a fine earthy fracture, and is very plastic. When of the best 
 quality, it is not remarkable either for its lightness, or weight. 
 By chemical examination it is found to consist of thirty-seven 
 parts of pure argillaceous or clayey earth, and sixty-three parts 
 of siliceous or flinty earth. 
 
 Of whatever description the earth intended for bricks may 
 be, it ought to be dug after midsummer, that is, between the 
 beginning of July and the latter end of October, before the first 
 frost appears ; it should be repeatedly worked with the spade 
 during the winter, and not formed into bricks till the following 
 spring. If the earth were not used till two or three years after 
 it had been dug, the quality of the bricks produced would be 
 very materially improved ; and in all cases, the oftener it is turned 
 and the more completely it is incorporated, the better will be 
 4he bricks. 
 
 The clay, before it is put into pits for soaking, must be 
 broken as small as possible, and allowed to lie at least ten days ; 
 every stratum of twelve inches should be covered with water, 
 ^n order that it may be uniformly softened. Two pits, at least, 
 will be necessary for every brick manufactory, so tliat, after 
 having been suffered to remain for five days, the second may be 
 prepared, and thus the manufacture carried ou without iuter- 
 
184 
 
 BUILDING. 
 
 TemJ)ering of brick earth. — Burning brick.s in kilns. — Gallon’s experiment. 
 
 ruption. The earth should as much as possible be divested of 
 stony particles, and other extraneous matter, and should have 
 sufficient time to mellow and ferment, otherwise it will be diffi- 
 cult to temper. On the treading and tempering, twice the cus- 
 tomary quantity of labour ought to be bestowed. Much of the 
 goodness of bricks depends upon the proper management of its 
 first preparation, for the earth itself, previous to its being wrought, 
 possesses very little tenacity ; but by long exposure to the air and 
 frost, and thoroughly working and incorporating it together, it is 
 converted into a tough, gluey substance, in which state alone it is 
 ht for moulding. 
 
 In the vicinity of London, coal ashes, and in other parts 
 of the country light sandy earth, is usually mixed with the clay,^ | 
 which, by such addition, is more easily and expeditiously/ i 
 wrought, and requiring rather less fuel, occasions some savingffi 
 in the expence of burning the bricks; but here the advantage^X 
 of it terminate ; in other respects it is injurious rather than other- U 
 wise. If in tempering the earth, too much water be used, the 
 bricks become dry and brittle ; but if duly tempered, they will 
 be smooth, solid, hard, and durable. A brick properly made,^ ^ 
 requires nearly as much earth as a brick and a half made in the] P 
 common way, when too great a proportion of water has been* | 
 added, which tends to render the bricks spongy, light, and full* “ 
 of flaws. As bricks made in the best manner are more solid 
 and ponderous than the common ones, they require a much 
 longer time to dry; they ought not to be burnt till they will 
 give a hollow’ sound on collision. Proper attention to the drying 
 of bricks is necessary to prevent their cracking and crumbling in 
 the kiln. 
 
 Of whatever materials the kiln be constructed, each burning of 
 from six to ten thousand bricks requires the Are to be kept up at 
 least for twenty-four hours, and double that time for a number of 
 from twelve to fifty thousand. The uniform increase of heat 
 deserves particular attention ; its duration should be regulated ae- ' 
 cording to the season ; in cold weather fire burns most fiercely. ' 
 During the last tw enty-four hours, the fire should be uninterrupt- ■ 
 edly supported by means of flues ; but afterwards the fire should 
 not be suddenly closed, as there is always some danger of bursting 
 the flues or melting the bricks. 
 
 The following experiment, by Gallon, made with a view 
 to ascertain the difierence in the quality of biicks diflerently 
 manufactured, deserves to be generally known. A certain 
 quantity of the earth prepared for moulding into bricks W'as 
 taken for the experiment; at the end of seven hours, it was 
 moistened and beaten for the space of thiity minutes. The 
 
BUILDING. 
 
 185 
 
 Strength of brick differently made. — Moulding of bricks. 
 
 next morning the same operation was repeated for an equal 
 length of time ; in the afternoon it was again beaten for fifteen 
 minutes. Thus this earth had not only been worked for an 
 hour and a quarter longer than usual, but at three different 
 times ; the consequence w as, that its density w as increased ; for 
 a brick made of it weighed five pounds eleven ounces, while 
 another brick made in the same mould, of the earth that had not 
 received this preparation, weighed only five pounds seven ounces. 
 The two sorts of bricks w^ere dried in the air, for the space of 
 thirteen days ; they were then burnt with others, without any par- 
 ticular precautions, and when they were taken from the kiln, it 
 was found that the bricks made of the earth which had been 
 most worked, still weighed foui ounces more than the others, 
 each having lost five ounces by the evaporation of the moisture. 
 They differed also very remarkably in strength ; for on placing 
 them with the centre on a sharp edge, and loading the two ends, 
 the bricks formed with the well-tempered earth were not broken 
 with a less weight than sixty-five pounds, or one hundred and 
 thirty pounds in all ; while the others w ere broken with thirty-five 
 pounds at each end, or seventy pounds in the whole. That the 
 quality of bricks should be improved, by bestowdng more labour 
 upon the preparation of the earth, will hardly excite surprise, 
 though the degree of the improvement, as just stated, may cer- 
 tainly be considered remarkable ; but there is another mode of 
 strengthening these artificial stones, still more extraordinary, and 
 not so easy to be accounted for. Goldham observes, that bricks 
 which have been once burnt, then steeped in water, and burnt 
 again, become doubly strong. We know not that this observation, 
 which is repeated without comment, by nearly all the writers w ho 
 have occasion to treat of this subject, will always be verified in 
 practice, but it deserves attention, from the number and respect- 
 ability of the writers who have contributed to give it currency. 
 
 When the earth is sufficiently prepared, it is brought to the 
 bench of the moulder, who works the clay into the brickmould, 
 which he has previously dipped in dry sand lying near him for 
 the purpose; he then strikes off the superfluous soil with a flat 
 smooth stick, which is dipped in water before it is used. I'he 
 bricks, as they are delivered from the mould, are ranged on the 
 ground, wdth a small interval betw een each ; in piling row 
 upon row, they are laid rather crosswise, and sand is thrown 
 over them, to prevent their adhering to each other. As soon as 
 they have acquired sufficient hardness to admit of handling, 
 they are dressed with a knife, turned, and reset more open, 
 A gang, consisting of six persons, will make twenty thousand 
 bricks in the course of a week. The time requiied for drying 
 8.— VoL. I, 2 B 
 
186 
 
 BUILDING. 
 
 Burning of bricks in kilns — clamps. 
 
 them necessarily varies with the state of the weather ; if this be 
 favourable, fourteen or sixteen days will often be sufficient. In 
 showci-y weather, the piles are usually protected from its bad 
 effects by some cheap covering, such as straw, or old light boards. 
 In grounds not extensive, sheds are often erected. 
 
 When the bricks have been sufficiently dried in the air, they 
 are burnt, which is done either in a kiln, or in a stack, tech- 
 nically called a clamp. The kilns are usually made large 
 enough to burn twenty thousand bricks, being about thirteen 
 feet long, ten feet broad, and twelve feet in height. The 
 aperture is diminished by contracting the walls towards the top, 
 where the area is about one-tenth smaller than below. The 
 thickness of the walls should at least be a brick and a half. 
 Bricks are burned in kilns with less fuel, and with greater 
 uniformity and expedition, than in clamps. When they have 
 been set or placed in the kiln, they are covered with pieces of 
 bricks or tiles, and dried by kindling a gentle fire, which is 
 kept up for two or three days, or till the smoke becomes light. 
 More fuel is then added, and the mouth or mouths of the kiln 
 are nearly closed with bricks and w'et clay ; as soon as the arches 
 of the kiln look white, and the fire begins to appear at the top, 
 they slacken the heat for an hour, and let all cool by degrees. 
 This they continue to do, alternately heating and slacking, till the 
 bricks are thoroughly burnt, which is usually effected in forty- 
 eight hours. 
 
 The stacks or clamps are built of the bricks themselves. 
 The foundation is commonly somewhat raised from the surround- 
 ing ground, and of an oblong form ; the sides slant inwards a 
 little towards the top ; hence the clamp, in its figure, is a trun- 
 cated pyramid. Flues, about the length of a brick in breadth, 
 are made entirely through the clamp; they are about six feet 
 apart when the burning is to be hastened, otherwise they are 
 made about nine feet from each other. The arching of the 
 flues is performed by laying the successive layers of bricks a 
 little over the edge of those below them, till they nearly meet, 
 and then a binding brick at the top finishes the arch. In every 
 direction, the bricks are separated from each other by a stratum 
 of coals and cinders. To facilitate setting fire to the clamp, a 
 quantity of wood is laid with tlie coal in the flues. When the 
 Are is kindled, if it burn strongly, or the weather is precarious, 
 they plaster the outsides of the clamp with clay, and close the 
 apertures of the flues. On the top of the clamp, a thick layer 
 of breese (cinders) are uniformly laid. When the whole of the 
 fuel is consumed, the manufacturer concludes that the bricks are 
 •ufficiently burnt. The operation requires from twenty to^thirty 
 
BUILDING. 
 
 187 
 
 Different kinds of bricks. — Cartwright’s bricks. 
 
 days, according to the quantity of fuel, tlie proximity of the flues, 
 and the state of the weather. 
 
 The different kinds of bricks made in this country are, princi- 
 pally, place bricks, grey and red stocks, marl facing bricks, and 
 cutting bricks. The place bricks and stocks are used in common 
 walling ; the marls are made in the neighbourhood of London ; 
 these are very beautiful bricks, of a fine yellow colour, hard, and 
 well burnt, and in every respect superior to the stocks. The 
 finest kind of marl and red bricks are called cutting bricks ; they 
 are used in the arches over windows and doors, being rubbed to a 
 centre, and gauged to a height. 
 
 An acre of land, including the ashes mixed with the earth, 
 is computed to yield about one million of bricks for every foot 
 in depth. The brick mould is ten inches in length, and three 
 in breadth, and the finished bricks are about nine inches long, 
 four and a half broad, and two and a half thick. Different 
 qualities of earth, however, produce bricks of different^ dimen- 
 sions from the same mould ; and even the same earth, in propor- 
 tion as it is more or less mought or burnt, exhibits similar 
 results. 
 
 It is extremely probable that bricks, properly made, would 
 prove superior in durability to almost every kind of stone. In 
 Holland, the streets are every where paved with a hard kind of 
 bricks, known by us under the name of clinkers, w hich are often 
 imported into this country, and used for paving stables and court 
 yards; and houses in Amsterdam which have stood more than 
 two centuries, so far from being decayed, appear perfectly fresh, 
 as if new. 
 
 The numerous patents which have been granted for the 
 making of bricks, appear to have had improvements in the 
 formation of the article for their principal object, wdtiiout much 
 regard to the materials of which it is composed. Cartwright’s 
 patent, the exclusive privilege conferred by which has now 
 expired, is perhaps one of the most important. His improve- 
 ment consists in giving bricks such a shape or form that they 
 shall mutually lock or cramp each other. The principle of his 
 invention may be understood by supposing the two opposite 
 sides of a common brick to have a groove or rabbet down the 
 middle, a little more than half the width of the side of the 
 brick in which it is made ; there will then be left a shoulder 
 on each side of the groove, each of which shoulders will be 
 nearly equal to one quarter of the width of the side of the 
 brick, or to one-half of the groove or rabbet. A course of these 
 bricks being laid shoulder to shoulder, they will form an indent- 
 ed line of nearly equal divisions; the grooves or rabbets being 
 
188 
 
 BUILDING. 
 
 Advantages of Cartwright’s bricks. 
 
 somewhat wider than the two adjoining shoulders, to allow for 
 mortar, &c. When the next course comes on, the shoulders of 
 the bricks which compose it, will fall into the grooves of the 
 first course ; and the shoulders of the first course will fit into the 
 grooves or rabbets of the second, and so on. This configura- 
 tion of the bricks is to be preferred, as it is perfectly simple ; 
 but the principle will be preserved by whatever form of inden- 
 ture they lock or cramp each other. For the purpose of turning 
 the angles, it may be expedient to have bricks of such a size 
 and shape as to correspond with each wall respectively ; though 
 this is not absolutely necessary, as the grooves in the bricks of 
 each w all, where they cross or meet each other, may be level- 
 led, and the bricks lap over as in the common mode. For the 
 purpose of breaking the joints in the depth of the wall, bricks 
 will be required of different lengths, though of the same wddth. 
 Buildings constructed with bricks of this principle, will require 
 no bond timber, one universal bond running through and 
 connecting the whole building together ; the w^alls of which can 
 neither crack nor bulge out, without breaking through the 
 bricks themselves. When bricks of this form are used for the 
 construction of arches, the sides of the grooves or rabbets, and 
 the shoulders, should be the radii of the circle of which the 
 intended arch is to be the segment. In forming an arch, the 
 bricks must be coursed across the centre on which the arch is 
 turned, and a grooved side of the bricks must face the w'ork- 
 man. It may be expedient, though not absolutely necessary, 
 in laying the first two or three courses at least, to begin at the 
 crow’ll and w'ork downwards. The bricks may be either laid in 
 mortar, or dry, and the interstices afterwards filled and w^edged 
 up, by pouring in lime putty, plaster of Paris, grouting, or 
 any other convenient material, at the discretion of the work- 
 man or builder. Arches on this principle, it is stated, having 
 no lateral pressure, can neither expand at the foot, nor spring 
 at the crown, consequently they will w'ant no abutments, re- 
 quiring only perpendicular walls to be let into, or to rest upon ; 
 and they will want no incumbent weight upon the crown to 
 prevent their springing up, a circumstance often of great 
 importance in the construction of bridges. Another advantage 
 attending this mode of arching is, that the centres may be 
 struck immediately ; so that the same centre (which in no case 
 need be many feet wide, whatever may be the breadth of 
 the arch,) may be regularly shifted as the work proceeds. But 
 the greatest and most striking advantage attending this inven- 
 tion, is the absolute security it affords (and at a very reasonable 
 rate) against the possibility of firej for, from the peculiar pro- 
 
BUILDING. 
 
 189 
 
 Fire bricks. — New method of dividing bricks. — Analysis of clay for bricks. 
 
 perties of this arch, requiring no abutments, it may be laid upon, 
 or let into, common walls, no stronger than what are required for 
 timbers, of which, precluding the necessity, it saves the expence. 
 A more particular account of this invention, illustrated by two 
 plates, may be seen in the third volume of the “ Repertory of Arts 
 and Manufactures.’^ 
 
 In 1798, Francis Farquharsoii, of Birmingham, obtained a 
 patent for making bricks and tiles by machinery, and indeed 
 the use of horse power, in working the clay, is now very 
 common. 
 
 Whitmore Davis, of Castle Comber, in the county of 
 Kilkenny, Ireland, observed some persons in the vicinity of a 
 colliery, to employ a mortar, for the backs of their grates, which 
 in a short time became very hard. This substance he found, 
 on inquiry, to be what miners term seat-coal, or that fossil which 
 lies between coal and the rock. It has been examined by Kir- 
 wan, who is of opinion that it will, when mixed with a due pro- 
 portion of clay, produce a kind of bricks, capable of resisting the 
 action of fire, and consequently w'ell calculated for furnaces, or 
 similar structures. The discovery of the use of this substance is 
 considered important, and it is fniiher observed, that seat-coal, 
 properly prepared, will answer every purpose of tarras, for build- 
 ings beneath w ater. 
 
 In building, a considerable waste of time arises from the 
 necessity of making bricks less than the common size, to suit 
 particular situations. Nor is the waste of time the sole loss; in 
 attempting to divide a brick, especially in the direction of its 
 length, one half of it is generally reduced to useless splinters ; 
 but bricks have lately been made, which in their soft state were 
 nearly cut through by pressing a wdre upon them ; they can 
 then be divided by a single blow : a proportion of them along 
 with the common sort, produces on the whole a saving of some 
 moment. 
 
 It is of considerable importance to examine clay before it is 
 made into bricks, in order to ascertain whether any addition can 
 be made to it which will improve its quality. According to 
 the observation of Bergman, the proportion of sand to be used 
 with any clay, must be greater, the more such clay is found 
 to contract in burning, but the best clays are such as require 
 no sand. This illustrious chemist recommends the following 
 mode of analysis to manufacturers : Nitric acid poured upon 
 unburned clay, detects the presence of lime, by producing an 
 effervescence. Calcareous clays, or marls, are often the fittest 
 materials for making bricks. In the next place, a lump of 
 clay, of a given weight, is to.be diffused in water by agitation. 
 
190 
 
 BUILDING. 
 
 Different kinds of tiles. 
 
 The sand will subside, and the clay remain suspended. Other 
 washings of the residue will carry off some clay, and by due 
 management in this way, the sand, or quartzose matter, may be 
 had separate. Nitric acid by digestion will take up the lime 
 from a part of the clay, previously weighed, and this may be 
 precipitated by volatile alkali. The clay, the sand, and the 
 lime, may thus be well enough ascertained by weight, so as to 
 indicate the quantity of sand or other material requisite to be 
 added, in order to form that compound, which, from other 
 experiments, may have been found best adapted to produce 
 good tiles and bricks. An examination with the microscope 
 will show whether the sand contain feld spar or other stones of 
 known figure. 
 
 Paving tiles are a long flat kind of brick, used for laying the 
 floors of kitchens, dairies, cellars, &c. Two or three sizes of 
 them are generally made ; the least of English manufacture are 
 about the same length and breadth as common bricks ; the largest 
 are about twelve inches square ; a middle size, about nine inches 
 square, is also in common use. The thickness of all the sizes of 
 paving tiles is only about an inch and a half. They are made of 
 the strongest kind of clay, and well burnt. Paving tiles or bricks 
 look the best when laid diagonally. 
 
 The most common tiles for roofs, are those called pan tiles, 
 which are thirteen inches long and eight broad, and about half 
 an inch thick ; they are curved in the direction of their length, so 
 that their transverse section is a figure of contrary curvature, like 
 the letter (n ; the hollow of one side serves as a channel for the 
 rain, and for that purpose is made of greater radius than the 
 other, which is employed to overlap the edge of the adjoining 
 tile. The tile, at the upper end of its under surface, has a 
 knob, by which it is hung to the lath. Tiles constitute a very 
 heavy covering, and require the laths by which they are sup- 
 ported to be proportionately strong. The latlis, for pan tiles, 
 are about three quarters of an inch thick, and an inch and a 
 quarter broad ; they are generally made of deal. The other 
 sorts of tiles are chiefly plain tiles, hip tiles, and ridge tiles ; 
 the latter resemble half a hollow cylinder, and are laid on the 
 ridges of houses. Ridge tiles are required by statute to be 
 thirteen inches in length, and six inches and a half in 
 breadth.* 
 
 • Brick-water, or water impregnated with the contents of bricks or tiles, is 
 possessed of properties so remarkable, and at the same time so pernicious in 
 their effects, when used for culinary purposes,^ that we cannot refuse a place to 
 
BUILDING. 
 
 191 
 
 Use of tar in preserving tiles. — Statute regulations. 
 
 In Holland, they frequently glaze their roofing tiles, which 
 increases their durability, but considerably enhances the price 
 of them. The early destruction of unglazed tiles is occasioned 
 by the moisture Urey imbibe ; for when the water has sunk into 
 them, they break with the action of the frost. Sonini has, how^- 
 ever, discovered an excellent preventive of this effect, nearly 
 equal to glazing, and very cheap : he directs us to brush over 
 the tiles with tar, after they have been well warmed in the 
 sun. Tiles which have been cracked by the frost, may be pre- 
 served from the further influence of the same cause, by the like 
 application. 
 
 By stat. 2, Geo. 2, cap. 15, any tw'o, three, or more per- 
 sons, appointed by the justices of the peace, are empowered, 
 within fifteen miles of London, to go in the day time, into any 
 grounds, sheds, or places, where any clay or earth shall be digged 
 or digging, for bricks or pan tiles, or any bricks or pan tiles 
 shall be making or made for sale, and there to search for, view, 
 and inspect the same, &c. — Offenders to forfeit twenty shillings 
 for every thousand of unstatutable bricks, and ten shillings for 
 every thousand of such tiles ; one half of the fine to the prose- 
 cutor, and the other to the poor of the parish in which the offence 
 shall be committed. 
 
 By stat. Geo. 2, cap 22, there may be mixed with the brick 
 earth any quantity of sea-coal ashes, sifted or skreened through a 
 sieve or skreen half an inch wide, and not exceeding tw enty loadi 
 to the making of 100,000 bricks, each load not exceeding thirty- 
 six bushels. And breese may be mixed with coal in the burning 
 of bricks in clamps for sale, &c. Stock bricks and place bricks 
 may be burned in the same clamp, provided that the stock bricks 
 be set in one distinct parcel, and not mixed and surrounded with 
 place bricks. 
 
 By stat. 10, Geo. 3, cap. 49, the earth for making bricks 
 must be turned at least once after it is dug, before it is made into 
 bricks. 
 
 the following curious experiment made by Dr. Percival, and stated in the first 
 volume of his Essays. He steeped two or three pieces of common brick, four 
 days in a basin full of distilled w’ater, which he afterwards decanted off, and 
 examined by various chemical tests. It was not miscible with soap ; struck a 
 lively green with syrup of violets, became slightly lactescent by the volatile 
 alkali, but entirely milky by the fixed alkali, and by a solution of sugar of lead. 
 No change was produced on it by an infusion of tormentil root. Hence the 
 Doctor justly concluded, that the lining of w ells with bricks, a practice very 
 common in various places, is extremely improper, as it cannot fail to render 
 Ihc water hard and unwholesome. 
 
192 
 
 BDILDIN®. 
 
 Duties on bricks. — Bricklayer’s trowel — hammer — plumb-rule. 
 
 By 17, Geo. 3, cap. 42, stock and place bricks made for 
 sale, shall, when burnt, be not less than eight and a half 
 inches long, four inches wide, and two and a half inches 
 thick. 
 
 By 43, Geo. 3, Cap. 69, (consolidating the excise duties,) 
 every thousand of bricks made in Great Britain, not exceeding 
 ten inches long, three inches thick, and five inches wide, is 
 liable to a duty of jive shillings, and if exceeding these dimen- 
 sions to ten shillings: and every thousand of bricks made in 
 Great Britain, and smoothed or polished on one or more sides, 
 not exceeding the superficial dimensions of ten inches long, and 
 five inches w ide, is subject to a duty of twelve shillings ; and if 
 such bricks exceed these dimensions, to the duty on paving 
 tiles. The said duties are to be paid by the, makers. An addi- 
 tional duty of tenpence per thousand was imposed on bricks and 
 tiles in 1805. 
 
 . Of the Tools used in Bricklaying, 
 
 The brick trowel, which is used for taking up and spreading 
 the mortar, is also used for cutting the bricks to any required 
 size, and should therefore be made of the best steel, and well 
 tempered. 
 
 The hammer used by the bricklayer, is adapted either to strike 
 a blow, or to divide the bricks, as may be required in cutting a 
 hole through a brick wall, or other operations. One end of the 
 head of it has therefore a face similar to that of any common ham- 
 mer, and the shape of the other end resembles that of a carpenter^s 
 axe, though far narrower in proportion to its length. The han- 
 dle is inserted much nearer the face of the hammer, than the other 
 extremity or edge. Another kind of hammer, often employed in 
 taking down brick-work, differs from the above only in having, 
 instead of the axe part, nearly the shape of an adze, but not so 
 broad for its length ; hence it may be driven with facility between 
 bricks to separate them. 
 
 The plumb-rule is similar to that used by carpenters and 
 other artizans. It consists of a well-seasoned board, the length 
 of which should at least be four feet ; its thickness and breadth 
 are not very material, provided they be sufficient to prevent its 
 warping. Down the middle of one of the broad surfaces of 
 the board is drawn a straight line, and at one extremity in this 
 line, is attached a small cord with a weight at the lower end. 
 If the long narrow sides of the rule be perfectly straight and 
 parallel with each other, and the line is equidistant from the 
 arris on each side of it, the plummet being hung in this line, 
 will form a correct instrument. Either of tlie long narrow 
 
BUILDING. 
 
 193 
 
 The level. — Tlie square. — The bevel. — The rod. 
 
 sides of the rule is applied to the wall, so that the plummet and 
 its cord may face the workman, who, by frequently using it, is 
 enabled to carry up his wall perpendicularly. If in any of 
 these trials, he observes that the cord of the plummet does not 
 coincide with the line on the rule, he sets the bricks further in or 
 t)ut, as may be required to rectify the error, taking care to do it 
 while the mortar they are set in is yet w^et. — As the plummet is 
 not made to hang below the rule, a hole is cut in the latter, to 
 alIo\v the cord to hang straight. 
 
 The hvel employed by the bricklayer, is also similar to that 
 of the carpenter. It is of various lengths from six to twelve 
 feet. If one end of the plumb-rule above mentioned were 
 joined at right angles to the middle of the long narrow edge of 
 another board of the same thickness, but about double its 
 length, it would become a level, the low'er edge or side of the 
 piece thus added to the rule, becoming the surface placed on 
 walls, particularly at wdndow^ sills and wall-plates, to ascertain - 
 whether they are horizontal or not. To try the correctness of 
 a level, place it vertically, that is, in the position in which it 
 is used, upon any fiat surface, or merely place each end of the 
 bottom edge upon a block of wood, and raise or low'er the sup- 
 ports till the cord of the plummet exactly coincides with the 
 line on the perpendicular rule or limb of the instrument. 
 When this is observed, reverse the ends, and if the same coin- 
 cidence then takes place, the level is true ; but if it does not, the 
 bottom must be planed, till the trial wall succeed. The perpen- 
 dicular and horizontal parts of the level are not only fastened toge- 
 ther by mortice and tenon, but for greater firmness, and to prevent 
 W'arping, tw^o braces are added, wdiich extend, in a slanting direc- 
 tion, from the horizontal piece nearly to the top of the perpendi- 
 cular one. 
 
 A large, square is employed in setting out the sides of build- 
 ings at right angles ; artd a small square for trying the bedding of 
 bricks, and squaring the soffits across their breadth. 
 
 A bevel is required for drawing the sofiit line on the face of 
 bricks. 
 
 Tlie rody for measuring, is either five or ten feet long ; the feet 
 are divided by notches, and one of those next the extremity of the 
 rule, is divided into inches. Dimensions may be more expedi- 
 tiously ascertained with the rod than w ith a pocket rule ; but 
 bricklayers are generally provided with a measuring tapey which is 
 coiled up by its winch into a cylindrical box, of such small dimen- 
 sions, as to unite a more convenient portability than the pocket 
 rule, with greater dispatch in the general operations of measure- 
 ment, than can be obtained by the use of the rod. 
 
 9- — VoL. I. C 
 
194 BUILDING. 
 
 Jointing-rule. — Raker. — Hod. — Line. — Rammer. — Crow and pick-axe. 
 
 The jointing-rule^ employed in running the joints of brick- 
 work, is eight or ten feet long, and four inches broad. When . 
 designed for the use of two bricklayers, the latter length is em- ; 
 ployed. ^ 
 
 The iron tool used along with the jointing-rule, to mark the i 
 joints of brick-work, is called 2 l jointer ; its form is nearly that , a 
 of the letter c/5 , though its flexure is not in proportion so consi- ' 1 
 derable. 
 
 The raker has its use designated by its name. It is employed 4 
 to rake or scrape loose and decayed mortar out of the joints of u 
 walls, the appearance of which is intended to be improved by J 
 pointing them afresh. The raker is made of iron, pointed with ■ 
 steel, and at about one-fourth of its length from each extremity, j 
 it is bent to a right angle, so that it would resemble a Z, if the ' 
 stroke connecting the top and bottom of that letter were perpendi- i 
 cular instead of slanting. ! 
 
 The hod is an angular wooden trough, closed only at onej'^ 
 €nd^ so that it resembles the half of a rectangular box, divided (| 
 in such a manner as to consist of two entire sides and one end. 
 From the middle of the angular ridge formed by the meeting of 
 the two sides, proceeds a pole or handle about four feet long ; 
 and so much of this ridge as lies between the handle and the end 
 piece, is covered with a cushion of several thicknesses of leather, 
 or leather stuffed with wool. In this utensil, the labourer carries 
 upon his shoulder the bricks and mortar with which he supplies 
 the bricklayer ; the cushion takes off* the sharpness of the ridge, 
 and by means of the handle, he can support it without difficulty, 
 either in walking on level ground, or ascending a ladder. It is 
 customary to sprinkle the inside of the hod with clean dry sand, 
 before it is filled with mortar, which is thereby prevented from 
 adhering to the wood. 
 
 A cord or line, to serve as a guide in laying the courses of 
 bricks exactly straight, is stretched close to the wall, and removed 
 at the proper intervals as the work advances. This line, to afford 
 the means of stretching it wherever it is wanted, is fastened to two 
 pointed iron pins, called the line pins, one being attached to each 
 • end of it. 
 
 The bricklayer’s rammer differs not from the paviour’s. If 
 the ground intended for a foundation is deemed not sufficiently 
 solid, it is compressed as much as possible by this tool, a due at- 
 tention to the use of w hich wdll prevent fractures that may endan- 
 ger the building. 
 
 The iron-crow and the pick-axe, are useful assistants to th« 
 bricklayer, of obvious utility; being sometimes employed in 
 4:onj unction, and sometimes alone, in digging, breaking through 
 
BUILDING. 
 
 195 
 
 The compasses and grindstone. — Chopping block. — Banker. — Camber slip,&o. 
 
 walls, raising heavy bodies out of the ground, and similar oper- 
 ations. 
 
 The compasses for traversing arches, and the grinding-stone for 
 sharpening tools, scarcely, perhaps, require to be mentioned. 
 
 The cutting and management of bricks for the construction of 
 gauged arches, add the following tools, besides the smalk square 
 and bevel above-mentioned, to the catalogue belonging this trade : 
 
 The chopping block, is made out of any piece of wood which 
 happens to be at hand, set or fixed with sufficient stability for 
 axing the bricks upon. Its height may be about thirty inches ; 
 the area of its upper surface is generally inconsiderable ; if six or 
 eight inches square, it is sufficiently large for one man to work at. 
 When several men are at work, it is better that they should have 
 separate chopping blocks than a single large one, unless the sin- 
 gle one be very firmly fixed, so that the vibrations communicated 
 by any of the workmen, may not inconvenience the rest, and un- 
 less also its dimensions be increased in a much greater proportion 
 than the number of hands. 
 
 The banker, is the appellation which designates a large bench, 
 upon which the bricks for arch-work are rubbed and gauged. It 
 is from two to three feet in breadth, and from six to twelve feet 
 in length, according to the number of men it is intended to ac- 
 commodate, or the dimensions of the timber which happens to be 
 at hand or most cheaply obtainable. The banker is generally 
 made thirty or thirty-two inches high : as it is not necessary for it 
 to be very thick, an old door may be converted into one, support- 
 ing it by four or five posts or piers, and for greater steadiness set- 
 ting one edge against a wall. 
 
 The camber slip is a piece of board cambered or made con- 
 vex on one or both edges, but not confined to any particular 
 length, breadth, or thickness, though the latter dimension is gene- 
 rally about half an inch. It is employed as a ruler. When only 
 one edge is curved, it rises about one inch in six feet, for drawing 
 the soffit lines of straight arches : when the opposite edge is 
 curved, it rises only about half an inch in six feet, and is used to 
 draw the upper side of straight arches. This small convexity is 
 an allowance for the settling of the arch, which many disregard, 
 and therefore make the upper side of the arches in question 
 straight. When the bricklayer has drawn his lines with the cam- 
 ber slip, he hands it to the carpenter, who by it forms the centre 
 to the curve of the soffit. To prevent the necessity of having 
 many camber slips, one is made large enough for the widest aper- 
 ture likely to be arched. 
 
 The mould is used to obtain the proper form of the brick, 
 tiiat it may be reduced to the requisite taper, one edge of the 
 
19o 
 
 BUILDING. 
 
 The scribe. — The tin-saw. — The brick-axe. — The rnbbing-stone, See. 
 
 mould being brought close to the bed of the brick previously 
 squared. There is a notch in the mould for every course of the 
 arch. 
 
 The scribe is any piece of iron, such as a large nail or the like, 
 ground to a point, for the purpose of marking the bricks -where 
 they are to be cut. 
 
 The thi-sazQ is used in cutting the bricks about the eighth of 
 an inch deep, at the lines marked with the scribe, so that an 
 entrance may be made for the brick-axe, and that they may be 
 reduced to the proper form for arches, without splintering or 
 jagging their edges. The false joints are also cut w ith this instru- 
 ment. 
 
 The brick-axe is used for reducing bricks to the saw cuttings 
 and lines drawn by the scribe. The subsequent labour of rubbing 
 the bricks is shortened in proportion to the dexterity shown in the 
 management of the axe. 
 
 When the bricks have been cut with the requisite exactness by 
 the axe, they are taken to the riibbin^-sioi^j upon which they are 
 rubbed smooth. The rubbing-stone is generally fixed upon a bed 
 of mortar at one end of the banker. To keep it tolerably level, 
 the bricks should be rubbed equally on every part of it, and in 
 different diiections ; it should not therefore be so large as to pre- 
 vent the w orkman from easily reaching over it, on whatever side 
 of it he stands : about tw^enty inches in diameter is a convenient 
 size. If the grain of it is not sharp enough to reduce the brick 
 with the necessary expedition, sand may be used. The lieaders 
 and stretchers in returns, which are not axed, are also dressed 
 upon the rubbing-stone. 
 
 The bedding-stone is a marble slab, about ten inches broad, 
 and twenty inches long, with one flat surface, which is used to try 
 whether the rubbed surface of the brick be straight, so as to fit 
 upon the leading skew back, or leading end of the arch. 
 
 The stone upon which bricks cut with curved surfaces are 
 rubbed, is called a jioat-stone, w'hich must itself necessarily be 
 curved in the reverse form, though of a radius equal to that in- 
 tended for the brick. 
 
 Of Foundations. 
 
 Foundations are either natural or artificial ; natural, where 
 the ground is rocky or good ; artificial, when, from its boggy, 
 sandy state, or from its having been lately dug up, piling, or 
 some other precaution, must be resorted to, for the support of tlie 
 building. Appearances are so often deceitful, that the prudent 
 builder will never depend upon them, but will examine the 
 ground intended for a foundation with the utmost attention. If 
 
BUILDING. 
 
 197 
 
 Founuatious. 
 
 tlie ground shake on being struck with the rammer, the nature 
 of it must be ascertained by piercing it with a w^ell-digger’s borer. 
 Having found how far the firm ground is beneath the surface, the 
 loose or soft parts must be removed, if not very deep : the exca- 
 vations made on these occasions, should widen upwards, and their 
 sides be cut in the form of steps : by which means a firmer bed 
 will be obtained for the wall, than if the sides of the trenches 
 were simply inclined planes. 
 
 Palladio directs the ground for the foundation to be penetrated 
 to a sixth part of the whole height of the building, unless there 
 be cellars, in which case he recommends digging deeper. It is a 
 good rule to make the foundation double the breadth intended 
 for the superincumbent wall, and the contraction of it should be 
 made alike on both sides. 
 
 If the earth is very unsolid, plies, close to each other, and long 
 enough to reach the good ground, must be driven in. The thick- 
 ness of these piles should not be less than one-twelfth of their 
 length ; they should be made to present as even a surface as pos- 
 sible, and should then be covered with planks. It is a curious 
 fact, that dry, straight-grained piles, may be driven much further 
 by the same force, than if they be made of wood in an opposite 
 slate. 
 
 When the infirmity of the ground is uniform, but not very 
 considerable, it may be made good by laying pieces of sound oak 
 about two feet apart, across the breadth of the trench in which 
 the wall is to stand, and when these have been firmly bedded and 
 rammed down, planks of the same timber, or of pitch pine, 
 (which is equally as durable in such situations,) must be laid 
 down and spiked upon them. The planks should be half a foot 
 wider than the base of the foundation wall. Ground of this de- 
 scription may also be maile good by ramming large stones upon 
 it closely together, and extending in breadth about a foot on each 
 side of the wall. Upon the first course of stones, another course, 
 raliier narrower, may be laid, taking care, as in walling generally, 
 to make the joints of one course fall on tlie middle of the stones 
 in the other. 
 
 If the ground be found defective in one place and good in 
 anollier, the unequal settlement which would be the inevitable 
 consequence of building upon it in such a slate, may be prevented 
 by a plan which is now becoming daily more common, and which, 
 if carefully executed, is always successful. It consists in the use 
 of aiches, either inverted or suspendc'd, according to circum- 
 stances, which we sliaLl now advert to. 
 
 When the soft parts of the ground are under the apeHures 
 only, inveited aiches are to be turned under such apertures, m the 
 
198 
 
 BUILDING. 
 
 Foundations. — Use of inverted and suspended arches. 
 
 manner represented by fig. 1, pi. 1. By this means the advantage 
 of one continued base is obtained, for the piers cannot sink with- 
 out carrying the arches, and consequently the ground upon which 
 they stand, along with them. The whole building will therefore 
 sink equally, and no fracture of the wails will ensue. So much 
 is the use of inverted arches under apertures approved, that they 
 are considered indispensable in all buildings of considerable 
 weight or consequence, even when the ground is not found to be 
 defective. It is sufficiently obvious, that the walls of all buildings, 
 the base of the foundation of which is not actually laid upon the 
 bare, solid rock, w ill sink a little ; and as the pressure of the piers 
 is incomparably greater than that of the low' piece of walling 
 under the apertures, the risk is extreme, that the resistance of the 
 ground against this low walling will not allow it to sink with the 
 piers, and consequently the fracture of the wall, and probably the 
 breaking of the window sills, will be the consequence. The 
 inverted arches prevent these bad effects ; and as they have so 
 important a service to perfuim, they should be thrown with the 
 greatest care, closely jointed, and their depth at least equal to. 
 half their width. IRD 
 
 When the reverse of the preceding case occurs, that is,, 
 when the solid parts of a foundation are only to be found under 
 apertures, then piers must be built in these places, and arches 
 suspended between them, as represented by fig. 2. It is best to 
 make the middle of the pier rest upon the middle of the summit 
 of the arch. If the pier does not cover the arch, the narrower 
 it is, the greater should be the curvature of the latter at the 
 apex. When arches are used in this way, the intrados ought to 
 be clear, that they may have their full effect. The uniform resist- 
 ance of the ground upon w'hich the piers are erected, is also of 
 greater importance than even its perfect hardness ; for if it resist 
 uniformly, the building wall sink in every part alike, and remain 
 uninjured. 
 
 When wood is laid in the trench of a foundation, the first . 
 course of stone or brick should be laid as close as possible, with- 
 out mortar, which injures the timber. If there be any difference 
 in the quality of the bricks, the strongest and closest, which 
 are least liable to imbibe moisture, should be selected for founda- 
 tion work. 
 
 Of Cements. 
 
 The bricklayer being provided with tools, with bricks, and 
 having prepared the trenches of his foundation for walling, finds 
 himself in immediate want of mortar or cement, a subject which 
 next claims our consideration. 
 
BUILDING. 
 
 199 
 
 Cements. — Importance of Higgins’s experiments — his mortar or stucco. 
 
 The nature and best methods of preparing calcareous ce- 
 ments, have been investigated by Dr. Higgins, with great 
 ability and success. He has advanced the most satisfactory 
 proofs, founded upon analysis, that the Romans, whose mortar 
 or cement, after a lapse of two thousand years, instead of 
 being decayed, has become as hard as the stones it binds toge- 
 ther, possessed no uncommon secret, which we are unable to 
 discover. His publication first appeared in 1780, and is 
 evidently the production of a liberal and intelligent mind. He 
 struck into a path with which we were but little acquainted, 
 though the knowledge of it is of very considerable importance 
 to the public collectively, as well as to individuals : for it is cer- 
 tainly lamentable to observe public or private edifices, insecure, 
 or prematurely mouldering away, from the ignorance or disre- 
 gard of a few particulars, which might not only be observed 
 with ease, but, in many instances, with a diminution of the 
 original expence. The Doctor’s conclusions, which were drawn 
 from innumerable experiments, constitute a great portion of 
 the best part of our knowledge on this subject at the present 
 time; but though so many years have elapsed since they were 
 communicated to the public, tliey are far from being yet gene- 
 rally known, and consequently are not reduced into general 
 practice. 
 
 Such is the neglect shown on this subject, that the timbers of 
 our houses last longer than the walls, unless the mouldering ce- 
 ment be frequently replaced by pointing. The following direc- 
 tions, for preparing durable mortar or stucco, contain the result 
 of the Doctor’s experience, and have been attended to with re- 
 markable success. 
 
 Sharp sand, free from clay, salts, calcareous, gypseous, or 
 other grains less hard and durable than quartz, is better than 
 any other. When a coarse and a fine sand, corresponding in 
 the size of their grains, to the coarse and fine sand hereafter 
 described, cannot be easily obtained native, the following 
 method of sorting and cleansing it must be resorted to. Let the 
 sand be sifted in streaming clear water, through a sieve which 
 will allow all grains not exceeding one-sixteenth of an inch to pass 
 through, and let the stream of water be regulated so as to wash 
 away the very fine parts of the sand, the clay, and every other 
 matter lighter than sand. The coarse rubbish left on the sieve 
 must be rejected. The sand which subsides in the receptacle 
 must then be further cleansed and sorted into two parcels, by 
 the use of a sieve which allows no grains to pass but what are 
 less than one-thirteenth of an inch in diameter. That part which 
 passes through this sieve^ we shall call fine sand, the remaining^ 
 
200 
 
 building.’ 
 
 Excellent mode of preparing mortar. 
 
 portion, coarse sand. These separate portions may then be dried 
 in the sun, or by means of a fire. 
 
 That sort of lime must be chosen which heats the most in 
 slaking, and slakes the quickest when duly watered ; which is 
 the freshest made, and has been the closest kept, which dissolves 
 in distilled vinegar with the least effervescence, and leaves tlie 
 smallest residue insoluble,- and in this residue the smallest quai> 
 tity of clay, gypsum, or martial matter. Put fourteen pounds 
 of the lime chosen according to these important rules into jl 
 brass-wire sieve still finer than the last mentioned. Slake the 
 lime by alternately plunging it into and raising it out of a butt 
 of soft water ; reject all the matter which does not easily pass 
 through the sieve, and use fresh portions of lime in a similar 
 manner, until as many ounces of lime have passed through the 
 sieve as there are quarts of water in the butt. This is the lime* 
 water, which contributes materially to the excellence of the 
 stucco. As soon as a sufficient portion of lime has been imparted 
 to it, it should be closely covered until it becomes clear, and then 
 be drawn off, by wooden cocks, placed at different heights as the 
 lime subsides, without breaking the crust formed on the surface. 
 The freer the water is from saline matter, the better will this 
 liquor be. Lime-water must be kept in air-tight vessels till the 
 moment it is used. 
 
 Slake fifty-six pounds of lime chosen as above directed, by 
 gradually sprinkling on it the lime-water. Sift the slaked part of 
 the lime immediately through the last mentioned fine brass-wife 
 sieve ; the lime which passes must be used instantly, or kept in 
 air-tight vessels, and the rest rejected. This finer, richer part of 
 the lime, may be called purified lime. It is always advisable to 
 sift the lime immediately after the slaking, otherwise much of the 
 ill-burnt lime and heterogeneous matter which it may contain, will 
 pass through the sieve. 
 
 The materials of the cement being thus prepared, take fifty- 
 six pounds of the coarse sand, and forty-tv\o pounds of the line 
 sand ; mix them on a large plank of hard wood placed hoi izon- 
 tally ; then spread the sand so that it may stand to the height of 
 six inches w ith a flat surface on the plank, and w et it with lime- 
 water, of which so much must be allowed to flow away off the 
 plank as the sand in the condition described cannot retain. To 
 the wetted sand add fourteen pounds of the purified linie in 
 several successive portions, mixing and beating them up toge- 
 ther with the instruments generally used in making fine mortar. 
 Then add fourteen pounds of bone-ashes in successive portions, 
 mixing and beating all together. The quicker and more per- 
 fectly these materials are mixed and beaten together, and the 
 
BUILDING. 
 
 201 
 
 Fine-grained mortar. — Coarse mortar. 
 
 sooner the cement thus formed is used, the better it will be. 
 As this cement is shorter than mortar or common stucco, and 
 dries sooner, it ought to be worked expeditiously in all cases, 
 and in stuccoing it ought to be laid on by sliding the trowel 
 upwards on it. The materials used along with it in building, 
 or the ground on which it is laid in stuccoing, ought to be well 
 wetted with the lime-water, at the instant of laying it on ; and 
 when the cement requires moistening, lime-water should always 
 be used. 
 
 The proportions above given are intended for a cement made 
 with sharp sand^ for incrustation in exposed situations, where it is 
 necessary to guard against the effects of hot w^eather and rain. In 
 general, half this quantity of bone-ashes wall be found sufficient ; 
 and although the incrustation in this latter case will not harden 
 deeply so soon, it wall be ultimately stronger, provided the w^ea- 
 ther be favourable. 
 
 When a mortar or cement of a fine texture is required, take 
 ninety-eight pounds of the fine sand ; w^et it with the lime-w^ater, 
 and mix it with the purified lime and the bone-ash in the quanti- 
 ties and in the manner above described, wdth this difference only, 
 that fifteen pounds or thereabouts of lime are to be used instead 
 of fourteen pounds, if the greater part of the sand be very fine. 
 This cement is suitable for the last coating of any work intended 
 to imitate the finer grained stones ; but it may be applied to all 
 the uses of the first mentioned composition. 
 
 When a mortar or cement is required, which shall be still 
 cheaper and more coarsely grained, much coarser sand than 
 the coarsest sort already spoken of may be made use of; for the 
 coarser the sand, the less the proportion of lime which will be 
 required. For example, of the coarsest sand alluded to, take 
 fifty-six pounds ; of the coarse sand which passes through the 
 meshes of a sieve one-sixteenth of an inch in diameter, twenty- 
 eight pounds ; and of the fine sand, fourteen pounds ; and after 
 mixing these and wetting them with lime-water, in the manner 
 already described, add fourteen pounds, or somewhat less, of the 
 purified lime, and then fourteen pounds, or somewhat less, of the 
 bone-ash. 
 
 When these cements are intended to be white, white sand, 
 white lime, and the whitest bone-ash, are to be chosen. Grey 
 sand, and grey bone-ash, formed of half-burnt bones, are to be 
 chosen to make the cement grey ; and any other colour may be 
 obtained either by chusing coloured sand, or by the admixture of 
 the necessary quantity of coloured talc in powder, or of coloured 
 vitreous or metallic pow^ders, or other ingredients of a similar 
 nature. 
 
 9. — VoL. I. 2 D 
 
202 
 
 BUILDING. 
 
 Artificial stone. — Water cement. — Fluid stucco. 
 
 These cements are applicable in forming artificial stone, by 
 making alternate layers of the cement and of flint, hard stone, 
 or brick, in moulds of the figure of the intended stone; the 
 stones thus formed being exposed to the open air to harden, but 
 not exposed to rain till they are almost as strong as fresh Portland 
 stone. They may be made very hard, and beautiful, by soaking 
 them, after they are thoroughly dry, in the lime-water, and repeat- 
 ing tliis process several times at distant intervals. Incrustations, 
 also, are greatly benefited by the application of lime-water, the 
 entrance of \vhich is facilitated by the use of bone-ashes in the 
 cement. 
 
 When any of the above cements are intended to be used for 
 water fences, two-thirds of the prescribed quantity of bone-ashes 
 are to be omitted, and an equal measure of powdered terras used 
 instead ; and if the sand employed be not of the coarsest sort, 
 more terras must be added, so that die terras shall be by weight 
 one-sixth part of the weight of the sand. 
 
 When a cement is required of the finest grain, or in a fluid 
 form, so that it may be applied with a brush, for the purpose 
 of smoothing and finishing the stronger crustaceous works, or 
 for washing walls to a lively and uniform colour, the fine pow- 
 der of calcined flints, or the powder of any quartzose or hard 
 earthy substance, may be used in the place of sand ; but in a 
 quantity smaller as the flint or other powder is finer; so that 
 the powder shall not be more than six times die weight of the 
 lime, nor less than four times its weight. The greater the 
 quantity of lime within these limits, the more the cement will 
 be apt to crack by quick drying, and vice versa. For washing 
 walls, the cement should not be made thicker than new cream, 
 and should be laid on briskly with a brush, in dry w^eather. 
 Fine yellow ochre is the cheapest colouring ingredient for such 
 a wash, when it is required to imitate Bath stone, or the warmer 
 white stones. 
 
 Where sand cannot be procured, any durable stony body, 
 or baked earth grossly pow^dered, and sorted as if it were sand, 
 may be used, measure for measure, but not weight for weight, 
 •tmless the same bulk of the gross pow'der be the same as that of 
 sand. But all substitutes for pure silicious sand, are imperfect 
 in proportion as the particles of which they are composed, are 
 less hard than those of that material. The scrapings of roads, 
 which consist principally of powdered calcareous stone, the 
 old mortar and other rubbish from ancient buildings, have often 
 been more strongly recommended than they deserve. These, 
 and all muddy, soft, minutely divided matters, require a large 
 proportion of lime, and never possess the hardness and durabi- 
 
BUILDING. 
 
 . 203 
 
 Use of lime-water — bone-ashes. — Sharp sand. 
 
 3 ; 
 
 r 
 
 lity which belong to good mortar. Sea sand, well washed in fresh 
 water, is as good as any other roxmd sand ; but if used without 
 being freed from salt, the mortar made with it is extremely liable 
 to be damp. 
 
 The proportion of lime may be increased without inconveni- 
 ence, when the cement or stucco is to be applied where it is not 
 liable to dry quickly ; and in the contrary circumstance, this pro^ 
 portion may be diminished. The defect of lime in quantity or 
 quality, is best supplied, by soaking the work, at distant intervals > 
 of time, with lime-water. It is proper to mix hair with these ce- 
 ments, when employed for interior work. 
 
 The powder of almost every well dried or burnt animal sub^ 
 stance may be used instead of bone-ash. 
 
 The bone-ashes facilitate the operation of plastering, by 
 increasing the plasticity of the mortar into which they enter. 
 They also render the mortar less liable to crack, and cause it to 
 acquire more quickly that state in which it is not easily injured 
 by unexpected rain. If employed in a less proportion than 
 one-fourth of the lime, they are of little use, and if they exceed 
 the lime in quantity they are injurious to the cement. Hence 
 the use of them should be regulated according to the follo\ying 
 circumstances : when the artist is more solicitous to secure an 
 incrustation from the effect of hot weather, to finish it quickly, 
 and to guard it against rain, than to make it durable in the 
 highest degree, he may use as much bone-ashes as lime; but 
 when the season, exposure, and other circumstances, permit him 
 to attend solely to the true excellence and duration of his work, 
 he must use, in Iris best calcareous cements, only one part of 
 bone-ashes for every four parts of lime. By these rules he may 
 chuse intermediate quantities adapted to his purposes. The bone- 
 ashes should not be in so coarse a powder as they are when used 
 for cupels, yet they should by no means be levigated or ground to 
 extreme fineness. 
 
 By shar'p sand, is meant such sand as consists of grains with 
 flat surfaces ; these flat surfaces, when enveloped and cemented 
 together by the lime paste, possess a much stronger cohesion than 
 if the grains were globular. 
 
 The preceding method of making mortar or stucco differs, 
 it will be perceived, from the common process in several essential 
 particulars ; among which, the purity and sorting uf the sand, the 
 use of lime-water, the newness of the lime, and the large propor- 
 tion of the sand to the lime, ought to be particularly noticed. It 
 may be useful to inquire into some of the causes of differences of 
 practice so remarkable. When the sand contains much clay, or 
 other impurities, the bricklayers find that the best mortar they can 
 
204 
 
 BUILDING. 
 
 Causes of the debasement of lime. 
 
 make, must contuin about one-half lime ; and in consequence pro- 
 nounce, \Mthout further investigation, l.alf sand and half lime, to 
 be the best composition. But wuh sand leqiuring so much lime, 
 they never can make durable mortar, ihougii of this fact it may 
 be dilticuit to convince those who are little disposed to investigate 
 causes. Too many artizans entertain an opinion that they have 
 nothing new to learn which is worth notice ; they are apt in efi'ect 
 to say, that having served an apprenticeship to tneir business they 
 ought to something ; and thus, because X\\ey ought to know 
 
 something, they seem to expect submission to their very errors. 
 To such characters we speak not; to convince them is impos- 
 sible, and therefore the attempt is folly, But those who consider 
 the interest of their employers, and that the warmth, dryness, and 
 salubrity, of a house, so far as the building is concerned, is com- 
 pletely in their power, will not despise any hint which may extend 
 their resources. 
 
 It is a common fault to build lime-kilns so high, that at the 
 bottom of the cavity, the lime is ready perhaps eighteen hours 
 before that in the upper part, and is greatly injured by its expo- 
 sure to the chaft of air passing through the fire. Lime-kilns 
 ought to be made much broader and shallower than customary, 
 with the cavity tapering upwards, and should terminate in a lofty 
 fiue, in order to accelerate the combustion, when required, by a 
 quick current of air. Calcareous stones acquire in the most emi-. 
 nent degree the properties of lime, when they are slow Iv heated in 
 small fragments of uniform size, until they appear to glow’ with 
 a white heat, and this is continued until they become non-effer- 
 vescent if steeped in an acid. The art of preparing lime consists 
 chiefly in attending to these particulars. The whiteness of lime 
 shows it to be free from metallic impregnation. Merely to keep 
 lime dry is not enough to preserve it ; it grows w orse for mortar 
 every day it is kept in heaps or untight casks, and is soon reduced 
 nearly to the state of chalk. It may be greatly debased, without 
 slaking sensibly, and such parts or fragments as fall to powder 
 merely by exposure to the air are unlit for mortar. It has been 
 found by experiment, that lime w ill absorb one-fourth of its weight 
 of water, and yet remain perfectly dry. Bishop atson found, 
 that, upon an average, every ton of limestone produced eleven 
 hundred w^eight, one quarter, and four pounds, of lime, weighed 
 before it w as cold ; and that, w hen exposed to the air, it in- 
 creased in weight daily, at the rate of a hundred weight per ton, 
 for the fijst flve or six days after it was drawn from the kiln. 
 Hence those who have to fetch lime from great distances, may 
 save even in point of cartage, by receiving it as it is taken out of 
 the kiln. 
 
BUILDING. 
 
 205 
 
 Drying of mortar. 
 
 Mortar which sets without cracking, whether this be owing 
 to the due proportion of sand, or to the slow exhalation of the 
 water from mortar containing less sand, never cracks afterwards, 
 whatever its faults, in other respects, may be. As it is the lime 
 paste, and not the sand, which contracts and produces fissures in 
 drying, so the more sand there is in the composition, the less 
 the cracks will be seen. Mortar which is liable to crack, be- 
 comes irreparably injured by frequent alternations of wetting and 
 freezing ; for the water imbibed by the smallest fissures, dilating 
 as it congeals, loosens its whole texture. Where it is composed 
 of seven parts of sorted sand, to one of lime, it is not disposed to 
 crack. 
 
 I'hat mortar may become indurated the soonest and in the 
 highest degree, and operate the most effectually as a cement, it 
 must be suffered to dry gently and set ; the exsiccation of it 
 must be effected by temperate air, and not accelerated by the 
 heat of the sun or fire. It must not be wetted till after it sets ; 
 and afterwards it ought to be protected from wet as much as 
 possible, until it is completely indurated ; the absorption of 
 carbonic acid must be prevented as much as possible till the 
 mortar is finally placed and quiescent, and then it must be as 
 freely exposed to the open air as the work Vvdll admit, in order to 
 supply carbonic acid, and enable it sooner to sustain the trials 
 to winch mortar is exposed in cementitious buildings and incrus- 
 tations. To show^ more clearly how much our slight buildings 
 are weakened by the agitations and percussions to which they 
 are exposed, first in erecting the walls and settling the timbers, 
 and then in driving those wedges to which the wainscots, mantle- 
 pieces, and other ornaments, are fastened, we must observe, that 
 the absorption of carbonic acid by mortar contributes nothing to 
 the strength of it, if it enter before it is finally fixed in a quies- 
 cent state. A little experience is sufficient to teach us, that the 
 same matter w hich assists in the induration of mortar, never serves 
 to repair the fissures, or solution of continuity betw-een the bricks 
 and cement, which happens after it is set. When mortar is set, 
 and before it is indurated, it may be easily severed from the bricks 
 and Cl Limbled ; and for w ant of softness, it cannot bend into the 
 fissures, or resume its former condition in any time. Hence by 
 heavy blows, and in wedging, our walls must be greatly weakened; 
 and the more so, as the houses are slight, quickly built, and has- 
 tily finished. 
 
 Nothing is more common than for bricklayers to keep their 
 mortar some time exposed to the air in heaps, before they con- 
 sider it proper to use; a practice which may perhaps be ac- 
 potmted for, if we consider that some portions of every kind of 
 
206 
 
 BUILDING. 
 
 Advantage of steeping bricks in lime-water. 
 
 lime used in this country, do not slake freely, by reason of their 
 not being sufficiently burned, or of the admixture of gypseous 
 or argillaceous matter ; which portions, like marl, slake in time, 
 though not so quickly as the purer lime. The plasterers, who 
 use a finer kind of mortar, made of sand and lime, observe 
 that their stucco blisters, if it contain small bits of unslaked lime, 
 and as smoothness of surface is with them of more consequence 
 than excessive hardness, they take care to secure the perfect slak- 
 ing of their lime by allowing sufficient time for the imperfect parts 
 to be penetrated by the moisture. The bricklayers, trusting, per- 
 haps, more to the judgment of the plasterers, in this respect, than 
 to their own, and considering it very convenient to slake a large 
 ' quantity of lime at once, follow the same practice, without caring 
 for or apprehending the real fact, that mortar is worse for every 
 hour it is kept, and that they are taking such measures as will pre- 
 vent it from ever acquiring that degree of hardness iu which its 
 perfection consists. 
 
 Among the circumstances which contribute to the speedy 
 ruin of modern buildings, it may also be observed, that mortar 
 made with bad lime, and a great excess of it, is used with dry 
 bricks, and not unfrequently with warm ones. These im- 
 
 mediately imbibe or dissipate much of the water, and as the 
 cement approaches nearer to be dry, whilst it is still liable to 
 be displaced by the percussions of the workmen, render it little 
 better than equivalent to a mixture of sand and powdered chalk. 
 To make strong work, the bricks ought to be soaked in lime- 
 water, and freed from the dust with w hich they are commonly 
 covered. By this means the bricks are rendered closer and 
 harder, the cement, by setting slowly, admits the motion which 
 the bricks receive when the workman dresses them, without being 
 impaired, and it adheres and indurates more perfectly. This 
 steeping of the bricks is an imitation of the practice of the plas- 
 terers, who always wet the w'all before they commence their work, 
 because they know the cement will not otherwise adhere. This 
 ought to be done as long as the wall is thirsty, and lime-water is 
 the most proper liquid they can use. The same advantage that 
 attends the soaking of bricks, would attend the soaking of bibulous 
 stones in lime-water. 
 
 Mortar made with sand containing one-seventh or one-eighth 
 of fat clay, moulders in w inter like marl ; a circumstance which 
 proves the propriety of freeing from clay the sand used in mortar. 
 The washing performed for this purpose, will be found a very 
 cheap operation, even in cities, if the water which carries off the 
 clay be directed into a receptacle where it may be depurated by 
 subsidence for repeated use. 
 
BUILDING. 2or 
 
 Chalk lime. — Water fittest for mortar. — Causes of damps on stucco. 
 
 Chalk lime may be easily prepared, so as to be fully equal if 
 not superior to stone lime. The reason why this is not generally 
 thought to be the case, probably is, that not being of so close a 
 texture, it is sooner spoiled by the absorption of carbonic acid, 
 when exposed to the atmosphere after it is made. A cask of 
 chalk lime should therefore never be opened till the moment it is 
 to be slaked, and the greatest expedition should be used in the 
 slaking, and in the making and applying the mortar to use. In 
 the quiescent air of a room, a pound avoirdupois of chalk lime, 
 becomes two ounces and a half heavier in two days ; and nearly 
 the whole of this increase of weight, consists of the carbonic acid 
 W'hich it has imbibed from the atmosphere. 
 
 The fittest water for making mortar, is rain water; river 
 water holds the next place ; land water the next ; spring water 
 the last ; sea water, and all waters noticed medicinally or other- 
 wise, for their saline contents, ought never to be used for this 
 purpose. 
 
 The compositions mostly used for stuccoing within doors, are 
 incapable of hardening considerably, and when they are laid 
 on the naked walls, soon become tarnished, unsightly, and in- 
 convenient, by the damps which workmen call swieating. Some- 
 times these damps are occasioned by the bad construction of the 
 walls, the joints of the facing bricks having become hollow by 
 the decay of the mortar, or when the copings or gutters are de- 
 fective : a damp by transpiration also occurs when the principal 
 walls are stuccoed before they are dried, or when the materials of 
 them are so spongy as to imbibe the rain, and the circulation of 
 air is not sufficient to waft away the transuding moisture. The 
 damp by condensation is also very common, and appears most on 
 the closest incrustations, how^ever perfect and old the walls may 
 be. In such instances, the damp is owing to the closeness of the 
 body, and a stucco pervious in a certain degree to air and mois- 
 ture, will be free from it, as well as from the damp before men- 
 tioned. The customary mode of avoiding these damps, is to case 
 the principal walls of houses wdth lathwork, on w hich the incrus- 
 tation or plaster is laid at some distance from the wall. The 
 narrowing of rooms and passages very perceptibly is the conse- 
 quence of this method, besides its expence. Bone-ashes, each 
 grain of which is tubulated in every direction, added to the stucco 
 in half the quantity of the lime, are a preventive of these damps 
 without lathwork. 
 
 The drying, induration, and texture of incrustations made on 
 brick walls and other irregular surfaces, are always so far unequal 
 as to exhibit visible traces which deform the work, and cannot be 
 effectually obliterated by any known method so convenient as 
 
208 
 
 BUILDING. 
 
 Colouring: of stucco. — Effect of painting it. — Water cements. 
 
 that of covering the first coarse incrustation, after it has dried, 
 with another coat which may be made finer and smoother. Thus 
 the expence of fine-grained, smooth, or coloured stucco, is ren- 
 dered moderate ; because the finer, or the colouring ingredients, 
 may be reserved for the exterior coat, which will last for ages, if 
 the cement be good. 
 
 To tinge a cement sufficiently, of any colour which is not 
 found in sand, so that the incrustation shall not be impaired, 
 and that the colour shall be as durable as the cement, the most 
 proper ingredients which can be used in lieu of sand, or of part 
 of it, are coloured glasses or coloured stones of the hardest kind, 
 beaten to coarse powder; the finer parts of which are to be 
 washed away, not merely because they are injurious to the 
 cement, but because they contribute very little to the intended 
 colour. 
 
 Stucco made with the best proportions of lime, sand,, and 
 lime-w'ater, is not bettered by painting as soon as it dries ; as this 
 covering retards the induration of it, by cutting off its com- 
 munication with the air. It therefore renders it liable to be 
 irreparably injured in wet weather, wherever the water can get 
 behind the paint. If paint or oil ought ever to be applied on 
 such stucco, it ought not to be laid on in less than a year after 
 the incrustation is made. The painting and sanding of the 
 common defective incrustations, contribute very little to their 
 duration, although it hardens them at the surface; for it does 
 not effectually prevent them from cracking, and it avails very 
 little to paint the cracked stucco again, because cracked stucco 
 is alw ays hollow, as the workmen term it ; that is, it parts from 
 the wall in the parts contiguous to the cracks, sounds hollow on 
 being struck with the knuckle, and falls off in a few^ years, if it be 
 so thick and large in extent, as to break the adhering portions by 
 its weight. 
 
 Mortar made of sand, water, and lime, whatever may be the 
 proportions of the mixture, cannot be employed in aqueducts, 
 reservoirs, and other aquatic buildings, unless sufficient time be 
 allowed for its perfect induration before the admission of the 
 w ater ; but when mixed up w ith a quantity of terras, as already 
 stated, it acquires the desirable property of hardening under 
 water. A few additional remarks on this subject will perhaps be 
 acceptable. A mortar made of terras powder and lime was 
 used in water-fences by the Romans, and has been generally 
 employed in such structures ever since their time. As it sets 
 quickly, and when set is impenetrable to w*ater, some people 
 have hasti'ly concluded that it is the best kind of mortar for any 
 purpose. But it is found by experience, that mortar made of 
 
BUILDING. 
 
 209 
 
 Water cements. — Brick bond. 
 
 terras powder, whether coarse or fine, will not grow so hard as 
 mortar made with lime and sand, nor endure the weather so 
 well ; on the contrary, it is apt to crack and perish in the open 
 air. Its efficacy in water-fences is experienced only where it is 
 always kept wet, and seems to depend principally upon the 
 property which the powder of terras has, in common with 
 Other argillaceous bodies, but in a higher degree, of expediting 
 the crystallization of the calcareous matter, by imbibing the 
 water in which it is diffused in the mortar, and of swelling, during 
 this absorption, so much as to render the mortar impenetrable 
 to any more water. It seems, also, that an acid of the vitriolic 
 kind, which is contained in terras, contributes to the speedy set- 
 ting of the cement, by reducing a part of the lime to the condition 
 of gypsum. Terras is a volcanic production, consisting chiefly 
 of clay and oxide of iron indurated together; and baked clay, 
 reduced to powder, communicates to mortar properties of a simi- 
 lar kind, 
 
 Pozzolana is another volcanic production differing little from 
 terras, as to the effect it produces in mortar. It is thrown out 
 of volcanoes in the form of ashes, and is found in many countries, 
 but most abundantly in the kingdom of Naples. The cement 
 used by Smeaton, in the construction of the Eddystone light- 
 house, was composed of equal parts by measure of linje and poz- 
 zolana; a mixture which was deemed the most suitable, as the 
 building was exposed to the utmost violence of the sea; but a 
 composition exceedingly proper for aquatic works in general, may 
 be composed of two parts of lime, one of pozzolana, and three of 
 clean sand. 
 
 It has lately been discovered, that manganese is a valuable 
 ingredient in water-cements, if used in the following manner; 
 mix together four parts of gray clay, six of the black oxide of 
 manganese, and ninety of good limestone reduced to fine powder ; 
 then calcine the whole to expel the carbonic acid. When this 
 mixture has been well calcined and cooled, it is to be worked 
 into the consistence of a soft paste with sixty parts of washed 
 sand. If a lump of this cement be thrown into water, it will 
 harden immediately. 
 
 Of Brick Bond and Walling. 
 
 When a brick is laid so that its length is in the direction of the 
 length of the wall, it is called a stretcher ; and when its length 
 crosses that of the wall, it is called a header. 
 
 The term bond is applied to any disposition of the bricks, 
 9—Vol. I. ^ E 
 
210 
 
 BUILDING. 
 
 English and Flemish bond. 
 
 by which the continuity, in a straight line, of the joints of a 
 wall is interrupted. It is obvious that a bond may be adopted, 
 which will interrupt the rectilinear direction of both the horizon- 
 tal and vei tical joints of a wall ; but in the two kinds of bond 
 which have liitherto prevailed, the horizontal joints are continued 
 in the same line round the whole building, and the vertical ones 
 only interrupted. When the wall is only intended to be half a 
 brick, or four inches and a half in thickness, the whole of the 
 bricks are laid so as to form stretchers, that is, their length is laid 
 in the direction of the length of the wall, and the bond is obtained 
 simply by making the vertical joints in every course exactly oppo- 
 site the middle of the bricks above and below. But when the 
 w all is intended to be the length of a brick or more in thickness, 
 it w^ould be apt to split into parts if it consisted only of two or 
 more walls separately bonded, as in the instance just mentioned of 
 the half brick wall. The bricks, therefore, in thick w^alls, must 
 be connected in their breadth as well as their length, and this is 
 done according to two principal modes, one of which is called 
 English, the other Flemish bond. 
 
 English bond consists of headers and stretchers crossing each 
 other in separate horizontal courses. In Flemish bond, the 
 headers and stretchers are placed alternately in the same horizon- 
 tal course. Flemish bond is now so common, that hardly any 
 other kind is to be seen ; it is preferred, for its appearance, to the 
 English, which is much superior in point of strength, and in 
 facility of execution. Many attempts have been made to unite 
 Flemish facings with complete bond, but without success. 
 Some have laid thin slips of iron occasionally in the horizontal ■ 
 joints betw^een the two courses; and others have laid diagonal 
 courses of bricks in the core of thick walls, so as to cross each 
 other at right angles in successive courses. By the latter means, 
 though the bricks in the middle of the core have a strong bond, 
 yet as they form triangular interstices with the bricks on each 
 side, the bond of the whole wall is very incomplete. As the 
 adjustment of the bricks in one course must depend on the course 
 beneatli, the latter, in making the Flemish bond, must be seen or 
 recollected by the workman. The view of the joints of the 
 under course is hindered by the mortar with which they are 
 covered, to bed the bricks of the succeeding course upon, and 
 it is perplexing for the workman to recollect the arrangement 
 of them, so that he is in danger of making the joints frequently . 
 to correspond, and thus rendering the bond imperfect. In the 
 old English bond, the outside of each course, points out the 
 proper disposition of the next, so that it is difficult for the work- 
 >mm to err. 
 
BUILDING. 
 
 211 
 
 English and Flemish bond. 
 
 * The following plans of walls in English bond, which well 
 deserve to be revived in this country, will render this subject 
 more intelligible ; 
 
 Fig. 3, pi. I. is the bond of a nine-inch wall. In order 
 that two vertical joints may not run over each other, at the 
 end of the first stretcher from the coiner, after placing the 
 return corner stretcher, which becomes a header in the face that 
 the stretcher is in below, half the length of which it covers, 
 a half brick is placed on its side, so that with the return corner 
 stretcher, it extends six inches and three quarters, and thus a 
 lap of two inches and a quarter is obtained for the next header ; 
 and the bond is continued by working up the wall with alternate 
 rows of headers and stretchers mutually crossing each other. 
 The half brick, or brick-bat, thus introduced, is called a closer, 
 and must be divided, it will be understood, through its two 
 broadest surfaces, in the direction of their length. "Hie same 
 effect might be obtained by the introduction of a three-quarter 
 brick at the corner of the stretching course, for then when the 
 corner header is laid over it, a lap of two inches and a quarter 
 will be left at the end of the stretchers below for the next 
 header, the middle of which, when laid, will be over the joint 
 below the stretcher, and thus constitute a bond as before. 
 The brick for the three-quarter bat, or closer, must be divid- 
 ed through its two broadest surfaces, in the direction of their 
 breadth. 
 
 Fig. 4, represents the English bond of a brick and a half or 
 fourteen-inch wall. Here the stretching course is so disposed, 
 that the middle of the breadth of the bricks in the same layer or 
 level, falls alternately upon the middle of the stretchers, and upon 
 the joints between the stretchers. 
 
 Fig. 5, represents the English bond of a two-brick wall. To 
 break the joints in the core of the wall, every alternate header, in 
 the heading course, is only half a brick thick. 
 
 Fig. 6, represents the English bond of a wall two bricks and 
 a half in thickness. The disposition of the bricks is similar to 
 that of those in the last example. 
 
 Fig. 7, part of the front of a wall in English bond, the un- 
 broken side being the corner. 
 
 Fig. 8, the Flemish bond of a nine-inch wall. Two stretchers 
 lie between tw o headers ; and bricks being tw ice as long as they 
 are broad, the breadth of the two stretchers is equal to the length 
 of the header, which is the whole thickness of the wall. M’he 
 dotted lines show the disposition of the bricks in the second 
 course. 
 
 9; the Flemish bond of a brick and a half or fourteea-' 
 
212 
 
 BUILDING. 
 
 Flemissh bond. — Vertical bond. — Straight arch. 
 
 inch wall. On one side the bricks are laid as in the last exam- 
 ple, and on the other side, half headers are placed opposite the 
 middle of the stretchers, and the middle of the stretchers opposite 
 the middle of the end of the headers. 
 
 Fig. 10, another example of Flemish bond, for a wall of the 
 same thickness as the last. Here the disposition of the brick* 
 is alike on both sides of the wall, the tail of the headers being 
 placed contiguous to each other, an arrangement that produces, 
 in the core of the wall, square spaces, which must be filled with 
 half bricks. 
 
 Fig. 1 1, part of the front of a wall in Flemish bond, reaching 
 on one side to the corner. 
 
 Fig. 1, 2, 3, and 4, pi. II. exhibit plans of brick piers in 
 Flemish bond. No. 1, in each figure, shows the bottom course, 
 and No. 2 the upper course; or, which amounts to the same 
 thing. No. 2 may be considered the low er course, and No. 1 will 
 then be the upper one. 
 
 Fig. 1, is a pier two bricks, that is, eighteen inches square; 
 for in speaking of the thickness of a w all or pier as consisting of 
 so many bricks, the length of a brick is always to be understood. 
 
 Fig. 2,. a two and a half brick pier. 
 
 Fig. 3, a pier three bricks square. 
 
 Fig. 4, a pier three bricks and a half square. 
 
 Before we take leave of the subject of bond, w^e must remark, 
 that a patent has very lately been taken out, by Moore and Co. 
 of Loudon, for a vertical bond, which is intended to supersede 
 the use of the bond timbers introduced to secure the equal set- 
 tlement of the wall. In case of fire, when the bond timbers 
 of a house are consumed, the falling of the wall almost necessa- 
 rily follows. The patentees, therefore, instead of these timbers, 
 place rows of hard strong bricks perpendicularly in the middle 
 of their walls, at short distances from each other in height as 
 well as horizontal measurement, and they place each row^ of 
 perpendicular bricks in such a manner, as to be opposite the 
 middle of the space between the row standing in the same posi- 
 tion immediately above or below' it. This plan for obtaining 
 a vertical bond, seems new and ingenious, and is applicable, as 
 indeed the patentees observe, to stone walls, as well as to those 
 of brick. 
 
 Fig. 5, represents a straight arch, which is usually made 
 the height of four courses of brick ; but in considerable build- 
 ings, it may with great propriety be made the height of five 
 courses. The manner of draw ing the joints of a straight arch 
 will be evident from an inspection of the figure. The joints of 
 tlie arcli-stones must all be made to lie in a direct line to the 
 
BUILDING. 
 
 ^13 
 
 Scheme, Semi-circular, and Elliptical arches. — Tapper’s mode of groining. 
 
 point C. The point C is easily obtained, as it is as far from the 
 points A and B, (which are separated by the whole breadth of 
 the aperture,) as A and B are from each other. Hence the 
 lines connecting A, B, and C, form an equilateral triangle. To key 
 the arch in, it is usual to have a brick, and not the joint betw’een 
 two bricks, in the centre ; and therefore the division of the arch 
 must be managed accordingly. Though the brick in the middle 
 tapers more in the same length than the extreme bricks, yet as the 
 ditference is very small, it is disregared, from the great conveni- 
 ence of drawing all the bricks with the same mould. It may, 
 however, be observed, that the real taper of the mould may be a 
 inediuin between that required for the middle and that for either 
 extreme distance. But whether this be done or not, the defect 
 will not appear in practice. 
 
 Fig. 6, a scheme arch, one brick, that is, nine-inches high. 
 
 Fig. 7, a semi-circular arch, one brick high. 
 
 Fig. 8, an elliptical arch, one brick high, the top of which 
 is divided into equal parts, and not the under side. It is struck 
 from three centres. A, B, and C. 
 
 The arches delineated in the last three figures are often made 
 a brick and a half, or even tw o bricks high ; but for crow ning 
 the apertures of ordinary dwellings, the height of one brick is 
 deemed sufficient, both for stability and appearance. In arches 
 of one brick high, when the walls are only half a brick thick, 
 it is evident there is no necessity for joints following the course 
 of the arch in every alternate brick. Accordingly, these joints, 
 in such w^alls, are generally false ones, being merely nicks of 
 little depth cut with the tin-saw ; and are made as a kind of deco- 
 ration, in arches exposed to view’, to give, w’hen pointed along 
 with the other joints, a more lively appearance. But when the 
 walls are of one brick or a greater thickness, the joints in question 
 must be real, and formed of two bricks disposed as headers, for 
 the sake of bond. 
 
 Fig. 9, a plan of Tappers improved method of groining. The 
 improvement consists in raising the angles from an octagonal pier 
 instead of a square one. By this means, the angles of the groins 
 are strengthened by carrying the band round the diagonals of 
 equal breadth, thus affording better bond to the bricks, which are 
 usually so much cut aw ay, that instead 'of giving support, they are 
 themselves supported by the adjacent filling-in arches. Square 
 piers are also often very inconvenient in cellars, by hindering the 
 turning of goods round their angles. 
 
 In different parts of the country, it will naturally be sup- 
 posed that many differences of practice will prevail in the 
 details of building. Several of these differences originate in local 
 
214 
 
 BUILDING. 
 
 Exterior and interior scaffolding. — Precautions to obtain dry walls. 
 
 peculiarities of materials, of one kind or another; but there is 
 one, not belonging to this class, respecting which we might have 
 expected to find a very general uniformity of practice. In Lon- 
 don, and a wide district around, the scaffolding for the workmen, 
 in erecting the walls of a building, is external ; but in Liverpool, 
 and several other parts of Lancashire, and adjacent counties, the 
 apcaffolding is wholly within the building, whatever may be its size 
 or consequence. On the merits of either plan we shall not offer 
 a decisive opinion ; yet we may remark, that external scaffolding ii 
 not only the most expensive, but has an air of insecurity to the 
 workman, and of incompactness, which is unpleasant to the ob- 
 server, especially when he is aware that- such cumbersome appur- 
 tenances may be dispensed with. In populous towns, or confined 
 places, also, the encroachment of external scaffolding upon the 
 street, is not a trifling inconvenience, particularly as the bricks 
 and other materials must be at some distance beyond its limits, to 
 prevent accidents. Interior scaffolding, on the contrary, being 
 supported on the joists of each door as the work proceeds, is 
 erected with little trouble or expence, the workman marks his joints^^ 
 with as much ease and regularity as if he were at the outside, such 
 a thing as a falling brick or splinter is hardly known, and the bricks, 
 and other materials, may, if necessary, be laid close to the wall„ 
 80 as to occasion little inconvenience in the street. 
 
 To obtain the desirable requisite of dry walls, we have 
 already observed, that the usual resort is the use of interior lath- 
 work, and have adverted to the means of preventing this expence,, 
 by a proper composition of the mortar or stucco. Another expe- 
 dient in common use, intended to secure the dryness of the walls 
 when lathwork is deemed too expensive, consists in leaving a por- 
 tion of the bricks in the core or middle of the walls, without mor- 
 tar from the top to the bottom. The interstices thus left, serve, 
 in some measure, the same purpose as the space betw^een the lath- 
 work and the bricks. Perhaps the reference to a figure may make 
 the nature of the plan more easily apprehended. In the nine-inch 
 wall, fig. 8. pi. I. the longitudinal joint a b, and the middle third, 
 or from e of the transverse joint c dy would be left without 
 mortar ; and the same thing is done to all the other joints or por- 
 tions of joints similarly situated. By this contrivance, the bond of 
 the wall is greatly weakened ; but strength, it will be perceived, is 
 not the object of it ; the walls in the inside are drier, they look as 
 well, the house will let for as high a rent as a stronger, and its in- 
 stability is left for the purchaser, or the next generation, to dis- 
 cover. It has not unfrequently happened, however, that the slight- 
 ness of modern houses has been quickly proved, by their having^ 
 tumbled down before they were even finished^ 
 
BUILDING. 
 
 215 
 
 Advantage of casing walls. — Directions for walling. 
 
 There is one advantage of casing external walls with lath- 
 work, which is independent of damp, and therefore not the 
 object, though the consequence, of that operation. As air is 
 one of the most imperfect conductors of heat known, the co- 
 lumn of it included between the plaster or stucco and the bricks, 
 lends to prevent tlie temperature of apartments from being af- 
 fected by sudden vicissitudes in the heat of the external atmo- 
 sphere. — Lath work casing should be composed of well- seasoned 
 heart laths, as the sap laths will shrink. Reeds are used instead 
 of laths in some parts of the country ; but they require a greater 
 quantity of mortar than laths, and produce on the whole little or 
 no saving. 
 
 Frost is exceedingly prejudicial to new brick-work, and its 
 effects ought to be guarded against with the utmost care. 
 When it is apprehended, the wall should not be left uncovered 
 at night; a capping of straw, or of weather boarding formed 
 like the roof of a house, to carry off the rain equally on both 
 sides, if any occur, is generally employed ; and sometimes, for 
 the more complete security, both the straw and the boarding- 
 are employed at the same time, the straw being placed next the 
 wall. In winter, the mortar should be used stiffer than at other 
 seasons, and if a quantity of lime, which is quite fresh, or has 
 been kept in tight casks till it is wanted, were reduced to pow- 
 der without slaking, and well beat up with it, the setting of it 
 would be very materially hastened. Where strong work is re- 
 quired, it is not expedient, even in winter, to relinquish the prac- 
 tice already recommended, of steeping the bricks in lime-water ; 
 and wLen this is done, the method just mentioned of preparing the 
 mortar, is the more useful. The bricks should not, however, be 
 laid so dripping wet in winter as in warm dry weather. When 
 the practice of steeping the bricks in lime-water is rejected as too 
 troublesome, the sprinkling of each course with common water 
 may be considered the easiest substitute for that operation. This 
 method of strengthening the work, was adopted in the building of 
 the College of Physicians, London, at the judicious suggestion of 
 Dr. Hooke. 
 
 If the mortar has been suffered to lie any time, previous to its 
 being used, the labourer should beat it up again to give it tenacity, 
 and to prevent the bricklayer from losing time in working it with 
 his trowel. 
 
 In working up the wall, it is by no means advisable to work 
 more than four or five feet in height at a time ; for as all walls 
 shrink a little soon after building, if the different parts of the 
 circuit of the W'alls or carcass, be carried up at distant intervals, 
 one part will sink by itself, and will consequently separate from 
 
216 
 
 BUILDING. 
 
 Directions for walling. — Measurement of brickwork. — Mason’s tools. 
 
 the Other part which has become fixed. No portion of a wall 
 ought to be carried up more than the height of one scaffold 
 above the rest, except in some case of pressing emergency. In 
 carrying up any particular part, each side on the right and 
 left should be sloped off, to receive the bond of the adjoining 
 work. 
 
 When the house the bricklayer is employed upon is intended 
 to have other houses parallel with it, half bricks in a straight line 
 with the front, should be left projecting from every alternate 
 course, at the corner or corners to which the addition is intended 
 to be made ; or, otherwise, an excavation equal to the breadth of a 
 brick in front, and to the thickness of the w^all from front to back, 
 should be left in the alternate courses. In either case, the two 
 fronts will be bonded together, and the gaps, so frequently de- 
 forming contiguous houses, when the fronts have been built inde- 
 pendently of each other, will be prevented. 
 
 Bricks, as a building material, have several advantages over 
 stone ; from their porous texture, they unite better with the ce- 
 ment, are much lighter, and the walls built with them are very 
 little subject to damps arising from the condensation of the mois- 
 ture in the atmosphere. 'When all materials are ready, a good 
 workman with his labourer wall lay a thousand or twelve hun- 
 dred bricks in one day. 
 
 Brickwork is measured by the square foot, reduced to the thick- 
 ness of one brick and a half ; thus a wall tw’o bricks thick, ten feet 
 Jong, and three feet high, and therefore containing only thirty 
 square feet of surface in front, would be called forty feet reduced. 
 It is valued by the rod of two hundred and seventy-two feet. 
 Facing and gauged arches are measured by the superficial square 
 foot ; and cornices by the foot running, or length. 
 
 MASONRY. 
 
 Masonry is the term used to designate the art of building with 
 stone, as well as the work itself when executed. 
 
 OJ' the Mason^s Tools. 
 
 The tools required by the mason consist principally of a 
 Level, a Plumb Rule, a Square, a Bevel, a Trowel, a Hod, a 
 pair of Compasses, a Saw, a Mallet, and various sorts of Ham- 
 mers and Chisels. The whole of these tools, except the four 
 sorts last named, are similar to those bearing the same name 
 among bricklayers, and therefore most of them have already bee* 
 sufficiently described. 
 
BUILDING. 
 
 Sir 
 
 Tile mason’s saw — mallet — chisels. 
 
 The masons generally employ labourers to make use of their 
 ^aWy as they can hire them at a cheaper rate than regular 
 artists. The saw is without teeth, and is stretched in a frame, 
 60 as nearly to resemble the joiner^s frame-saw. It is generally 
 made from four to six feet in length, and for cutting through 
 slabjs of uncommon size, it is occasionally made much larger. 
 Its progress through the stone is facilitated by the use of sharp 
 silicious sand and water. The sand is placed upon an inclined 
 plane, and the water, exuding drop by drop from a small bar- 
 rel or other convenient vessel, runs over this plane, and carries 
 along with it a portion of sand into the kerf. The workman, 
 in the mean time, slowly drags the saw backwards and forwards 
 horizontally, taking a range of about twelve inches before he 
 makes the return stroke. By this simple process, the hardest 
 calcareous stones are cut into slabs of the required thickness, 
 with very little loss of their substance. But though the prac- 
 tice of sawing stone by hand may be considered easy, it is at 
 the same time slow and expensive. — Mills have therefore been 
 erected in various parts of the kingdom, for sawing and polish- 
 ing marble, particularly at Ashford in Derbyshire, and at Ken- 
 dal in Westmoreland. At the latter place, the machinery 
 produces on stone, every moulding which the turner can 
 produce on metal, or the joiner with his plane on wood. The 
 beautiful marbles of the neighbourhood, are wrought at a cheap 
 rate, into various forms, which, from the great expence that 
 would be incurred, would never be attempted by manual labour. 
 It seems to be a desideratum in the management of such ma- 
 chinery, to produce the return of the mouldings. Artists in w ood 
 do not require this return, from the facility with which they can 
 make use of mitre-joints ; but to the mason, w’ho can rarely adopt 
 joints of this nature, the return of the moulding is mostly an in- 
 dispensable requisite. In a chimney-piece, or the architrave of 
 a door or window, for example, the return is requiied on the hori- 
 zontal piece. 
 
 The shape of the mason’s mallet differs from that of other ar- 
 tists. Its contour is not unlike that of a bell, except that a small 
 portion of the broadest part of it is cylindrical ; this part is usually 
 about eight or ten inches in diameter. The handle is just long 
 enough to be firmly grasped in the hand. The chisels are struck 
 with any part of the cylindrical surface. 
 
 The chisels used by the masons are required to be of various 
 breadths on the cutting edge, but three inches for the broadest, 
 and a quarter of an inch for the narrowest, may be considered 
 the general limits of their sizes in this respect. They are usually 
 made of iron and steel welded together, and the steel extends no 
 10.— VoL. I. 2 F 
 
218 
 
 BUILDING. 
 
 Different kinds of chisels — hammer. — Marble. 
 
 farther than they are likely to be ground for use. When new, 
 they are mostly about eight or nine inches long. The part held 
 in the hand, is generally about live-eighths or three quarters of an 
 inch in diameter, and is forged to an octagonal shape. When 
 the cutting edge is broader than this octagonal portion for the 
 hand, the lower part is first expanded in a dove-tail form, and then 
 the sides for a short distance are parallel to the edge. When the 
 cutting edge is narrower than the octagonal part, the lower end is 
 sloped oft in a pyramidal form. The larger sizes of chisels ob- 
 tain the name of toolsj the act of using them is called tooling, and 
 the stone to which they have been applied is said to be tooled. 
 The chisel with the nanowest edge, which is seldom broader, and 
 often not so broad, as a quarter of an inch, is called a point. The 
 point is employed to reduce the larger irregularities of a surface, 
 which is afterwards made tolerably regular with the broad chisels 
 or tools. In chipping stone intended to be finished smooth and 
 neat, great care should be taken to avoid splintering the arris, 
 which will almost certainly happen if the edge of the chisel be 
 directed outwards in making the blow . Even for the edge of the 
 chisel to be at right angles to the arris it is applied to form, is 
 not always a Safe position ; but if directed inw ards, so as to form 
 an angle of forty-five degrees with the line of the arris, which it 
 should overhang a little, the chipping may be safely executed. 
 To direct the workman in the use of the chisel, a thin board, 
 planed true, is used as a straight-edge, to point out cross-windings 
 and other inequalities of surfaces, the prominent defects of which 
 have previously been removed. 
 
 On this occasion, we shall make an observation of which per- 
 haps very few^ artists are aw are. A chisel made entirely of steel, will 
 produce a greater effect with a given impulse than one composed 
 partly of iron and partly of steel, although the steel of the latter 
 forms its edge, and is there equally as good and hard as the steel 
 of the former. But tempered steel is more elastic than iron, and 
 therefore transmits more faithfully the impulse it receives. 
 
 In some parts of the country, masons are provided with such 
 hammers as they can use (particularly when they are employed 
 upon the harder kinds of calcareous stones) instead of chisels, as 
 well as for the general purpose of dividing stones. A heavy 
 pointed hammer serves instead of the point, and another which is 
 also heavy, and has an edge like that of a chisel, serves very effec- 
 tually to produce those narrow marks of furrows left upon hewn 
 stone-work w'hich is not ground on the face. 
 
 Of Stones. 
 
 All calcareous stones, namely, those that burn to lime, if hard> 
 
BUILDING. 
 
 219 
 
 Marble. — Limestone. — Portland and Purbeck stone. — Freestone. 
 
 of a close texture, and beautiful appearance, from the variegation, 
 or clearness and uniformity of their colours, are called marble. 
 The names of the different kinds of marble are generally derived 
 either from their colour, or the place where they are obtained. 
 The most valuable kind of milk-white marble is obtained in Italy, 
 and is too costly to be often used for any but the smaller orna- 
 mental parts of buildings, lliis, when cut into thin slices, is 
 semi-transparent. Many parts of the united kingdom abound 
 with marble, which is mostly more or less coloured,, often close 
 in its texture, and capable of receiving a high polish. Derbyshire 
 and Westmoreland, in England, are the counties which supply the 
 greatest quantity and variety of marbles, some specimens of which 
 are highly esteemed. 
 
 When marble that has been faced with a chisel, is intended to 
 be polished, it is ground by rubbing it with rough-grained free- 
 stone, assisted by sand and water, until the chisel marks are re- 
 moved. Finer and liner-grained freestone, with water, but no 
 sand, is then used, till the surface becomes very smooth. If the 
 finer grit- stones be found too slow in their operation, a little fine 
 flour of emery is used. The last and highest lustre is given with 
 oxide of lime, w ell known under the name of putty. When the 
 surface of the marble to be polished has been cut with a saw, the 
 very rough freestone and sand are not necessaiy. In other re- 
 spects it must be hnished by the same process. 
 
 Limestone is a coarse kind of marble, and is cut and polished 
 in a similar manner. In many districts, it forms immense strata. 
 From its great hardness, it is only hammer-dressed when used 
 for the fronts of buildings ; but when this is done in the best man- 
 ner, the etfect is very fine. 
 
 The stone most commonly used in London, is Pcrtiaiid stone, 
 which is brought from the island of Portiand in Dorsetshire. It 
 is of a dull whitish colour, though the buildings constructed with 
 it have a handsome appearance. Vfhen recently dug, it is soft, 
 and easy to work, but acquires hardness with age. Although it 
 contains silex, the hardness it acquires is not so great that it wall 
 strike fire with steel. Under great pressure, Portland stone is apt 
 to splinter at the joints. 
 
 Purbeck stone is brought from the island of Purbeck, also in 
 Dorsetshire ; it is mostly employed in rough w ork, such as steps, 
 paving, &c. 
 
 Freestone is a general name for stones of very different quali^ 
 ties as to their value in building. It consists of clay and silex, 
 sometimes the silex amounts to nearly one-half its weight, but ge- 
 nerally it is considerably less, and the hardness of the stone varies 
 with the proportions of its component parts. It is often caUed 
 
220 
 
 BUILDING. 
 
 Freestone. — Strength of stone ditferent in different positions. 
 
 grit or sand stone. It is a very plentiful stone ; the strata of entire 
 districts, under a slight covering of soil, appearing, in various in- 
 stances, to be composed of it. The particles of some kinds of it 
 have so little cohesion, that small bits may be granulated be- 
 tween the fingers ; this is the sort commonly used for fiftering- 
 stones. All kinds of it are softer when taken out of the quaiVy, 
 than they afterwards become when dry, or after long exposure to 
 the atmosphere. Freestone is generally about two and a half 
 times the weight of water. That from H9llington, near Utoxeter, 
 is of a whitish or yellowish gray ; that from Knipersly, in Stafford- 
 shire, is of a bluish gray, and so infusible as to be used as a fife 
 stone. The colour of freestone is often a dull red, but sometimes 
 so nearly white, that buildings constructed with it look as well as 
 those of Portland stone. Different parcels of freestone, taken 
 from the same quarry, frequently exhibit a considerable diversity 
 of colour ; a circumstance which gives a motley appearance to 
 buildings in other respects perhaps faultless. When the stone has 
 been recently dug, and is damp, these differences are often not 
 perceptible. If, therefore, uniformity of appearance be desired, 
 the stones should be faced, dried, and sorted before they are used. 
 Freestone is incapable of receiving a polish, and therefore, when 
 it has been cut with a saw, it is rarely submitted to any subse- 
 quent operation to produce greater smoothness ; but when it has 
 been reduced with the chisel, it is made smooth w ith another piece 
 of stone of the same kind, along with sand and water. When 
 the freestone has a laminated texture, it is called dag-stone, and i^ 
 divided into thin pieces for the purpose of covering houses, and 
 for flooring. 
 
 The position which stones have had in the quarry, is not a 
 matter of indifference to the attentive mason. Stones intended to 
 sustain great vertical pressure, as pillars, should stand in a build- 
 ing as they stood in the quarry from w hich they were taken ; for 
 pillars, the axis of which were horizontal in the quarry, when 
 placed perpendicularly, are apt to split under a great strain. Per- 
 haps all stones, however solid they may appear, possess more or 
 less of a laminated texture, — a property doubtless occasioned by 
 separate depositions, or crystallizations of their peculiar matter. 
 V» hat kind of a pillar any stone well known to be laminated, 
 'would produce, in different positions, it is not difficult to conjec- 
 ture. If, for example, a block of flag-stone were converted into a 
 pillar, so as to leave each lamina or flag of wliich it is composed 
 posited hcrizonLally, it would sustain aOy weight not capable of 
 crushing it to atoms ; but if the lamina were placed in an incllhed 
 position, so as to form an acute angle with the axis, an incoiisidef- 
 -abie pressure would occasion diem, where the cohesion was slightest. 
 
BUILDING. 
 
 221 
 
 Mortar. — Oil cement. — Plaster of Paris. 
 
 to slide over each other ; lastly, if the lamina were placed parallel 
 to the axis, the pillar, under sufficient pressure, would divide verti- 
 cally into several parts, and though rather stronger than in the 
 last instance, would still be comparatively weak. An attention 
 therefore to the position of stones, and to veins or other signs 
 which may indicate the existence of lamina, w^ell deserves the 
 mason’s regard. 
 
 Of Cements. 
 
 The cement used by the mason, for the ordinary purposes of 
 walling, is mortar, differing not from that used by the bricklayer ; 
 and as we have already treated of this subject rather at length, a 
 few remarks in this place will suffice. For the joints of hewn 
 stone, the mortar should be much finer than the bricklayer requires; 
 and sometimes a mixture of oil-putty, or vei^ thick white lead 
 paint, is used as the cement. These compositions will last longer 
 than almost any stones, and will remain prominent w hen the face 
 of the softer kinds of stone has been corroded by age. 
 
 The cement used in setting column stones, is mostly oil- 
 putty, or white lead mixed with chalk-putty, or fine mortar. 
 Sometimes columns are set upon milled lead ; in this case, the 
 lead should not be quite equal to the column in diameter, but so 
 as to leave a narrow ring externally, wiuch must be tilled wdth 
 oil-putty. 
 
 In situations not exposed to damp, plaster of Paris is em- 
 ployed as a bedding for stones or marble. When a mantle-piece 
 is composed of valuable marble, the various pieces are commonly 
 very thin. In this case, a considerable thickness of plaster cff 
 Paris is laid on the back, and a slate or some ordinary stone 
 bedded in it, to give greater strength. Good plaster of Paris is 
 scarcely to be met with, nor are the causes of its imperfections 
 generally understood by workmen. We shall therefore point them 
 out, and give directions for preparing it, of a uniform and excel- 
 lent quality, when we treat of casting in plaster. 
 
 I'he Greeks and Romans constructed their works of wrought 
 stone without cement; but as they used a profusion of cramp* 
 and bands of iron and bronze, and the beds of the enormous 
 stones they used were finished w ith almost mathematical precision, 
 their edifices were substantial and durable in the highest degree.^ 
 IVletailic bands and cramps are still used in aquatic works, as 
 well as for lofty steeples, and other slender buildings much ex- 
 posed to the action of the winds, also to connect the different 
 stones composing mantle-pieces, &c, with the wall. 
 
222 
 
 BUILDING. 
 
 Definitions. — Footings of trails. 
 
 Of Stone Walls. 
 
 Tlie propriety of erecting suspended or inverted arches, 
 according to circumstances, and other general directions already 
 given respecting foundations, being as applicable to stone walls 
 as to those of brick, need not be adverted to again. The ex- 
 planation of a few technical terms will therefore be our first 
 object. 
 
 Stones which run through the thickness of a wall, in order to 
 bind it, are called bond stones ; in some parts of the country they 
 obtain the name of through stones. 
 
 When the side or sides of a wail lean back, so that the plumb 
 would fall within the base of the wall, the inclination is called 
 battering ; it is generally made about one inch in a foot. 
 
 The large stones at the base of a foundation, which project 
 beyond the vertical surface or front of the superincumbent wall, 
 are called ybo^//2gs. 
 
 The parts of a \vall between apertures, or between an aper* 
 ture and the corner, are called yj/ers. 
 
 The beds of a stone are its upper and under surface, which 
 are generally in a horizontal [)Osition within the w'all. 
 
 Walls built with unhewn stone, with or without mortar, are 
 Called rubble zcalls. Rubble walls are of two kinds, the 
 coursed and the imcoursed. In the coursed, the stones are 
 hammer-dressed or axed, and adjusted by a sizing rule, so that 
 each row of stones forms a horizontal surface. In the uncoursed, 
 the stones are used in a rough state, nearly as they come out of 
 the quarr}'. 
 
 Walls which are faced with squared stones, hewn or rubbed, 
 and backed with rubble, stone, or brick, are called ashlar. 
 
 AVali-plates, are horizontal pieces of timber, commonly laid 
 even w ith the interior of walls, for the ends of the joists and other 
 timbers to rest upon. 
 
 The footings of walls ought to consist of the largest stones 
 which can be conveniently procured. It is better to have them 
 of a rectangular form than any other, and if not square, their 
 largest surfaces should be laid horizontally. With this shape 
 and disposition, they will make tlie greatest resistance to sink- 
 ing. If the stones, intended to be employed as footings, 
 deviate materially from a rectangular figure, when received 
 from the quarr}', they ought to be hammer-dressed ; as, if they 
 taper downwards, or rest upon angular ridges, they will be 
 apt to give w ay under the weight of the superstructure. When 
 the footings can be obtained the full breadth of the w all in one 
 piece, they are to be preferred j but when a sufficient number of 
 
BUILDING. 
 
 2^3 
 
 Uncoursed and coursed rubble walls. — Ashlar walls. 
 
 stones of this description cannot be obtained, then every alternate 
 stone in the course may be the whole breadth, with two stones 
 next to it, disposed like two stretchers in a nine-inch wall of 
 Flemish brick-work. When the largest stones which can be con- 
 veniently obtained, are insufficient even for the latter arrangement, 
 the most suitable which can be procured, must be disposed so as 
 to break in the best manner circumstances will admit, the vertical 
 joints in the same course, as well as those of the different courses 
 with respect to each other. Each course, also, should be well 
 bedded in mortar. 
 
 When bond timber will be required, the uncoursed rubble is 
 an inconvenient mode of building, as the heights on which they 
 are disposed must be levelled. The best kind, or coursed rubble, 
 admits of bond timbers without difficulty, for though the different 
 courses are not of the same height, the surface of each of them is 
 level ; but as the walls in which bond timbers are introduced, are 
 apt to warp or even fall in case of fire, the use of them should be 
 avoided in strong w'ell-built walls. 
 
 The stones of an ashlar front should have their upper and 
 under surface correctly parallel with each other, and correctly 
 at right angles to the face. If these surfaces be carelessly left 
 concave, they will be apt to splinter near the edge under great 
 pressure. On the right and left they should taper inwards, but 
 the taper should not be continued quite to the face, though it 
 may reach the face within an inch or two. The ashlar stones 
 having the form of a truncated wedge, they will, in each course, 
 present a series of angular indentations within the wall, like the 
 spaces between the teeth of a saw. The stones are so selected 
 and disposed that the vertical or upright joints, and consequently 
 the angular spaces, of one course fall on the middle of the stones 
 below. By this means the ashlar face is bonded to the rubble, 
 brick, or rough stone of the back, and the strength of the w^all 
 much greater than if each stone was of an equal rectangular 
 figure. Strength is also to be promoted by adopting a plan not 
 commonly regarded, that of sorting the stones, so that in each 
 alternate course they will extend farther into the wall than those 
 of the ^course immediately above and below. In ashlar work, 
 the bond stones, which ought frequently to be introduced, can- 
 not like the other stones have a wedge-like form ; they must be 
 rectangular; and they produce the best effect, when so disposed 
 in each course that they will be opposite the middle of the space 
 between the two bond stones in the course immediately above and 
 beneath them. 
 
224 
 
 BUILDING. 
 
 Working of the stones for columns. 
 
 Of Stone Columns. 
 
 When largfe stone columns are made in one piece, their eflft^ct, 
 from that circumstance alone, is very striking ; but as tliis advan- 
 tage is not always obtainable, the next object is to make the joints 
 as few and as minute as possible, as well as to be very attentive 
 in selecting the different stones to be combined, that the joints 
 may not l>e descried at a distance, by the commencement of a dif- 
 ferent colour. From what has been said in the section on the dif- 
 ferent kinds of stone, it will be understood, that none but hori- 
 zontal joints can be allowed in any shaft ; all others being incon- 
 sistent with the laws of strength. 
 
 The stones proper for an intended column being procured, 
 and the order in which they are to succeed each other being 
 determined, the next consideration will be to ascertain the exact 
 diameter proper for each end of every one of them. For this 
 purpose, draw an elevation of the proposed column to the full 
 size, divide it by lines parallel to the base, into as many heights 
 as the column is intended to contain stones, taking care that 
 none of the heights exceed the length that the stones will pro- 
 duce. The working of the stones to the diameters thus 
 obtained then becomes easy. The ends of each stone must first 
 be wrought so as to form exactly true and parallel planes. The 
 two beds of a stone being thus formed, find their centres, and 
 describe a circle on each of them. Divide these circles into the 
 aame number of equal parts, which may, for example, amount 
 to six or eight. Draw lines across each end of the stone, 50 
 that they will pass through the centre, and through the oppo- 
 site divisions of the same end. The extremities of these lines 
 are to regulate the progress of the chisel along the surface of 
 the stone, and therefore when those of one end have been 
 diawn, those of the other must be made in the same plane, or 
 opposite to them respectively. The cylindrical part of the 
 Slones must be wrought with the assistance of a straight-edge; 
 but for the swell of the column, a diminishing rule, that is, one 
 made concave to the line of the column, must be employed. 
 This diminishing rule will serve to plumb the stones in setting 
 them. If it be made the whole length of the column, the heights 
 into which the elevation of the column is divided, should be mark- 
 ed upon it, so that it may be applied to give each stone its proper 
 curvature. But as tlie use of a long diminishing rule, when die 
 stones are in many and short lengths, would be inconvenient, rules 
 corresponding in length to that of the different heights, may be 
 employed with advantage. 
 
BUILDING. 
 
 225 
 
 To draw tlie curve of a shaft. 
 
 Of the different methods which may be practised, to obtain 
 the curvature of the rule to be used in the diminution of the 
 shaft of a column, the following may be considered the easiest 
 and best adapted for general use : Let A B, fig. 12, pi. I. re- 
 present the height of the columny e f the semi-diameter of the 
 lower part, and g h the semi-diameter of the upper part, ac- 
 cording to the customary proportion of the diminution. As 
 the lower one-third of the column must be cylindrical, draw 
 a line from e to f, parallel to A B. What we now want 
 is, to obtain a curved line from i, which will fall into g 
 without making the diameter of the upper two-thirds of the 
 column in any point greater than that of the lower or cylindri- 
 cal third. With the radius i /c, draw the quarter of a circle C. 
 Draw a line from the narrowest* part of the column, that is from 
 g, parallel to the axis or middle line A B, till it cuts the quad- 
 rant of a circle C. Divide the arc contained between i and the 
 point where g cuts C, into four equal parts, as /, m, u, o, and 
 divide the height A k into the same number of divisions as this 
 arc, as 1, 2, 3. From the point m, draw a line parallel to the 
 axis, which will cut the transverse line 3 at v ; from 7i draw' ano- 
 ther line parallel to the axis, which will cut the transverse line 2 
 in w ; in the same manner draw a line from o to the transverse 
 line 1, which it will cut in x. Now' the curved line of tlie co- 
 lumn, between i and g, must pass through the points ar, Zi), v; 
 therefore at or near all the points from i to g inclusive, drive in 
 two pins or nails, in such a maimer that the direction in which 
 each pair of nails stand shall be the same as the transverse lines 
 of the column. Between each pair of nails, also, there must be 
 just sufticient space left to admit a thin slip of wood, like a lath, 
 or some other equally flexible substance, and care must be taken 
 to fix the nails in such a position, that w hen this slip of wood is 
 placed between them, either its inner or its outer perpendicular 
 surface shall exactly coincide with the several points of diminu- 
 tion /, X, Zi)y V, g. This being done, it is easy, with a pencil or 
 any marking instrument suited to the surface worked upon, to 
 draw' a line along the slip of wood, which line will be the curve 
 of the shaft. The piece intended to be used as a diminishing 
 rule may have the curve made on it, or transferred to it, as may 
 be deemed most convenient. It will also be understood, that the 
 number of the parts into which the arc / z, and the height A k 
 are divided, and which determine the number of diminishing 
 points obtained, may be varied at pleasure. In no case, how-' 
 ever, can it be considered advisable to make these divisions’ 
 few'er than four, though for lofty columns they may be made 
 two or three times that number, as they constitute so many* 
 
 10. — VoL. I. 2G 
 
226 
 
 BULtDtNG. 
 
 To draw the curve of a shaft. — To draw flutes and fillets on a shaft. 
 
 checks upon the irregular or imperfect flexibility of the lath 
 or spring rule. so much of the beauty of a shaft depends 
 upon the easy and imperceptible transition from the cylindrical 
 to the tapering part, it may be advantageous to divide the arc 
 from i to o into two parts, and the division /c 1, of the shaft, 
 also into two parts ; a diminishing point will then be obtained 
 between i and x, which will be of use to lessen the possibility of 
 imperfection. 
 
 Another mode, which we shall recite, of diminishing a shaft, 
 is not essentially different from the preceding : divide the shaft 
 from D to E, fig. 17, into four parts, as 1, 2, 3; divide the space 
 EF, which is the difference between the superior semi-diameter 
 E r, and the inferior or lower diameter D q, into the like number 
 of parts, viz. four. Draw lines from each division on EF tending 
 to the point D. The first line next to F will reach to D, the 
 point from which the diminution commences, in a direction pa- 
 rallel to the axis ; the next line, reckoning from the same side, 
 will give the point a ; the third line the point b ; and the fourth 
 line the point c ; a line from the point E, which is the limit of the 
 diminution, need not be drawn. The points E, c, 6, Uy D, consti- 
 tute the places where the nails must be driven in, to direct the 
 path of the lath as in the last example. Some artists prefer this 
 mode of diminution. Here, also, as before, the number of the di- 
 visions is optional, but a greater error may be committed by mak- 
 ing them too few than too numerous. 
 
 To draw the flutes and fillets on the shaft of a column, the 
 following will be found an easy and effectual method : let A, fig. 
 10, pi. II. be the shaft; make a hole exactly in the centre of both 
 ends, and into each hole drive a piece of wood, so as to be quite 
 tight, and to project five or six inches. The projecting parts, 
 b by of the pieces of wood, must be w ell rounded off, and made 
 exactly in the middle of the ends. Being provided with a di- 
 minishing rule, made as above-mentioned to the curve of the shaft, 
 it may here be used as the ruler, by fixing it in a groove in two 
 pieces of wood, c c, so as to revolve w ith them upon the pins b b. 
 The edge of the rule must be brought very near the shaft, and one 
 side of it must tend exactly to the centre, which is done by giv- 
 ing that direction to one side of the grooves in c c, as shown by 
 the dotted lines a a. As the diminishing rule d, is commonly 
 made rather slender, and therefore will be apt to bend with the 
 force employed to draw the lines, it will be proper to fix a piece 
 of w'ood, of sufficient thickness to keep it straight, on the back, 
 or that side of it which does not tend to the centre. As in 
 marking a long column, there may be some difficulty in keep- 
 ing the rule steady, while the lines are drawn, the strong piece 
 
BUILDING. 
 
 227 
 
 To draw the flutes and fillets on a diminished pilaster. — Stone bridges. 
 
 of wood thus attached to the back of the diminishing rule, may 
 have one, two, or more screws, according to its length, passing 
 through it, as ff, and pressing against the shaft, by which 
 means the rule will be staid at any part of a revolution, much 
 more effectually than with the assistance of several men. The 
 screws ought to be of wood, or, if iron, their extremities should 
 be prevented from injuring the stone, by the interposition of a 
 flat piece of metal, or some other suitable substance. These 
 arrangements being made, the lines desired may be drawn with 
 any sharply-pointed steel instrument kept close to the rule, with 
 the greatest ease and precision, from divisions previously mark- 
 ed at one end. To obtain these divisions, suppose it were 
 desired to flute the Ionic, the Corinthian, or Composite co- 
 lumns, the circumference at one end will be divided into six 
 equal parts, by taking half the diameter at that end, and apply- 
 ing it round the said circumference ; and if each of these divisions 
 be divided into four, the whole circumference will be divided into 
 twenty-four. In order to make the proportion of a flute to a fillet 
 as one to three^ divide any one of these last divisions, or twenty- 
 fourth part of the circumference, into four equal parts, and one of 
 these parts will be the breadth of a fillet; which being set off* from 
 the same side of each division, the whole shaft will be properly 
 divided for flutes and fillets, and consequently prepared for the use 
 of the rule. 
 
 To draw the flutes and fillets on a diminished pilaster, make 
 a line down the middle of the face of it, from top to bottom ; 
 divide this longitudinal line into any convenient number of 
 equal parts, and through the points of division draw transverse 
 lines crossing the breadth of the pilaster ; set off the flutes and 
 fillets on each transverse line; fix pins or nails at each correspond- 
 ing point of the transverse line, and draw the lines with the help of 
 a pliable slip of wood, as in obtaining the diminution of a shaft. 
 
 Stone Bridge^. 
 
 Here it* will be proper, in the first place, to give an explanation 
 of the principal terms used in speaking of biidges. 
 
 The abutments of a bridge are the walls adjoining to the land, 
 each of them supporting the end of an extreme arch. — The walls 
 supporting the ends of the arches betw'een the abutments, are 
 called piers. 
 
 The imposts are the highest course of the stones of piers, from 
 which an arch immediately springs, and which project a little from 
 the piers. ' 
 
 Batterdeau, or Coffsr-’dam, a case of piling, to divert the 
 stream of a river while the piers are building. 
 
228 
 
 BUILDING. 
 
 Definitions. — Requisites of a good bridge. 
 
 The i^paii of an arch is its greatest horizontal width. 
 
 Caissons are large hollow vessels, framed of strong timbers, 
 and made water tight, which being launched and floated to a pro^ 
 per position in the river, where the bed has been previously exca- 
 vated and levelled, are there sunk. The piers of the bridge are 
 built within them, and when carried up above or nearly to the level 
 of the water, the sides of the caisson are detached from the bot- 
 tom, and removed ; the bottom is left remaining, and serves for a 
 foundation to the pier. 
 
 Drift, push, shoot, thrust, terms used indiscriminately, to ex- 
 press the horizontal force of an arch, by which it endeavours to 
 spread itself out and overset the piers and abutments. 
 
 - . The extrados of an arch is the outside or convex cu.»'ve on the 
 
 top of the arch stones : hence the extrados of a bridge is the curve 
 of the road-way. Extrados is the term opposed to i)itrados, which 
 signifies the concave side of an arch. 
 
 Those stones, one side of which is in front, and another side 
 forms part of the intrados of an arch, are called voussoirs or arch-- 
 stones ; the middle voussoir, at the summit of the arch, which is 
 put in last, for wedging and closing the whole, is also, and indeed 
 most commonly, called the key-stone. 
 
 Parapets are the low walls erected on the sides of the extra- 
 dos or road-way of a bridge, for the safeguard of passengers. 
 
 These explanations being premised, we may proceed with the 
 subject. Palladio observes, that bridges ought to have the same 
 qualifications which are deemed necessary in all buildings of 
 consequence, namely, that they should be convenient, beautiful^ 
 and lasting. To make a bridge lasting, the goodness of its 
 foundation is one of the most important things to be attended to; 
 to make it convenient, an easy ascent, and a width suited to its 
 situation, are absolutely necessary ; and to make it beautiful, its 
 workmanship must be good, and its several parts well designed 
 and proportioned. 
 
 Bridges ought always to be constructed at right angles to the 
 current. The piers ought to be so proportioned, as to enable 
 them to withstand the thrust of the adjoining arches, though the 
 rest were throwm dowm ; yet to fall short of the size required for 
 this purpose, would hardly be a greater fault than materially to 
 exceed it ; for w’hen the piers are unnecessarily lajge, the current 
 being contracted, increases its velocity, and is apt to undermine 
 the foundation. 
 
 A judicious regard to local circumstances, is aUvays of great 
 importance to the durability of a bridge. Over rivers which 
 are subject to great inundations, arches that will be dry at ordi- 
 nary seasons will often be necessary ; and w'hen a ciioice with 
 
BUILDING. 
 
 229 
 
 Difference of opinion respecting the proper curve for arches. 
 
 respect to the situation can be made, a wide part of the 
 stream should he preferred to the narrow part, because the water 
 at the narrow part has not only a greater velocity than at the 
 other, but that velocity would be increased by the space which the 
 piers would occupy. Another point to be duly regarded in select- 
 ing the site of a bridge, is the solidity or otherwise of the ground 
 in ditferent parts of the river, for the foundations of piers and 
 abutments. An immense expence in piling, and other artificial 
 means of improving inadequate ground, may often be saved by 
 proper attention in this respect. 
 
 I'he arches in a bridge ought to be an odd number, that one 
 of them may stand in the middle, where the velocity of the stream 
 is greatest. The middle arch is also to be made the largest. As 
 the expeiice of piers is very considerable, besides the perpetual in- 
 convenience of them in navigable rivers, bridges should always be 
 constructed with the fewest arches possible. 
 
 With regard to the most suitable curve for the arches of a 
 bridge, the subject has been much controverted. Not only 
 architects, but mathematicians have, on this point, differed 
 widely from each other. 'I his diversity of judgment seems to 
 have been perpetuated by the fact, that bridges of acknow- 
 ledged, and to all appearance of equal excellence, have been con- 
 structed with semi-circular, semi-elliptical, and curves of various 
 other denominations, and consequently possessing very differ- 
 ent properties. Some have contended that semi-circular arches 
 ought in most cases to be preferred, because they press more 
 perpendicularly upon the piers than smaller portions of circles, 
 and in proportion to their number will diminish the pressure on 
 the abutments. Others have preferred the elliptical curve, 
 where the arches were to be large and but few in number, 
 because the extensive radius of the semi-circular arch w'ould 
 occasion the central part of the extrados or road-way to be very 
 inconveniently elevated. This objection to the semi-circular 
 form is obviated by adopting the elliptical one, and Muller as- 
 serts that the elliptical arch does not press against the piers with a 
 greater force than a circular one ; and being lighter, from requir- 
 ing a smaller quantity of materials, will consequently be more 
 lasting. 
 
 As the strength of a bridge depends so much on mathemati- 
 cal principles, it seems natural to look for, and to regard with 
 deference, the opinions of the most eminent mathematicians. 
 If mere theorists differ from each other, those who enjoy alike 
 the light of theoretical and practical knowledge, may discover 
 the sources of the errors which have occasioned contradictory 
 deductions. It was the opionion of the celebrated Dr. Hutton, 
 
230 
 
 BUILDING. 
 
 Arch of equilibration. 
 
 late professor of the mathematics to the royal military academy, 
 Woolwich, that the mechanical arch of equilibration is the 
 only perfect one adapted to the principles of bridges. This 
 arch, it is asserted, being in exact equilibrium in all its parts, 
 can have no tendency to break in one part more than another, 
 and must therefore be the safest and strongest. A table for 
 constructing the arch of equilibration, is given in the Doctor^s 
 Mathematical and Philosophical Dictionary, under the article 
 Bridge. To explain the nature of the equilibrated arch, we 
 shall advert, in the first place, to the construction of the 
 catenarian curve. If a perfectly flexible rope, of equal thick- 
 ness and density in every part, were fastened by its ends to 
 hooks, or some other supports, at any distance insufficient to 
 draw it tight, the position which it would assume is called the 
 catenarian curve. This is a curve which has had many advo- 
 cates, among whom is found the celebrated Emerson, who con- 
 sidered it of all curves the best adapted to bridge building. 
 The fact is, that if the arch of a bridge were, like the rope, 
 of uniform thickness (which for the present we will suppose to 
 imply uniform density) in every part, the catenarian curve 
 would be the most proper for it, because that curve, under such 
 circumstances, is a true arch of equilibration. Hence, as the 
 uniform thickness of an arch can rarely be made c.ompatible 
 with other objects in constructing a bridge, the catenarian 
 curve can as rarely be the proper arch to use. Every particu- 
 lar form of the extrados of an arch, requires, according to the 
 principles of equilibration, a difference in the form of the 
 intrados. I'o render this more clear, we will return to the con- 
 sideration of the rope, suspended as above stated. Supposing 
 it to hang along a wall, the curve it exhibits may be traced. 
 When this has been done, let the substance of the rope, on the 
 convex side, be increased in some parts, for example at the 
 haunches ; it will then no longer describe a catenary, and it 
 would be found that every difference in the additions made to it 
 will produce a difference in its curvature. If the experiment 
 could be made with materials really possessing perfect flexibility, 
 these differences would be very perceptible, and if the points of 
 suspension were at a distance from each other just equal to the 
 diameter of a circle which the rope would half encompass, it 
 would be found that the rope would require the additions made 
 to it to be as represented by No. 2, fig. 11. pi. II. before the 
 concave side of it would describe a semi-circle. But when these 
 additions were made, the contour of the figure, if introverted, 
 as shown by No. 1, would represent the section of an equili- 
 brated arch, the intrados of which should be semi-circular. As 
 
BUILDING. 
 
 231 
 
 Arch of equilibration. 
 
 an extrados or terminating line of wall running up at both ends 
 in this manner, is far remote from any form that is admissible 
 in practice, semi-circular arches are never balanced or equili- 
 brated ; and the advocates of the equilibrating system assert, that 
 though such arches have stood for centuries, this event has only 
 happened in consequence of the cement and friction w^hich have 
 prevented the stones from being pushed out of their places. 
 Hence, they add, that if the materials were only placed in 
 contact, without cohesion or friction, they would not stand when 
 the road-way was straight, or a convex curve throughout the 
 length of the arch, or in any other way different from the figure 
 above referred to. 
 
 These observations will, w^e hope, communicate to the 
 reader, an idea of the nature of equilibrated arches. The 
 subject is interesting and important, and to dismiss it without 
 some explanation of this kind, would be denying it the notice 
 to which it is fairly entitled. Here, however, it may be ob- 
 served, that the subtle refinements of theory, whatever may be 
 their general utility, are not always found to be directly 
 applicable to practice; and w^e may therefore sometimes act in 
 opposition to them, not only with safety, but with advantage. 
 Bridges, which have not been equilibrated, have, as before 
 stated, endured for ages, and appear likely to endure till the 
 materials of which they are composed crumble away. Hence 
 we are afforded a reasonable encouragement to construct again 
 such arches as they are composed of, whenever their form 
 suits us in other respects. Though the mathematical reasoning 
 on arches of equilibration be just, yet it applies with strict 
 propriety only to homogeneous materials, and to structures 
 acted upon by no pow'er but that of gravitation. If the form 
 it prescribes w ere attended to, it would cease to be an arch of 
 equilibration, under the pressure of a waggon or any other load ; 
 and without the friendly aid of cement and friction, would no 
 more be able to stand than any other kind of an arch. In the 
 practical operations, also, of constructing a bridge, the proper 
 configuration of the stones for many kinds of curves, either 
 cannot be determined by any unexceptionable rule, or if 
 determinable, is apt to be mistaken and subject to considerable 
 difficulties of execution. The aggregate of small deficiencies 
 originating in this \vay, is, in extensive structures, incalculably 
 subversive of strength. Serai-circular arches, or those of any 
 less portion of a circle, having all their stones of the same 
 figure, and which therefore it is easy to form with correctness, 
 and impossible to misarrange in the work, are, there can 
 hardly be a doubt, more uniformly built strong, than any other 
 
232 
 
 BUILDING. 
 
 Elliptical arches. — Piers. 
 
 kind of arches. Next to circular curves, elliptical ones admit 
 the stones of which they must be composed to be formed in the 
 most certain and satisfactory manner, and though it may be 
 much more difficult in them, than i)i circular arches, to make 
 the surfaces intended to be in contact, press against each other 
 simultaneously and uniformly in every part, under any constant 
 or occasional weight, yet with care it may be done, or at least 
 the imperfection may be so small as to produce little ultimate 
 disadvantage. For other curves, it appears difficult to contrive 
 an easy and unexceptionable mode of forming the stones, or to 
 determine from what point or points, their joints should appear 
 to radiate ; besides the constant risk, from their varied shapes, of 
 putting them in wrong places. But as semi-elliptical arches 
 approximate very nearly to the form of equilibrated arches, 
 as their contour is not only graceful, but, in navigable rivers 
 especially, very convenient, from the elevation of their haunches, 
 as they can be made also to so great a variety of heights with the 
 same span, and combine with these properties the greatest facility 
 of execution, next to the circular form, the opinion of architects 
 generally, is, for extensive structures, decidedly in their favour. 
 In small bridges, a semi-circular arch may be adopted with pro- 
 priety, where the extra quantity of materials required by that form 
 is no object, and when a strong arch of the easiest execution is 
 desired. When a semi-circular arch would be too elevated for the 
 situation, a segment of a circle,' not exceeding sixty degrees, may 
 be employed. — All arches, less than a semi-circle, are called 
 scheme-arches. 
 
 The dimensions of the piers of bridges are commonly deter- 
 mined by those of the arch. Their breadth should not exceed 
 a fourth, nor be less than one-sixth of its span. To break 
 the force of the current, they have mostly angular ends so as to 
 present an edge in front ; though sometimes their ends are semi- 
 circular, in order that such bodies as are impetuously brought 
 down the river, when they strike against them, may be deflected 
 towards the middle of the arch. To prevent the stability of the 
 piers from being endangered, by the washing away of their 
 foundations, occasioned by an increased velocity of the current, 
 the bed of a river must be sunk or hollowed in proportion to 
 the room taken up by the piers, in order that the water may 
 gain in depth what it loses in breadth ; or the current may be 
 broken, by stopping the bottom with rows of planks, stakes, or 
 piles. The celebrated Palladio gives the following proportions 
 of a bridge designed by him : the river M as one hundred and 
 eighty feet wide ; making three arches in the bridge, he gave 
 sixty feet to the centre arch, and to the other two forty-eight foet 
 
BUILDING. 
 
 233 
 
 Semple’s method of laying the foundations of Essex-bridge. 
 
 each. The piers were twelve feet thick, or one-fifth part of the 
 span of the middle arch, and a fourth part of that of the smaller 
 ones. The arches were a little less than a semi-circle, and their 
 keystone one-seventeenth part of the span of the middle arch, 
 and one-fourteenth part of the other two. In an arch of twenty- 
 four feet, Palladio makes the length of the keystone about sixteen 
 inches, which is deemed a very eligible size. 
 
 Autumn is in general the most favourable time to lay the 
 foundations of a bridge, as it is commonly the driest season of 
 the year, and the consequent lowness of the waters, is favourable 
 to the work. The simplest mode of overcoming the inconveni- 
 ence of the water, in laying the foundations of piers, consists in 
 turning the river out of its course, above the place designed for 
 the bridge, into a new channel cut for it near where it makes 
 an elbow or turn. As this plan is, however, seldom expedient, 
 and often not practicable at a reasonable ex pence, the use of 
 the cofter-dam, or enclosure to keep off the w^ater, is much more 
 common. By the coft'er-dam, a part only at a time of the bed 
 of the river is enclosed from the water, which flows in a free 
 current along the unenclosed parts of its bed. An account of 
 the method employed by the ingenious Robert Semple in lay- 
 ing the foundations of Essex-bridge, in Dublin, wdll illustrate 
 this subject. Round the place where the intended pier was to 
 be built, two rows of strong piles were driven, about thirty 
 inches from each other, and which were left at low water-mark. 
 These piles were lined with planks, between w hich was rammed 
 a quantity of clay, and thereby the wall of the coffer-dam 
 w'as formed. Within this wall were driven a row^ of piles, dove- 
 tailed at their edges, so as to receive each other, and which 
 formed the extremities of the plan of the piers at the level of the 
 bed of the river. After having dug to a flue stratum of sand, 
 about four feet lower, within these a great number of other piles 
 W’ere driven as deep as they could possibly be made to penetrate. 
 Tlie intervals of these piles were lilled up, and in order to pro- 
 duce a solid foundation, the first course was laid with mortar 
 made of roach-lime and sharp gravel, and on this large flat 
 stones were rammed to about a foot in thickness. On this 
 first course w^as laid a thick coat of dry lime and gravel of the 
 same quality, on which were again laid stones and the mortar 
 as at first ; and so on alternately, until the pier arrived at a level 
 with the piles. Three beams, stretching the whole length of 
 the pier, from sterling to sterling, w'ere fastened down to the 
 ends of these piles, and their intervals filled up with masonry. 
 On this platform, which was four feet six inches under low-water- 
 iiiark, was laid the first course of stones for the pier, cramped 
 10.— VoL. L H 
 
234 
 
 BUILDING. 
 
 Marnier of constructing and using caissons. 
 
 together, and jointed ^vith terras mortar as usual ; courses of 
 stones were laid in tliis maimer, until the piers were on a level 
 with the water at ebb tide. 
 
 The caisson is a contrivance of still more extended utility 
 than the coffer-dam, from being better suited to very deep and 
 rapid streams. The most considerable work where caissons 
 have been employed, was in the building of Westminster-bridge, 
 of which, therefore, a particular account may be interesting. 
 Each of the caissons contained one hundred and fifty loads of 
 fir timber, and was of greater tonnage than a frigate of forty 
 guns ; their size was nearly eighty feet from point to point, 
 and thirty feet in breadth ; the sides, which were ten feet in 
 height, were formed of timbers laid horizontally over one ano- 
 ther, pinned with oak trunnels, and framed together at all the 
 coiners, except the salient angles, where they were secured by 
 proper iron-work, which being unscrewed, would permit the 
 sides of the caisson, had it been found necessary, to divide 
 into two parts. These sides were planked across the timbers^ 
 inside and outside, with three-inch planks in a vertical position. 
 The thickness of the sides was eighteen inches at the bottom, and 
 fifteen inches at the top ; and in order to strengthen them more, 
 every angle, except the two points, had three oaken knee tim- 
 bers properly bolted and secured. These sides, when ffnished, 
 were fastened to the bottom or grating, by twenty-eight pieces 
 of timber on the outside, and eighteen within, called straps, 
 about eight inches broad and three inches thick, reaching and 
 lapping over the tops of the sides. The lower part of these 
 straps were dove-tailed to the outer curb of the grating, and 
 kept in their places by iron wedges. The purpose to be answer- 
 ed by these straps and wedges was, that when the pier w^as built 
 up sufficiently high above lovv-water-mark, to render the caisson 
 no longer necessary for the masons to work in, the wedges being 
 drawn up, gave liberty to clear the straps from the mortices, in 
 consequence of which the sides rose by their own buoyancy, leav- 
 ing the grating under the foundation of the pier. The pressure 
 of the water upon the sides of the caisson, w’as resisted by means 
 of a ground timber or ribbon, fourteen inches wide and seven 
 inches thick, pinned upon the upper row of timbers of the grating; 
 and the top of the sides was secured by a sufficient number of 
 beams laid across, which also served to support a floor on which 
 the labourers stood to hoist the stones out of the lighters, and to 
 lower them into the caisson. 
 
 The caisson was provided with a sluice to admit the water. 
 T.lie method of working w-as as follows : a pit being dug and 
 levelled in the proper situation for the pier, of the same shape as 
 
BUILDING. 
 
 235 
 
 Stone caissons suggested. 
 
 caisson, and about five feet wider all round, the caisson was 
 brought to its position, a few of the lower courses of the pier 
 ; were built in it, and it was sunk once or twice to prove the level 
 kof the foundation; then, being finally fixed, the masons worked 
 !(i in the usual method of tide-work. About two hours before low 
 water, the sluice of the caisson, kept open till then, lest the 
 !l water, flowing to the height of many more feet on the outside 
 than the inside, should float the caisson and all the stone-work 
 ,,, out of its true place, was shut down, and the water pumped 
 , low enough, without w'aiting for the lowest ebb of the tide, for 
 t) the masons to set and ciamp the stone-work of the succeeding 
 )i] courses. '^1 hen, when the tide had risen to a considerable 
 1*1 height, the sluice was opened a^ain, and the water admitted ; and 
 fii as the caisson was purposely built hut ten feet high to save useless 
 ]:i expence, the high tides flowed some feet above the sides, but 
 il without any damage or inconvenience to the works. In this man- 
 jj ner the work proceeded, till the pier rose to the surface of the 
 II caisson, when the sides were floated away, to serve the same pur- 
 !i pose at another pier. 
 
 I To change altogether, for a time, the course of a river, by 
 I providing it with a new channel ; or to divei t a part of the 
 P current at once by the erection of a cofl'er-dam, may be classed 
 il among the simple and obvious expedients, to obtain the means 
 ,f of laying the foundations of a bridge, which would suggest 
 
 f themselves in the infancy of aquatic architecture. Improve- 
 
 ! inent seems to have gone no further for a long time. In early 
 periods, if the first means that suggest themselves to accomplish 
 the end in view, possess the two grand requisites of possibility 
 ] and efficiency, the march of invention is impeded by a willing 
 ; prodigality of labour. The caisson, an invention of greater re- 
 j linement, and bolder aspect, probably originated in a much 
 
 j later perioct; but when the success which attended the use of 
 ! it once became familiar to architects, the transition seems natural 
 i and easy, fiom a caisson of wood, which is floated away when 
 I no longer wanted, to a caisson of stone, which should perma- 
 ' nently remain the external part of the pier. It would soon 
 
 appear to the mind of him who was first struck with this happy 
 
 idea, that by the common well-known means, the stones might 
 be cemented so perfectly as to be impervious to w ater ; that 
 they might be cramped so firmly together, likewise, that no force 
 to which they need be exposed would injure the work ; and 
 that as the weight of the heaviest stones employed in building 
 is only about two and a half times greater than that of water, 
 the proportion which must subsist between the solid and the 
 hollow part of any mass of masonry intended to float, could 
 
236 
 
 BUILDING. 
 
 General Bentham’s patent. — Hawkins’s claims and inventions. 
 
 not only always be determined with facility, but was so small 
 as not to occasion a fear of success. Indeed, so great appears 
 to be the practicability of the plan, that the praise of ingenuity 
 exclusively belongs, not to him who shall carry it into execu- 
 tion, but to him by whom it was first suggested. We shall 
 proceed concisely to notice a few' particulars respecting it. 
 General Bentham took out a patent, dated April 2, 1811, for 
 a new and economical method of laying the foundations for 
 bridges, wharfs, &c. He proposes the construction of hollow 
 masses of masonry, brickwork, &c. which he would afterwards 
 float to and sink at the place desired. He observes that these 
 masses, if filled with casks, might be floated without having them- 
 selves any bottom ; and directs a calculation to be made of the 
 weight which any of them will have to bear when employed as 
 piers, or for any other purpose, so that vessels properly loaded 
 may be grounded upon them, and by that means sink them, w hen the ^ 
 tide retires, as low as they would otherwise have been ultimately 
 sunk by the w^eight they are to sustain, and thus prevent their 
 sinking after the structure is finished. J. 1. Hawkins, in a letter 
 to the Editors of the Repertory of Arts, &c. states, that, several 
 years previous to the date of the above specification, he had men- 
 tioned the leading feature of the General’s plan, and also of that 
 with respect to the suspension of centering, proposed by I'elford, 
 in liis repoi t to the House of Commons, for throwing a bridge 
 over the straits of Menai, to several of his friends, who highly 
 approved of them. He attributes not disingenuity to these gen- 
 tlemen, in borrowing his plans, being aware of the similarity of 
 ideas which may occur to different persons, independent of each 
 other, when engaged with the same subject; but lays claim to 
 priority of invention, and developes to the public the particulars 
 of a plan which he had long before circulated among his friends. 
 He would build his piers on shore, in some situation where they 
 might be launched like a ship ; he w ould cramp the outside 
 stones strongly with iron, and would make the walls of such a 
 thickness that they might float in water. He would have a valve 
 in the wall, to admit water whenever it might be proper to 
 sink the pier, and a pipe fixed through the top, communicating 
 with the lower part of the inside, on which pipe a pump must 
 be fixed, for drawing the water out, should that measure be ne- 
 cessary. 
 
 '1 o form a good foundation, a space in the bed of the river, 
 rather larger than the base of the pier, must be excavated and le- 
 velled by the means used in taking up ballast, and the spot must 
 be piled, if necessary, as on other occasions of the kind. He 
 details the means of making a platform over the place intended 
 
BUILDING. 
 
 23f 
 
 Hawkius’s experiments to float brickwork, 
 
 for erecting the pier, which, when sunk, must have bricks or stonei 
 let down into it equal at least in weight to the water required to 
 sink it. The watei must then be pumped out, when the workmen 
 may descend, to lay the stones or bricks in mortar, and fill up the 
 whole interior : thus he obtains a solid pier. 
 
 In situations where, from the state of navigation, or any 
 other cause, it may be inconvenient to erect centering in the cus- 
 tomary way, he would not hesitate to adopt suspending chains. 
 However bold the idea may appear, it will not be deemed im- 
 practicable, when it is considered that the weight of an arch of 
 any giv'cii dimensions is easily calculated, and the suspending 
 power of iron is known ; consequently no arch can be so heavy, 
 but that a sufficient number of chains may be provided to bear 
 more than the weight. The chains may be advantageously com- 
 posed of long bars of iron, merely turned up at the ends, so that 
 w hen done with, they may have these ends cut off, and be sold as 
 bar iron again. 
 
 Thai the piers of bridges may be built hollow^, and rendered 
 perfectly manageable in or under water, at a considerable depth, 
 2s put beyond a doubt by an experiment which J. 1. Hawkins 
 conducted for the 'i’hames Archway Company. l\vo hollow 
 cylinders of brickwork, upwards of eleven feet diameter, and 
 twenty -five long each, were sunk through thirty feet of water in 
 the river Thames, and bedded precisely at the spot proposed. 
 These cylinders were built in a barge, in October and ^^ovem- 
 ber 1810, and launched into the water, where they remained 
 floating all winter, and were sunk in the river in the Spring 
 of 1811. They were under such perfect command, that from 
 a stage erected on the bed of the river, and supplied with 
 suitable windlasses, pullies, ropes, &c. they were lowered, 
 raised, or moved, in any lateral direction without difficulty. 
 These experiments left not a doubt, that masses of masonry, of 
 large dimensions, might be fixed through the water, in the bottom 
 of a river or harbour, to the depth, if requisite, of one hun- 
 dred or more feet. They were instituted for the purpose of 
 ascertaining the practicability of forming a tunnel under the 
 Thames, upon a cheaper and more certain plan than undermining 
 it, as was first projected. I he result proves, that by this method, 
 communications may cheaply and to a certainty be formed be- 
 tween the opposite shores of rivers, in almost all situations where 
 the navigation, or any other circumstance, renders bridges inad- 
 missible. 
 
 The inventions recorded in the preceding details, promise 
 the dawn of a new era in the science of aquatic architecture. 
 They extend incommensurably the limits of our views, and will 
 
238 
 
 BUILDING. 
 
 Breadth of bridges. — Hollow piers and abutments, See. 
 
 lead to undertakings of the boldest and most novel character. 
 Telford, in his report to the House of Commons, on throwing 
 an iron bridge of one arch over the straits of Menai, to connect 
 the Isle of Anglesea with the county of Caernarvon, proposes to 
 suspend the centering, although the span of the required arch will 
 be six hundred feet. 
 
 The common breadth of bridges is about thirty feet ; but 
 when they form the avenues to large towns, the carriage road is 
 nearly this breadth, and they have besides a raised foot-path 
 from six to nine feet broad on each side. The bre^adth of 
 Westminster bridge over the 1 hames is forty-four feet, and the 
 breadth of Blackfriar’s bridge, over the same river, forty-two 
 feet. In large structures, the parapets are about eighteen feet 
 in thickness ; they often project from the bridge with a cornice 
 underneath. The parapets are parallel for the greater part of 
 their length, in the middle ; but at each end they diverge, in 
 order that the increased width of the road thus obtained may 
 render the entrance upon the bridge more free, and unite better 
 with streets, which are commonly wider than bridges. These 
 wide parts of the ends are generally supported by the solid abut- 
 ments alone. 
 
 Piers and abutments are not always built solid throughout. 
 By the judicious use of arches or vaults, the structure may be 
 considerably lightened, without having its strength or durability 
 impaired. The arch-work or hollows of the piers are generally 
 made a little above the spring of the arch. When the abutments 
 are vaulted, care must be taken to do it in such a manner that 
 they will bear the push of the arch as completely as before. The 
 sides of the foundations of abutments should be concave or dished, 
 as this form will conduce the more effectually to their resisting 
 the pressure upon them. 
 
 With regard to the superstructure of a bridge, it may be 
 observed, that when the first course of stones has been disposed 
 upon the centering, or wooden frame-work, the remaining courses 
 may be laid wdth their bedding joints horizontal, in w'hich case 
 they will press upon the first course as a dead weight, without 
 conti'ibuting, except by their pressure, to the strength of the arch ; 
 or the subsequent courses may be laid in the same manner as the 
 first course, so as to be like a series of arches built immediately 
 upon one another. The latter is the mode most consonant to the 
 laws of strength. 
 
 As the principal objects of this work are of a practical nature, 
 we confine ourselves to few and transient sketches of historical 
 research ; yet in this place it wdll be interesting to many, and 
 afford matter for deductions which will supersede the dryness of 
 
BUILDING.' 
 
 239 
 
 An account of the most remarkable stone bridges. 
 
 further precept, if we notice some of the most remarkable stone 
 bridges which have been erected at different periods. 
 
 Of all the bridges of antiquity, that built by the Roman 
 emperor Trajan over the Danube, is allowed to have been the 
 most magnificent. It was demolished by his immediate suc- 
 cessor Adrian, and the ruins are still to be seen in the middle of 
 the Danube, near the city of Warhel, in Hungary. It had 
 twenty piers of stone, and each pier was one hundred and fifty 
 feet high above the foundation, sixty feet in breadth, and one 
 hundred and seventy feet distant from one another, which was 
 the span or width of the arches, so that the whole length of the 
 bridge, exceeding fifteen hundred and ninety yards, was nearly 
 one mile. 
 
 In France, the Pont de Garde is a very bold structure ; the 
 piers being only thirteen feet thick, yet serving to support the im- 
 mense weight of a triplicate arcade, and joining two mountains. 
 It consists of three bridges, one over another; the uppermost of 
 which is an aqueduct. 
 
 The bridge of Avignon, which was finished in the year 
 1188, consists of eighteen arches, and measures thirteen 
 hundred and forty paces, or about one thousand yards in 
 length. 
 
 The famous bridge at Venice, called the Rialto, has been 
 usually regarded as a master-piece of art. It consists of only 
 one very fiat, bold arch, nearly one hundred feet span, and 
 only twenty-three feet high above the water. It was built 
 in 159 !• Yet such structures sink to insignificance when 
 compared with a bridge in China, built from one mountain to 
 another, consisting of a single arch, six hundred feet long, and 
 seven hundred and fifty feet high ; whence it is called the flying 
 , bridge. 
 
 London bridge was built about the commencement of the 
 thirteenth century. It consisted at first of twenty arches, each 
 of them only twenty feet wide ; but the two middlemost were 
 thrown into one in 1758, and another next one side is concealed 
 or covered up. It is nine hundred feet long, sixty feet high, 
 and seventy-four feet wide. The piers are from twenty-five to 
 thirty-four feet broad, with sterlings projecting at the ends ; so 
 that the great water-way, when the tide was above the sterlings, 
 was four hundred and fifty feet, scarcely half the breadth of 
 the river and below the sterlings, the water- w^ay was reduced 
 to one hundred and ninety-four feet, before the opening of the 
 centre. At the time the two middle arches were thrown into 
 one, the houses which had been erected upon the bridge were 
 removed, and various} other alterations and repairs undertaken 
 
240 
 
 BUILDING. 
 
 An account of the most remarkable stone bridges. 
 
 at an expence of c£ 80 , 000 . But so large a sum is still annually 
 required to keep it in repair, that tlie preservation of it, instead of 
 erecting a new one, may be considered an act of national impro- 
 vidence. In consequence of the water-way being so much con- 
 tracted, the tide, at its ebb and flow, rushes through the arches 
 with such prodigious violence, as to occasion a cataract exceeding 
 the height of four feet, and which prevents or renders highly dan- 
 gerous the passage of boats. llie fall of the water passing 
 through Westminster bridge is only one inch. 
 
 The longest bridge in England, is that over the Trent at Bur- 
 ton, built in the twelfth century, of squared free-stone. It is ra- 
 ther low, but strong, and contains thirty-four arches. Its whole 
 lengtli is fifteen hundred and forty-five feet. 
 
 One of the most singular bridges in Europe, is that built in 
 the year 1756, over the Taaf, in Glamorganshire, by William 
 Edw ard, a poor country mason. It consists of only one stupen- 
 dous arch, which though only eight feet broad, and thirty-five feet 
 high, is no less than one hundred and forty feet span, being part 
 of a circle of one hundred and seventy-five feet in diameter. It 
 is the last of three bridges erected on the same spot by tlie same 
 person, and it deserves to be mentioned, were it only to record the 
 amazing perseverance of the builder, under very discouraging cir- 
 cumstances. The first bridge he erected consisted of two arches, 
 and he was bound to uphold it for seven years. It was swept 
 away before the expiration of the term, by a flood, that brought 
 dow n a prodigious mass of floating materials, which obstructed 
 the progress of the torrent through the arches. Edward, whose 
 perseverance increased, and whose genius expanded, under his 
 misfortunes, determined to prevent the recurrence of a similar 
 event, from the like cause, by making only one arch. The first 
 bridge attempted upon this plan, sprung at the crown, and fell be- 
 fore it w as finished, from the great weight of the ends. This se- 
 vere blow again called forth the resources of his ingenuity ; he 
 disdained to give up the project of having one arch only, but 
 guarded against his late accident, by making the ends of the bridge 
 hollow, and thus he succeeded. 
 
 Of modern bridges, those of Westminster and Blackfriars, 
 over the Thames, at London, are esteemed the tw'O finest in 
 Europe. The former is upwards of four hundred yards long. 
 It consists of thirteen large and two small arches, all semi- 
 circular, with fourteen intermediate piers. The arches all spring 
 from about two feet above low-water mark. The middle arch is 
 seventy-six feet wdde, and the others on each side decrease always 
 by four feet at a time. The two middke piers are each seventeen 
 feet thick at the springing of the arches ; and the others decrease 
 
BUILDING. 
 
 241 
 
 Blackfriar’s bridge. — Situation of houses. 
 
 equally on each side by one foot at a time ; every pier terminating 
 with a salient right angle against either stream. This bridge is 
 built of the best materials, and in a neat and elegant taste, but the 
 Inarches are too small for the quantity of masonry it contains. It 
 was begun in the year 1738, and opened in 1730. It was exe- 
 cuted at an expence of c£389j300. 
 
 Blackfriar’s bridge, which is nearly opposite the centre of 
 the city of London, was begun in 1760, and completed 
 within eleven years from that time. It is an exceedingly light 
 and elegant structure ; but the materials do not seem to have 
 been well selected, as some of the arch-stones are already decay- 
 ing. It consists of nine elliptical arohes. The centre arch is one 
 hundred feet wide, and those on each side decrease in regular 
 gradation, to the smallest, at each extremity, which are seventy 
 feet w'ide. Its length, from wharf to wharf, is about nine 
 hundred and ninety-five feet. The extrados is a portion of a 
 very large circle, of convenient passage over it. It cost 
 (o£’130,840. This amount being so much less than the cost of 
 I Westminster bridge, even after making an ample allowance for 
 the greater length of that bridge, is a proof of the advantage of 
 few and elliptical arches orer iiuiuy and semi-circular onc5. 
 
 Miscellaneous Remarks relative to Building. 
 
 Under the present title, w^e shall include a variety of parti- 
 culars, either not belonging, or not exclusively belonging, to 
 Masonry or Bricklaying. Under the first head of this subdivision 
 of the chapter on Building, the earliest notice seems due to a 
 sketch of the general rules proper to be observed with respect 
 to the 
 
 Situation and Plan of Houses. 
 
 With regard to situation, when the person wdio is intending 
 to build enjoys the advantage of choice, the proximity of 
 marshes, fens, of a boggy soil, or stagnant water, should be 
 avoided. If a river be very near, the site of the house should 
 be on elevated ground, so as to be out of the reach of the 
 unwholesome fogs and mists arising from it early in the morn- 
 ing. A neighbourhood w'here cattle thrive, and where the 
 inhabitants are healthy and cheerful, or are remarkable for their 
 longevity, may be regarded as possessing a salubrious air. The 
 most essential requisites of a good situation, or those which are 
 most conducive to health, being obtained, minor advantages 
 may be regarded according to their importance. Abundance 
 of water, fuel, and ways of easy access to arrive at the house, 
 11. — Vol. I. 2 I 
 
242 
 
 BUILDING. 
 
 Situations and plans of houses. 
 
 are conveniences of lasting value. The advantages of a situa- 
 tion, with regard to prospect, are of a more problematical 
 nature, as they are so differently valued by different persons, 
 
 A man of taste, will, however, undoubtedly prefer a spot the 
 prospect from which is most agreeably diversified in the distribu- 
 tion of its land, wood, and water ; and those who have little or 
 no relish for the charms of nature, will, perhaps consult their own 
 comfort more than they may be aware of, by making the same 
 choice. There are very few so obstinately morose, as to be unin- 
 fluenced by the opinions of others ; and to observe those about 
 them, particularly visitors, warm in their admiration of the sur- 
 rounding scenery, may create a beneficial complacency which they 
 would otherwise want. 
 
 Trees at a little distance from a house are better than hills, 
 as they yield during the day, in summer, a cooling, refreshing, 
 sw eet, healthy air ; and, in winter, break and diminish the keenness .g 
 of the winds. Hills on the east and south side are the most in^ |l 
 conveniently situated. If the site of a house be low, the first floor 
 should be set so much the higher. Cellars contribute to the dry- 
 ness of a house, if the ceilings and floors be good. 
 
 With respect to the situation of the parts of a house, the 
 studies, libraries, and chief rooms, particularly the bed-cham- 
 bers, should face the east ; those offices w'hich require heat, as 
 kitchens, brewhouses, bakehouses and distilleries, should have 
 southern aspects ; and those which require a cool fresh air, as cel- 
 lars, pantries, dairies, granaries, a northern one, which is also 
 proper for galleries, paintings, museums, &c, which require a 
 steady light. 
 
 When a situation has been fixed on, the plan and elevation 
 of every part should be made by some person well acquainted 
 with the theory and practice of building. A skilful architect 
 will not only make the structure handsome and convenient, but 
 will save the great expences often incurred in rectifying the 
 blunders of hasty and injudicious management. For a small 
 building the elevation of each front, with a plan, may indicate 
 wdth sufficient correctness its ultimate advantages ; but for a 
 large mansion, or structure of any description consisting of 
 many complex parts, the most certain way to prevent mistakes, 
 is to have a perfect model of the whole made to a regular scale. 
 
 In order that such a model may not mislead the judgment by , 
 pleasing the eye, it should be made of plain w'ood, of one 
 colour. 
 
 Lodges and small houses, standing alone, have an agreeable 
 appearance when of a cubical figure ; but large mansions of 
 this shape look rather clumsy ; an oblong is therefore conuuonly 
 
BUILDING. 
 
 243 
 
 Forms of rooms. 
 
 preferred, care being taken not to make the plan so long as to 
 lose much room in the passages which will be required, and 
 which will be difficult to light. When the length of a mansion 
 does not exceed the breadth more than one-third, the proportion 
 P good. 
 
 y. Of Rooms, 
 
 “ ' The principal objects to be regarded in the arrangement and 
 proportions of rooms, are doubtless those of conveniency, and 
 their adaptation to health. Rooms, the plan of which are 
 rectangular, give the greatest facility to convenience of arrange- 
 ment, without the disadvantage of losing the space rendered 
 unavoidable by adopting circular or other curved forms ; but 
 with regard to health, the height of a room should at least be 
 ten or twelve feet, and this point should be regarded even in 
 apartments too small to render such an elevation proper in an 
 architectural point of view. A square is an agreeable form, 
 but is most proper for rooms not exceeding a moderate size, as 
 it cannot well, if very large, be completely lighted from one 
 wall, and the company, while ranged on each side, are too far 
 apart. For spacious apartments, a rectangular parallelogram or 
 oblong is a more convenient figure, and, with regard to beauty, 
 every variation in the proportion, from nearly a square to a 
 square and a half or sesquialteral, may be employed. If the 
 length of the plan be extended materially beyond a sesquialteral, 
 the apartment obtains rather the appearance of a passage or gal- 
 lery, and it becomes impossible to adjust the height so as to suit 
 both the length and breadth. In square rooms of the first story, 
 the height may be from four-fifths to five-sixths of the breadth of 
 the side ; and in oblong rooms, the height may be equal to the 
 width. An error in favour of height, is preferable to making a 
 room too low\ 
 
 The height of rooms on the second story may be one-twelfth 
 part less than that of the chambers below : and if there is a third 
 story, divide the height of the second into twelve equal parts, of 
 which take nine for the height of such rooms. 
 
 An eligible length for galleries is five times their breadth, and 
 they should rarely exceed eight times their width in length ; their 
 height may exceed their breadth in the proportion of a third or 
 even three-fifths, according to their length. 
 
 As, however, in modern houses, all the rooms of the same 
 story are commonly of the same height, and convenience 
 requires them to be of different sizes, according to their use, it 
 follows that the best proportion of the height to the other 
 dimensions, cannot always be observed, without incurring some 
 
244 
 
 BUILDING. 
 
 Arrangements for the comfort of rooms. 
 
 extraordinary expence. Where expence is not a hinderance, 
 the height of the story may be suited to the principal rooms, 
 and the middle-sized rooms may be reduced by coving the 
 ceilings, ^^ith a flat in the middle, or by groins or domes, which 
 will add to their beauty, independently of bettering their pro- 
 portions. 
 
 Precautions should be taken to prevent the effluvia from the 
 kitchen, brewhouse, and other offices, from penetrating to the 
 bed-chambers and dining-rooms. The most difficult object to 
 attain of this description, is to prevent the effluvia of the 
 kitchen from annoying the dining-room, to which the access 
 from it should be as easy and short as circumstances will allow, 
 for the convenience of the servants waiting at table. In country 
 mansions, which admit of the greatest liberty of plan, and 
 where the kitchens are above ground, this may generally be 
 done, by such an arrangement of the doors and passages of com- 
 munication, that no current of air from the kitchen can proceed 
 directly towards the dining-rooms. But in town houses, where 
 the kitchen is beneath the parlour floor, and therefore not only 
 far nearer in point of situation (though not perhaps of access 
 for persons) than it is usually placed in the country, but on a 
 lower level, the lighter warm air, charged with the smell of 
 the various operations of cookery, is apt to be felt above, 
 from its disposition to ascend. It may, however, be effectually 
 removed by a small separate funnel, carried up in the stack with 
 the rest, and next to that of the kitchen. This funnel, to be 
 used for no other purpose, must have its throat or lower open- 
 ing level with the ceiling of the kitchen. The lighter air, charged 
 with the vapours of the cooking, wdll then pass off into the 
 external atmosphere by this aperture, instead of accumulating 
 under the ceiling of the kitchen, until it forms a stratum as low 
 as the top of the kitchen door, and then ascending through the 
 house by the stairs and passages. The opening of this J funnel 
 or pipe may be closed by a hinged door, when no operation is 
 going on in the kitchen which can create a disagreeable 
 smell. 
 
 Every chamber in a house should, if possible, have a fire- 
 place, the place of which, in those employed' as bed-rooms, if 
 they are not very spacious, should be about two feet or two feet 
 and a half out of the middle, to allow room for the bed. In 
 apartments of twenty or twenty-four feet a side, this arrangement 
 need not be studied, as the bed can without it be placed sufficiently 
 far from the fire. 
 
BUILDING. 
 
 245 
 
 Best form for chimneys. — Definitions. 
 
 I Chimneys. 
 
 That no apartment can be comfortable whkh is incommodea 
 with smoke, will not be contradicted ; yet we find a very general 
 disregard of the precautions which would secure a strong draft 
 up the chimney. Masons and bricklayers follow their own 
 fancy or judgment, which are often influenced by their conve- 
 nience, or by local customs, with little regard to rational prin- 
 ciple. It frequently happens that the smoking of chimneys is 
 occasioned by their being carried up narroNver at the top than 
 below^, or by their having one or more sharp angular turns. 
 AVhen chimneys are constructed in a pyramidal or tapering 
 form, and are besides left rough, or unplastered, with stones 
 or bricks projecting into them, as well as the mortar pressed 
 from the joints in the Wall, their smoking is almost certain. The 
 air, rarefied by the fire, passes up a chimney with the smoke ; 
 but as it recedes from the impelling power, or fire, it moves 
 slower, and requires a greater portion of space to circulate 
 through ; if, then, the upper part of a chimney, instead of 
 being wider than below, be contracted, and if the roughness of 
 the walls concur at the same time to increase the obstruction, it is 
 no wonder that the smoke, taking the path of least resistance, 
 should find its way into the room, whenever assisted by a cur- 
 rent from above. It may be urged, that a chimney wider at the 
 top than below, allows the wind more liberty to blow down ; but 
 it must be considered, that the w'ind having no adequate resistance 
 from above to overcome, must necessarily return, and thus facili- 
 tate the free egress of the smoke. On the other hand, when a 
 current of air rushes down a pyramidal chimney, it becomes 
 confined or w'edged in, and if urged by a constant wind, the rare- 
 fied air from the fire cannot make head against it, and therefore 
 the smoke bursts into the room. 
 
 Experiments, in endless variety, and often at great expence, 
 have been made to prevent or cure smoky chimneys : we shall 
 notice some of the most simple and efficient ; but it will be 
 proper to explain, in the first place, some of the terms which 
 are used in speaking of chimneys. The aperture from a chim- 
 ney into the room, is called the Jire-ptace. The projecting 
 parts of the wall, on the right and left of the fire-place, are 
 called jambs. The head of the fire-place, resting upon the 
 jambs, is called the mantle. The mantle, and the whole side 
 of the chimney above it up to the top, are called the breast. 
 The side of the chimney, called the breast, being pointed out, 
 the application of the term back, to the opposite side, explains 
 Itself The sides of the fire-place contained between the jambs 
 
246 
 
 BUILDING. 
 
 Precautions to prevent smoky chimneys. 
 
 and the back, are called covings. The throat is that part of the 
 opening immediately above the fire, and contained between the 
 mantle and the back. 
 
 When chimneys are bounded on the top by a zigzag line, so 
 as to resemble the teeth of a saw, they are found to be far less 
 liable to smoke than those in other respects under the same cir- 
 cumstances ; and in a great variety of cases, the cheap and easy 
 expedient of altering the tops of smoky chimneys to this form, has 
 been attended with complete success. The partitions in a stack 
 should be indented as well as the outside wall. 
 
 The fire-place is generally an exact square. Its height, in 
 large rooms, is often very properly made less than a square, 
 and in small rooms, particularly when the chimney is in a 
 corner, it is rather more. In rooms from twenty to twenty- 
 four feet square, or of equal area, it may be from four feet t^| 
 four feet and a half broad; in rooms from twenty-four t^B 
 thirty feet square, it may be from four and a half to five feet;W 
 and in rooms still larger, it may be extended in a similar pro- v 
 portion to six feet. If much beyond six feet, whatever may^ 
 be the size of the room, the effect will not be good ; for if the fire 
 • be proportionate, it will excite rather the idea of a furnace. Two ! 
 fire-places will certainly be better than one of such overgrown j 
 dimensions. As to the effect of the form of this aperture on i 
 the draft, its breadth is not very important, provided it be not r 
 so narrow as to prevent the covings from standing with their 
 greatest power of reflection towards the room ; but the height 
 should seldom exceed two feet six inches to the under side of the 
 mantle. The throat should not be more than four or five inches 
 wide ; but should be contracted by a part moveable at the back, 
 W'hen the chimney requires sweeping. The nearer the throat 
 is brought to the fire, the stronger the draft will be. For fire- 
 places about three and a half feet wide in front, the flues of 
 chimneys, above the throat, are usually made equal to about 
 twelve inches square ; and the general rule is, to make the area 
 of the horizontal section of the flue, equal to the area of the 
 horizontal section of the fire. If the flue w ere smooth and cir- 
 cular, this mode of proportioning its size, would doubtless be 
 found to allow it unnecessarily large. Where bends are required 
 in a flue, they should be in a curved, and not an angular direc- 
 tion. High chimneys always draw better than low ones, as, in 
 proportion to their, length, the influence of the wind extends a ^ 
 shorter way down them. An apartment mad^wind- tight, by 
 listing the doors and other contrivances, is liable to be incom- 
 moded with smoke, from the want of draft, even when the 
 chimney is constructed in the most proper manner : a few 
 
building; 
 
 ^47 
 
 Precaution to prevent a smoky chimney. — Smart’s sweeping apparatus. 
 
 ninute holes, communicating with the exterior, will, in such cases, 
 :oustitute an effectual remedy. 
 
 Another method of increasing the draft of a chimney, con- 
 dsts in setting the grate, if a Bath stove, eleven or twelve inches 
 fom the fender ; and in cutting away the back of the chimney, 
 50 as to leave a space of two inches between it and the back of 
 :;he grate. If the grate be of the common form, the sides should 
 be filled up with brickwork. By this construction the air that 
 passes behind the back of the grate, assisting to impel the smoke, 
 prevents its bursting into the room, — That a chimney clogged 
 with soot will be apt to smoke, is so well known, as to require no 
 I notice here. 
 
 The evils attending the practice of cleaning chimneys by 
 means of climbing boys, are of the first magnitude. The 
 'degraded situation of the children in this employment, so de- 
 structive to health at any period of life, but especially in early 
 youth, has often and strongly excited the commiseration of 
 philanthropical men, and many schemes have been proposed to 
 fender their ascending unnecessary, and thus abolish a practice 
 so offensive to humanity. Of these plans, the most feasible 
 consists in the use of brushes, which are drawn up and down 
 by men at the top and others at the bottom, or are pushed up 
 from the bottom, and drawn down by persons within the 
 apartment. The former method is inconvenient and expensive ; 
 the latter, which is the invention of Smart, is often practised, 
 and has justly received much approbation. The rods by 
 which the brush is pushed upwards, are made hollow and in 
 short lengths, and are connected together by a cord passing 
 through them. As soon as the lower end of the first rod is 
 pushed about the height of the mantle, another rod is 
 slipped over the cord, and the lengthening of the whole is 
 thus continued, till the brush clears the top of the chimney, 
 where it spreads itself out, and is prevented from contracting 
 by a spring ; so that the soot is brought down along with it. 
 But as long as the practice is continued, of making chimneys 
 square, crooked, and rough, it is to be feared that no sweeping 
 apparatus can be contrived which will be found completely 
 successful. Inventions recommended merely by their huma- 
 mty, are often so slow' in their progress towards general 
 adoption, that an eminent service will be rendered to society, 
 by the man who makes the convenience and interest of indivi- 
 duals conspire with their benevolence, to promote the purpose 
 before us. The Author of the treatise on Architecture in this 
 work, has furnished us with the following remarks on the 
 present subject, which w e have pleasure in laying before the 
 
248 
 
 BUILDING. 
 
 New applications of iron. — Use of iron for chimneys proposed. 
 
 public. He commences 'with a few observations on the late ra- 
 pidly extended use of iron : 
 
 Amongst the various improvements of modern times, per- 
 haps few have been more really beneficial than the use of metals, 
 for purposes to which, only half a century back, they were 
 hardly thought applicable. It is perhaps not generally known, 
 that the great variety of articles of fittings now constantly kept 
 in ironmongers! shops, in the remotest villages, were hardly 
 know n in country places at the beginning of the last century ; but 
 wherever a house was built, its bolts and latches w^ere mostly 
 of w ood ; its window s, if sashes, w^ere massive, and if one sash 
 run, the sash-cord ran over a w’ooden pulley ; and many other 
 minute fittings were either of wood, or, if of metal, were 
 individually made by the adjacent smith, who produced a 
 clumsy article at a great waste of time and labour. We now 
 see the use of iron and brass extensively superseding timber and 
 stone in the most advantageous manner. At Eaton Hall, the 
 magnificent mansion of Lord Grosvenor, near Chester, the 
 tracery of the window's, many shields, a stair-case, and a 
 variety of ornaments, are of cast iron. A large portion of 
 tracery pannelling, for a terrace balustrade, are of the same 
 material, and being painted and sanded, have all the beauty of 
 stone. Hollow Grecian balustrades, for a public building in 
 Ireland, have been made of iron, in Liverpool. Two patents 
 have been taken out for roof framings of iron, one of which, at 
 least that of J. Cragg, Liverpool, is particularly remarkable 
 for its simplicity, and the beauty of its execution. This 
 gentleman also executes stair-cases of iron, in so accurate a 
 manner that they may be put up at any place in a few' hours. 
 Cast iron bridges have long been deservedly celebrated, and the 
 dearness of wood, and the supposed inefficiency of the patent 
 stone pipe, have caused cast iron w’ater- pipes to be used in 
 London, and many other places. Cast iron has also been 
 advantageously used for beams and their supports in a large 
 way, the Trustees of the Duke of Bridgewater’s canal having 
 used it for beams in a new' warehouse at Liverpool, of more thau 
 thirty feet clear span ; and the Carron Company have used some 
 excellently arranged hollow iron stanchions in a new warehouse 
 at Liverpool. 
 
 There is yet another purpose, for w’hich iron seems so well 
 adapted, that it deseiwes at least a fair trial ; it is that of cast 
 pipe for chimneys. The present mode of building chimneys is 
 not only extremely unsafe, but also takes up a great deal of 
 room ; and from their being square, there is a great difficulty in 
 cleaning them thoroughly, except by climbing boys, or, as ii 
 
BUILDING. 
 
 249 
 
 Advantages of iron-pipe for ordinary chimneys. 
 
 frequently practised, by purposely firing them. Smart’s appa- 
 ratus does not completely answer, because the corners which 
 contain most soot do not get swept completely, and are often 
 not touched. Another objection to ordinary brick square chim- 
 neys is their smoking, or, in other words, not drawing ; an evil 
 not considered, by those who suffer from it, of the most trivial 
 magnitude. All these inconveniences might be remedied by 
 using for chimneys cast iron pipes three eighths of an inch thick, 
 and when once the utility of the plan became known, the article 
 would be as common in iron shops as fronts of grates. The 
 present openings into the room are built far loo large, and are 
 now generally contracted by various means when the grate is set, 
 but while the present arrangement of biick chimneys for climbing 
 boys remains, they cannot well be much altered ; but if iron pipe 
 were used, an iron fire*place top would be cast for the pipe to fit 
 into, and would of course be made of various sizes and shapes like 
 grate fronts, and iron articles in general, cast by the Canon Com- 
 pany and other founderies. 
 
 More clearly to exemplify the advantages of this invention, 
 it will be proper to detail the arrangements necessary. Preju- 
 dice will at first perhaps require the pipe to be eight inches in 
 diameter, though there is little doubt that seven, six, or even 
 five inches diameter, would be amply sufficient for the largest fire. 
 However this may be, it should be cast in lengths of from three 
 to five feet, which at three-eighths of an inch thick would render 
 the pieces of a manageable weight ; they should be carried up as 
 close as possible to each other, and as four inches of brick-werk is 
 more than enough on each side of them, in walls of two bricks 
 thick they weuld cause no projection into the apartment. The 
 interstices between the bricks and pipe should be filled with earth. 
 Or, what would be still better, rubbish and liquid cement. A bevel 
 joint, cr, fig. 15, pi. I. might be made to the pipe, to fit so closely 
 that no crevice would be left to lodge soot, when the cement sur*» 
 rounded the exterior. 
 
 If one, or indeed all the chimneys in a stack, constructed 
 upon this plan, were to get on fire, it would be of little conse- 
 quence, as the heat could never be so great as to act through 
 the pipe and such a solid mass as the stack would be. The soot 
 would not lodge in such chimneys as in the common ones, for in 
 a round chimney the draft would clear more smoke out of the 
 chimney before it was condensed ; and when they required 
 cleaning, Smart’s apparatus w'ould sweep them completely. — 
 When a house with ordinary chimneys is burnt, the stack 
 usually falls down, tearing the walls, and rendering it necessary 
 to rebuild them ; but if a house with an iron-pipe stack wai 
 11— Vol.L 
 
250 
 
 miuniNG. 
 
 Advantages of iron-pipe for ordinary chimneys. 
 
 burnt, it would not only in all probability stand, but act as a 
 buttress to sustain the walls ; or, in case it fell, the greater part 
 of the pipe would remain as fit for use as at first. Although it 
 may be least trouble generally to leave the breadth of half a 
 brick round the pipe, yet, in small buildings, and in cases where 
 room is wanted, a flat brick would, there is no doubt, be quite 
 sufficient for all purposes of safety. In all cases, too, where 
 iron pipe may be used, the labour of plastering chimneys will be 
 saved, which, though in many places omitted, is done to all well- 
 built houses. 
 
 The saving of room which w'ould be produced by the use of 
 iron pipe, in a stack of four eight-inch chimneys, will be imme- 
 diately seen by the inspection of fig. 13 , No. 1 and 2 , pi. I; and 
 fig. 14 , No. 1 and 2 , exemplifies the same thing with respect to 
 a stack of five chimneys. If nine inches be allowed for the ex- 
 terior diameter of the pipe (and such an allowance will be enough 
 for all contingencies,) it may be carried up, in all walls exceeding \} 
 two bricks thick, without any projection into the apartments. In 
 the corners of a two brick, or even a one and a half brick wall, 
 there is sufficient room, as may be seen by referring to the plans 
 of such walls in pi. I, to carry up such a chimney, which would 
 rather increase than diminish the strength of the w^all. By the 
 figures of the plate just referred to, it will be seen that nearly one- 
 half of the space taken up by chimneys in the common w'ay will 
 be saved, under circumstances the most unfavourable to showing 
 the advantages of the new' plan ; for the diameter of the pipes is 
 reckoned at the largest that can be required ; and the brick-work 
 surrounding the square chimneys at the thinnest it can with any 
 propriety be made. 
 
 The increased expence of round work, either in brick or stone, 
 is probably the principal reason that round chimneys are not in 
 general use, though the advantages of them, particularly in being 
 free from the currents or eddies of air occasioned by square chim- 
 neys, are not unknown to intelligent builders. 
 
 Iron pipe, it is well known, answers perfectly well for stoves, 
 arid the portable furnaces of chemists, although the area of it is 
 seldom more than half that of the fire, and the quantity of air re- 
 quired to maintain the vehement heat w'hich can be excited at 
 pleasure, occasions a much greater draft than that of ordinary do- 
 mestic fires of twice the size. Hence the above plan only pro- 
 poses the extended use of a kind of cliimney, the adequateness of 
 which is already proved. 
 
 
BUILDING. 
 
 251 
 
 Proportions of doors. 
 
 Doors. 
 
 The ancients, according to Vitruvius, frequently made their 
 doors rather narrower in breadth at the top than at the bottom. 
 These trapezoidal doors were probably adopted from their having 
 the property of closing themselves, and in modern times they are 
 useful besides as the simplest mode of raising the door, in the act 
 of opening, above the floor, in order to keep it clear of the carpet. 
 There are examples of them in the Bank of England; but they arc 
 not often introduced by architects. 
 
 Doors are varied in their dimensions according to the height 
 of the story and the magnitude of the building in which they are 
 placed. In private houses, they can rarely with propriety be 
 made wider than four feet, and in general three feet will be suf- 
 ficient. For small doors, when the height is to the breadth in the 
 ratio of seven to three, the proportion may be considered good ; 
 but the height of large doors need not be more than double their 
 breadth. The enti*auce doors of palaces and the mansions of 
 noblemen, where much company resort, are often made from four 
 to six feet w ide ; and those of public edifices may be from six to 
 ten feet wide. Doors much exceeding three feet in width, should 
 have folding leaves. In modern houses,, it is not uncommon to 
 have large folding doors, the opening of which serves, instead of 
 removing a whole partition, to throw two rooms into one. In 
 such cases, the width of the aperture will generally be of less 
 height than twice its breadth, as all the doors of the same story 
 are commonly of the same height. 
 
 When the principal door of entrance is in the middle, its 
 communication with every part of the building is not only the 
 most readily effected, but it contributes so much to the sym- 
 metry of the front, that when the plan renders such a position 
 inadmissible, a blank door is frequently substituted for a real 
 one, which is then made in the most convenient place. The 
 entrance doors of stately houses, are frequently adorned with 
 porticoes, in the Grecian or Roman taste ; but the most com- 
 mon mode of adorning entrance doors, is to surround them with 
 an architrave, surmounted with a cornice, or with a frieze and 
 cornice forming a complete entablature. These decorations are 
 made . of stone, wherever a suitable kind can be had at a reason 
 able price. 
 
 Windows. 
 
 In determining the number and size of windows, regard 
 must be paid to the destination, local position, and elevation of 
 the building, as well as to the cubature and height of the stfir^' 
 
"building. 
 
 ^52 
 
 Proportions of windows. 
 
 in the rooms to be lighted, and the thickness of the walls* 
 With respect to private houses, though considerable latitude 
 may be allowed in the determination of this subject, still there 
 are limits which cannot be disregarded without losing the 
 beauty of proportion, and the convenience of a due quantity of 
 light. Id general, the piers should not be of less breadth 
 than the apertures, nor more than twice such breadth. I’he 
 windows in all the stories of the same aspect, should be of 
 the same breadth, unless a variation be required from this 
 rule for the convenience of particular offices in the lowest 
 story. The laws of symmetry and strength alike require them 
 to be exactly one above another ; this practice, so strangely 
 neglected by our ancestors, is now, indeed, duly attended to. 
 The apertures of windows should widen inwards on each side, by 
 which means the quantity of light admitted w ill be nearly as much 
 as if they w ere externally of the same size as the increased internal 
 dimensions. 
 
 To determine the aggregate area of the window^s proper to 
 be made in an apartment, extract the square root of the cubature 
 of such apartment, and the quotient will be the answer. For ex- 
 ample, suppose the room to be forty feet long, thirty feet broad, 
 and sixteen feet high, then 40 x 30 x 16=19^200, w hich product 
 is, in feet, the cubature sought, and the square root of it, neglect- 
 ing a small remainder, is one hundred and thirty-eight feet for 
 tlie aggregate area of the apertures. One hundred and thirty- 
 eight feet will make four w'indows of a handsome size and shape, 
 adapted to the apartment in question ; and if divided accordingly 
 into four parts, thirty-four feet and a half w ill be the area of one 
 of them. The area thus obtained, when set out, for a ground 
 floor, according to the customary rule, which allows rather more 
 than two squares in height, each window may be about eight feet 
 eight inches high, by four feet broad. By the same rule, the di- 
 mensions of the apertures of windows for rooms of any other cu- 
 bature may be determined. 
 
 The sills of windows have been mostly made from three 
 feet to three feet six inches distant from the level of the floor, 
 as at that height they formed a convenient parapet to lean upon ; 
 but the French fashion having been introduced, of having the 
 windows, at least in the principal drawing rooms, down to the 
 floor, window-sills are now, partly in imitation of it, made 
 lower than formerly, and in ordinary dwellings are frequently not 
 higher than two feet, and in the extreme not more than two feet 
 six inches. 
 
 It will be proper to remind those who are partial to spaciou.s 
 and iiuinerous windows, and who are not disposed to modify 
 
BUILDING. 
 
 253 
 
 Utility of double windows. — Requisites of a good staircase. 
 
 their choice by motives of economy, that as the aggregate area 
 of the windows is enlarged, it becomes increasingly difficult 
 in winter to keep apartments warm, the heat produced in them 
 being so very speedily communicated through the glass to the 
 atmosphere without. It is for this reason, that in Russia they 
 often make their windows double, and as air is a bad conductor 
 of heat, the stratum of it interposed between the two windows 
 in the same frame, tends very materially to prevent the temper- 
 ature of the room from being carried off. The cold season is 
 not so severe, or of such long continuance, as to have occasioned 
 the introduction of lliis practice for front windows into the 
 United Kingdom, but it might be advantageously acted upon 
 with respect to the skylights employed to light staircases, as 
 such windows, when only single, contribute greatly to the 
 speedy dissipation of the warm air which ascends to the top of 
 the house. 
 
 The number of windows on each side of the entrance door 
 should be equal, and an odd number of windows in an apartment, 
 when they are all on one side of it, is better than an even number, 
 as it avoids the necessity of having a pier opposite the middle of 
 the floor. 
 
 The windows of the principal floor are generally the most 
 enriched. The simplest mode of adorning them, is, to surround 
 them with an architrave, which sometimes has, and sometimes is 
 left without, a frieze and cornice ; frequently, the whole of the 
 w indows are left plain, except the central one of the second story. 
 When the windows of the principal story have pediments, those 
 of the story immediately above should have architraves surmounted 
 by a frieze and cornice, and those of a next higher story, an ar- 
 chitrave only. The sills of all the windows in the same floor 
 should be on the same level. 
 
 Stairs. 
 
 To unite the requisites which a good staircase requires, name- 
 ly, convenience in situation and form, w ith a sufficiency of light, 
 affords one of the strongest proofs of an architect’s skill. In 
 stately mansions, the steps should not be less than four, nor more 
 than six inches high ; nor more than eighteen or less than twelve 
 inches broad; and the width should not be less than six or exceed 
 fifteen feet. In ordinary houses, the steps are generally made 
 higher, and are almost necessarily narrower, both in width and 
 length ; but, while any thing like handsomeness of appearance and 
 convenience of ascent are studied, they should not exceed seven 
 inches in height, nor be less than ten inches broad, and three feet 
 Jong. Stairs are ascended with more ease when laid somewhat 
 
254 
 
 BUILDING. 
 
 Pitch of roofs. 
 
 sloping, or a little higher at the back. The ancients were partial ^ I 
 to an odd number of steps; one consequence of which choice 
 was, that the same foot whicli was placed on the first step was ^ 
 first placed at the top on the landing. 
 
 Roofs. 
 
 Architects include, under the term roof, not only the exterior 
 covering of a house, but all the beams and other parts necessary 
 to support that covering. 
 
 Among the ancients, in those countries where it seldom rained, • ; 
 roofs were made quite flat; but the Greeks, perceiving the incon-** j 
 venience of this form, deviated from it a little, by inclining their - 
 roofs, as appears from several ancient remains, abo\it one-eighth j ! 
 or one-ninth part of the span. The Romans, who had rather A ! 
 more occasion than the Greeks to provide for the speedy Ais-^ 
 charge of rain from their houses, made the height of the inclina-^ 
 tion of their roofs from one-fifth to two-ninths of the span.*| 
 Among the northern nations of Europe, after the decline of the 
 Roman empire, high-pitched roofs began to be in general request, 
 and that was considered the standard form, the vertical section of 
 which was an equilateral triangle. No part of the art of building 
 has been more subject to caprice, than the height of the inclina- 
 tion, or, as it is usually expressed, the pitch of roofs. In the pre- 
 sent day, a great variety of pitch is employed, but the equilateral , 
 one is seldom seen. In ordinary dwellings, the pitch of the roof 
 is from one-third to one-fourth part of the span; but mansions 
 and public edifices are still executed in every diversity of style i * 
 that fancy or particular views can dictate. 
 
 High-pitched roofs discharge rain and snow more quickly : 
 than others ; they are also not so easily stripped by the wind ; 
 the rain is not so easily blown between the slates, and from their ' 
 approach towards perpendicularity of pressure, they are not so ' 
 great a burden to the walls. They are, however, more expen- ' 
 sive than low roofs, as they require longer and stronger timbers ; 
 and from their greater surface, they require a larger quantity of 
 the covering material : but though low roofs have the advantage 
 in point of cheapness, they require large slates, and great care of 
 execution. 
 
 The roof is one of the principal ties of a building, when skil- 
 fully executed, in connecting the exterior walls. Some idea may 
 be formed of the success with which scientific knowledge and ex- 
 perience may be employed in the construction of roofs, when it 
 is observed, that roofs have been constructed sixty feet wide, al- 
 though they have not contained a single piece of timber more 
 than ten feet long and four indues square. 
 

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BUILDING. 
 
 255 
 
 Pitch of the rafters for lead — pantiles — plain tiles. — Experiment on tiles & slate. 
 
 In determining the pitch of rafters, when mere fancy is not to 
 be our guide, the nature of the intended covering should be taken 
 into consideration, and the inclination proportioned accordingly. 
 The following rules may be observed with propriety : 
 
 For Lead. — Divide the width first into two parts, see fig. l6, pi. 
 I. and one of these parts into four, as 1, 2, 3, 4 ; with two of these 
 parts, describe a quarter circle, 2, which gives a proper pitch or 
 slope to be covered with lead, and is called a pediment pitch. 
 
 For Pantiles. — Divide the width as before into two parts, and 
 one of these two parts into four, as 1, 2, 3, 4; with three parts, 
 describe a quarter circle, 3, which gives a proper pitch for use. 
 
 For Plain Tiles. — Divide the width into two parts ; with one 
 of them make a quarter circle, which gives a pitch or slope pro- 
 per for the roof. — The lighter the covering material, the lower the 
 roof may be, and therefore the pitch for slates, may be the same as 
 that for the covering which the particular quality used most nearly 
 approaches to in weight. 
 
 Tiles, though extensively used in many parts of the country, 
 constitute a very heavy covering for houses, and, what is still worse, 
 they injure the timber upon which they are laid, and tend to make 
 a house damp, from the facility with which they are penetrated 
 with moisture. The following experiment, by the bishop of Llan- 
 daff, decisively proves their great porosity, and their inferiority to 
 slate. The Bishop observes, that sort of slate, other circum- 
 stances being the same, is esteemed the best which imbibes the 
 least water ; for the imbibed water not only increases the weight 
 of the covering, but, in frosty weather, being converted into ice, it 
 swells and shivers the slate. This effect of frost is very sensible 
 in tiled houses, but is scarcely felt in those which are slated ; for 
 good slate imbibes but little water ; and when tiles are well glazed, 
 they are rendered in some measure, with respect to this point, 
 similar to slate. He took a piece of W estmoreland slate, and a 
 piece of common tile, and weighed each of them carefully ; the 
 surface of each was about thirty square inches. Both the pieces 
 were immersed in water for about ten minutes, and then taken 
 out, and weighed as soon as they had ceased to drip. The tile 
 had imbibed above a seventh part of its weight of water ; and the 
 slate had not imbibed above a two-hundredth part of its weight ; 
 indeed the wetting of the slate was merely superficial. He placed 
 both the wet pieces before the fire ; in a quarter of an hour the 
 slate was become quite dry, and of the same weight it had before 
 it was put into the water ; but the tile had lost only about twelve 
 grains of the water it had imbibed, which was, as nearly as could 
 be expected, the very same quantity that had been spread over its 
 surface ; for it was the quantity which had been imbibed by the 
 
256 
 
 BUILDING. 
 
 Prices of slate. — Statement of the weight of ditferent coverings. 
 
 slate, the surface of which was equal to that of the tile. The tile 
 was left to dr}^ in a room heated to sixty degrees, and it did not 
 lose all the water it had imbibed in less than six days. If, then, 
 tiles imbibe a seventh part of their weight of w^ater in ten minutes, 
 and cannot be deprived of this water w ithout a degree of heat equal 
 to sixty degrees continued for six days, it must be obvious that a 
 roof covered with them, can, in this country, seldom be dry. The 
 timbers also of the roof must be calculated to support their w eight 
 in their wet state. 
 
 The finest sort of blue slate, which is obtained in the neighbour- 
 hood of Kendal, is sold there for 3s. 6d. per load, which comes 
 to e£l. los. per ton, the load weighing two hundred weight. The 
 coarsest sort may be had for 2s. 4d. per load, or <£l. 3s. 4d. per 
 ton. Thirteen loads of the finest soi t will cover forty-two square, 
 yards of roof, and eighteen loads of the coarsest sort will cover 
 the same extent. Hence there is half a ton less weight uponj 
 forty-two square yards of roof when the finest slate is used, than! 
 W'hen it is covered with the coarsest kind, and the difference in the"] 
 expence of the material is only 3s. 6d.; yet as fine slate owes its 
 lightness, not so much to a difference in the quality of the stone 
 from which it is split, as to the thinness to which it is reduced, it 
 is inferior to the heavier kind in point of durability. 
 
 The following statement shows the average weight of the co- 
 vering laid upon forty’-tw-o square yards of building, according to 
 the material employed : 
 
 CWT. 
 
 Copper 4 
 
 Fine Slate 26 
 
 Lead 27 
 
 Coarse Slate 36 
 
 Tiles 54 
 
 Such are the advantages of slate as a covering, that 
 wherever it can be procured without much land carriage, it 
 obtains the preference of all other materials. Its durability is 
 so great, that it has been known to continue sound and good for 
 centuries ; but all kinds of it are not equally excellent in this 
 respect. Its most usual colours are white, brown, and blue, 
 and the colour aftbrds some index to the quality of the slate. 
 The light blue sort is always the least penetrable to water, 
 which the deep black blue is apt to imbibe rather freely. 
 Several methods may be practised to ascertain the goodness of 
 slate, when not brought from a quarry of well known character. 
 If a slate, when struck shaiply against a large stone, produce 
 a complete sound, it is a mark of goodness ; and if it does not 
 
BUILDING. 
 
 557 
 
 If Tests of the quality of slates. — Patent roofing. 
 
 — — — — ' ^ 
 
 ihatter before the edge of the zax, or instrument used for hew- 
 ng it, the criterion is decisive. Another method is, to place 
 he slate lengthwise and perpendicularly in a tub of water, 
 ibout half a foot deep, care being taken that the unimmersed 
 Dart of the slate be not in any way accidentally wetted. Let it 
 emain in this state twenty-four hours : at the end of that time, 
 f the slate be good, it will not have drawn water more than 
 lalf an inch above the surface of that fluid, and that perhaps 
 it the edges only, where the texture has been a little loosened in 
 :he hewing ; but a spongy defective stone will draw water to the 
 very top. Another mode of trial, on the result of which full 
 reliance may be placed, is to weigh two or three of the most 
 juspected slates, and note their precise weight; then immerse 
 them entirely in water for twelve hours ; take them out, wipe 
 them as clean as possible with a linen cloth, and if their weight 
 differs not, or differs but very little from what it was at first, 
 they may be considered good ; a drachm in a dozen pounds is al- 
 lowable, but not more. The principal reason of the inferiority of 
 the slates which imbibe much moisture, being that they are shiver- 
 ed by frost ; when none but a porous kind can be easily obtained, 
 they might doubtless be improved by the application of tar, as al- 
 ready mentioned for tiles, when treating of the manufacture of the 
 latter article. 
 
 John’s tessera is perhaps the best of those artificial compo- 
 sitions which are designed for roofing, and it certainly has 
 advantages of the first class. It is cheaper than any covering 
 hitherto known ; it is superior in lightness and evenness of sur- 
 face to lead, and is said to equal that metal in point of durability. 
 It is equally adapted to the flat roof intended to be walked on, 
 and to angular roofs. The roofs intended to be covered with it 
 must in all cases be boarded as for lead, except that a less 
 thickness of timber will suffice. For a flat roof, it may be laid 
 on three-quarter inch deal ; but for an angular roof, half-inch 
 will be sufficient. This roofing, for which a patent was taken out, 
 dated the 22d December, 1806, consists of calcareous stone and 
 tar, with a little of the powder of burnt bones. The sheets are 
 made of an equal thickness by rolling. From the quantity of tar 
 entering into the composition, it may be considered very com- 
 bustible; but the contrary has been proved by experiment. j[t 
 is found to resist the effects of heat two or three times as long 
 as lead. Tessera is made in sheets about four feet long, by two 
 feet wide ; and the sheets are united by solder of the same com^ 
 position. 
 
 2 L 
 
 ll—VoL. I. 
 
258 
 
 BUILDING. 
 
 Qualities and methods of jointing flooring boards. 
 
 Floors. 
 
 Flooring boards are mostly made of fir. The first class are 
 selected free from knots, shakes, sap-wood, or cross-grained 
 stuff ; the second class consists of boards also free from shakes 
 and sap-wood, but not fiom small sound knots ; the third class 
 contains the residue of any parcel, or such boards as cannot be " 
 included in either of the preceding classes. When an agreement q 
 is entered into for the erection of a building, the quality of the 
 boards should be specified, to prevent subsequent disputes. As . 
 all boards shrink in the course of time, and as the quantity of i| 
 their contraction increases with their dimensions, floors which ® 
 are laid with very broad boards, soon exhibit, at the joints, 
 wide fissures that have an unpleasant appearance. It is there- 
 fore the practice in good houses, not only to select the best part 
 of the wood, but to cut the boards into narrow scantlings ; so thaf, |j 
 if properly seasoned, and laid close at first, their shrinking after- |ji 
 wards is so small as to make no openings of consequence. Boards 
 about five inches broad may be reckoned narrow, but when they 
 measure nine inches or more in the same direction, they must be « 
 considered broad. HH 
 
 The manner of jointing flooring boards, and fastening thenW 
 down upon the joists, is performed in a variety of ways, the 
 most usual of which is, to plane the edges of the board quite 
 square, that is, at right angles to the upper and under surface, 
 and then, placing them as closely to each other as possible, to 
 nail them down from the upper surface. Sometimes, particularly 
 when the wood is known to be insufficiently seasoned, after the 
 first board has been fastened down, the fourth board k secured 
 in like manner, the two intermediate boards are then made 
 somewhat wider than the space to receive them, and forced into 
 their places by jumping upon them. To do this with the most 
 ease and advantage, the intermediate boards are laid aslant, so 
 as to be highest in the middle, and those edges which are placed 
 together being sloped a little, so as to form rather less than a 
 right angle with their respective upper surfaces, they are, by an 
 adequate weight, at once compressed and levelled. The fourth 
 board of the last series becomes the first of the next, and the 
 operation, which is called folding the boards, is repeated till the 
 floor is finished. The nails are driven in a little below the sur- 
 face of these boards, and the cavity is filled with glazier’s 
 putty. ■ But in 'rooms not intended to be carpeted, and yet 
 where a neat and clean appearance is indispensable, the use of 
 putty must be avoided, and the nails must not be driven in g 
 kom the top. This object is obtained by doweling the joints, 
 
BUILDING. 
 
 
 Jointing of flooring boards. — Mortar floors. 
 
 hat is, driving wooden pins into them in the middle of their thick- 
 less, and parallel to the surface, in the same manner as the coopers 
 oint the boards forming the ends of their casks. In this case, 
 )ne-half of each pin entering the edges placed together, the 
 joards, if the dowels be sufficiently numerous and properly 
 alaced, cannot rise or sink but in conjunction. The best place 
 for the dowels is in the middle of the space between the joists. 
 In the best doweled work, the nails are concealed when the floor 
 ,s finished, for they are driven in slantwise through the outer edge 
 silly of each board. Sometimes the joints of flooring boards are 
 rabbeted, that they may lap over each other a little way, and 
 sometimes toothed into each other, or, as it is technically ex- 
 pressed, ploughed and tongued. When either of these methods is 
 adopted, the boards are not separated on their contraction so as 
 to leave an aperture between each pair, through which any thing 
 can drop ; but such floors are more costly than others, not only on 
 account of the extra labour, but the greater quantity of wood which 
 they require. 
 
 It is always desirable to cover a floor with boards in one length; 
 but as this may not always be convenient, when it is not done, 
 the ends of the two boards that meet are called headings. The 
 headings should invariably be upon a joist, and two of them should 
 never be together in the same line. 
 
 Before the boards are laid, it is necessary to examine whether 
 the upper sides of the joists all lie in the same plane. The defect 
 they are most liable to, is that of being depressed in the middle; 
 in which case they must be raised by the addition of suitable pieces, 
 but if found too protuberant, they must be I'educed by the adze. 
 
 Yellow deal, well seasoned, is one of the best woods that can 
 be selected for .floors, and retains its colour for a long time; 
 whereas the white sort, by frequent washing, becomes blackish 
 and disagreeable in its appearance. 
 
 In the habitations of the labouring classes of society, the 
 ground floors are often made of a kind of mortar. The best ma- 
 terials for this purpose are two-thirds of lime, one of coal-ashes, 
 and a small portion of clay. These ingredients are to be well 
 tempered with water, left to subside for a week or ten days, and 
 then well worked up again. This operation should be repeated 
 in the course of three or four days, till the mixture becomes 
 smooth and glutinous, when it is fit for use. After the ground is 
 made perfectly level, the composition is to be laid on to the depth 
 of two and a half or three inches, and carefully smoothed with a 
 trowel. The hottest season of the year is the most proper for ap- 
 plying this composition, which, when completely dried, will make 
 a most durable floor. 
 
260 
 
 BUILDING. 
 
 Proportions of timbers. 
 
 Proportions of Timbers, <5fc. 
 
 In the treatise entitled the British Carpenter/’ already re- 
 ferred to, are given the following Tables to show the proportions 
 of timbers for small and large buildings : 
 
 PROPORTIONS OF TIMBERS FOR SMALL BUILDINGS. 
 
 Beanng i 
 Height 
 if 8 feet 
 10 
 12 
 
 ^osts of Fir, 
 
 Scantling 
 
 4 inches square 
 
 5 
 
 6 
 
 Bearing P 
 Height 
 if 10 feet 
 12 
 14 
 
 osts of Oak. 
 Scantling 
 6 inches square 
 8 
 10 
 
 Girdei 
 Bearing 
 if 16 feet 
 20 
 ,2.4 
 
 s of Fir. 
 
 Scantling 
 8 inches by 11 
 10 12| 
 
 12 14 
 
 Girden 
 Bearing 
 if 16 feet 
 20 
 24 
 
 f of Oak. 
 
 Scantling 
 10 inches by 13 
 12 14 
 
 14 15 
 
 Joists 
 Bearing 
 if 6 feet 
 9 
 12 
 
 (f Fir. 
 
 Scantling 
 5 inches by 2| 
 f a 2|; 
 
 8 2f 
 
 Joists 
 Bearing 
 if 6 feet 
 9 
 
 12 
 
 of Oak, 
 
 Scantling 
 5 inches by 3 
 7| 3 
 
 10 3 
 
 Bridging 
 Bearing 
 if 6 feet 
 8 
 
 10 
 
 of Fir, 
 Scantling 
 
 4 inches by 2| 
 
 5 
 
 6 3 
 
 Bridging 
 Bearing 
 if 6 feet 
 8 
 10 
 
 s of Oak. 
 
 Scantling 
 4 inches by 3 
 5| 3 
 
 7 3 
 
 , Small Raj 
 
 Bearing 
 if 8 feet 
 10 
 12 
 
 ■ters of Fir, 
 
 Scantling 
 inches by 2| 
 ^ 2 
 
 51 oX 
 
 ’"a -‘a 
 
 Small Rafte 
 Bearing 
 if 8 feet 
 10 
 12 
 
 rs of Oak. 
 
 Scantling 
 4| inches by 3 
 5| 3 
 
 6\ 3 
 
 Beams of 1 
 Length 
 if 30 feet 
 45 
 60 
 
 "ir, or Ties. 
 
 Scantling 
 6 inches by 7 
 
 9 8i 
 
 12 11 
 
 Beams of 0 
 Length 
 if 30 feet 
 45 
 60 
 
 aky or Ties. 
 Scantling 
 7 inches by 8 
 10 111 
 
 13 15 
 
 Princi 
 
 Length 
 if 24 feet 
 36 
 
 48 
 
 pal Rafters t 
 Scam 
 Top 
 
 5 ins. & 6 
 8 
 
 8 10 
 
 f Fir. 
 tling 
 Bottom 
 6 ins. Sc 7 
 8 10 
 
 10 12 
 
 Princii 
 
 Length 
 if 24 feet 
 36 
 48 
 
 pal Rafters 
 Scant 
 Top 
 
 7 ins. & 8 
 
 8 9 
 
 9 10 
 
 f Oak. 
 ling 
 Bottom 
 
 8 ins. Se 9 
 
 9 10| 
 10 12 
 
) 
 
 BUILDING. 
 
 
 Proportion® of timbers. 
 
 261 
 
 PROPORTIONS OF TIMBERS FOR LARGE BUILDINGS. 
 
 [r : 
 
 '■ Bearing F 
 
 ' Height 
 
 if 8 feet 
 12 
 16 
 
 osts of Fir. 
 
 Scantling 
 5 inches square 
 8 
 10 
 
 Bearing Pi 
 Height 
 if 8 feet 
 12 
 16 
 
 )sts of Oak. 
 Scantling 
 8 inches square 
 12 
 16 
 
 Girder 
 Bearing 
 if 16 feet 
 20 
 24 
 
 s of Fir. 
 
 Scantling 
 9f inches by 13 
 12 14 
 
 13| 15 
 
 Girders 
 Bearing 
 if 16 feet 
 20 
 24 
 
 of Oak. 
 
 Scantling 
 12 inches by 14 
 15 15 
 
 18 16 
 
 Joists 
 Bearing 
 if 6 feet 
 9 
 
 12 
 
 of Fir. 
 
 Scantling 
 5 inches by 3 
 
 71 s 
 
 10 3 
 
 Joists 
 Bearing 
 if 6 feet 
 9 
 
 12 
 
 of Oak. 
 
 Scantling 
 6 inches by 3 
 9 * 3 
 
 12 3 
 
 Bridgin, 
 Bearing 
 if 6 feet 
 8 
 
 10 
 
 of Fir. 
 
 Scantling 
 4 inches by 3 
 31 3 
 
 7 3 
 
 Bridging 
 Bearing 
 if 6 feet 
 8 
 
 10 
 
 'S of Oak. 
 
 Scantling 
 5 inches by S| 
 6| 3| 
 
 8 3| 
 
 Small Raj 
 Bearing 
 if 8 feet 
 10 
 12 
 
 fters of Fir. 
 
 Scantling 
 41 inches by 3 
 5| 3 
 
 6\ 3 
 
 Small Rafti 
 Bearing 
 if 8 feet 
 10 
 12 
 
 rs of Oak. 
 
 Scantling 
 5| inches by 3 
 7 3 
 
 9 3 
 
 Beams of j 
 Length 
 if 30 feet 
 45 
 60 
 
 Fir, or Ties. 
 
 Scantling 
 7 inches by 8 
 10 ' 111 
 
 13 . 15 
 
 Beams of C 
 Length 
 if 30 feet 
 45 
 60 
 
 )ak, or Ties. 
 Scantling 
 8 inches by 9 
 11 12| 
 
 14 16 
 
 Princi 
 
 Length 
 if 24 feet 
 36 
 48 
 
 pal Rafters (. 
 Scam 
 Top 
 
 7 ins. & 9 
 
 8 9 
 
 9 10 
 
 f Fir. 
 lling 
 
 Bottom 
 
 8 ins. & 9 
 
 9 10| 
 
 10 12 
 
 Princij 
 
 Length 
 if 24 feet 
 36 
 48 
 
 pal Rafters oj 
 Scant 
 Top 
 
 8 ins. & 9 
 
 9 10 
 
 10 13 
 
 r Oak. 
 ling 
 Bottom 
 9 ins. & 10 
 10 12 
 
 12 14 
 
 The Author of the preceding Tables observes, that though 
 they seem so plain as not to need explanation, yet a few remarks 
 might be subjoined with propriety. All binding or strong joists, 
 he then adds, ought to be half as thick again as common joists ; 
 
. 262 
 
 BUILDING. 
 
 Proportions of timbers. — Camber of timbers. 
 
 that is, if a common joist be given three inches thick, a binding joist 
 should be four inches and a half thick, although of the same depth. 
 
 If it be not convenient to allow the posts in partitions to be 
 square, which is the best form, in such cases, multiply the 
 square of the side of the posts, as here given, by itself : for 
 instance, if it be six inches square, then as six times six is 
 thirty-six, to keep this post nearly to the same strength, find- 
 two numbers producing the same amount; as suppose the parti- '■ 
 tion to be four inches thick, then let the post be nine inches th^; 
 other way, so that nine times four being thirty-six, the area of its' 
 horizontal section is the same, and its strength nearly equal to the 
 square post. I 
 
 Posts that go to the height of two or three stories, need not 
 hold the proportions given in the table, because at every floor 
 they meet with a tie. Admit a post to be thirty feet high, and 
 that in this height there are three stories, two of ten feet and one ** 
 of eight feet ; look for posts of fir ten feet high, their scantling is 
 five inches square, that is, twenty-five square inches, which double 
 for the two stories ; and also take that of eight feet high, being 
 four inches, that is, sixteen inches square, all which being added 
 together, make sixty-six inches ; so that such a post would be la? || 
 ther more than eight inches square. On occasion it may be les? ‘ -■ 
 sened in each story as it rises. 
 
 All beams, ties, and principal rafters, ought to be cut or forced “ 
 in framing to a camber, or roundness, on the upper side, and the 
 
 convexity may be about one inch in eighteen or twenty feet. The 
 
 reason is, that all timber, partly from its own weight, but princi- 
 pally from the weight of the covering or other burden it has to 
 bear, will swag ; and unless prepared in this manner, that it may 
 never become concave, a degree of unsightliness, and often of in- 
 convenience, will be pixiduced. 
 
 The joists in floors, the purlines (or timbers into wdiich the 
 small rafters are tenoned in roofs,) &c. should not exceed tw'elve f 
 feet in the length of their bearing, or from support to support.lBfl 
 The strong joists of floors should not be at a greater distance than 
 five feet, nor common joists more llian ten or twelve inches apart. A 
 
 According to the experiments of Muschenbroek, fir is able to 
 bear compression in the direction of the length of its fibres, or to 
 sustain as a post, a much greater weight than oak, but is far infe- 
 rior to oak when the weight is suspended. In the preceding ta- 
 bles, therefore, the scantlings of fir bearing posts and principal 
 rafters are properly made less than those of oak ; but for other 
 timbers, particularly for ties, many are of opinion that the propor- 
 tions of the Author’s tables should be reversed, and the sdantling 
 which he has assigned to fir should be given to oak. 
 
BUILDING. 
 
 263 
 
 Provisions of the Building Act affecting the Carpenter. 
 
 J Building Act. 
 
 All the buildings erected in London and the several Parishes 
 within the Bills of Mortality, are subjected to the regulations of 
 in Act of Parliament, of the 14. Geo. III. the main object of 
 which is to lessen the danger to be apprehened from fire. As 
 iiany of the provisions of this act are of great importance, and de- 
 serve to be universally known and acted upon, we shall conclude 
 :he subject of Building by an abstract of them. Those which re- 
 ate to the Carpenter are the following : 
 
 Timber partitions between building and building, erected or 
 3 recting before the passing of the act, may remain till one of 
 he adjoining houses is rebuilt, or till one of the fronts, or two- 
 hirds of the fronts which abut on such timber partition, is 
 aken down to the bressummer, or one pair of stairs floor, and 
 ebuilt. 
 
 Three months’ notice of the pulling down of such wooden 
 partition, when decayed or of insufficient thickness, to be given by 
 I he proprietor to the owner or occupier of such a house, and if 
 ;he house be empty, such notice to be stuck up on the front or 
 front door of it. 
 
 No timber hereafter to be laid in any party arch, or party wall, 
 ^except for bond to the same; nor any bond timber within nine 
 inches of the opening of a chimney, nor within five inches of the 
 due ; nor any timber within two feet of any oven, stove, copper, 
 still, boiler, or furnace. 
 
 The wood work of chimney breasts to be fastened to the 
 'said breast with iron wall hooks, spikes, nails, or holdfasts, 
 which must not be driven more than three inches into the wall, 
 or nearer than four inches to the inside of the opening of the 
 chimney. 
 
 No timber bearer to wooden stairs let into an old party wall, 
 must come nearer than eight inches and a half to the flue, nor 
 nearer than four inches to the internal finishing of the adjoining 
 building. 
 
 No timber to be laid under any hearth to a chimney, nearer 
 than eighteen inches to the upper surface of such hearth. 
 
 No timber must be laid nearer than eighteen inches to any 
 door of communication through the party walls of warehouses and 
 stables. 
 
 Bressummers, story posts, and plates thereto, are only 
 permitted in the ground story, and may stand even with the 
 outside of the wall, but must go no deeper than two inches into 
 a party wall, nor nearer than seven inches to the centre of a party- 
 wall, when it is two bricks thick, nor nearer than four inches and 
 
264 
 
 BUILDING. 
 
 Provisions of the Building Act affecting the Bricklayer. 
 
 a half, provided the party wall does not exceed one brick and a 
 half in thickness. 
 
 Every corner story post must be of oak, and at least twelve 
 inches square, when employed for the support of two fronts. 
 
 Window frames and door frames to the first, second, third, 
 and fourth rate classes, are to be recessed in reveals, four inches 
 at least. 
 
 Door-cases and doors to warehouses only of the first, second, 
 third, or fourth rate classes, may be even with the outward face of 
 the wall. 
 
 No external decoration to be of wood, except cornices or 
 dressings to shop windows ; frontispieces to door-ways of the 
 second, third, and fourth rate classes, and covered ways or porti- 
 coes to buildings ; but not to project beyond the original line of 
 the house in any street or way. Such covered way or portico not 
 to be covered w ith wood ; nor such cornice, covered way, or roof 
 of the portico, to be higher than the under side of the sill to the 
 windows of the one pair of stairs floor. No flat gutter or roof, 
 nor any turret, dormer, or lantern light, or other erection placed 
 on the flat of the roof belonging to the first, second, third, fourth, 
 and fifth rate classes, to be of wood. 
 
 No wooden water tanks must be higher from the ground than 
 the tops of the windows of the ground story. 
 
 Those provisions of the Act which relate to the Bricklayer are 
 the most numerous. Every master bricklayer must give twenty- 
 four hours’ notice to the Surveyor of the district, concerning the 
 building to be altered or erected ; but if the building is to be piled 
 or planked, or begun with wood, it becomes the business of the 
 carpenter to give such notice. 
 
 The footings of the walls are to have equal projections on 
 each side ; but where any adjoining building will not admit 
 of such projections to be made on the side adjoining to such 
 building, this direction to be complied with as nearly as pos- 
 sible. 
 
 The timbers in each rate, as girders, beams, trimming joists. 
 Sec. may have as much bearing as the nature of the wall will ad- 
 mit, provided four inches be left between the ends of such timber 
 and the external surface of the w all. 
 
 External Walls. 
 
 Every, front, side, or end w all, not being a party wall, is called 
 an external wall. 
 
 External w^alls, and other external inclosures to the first, 
 second, third, fourth, and fifth rates of buildings, must be of 
 brick, stone, artificial stone, lead, copper, tin, slate, tile, o 
 
BUILDING. 
 
 • 265 
 
 Provisions of the Building Act affecting the Bricklayer. 
 
 iron ; or of some or all of these materials in conjunction, except 
 the planking, piling, &C. for the foundation, which may be of 
 wood. 
 
 If any part to an external wall of the first and second rate is 
 built wholly of stone, it is not to be less in thickness than as fol- 
 lows : first rate, fourteen inches below the ground floor, nine 
 inches above the ground floor ; second rate, nine inches above the 
 ground floor. 
 
 Where a recess is meant to be made in an external wall, it 
 must be arched over, in such a manner, that the arch and the back 
 of such recess, shall respectively be of the thickness of one brick 
 in length ; hence no walls are allowed to be recessed which are 
 not more than one brick in thickness. 
 
 No external wall to the first, second, third, and fourth rate, is 
 ever to become a party-wall, unless the same shall be of the height 
 and thickness above the footing, as is required for each party-wall 
 to its respective rate. 
 
 "Party Walls. 
 
 Buildings of the first, second, third, and fourth rate, which are 
 not yet designed by the owner thereof to have separate and dis- 
 tinct side wails, on such parts as may be contiguous to other build- 
 ings, must have party-w^alls ; and they are to be placed half and 
 half on the ground of each owner, or of each building respectively, 
 and may be built thereon, without any notice being given to the 
 owner of the other part, the first builder having a right so to do, 
 when building against vacant ground. 
 
 Party-walls, chimneys, and chimney shafts hereafter to be 
 built, must be of good sound brick or stone, or of sound 
 bricks and stone together, and mnst be coped with stone, tile, 
 or brick. 
 
 Party-walls, or additions thereto, must be carried up thirteen 
 inches above the roof, measuring at right angles with the back of 
 the rafter, and twelve inches above the gutter of the highest build- 
 ing which gables against it ; but where the height of a party-wall 
 so carried up, exceeds the height of the blocking course or para- 
 pet, it may be made less than one foot above the gutter, for the 
 distance of two feet six inches from the front of the blocking 
 course or parapet. 
 
 Wlvere dormers (the term for windows in roofs, differing from 
 sky-lights by their being vertical,) or other erections are fixed in 
 any flat or roof, within four feet of any party-wall, such party-wall 
 is to be carried up against such dormer, and must extend at least 
 two feet wider, and to the full height of every such dormer or 
 erection. 
 
 12.— VoL. I. 2M 
 
266 
 
 BUILDING.’ 
 
 Provisions of the Building Act affecting the Bricklayer. 
 
 No recess is to be hereafter made in any party-wall of the 
 first, second^ third, and fourth rate, except for chimney flues, 
 girders, &c. and for the ends of walls or piers, so as to reduce 
 such wall in any part of it to a less thickness than is required by 
 the act, for the highest rate of building to which such wall 
 belongs. 
 
 No opening is to be made in any party-wall, except for com- 
 munication from one stack of warehouses to another, and from 
 one stable building to another, and the communications allowed 
 must have wrought iron doors, and the pannels thereof are not 
 to be less than a quarter of an inch thick, and must be fixed in 
 stone door-cases and sills. But there may be openings for pas- 
 sages or w'ays on the ground, for foot passengers, cattle, or car- 
 riages, which must be arched over throughout with brick or stone, 
 or brick and stone together, of the thickness of a brick and a half 
 at the least, to the first and second rate, and one brick to the third 
 and fourth rate. And if there is any cellar or vacuity under such 
 passage, it is to be arched over tluoughout in tlie same manner as 
 the passage over it. 
 
 No party-wall, or party-arch, or shaft of any chimney, new 
 or old, must be cut into, except for the following purposes : if 
 the fronts of buildings are in a line with each other, a recess 
 may be cut, both in the fore and back front of such buildings, 
 (as may be already erected,) for the purpose of inserting the 
 end of such other external wall, which is to adjoin thereto. 
 This recess must not be more than nine inches deep from the 
 outward faces of such external walls, and not be cut beyond 
 the centre of the party-wall. And for the purpose of inserting 
 bressummers and story-posts, that are to be fixed on the ground 
 floor, either in the front or back wall, the recess may be cut from 
 the foundation of such new wall to the top of such bressummer, 
 fourteen inches deep from the outw^ard face of such wall, and four 
 inches wide in the cellar story, and two inches wide on the ground 
 story. The same may also be done for the purpose of tailing-in 
 stone steps, or stone landings, as for bearers to wood stairs, or for 
 la}ing-in stone corbels for the support of chimney jambs, girders, 
 beams, purlines, binding or trimming joists, or other principal 
 timbers. 
 
 Perpendicular recesses may also be cut in any party-wall, 
 whose thickness is not less than thirteen inches, for the purpose of 
 inserting walls and piers therein ; but they must not be wider than 
 fifteen inches, or more than four inches deep ; and no such recess 
 is to be nearer than ten feet to. any other recess. All such cuttings 
 or recesses must be immediately made good, and effectually pinned 
 wp, with brick, stone, slate, tile, shell, or iron, bedded in mortar. 
 
BUILDING. 
 
 267 
 
 Provisions of the Building Act affecting the Bricklayer. 
 
 No party-wall must be cut for any of the above purposes, if 
 ' the same will injure, displace, or endanger the timbers, chimneys, 
 flues, or internal finishings of the adjoining buildings. 
 
 ' The footing may be cut off on the side of any party-wall, 
 where an independent side wall is intended to be built against 
 such party-wall. 
 
 When any buildings (inns of court excepted) that are erected 
 over gate-ways, or public passages, or have different rooms and 
 floors, the property of different owners, are to be rebuilt, they 
 must have a party-wall, with a party-arch or arches of the thick- 
 ness of a brick and a half at the least, to the first and second rate, 
 and of one brick to the third and fourth rate, between building and 
 building, or between the different rooms and floors that are the 
 property of different owners. 
 
 Inns of court are required only to have party-walls where any 
 room or chamber communicates to each separate and distinct 
 staircase, and which are also subject to the same regulations as 
 respect other party-walls. 
 
 If buildings of different rates adjoin each other, and any. addi- 
 tion is intended to be made to the low^er rate, the party-wall of 
 ‘ such building must be such as is required for that of the higher 
 rate adjoining. 
 
 When any party-wall is raised, it is to be made of the same 
 thickness as the wall in the story next below the roof of the highest 
 ’ building adjoining, but it must not be raised at all, unless it can 
 be done with safety to such wall, and the building adjoining 
 thereto. 
 
 Every dwelling-house built four stories high from the founda- 
 tion, exclusive of rooms in the roof, must have its party-wall built 
 according to the third rate, although such dwelling-house may be 
 of the fourth rate. Every dwelling-house, also, exceeding four 
 stories in height from the foundations, exclusive of the rooms in 
 the roof, must have its party-wall built according to the first rate^ 
 although such house may not be of the first rate. 
 
 Chimneys, 
 
 No chimney is to be erected on timber, except on the piling, 
 planking, &c. of the foundations of the building. 
 
 Chimneys may be built back to back in party-walls ; but when 
 this is done, they must not be less in thickness from the centre of 
 such party-wall than as follows : first rate, or adjoining thereto, 
 must be one brick thick in the cellar story, and half a brick in all 
 the upper stories. Second, third, and fourth rate, or adjoining 
 thereto, must be three-quarters of a brick in the cellar story ; and 
 half a brick in all the upper stories. Such chimneys in party-wail& 
 
268 
 
 BUILDING. 
 
 Provisions of the Building Act affecting the Bricklayer. 
 
 of any of the four rates, as do not stand back to back, may be 
 built as folio\v.s : Ironi the external face of the party-wall to the 
 inward face of the back of the chimney in the cellar story, one 
 brick and a half thick, and in the upper stories, one brick thick 
 from the hearth to tnelve inches above the mantle. If such chim- 
 ney is built against anv other wall, the back may be half a brick 
 tliinner than above stated. 
 
 Those backs of chimneys which are not in party-walls of th^ 
 second, third, and fourth rate, must be in every story one brick 
 thick at least, from the hearth to twelve inches above the mantle. 
 These backs may also be half a brick thinner, if such chimney be 
 built against any other wail. 
 
 The breasts of chimneys, whether in party-walls or not, are 
 not to be less than one brick thick in the cellar story, and half a 
 brick thick in every other story. 
 
 All partitions between flues must not be less than half a brick 
 thick. 
 
 Flues may be built opposite to each other in party-walls, but 
 they must not approach nearer to the centre of such w'all than 
 two inches. 
 
 All chimney breasts next to the rooms, and chimney backs, 
 and all flues, are to be rendered or pai^eled. 
 
 Backs of chimneys, and flues in party-walls against vacant 
 gi'oimd, must be lime whited, or marked in some durable manner, 
 but must be rendered or pargeted as soon as any other building is 
 erected to adjoin them. 
 
 Isto timber must be over the opening of any chimney for sup- 
 porting the breast ; but all chmmeys must have a brick or stone 
 arch, or iron bar or bars. 
 
 All chimneys must have slabs or foot paces of stone, marble, 
 tile, or iron, at least eighteen inches broad, and at least one foot 
 longer than the opening of the chimney when finished; and such 
 slabs or foot paces must be laid on brick or stone trimmers at 
 least eighteen inches broad from the face of the chimney breast, 
 except there be no room or vacuity beneath, in which case they 
 may be bedded on the ground. 
 
 Brick funnels must not be made on the outside of the first, 
 second, third, or fourth rate, next to any street, square, court, 
 road, or w ay, so as to extend beyond the general line of the build- 
 ings in such situations. 
 
 No metallic funnel or other pipe, for conveying smoke or 
 steam, is allowed to be fixed near any public street, square, court, 
 or way, to the first, second, third, or fourth rate, and no such pipe 
 is to be fixed on the inside of any building nearer than fourteen 
 indies to any timber, or other combustible material. 
 
MECHANICS. 
 
 269 
 
 Definitions. 
 
 MECHANICS. 
 
 The science of ^fechanics has been very concisely defined, 
 the geometry of motion. It is divided by Sir Isaac Newton 
 into the two branches of practical and rational mechanics. 
 Practical mechanics treats of the six mechanical powers, of one 
 or more of which every machine is composed ; and rational 
 mechanics comprehends the whole theory of motion, shows 
 how to determine the motions produced by given powers or 
 forces ; and, conversely, when the phenomena of the motions 
 are given, how to trace the powers or forces from which they 
 arise. 
 
 Of Matter. 
 
 Every branch of natural philosophy acquaints us with some 
 new properties of matter, the general nature of which, and those 
 properties of it which respect mechanical science, it will here be 
 requisite for the reader to consider. 
 
 I'he terms matter^ substance, and body, though so nearly allied 
 that they are occasionally employed in the same sense, without 
 creating much confusion of ideas, have different significations, 
 which ought to be understood. 
 
 Matter is the most general term of the three, and compre- 
 hends whatever is capable of making resistance, and is possessed 
 of weight, without regard to figure or quantity 
 
 The word substance is compounded of the Latin preposition 
 sub (under,) and the verb stare (to stand,) and approaches very 
 nearly to the signification of the w'ord matter, as it implies that 
 which supports or stands under the different forms and appear- 
 ances which are pre>sented to our senses. Its meaning is, how- 
 ever, more restricted, and it is generally accompanied by the arti- 
 cle, to denote a particular portion of matter. 
 
 The term body comes from the Saxon, and originally signi- 
 fied the person or form of a man, or other creature ; hence it 
 ought to be applied only to a substance possessing a definite 
 form. 
 
 An examination of the general properties of matter wdll afford 
 us a better idea of its nature, than can possibly be given by a de- 
 finition. Some kinds of matter, as metals, wood, stone, &c. are 
 ▼isiWe, a property dependent upon their opacity, or power of re- 
 
270 
 
 MECHANICS. 
 
 Properties of matUr. — Extension. — Divisibility. 
 
 fleeting to our eyes, some or all of tire rays of light which fal 
 upon them. Other kinds of matter are invisible, on account o 
 their perfect transparency, and their existence is ascertained onl 
 by their effects. Of this class are tlie various kinds of gases ; fo 
 example, the atmosphere or air that we breathe, which, thougl 
 totally invisible when dry and pure, yet is matter, as much as iroi 
 or the hardest body in natuie. 
 
 Among the properties attributed to all matter, the followinj 
 are too important to be passed over without particular notice, viz 
 SOLIDITY, EXTENSION, DIVISIBILITY, MOBILITY, INERTIA 
 ATTRACTION, and REPULSION. 
 
 The solidity of matter here meant is not opposed to fluidity 
 but expresses that property which every body possesses of no 
 permitting any other body to occupy the same place with it a 
 the same time. This fact is an axiom in philosophy of tin 
 most incontestable kineJ. If a piece of wood or stone occupy 
 a certain space, it must be removed before another body cai 
 be put into that space, and though the tyro may suggest tha 
 fluids do not oppose such resistance, it is only the facility will 
 which they escape that induces the supposition of their being ai 
 exception to this universal property of matter. Under prope. 
 circumstances, their solidity is as obvious as that of the most solk 
 substance : the piston of a syringe drawn full of water, cannot b< 
 thrust down if the aperture for the jet be stopped ; and a pair o 
 bellows filled with air, cannot be compressed if the pipe b< 
 closed. I'he solidity of matter thus understood, is the same watl 
 what some writers call its impenetrability. These words, in com- 
 mon language, denote the property of not being easily, separatee 
 into parts ; a meaning very different from that attached to them ir 
 the sense just explained, and wLicii should therefore be carefull} 
 remembered. 
 
 Extension is another property of matter inseparable from iff 
 existence. 'Ihe idea which we obtain of. solidity by the resist- 
 ance of bodies, and the impossibility of two bodies co-existing 
 in the same identical place, immediately suggest and prove tc 
 us that matter is extended, or occupies a certain portion oi 
 space. 
 
 Divisihility is that property by which matter is capable ol 
 being separated into parts removeabie from each other. We 
 cannot conceive a particle of matter to Le so small, as not to 
 consist of two halves ; this being the case, we are directly led to 
 the conclusion that matter is capable of being divided to infinity. 
 But, however natural this mode of reasoning appears, it has had 
 many opponents, and those who suppose it just, have been consb 
 dered as involving themselves in a cloud of palpable contradic? 
 
MECHANICS. ’ 
 
 271 
 
 Properties of matter. — Divisibility. 
 
 tioiis. To assume, it has been said, as a first principle in philo- 
 sophy, that matter is infinitely divisible, is to assert that it has no 
 beginning of substance ; that there are no limits between matter 
 and nothing ; and that a finite thing has infinite properties. Such 
 being the difficulties attending the assumption' of the infinite di- 
 visibility of matter. Sir Isaac Newton closes an admirable disqui- 
 sition on the nature, laws, and constitution, of matter, by stating 
 the great probability that God in the beginning formed matter 
 into solid, massive, impenetrable, moveable particles or atoms, of 
 such sizes and figures, and with such other properties, and in such 
 proportion to space, as most conduced to the end for which he 
 formed them ; and that these primitive particles being absolute 
 solids, are incomparably harder than any of the bodies compound- 
 ed of them, even so hard as to be incapable of wearing or break- 
 ing in pieces, nothing but Infinite Power being able to destroy 
 what Infinite Power made one in the first creation. That nature 
 may be lasting, the changes of corporeal things are to be 
 attributed only to the various separations and new associations of 
 tliese permanent particles, and when compound bodies break, 
 it is not in the midst of solid particles, but where these are laid 
 ' together and touch only in a few points. By adopting this 
 theory of ultimate atoms, we avoid the toils of metaphysics, 
 although, at the same time, we take for granted what we cannot 
 directly prove. But those propositions which are proved by the 
 absurdity of supposing the contrary, are often as important, and 
 nearly as well entitled to be received, as those which admit of 
 strict demonstration; and although Sir Isaac Newton’s conjecture 
 respecting solid, indestructable atoms, has been buffeted among 
 men of science for about one hundred years, it remains to be the 
 prevalent opinion at this day ; not because much new light has 
 been thrown upon the subject, since the time of that justly ad- 
 mired philosopher, but because it comports so well with the phe- 
 nomena of nature, that an assent to it can scarcely be denied. 
 The extreme tenuity of certain substances to our general percep- 
 tion, is no proof against their being composed of particles per- 
 fectly solid ; if a wet bladder be tied over the mouth of a pneu- 
 matic jar (that is, a jar open at the bottom,) and then gently dried, 
 so as to remain well stretched, and the jar be then placed upon 
 the air-pump, as soon as it is exhausted of air, the atmosphere 
 pressing upon the exterior, will burst the bladder, and falling upon 
 the pump-plate, will produce a loud report, like a gun. This 
 effect could not be produced without the intervention of solid par- 
 ticles in air. Further, it seems impossible to account for the 
 power of the most subtile agents of nature, if their ultimate atoms, 
 however few tbev may be in a given compass, were not equal in 
 
272 
 
 MECHANICS. 
 
 Properties of matter. — Divisibility. 
 
 solidity to those which appear to us the hardest. Though tht 
 velocity of the electric fluid is immeasurably great, that velocit 
 alone would be insufficient to produce its well-known effects oi 
 bodies of the closest texture, if it contained no principle of hard 
 ness within itself. 
 
 Having premised these considerations, we shall now' take : 
 view of parts actually separate, and if we shall find that thest 
 are so small and so numerous as to surpass imagination, wi 
 shall have approximated as nearly as the human understandiiij 
 can do, to the attainment of an idea of the inconceivabl. 
 minuteness of those solid, ultimate, indivisible atoms, wind 
 constitute matter. A pound of so gross a substance as cotton 
 may be spun into a thread exceeding one hundred miles ii 
 length ; and the celebrated Boyle speaks of a thread of sili 
 three hundred yards in length, which weighed no more thai 
 three grains and a half. But the ductility of gold is still nion 
 astonishing ; a grain of gold can be hammered by the gold 
 beaters, until it will cover fifty square inches, and may bi 
 divided into two millions of visible parts. The gold whid 
 covers the silver wire used in making silver lace, is spread ove 
 a surface twelve limes greater than in the last mentione* 
 instance. In making this wire, a cylindrical bar of silver i 
 strongly gilt, and afterwards converted into wire by drawing i 
 successively through holes diminishing in magnitude, formei 
 in plates of steel. By this means the surface is prodigiousl 
 augmented ; but the wire still remains gilt, and preserves . 
 uniform appearance, even when examined by the microscope 
 Sixteen ounces of gold, which w'ould not occupy more spac- 
 than one cubical inch and a quarter, will completely gild : 
 W'ire sufficient to encompass the whole globe of the earth. Th» 
 metallic particles in acid solutions are still more minutely di 
 vided. A single grain of copper dissolved in an ounce of dilute* 
 nitrous acid, will impart a green colour to a gallon of w ater, o 
 cover one thousand square inches of bright iron with a coat o 
 copper. 
 
 The odour of all bodies that excite the sensation of smel 
 cannot be given out without a w aste of their substance ; yet thi 
 waste is so very small, that is, the fragrant parts of bodies an 
 endued with such prodigious divisibility, as in general, for lonj 
 periods, to occasion no perceptible diminution of the substanc* 
 by which it is sustained. The odour of a grain of musk wil 
 continue for tw'enty years in an apartment where fresh air i. 
 admitted every day. Instances of the wonderful divisibility o 
 matter are very abundant. Gunpowder, when exploded, ex 
 pands to two hundred and forty-four times the bulk it occupiet 
 
MECHANICS. 
 
 273 
 
 Properties of matter. — Divisibility. — Mobility. 
 
 in a solid state; and water, wiien converted into vapour or 
 steam, fills a space eighteen hundred times greater than in its fluid 
 form. 
 
 The wondei's of the organized creation, as laid open to ns 
 by the microscope, are not less extraordinary than any thing we 
 have yet contemplated, as exemplifying the divisibility of mat- 
 ter. Lewenhoeck discovered in the melt of a single cod-fish 
 a greater number of animaiculae than there are inhabitants upon 
 the face of the earth ; and he calculates tliat one thousand 
 millions of such animalculae as are discovered in common water 
 would not equal in magnitude a grain of common sand. 
 Thousands of these minute beings might be contained on the 
 point of a fine needle ; yet, if we suppose them to be furnished 
 with blood, like other animals, and if the globules of their 
 blood bear the same propoition to their bulk as those of a man 
 bear to his body, it may be proved that the smallest visible 
 grain of sand would contain more of these globules than ten 
 thousand of the largest mountains in the world would contain 
 grains of sand. When we have w ell considered the minuteness 
 of the smallest object in the microscopic world, and are 
 apprized that innumerable particles of light must proceed to 
 our eye from every part of that ol^ect, or it would not be 
 visible, w'e may perhaps be led to institute an inquiry respecting 
 the size of the particles of light itself. The result of such an 
 investigation will mock our conception, even if nothing we 
 have previously learned has had this effect. By a calculation 
 apparently w ell conducted, a particle of light has been estimated 
 
 at 
 
 o0,831,z30,1 “48,000 
 
 part of a grain. 
 
 From the preceding statement it is clear, that matter is 
 actually divisible to a greater extent than we can conceive. On 
 the other hand, it is next to an absolute certainty, tliat the 
 hardest and most compact bodies are full of pores or interstices, 
 their particles being either no-where in actual contact, or touch- 
 ing only in a few points. One proof of this is, that the hardest 
 bodies are known to contract by cold, which contraction would 
 be impossible, if their particles were incapable of a nearer ap- 
 proach to each other. 
 
 Mobility expresses the capacity of matter to be moved from 
 one position or part of space to another. 
 
 Space is an abstract idea, and must be described princi- 
 pally by its want of properties. Its extension or capacity is 
 without limits, and it does not consist of parts capable of actual 
 separation from each other ; the division of it therefore is 
 always merely hypothetical. It is incapable of resisting in 
 
 12.— Vo L. I. 2N 
 
274 
 
 MECHANICS. 
 
 Properties of matter. — Inertia. — Attraction. 
 
 any degree the passage of bodies through it; and being per- 
 fectly uniform in all its parts, these cannot be distinguished 
 from each other except by the bodies placed in them. When anv 
 given length, as a yard or a mile, has become familiar to us, we 
 can reduplicate these measures as often as we please, without join- 
 ing them to the idea of body, and we thus obtain our ideas of im- 
 mensity. 
 
 Inertia is the term which designates the passiveness of 
 matter, which, if at rest, will for ever remain in that state until 
 compelled by some cause to move ; and, on the contrary, if in 
 motion, that motion will not cease, or abate, or change its direc- 
 tion, unless the body be resisted. That a body at rest will not 
 move of itself, wdll be readily admitted ; but its tendency to 
 continue the motion once communicated to it, contradicting 
 our ordinary experience, requires a little explanation. We can 
 indeed produce no species of motion which will fully illustrate 
 the proposition by experiment ; but the conclusion seems unde- 
 niable, when we consider the effect produced by diminishing the 
 obstructions to a body in motion. These obstructions are, 
 gravitation, the resistance of the air, and friction. Gravita- 
 tion, as will be understood when we come to treat of it, 
 operates according to established laws, unsusceptible of change 
 or modification by human art ; most of the resistance of the 
 air may be removed by means of the air-pump, but experi- 
 ments with this machine can only be of small extent ; the last- 
 named obstruction to motion, viz. friction, is therefore the only 
 one we have in general the power of diminishing ; and yet in 
 proportion as this one is diminished, we find the motion com- 
 municated to a body by a given impulse, so much increased that 
 we cannot hesitate to consider the action of gravitation and the 
 resistance of the air, combined with the friction yet unde^ 
 stroyed, as the sole causes of its ever ceasing. If a ball be 
 projected along a rough pavement, it will soon stop ; if pro- 
 jected on a level floor, the same force will send it much further ; 
 and on a surface perfectly plane, hard, and smooth, a ball 
 also perfectly hard and smooth, as well as globular, would be 
 carried perhaps five hundred yards, by the same force that would 
 scarcely carry it twenty yards upon the rough pavement. So far, 
 also, as reasoning confirms the explanation given of the inertia of 
 matter, it seems as absurd to suppose that matter once put in mo- 
 tion can stop without a cause, as that when at rest it can move 
 without a cause. 
 
 Attraction denotes the property which bodies have to ap- 
 proach each other. Philosophers enumerate five kinds of attrac- 
 tion, viz. the attraction of cohesion, of gravitation, of electricity, of 
 
MECHANICS. 
 
 275 
 
 Properties of matter. — Attraction of cohesion — gravitation. 
 
 magnetism^ and chemical attraction. Only the two first kinds of 
 attraction belong to our present subject. 
 
 The attraction of cohesion takes place between bodies only 
 when they are at very short distances from each other, and may be 
 exemplified in a variety of ways. If two pieces of lead be scraped 
 clean and smooth, and then strongly compressed, they will cohere 
 almost as firmly as if united by solder. Planes of glass, marble, 
 and other substances, exhibit similar phenomena. That this cohe- 
 sion is not owing merely to the exclusion of the atmosphere, from 
 the evenness of the surfaces employed, is proved by its taking 
 place in vacuo. 
 
 The strength of the attraction of cohesion differing in 
 different kinds of matter, is supposed to be the cause of the 
 relative degrees of hardness in different bodies. It is therefore 
 weakest in fluids; yet substances of this description indicate a 
 disposition to unite. A fluid may be poured into a vessel till it 
 rises above the brim, because the attraction between its particles 
 resists, to a certain extent, its overflowing. From the same 
 cause, the drops of dew upon the leaves of plants, and water 
 thrown upon a dusty floor, where it is prevented from spreading, 
 assume a globular form ; small portions of quicksilver, also, when 
 brought near each other, coalesce, and assume the same globular 
 appearance. 
 
 A fluid contained in a vessel not full to the brim, assumes a 
 concave form, being highest at the edge. If a plate of glass 
 be immersed in water, the water immediately surrounding it 
 will rise above the general surface. If a second plate of glass 
 be immersed in the water, parallel to the former, and at a very 
 short distance from it, the fluid will rise above its level between 
 the two plates, and the nearer they are brought to each other, 
 so as not to touch, the higher will the water rise. If the water 
 be darkened, as by the addition of a little ink, and a glass tube 
 with an exceedingly small bore be placed perpendicularly in it, 
 the rise may be distinctly detected to be very considerable. All 
 effects of this description, to which may be referred the rising 
 of water in a sponge, or other porous bodies, are usually attri- 
 buted to what is called capillary attraction ; but capillary attrac- 
 tion is only a particular modification or branch of the attraction 
 of cohesion. 
 
 The attraction of gravitation, differs from the attraction of 
 cohesion in this respect, that it is exerted at all distances, and 
 by every particle of matter upon every other particle. This 
 principle is the basis of the Newtonian philosophy. The planets 
 and comets all gravitate towards the sun, and towards each 
 other, as well as the sun towards them. The gravitating power 
 
276 
 
 MECHANICS. ' 
 
 Properties of matter. — Attraction of gravitation. 
 
 of a body, is always proportionate to its quantity of matter ; and 
 all the heavenly bodies are retained in their places by the due ba- 
 lance of their action on each other. An effect of gravity, or gra- 
 vitation, familiar to all mankind, is the tendency of bodies to fall 
 to the earth. This tendency is always towards a point, which is 
 either accurately or very nearly in the centre of the earth ; conse- 
 quently bodies fall every-where perpendicularly to the surface, and 
 on opposite sides of the globe, they fall in opposite directions or 
 towards each other. It is not meant that there is any peculiar 
 virtue or charm in the point called the centre ; but because such 
 is the result of the gravitation of bodies towards all parts of which 
 the earth consists. The pressure of bodies to attain, in all cases, 
 the lowest situation possible, or that nearest the centre of the 
 earth, is what constitutes their weight. All substances having a 
 certain degree of gravity ; they have consequently all w eight. 
 Even smoke and vapours are possessed of it, the reason of their 
 rising from the earth being the same as that which causes a piece 
 of w ood to sw im in w ater, viz. they are lighter than an equal bulk 
 of the atmosphere or fluid in which they are disengaged, and there- 
 fore their falling to the ground is as effectually resisted as the fall- 
 ing of a stone supported by the hand. 
 
 As the gravitating force is always proportionate to the quan- 
 tity of matter, the most compact and the most loose, the 
 greatest and the smallest bodies, descend through equal spaces 
 in equal times, unless they fall through a resisting medium, 
 which operates most upon those which have the greatest exten- 
 sion for their weight. If a guinea and a feather were dropt at 
 tlie same instant from the top of a house, no one will be at 
 a loss to say, which would soonest reach the ground; but in 
 the exhausted receiver of an air-pump, these tw'o bodies fall 
 together. The guinea containing more solid matter than the 
 feather, requires more force to put it in motion ; but the attrac-. 
 tive power being proportioned to the quantity of matter, its ve- 
 locity is not greater than that of a body w hich requires less force 
 to put it in motion. Another proof that the gravity of bodies is 
 proportionate to their quantity of matter, is derived from experi- 
 ments on the motion of pendulums. When the lengths of pen- 
 dulums are equal, and they vibrate in equal arcs, they always ac- 
 quire equal velocities at the corresponding points of those arcs, 
 and their vibrations are consequently performed in times exactly 
 equal, however diflerent the bulk and texture of the material of 
 which they are composed. I'he resistance of the air must be un- 
 derstood to be excluded m this experiment, because it acts un- 
 equail) on different bodies, as already exemplified in the guinea 
 and feather experiment. 
 
MECHANICS.' 
 
 277 
 
 Properties of matter.^ — Attraction of gravitation. 
 
 In all places equally distant from the centre of the earth, 
 the force of gravity is nearly equal. The earth is, however, 
 not a perfect globe, btit a little depressed on two opposite sides, 
 partly like an orange. These depressed parts are at the poles, 
 and the polar diameter of the earth has been found to be about 
 thirty-four miles shorter than the equatorial one. The surface 
 of the earth at the equator being therefore seventeen miles fur- 
 ther from the centre than at the poles, the force of gravity there 
 is less than at the poles. It is for this reason that a pendulum 
 calculated to sw ing seconds in the polar regions, must be shortened 
 before it will swing seconds at the equator ; and that bodies at the 
 equator lose part of the weight which they would have at 
 the poles. 
 
 The power of gravity, at any given place, is greatest at 
 the earth’s surface, from whence it decreases both upwards and 
 downwards, but not both ways in the same proportion. The 
 force of gravity upw'ards decreases as the square of the distance 
 from the centre increases ; so that at a double distance from the 
 centre above the surface, the force would only be one-fourth of 
 what it is at the surface. The surface of the earth is, in 
 round numbers, four thousand miles from the centre ; if then 
 a body at the surface weighs four pounds, and falls through 
 sixteen feet in a second of time, it will at double this distance 
 from the centre weigh but one pound, and will fall through but 
 four feet in a second of time. Below the surface of the earth, 
 the power of gravity diminishes in such a manner that its inten- 
 sity is in the direct ratio of the distance from the centre, and 
 not as the square of the distance ; so that at the distance of two 
 thousand miles, which is half a semi-diameter from the centre, the 
 force w ould be but half what it is at the surface ; at one-third of a 
 semi-diameter, the force would be one-third, and the same ratio is 
 applicable to all other distances. But although the force of gra- 
 vity, strictly speaking, varies in the manner just stated, in receding 
 from the surface, its operation at short distances is considered uni- 
 form, a quarter or even half a mile bearing so small a proportion 
 to the earth’s radius, that the difference is too insignificant to be 
 noticed in calculations. 
 
 As the power of gravity appertains to every particle of 
 matter, and the gravitating power of entire bodies consists of 
 that of all their parts, under certain circumstances the gravity 
 of a part of the earth somewhat counteracts that of the whole 
 earth. Thus, the attraction of a lofty mountain is found to 
 draw a plumb-line at the foot of it a little out of the perpendicu- 
 lar, so that in such a situation it does not tend to the centre of the 
 tarth. 
 
278 
 
 MECHANICS. 
 
 Properties of matter. — Attraction of gravitation. 
 
 The space which bodies actually fall through is sixteen feet 
 and one-twelfth in the lirst second of time, in the latitude of 
 London ; and for other times either greater or less than that, the 
 spaces descended from rest, are directly proportional to the 
 squares of the times, while the falling body is not far from the 
 earth’s, surface. 
 
 If two bodies, which contain equal quantities of matter, were 
 placed at ever so great a distance from one another, and then left 
 at liberty in space, and if there were no other bodies in the uni- 
 verse to affect them, they would fall equally swift towards one ano- 
 ther, with a velocity continually accelerated, and would meet in a 
 point which w as at first exactly half way between them. But if 
 two bodies, containing unequal quantities of matter, were placed 
 at any distance, and left in the same manner at liberty, they would 
 fall towards one another, with velocities which would be in an in- 
 verse proportion to their respective quantities of matter ; moving 
 as ill the former case with an accelerated motion, they would meet 
 in a point as much nearer to the place from wdiich the heavier 
 body began to fall, than to the place from which the lighter began 
 to fall, as the quantity of matter in the former exceeded that in the 
 latter. 
 
 That gravity should accelerate the descent of falling bodies, is 
 an effect of its uniform action under all circumstances. Let us 
 suppose that it causes a body to descend through the space of one 
 mile in a minute ; at the end of this time, the body w ill have ac- 
 quired a velocity sufficient to carry it through two miles the next 
 minute, although it received no new impulse from gravity ; but as 
 this accelerating cause remains, it adds another mile to its effect in 
 the first minute, and therefore, at the expiration of two minutes, 
 the body will have descended through four miles. 
 
 The spaces described by a uniformly accelerated motion, are 
 always as the odd numbers, 1, 3, 5, 7, &c. and consequently the 
 whole spaces are as the squares of the times, or of the last acquir- 
 ed velocities ; for the continued addition of the odd numbers yields 
 the squares of all numbers from unity upwards. Thus 1 is the 
 first odd number, and 1 is the square of one ; 3, the next number, 
 added to 1, makes 4, which is the square of two ; 3, added to four 
 makes 9, the square of 3, and so on. The times and velocities pro- 
 ceeding evenly and constantly, as 1,2, 3, 4, &c. and the spaces de- 
 scribed as 1, 3, 3, If &c. it follows that the spaces described 
 
 In 1 minute will be 1, which is the square of 1 
 
 In 2 minutes will be....l-{-3 = 4, — — 2 
 
 In 3 minutes will be . . 1-f 34-5—9, — — 3 
 
 In 4 minutes will be 14-34-54-7 = 16, — — 4 
 
MECHANICS. 
 
 279 
 
 Distinction between gravity anJ weight. 
 
 Hence it is apparent that the spaces described in different 
 times by a falling body, are to each other as the squares of the 
 times from the beginning of the descent, or, which produces the 
 same result, they are as the squares of the velocities acquired at the 
 ends of those times. 
 
 The motion of a falling body being uniformly accelerated by 
 gravity, the same cause uniformly retards the motion of a body 
 thrown directly upwards. A body projected perpendicularly, with 
 a velocity equal to that which it would have acquired by failing 
 from any height, will ascend to the same height before it loses all 
 its velocity. 
 
 Gravity and weighty it ought to be understood, are not inter- 
 changeabie terms. Gravity is a power of which weight is the ef- 
 fect. Gravity has a constant tendency to impress, on every parti- 
 cle of bodies, a certain velocity, which would cause them to fall if 
 they were hot supported ; weight is the resistance necessary to de- 
 stroy this velocity, or produce this support. 
 
 When the many and wonderful discoveries which signalize 
 the present age are considered, it seems presumptuous to mark 
 the boundaries of success to human inquiry ; but we may very 
 safely assert, that no researches have ever yet been made, which 
 enable us to discover, in the essential properties of matter, the 
 cause of gravity. Of the existence of such a power, we are 
 continually surrounded by the most indubitable proofs ; and 
 that its influence extends over and governs the solar system, 
 and if the solar system, the whole material universe, there 
 seems as little reason to deny. Sir Isaac Newton has conjec- 
 tured that matter is composed of indivisible, perfectly solid 
 particles or atoms, a theory that has been explained in the 
 former part of this subject ; but if the ultimate particles of 
 matter be perfectly solid, they cannot be pervaded ; if they be 
 incapable of wearing or separation, they can throw nothing 
 off; and if no single atom can receive or part with any thing, 
 how can it act at all distances upon every other portion of 
 matter in the universe ? or how can any aggregation of atoms 
 possess a power incompatible with the nature of its component 
 parts f Such is a slight view of the argument on this subject ; 
 it is not our object to involve the reader in the mazes of useless 
 theories, though we may, for his amusement, and the exercise 
 of his judgment, occasionally glance at them and pass on. 
 In various branches of knowledge, we discover abundant 
 proofs that the effects we observe are produced by secondary 
 causes, that is, these effects spring bom the inherent properties, 
 the properties originally impressed upon things by the Creator ; 
 but in the absence of the secondarv cause of gravity, or until 
 
280 
 
 MECHANICS. 
 
 Properties of matter. — Repulsion. 
 
 we can prove it to be dependent upon the essential properties of 
 matter, gravity should be considered as a term expressing 
 merely a fact or phenomenon ; and it is wise to refer the cause 
 of it at once to the Final Cause of all things, and to regard 
 it as the Finger of God, the constant impression of Divine 
 povver.’^ 
 
 Repulsion is the last property of matter which we have 
 enumerated. A variety of considerations induce philosophers 
 to admit that there is a .sphere of repulsion which extends to a 
 small distance round bodies, and prevents them from coming 
 into actual contact with each other, except some force is 
 exerted to overcome this resistance, and then the attraction of 
 cohesion takes place. Dr. Kniglit defines repulsion to be that 
 cause which makes bodies mutually endeavour to recede from 
 each other, witli different forces at different times ; and that 
 such a cause exists in nature, he thinks evident for the follow- 
 ing reasons : 1. Because all bodies are electrical, or capable of 
 
 being made so ; and it is well known that electrical bodies both 
 attract and repel. 2. Both attraction and repulsion are very 
 conspicuous in all magnetical bodies. 3. Sir Isaac Newton has 
 shown from experiments, that the surfaces of two convex glasses 
 repel each other. 4. The same great philosopher has explained 
 the elasticity of the air by supposing its particles mutually to 
 repel each other. 5. The particles of light are, in part at least, 
 repelled from the surfaces of all bodies. 6. Lastly, it seems 
 highly probable that the particles of light mutually repel each 
 other as well as the particles of air. The Doctor ascribes the 
 cause of repulsion, as well as that of attraction, to the imme- 
 diate effect of the will of God ; and as attraction and repulsion 
 are contraries, and consequently cannot at the same time belong 
 to the same substance, he supposes there are in nature two 
 kinds of matter, one attracting, the other repelling ; and that 
 those particles of matter which repel each other, are subject to the 
 general law of attraction in respect of other matter. A repellent 
 matter being thus supposed equally dispersed through the whole 
 universe, the Doctor refers to its operation a variety of natural 
 phenomena ; but whether, on his hypothesis, all the particular 
 effects of repulsion can be accounted for, time and experience 
 alone must determine. 
 
 In the instance which has already been adduced of the 
 round drops of dew upon the leaves of plants, it is supposed 
 not only that there exists an attractive force between the par- 
 ticles of the fluid, but a repulsive force between them and the leaf 
 upon which they are suspended. That the drops are not in 
 actual contact with the leaf, is evident from their rolling off in 
 
MECHANICS.' 
 
 281 
 
 Motion either absolute or relative. 
 
 a compact body with the greatest ease ; as well as from their 
 white or pearly appearance, which is an etfect of the copious 
 reflection of white light from the flattened part of the surface 
 contiguous to the plant, and could not take place unless there 
 was a real interval between the under side of the drop and the 
 surface of the leaf. The power of repulsion will, in certain 
 cases, cause metals to swim in fluids much lighter than them- 
 selves. A fine needle, if gently laid on the surface of water, 
 will swim, and may be drawn off again by a magnet, w'ithout 
 having in reality touched the fluid. In this instance, the needle 
 is not heavy enough to overcome the power of repulsion between 
 itself and the water, the attraction of cohesion cannot therefore 
 operate, and though as much heavier than its own bulk of water 
 as the largest piece of steel, it will float till pressed down by a 
 greater force than its own w^eight. It would appear that, from 
 the same cause, flies walk upon water, and oil refuses to mix with 
 that fluid. Hence the feathers of water-fowl, which are covered 
 with a thin coating of subtile oil, actually repel the surrounding 
 water. 
 
 Of Motion. 
 
 No definition can be given of the term Motion which will sa- 
 tisfy the casuist. It expresses a simple idea, and cannot be ex- 
 plained by words more simple than itself. It has been called a 
 change of place,” or the act by which a body corresponds w'ith 
 different parts of space at different times ; but it would require 
 great ingenuity to prove that these definitions amount to more than 
 the assertion that motion is motion.” Perhaps that philosopher 
 answered the question of “ what is motion ?” with as much per- 
 spicuity at least as any other, who began to walk, and observed to 
 his inquirer that “ that w as motion.” 
 
 It is by motion alone that we know the existence of bodies, 
 and that a relation is established between them and our senses. 
 Nothing can be produced or destroyed without motion, and every 
 thing that happens depends upon it. 
 
 Space being nothing but an absolute and infinite void, the 
 place of a body is that part of the immense void which it takes 
 up or possesses ; and this place may be considered absolutely or 
 in itself, in which case it is called the absolute place of the 
 body ; or else with regard to the place of some other body, and 
 then it is called the relative or apparent place of the body. As 
 the place of a body may be considered absolutely or relatively, 
 so may the motion of a body be distinguished in like manner. 
 All motion is in itself absolute, or the change of absolute space j 
 but when the motions of bodies are considered and compared 
 12. — VoL. I. 2 0 
 
282 
 
 MECHANICS. ' 
 
 Difficulty of judging correctly of motion. 
 
 with each other, then are they usually denominated relative and ap- 
 parent only : they are relative, as they are compared to each other; 
 and they are apparent only, insomuch that not their true or abso- 
 lute motion, but the sum or difference of the motions only is per- 
 ceivable to us. Hence the absolute and relative motions of bodies 
 may be different and even contrary. 
 
 If two ships set out and sail together in the same direction and 
 with the same velocity, neither of them will appear to the other to 
 move. Hence it is, that though the earth is continually revolving 
 about its axis, and advancing in its orbit, yet, as all objects on its 
 surface partake of the same common motion, they appear not to 
 move at all, but are relatively at rest. 
 
 If two ships set sail at the same moment, in the same direc- 
 tion, but one of them sails only three miles while the other sails 
 five miles an hour, the difference of their velocities, viz. two miles 
 per hour, will alone be perceptible to a spectator in either of 
 them, looking at the other. 
 
 But if two ships pass each other, the one will appear to the 
 other to move with the sum of both velocities ; so that in this case 
 the apparent motion exceeds the true, as in the other instances it 
 fell short of it. The reason of these phenomena of motion will 
 be evident, if we consider that we must be absolutely at rest, if we 
 would discern at once the true or real motions of bodies about 
 us. But as at absolute rest we can never be, from the motion 
 of the earth, we must detect the real and absolute motions of bo- 
 dies in general by means of observations made on their relative 
 motions. 
 
 We are best acquainted with that kind of relative motion 
 which consists in the transfer, from one place to another, of 
 entire bodies, as the falling of a stone, or the flight of an arrow. 
 But, besides this, there is another kind of relative motion, 
 which, though not so obvious, is not less common or import- 
 ant. We allude to the motion of the parts of bodies among 
 themselves, which though sometimes the object of our senses, 
 yet, in other cases, we require the aid of reflection to be con- 
 vinced of its existence. It is by this imperceptible motion 
 that plants and animals grow, and by which the greatest num- 
 ber of the compositions and decompositions of the globe take 
 place. We may form some idea of this, by observing the 
 continual motion of the light particles which sometimes float 
 about in water, when it is held in the rays of the sun ; which 
 proves that the parts of the water are in constant motion among 
 tliemselves. But if we reflect a little, we shall discover that the 
 particles of the most solid substances are also continually 
 changing their situations. Heat expands and cold contract* 
 
MECHANICS/ 
 
 28S 
 
 The various circumstances attending motion. 
 
 the size of all bodies ; and as we know from experience, that the 
 temperature of bodies is constantly varying, their particles 
 must consequently be in continual agitation, in order to adapt 
 themselves to the ever-changing size of the body. This is one 
 of the causes of the perpetual motion of the particles of matter ; 
 but there are no doubt an infinite number of other causes which 
 escape our observation, and which we are perhaps incapable of 
 discovering. The gradual changes that take place in all bodies 
 during a series of years, sufficiently prove that they are constantly 
 acting on each other ; and from a view of all that we know on this 
 subject, we are compelled to conclude that no particle of matter 
 is in a state of absolute rest. 
 
 In considering motion, the several circumstances attending tht 
 communication of it from one body to another may be classed af 
 follows : 
 
 1. The force which impresses the motion. 
 
 2. The quantity of matter in the moving body. 
 
 3. The velocity and direction of the motion. 
 
 4. The space passed over by the moving body. 
 
 5. The time employed in going over this space. 
 
 6. The force with which the moving body strikes another 
 which is opposed to it. 
 
 In a mechanical sense, the inertia of every body causes it to 
 resist all change of state. If at rest, it will not begin to move of 
 itself ; and if motion is communicated to it by another body, it 
 will continue to move for ever uniformly, except it be stopped by 
 an external agent. This has been shown in treating of inertia. 
 The causes by which bodies are put in motion, are called motive 
 powerSy of which the following are those generally used in me- 
 chanics : the action of men and other animals, wind, water, gra- 
 vity, the pressure of the atmosphere, and the elasticity of fluids 
 and other bodies. 
 
 The velocity of motion is estimated by the time employed in 
 moving over a certain space, or by the space moved over in a 
 certain time. The less the time, and the greater the space 
 moved over, the greater of course is the velocity ; and, con- 
 versely, the greater the time, and the less the space moved over, 
 the less is the velocity. As no motion can be instantaneous, every 
 body in motion must have a determinate velocity. To ascertain 
 the degree of this swiftness or velocity, the space run over must 
 be divided by the time. For example, suppose a body moves 
 over 1000 yards in ten minutes, its velocity is 100 yards per mi- 
 nute, because 100 is the quotient of 1000 divided by 10. If we 
 would compare the velocities of two bodies, A and B, of which A 
 moveip ^4 yards in 9 rainjates, and B 96 yards in 6 minutes, th« 
 
^84 
 
 MECHAKICS. 
 
 Compound motion. 
 
 velocity of A will be to that of B in the proportion of 6 to 16, 
 because the quotient of 54 divided by 9, is 6 ; and the quotient 
 of 96 divided by 6, is 16. 
 
 To know the space run over, the velocity must be multi- 
 plied by the time ; for it is evident, that if either the velocity 
 or the time be increased, the space run over will also be 
 increased. If the velocity be doubled, then the body will 
 move over twice the space in the same time ; or if the time be 
 twice as great, then the space will be doubled ; but if the 
 velocity and time be both doubled, then will the space be four 
 times as great. Hence when two bodies move over unequal 
 spaces in unequal times, their velocities are to each other as the 
 quotients arising from dividing the spaces run over by the times. 
 If two bodies move over unequal spaces in the same time, their 
 velocities will be in proportion to the spaces passed over. Again, 
 if tw'O bodies move over equal spaces in unequal times, then 
 their respective velocities will be inversely as the time employed ; 
 that is, if A in one minute, and B in two minutes, run over 
 one hundred yards, the velocity of A will be to that of B as two 
 to one. 
 
 A body in motion must every instant tend to some particular 
 point. It may either always be to the same point, in which case 
 the motion will be rectilinear ; or it may be continually changing 
 the pomt to which its motion is directed and this will produce a 
 curvilinear motion. 
 
 AVhen a body is acted upon by one force, or by several 
 forces in the same direction, its motion will be in the same 
 direction as that in which the moving force acts ; as the motion 
 of a boat which a man draws to him with a rope. But if 
 several powers differently directed, act upon a body at the 
 same time, it will not exactly obey any of them, but will move 
 in a direction somewhere between them. This subject may be 
 rendered more plain by a diagram, and the illustration of it 
 oguht to be w'ell considered by the young mechanic. Let a 
 body A, fig. 1. pi. I. be impelled by a force acting on it in the 
 direction AC. At the same instant, let it be impelled towards 
 B, by another force that will carry it direct from A to B in the 
 Same time that the former force would carry it from A to C. 
 Complete the parallelogram, ACBD, and draw the diagonal 
 AD, and this line will represent the direction and distance the 
 body will move in the same time when acted upon by both forces 
 conjointly; for let us suppose a tube, equal to AB in length, 
 in which a ball. A, can move freely ; and that in the same time 
 that the ball is moving uniformly from A to B, the tube is also 
 moving uniformly from A to C, but so as to be ^ways oaralkl to 
 
MECHANICS. 
 
 285 
 
 Compound motion. 
 
 AB, and its extremities describing the lines AC to BD. The 
 ball has moved from A to B in the tube, in the same time that 
 the tube has descended to CD, and therefore when the tube 
 coincides with the line CJ), the ball will be at the extremity, 
 D, of that line, where it has arrived in the same time that it 
 would have taken to describe either side. It is obvious also, 
 that the ball thus subjected to the impulse of different forces, 
 can have described no other line than the diagonal one ; for by 
 assuming smaller forces, and forming the parallelograms A ef g, 
 A h i kf &c. it will be found at every interval, in the diagonal 
 I of the parallelogram. The motions along AB, AC, may be 
 called the simple or constituent motions ; the motion along AD 
 is called the compound or resulting motion. Hence if we know 
 the effect which the joint action of two forces have upon a body, 
 and the force and direction of one of them, it is easy to find 
 that of the other : for suppose AD to be the direction and force 
 with which the body moves, and AB to be one of the impelling 
 forces, then, by completing the parallelogram, the other power is 
 found. 
 
 The practice of reducing compound forces to simple, and that 
 of finding two or more forces equivalent to one, is called the com- 
 position and resolution of forces, the whole theory of which is 
 comprised in and may be deduced from the following princi- 
 ple : two forces acting at the same time on a body, in directions 
 which are oblique to each other, do not move the body by that 
 part of their force, which, on account of their obliquity, is oppo- 
 site and contrary, but by what remains after the opposite force# 
 are deducted. 
 
 Instances of motion produced by several powers acting at the 
 same time are innumerable, and the application of this useful 
 principle, by which they are governed, is therefore very extensive. 
 A ship impelled by the wind and tide is one well known. A kite, 
 acted upon by the wind and the string ; the rain and snow that 
 fall more or less obliquely according to the action of the wind ; 
 are other instances not less familiar. A fish, by striking the water 
 with its tail, advances forward in a mean direction betw^een the 
 two impulses. — In jumping out of a carriage in motion, accidents 
 frequently occur, and the adventurer falls short of the spot he aims 
 at, for want of duly considering, or not knowing, that the lateral 
 impulse he gives himself must, in a ratio proportionate to the 
 swiftness of the carriage, be greater than if he sprung from a state 
 of rest. 
 
 Motion is said to be accelerated if its velocity continually in- 
 creases ; and it is said to be uniformly accelerated, if its velocity 
 increases equally in equal times. 
 
286 
 
 MECHANICS. 
 
 Illustration of accelerated motion. 
 
 Motion is said to be retardedy if its velocity continually 
 f?ccreases ; and to be uniformly retarded, if its velocity decrease 
 equally in equal times. 
 
 If a body be put in motion by a single impulse, and, 
 moving uniformly, receive a new impulse in the same direction, 
 its velocity will be augmented, and w\th that augmentation of 
 velocity it will again proceed uniformly. But if at each 
 instant of its motion it receives a new impulse, its velocity will 
 be continually increasing; and if this impulse is always equal, 
 and acts in equal times, the velocity will be uniformly accele- 
 rated. 
 
 On the contrary, if a certain velocity be given to a body, and 
 it loses equal portions of that velocity, at each equal instant, by 
 new impulses acting in a direction exactly opposite to its motion, 
 it wull be uniformly retarded. 
 
 The effect of gravitation in uniformly accelerating the 
 descent of a body, and uniformly retarding one thrown directly 
 upwards, has been shown in the last section ; but as a right 
 notion of this doctrine is very important, in adverting to it 
 again, w^e shall endeavour to exhibit it to the eye as well as the 
 understanding. Let the perpendicular line AB, of the right- 
 angled triangle ABC, be considered as expressing the time 
 which a body takes in falling, under the influence of gravity or 
 any accelerating force, and the base line BC, as expressing the 
 velocity acquired at the end of the fall. The time expressed by 
 the line AB is divided into four equal parts or moments, A r,r s, 
 St, t'Q. The close parallel lines in the triangle A r k, repeated 
 at equal intervals, and from the nature of the triangle, regularly 
 increasing in length as they recede from the point A, denote equal 
 accelerations of the velocity from the instant in wdiich the body 
 begins to fall. The line r k, therefore, will represent the velo- 
 city acquired by a falling body in the first moment of time ; s I, 
 the velocity acquired at the end of the second moment of time ; 
 t 0, the velocity at the end of the third moment, and BC the 
 velocity at the expiration of the fourth moment, or termination of 
 the fall. 
 
 The body, during the second moment of time, if retaining 
 only the velocity r k, which it had acquired at the end of the 
 first, wdll describe the square surface r k sm; for this surface is 
 generated by a continual repetition or motion of the line r k, 
 during the time expressed by rs; as the area of the triangle 
 a r k, is described by a uniformly increasing velocity during 
 the time A r. But the area of the square is manifestly double 
 the area of the triangle above it ; whence it appears, that a 
 body moving on, during the second moment, with the velocity 
 
MECHANICS. 
 
 287 
 
 Illustration of accelerated motion. 
 
 acquired at the end of the first, will fall twice as far in the second 
 moment as in the first ; and the rule deducible from this instance 
 will universally hold, that is, the velocity acquired at the end of 
 any given time, will carry the body twice as far in the same time. 
 In pursuing the illustration of the figure, this will still further 
 appear. 
 
 If the velocity continue to increase uniformly during the second 
 moment, then the space wdll be as expressed by the area r s I k, 
 and will be equal to three times the triangle A r k. 
 
 The whole space described by the body in the two first mo- 
 ments will be as the area A si, which is four times greater than 
 that of Ark; rendering it apparent that the space described by a 
 body in its fall, is as the squai e of the time in which it falls ; for 
 here the time is 2, (because A s expresses two moments of the de- 
 scent,) and the square of two is 4. 
 
 In the third moment, were the body to fall wuth the velocity 
 s /, during the time s t, the space described will be as the rectan- 
 gle under the time and velocity, that is, as the rectangular space 
 sltn, on which rectangle may be described four triangles, each 
 equal to A r A' ,• but as the velocity is still uniformly accelerated 
 by the continued action of gravity, the space fallen through in 
 the time s t, or third moment, will be as the area st ol, or five 
 times as great as A r k. 
 
 As the triangles Ark, A si, A to, ABC, are all similar; 
 as A s is twice as much as A r, si will be twice as much as r k; 
 and as A s expresses the time, and s I the velocity, w here the 
 time is double the velocity is double. This rule applies to 
 every part of the descent, and proves that the velocity is as the 
 time. 
 
 If the spaces described in each moment be considered sepa- 
 rately, the space in the first moment being 1, the space in the se- 
 cond moment, it will be obvious to inspection, is 3, in the third 5, 
 in the fourth 7, the difference each time being 2. 
 
 The motion of a body ascending from B to A, and therefore 
 uniformly retarded by the action of gravity, may be illustrated by 
 the same figure, if we change only a few of the terms of expla- 
 nation ; thus BA will express the time which the body takes to 
 rise to A, and any horizontal line compared with the base, as t o, 
 si, r k, will show the velocity lost at the height at which it is 
 drawn. 
 
 It is to be understood that the velocity above assigned to fall- 
 ing bodies, is that which they would acquire if they passed through 
 a space where there was no air ; but in fact the resistance of that 
 fluid considerably diminishes the velocity acquired in falling, even 
 when the body, from its density, is of a kind least affected by its 
 
288 
 
 MECHANICS. 
 
 Motion of projectilee. 
 
 action. A leaden bullet dropt from an altitude of two hundred 
 and seventy-two feet, was found by Dr. Desaguliers to reach the 
 ground in four seconds and a half; in which time, from theory, it 
 should have descended through 325.6 feet, which makes a differ- 
 ence of about one-fifth of the actual descent between the experi- 
 ment and the theoiy. 
 
 It has already been shown, that if two forces act uniformly 
 upon a body, they will cause it to move in a straight line ; but 
 if one of the forces is not uniform, but either accelerating or 
 retarding, the moving body will describe a curve. A ball 
 projected from a cannon would always proceed in a right line, 
 if it were acted upon by no force except the impulse it received 
 from the powder; but as soon as it leaves the mouth of the can- 
 non, gravity acts upon it, and changes its direction. The ball, 
 acted upon only by gravity and the original impulse, would 
 describe a peculiar curve, called a parabola; but as the resist- 
 ance of the air also contributes to the variation of the line de- 
 scribed, and this resistance differs with the velocity of the ball, 
 its path is not exactly determinable. In some cases, the resist- 
 ance amounts to more than twenty times the weight of the ball ; 
 and when a ball moves with a velocity of two thousand feet per 
 second, the amount of this resistance has been found to be one 
 hundred times its weight. Hence the parabolic theory of projec- 
 tiles is inapplicable to practice. Sir Isaac New ton has, indeed, 
 shown that the curve described by a projectile approaches more 
 nearly to an hyperbola than a parabola ; and that the resistance to 
 the body is not proportional to the velocity itself, but to the square 
 of the velocity. About two hundred years ago, philosophers took 
 the line described by a body projected horizontally, such as a ball 
 out of a gun, while the force of the powder greatly exceeded the 
 W'eight of the ballet, to be a right line, after which they allow'ed it 
 became a curve. Nicholas Tartaglia was the first who maintained 
 that its path w^as a curve through the whole of its extent ; but it 
 was Galileo w ho determined the curve to be a parabola in a non- 
 resisting medium. 
 
 The force with which a body moves, or wdiich it would exert 
 upon another body opposed to it, (force being constantly measured 
 by its effects,) is always in proportion to its velocity multiplied by 
 its weight or quantity of matter. ’^Flns force is called the momen- 
 tum of the body. If two equal bodies move with different velo- 
 cities, their forces or momenta are as their velocities ; and if two 
 bodies move with the same velocity, their momenta are as their 
 quantities of matter ; therefor.e, iu all cases, their momenta must 
 be as the products of their quantities of matter and their velocitiei. 
 This rule is the foundation of mechanics. 
 
mechanics/ 
 
 289 
 
 Proofs that action and re-action are always equal. 
 
 Lam of Motion^ 
 
 A summary of the doctrine of motion has been reduced to 
 three axioms, which are usually called the laws of motion, and are 
 as follow : 
 
 I. All bodies are perfectly indifferent to motion and rest, that 
 is, they are incapable when at rest of moving, and when in mo- 
 tion of stopping, without the action of an external cause. 
 
 II. The alteration of the state of any body, whether from rest 
 to motion, or from one degree of motion to another, is always 
 proportionate to the force which is impressed, and in the direction 
 of that force. 
 
 III. Re-action is always equal to action; or, in other words, 
 the actions of two bodies on each other are always equal, and 
 exerted in opposite directions. 
 
 The proofs of the first and second of these laws have been 
 shown in the preceding sections, from which they are obviously 
 deducible. The last is not so clearly implied ; though as easily 
 admitted to be true on a little consideration. That any body 
 acting upon another loses as much force as it communicates. 
 Mill be evident, if with a bullet suspended from a string we strike 
 another bullet which is at rest, in which case the striking body 
 will lose half its quantity of motion, and what it loses will be 
 communicated to the other body. If the finger be pressed upon 
 one scale of a balance, to counterpoise a weight in the other 
 scale, the scale pressed by the finger acts against the finger with 
 a force equal to that with which the other scale endeavours 
 to descend. A great variety of facts might be adduced to the 
 same purpose. If a man in a boat draws another boat to him, by 
 means of a rope, the two boats will approach each other with 
 equal quantities of motion. When a load is drawn by a horse, 
 the load re-acts against the motion of the horse, and the progres- 
 sion of the animal is as much impeded by tlie load, as the mo- 
 tion of the load is promoted by the efforts of the horse; and sup- 
 pose the animal to possess a force equal to one hundred, and the 
 force necessary to keep the traces tight be equal to fifty, it will 
 only be able to draw with the remaining force of fifty. When a 
 cannon is discharged, the rarefied powder presses it backwards 
 and the ball forwards with equal force, though the velocities are 
 very different. If the ball weighs lOlbs. and the cannon and car- 
 riage 10,000lbs. the velocity of the ball will be a thousand times 
 greater than that of the cannon, but the quantity of motion equal. 
 The water by its re-action communicates to an oar as much motion 
 as it receives, and therefore impels a boat: fishes swim and biids 
 ffy upon the same principle. 
 
 13.~Vol. I. 2 P 
 
290 
 
 MECHANICS. 
 
 Central forces. — Centre of gravity. 
 
 Of Central Forces, 
 
 All moving bodies endeavour to obtain a rectilinear motion, 
 because it is the shortest and most simple ; whenever, therefore, 
 they move in a curve, there must be something that draws them 
 from their rectilinear motion, consequently we may be certain they 
 are acted upon by two powers at the least, and were the detaining 
 power to cease, the moving body would instantly fly off in a 
 straight line. 
 
 That force by which a body describing a curve endeavours to 
 fly off in a straight line, is called the centrifugal force ; and the 
 opposite force, or that by which a body is every-where impelled, 
 or in any manner tends tow’ards some point as a centre, is called 
 the centripetal force. If a bullet fastened to a string held in the 
 hand be whirled round, and the string be broken or let loose, the 
 bullet immediately flies off in a tangent to the circle it was pre- 
 viously describing ; in this case, the string represents the centri- 
 petal force, and the power it has acquired to fly off, is the centri- 
 fugal one. 
 
 The centrifugal and centripetal forces are called together cen- 
 tral forces. 
 
 Of the Centre of Gravity. 
 
 ^ In every body, there is a certain point called the centre of gra- 
 vity, the nature and properties of which require our attention, be- 
 fore we begin to treat of the mechanical pow ers. 
 
 The centre of gravity is that point in a body, about w hich all 
 its parts exactly balance each other in every position. 
 
 If a body be suspended or supported by the centre of 
 gravity, it will remain at rest in any position indifferently ; and 
 whatever supports this point bears the whole weight of the body, 
 which, while so supported, cannot fall. The whole weight of 
 a body may therefore be considered as centred in this point ; 
 and mathematicians, by the place of a body, often mean that 
 point where the centre of gravity is situated. A body suspended 
 by any otlier point, can rest only in two positions, viz. when the 
 centre in question is either exactly above or below the point of 
 suspension. 
 
 , To balance a stick which is of equal thickness throughout 
 across a finger, or any other narrow support, every child 
 knows that he has only to find the middle of it ; to balance one 
 which is thicker at one end than the other, he knows that he 
 must place the support nearer the extremity of the heavy end 
 „ than the other, and that the difference of length on each side 
 of the balancing point must be proportionate to the difference 
 
MECHANICS/ 
 
 2.91 
 
 Centre of gravity. — Line of direction. 
 
 n the weight of the two ends. On the same principle, the 
 common centre of gravity of t\vo equal bodies, is just midway 
 between them ; but when the two bodies are unequal, it is 
 flearer the greater body in proportion as it is heavier than the 
 other or the distances from the centre are inversely as the 
 weights of the bodies. Let A, fig. 3, pi. I. be greater than B ; 
 join AB, upon which take the point C, so that CA : CB : : B : A, 
 that is, if the weight of A be multiplied by the distance AC, and 
 the product be the same as that of the weight of B multiplied by 
 the distance BC, then C is the centre of gravity of the two bodies 
 A and B. If the common centre of gravity of three bodies be 
 required, first find C, the centre of gravity of A and B ; and sup- 
 posing a body to be placed there equal to the sum of A and B, 
 find G, the common centre of gravity of it and D ; then will G 
 be the common, centre of gravity of the three bodies A, B, and D. 
 In a similar manner the centre of gravity of any number of bodies 
 may be determined. 
 
 As gravity always acts in a direction perpendicular to the 
 horizon, and as if the whole weight of a body were collected in 
 the centre of gravity, this point always endeavours to descend 
 in a vertical line, and with a force equal to the body’s weight. 
 Hence a vertical line passing through the body’s centre of 
 gravity, is called the line of direction. While the line of direc- 
 tion falls within the base upon which a body stands, the body 
 cannot descend, but if it fall without the base, the body will 
 fall. Thus the inclining body ABCP, fig. 4, pi. I. whose 
 centre of gravity is E, stands firmly on its base CD, because 
 the line of direction, Elf> falls within the base. But if we 
 attempt to set up a longer body, GHIK, fig. 5, with the same 
 inclination, the centre of gravity L will be more elevated, and 
 the line of direction LM falling without the base, it will drop 
 the instant it is left to itself. The same effect would be pro- 
 duced by placing a sufficient weight on the top of the former 
 body, fig. 4, for every such addition of weight would raise the 
 centre of gravity, and therefore the weight w^ould be sufficient 
 to cause the fall of the body, the moment it was high enough to 
 cause the line of direction to fall without the base, as shown by 
 fig. 5. 
 
 From the preceding observations, we may deduce the 
 danger and absurdity of people’s rising in a boat or other 
 vehicle which is likely to be overset; for by that means they 
 raise the centre of gravity, and a swing which would not have 
 been attended with the least hazard while they were sitting, will 
 then throw the line of direction beyond the base, and thus 
 render their being overset inevitable. If, instead of rising, th^ 
 
292 
 
 MECHANICS. 
 
 Line of direction. — Experiment with a double cone. 
 
 people had crouched as low as possible, many a boat’s party 
 would have saved themselves. 
 
 As the broader the base, and the nearer the line of direction 
 is to the middle of it, the more firmly does the body stand ; so, 
 on the contrary, the narrower the base, and the nearer the line 
 of direction is to the side of it, the more easily may the body 
 be overthrown, because a less change of position is sufficient to 
 remove the line of direction out of the base, and a less force is 
 required to effect that change of position. It is for this reason 
 that a sphere is so easily rolled upon a horizontal plane, and 
 that it is so difficult to make any body stand upright on a 
 point. 
 
 Various contrivances have been executed, taking their rise 
 from the principle, that the centre of gravity always tends to 
 the lowest place possible, as the manner of suspending the 
 marine barometer, compasses, &c. An experiment frequently 
 shown by lecturers on natural philosophy, in illustration of the 
 principle, is that made with a double cone, which appears to 
 roll up two .inclined planes, forming an angle with each other, 
 and lying in the same plane. In this case, the double cone 
 actually sinks as it advances, and by that means the centre of 
 gravity keeps continually descending. It is necessary to this 
 effect, that the height of the planes be less than the radius of 
 the base of the cone. If the height be equal to the radius, the 
 body will rest in any part of the plane ; if the height be greater 
 than the radius, it will descend. The two rules AB, CD, fig. 6, 
 pi. I. are united by a hinge CA. The lower sides are straight ; 
 on the upper sides, one end is wider than the other, so that when 
 opened they form two inclined planes. If a double cone EF, be 
 placed near the hinge, it will roll towards the upper end of the 
 planes, and thus apparently ascend ; but in reality it is let down, 
 because as the rules widen, the cone touches them in parts nearer 
 and nearer the apex on each side. 
 
 When the line of direction of a body upon an inclined plane falls 
 within the base, the body will slide down the plane ; but it will 
 roll down, if the friction of the surfaces be sufficient, when that 
 line falls without the base. Thus the body C, fig. 7, will only 
 slide down the inclined plane AB ; but the body D will roll down 
 the same surface. A sphere would descend upon an inclined 
 plane without rolling, if there were no friction, which is the only 
 cause of its rotation. 
 
 When a man is standing, the line of direction passes 
 between his feet ; when he walks, most of the motion is to 
 preserve this line in the same position. The various methods 
 and postures which we instinctively use to retain or to recover 
 
MECHANICS. 
 
 293 
 
 Mechanical rule for finding the centre of gravity. — Mechanical powers. 
 
 that position of the line of direction which ensures our stability^ 
 might afford matter for curious reflection. We bend our body 
 forward when we rise from a chair, or go up stairs; and in 
 carrying a load, we always lean from it. Thus a man leans 
 forward when he carries a burden on his back ; backward, when 
 he canies a burden on his breast ; and to the right or left 
 according to the situation of his load. The purpose of all 
 these changes, is to make the line of direction fall between 
 his feet. 
 
 If a body be suspended freely from different centres, its 
 centre of gravity will be in the intersection formed by lines 
 drawn from those centres perpendicular to the horizon. Hence 
 we obtain an easy practical method of finding the centre of 
 gravity of any irregular plane figure : suspend it by any point 
 with the plane perpendicular to the horizon ; from the point of 
 suspension hang a plumb-line, and draw a line upon the body 
 where the string passes over. Do the same for any other point 
 of suspension, and where the two lines meet must be the centre 
 of gravity. For example, supposing AB, fig. 8, pi. I. to be 
 the body of which the centre of gravity is to be found. Sus- 
 pend it in the first instance from any point, as D, so that it may 
 move freely on that part : let a plumb-line hang from the pin 
 on which it is suspended, and mark correctly the direction of 
 the plumb-line on the body. Then suspend the body by ano- 
 ther part, as F, and use the plumb-line as before. The line last 
 drawn will intersect the first line DE in C, which is the centre of 
 gravity. 
 
 Of the Mechanical Powers. 
 
 The simple machines, of a combination of two or more of 
 which all complex engines must consist, are called, by way of 
 distinction, the Mechanical Powers. Tliey are six in number, 
 viz. the LEVER, the pulley, the wheel and axle, the in- 
 clined PLANE, the WEDGE, and the screw. Some authors 
 are of opinion that we ought only to reckon two simple ma- 
 chines, the lever and the inclined plane ; for the pulley and the 
 wheel and axle may be considered as compound levers, and the 
 wedge and the screw are only modifications of the inclined plane ; 
 but as this enumeration is not in general use, and as it is in fact 
 calculated rather to confuse than to simplify the subject, we shall 
 pass it by. 
 
 If the abilities of man were limited by the extent of his 
 natural strength, small indeed would be his knowledge of the 
 works of nature, and few the refinements and comforts of civi- 
 lised society. We can hardly look upon any production of art 
 
294 
 
 MECHANICS. 
 
 Postulata. 
 
 which could have been obtained without the aid of mechanical 
 contrivances. Hence we may conclude, that the construction of 
 machines must have been long antecedent to a knowledge of the 
 theory upon which their principles depend. The remains of 
 Egyptian architecture exhibit the most surprising marks of mecha- 
 nical genius. The stones laid upon the tops of the pyramids of 
 Egypt, are each of them equal in size to a small house. The ele- 
 vation of such immense and ponderous masses, to the tops of these 
 and other stupendous fabrics, must have required an accumulation 
 of mechanical power, w hich the architect of the present day cannot 
 regard without astonishment. 
 
 In establishing the theory of the science of mechanics, some 
 assumptions are made and taken for granted, though not strictly 
 true, and when the theory has been explained, the proper allow- 
 ances are made. The assumptions alluded to, are commonly called 
 postulata, and are principally the four follow ing : 
 
 1. That a small portion of the surface of the earth, although 
 in reality convex, may be considered as a plane. 
 
 2. That heavy bodies descend in lines parallel to each other; 
 for though all bodies tend towards the centre of the earth, yet the 
 distance from which they fall is so inconsiderable, when compared 
 with their distance from the centre of the earth, that their inclina- 
 tion is very tiifling. 
 
 3. That the effort of any given power or weight, is the same 
 in all points of its direction ; or if a body be acted upon by any 
 power in a given direction, the action will be the same, in what- 
 ever part of that direction it be applied. 
 
 4. That though all surfaces are more or less rough, and all 
 machines imperfect, yet we must suppose all planes to be per- 
 fectly even ; all surfaces quite smooth ; all levers straight and in- 
 flexible, and without thickness and weight ; all cords perfectly pli- 
 able, and all machines without friction and inertia. 
 
 Three things are always to be considered in treating of mecha- 
 nical engines ; a zceight to be raised ; the poxcer by which it is to 
 be raised ; and the instrument or engine by which that pow'er acts 
 upon the weight. 
 
 The artifice in all mechanical contrivances is, to dbtribute 
 the w’eight among such a number of agents, that the part sus- 
 tained by the power may bear only a small proportion of the 
 whole. 
 
 In calculating the power of a machine, it is usually con- 
 sidered in a state of equilibrium ; that is, in the state when the 
 power which has to overcome the resistance, just balances it. 
 Having discovered how much pow'er will be requisite for this 
 purpose, it will then be necessary to add so much more as will 
 
MECHANICS. 
 
 295 
 
 ^ 1 ^ 
 
 Three kinds of levers. 
 
 overcome the friction and weight of the machine itself, and give 
 the necessary velocity. 
 
 Of the Lever. 
 
 The lever is of all machines the most simple ; it is merely a 
 bar of iron, of wood, or any solid material, by means of 
 which a certain force, when one part of it rests against a ful- 
 crum or prop, is capable of overcoming or resisting a greater 
 force. 
 
 In the lever there are three circumstances to be particularly 
 noticed : 1. The fulcrum or prop by which it is supported, or on 
 which it turns as an axis or centre of motion. 2. The power to 
 raise and support the weight. 3. The resistance or weight to be 
 raised or sustained. 
 
 The points of suspension are those points where the weights 
 really are, or from which they hang freely. 
 
 The power and the weight are always supposed to act at 
 right angles to the lever, unless it be otherwise expressed. 
 
 The lever is distinguished into three sorts, according to the 
 different situations of the fulcrum or prop, and the power, with 
 respect to each other. 
 
 if The lever is of the first order, when the fulcrum or prop is 
 
 [ placed between the power and the weight. 
 
 The lever is of the second order, when the prop is at one end, 
 ^the power at the other, and the weight between them. 
 
 i The lever is of the third order, when the power is applied be- 
 tween the weight or resistance and the fulcrum. 
 
 The greater number of instruments in general use are levers 
 of one kind or other. A poker, in stirring a fire, is a lever of 
 the first order ; the bar of the grate upon which it rests, is the 
 j fulcrum; the fire, the weight or resistance to be overcome; and 
 'the hand, is the power. Of this kind of lever, are constructed 
 balances, steelyards, scissars, pincers, snuffers, 8cc. The 
 instrument commonly called the iron-crow, by which large 
 stones are loosened, and great weights raised to small heights, 
 in order to get ropes under them, for raising them still higher, 
 is also a lever of the first kind. AB, fig. 9« pi* L represents 
 this lever, in which C is the fulcrum, A the end at which the 
 power is applied, and B the end where the weight acts. The 
 parts AC and CB, on the right and left of the fulcrum, are 
 called the arms of the lever. To find when an equilibrium 
 will take place between the power and the weight, we must 
 recur to what was formerly premised respecting the momenta 
 of bodies, viz. that their momenta are always as the products of 
 their quantities of matter multiplied by their velocities; and 
 
296 
 
 MECHANICS, 
 
 Lever of the first order. 
 
 therefore that the momentum of a small body will be equal to 
 that of a large one, if its velocity, or the space it passes through, 
 be such as to make their respective products equal. Now let 
 us consider when this equilibrium will take place in the lever. 
 Suppose the lever, AB, tig. 10, to be turned on its axis or ful- 
 crum, so as to come into the situation DC ; as the end D is at 
 the greatest distance from the centre of motion, and as it has 
 moved through the arch AD in the same time that the end B 
 moved through the arch BC, it is evident that the velocity of the 
 end A must have been greater than that of B, and for this reason 
 it requires less weight or quantity of matter to produce an equili- 
 brium than B. 
 
 Let us novr ascertain how much more weight B will require 
 than A to balance. As the radii* of circles are in proportion 
 to their circumferences, they are also proportionate to similar 
 parts of them ; therefore as the arches AD, CB are similar, 
 that is, are both equal portions of the circles to which they re- 
 spectively belong, the radius or arm DE, bears the same pro- 
 portion to EC, that the arch AD bears to CB. But the arches 
 AD and CB represent the velocities of the end of the lever, be- 
 cause they are the spaces which they moved over in the same 
 time ; therefore the arms DE and EC may also represent these 
 velocities. 
 
 It is evident, then, that an equilibrium w’ill take place, when 
 the length of the arm AE multiplied into the power ai A, shall 
 equal EB multiplied into the weight at B ; and, consequently, 
 that the shorter EB is, the greater must be the weight at B to 
 produce the balance ; that is, the pow'er and the w'eight must be 
 to each other inversely as their distances from the fulcrum. Thus 
 supposing AE, the distance of the power from the fulcrum, 
 to be twenty inches ; and EB, the distance of the weight from 
 the fulcrum, to be eight inches ; and the weight to be raised at B 
 to be five pounds; then the power to be applied at A must be 
 tw^o pounds ; because the distance of the w eight from the fulcrum, 
 viz. eight, when multiplied into the weight five, makes forty; 
 therefore twenty, the distance of the power from the prop, must 
 be multiplied by two to get an equal product, which will produce 
 an equilibrium. 
 
 It is obvious, that while the distance of the power from the 
 prop exceeds that of the weight from the prop, a power less 
 than the weight will raise it ; so. that then the lever affords a 
 
 • The radius of a circle U a line proceeding directly from the centre to the 
 circumfecenee* 
 
MECHANICS. 
 
 297 
 
 Hammer lever. — Second kind of lever. 
 
 mechanical advantage. Bui when the distance of the power is 
 less than that of the weight from the prop, the power must be 
 greater than the weight, or it will not raise the latter ; when both 
 the arms are equal, the power and the weight must be equal, to 
 be in equilibrium. 
 
 The hammer lever differs in nothing but its form from a 
 lever of the first kind. Suppose the handle of the hammer to 
 be ten times the length of the iron part that draws the nail, then 
 pulling backwards the handle, while the lower end rests upon the 
 board as a fulcrum, the nail may be drawn with one-tenth part of 
 the power which would be required to pull it out with pincers ; 
 because in using the pincers, the nail would move as fast as the 
 hand, but in using the hammer the hand moves over ten times the 
 space of the nail. A pair of scissars is composed of two levers 
 of the first kind, the centre of motion being the rivet. If the hand 
 or power be applied three times as far from the rivet as the mate- 
 rial to be cut, each lever acting with a force of three, the scissars 
 will act on the material six times more strongly than if the same 
 power were applied directly to it. 
 
 The second kind of lever, that is, a lever employed so as to 
 have the weight between the fulcrum and the power, is repre- 
 sented by fig. 11, pi. I.; A is the fulcrum, B the weight, and C 
 the power. The advantage gained by this lever, as in the first, is 
 as great as the distance of the power from the prop exceeds the 
 distance of the weight from the prop. Thus if the point a, on 
 which the power acts, be seven times as far from A as the point 
 on which the weight acts, then one pound applied at C will raise 
 seven pounds at B. 
 
 From the properties of this lever it is evident, that if two 
 men carry a burden upon a stick between them, the shares of 
 the weight which they bear, are to one another in the inverse 
 proportion of their distances from it. The fact, indeed, is 
 well known, that the nearer either of them is to the burden, the 
 greater share he bears of it; and if one of them go directly 
 under it, he bears the whole. If one man be at A and the 
 other at a, with the pole or stick resting on their shoulders, and 
 the burden B is placed five times as near the man at A, as it is to 
 the man at a, the former will bear five times as much weight as the 
 latter. Upon the same principle, two horses of unequal strength, 
 may be yoked so that each horse may draw a part proportionable 
 to his strength ; for the beam they pull may be divided in such a 
 manner, that the point of traction may be as much nearer to the 
 stronger horse, as will be required to balance the different effects 
 of their strength. 
 
 The oars and rudders of vessels are levers of the second kind ; 
 
 13. — VoL. I. SQ 
 
398 
 
 MECHANICS. 
 
 Third kind of lever. 
 
 the vessel is the weight or resistance, the water is the fulcrum, and 
 the man who governs their motions is the power. A door is a le- 
 ver of the second kind ; the hinges are the centre of motion, the 
 body of the door is the weight, and the hand by which it is moved 
 is the power. A pair of bellows, nut-crackers, &c. are composed 
 of two levers of the second kind. 
 
 In the third kind of lever, that is, when the power is between 
 the weight and the prop, the power and the weight are in equili- 
 brium, when the intensity of the power exceeds the intensity of 
 the weight just as much as the distance of the weight from the 
 prop exceeds the distance of the power. Thus, let E, fig. 12, be 
 the prop of the lever EF, and W a weight of one pound, placed 
 five times as far from the prop as the point at which the power, 
 P, acts, by the cord going over the fixed pulley D ; in this case, 
 the power must be equal to five pounds, in order to support the 
 weight of one pound. 
 
 The third kind of lever is used as little as possible, on 
 account of the disadvantage to the moving power ; but it can- 
 not always be avoided ; an example of it is seen in rearing a tall 
 ladder against a wall, where the power or strength of a man is ex- 
 erted at a short distance from one end, and the task cannot be 
 performed without more effort than would be necessary to bear 
 the ladder. 
 
 The bones of a man’s arm, and the limbs of animals gene- 
 rally, are levers of the third order ; for w hen we lift a weight by 
 the hand, the muscle that exerts its force to raise that weight, is 
 fixed to the bone about one-tenth part as far below the elbow as 
 the hand is, and the elbow being the centre round which the lower 
 part of the arm turns, the muscle must therefore exert a force ten 
 times as great as would be required simply for the suspension of 
 the weight that is raised. The principle of vitality has an influence 
 upon the strength of the muscles for which we are wholly unable 
 to account ; and a weight which would instantly break a muscle 
 the moment it was dead, can be raised by that muscle while alive, 
 without the smaltest pain or difficulty. Vitality, therefore, being 
 ordained to communicate so much energy to such flexible mate- 
 rials as flesh and blood, a lever of the third sort became most ad- 
 mirably adapted to the animal frame, because, if the power be suf- 
 ficient, its operations are quick, and it is exerted or resides in a 
 small compass. 
 
 In every species of lever there will be an equilibrium, when 
 the power is to the weight as the distance of the weight from the 
 fulcrum is to the distance of the power from the fulcrum. 
 
 In making experiments to prove the truth of the theory of 
 the mechanical powers, as it is impossible to obtain materials 
 
MECHANICS. 
 
 299 
 
 Effects of changing the position of the weight supported by a lever. 
 
 void of weight, the young mechanic ought to be apprized of 
 the necessity of taking the only precaution in his power, that 
 of perfectly balancing the levers, &c. before the weights and 
 powers are applied, otherwise his results will not be satisfactory. 
 Thus a lever made to exemplify the theory explained by fig. 9, 
 pi. I. should from B to C be so much thicker than from C to A, 
 tliat BC will balance CA when the fulcrum is at C. In like 
 manner, for every other position of the fulcrum, the shorter 
 arm of the lever should be made of the same weight as the 
 longer. 
 
 If the weight to be raised be of considerable bulk, and if it 
 be fixed either above or below the end of the lever, it will vary 
 in its intensity according to the position of the lever. Let AB, 
 fig. 13, represent a lever having a weight fixed above it, as A, 
 of which the centre of gravity is «, and the line of direction 
 ab ; then « ^ is the point in the lever upon which the weight 
 acts ; but if the lever is moved into the position CD, the line of 
 direction of the weight will fall nearer to the fulcrum of the 
 lever, and consequently act with less force upon it; but if the 
 lever is placed in the direction EF, the line of direction will fall 
 further from the fulcrum, and therefore its action on the lever 
 will be increased. On the contrary, opposite effects will take 
 place when the weight is below the lever, as represented by 
 fig. 14. 
 
 When the weight is suspended from the lever by a rope, or 
 any flexible material, no alteration, as exemplified by fig. 13, 
 can take place, because the point of suspension or point of 
 action is not altered. When, therefore, two draymen carry a 
 barrel on a coulstaff, to which it is suspended by a chain, the 
 point on which the weight acts not being altered by inclining 
 the staff in going up or down hill, each man will sustain the 
 same weight as if on level ground. But if they carry the 
 barrel upon two dogs, then the weight does not swing, and 
 the centre of gravity is below the lever ; therefore the point on 
 which the wei^t acts, will, by inclining the lever, be made to 
 approach the highest end ; and the first man, in going down 
 hill, by having this point removed from him, will be eased in 
 part of his burden, and the last man will have his equally in- 
 creased. 
 
 If several levers be combined together, so that a weight 
 appended to the first lever, may be supported by a power 
 applied to the last, as in fig. 13, pi. I. where three levers of 
 the first kind are so disposed that a power applied to the point 
 L of the lever C, may sustain a weight at the point S of the 
 lever A, the power must be to the weight, in a ratio, or propor- 
 
300 
 
 MECHANICS. 
 
 Power of levers in combination. — The balance. 
 
 tion compounded of ihe several ratios which those powers that 
 can sustain the weight by the help of each lever, when used singly 
 and apart from the rest, have to the weight. For instance, if 
 the power which can sustain the weight W by the help of the 
 lever A, be to the weight as 1 to 5 ; and if the power which can 
 sustain the same weight by the lever B alone, be to the weight 
 as 1 to 4 ; and if the power which could sustain the same 
 weight by the lever C, be to the weight as 1 to 5 ; then the 
 power which will sustain them by the help of the three levers 
 joined together, will be to the weight in a proportion consisting 
 of the several proportions multiplied together, of 1 to 5, 1 to 4, 
 and 1 to 5 ; that is, as 5 x 4 x 5, or of 1 to 100. For since 
 in the lever A, a power equal to one-fifth of the weight W, 
 pressing down the lever at L, is sufficient to balance the weight; 
 and since it is the same thing whether that power be applied to 
 the lever A at L, or the lever B at S, the point S bearing on the 
 point Ij, a power equal to one-fifth of the weight W, being 
 applied to the point S of the lever B, will support the weight; 
 but one-fourth of the same power being applied to the point L 
 of the lever B, and pushing the same upward^ will as effectu- 
 ally depress the point S of the same lever, as if the whole power 
 were applied at S ; consequently a power equal to one-fourth of 
 one-fifth, that is, one-twentieth of the w^eight W, being applied 
 to the point L of the lever B, and pushing up the same, will 
 support the weight. In like manner, it matters not whether 
 that force be applied to the point L of the lever B, or to the 
 point S of the lever C ; since if S be raised, L, which rests on 
 it, must also be raised ; but one-fifth of the power applied at the 
 point L of the lever C, and pressing it downwards, will as 
 effectually raise the point S of the same lever, as if the whole 
 power were applied at S, and pushed that lever up ; conse- 
 quently a power equal to one-fifth of one-twentieth, that is, one- 
 hundredth part of the weight W, being applied to the point 
 L of the lever C, will balance the weight at the point S of the 
 lever A. This method of combining levers is frequently used 
 in machines and instruments, and is of great service, either in 
 obtaining a greater power, or applying it with more conve- 
 nience. 
 
 Of the Balance* 
 
 The common balance, the extensive utility of which, in com- 
 paring the weights of bodies, is so w ell known, consists of a lever 
 of the first kind, the arms of which are equal in length. The 
 points, therefore, from which the weights are suspended, being 
 
MECHANICS. 
 
 301 
 
 The balance. 
 
 equally distant from the centre of motion, will move vyith equal 
 velocity, consequently, if equal weights be applied, their momenta 
 will be equal, and the balance remain in equilibrium. 
 
 In order to have a balance as perfect as possible, it is neces- 
 sary to attend to the following circumstances : 
 
 1. The arms of the beam ought to be exactly equal both 
 as to weight and length, and should at the same time be as long 
 as possible, relatively to their thickness, and the weight they are 
 intended to support; because the further the points of suspen- 
 sion are from the centre of motion, the more the momentum of 
 the weights is increased, and the more sensible will be the in- 
 strument. 
 
 2. The points from which the scales are suspended, should 
 be in a right line, passing through the centre of gravity of the 
 beam ; for by this means the weights will act directly against each 
 other, and no part of either will be lost, on account of any oblique 
 direction. 
 
 3. If the fulcrum, or axis of motion, passes through the 
 
 centre of gravity of the beam, and if the fulcrum and the 
 
 points of suspension be in the same right line, the balance will 
 
 have no tendency to one position more than another ; but will 
 
 rest in any position it may be placed in, whether the scales be 
 
 on or off, empty or loaded, provided the weight in each scale 
 be the same. The equality of two weights suspended from a 
 beam, where the centres of gravity and of motion are thus coin- 
 cident, being show n by their quiescence, and not by any parti- 
 cular position, such a beam is evidently inapplicable to common 
 use, for which but one position ought to denote the equality of 
 the weights ; and for that one, the horizontal position is the most 
 convenient. 
 
 If the centre of gravity of the beam, when level, be immedi- 
 ately above the fulcrum, it will overset with the smallest action ; 
 that is, the end which is the lowest wall descend and not rise 
 again, and it will descend thus with more swiftness, the higher the 
 centre of gravity, and the less the points of suspension are loaded. 
 Hence such a beam will make equal weights appear unequal. If 
 the centre of gravity of the beam be be/ow the fulcrum, the beam 
 will not rest in any position but when level ; but if disturbed from 
 that position, and then left at libert}’, it will vibrate, and at last 
 come to rest on the level. In a balance, therefore, the fulcrum 
 ought always to be placed a little above the centre of gravity. Its 
 vibrations will be quicker, and its horizontal tendency stronger, 
 the lower the centre of gravity, and the less the weight upon the 
 points of suspension. 
 
302 
 
 MECHANICS. 
 
 balance. 
 
 4. The friction of the beam upon the axis ought to be as 
 little as possible ; because should the*, friction be considerable, 
 the force required to overcome it will much injure the sensibi- 
 lity of the instrument ; upon which account, though one weight 
 should a little exceed the other, the greater will not preponde- 
 rate, if the excess be insufficient to overcome the friction and 
 bear down the beam. The axis of motion should be formed 
 with an edge like a knife, and made very hard. These edges, 
 in small balances, are at first made sharp, and then rounded 
 upon a fine hone, or piece of buff leather, covered with some 
 impalpable cutting powder. This operation causes a sufficient 
 bluntness or rolling edge. The excellence of the instrument 
 depends in a great measure upon the regular form of this round- 
 ed part. The scales should be hung upon an edge of the same 
 kind. 
 
 5. The pivots, which form the axis of motion, sliould be in a 
 straight line, and at right angles to the beam. 
 
 6. The rings, or pieces on which the axis bears, should be very 
 hard and well polished, parallel to each other, and of an oval 
 figure, that the axis may keep its proper bearing, or always remain 
 at the lowest point. 
 
 7. The beam should alw^ays be made so strong, as to be 
 inflexible by the greatest weight it is likely to sustain ; as if it 
 bend it w'ill be rendered less sensible, and as the arms will pro- 
 bably bend unequally, the balance will cease to give a correct 
 result. That the beam may not be thick or clumsy without 
 utility, it should be strongest at the middle, from which part to the 
 points of suspension it should be gradually diminished in thick- 
 ness, because the strain upon it is likewise so diminished. In 
 small balances, the beam is mostly round, but in large ones it is 
 generally rectangular, its transverse section being a parallelogram, 
 of which the long sides are vertical. A square, a round, or any 
 other form, would require a greater quantity of metal to possess 
 the same strength. 
 
 8. Very delicate balances are not only useful in nice experi- 
 ments, but are likewise much more expeditious than others in 
 common w'eighing. If a pair of scales, with a certain load, be 
 barely sensible to one-tenth of a grain, it will require a consider- 
 able time to ascertain the weight to that degree of accuracy, be- 
 cause the turn, being very small, must be observed several times 
 over. But if no greater accuracy were required, and a balance ■ 
 were used wffiich would turn with the one-hundredth part of a grain, 
 a tenth of a grain more or less would make so great a difference in 
 the turn that it would be seen immediately. 
 
MECHANICS. 
 
 303 
 
 The balance. 
 
 9. A curious effect caused by exciting a tremulous motion in 
 the beam, deserves to be noted. If a balance be found to turn 
 with a certain addition, and is not moved with any smaller weight, 
 a greater sensibility may be given to that balance by drawing a 
 file, a saw^, or any similar instrument, along any part of the beam 
 or its support ; for the jar produced by this operation w ill diminish 
 the friction on the moving parts so much, that the turn w ill be evi- 
 dent with one-third or one-fourth of the addition that would else 
 have been required. In this way, a beam w hich would hardly turn 
 tvilh the addition of one- tenth of a grain, wdll turn w ith one-thirtieth 
 or fortieth of a grain. 
 
 10 . When the arms of a balance are unequal, the balance is 
 said to be false, because it does not give the true weight of the 
 body, whether it be suspended from the shorter arm or the longer 
 one. There are, however, several properties of the false balance, 
 which are extremely useful in the estimation of weights, as w ell as 
 in correcting errors which may have arisen in the adjustment of 
 the true balance. 
 
 A balance with unequal arms will weigh as accurately as 
 another of the same workmanship, provided the standard weight 
 be first counterpoised, then taken out of the scale, and the thing 
 to be weighed be put into the scale, and adjusted against the coun- 
 terpoise. Or when proportional quantities only are considered, as 
 in chemical and other philosophical experiments, the bodies and 
 products under examination may be weighed against the w^eights, 
 taking care alw ays to put the w eights into the same scale ; for 
 then, though the bodies may not be equal to the weights, yet their 
 proportions among each other w ill be the same as if they had been 
 accurately so. 
 
 A weight which counterpoises an ounce w'hen suspended from 
 the longer arm of a false balance, being added to the weight 
 which counterpoises an ounce suspended from the shorter arm, 
 will always be greater than two ounces. The excess is that 
 part of an ounce which is expressed by a fraction, of which the 
 numerator is the square of the difference of the arms, and the 
 denominator the product of the arms. If any substance be suc- 
 cessively weighed from the longer and shorter arms of a false ba- 
 lance, the true weight will be a geometrical mean between the 
 false weights. 
 
 11 . But though the equality of the arms may not aUvays 
 be essential, yet it is indispensably necessary that their relative 
 lengths, whatever they may be, should continue invariable. For 
 this purpose, it is requisite, either that the three edges be all truly 
 parallel, or that the points of suspension and support should be 
 
304 
 
 MECHANICS. 
 
 Accuracy obtained in constructing balances. 
 
 always in the same part of the edge. The last point is the most 
 easily obtained. 
 
 The index of a balance is that slender rod rising perpendicu- 
 larly from the middle of the beam, of which it shows the 
 inclination from the horizontal position. A sliding weight is 
 sometimes fitted upon the index, in order to raise or depress the 
 centre of gravity of the balance. This contrivance is useful in 
 adjusting the best distance between the centre of gravity and the 
 fulcrum. 
 
 The lever, we have observed, is of all machines the most sim- 
 ple ; the balance is merely a lever, yet, in applying it to practice, 
 it appears, from the preceding considerations, that many difficul- 
 ties are to be encountered before it becomes fitted to afford 
 any thing like accurate results. The workman finds it a severe 
 check upon him, to keep the arms equal, while he is making 
 the other adjustments, and this point, though certainly not 
 the easiest to manage, is but one of many in which he ought 
 perfectly to succeed. If then the adjustment of a simple lever be 
 so difficult a matter, the sources of imperfection in complex ma- 
 chines may well be supposed to be very considerable. 'Yet when 
 W'e regard what has been actually executed, we find no reason to 
 coudude that the ingenuity of man is too limited for his happi- 
 ness ; or that most of the obstacles which, at present baffling us, 
 form the desiderata of human knowledge, will not in time be re- 
 moved. In this place, it may gratify the reader to notice the near 
 approximation to perfection which has been shown in the con- 
 struction of the balance. 
 
 Muschenbroek mentions his being possessed of a balance 
 which turned with one-fortieth of a giain. The substances he 
 weighed were between 200 and 300 grains. The balance there- 
 fore weighed to part of the whole. 
 
 Two accurate balances of Bolton’s are mentioned in the 
 Philosophical Transactions, vol. 66. One of them, it is said, 
 would weigh a pound, and turn with one-tenth of a grain. This, 
 supposing the pound to be avoirdupoise, is ^rr^rro of the weight, 
 and shows that the balance could be depended on to four places 
 of decimals, and probably to five. The other balance weighed 
 half an ounce, and turned with of a grain, which is about 
 
 of the weight. 
 
 In the same volume of the Philosophical Transactions, two 
 other balances, one of them Read’s, and the other Whitehurst’s, 
 are noted. Read’s, when loaded with 55 pounds, readily 
 turned with one pennyweight; but very distinctly turned with 
 four grains when tried more patiently. This being about 
 
MECHANICS. 
 
 305 
 
 Accuracy obtained in constructing balances. — Steel-yard. 
 
 part of the weight, the balance may be well depended on to live 
 places of figures. — Whitehurst’s balance weighs one penny-weight, 
 and is sensibly affected with of a grain, which is part 
 
 of the weight. 
 
 W. Nicholson, so well known as the intelligent author of many 
 scientific works, observes, that he has a balance made expressly 
 for him by a skilful artist in London. With 1200 grains in each 
 scale, it turns with one-seventieth of a grain. This is of 
 
 the whole ; and therefore about this weight may be known to five 
 places of figures. The proportional delicacy is less in greater 
 weights. '^I’he beam will weigh nearly one pound troy, and when 
 the scales are empty, it is atfected by of a grain. On the 
 
 whole, it may be usefully applied to determine all weights between 
 100 and 4000 grains to four places of figures. 
 
 A balance in the possession of Dr. George Fordyce, is men- 
 tioned in the Philosophical Transactions, vol. 75. It was made 
 by Ramsden, and turns upon points instead of edges. Witli a load 
 of four or five ounces, a difference of one division in the index was 
 inade by -reW ^1 ^ grain. This is y-g-rW- P^i't of the weight, and 
 consequently this beam will ascertain such weights to five places 
 of figures, besides an estimate figure. 
 
 The Royal Society’s balance, wdiich was also made by Rams- 
 den, turns on steel edges, upon planes of polished chrystal. Ni- 
 cholson, who speaks of it in his Chemical Dictionary, says he was 
 assured, that it ascertained a weight to the seven-millionth part. 
 He was not present at this trial, which must have required great 
 care and patience, as the points of suspension could not liave 
 inoved over much more than one-fiftieth of an inch in the first 
 half minute : but from some trials which he saw, he considered it 
 probable, that it might be used in general practice to determine 
 weights to five places and better. 
 
 Tables of specific gravities, are sometimes carried to five, 
 six, and even seven places of decimals ; but from the preceding 
 account of balances it is obvious, that experience does not 
 authorise the practice ; and that deductions founded on such a 
 supposed accuracy in weighing, arc of a very questionable nature. 
 In general, wdiere weights are given to five places of figures, the 
 last figure is hypothetical ; and where they are carried further, we 
 have reason to doubt the veracity or competence of the expeii-. 
 inentalist. 
 
 Of the Steel-yard, 
 
 The statera, or Roman steel-yard, is a lever of the first 
 kind, and is used for finding the weights of different bodies, by 
 one single w'eight, placed at different distances from the prop 
 
 13. — Vol. f. 
 
 2 R 
 
306 
 
 MECHANICS. 
 
 Danish and Swedish steel-yard. — Pulley. 
 
 or centre of motion U, fig. 16, pi. I. The shorter arm UM, is of 
 such a weight as exactly to counterpoise the longer arm UN. 
 If the longer arm be divided into as many equal parts as it will 
 contain, each equal to UO, the single weight Q (which we 
 may suppose to be one pound) will serve for any thing as heavy 
 as itself, or as many times heavier as there are divisions in the 
 arm UN equal to the distance UO ; or any quantity between its 
 own weight and that quantity. For example, if Q be one 
 pound, and be placed at the first division 1, in the arm UN, 
 it will balance one pound in a scale or on a hook at P ; if it be 
 removed to the second division at 2, it will balance two pounds 
 at P ; if to the third, three pounds, and so on to the end of the 
 arm UN. If these integral divisions be subdivided into as many 
 equal parts as a pound contains ounces, and the weight Q be 
 placed at any of these subdivisions, so as to counterpoise the 
 goods in the scale, the pounds and odd ounces of those goods will 
 by that means be ascertained. 
 
 In the Danish and Swedish steel-yard, the body to be weighed, 
 and tha constant weight, are fixed at the extremities of the steel- 
 yard, but the point of suspension or centre of motion, moves along 
 the lever till the equilibrium takes place. The centre of motion 
 therefore shows the weight of the body. 
 
 Of the PuLLElf. 
 
 The pulley is a small wheel tunnng on an axis, with a 
 drawing rope passing over it. The circumference of the pulley 
 is generally hollowed to receive the rope, which is attached on the 
 one hand to the moving power, and on the other to the resisting 
 force. 
 
 The pulley is usually called a sheave, and is so fixed in a 
 frame or block, as to be moveable on a pin passing through its 
 centre. When pulleys are made of wood, a ring of iron or 
 brass is generally let into the middle of them, to work upon 
 the pin, as they w'ould otherwise wear unequally, and their 
 motion would then be impeded by an increased degree of 
 friction, 
 
 A fixed pulley is one which has no motion except upon its 
 axis : a moveable pulley is one which rises and falls with the 
 weight. The expression moveable pulley” is clear enough, but 
 the epithet fixed being rather calculated to exclude the idea of mo- 
 tion entirely, the expression fixed pulley,” requires more parti- 
 cular notice from those to whom the subject is new. 
 
 The gorge or groove of a pulley, is the hollow part of the cir» 
 cumference which receives the rope or cord ; it is frequently hoi- 
 
MEriiAxir s. 
 
 PL. /. 
 
 J. Smith f/r/ . SJ‘orfrPs\'t'. 
 
 in' .Vnfhiil. FUitvn fiiroti.fJi rrjHnil 
 
t 
 
MECHANICS. 
 
 307 
 
 The pulley. 
 
 lowed out angularly, so that the rope is, by the pressure, so wedged 
 in the angle, that it cannot glide or slip in its motion. 
 
 A pair of blocks, with the rope fastened round it, is commonly 
 called a tackle. 
 
 Two equal weights attached to the ends of a rope going over 
 a fixed pulley, as fig. 1, pi. II. will balance each other, for 
 they stretch the rope equally, and if either of them be pulled 
 down through any given space, the other will rise through ai> 
 equal space in the same time, and consequently as their velocities 
 are equal, they must balance each other. This kind of pulley, 
 therefore, gives no mechanical advantage, but the use of it is a 
 source of great convenience. It serves to change the direction of 
 draught; it gives a man an opportunity of applying his weight 
 instead of his muscular strength, but not of lifting more than his 
 weight ; it also enables a man to raise a weight to any point, with- 
 out moving from the place he is in, whereas he would otherwise 
 have been obliged to ascend with the weight; and, lastly, by it 
 several men may apply their strength to the weight by means of 
 the rope, with as much facility, under the same circumstances, as 
 one person only. 
 
 In treating of the lever, it might have been observed, that 
 the prop may be regarded as a third power, which keeps in 
 equilibrium the motive force and the resistance, or which con- 
 curs with the one, to enable it to sustain the effort of the other. 
 If the lever of the second order, AB, fig. 3, have its fulcrum 
 at B, the weight in the middle at C, and the power at A, half 
 the weight being supported by the fulcrum, a power equal to 
 the other half will keep it in equilibrium. This will apply to 
 the illustration of the action of pulleys, which, when the weight 
 is appended to the circumference, may be considered as levers 
 of the first kind, and when the weight is appended to the 
 centre, they may be considered as levers of the second kind : 
 hence the ropes a b, fig. 1, hanging at equal distances from the 
 centre, c, (which must be regarded as the fulcrum,) equal 
 weights must be in equilibrium, exactly as they w ould be if placed 
 in the scales of a common balance. But if one weight be fur- 
 ther from the centre or fulcrum than the other, they will balance 
 each other only as they w'ould in a steel-yard, and, therefore, 
 though still a lever of the first kind, a less weight will suspend 
 a greater. Thus, if the pulley, as in fig. 2, have different 
 gorges, and the weight R of six ounces, be hung at the dis- 
 tance of one inch from the fulcrum, c, and the weight S of 
 three ounces be hung at the distance of two inches from the 
 same centre ; the two weights R and S, though in the propor- 
 tion of 2 to 1, will balance each other. If the weight S were 
 
£i08 
 
 MECHANICS. 
 
 The pulley. 
 
 only two ounces, it would produce the same effect upon R, pro- 
 vided its distance from the fulcrum were proportioned to the dimi- 
 nution of its weight ; that is, if it were three times as far from tlie 
 centre c, as R. 
 
 We have now to show that the moveable pulley acts like a 
 lever of the second order. Let the moveable pulley fig. 4, 
 pi. II, be fixed to the weight W, with which it rises and falls. 
 In comparing it with the lever alluded to, the fulcrum must be 
 considered as at F ; the weight acts upon the centre c, by meanj 
 of the neck ch ; the power is applied at D ; and the line DF 
 will represent the lever. The power, therefore, as in fig. 3, is 
 twice as far from the fulcrum as the weight, and the effect in 
 both cases is alike, viz. the proportion between the power and 
 the weight, in order to balance each other, must be as 1 to 2. 
 It is evident, therefore, that the use of this pulley doubles the 
 power, and that a man may raise twice as much by it, as by 
 his strength alone. Or, as variety in illustration will some- 
 times catch the attention, and familiarize a subject to some 
 whose ideas of it would not otherwise be distinct, the action of 
 this pulley may be viewed in a light somewhat different from the 
 above. Every moveable pulley may be considered as hanging 
 by tw^o ropes equally stretched, and which must consequently 
 bear equal parts of the weight; the rope FG being made fast 
 at G, half the weight is sustained by it, and the other part of 
 the rope, to which the power is applied, has only the other half 
 of the w'eight to support ; consequently the advantage gained is as 
 2 to 1. 
 
 When, as in fig. 5, the upper and fixed block, or pulley- 
 frame, contains tw'O pulleys, which only turn upon their axes, 
 and the lower moveable block contains also two, which not only 
 turn on their axes, but rise with the weight the advantage 
 gained is as 4 to 1 ; for each low'er pulley will be acted upon by 
 an equal part of the weight ; and because each pulley that 
 moves with the weight, diminishes one-half the power necessary 
 to keep the w'eight in equilibrium, the power by which W may 
 be sustained will be equal to half the weight divided by the 
 number of lower pulleys ; that is, as twice the number of the 
 lower or moveable pulleys is to 1, so is the weight suspended to 
 the power. But if the extremity A, fig. 6, be fixed to the 
 lower block, it will sustain half as much as a pulley ; conse- 
 quently here the rule will be, as twice the number of moveable 
 pulleys, adding unity, is to 1, so is the weight to the powder. 
 To prevent the ropes a and b from rubbing against each other, 
 • the upper fixed pulley may have a double gorge. The pulley 
 d belongs not to the system of pulleys, it is merely used in the 
 
MECHANICS. 
 
 309 
 
 The pulley 
 
 plate, to separate from the ropes, and show more distinctly the 
 power, P. 
 
 If instead of one rope going round all the moveable pulleys, 
 the rope belonging to each of them be made fast at the top, as in 
 lig. 7, a different proportion between the power and the weight 
 will take place. Here it is evident, that each pulley doubles the 
 power ; thus, if there are two pullies, the power will sustain four 
 times its own force or weight ; if three pulleys, eight times its own 
 weight; if four pulleys, sixteen times its own weight, as in the 
 figure, where the weight W, of sixteen ounces, is supported by the 
 power P, of only one ounce. This arrangement of pulleys takes 
 up much room, raises the weight very slowly, and is not conve 
 nient to fit up. It is therefore seldom used, notwithstanding the 
 great power gained. 
 
 These rules are applicable, whatever may be the number of 
 pulleys employed. 
 
 The large space occupied by pulleys, when arranged under 
 each other, as in fig. 5 and 6, is an inconvenience that would often 
 render them useless, and such an arrangement would increase the 
 liability to entangl^mexiJ:, particularly on shipboard ; it is therefore 
 common to plac^e all the pulleys in each block on the same pin, 
 by the side of each other, as in fig. 8. The advantage, and the 
 rule for the power, are the same here as in fig. 5. In this kind of 
 tackle, the ropes are not exactly parallel, a direction which should 
 be preserved as much as possible ; but the defect is not very con- 
 siderable. 
 
 The reason of the parallel direction of the ropes being 
 better than an oblique one, is that less power is required to sus- 
 tain the same weight ; and in proportion to the obliquity of the 
 ropes must be the increase of the power. When there are 
 many pulleys in the same block, and the end of the rope to 
 which the power is applied terminates over one of the outside 
 pulleys, that pulley always endeavours to get into a line with 
 the centre of suspension or middle of the moveable pulleys, 
 from which the weight hangs. In consequence of this, the 
 friction of the pulleys against the sides of the block is so great 
 as sometimes to equal the power. Hence the multiplication of 
 pulleys thus used, soon ceases to be advantageous ; they are 
 seldom effective, if their number exceeds three or four. 
 Smeaton, the eminent engineer, was the first who disencumbered 
 himself of the difficulty here stated, by making the -rope ter- 
 minate over the middle pulley or sheave in the fixed block, 
 which is thereby kept perpendicularly under the other, and the 
 friction of the sheaves is on their centres of motion only. The 
 
310 
 
 MFXHANICS. 
 
 The pulley. 
 
 number of sheaves must always be uneven, or this improvement 
 cannot be adopted. 
 
 To avoid as much as possible the friction and shaking 
 motion of a combination of pulleys, James White, a very able 
 mechanic, invented and obtained a patent for the concentric 
 pulley, fig. 9* M and N are two of these pulleys, one of them 
 being fixed, the other moveable. They are usually made of 
 brass, and answer the purpose of as many distinct pulleys as there 
 are grooves. In this case, as in fig. 5, the weight being divided 
 among the number of ropes, a power of 1 will support a weight 
 of 12. 
 
 In speaking simply of a system of pulleys, the common ar- 
 rangement of them is meant, viz. that where the number of ropes 
 is just twice the number of the moveable pulleys. Fig. 4, 6, and 
 8, are all systems of this kind. The ropes are spoken of as if 
 they were in different lengths, but it can hardly require an obser- 
 vation, that the expression is used merely because it is convenient, 
 and that there is in fact but one rope, the parts of which are al- 
 luded to as if they were separate. 
 
 It has been shown, in illustrating fig. 2, pi. II, that by 
 means of a pulley of several grooves, the actions of two 
 unequal powers may be made to balance each other. In like 
 manner, a constant equilibrium or relation may be preserved 
 between two powers, the relative forces of which continually 
 change. Watchmakers derive great advantage from the appli- 
 cation of this principle to their wojJc. The spring of a watch 
 always acts with the greatest power immediately after it has 
 been wound up, and its powder is continually but gradually 
 diminising, till the watch stops. If this inequality of the 
 maintaining power operated upon the wheels, the watch would 
 not go two successive hours at the same rate ; but the effects of 
 it are completely avoided by the peculiar conformation of the 
 pulley off which the spring draws the chain. Instead of many 
 concentric gorges upon the fusee, they make only one, but that 
 one is in a spiral form upon a truncated cone, see fig. 10. 
 When the watch is wound up, the chain extends from the upper i 
 or narrowest part e, to the spring-box. The spring is then at 
 the strongest ; but it acts on a part so near the centre of motion, 
 or axis F G, that its power on the w heels is the same as just 
 before it stops, when its absolute strength is much diminished ; 
 and its weakness is favoured by pulling at a longer lever, or at 
 a greater distance from the centre of motion, that is, at y*. Now 
 as the alteration in the power of the spring from its greatest to 
 its least strength is gradual, so is the extension of the lever, or 
 
MECHANICS. 
 
 311 
 
 Convenience, and not absolute increase of power, gained by machines. 
 
 increase of the distance from the centre of motion, FG, also gra- 
 dual between the extremes e f; therefore the spring and the fusee 
 may be so adjusted to each other, that the power operating upon 
 the wheels will always be the same. 
 
 We have postponed till now one remark which thd reader 
 has probably anticipated, viz. that it is convenience alone, and 
 not any actual increase of power, which we gain by machines ; 
 for in all contrivances by which power is gained, a proportional 
 loss of time is suffered. This is evident from a consideration 
 of the properties of the lever; and is still more evident, if 
 possible, in considering the action of pulleys. If one man, 
 by means of a tackle, can raise as much weight as ten men could 
 by their unassisted strength, he will be ten times as long in per- 
 forming his task. Suppose a man at the top of a house draws up 
 ten w^eights, one at a time, by a single rope, in ten minutes, let 
 him have a tackle of five moveable pulleys, and he will draw up 
 the whole ten at once with the same ease as he before raised up 
 one; but in ten times the time, that is, in ten minutes. Thus we 
 see the same work is performed in the same time, whether the 
 tackle be used or not ; but the convenience is, that if the w hole of 
 the ten w^eights were in one mass, they may be raised vvith the 
 tackle, though it would be impossible to move them by the unas- 
 sisted strength of one man. 
 
 Or suppose, instead of ten weights, a man draws ten 
 buckets of water from the hold of a ship in ten minutes, and 
 that the ship being leaky, admits an equal quantity in the 
 same time. It is proposed, lhat by means of a tackle he shall 
 raise a bucket ten times as capacious with tlie same exertion of 
 strength. With this assistance, he draws up the large bucket, but 
 in as long a time as he employed to draw up the ten, and there- 
 fore he is as far from gaining on the water in the latter case as in 
 the former. 
 
 The remark, that whatever is gained in power is lost in time, 
 would be perfectly correct, even if machines were destitute of 
 friction and of inertia ; but as these hinderances are always 
 present, the truth is, as we shall afterwards see, that power, in- 
 stead of being increased by the use of machines, is actually dimi- 
 nished. The convenience, however, which is obtained by a well 
 constructed machine, is an ample recompence for the actual loss 
 of power. 
 
 Of the Wheel and Axle. 
 
 The wheel and axle, sometimes called the axis in peritrochiOf 
 IS a machine of the most extensive utility, and, to suit different 
 purposes, is greatly diversified in form. It is nearly allied to the 
 
312 
 
 MECHAKICS. 
 
 AVheel and axle. 
 
 pulley ; the same illustrations will frequently serve for them both ; 
 and, like the pulley, it may be considered as an assemblage of 
 levers, or a perpetual lever, because from its constant renewal of 
 the points of suspension or resistance, it frees us from the great 
 defect of the simple lever, which can only be used to raise weights 
 to small elevations. 
 
 This mechanical power consists generally of a wheel with an 
 axis fixed to it, so as to turn round with it ; the power being ap- 
 plied at the circumference of the wheel, the weight to be raised is 
 fastened to a rope which coils round the axis. But a machine 
 which has in reality no wheel, comes under the denomination of 
 the wheel and axle ; such is a windlass, where an axle is turned by 
 a winch or handle ; here the handle is virtually the wheel, its re- 
 volution renders it a perpetual lever, its power is the same as that 
 of a wheel whose circumference is equal to the cirumference of 
 the circle the handle describes, and therefore the machine is iden- 
 tified with the wheel and axle. 
 
 AB, fig. 11, is a wheel, and CD an axle lixed to and mov- 
 ing round with it. If the rope which goes round the wheel be 
 pulled, and the wheel be turned once round, it is evident that so 
 much rope will be draw n olf as would encompass the wheel ; but 
 while the wheel turns once round, the axle turns onCe round, and 
 consequently the rope by wdiich the weight is suspended will wind 
 once round the axle, and the weight will be raised through a space 
 equal to the circumference of the axle. Hence the velocity of 
 the power will be to that of the w^eight, as the circumference of 
 the wheel is to the circumference of the axle. This being the 
 case, the weight and the pow'cr will be in equilibrium, when the 
 one is to the other as the circumference of the wheel is to the cir- 
 cumference of the axLe. 
 
 IMathematicians demonstrate, that the circumferences of 
 different circles bear the same proportion to each other as their 
 respective diameters ; consequently, the power and the weight 
 will balance each other, when the power bears the same propor- 
 tion to the weight that the diameter of the axis bears to the dia- 
 meter of the wheel. Thus, let fig. 2, pi. ^II, which was 
 formerly considered as a pulley of tw'o concentric grooves, be 
 now' considered as a wheel and axle ; d e representing the diame- 
 ter of the axle, w hich we w ill suppose to be one inch ; and jf g 
 the diameter of the wheel, which we will suppose to be six 
 inches ; then ofie ounce acting as the power, S, will balance a 
 weight or resistance of six ounces acting as a weight, R. In 
 whatever form the machine presents itself, this proportion 
 between the powder and the weight will subsist, when the power is 
 applied to the circumference of the w heel, or to a wiiich, as at 
 
Me c h its. n . ji . 
 
i 
 
MECHANICS. 
 
 Wheel and axle. 
 
 . E, fig. 11, and the weight to the axle. Therefore if W be 100 
 k Libs, and the power, P, or a force acting at E, be equal to 10 lbs. 
 i i supposing the diameter of the wheel, or the diameter of the circle 
 1 1 described by the winch, to be ten times greater than the diameter 
 ' of the axle, they will be in equilibrium, and a small additional 
 force will cause the wheel to turn with its axle, and raise the 
 [ i weight, and for every inch which the weight rises, the power will 
 fall ten inches. 
 
 When the wheel and axle is considered as a perpetual lever, 
 the fulcrum is the centre of the axle, the longer arm is the 
 radius of the wheel, and the shorter arm the radius of the axle. 
 I; From this again it is evident, that the longer the wheel, and the 
 j smaller the axle, the stronger is the power of the machine ; but 
 I then, as in other instances, the drawback on the power gained is 
 j the time lost, which is always proportionate to the disparity be- 
 t tween them. 
 
 A capstan is an axle or cylinder of wood, with holes in it, in 
 which levers are inserted, to turn it round ; these are like the 
 spokes of a wheel without the rim. 
 
 In some cases, the weight is not connected with the axle by a 
 rope, but is immediately affixed to the axle itself. A bell, which 
 I is moved in ringing by a wheel and axle, is an example of this ; 
 
 here, in once turning the bell round, the velocity of the bell is »» 
 ; the circumference described by its centre of gravity. 
 
 On the other hand, in what is called the circular crane, the 
 power is not applied to the wheel by means of a rope, nor does 
 it act upon any handle or spokes, but by a man walking 
 ; within the wheel ; as the man steps forward, the part upon 
 which he treads becomes the heaviest part of the w'heel, and 
 descends till it is the lowest. Thus he keeps going on, and by 
 treading upon every part of the wheePs circumference in its turn, 
 the revolution is effected. This mechanical contrivance is unwiel- 
 • dy, from the necessity of employing a large wheel, and its power 
 f insignificant, because the man can ascend so little from the bottom 
 j of the wheel. It is also unsafe for the man, for if the rope sus- 
 pending the weight give way, or his feet slip, he is exposed to the 
 ! greatest danger. 
 
 In considering the theory of the wheel and axle, the rope is 
 supposed to have no sensible thickness ; but if the rope is thick, 
 or if there is several folds of it round the axle, in obtaining the 
 radius of the axle, the measurement must be made to the middle 
 of the outside rope ; for it is plain, that the distance of the weight 
 is as effectually increased by the coiling of the rope as by any 
 other means. 
 
 14. — VoL. I. 
 
314 
 
 MECHANICS. 
 
 M’^lieel and axle. 
 
 If teeth are cut in the circumference of a wheel, and if they 
 W'ork in the teeth of another wheel of the same size, as fig. 1, 
 pi. Ill, it is evident that both the wheels will revolve in the same 
 time ; and the weight appended to the axle of the wheel B, 
 will be raised in the same time as if the axle had been fixed to 
 the wheel A. But if the teeth of the second wheel be made to 
 work in teeth made in the axle of the first, as fig. 2, every part 
 of the circumference of the wheel D, is applied successively to 
 the circumference of the axle E, of the first wheel C ; and as E 
 is much less than D, it is evident that it must go round as many 
 times oftener than D, as the circumference of D exceeds its own 
 circumference ; or, which amounts to the same thing, if the 
 number of teeth in the axle E, be divided by the number of 
 teeth in the wheel D, the product will shew the number of 
 revolutions which E will make for one revolution of D. In 
 order to obtain a balance here, between the power P, and the 
 weight W, the power must be to the weight, as the product of 
 the circumferences or radii of the two axles multiplied together, 
 is to the circumferences or radii of the two wheels. This will 
 become sufficiently clear, if the whole be considered as a com- 
 pound lever, the explanation of which, as given in its proper 
 place, will shew that fig. 2, requires the same proportion between 
 the weight and the power, and therefore is represented by the 
 compound lever, fig. 3. The dotted lines shew the half of the 
 wheels and axle, of which the different parts of the lever are 
 equal to the radii. 
 
 Instead of a combination of two wheels, three or four, or 
 any number of wheels, may work in each other, and by thus in- 
 creasing the number of wheels, or by proportioning the wheels to 
 the axles, any degree of power may be acquired. By increasing 
 the length of the axle, by varying the sizes of the wheels, and plac- 
 ing their teeth sometimes on the circumference, and sometimes on 
 the side of the rim, the action of the power may be transmitted 
 to a distance, the direction of the movement changed, and any 
 given velocity assigned to particular parts. 
 
 What we have uniformly called the teeth of wheels, are not 
 in all cases distinguished by that name, though they always are 
 so w'hen the work is small, as clockwork, and generally when 
 the wheels are made of metal, whatever be the size of the 
 work ; but in large works, where the wheels are of wood, and 
 the teeth are separate pieces morticed into the rim, they are 
 called cogs. 
 
 It may also be observed, that we have called the small 
 wheel E, fig. 2, pi. Ill, an axle, because in point of reasoning 
 as to the effect, it is the same thing whether it is really a toothed 
 
5IEGHANICS. 
 
 315 
 
 Wheel and axle. — App4icatiou of the wheel and axle to the crane. 
 
 axle, or a wheel of that size upon a small axle. For the sake 
 of distinction, the small wheel E is called by mechanics a 
 pinion, and sometimes a nut. Its teeth are also called leaves. 
 Ill large machines, trundles are frequently substituted for 
 pinions or nuts, being of easier manufacture, and performing the 
 same office. These trundles are cylinders or spindles, parallel 
 to each other, and placed circularly in two plain pieces of wood 
 at the top and bottom. The teeth of the wheel then catch the 
 spindles of the trundle, as they would do the teeth of a nut or 
 pinion. 
 
 That useful machine, the crane, so much employed in 
 raising or lowering goods, is indebted for its value principally 
 to the wheel and axle. Cranes are very variously constructed) 
 but the object in all of them is to manage a great weight with 
 an inconsiderable power. An elevation of a crane, the general 
 construction of which is now becoming very prevalent in 
 London, is shown in fig. 4, pi. III. It is approved, from its 
 requiring no very expensive frame-work, and because it can be 
 turned ail round. AB is a stout beam, turning in a cast-iron 
 collar at B, affixed to the beams in the floor of the wharf; -it 
 goes down about twelve feet below this, and has a steel pivot at 
 the lower end, which works in a brass socket or collar, so that 
 the beam AB can turn freely round without shaking. CD are 
 the two beams of the jib, with a fixed pulley at E, over which 
 the chain for hoisting the goods w'orks. The other end of thia 
 chain winds round the axle e, of the great wheel F, of 98 
 teeth ; this wheel works in a pinion of seven leaves, on the same 
 axle with the wheel G of 35 teeth. The wheel G, works in a 
 pinion, H, of 14 teeth. When a great pow'er is required, the 
 winch handle is applied to a square on the end of the axis or 
 spindle of this pinion ; but for a less weight, the winch is put 
 on the axle of the wheel G. In this case, to lessen the friction, 
 the pinion may be disengaged, or, as it is usually called, thrown 
 out of gear, by sliding its axle lengthways. For this purpose, it 
 must be provided with two grooves, and must have a clip or catch 
 to fall into one of these in either of its situations. The frame 
 containing the wheels is formed by two cast-iron crosses, bolted 
 to the main beam, AB, by their vertical arms, of which IK are the 
 two in front. 
 
 Machines of this description should be furnished with a 
 ratchet wheel, as in, with a catch to fall into its teeth. This 
 wheel will at any time support the weight, and keep it from 
 descending, if the person who turns the handle should, through 
 inadvertence or carlessness, quit his hold while the weight is sus- 
 pended. This very easy mode of preventing the danger which 
 
316 
 
 MECHANICS. 
 
 The inclined plane. 
 
 would result from the running down of the v/eight, if left at liber- 
 ty, should never be omitted. 
 
 Of the Inclined Plane. 
 
 The inclined plane is any flat surface which forms an angle 
 less than a right angle, with the plane of the horizon. When 
 hogsheads, or pipes of wine, &c. are to be let down into a cellar, 
 or brought up out of it, a plank is laid along the stairs, and thus 
 forms an inclined plane. 
 
 The time which a rolling body takes to descend upon an in- 
 clined plane, is to the time in which it would descend vertically 
 by its absolute gravity, in free space, from the highest part of the 
 plane, in the ratio or proportion which the length of the plane 
 bears to its perpendicular height. The whole theory of the in- 
 clined plane rests upon this principle. 
 
 Suppose the plane AB, flg. 5, pi. Ill, to be parallel to the 
 horizon, the cylinder, C, will remain at rest on whatever part of 
 the plane it is laid. If the plane be placed perpendicularly, as 
 AB, fig. 6, the plane will contribute nothing to the support of the 
 cylinder, C, which will therefore descend with its whole force of 
 gravity, or require a power equal to its whole weight to keep it 
 from descending. But suppose AB, fig. 7, to be a plane parallel 
 to the horizon, and AD a plane inclined to it ; if the whole length 
 AD be three times as great as the perpendicular DB, the cylin- 
 der C will be supported upon the plane, or kept from rolling 
 down, by a power equal to a third part of the weight of the cylin- 
 der ; it is clear, therefore, that a weight may be rolled up this in- 
 clined plane by a third part of the power which would be requir- 
 ed to draw' it up by the side of an upright plane, as AB, fig. 6, 
 where the force required must be equal to its whole weight ; but 
 it is to be noticed, that upon the inclined plane the weight goes 
 over three times the space ; which proves that, as in all other cases, 
 w e only substitute time for power. 
 
 As the horizontal plane supports the whole weight of the cy- 
 linder, C, and the vertical plane, or plane perpendicular to the 
 horizon, supports none of it ; so, in all cases, the less the angle 
 of elevation, or the gentler the ascent is, the greater will be the 
 weight which a given power can draw' up ; for the steeper the in- 
 clined plane, the less does it support of the weight, and of course 
 the greater the tendency the weight has to roll. 
 
 The weight is alw'ays most easily either drawn or pushed in 
 a line g r, parallel to the plane, and passing through the centre 
 of the weight ; for if one end of the line be fixed at g, and the 
 other end inclined towards D, the cylinder C would be drawn 
 
MECHANIC'?. 
 
 317 
 
 The wedge. 
 
 against the plane, and the power must be increased in propor- 
 tion to the greater difficulty of the line of traction ; also if the 
 line be carried above the power must be increased, but in 
 that case only in proportion as it endeavours to lift the body off 
 the plane. In stating the power necessary to support the 
 cylinder C, on the inclined plane AD, the parallel direction of 
 the line of traction, as it is the most favourable one, is taken for 
 granted. 
 
 In estimating the draught of a waggon or other vehicle up- 
 hill, the draught on the level must be added. Suppose the hill 
 rises one foot in four, then one-fourth part of the weight must be 
 added to the draught on level ground. If the weight were 12 cwT. 
 and its draught on the level was If cwt. then one-fourth of 12 
 cut. or 3 cwt. added to If cwt. would give cwt. for the real 
 draught necessary to draw 12 cwt. up a hill rising one foot in four. 
 Hence may be discerned the great disadvantage of hilly roads, and 
 die propriety of losing a little time in winding about hills where 
 it is possible, rather than exhaust the power of the horses in pass- 
 ing over them. 
 
 Of the Wedge. 
 
 The fifth mechanical power or simple machine, is the wedge, 
 which is a piece of w ood, metal, or any firm material, thin at one 
 end and thick at the other. The thin end is called the point 
 or edge, and the thick end is called the head or base of the 
 wedge. 
 
 The action of the wedge resembles most that of the inclined 
 plane, but the theory of it is by no means complete. The wedge 
 when driven, as it mostly is, by percussion, for example, by the 
 blow of a hammer, produces an effect differing considerably from 
 that of pressure, so as not to admit of exact calculation. A 
 weight of five hundred pounds, pressing on the back of a wedge, 
 will often make very little impression upon a body, which a ham- 
 mer weighing only two pounds, if, when the force of the blow is 
 extinguished, it have acquired a velocity rendering its momentum 
 equal to five hundred pounds, would instantly sever. This great 
 difference of effect between percussion and pressure, when the 
 momenta are equal, is perhaps owing to the tremulous motion or 
 vibration produced by percussion among the parts of the body, 
 which, when so agitated, and the wedge has once entered, has the 
 friction between its sides and that of the wedge lessened, as w'ell 
 as the cohesion of its parts diminished. Until therefore we are 
 thoroughly acquainted with the nature and force of the tenacity 
 of bodies, and the activity with which they vibrate under a 
 
318 
 
 MECHANICS. 
 
 The Avedge. 
 
 given impulse, (and these are branches of knowledge to which 
 philosophers are as yet nearly strangers,) the theory of the wedge 
 will not be susceptible of much precision. We are not, however, 
 wholly in the dark, and the following mode of regarding the sub- 
 ject is generally approved. 
 
 ABf fig. 8, pi. Ill, is a wedge driven into the cleft CDE, 
 of the wood FG. When the wood does not cleave at anv 
 distance before the wedge, there will be an equilibrium between 
 the power impelling the w^edge downward, and the resistance 
 of the wood acting against the two sides of the wedge, when the 
 power is to the resistance as half the thickness of the wedge at its 
 back, that is, from A to B, is to the length of one of its sides ; 
 because the resistance then acts perpendicularly to the sides of the 
 wedge. 
 
 When the w'ood cleaves at any distance before the wedge, 
 which is generally the case, the powder impelling the wedge will 
 not be to the resistance of the w ood as the thickness of the back 
 of the wedge is to the length of both its sides, tut as half the 
 thickness or length of the back is to the length of either side of 
 the cleft, estimated from the top or acting part of the wedge. 
 The reason of this is, that if w^e suppose the wedge to be 
 lengthened, so as to reach the bottom of the cleft at D, the 
 same proportion w ill hold ; namely, the pow er w ill be to the 
 resistance as half the length of the back of the wedge is to the 
 length of either of its sides ; or, which amounts to the same 
 thing, as the whole length of the back is to the length of both the 
 sides. 
 
 The thinner a wedge is at the back, that is, the more acute 
 the angle of its longitudinal section, the more powerful is its 
 action, or the greater the effects w hich may be produced by the 
 same forge. 
 
 When this instrument is employed to cleave a hard body, the 
 parts of which strongly adhere together, its advantage is augment- 
 ed in proportion as the w edge is sunk or driven deeper between 
 these parts. For example, if the piece of w ood, FG, have three 
 bandages, r, s, f, all of equal strength, and w hich may represent the 
 strength with which the parts of the wood cohere, the wedge may 
 be considered as acting by the arms DE, DC, of two angular 
 levers. If then the force of the w edge, exceeds a little that of the 
 first bandage r, this bandage w’ill be severed. The second ban- 
 dage, s, though as strong as the first, will be more easily broken 
 by the action of the same w edge, because the arms of the* lever by 
 w hich it then acts are lengthened by the quantity r s ; and the in- 
 creased facility with which the last bandage will be broken, will. 
 
MECHANICS. 
 
 319 
 
 The screw. 
 
 in like manner, be proportionate to the increased length of the 
 arms of the lever formed by the sides of the cleft. 
 
 The wedge is a mechanical power of singular efficacy, and the 
 percussion by which its action is obtained, is precisely that force 
 which we can, with the greatest convenience, almost indefinitely 
 increase. By means of the wedge, the walls of houses may be 
 propped, rocks split, and the heaviest ships raised up ; — operations 
 to which the lever, the wheel and axle, and the pulley, are either 
 ill-adapted, or entirely incompetent. 
 
 To the wedge are referred the axe, the spade, chisels,'iieedles^ 
 knives, punches, and, in short, all instruments which, begin- 
 ning with edges or points, grow gradually thicker. A saw is a 
 number of chisels fixed in a line; and a knife, if its edge be 
 examined with a microscope, will be found to be only a fine 
 saw. 
 
 Of the Sceew. 
 
 The sixth and last mechanical power which we have to notice, 
 is the screw. 
 
 The screw, strictly speaking, consists of two parts, which 
 , work within each other. One of these parts, and which is 
 always meant when the w'ord screw is used alone, is a solid cylin- 
 der on the circumference of which is cut a spiral groove ; it is, as 
 , we have formerly observed, when specifically named, called an 
 , outside or convex screw. The other part, is a hollow cylinder, or, 
 at least, whatever its external form may be, it contains a cylin- 
 drical hole, within which is cut a spiral groove corresponding to 
 that of the convex screw, which can be turned wdthin it, and the 
 spiral projections of the one lock into the spiral hollows of the 
 other. For the sake of necessary contradistinction, this latter 
 part is called an inside, a concave, or socket screw, w hen spoken 
 of generally, without reference to any other use than its principal 
 one, of an indispensable companion to the convex screw'; but 
 when it consists of a small piece of metal, as for drawing 
 tight bolts of any description, it is most commonly called a nut ; 
 and when it is of considerable size, as for a large press or vice, it 
 is usually called a box. 
 
 The thread of a screw is its spiral projection ; the pace or step 
 of a screw is the distance between the threads ; and the groove or 
 gorge is the hollow betw'een the threads. 
 
 To obtain an idea of the nature of the screw, and of its 
 affinity to the inclined plane, cut a piece of paper in the form 
 of an inclined plane, or half wedge, as LMN, fig. 9, ph HI, 
 and then wrap it round- a cylinder, fig. 10 ; the edge of this 
 plane or paper, LMN, will form a spiral round the cylinder/ 
 
320 
 
 MECHANICS. 
 
 The screw. 
 
 which will give the thread of the screw. The height of the 
 plane is the pace of the screw, or distance of one thread from 
 another ; its base is the circumference of the screw, and its 
 length is estimated by this circumstance and the height of the 
 pace. 
 
 A screw is seldom used without the application of a lever 
 to assist in turning it ; it then becomes a compound machine of 
 great force, either in compressing the parts of bodies together, 
 or in raising great weights. As the lever or winch must turn 
 the cylinder once round, before the weight or resistance can be 
 moved from one spiral winding to another, or before the screw 
 working in its box can rise or sink the distance between the 
 threads as from a to 6, therefore as much as the circumference 
 of the circle described by the lever is greater than the pace of 
 the screw, or distance between the threads, so much does the 
 force of the screw exceed the motive force. For example, 
 suppose the pace or distance of the threads to be half an 
 inch, and the length of the lever 12 inches, the circle described 
 by the extremity of the lever where the power acts, will be about 
 76 inches, or 152 half inches, consequently 152 times as great 
 as the distance betw’een tw'O contiguous threads ; therefore, if the 
 intensity of the pow'er at the end of the lever, be equal to one 
 pound, that single poundvwilJ balance 152 pounds acting against 
 the screw. If as much additional force be exerted as is sufficient 
 to overcome the friction, the 152 pounds may be raised; and the 
 velocity of the powder will be to the velocity of the weight as 152 
 to 1. Hence we may clearly perceive, that the longer the lever, 
 and the nearer the threads to one another, so much the greater is 
 the force of the screw. 
 
 The friction of the screw is very great, but we are indebted 
 to this circumstance for a peculiar advantage in the use of this 
 machine, which will sustain a weight, or press upon a body 
 against which it is driven, after the power is removed or ceases 
 to act. To enumerate all the uses of the screw would be impos- 
 sible. Among other purposes, it is applied to great advantage 
 for measuring or subdividing small spaces ; when thus applied it 
 is called a micrometer, w hich may be made to indicate on an index 
 plate, a portion of a turn, advancing the screw less than the fifty 
 thousandth part of an inch. 
 
 The threads of screws are differently formed, according to 
 the materials of w hich they are made, or the use for which they 
 are intended. The threads of wooden screws are generally angu- 
 lar, that they may rest upon a broad base, and thereby have 
 their strength increased to the utmost. Small screws, whatever 
 material they are made of, are generally angular also, not only 
 
MECHANICS. 
 
 321 
 
 The endless screw. 
 
 for the same reason as the wooden ones, but because the angular 
 thread is the most easily made. The metal screws which are 
 used for large presses, vices, &c. generally have a square thread, a 
 form which augments the surface of each thread, and occasions an 
 increase of friction, but is attended with the advantage of greater 
 steadiness in its motion. 
 
 In the common screw, to which the preceding observations 
 are exclusively applicable, the threads are one continued spiral, 
 from one end to the other; but where there are two or more 
 separate spirals running up together, as in the worm of a jack, 
 or the principal screw of a common printing-press, the descent 
 of the screw in a revolution will be proportionately increased ; 
 and therefore whatever be the number of the spirals, they must, 
 in calculating the power, be measured and reckoned as one 
 thread. 
 
 Of the Endless Screze* 
 
 A screw is sometimes cut on an axle, to serve as a pinion, and 
 either turns or is turned by a wheel ; it is then called an endless 
 screWf because it may be turned perpetually without advancing or 
 receding, that is, without any other motion than a rotary one. 
 The threads of this screw are of the square form, and fit exactly 
 into the spaces between the teeth of a wheel, which teeth are cut 
 obliquely to answer to the threads. When the enless screw has 
 been turned once round, the wheel has only made a portion of a 
 turn equal to the distance between one of its threads, that is, the 
 wheel has moved one tooth, and therefore the number of its teeth 
 is always the same as the number of the revolutions made by the 
 screw before it is once turned round. 
 
 The construction and mechanical advantage gained by this 
 screw may be best illustrated by a figure ; let the wheel C, fig. 
 11, pi. Ill, have an endless screw B, on its axis, working in a 
 wheel D, of 48 teeth. The screw B, and the wheel C, being on 
 the same axis, every time they are turned round by the winch, 
 the wheel D will be moved one tooth forward by the screw, 
 and therefore 48 revolutions of the winch will be required to 
 turn the wheel D once round. Then, if the circumference of 
 the circle described by the handle of the winch A, be equal to 
 the circumference of a groove round the wheel D, the velocity 
 of the handle will be 48 times as great as the velocity of any 
 given point in the groove ; consequently, if a line G, goes 
 round the groove, and has a weight of 48 pounds hung to it, 
 a power equal to one pound at the handle will balance and 
 support that weight. If an apparatus were constructed for the 
 14. — VoL. I. 2T 
 
322 
 
 MECHANICS. 
 
 The endless screw. 
 
 purpose, this might be proved by making the circumferences of 
 the wheel C and D equal to one another ; and then if a weight 
 H, of one pound were suspended by a line going round the groove 
 of the wheel C, it would balance a weight of 48 pounds hanging 
 by the line G, and a small addition to one of the weights will 
 cause it to move the other, as in every other case after the equili- 
 brium takes place. 
 
 If a line G, instead of going round the groove of the wheel 
 D, goes round its axle I, the power of the machine will be as 
 much increased as the circumference of the groove exceeds the 
 circumference of the axle. If then the circumference of the 
 groove, be six times greater than the circumference of the axle, 
 one pound at H will balance six limes 48, or 288 pounds, hung 
 to the line on the axle ; the power gained will therefore be as 288 
 to 1 ; and a man whose natural strength would enable him to lift 
 one hundred weight, will be able to raise 288 hundred weight by 
 this engine. 
 
 The use of the endless screw' affords a very ready means of 
 greatly diminishing or increasing a rotary motion, and ac- 
 complishes at once what would otherwise require the inter- 
 vention of two or three wheels. It possesses the advantage, 
 too, of moving or being moved by a wheel with much more 
 steadiness than a pinion, when the workmanship of both is of 
 equal quality. This circumstance is not so much regarded by 
 mechanics as perhaps it ought, and therefore the endless screw 
 is often not used when it would be advantageous. Cast iron 
 wheels are becoming increasingly general for all kinds of large 
 machinery requiring w'heel-work. Their durability, and the 
 little room they take up, when compared with the wooden ones, 
 to answer the same purpose, entitle them to a decided prefer- 
 ence ; but still there are many great impediments to their being 
 made true. They must necessarily partake of the defects of 
 the pattern made for casting them ; very few millwrights make 
 good patterns, and even their best efforts are sometimes rendered, 
 in some measure, abortive by the warping or shrinking of the 
 wood they have used. But supposing all difficulties with 
 respect to the pattern to be conquered, the liability to incorrect 
 work from the imperfections of the mould, the presence of stag- 
 nated air, and the unequal contraction of different portions of 
 the metal, are very considerable, and never absolutely overcome 
 by the most careful workmen. It is true that cast iron wheels 
 are better than any other, and that they may be rectified by 
 hand ; but this operation, if carried beyond a certain point, would 
 prove very expensive ; it is besides for other resons indesirable, 
 and seldom attempted in any great degree, because, as the surface 
 
MECHANICS. 
 
 323 
 
 The nature of the advantages of machines. 
 
 of cast iron is far harder than the interior, it would remove that 
 portion of the metal which always wears the best. Whoever, 
 therefore, attends to the motions of machinery, will frequently 
 observe some parts to move by jolts or starts ; the cause of which 
 is, the inaccuracy of the teeth or cogs. In consequence of this, 
 engineers, when they want a steady motion, as for a lathe, resort 
 to the expedient of employing a large train of wheels. They thus 
 commonly attain their prime object, for the probability is greatly 
 in their favour, that opposite imperfections will balance each 
 other ; but they have introduced a great increase of friction, and 
 a consequent necessity for a greater motive force, not to mention 
 the expence of the additional parts of the machinery. To avoid 
 in part these disadvantages, they might more frequently employ 
 the endless screw, for the reasons already alleged. The princi- 
 pal restriction to the use of this screw is, its being so liable to 
 wear when its motion is very rapid ; a rapid motion therefore, 
 should not be assigned to it, unless it be made of hardened steel, 
 when the objection will not apply. 
 
 Of Compound Machines. 
 
 When two or more of the simple mechanical pow’ers are 
 made to act in conjunction, to produce a given effect, the contri- 
 vance resulting from the union is called a compound machine or 
 engine. 
 
 Though any one of the mechanical powers is capable of 
 overcoming the greatest possible resistance in theory ; yet, in 
 practice, if used singly, they would frequently be so unma- 
 nageable as to render their properties nugatory. It is therefore 
 generally found best to combine them together ; by which means 
 the power is more easily applied, and many other advantages ob- 
 tained. 
 
 As the mechanical powers, in whatever manner they are com- 
 bined, still preserve their properties ; so, in compound as in sim- 
 ple engines, whatever is gained in power is lost in time ; conse- 
 quently, if a given power will raise one pound with a given velo- 
 city, it will be imposible for that power, by the help of any ma- 
 chine, to raise two pounds with the same velocity ; yet, by the as- 
 sistance of a machine, two pounds may be raised with half that 
 velocity, or one thousand pounds with a thousandth part of it ; but 
 still there is no greater quantity of motion produced when a thou- 
 sand pounds are moved, than when only one pound is moved ; 
 because the greater weight moves proportionately slower than the 
 lighter. The power, then, of machines, consists only in this, that 
 by their means the velocity of the weight may be diminished at 
 
324 
 
 MECHANICS. 
 
 Simplicity of machines to be studied. — Models not always to be depended on. 
 
 pleasure, so that with a given force any given resistance may be 
 overcome ; and the advantages they afford us are confined to con- 
 venience ; for instance, by machines, we are enabled to assign a 
 convenient direction to the moving power, and to apply its action 
 at some distance from the body to be moved, which are circum- 
 stances of the highest importance. By machines, also, we can so 
 modify the energy of the moving power, as to produce effects not 
 otherwise obtainable. 
 
 In contriving machines, simplicity of parts should always be 
 studied ; for in general the more complex they are, the more fre- 
 quently they are out of order, and the more difficult to repair ; 
 the more expensive also at first, and the greater their friction, 
 from the number or extent of their rubbing parts. A very com- 
 plex engine may indeed be a proof of the ingenuity of the inventor; 
 but if that be all, as soon as a more simple mode of effecting the 
 same purpose shall be known, the precarious tenure of his celebrity 
 will soon be evident, and he will be convinced that ingenuity 
 w ithout science is exerted to very little purpose ; and the display 
 of science is always the more complete, the fewer the parts of a 
 machine, or the more simple the means by which a given purpose 
 is attained. 
 
 To those who have a mechanical invention in view, and 
 who are as yet possessed of little practical knowledge, it may 
 prevent some disappointment to observe, that the performance 
 of the models, or small machines made for the sake of trial, may 
 equal their expectations, yet the action of such a machine 
 upon a large scale, may not prove permanently advantageous, 
 nor perhaps capable even for a short time of supporting an 
 analogous action. A large machine has not the same relative 
 strength as a small one ; it frequently admits not of the same 
 excellence of workmanship, and in general it cannot be made 
 of materials so , durable or possessed of so little friction. On 
 the contrary, it will sometimes happen, that a model will perform 
 indifferently, though a large machine on the same plan will 
 give satisfaction ; this may occur, when some of its parts are 
 so minute, and of such a description, that a common degree of 
 manual skill is insufficient to form them correctly. Experience 
 alone can effectually teach the art of making the proper allow- 
 ances in these different cases ; and the mere study of the theory 
 of the mechanical powers, will therefore supply the mechanic 
 with but a small portion of the knowledge which he ought to 
 possess. 
 
 To discover the mechanical power of any engine, it will be 
 sufficient to measure the space described in the same time by the 
 power and the resistance, or weight ; for the power always 
 
MECHANICS. 
 
 325 
 
 Wheel-work. 
 
 i; balances the weight, when it is in the same proportion as the 
 j velocity of the weight to the velocity of the power. Or, divide 
 the machine into all the simple ones of which it is formed ; then 
 begin at the power and call it one, and by the properties of the 
 mechanical powers find the forces in numbers which the first sim- 
 ).ple machine exercises upon the second. Call this force one, and 
 find the force in numbers with which it acts upon the third ; and 
 putting this force as one, ascertain its action on the fourth in num- 
 bers, and so on to the last. Then multiply all these numbers or 
 individual ratios of the power to the weight together, and the pro- 
 duct will be the force of the machine, supposing the first power 
 to be one. This has been exemplified by the method of calcu- 
 lating the power of a compound lever. 
 
 In wheel-work, it is evident from the principles already laid 
 down, that the velocity of a wheel is to that of a pinion, or smaller 
 wheel which it drives, or by which it is driven, in proportion to 
 the diameter, circumference, or number of teeth, in the pinion to 
 that of the wheel. If then the teeth in a wheel amount to 80, and 
 the leaves in a pinion to 10, the pinion will go 8 times round for 
 one revolution of the wheel, because 80 divided by 10, gives 8 for 
 the quotient. 
 
 If the product of the teeth in any number of wheels acting 
 on so many pinions, be divided by the product of the teeth or 
 leaves in the pinions, the quotient will give the number of turns 
 of the last pinion in one turn of the first wheel. Thus, if a 
 wheel Ay (fig. 12, pi. Ill,) of 48, acts on a pinion B of 8, on 
 the axis of which pinion there is a wheel C of 40, driving a 
 pinion D of 6, carrying a wheel E of 36, which moves a 
 pinion F of 6, carrying an index ; then the number of turns made 
 by the axis of the last pinion, and consequently by the index, 
 will be found in this manner: x x =240, which 
 
 are the number of turns made by the index, for one turn of the 
 wheel A. 
 
 It will be evident on a little consideration, that whatever 
 may be the number of teeth in the wheels and pinions, if they 
 bear the same ratio, they will give the same number of revolu- 
 tions to an axis. Thus, x y x ^-/ = * =240, as before. 
 
 The numbers, therefore, may be varied at the discretion of the 
 engineer, whose design must of course regulate his choice. One 
 wheel and pinion, it is also evident, will give the same motion 
 as many wheels and pinions, if the number of teeth contained 
 respectively in the single wheel and pinion, bear to each other the 
 same proportion as the product of the teeth in a train of wheels 
 bears to the product of the teeth in the pinions belonging to that 
 train. 
 
S26 
 
 MECHANICS. 
 
 Example of the mode of calculating the power of a compound maebioe. 
 
 When a wheel is moved immediately hy the power, it is called 
 a leader ; and if there be another wheel or pinion on the same 
 axis, it is called afollozcer. The leader receives, and the follower 
 imparts the motion. 
 
 As an example of the method of calculating the power of a 
 compound machine, we shall refer to the crane, fig. 4, pi. Hi. 
 When the handle is affixed to the axis of the wheel H of four- 
 teen teeth ; if the length from I to the centre of the square end 
 of the axis upon which it is fastened, be four times the semi- 
 diameter of the pinion H, then is the acting part of the lever 
 four times the length of the resisting part ; and it will act upon 
 the wheel G with four times the effect of the same power 
 applied directly to G, as for example in pulling it round by its 
 teeth ; therefore, if the man at the winch exert a power equal to 
 thirty pounds, the effect of it on the wheel G will be equal 
 to one hundred and twenty pounds. If then G be turned by H 
 with a force of 100, and having two and a half times as many 
 teeth, it only makes one revolution while H makes two and ^ 
 half, its pinion w ill exert a force of twice 120 added one half, or 
 300 on the wheel F. But as the wheel F has 14 times as 
 many teeth as the pinion by which it is driven, its revolutions 
 will be 14 times fewer than that pinion, and the power will be 
 proportionately increased ; therefore 14 times 300, which 
 amounts to 4,200, expresses the weight which, if appended to 
 the circumference of the wheel F, w ould keep a power of 30 at 
 the wienh in equilibrium. The weight, however, is not 
 appended to the circumference of the wheel F, but to the cir- 
 cumference of its axle, which being four times less, its velocity 
 is in the same proportion diminished, and its intensity must be 
 quadrupled to have the same effect on the pow'er ; so that 4 times 
 4,200, or 16,800 pounds will express the wFole weight w'hich a 
 power of 30 pounds applied to the winch when on the axis of 
 the wheel H, will be able to keep in equilibrium ; and when the 
 equilibrium is produced, so small an addition to the power gives 
 motion to the weight, that it is seldom mentioned, the power 
 W'hich effects the equilibrium being generally spoken of as if it 
 were actually sufficient to raise the weight. We have supposed 
 the crane to have but one winch, but such machines are gene- 
 rally provided with two, one at each extremity of the same 
 axle ; and it follow's, that a double intensity of pow'er will over- 
 come a double resistance. We have also supposed the power 
 to be 30 pounds, because that is about the intensity of the 
 power which one man could exert on the winch ; but as, in the 
 customary way of expressing the advantage gained by a 
 machine, the power is considered to be 1, to suit this mode of 
 
MECHANICS. 
 
 327 
 
 Importance of machinery to Great Britain, 
 
 expression, 16,800 must be divided by 30, and we shall then have 
 a quotient of o60, which number is to 1, as 16,800 is to 30, and 
 it shews how much the velocity of the power exceeds that of the 
 weight, when the velocity of the latter is 1, — the difference of 
 these velocities denoting the power of the machine, and being sy- 
 nonimous wdth the expression, that a power of one pound would 
 raise 560 pounds. 
 
 Another mode of estimating the power of the crane, fig. 4, pi. 
 Ill, will be, instead of proceeding from wheel to wheel, to divide 
 3420, the product of the teeth in the wheels F and G, by 98, the 
 product of the teeth in the wheel H, and the pinion on the axis of 
 G; the quotient of this division will be 35, which, if multiplied by 
 4, the difference between the radius of the circle described by the 
 winch and the radius of the wheel H ; and again by 4, or the dif- 
 ference between the radius of the axle round w hich the chain coils, 
 and the radius of the w heel F ; — the product will as before be 560 
 for the power gained. 
 
 The practical application of mechanics to the construction 
 of machinery, is a subject of the utmost importance to our 
 country; the prosperity of which materially depends upon its 
 commerce ; its commerce is derived chiefly from its manufac- 
 tures ; and some of its most important manufactures are 
 indebted to the general introduction of machinery for the pre- 
 ference they every-where receive. It is to this source we must 
 attribute the increase of property of every description, as the 
 introduction of a machine is a virtual creation of all the work 
 it will perform without further increasing human labour. 
 Among many, it is a prevalent opinion, that machinery is pre- 
 judicial to the interests of mankind, from its supposed tendency 
 to diminish the amount of that labour by which the lower 
 classes of society can alone purchase the means of subsistence. 
 This opinion is, however, erroneous, as applied to society in 
 general ; though individuals, whose labours are superseded by 
 machines, may suffer inconvenience for a time, yet it is only for 
 a time, and until they, or others more intelligent, discover a 
 new channel for the exertion of their industry. If improve- 
 ments in machinery enable our manufacturers to offer their 
 goods at lower prices than before, these goods will command a 
 sale in foreign markets w here it was previously useless to carry 
 them. The result of their success wall give them, and conse- 
 quently their country, an accession of capital; and the inevit- 
 able operation of an accession of capital, will be for the 
 advantage of the labouring^ classes, because it can no way be 
 expended, without increasing the general amount of labour, 
 and therefore making them in the end full participators in its 
 
328 
 
 MECHANICS. 
 
 Value of machinery. — Importance of uniformity of motion. 
 
 benefits. Thus are the real interests of all ranks inseparately 
 connected; and a retrospect of the last forty years ^vill shew, 
 that though so many machines have been employed in all 
 trades and manufactures, as probably to do more work than the 
 whole population could do previous to that period, yet the value 
 of human labour has increased in the same proportion as other 
 articles have advanced in price, except .so far as this natural 
 tendency of things has been checked by the regulations of war. 
 As machines tend to increase the quantities of those luxuries and 
 necessaries of life which mankind are so anxious to obtain, it 
 only requires that an equitable division of these benefits should 
 be effected, and then every objection to them will be obviated. 
 But such a division is not to be obtained by the infatuated 
 violence of individuals, nor by legislative or municipal con- 
 straints. On the contrary, this benefit can only spring wdth full 
 vigour from the ashes of all monopolies, combinatiems, or ex- 
 clusive immunities in trade. Let every individual be at ail 
 times free from the least restriction or check in the pursuit of 
 any business consistent with the common weal, every one would 
 be a gainer by the change ; if one source of employment failed, 
 abundance of other sources would still be open ; rew'ard would 
 be proportioned to ingenuity ; every branch of art would be occu- 
 pied by those who were led to it more by choice than accident, 
 and who were anxions to excel as much for their credit as from 
 necessity. The aspect of the country would then be improved ; 
 more cheerful industry would enliven it ; that laxity of moral con- 
 duct which is produced by insubordination, and fostered by the 
 protecting arm of injudicious immunities, would cease to be so 
 flagrant, if it did not disappear; and the advancement of ma- 
 chinery, far from being viewed by any class with malignant eyes, 
 would be regarded as an increasing and inexhaustible source of 
 national prosperity. 
 
 If we suppose the preceding observations on the utility of ma- 
 chinery were not radically true, it would be an invidious task to 
 pursue the present subject ; but unassailed by any apprehension 
 of this kind, we shall make them the prelude to some general re- 
 marks which may be useful to the young mechanic. 
 
 In contriving machinery, it should always be remembered, 
 that nothing will contribute more to its perfection, especially 
 if it be massive and ponderous, than great uniformity of 
 motion. Every irregularity of motion wastes some of the 
 impelling power ; strains, jolts, and whatever occasions a vibra- 
 tory motion of the parts within themselves, weaken the cohesion 
 of the most solid substances, and are particularly injurious to 
 cast iron, and the pressures at the communicating points are 
 

 PuNuf/tvJ iy .JUu r A* v/ .//Vt // * *<’/,- //vv'/.y 
 
MECHANICS. 
 
 329 
 
 Communication of motion by levers — cranks — toothed wheels. 
 
 inconstant and unequal. A great engine, constructed without 
 due regard to the uniformity of its motion, will shake the firmest 
 building ; but when uniform motion pervades the whole, the 
 inertia ©f each part tends to preserve this uniformity, and all goes 
 smoothly. 
 
 In modifying the motion of the first mover, and communi- 
 cating it in a proper mannei' to the subject to be operated upon, 
 the slow rotative motion of a water-wheel is, by the machinery 
 of cranks, levers, and toothed wheels, converted into a rapid 
 reciprocating motion for w orking saw s ; and the velocity of the 
 motion is increased or diminisJied, as the occasion requires 
 either great pow er or great speed. In like manner, the rectilinear 
 motion of the piston rod of a steam engine is, by the machi- 
 nery of parallel levers, working beam, connecting rod, crank and 
 fly-wheel, converted into a rotative motion ; and this motion is 
 again, by the machinery of wheel-work, adapted to work grinding 
 stones, circular saw'-s, threshing mills, and other similar machines 
 w hich require great velocity ; or flatting mills, boring machines, 
 machines for rasping dyewoods, draw ing lead pipe, &c. which re- 
 quire great power to give them motion, and are therefore per- 
 formed with less velocity. 
 
 In modifying a rotary motion, toothed wheels are most 
 generally employed ; and if the teeth are properly formed, 
 wheels, perhaps, consume less force in friction than any other 
 mode of transmitting motion. In forming the teeth of wheels, 
 a deviation from the perfect form is of most importance w here a 
 very large wdieel drives a very small one, a case the judicious 
 engineer should always endeavour to avoid. It is of great im- 
 portance to make all the teeth of a wheel precisely equal, and to 
 make as great a number of them as the necessary strength will 
 allow^ The greater the number of the teeth, the less w ill be the 
 time that any one of them will act upon its fellow, and several 
 teeth being in action at once, will cause the communication of the 
 motion to be extremely smooth and uniform. To obtain 
 
 strength, when the cogs are made flne, the width or thickness 
 of the wheel must be increased ; and this is one of the greatest 
 practical improvements which have been made in machinery for 
 twenty years past. When the teeth are far apart, three, four, 
 or five inches, for example, they always act unequally upon each 
 other, in consequence of the point of contact altering its posi- 
 tion, becoming alternately nearer or farther from the centre of 
 one or other of the wheels ; the acting radius of one is thus in- 
 creased, while that of the other is diminished, and their 
 velocity and powers varying in consequence with every cog 
 that passes by, the machine works by starts and ierks, 
 14.— VoL.I. 2U 
 
330 
 
 MECHANICS. 
 
 Communication of motion by endless straps and bands. 
 
 Wheels are found to work most smoothly, when the teeth of the 
 large w'heel are made of hard wood, and the teeth of the small 
 one of cast iron. 
 
 A rotary motion is very frequently transmitted by means 
 of an endless strap, or belt, particularly when a very quick 
 motion / is to be created, and the re-action to be overcome is 
 nearly equable. In such cases, it has the advantage of wheel- 
 work from its simplicity, the ease of its motion, and the 
 distance to which it may be conveyed. A strap should always 
 work on a pulley which is highest in the middle of its circum- 
 ference, otherwise it will be exceedingly apt to slip off. If one 
 of the pulleys is stopped while the strap is moved round by the 
 motion of the other, the strap instantly flies off its pulley, 
 unless the breadth of the circumference of the pulley greatly 
 exceeds that of the strap. This property is a great recom- 
 mendation of it on many occasions, where the machines might 
 be much injured or destroyed if driven by wheel-work and ac- 
 cidentally stopped. Straps should be of an equal thickness 
 and breadth throughout. They are mostly joined by sewing; 
 but the best method is by gluing them together, with a glue com- 
 pounded of Irish glue, isinglass, ale grounds, and boiled linseed 
 oil. The ends that overlap should be pared thin, tapering to the 
 edge in the form of an inclined plane, so that the juncture, w hen 
 they are placed upon each otlier, shall be no thicker than the rest 
 of the strap. 
 
 The wheels which are turned by straps never make quite so 
 many revolutions as they ought to do from a calculation of the 
 diameters of the pulleys over which the straps pass. Sometimes 
 the straps may slip a little, but the principal source of the error 
 has been ingeniously attributed to their elasticity, which per- 
 mits them to stretch on that side which bears the strain (called 
 the leading side) and to collapse on the returning side. This 
 error, if elasticity be the cause of it, w’ill augment with the 
 intensity of the strain, and the distance between the pulleys ; but 
 its utmost amount is so small as to be rarely of any great conse- 
 quence. 
 
 Bands of rope or catgut are frequently employed to com- 
 municate motion, particularly to the mandrels of lathes. Catgut 
 is the best material known for a band, and always to be pre- 
 ferred when it can be had of sufficient strength for the purpose : 
 the ends are united by a small steel hook and eye, each of these 
 has a socket screws to receive the band, which is tapered a little, 
 and screwed into it with a little rosin. If the hook and eye be 
 made warm enough to keep the rosin fluid during the fastening, 
 the band will be verv firm, though it may, for further security, 
 
MECHANICS. 
 
 331 
 
 Communication of motion by endless chains and by friction^ 
 
 be seared with a hot wire on the extremity projecting through 
 the socket. The groove of pulleys intended to receive a band, 
 should be of a sharp angular form, that the band may not 
 touch the bottom of it, in which case it would be liable to slip. 
 Wood is the best material for pulleys, because the polish soon 
 ' acquired by metals, prevents the band from holding firmly, 
 
 I The wood for a pulley should be cut endways, that is, with 
 its grain in the direction of its axis, so that every part of the 
 circumference being of a similar texture, it will wear equally. 
 Two methods of applying a band present themselves : in one of 
 them it is carried over parts of the wheel and pulley corresponding 
 in position, the upper part of the band passing directly to the upper 
 part of the pulley, and the lower part to the lower part ; but in 
 the other method, the band is crossed, and therefore it passes from 
 * the upper part of the wheel to the under part of the pulley. A 
 crossed band answers the best end ; for as it envelopes a larger 
 portion of the pulley, it produces a better effect than the other, 
 
 I even when not so tight. 
 
 Endless chains are sometimes used to communicate motion, 
 
 ■ and where their slipping would be injurious, cogs are frequently 
 ! formed on the wheels, to be received into the links of the chain. 
 
 The links should be formed with great exactness. 
 
 ^ For very light machinery, a very neat and even elegant 
 j mode of communicating motion has long been partially in use ; 
 
 it consists in covering the circumferences of the wheels with 
 I buff leather, which creates sufficient friction to make them turn 
 each other freely, although not pressed very hard together. The 
 i same principle has been adopted upon a large scale for a saw- 
 mill, in which the wheels acted upon each other by the contact of 
 the end grain of wood instead of cogs. The machinery wore 
 I well, made little noise, and was in use tw'enty years. When this 
 mode of transmitting power is adopted, a contrivance to make the 
 wheels bear firmly against one another, either by wedges at the 
 socket, or by levers, must be included. 
 
 It requires all the art of the engineer, when reciprocating 
 and desultory motions are required, to introduce them in the 
 most advantageous manner. Eccentric wheels are frequently 
 employed, but the common crank is perhaps the most lastingly 
 useful (principally on account of its being the most simple) con- 
 trivance, for controverting a reciprocating motion into a circular 
 one, or the contrary. Attempts have been made to move the 
 pistons of pumps by means of a double rack on the piston rod ; 
 a half wheel takes hold of one rack and raises it to required 
 height ; the moment the half wheel has quitted that side of 
 the rack^ it lays hold of the other side, and forces the piston 
 
332 
 
 MECHANICS. 
 
 Methods of raising stampers. 
 
 down again. Tl'his has been proposed as a great improvement, bj 
 correcting the unequal motion ot' the piston, moved in the com- 
 mon way by a crank motion; but it occasions stich abrupt changes 
 of motion, as to be wholly inadmissible in practice; the more 
 ponderous the machine, and the more correct its workmanship, 
 the sooner it would shake it to pieces. 
 
 When heavy stampers are to be raised, in order to drop on the 
 matter to be pounded, the w ipers by which they are lifted, should 
 be made of such a form, that the stamper may be raised by a uni- 
 form pressure, or with a motion almost imperceptible at tirst. if 
 this is neglected, and the wiper is only a pin sticking out from the 
 axis, the stamper is forced into motion at once. This occasions 
 a violent jolt to the machine, and great strains on its moving parts 
 and their points of support ; whereas, when they are gradually 
 lifted at tirst, the inequality of desultory motion is never felt at the 
 impelled point of the machine. The principle, how ever, of com- 
 municating the motion gradually may be carried too far. In order 
 to avoid the great inconvenience arising from the abrupt motion 
 given to a great sledge hammer of seven hundred weight, resisting 
 with a five-fold momentum, an engineer formed the wipers for 
 lifting it into spirals, which communicated motion to the hammer 
 with scarcely any jolts whatever; but the result was, that the 
 hammer rose no highei\than it had been raised in contact with the 
 wiper, and then fell on the iron with very little effect. Wipers of 
 the common form were therefore of necessity substituted for the 
 spirals ; for in this operation the rapid motion of the hammer, 
 during the greater part of its progress, is absolutely necessary ; it 
 is not enough to lift it up ; it must be tiung up so as to rise higher 
 than the wiper lifts it, and to strike with great force the strong 
 oaken spring which is placed in its way. It compresses this spring, 
 and is reflected by it with a considerable velocity, so as to hit the 
 iron as if it had fallen from a great height; had it been allowed to 
 fly to that height, it would have fallen upon the iron with some- 
 what more force, (because no spring is perfectly elastic) but twice 
 the time would have been required. 
 
 All ponderous movements should be supported by a frame- 
 work of wood, or of iron upon w ood, independent of the build- 
 insf of masonrv or brick-work containing them. The want of 
 attention in this respect, has not unfiequently occasioned build- 
 ings to be shaken to pieces. If the gudgeons of a water-wheej, 
 for example, rest upon the wall of a building recently erected, it 
 can scarcely fail to prevent the perfect induration of the mortar, 
 and the strength of the wall will thus be completely crippled. 
 If such a situation must be selected, the gudgeons should be sup- 
 ported upon a block of oak laid a little hollow. This will soften 
 
MECHANICS. 
 
 335 
 
 Detaching and reversing motions. 
 
 all tremors, like the springs of a carriage, and a prudent extension 
 of the principle would be very serviceable in many parts of the 
 construction. 
 
 To avoid the injurious effects occasioned by urging sud- 
 denly or by jerks the parts of a ponderous machine from a state 
 of rest into a state of motion, an ingenious contrivance has lately 
 been introduced, which deserves to be generally known. The 
 arm which gives motion to the machine, when the clutch or con- 
 necting part of the running spindle is engaged with it, is not 
 fixed fast upon the spindle, but is made in two halves screw'ed 
 together upon a cvlmdrical part of the spindle, and so closely 
 pinched upon it by screw's, that it will have sufficient friction to 
 turn the machine round in tfie ordinary course of its w'ork, but 
 slips round upon the spindle, if the resistance is greater than this 
 friction, which thus becomes the measure of the power exerted 
 upon the machine. 
 
 Contrivances for uniting or detaching motions are very vari- 
 ous. The supports of the gudgeons of toothed wheels are some- 
 times fitted up so as to be moveable, so that the wheels can be 
 separated so far as to relieve each other’s teeth. At other times 
 one of the wheels is fitted on a round part of its axis, and united 
 with it at pleasure, by what is called a clutch-box. Thus the 
 wheels are always in motion, but one of them can be detached at 
 pleasure from its axis, on which it slips freely. Bevelled cog- 
 wheels are easily disengaged, by moving the axis of one of them 
 a little endways. For disengaging the motion of a strap, the con- 
 trivance called the live and dead pulley is very ingenious and 
 effectual : it consists of two pulleys placed close together upon 
 any axis w hich is to receive a circular motion. One of them is 
 fast upon the spindle, and the other loose, so as to slip round. 
 It is necessary that the wheel by which these pulleys are turned, 
 should have its nm at least equal in breadth to that of both the 
 pulleys I'his contrivance is extensively used, and as applied to 
 a lathe, has been described in the section on Turning. 
 
 Jt is liequently necessary in machinery to have the power of 
 reversing the motion of a wheel or axis at any required interval. 
 Various means are used for effecting this object. The most 
 common is by two e-qual and similarly bevelled or contrate 
 wheels, situated on the same axis, with their teeth towards each 
 other. A third bevelled wheel is applied with its axis perpendi- 
 cular to these ; and as its teeth, by simply moving a lever, can be 
 made to engage either of them at pleasure, they will, as they act 
 on contrary sides of this third wheel, communicate to it opposite 
 motions. Smeaton applied this movement to draw' coals from 
 coal-pits. 
 
334 
 
 MECHANICS. 
 
 Utility of fly-wheels. 
 
 Of Fly Wheels. 
 
 In all machines, the moving power and the resistance are sub- 
 ject to fluctuations of intensity. It becomes, therefore, an object 
 of great moment, to have, in most compound machines, some 
 means of accumulating the excess of the motive power, and of 
 expending this excess, when the motive power operates too feebly. 
 This equalization of motion is usually obtained by what is called a 
 jiyy which is generally made m the form of a wheel, though some- 
 times it is merely two bars crossing one another at right angles in 
 the middle, with weights at the four extremities. 
 
 A fly being made to revolve about its axis, keeps up the 
 force of the power, and distributes it equally in all parts of its 
 revolution. On account of its weight, a small variation in 
 force does not sensibly alter its motion ; whilst its friction, and 
 the resistance of the machine, prevents it from accelerating. 
 If the motive power slackens, it impels the machine forward, 
 and if the power tends to move the machine too fast, it keeps it 
 back. 
 
 In all machines in which flies are used, either a considerably 
 greater force must be applied at first than what is necessary 
 to give motion to the machine w ithout it, or the fly must be set 
 in motion some time before the force is applied to the machine. 
 This superfluous power is collected in the fly, which is in fact a 
 reservoir of motion. A man, working at a common windlass, 
 exerts a very irregular pressure on the w inch. In two of his posi- 
 tions, during each turn, he can exert a force of nearly seventy 
 pounds without fatigue, but in other positions he exerts a force of 
 little more than twenty-five pounds ; nor must he in general have 
 to oppose much above this ; but if a large fly be properly con- 
 nected with the windlass, he will act with eq^ual ease and speed 
 against thirty or even forty pounds. 
 
 The motion communicated to a fly-wheel by means of a 
 small force, may be accumulated to such a degree as to produce 
 eft’ects which the original force would never have accomplished. 
 Atwood has demonstrated in his Treatise on Rectilineal and 
 Rotary motion,” that a force equivalent to 20 pounds, applied 
 for the space of 37 seconds to the circumference of a cylinder 
 20 feet in diameter, which weighs 4713 pounds, would, at the 
 distance of one foot from the centre, give an impulse to a mus- 
 ket-ball equal to that which it receives from a full charge of 
 powder. In the space of six minutes and ten seconds, the same 
 efl'ect would be produced, if the wdieel were driven by a man, 
 who constantly exerted a force of 20 pounds at a winch one 
 
MECHANICS. 
 
 355 
 
 Power accumulated, not increased by a fly. 
 
 foot long. This accumulating power of a fly, induces among 
 many a supposition, that a fly really adds power or mechanical 
 force to an engine ; accordingly, from not understanding on what 
 its efficacy depends, nor considering, that if it communicated a 
 power which it did not receive, it must, contrary to the nature of 
 matter, possess a principle of motion within itself, they often place 
 the fly in a situation where it only adds a useless burden to the 
 I machine. If intended for a mere regulator, it should be near the 
 I first mover : if intended to accumulate force in the working point, 
 
 ; it should not be far separated from it. 
 
 It is certain, that a fly does not communicate any absolute 
 I increase of motion to the machine ; for if a man, or any animal, 
 is not able to set an engine in motion without a fly, he will not 
 be able to do it though a fly be applied ; nor will he be able to 
 keep it in motion, though set to work with a fly by means of a 
 I greater power. The apparent creation of power by a fly con- 
 sists in its accumulating into one moment, the exertions of 
 I many. A man, caught by some of the movements of a country 
 mill, may be instantly deprived of a limb or of life. In this 
 case, the power of the stream is conceived to be prodigious, 
 and yet we are certain, upon examination, that it amounts to 
 the pressure of no more than fifty or sixty pounds ; but this 
 force has been acting for some time, and there is a millstone of 
 a ton weight whirling twice round in a second ; the effect, 
 
 • therefore, not of any self-derived but of the accumulated 
 power, is enormous. Contrivances to prevent accidents from the 
 ! force of machinery, deserve every eiico*uragement ; and per- 
 haps among the improvements yet to be made in practical 
 mechanics, they will be more conspicuous than they have 
 hitherto been. It has been asserted, that in the neighbourhood of 
 ' Elbingroda, in Hanover, there was a contrivance which disen- 
 I gaged the millstone when any thing got entangled in the teeth of 
 I the wheels. On being tried w ith the liead of a cabbage, it crushed 
 it, but not violently, and would by no means have broken a man’s 
 arm. 
 
 The resistance which the air opposes to any body in motion, 
 and the friction of the pivots which support the axis of a fly, 
 
 ' are considerable deductions from the power communicated to 
 this appendage of machinery, so that instead of really gaining 
 power, a fly-wheel requires a constant exertion to keep it in 
 motion, even when no further resistance is applied to prevent it. 
 For this reason, a fly-wheel should never be introduced into a 
 machine unless the advantages to be derived from its action are 
 greater than the actual loss of power it occasions. In general, 
 ^'here the power is tolerably uniform in its action, if the resist- 
 
S36 
 
 MECHANICS. 
 
 Best form for a fly. — Method of calculating the effect of a fly. 
 
 ance can be made so too,, a fly is not necessary. If two ham- 
 mers are raised at the same moment by a water-wheel, during 1 
 the interval of their descent much power would be lost, unless 
 it were collected by a fly ; but if the two hammers, or any other | 
 number, were raised in succession, the resistance would be ren- 
 dered nearly as uniform as with a fly-wheel, without its inconve- 
 niences. 
 
 A fly for the accumulation of power, should be so constructed 
 as to present the smallest resistance to the air. A wheel is the 
 best form ; it should be made of metal, that it may have a great 
 weight under a small surface, and it should be smooth and 
 truly circular, without any projecting nuts. If the transverse 
 section of the rim be a circle, and the transverse sections of the 
 arms connecting the rim with the centre, be ellipses, presenting 
 their thin edge to divide the air, the fly will be less resisted by 
 the air than under any other form. This configuration of a fly 
 is included in a patent taken out by Murray and Wood, of 
 Leeds. 
 
 FI). -wheels are usually made of iron; when they are too large 
 to be cast in one piece, it is very important to unite the parts in 
 the most substantial manner ; for the centrifugal force of a great 
 wheel in rapid motion is prodigious, and if the bolts be insuflicient 
 to withstand the strain upon them, the parts broken oflp will be pro- 
 jected with a velocity giving them the destructive pow er of a can- 
 non-shot. 
 
 Su})pose a fly be employed in a machine required to raise a 
 pestle of thirty pounds weight to the height of one foot sixty 
 times in a minute ; here the weight of a fly is a principal object, 
 and its eft’ect i? calculated by a comparison with the weight to m 
 be raised. Let the diameter of the fly be seven feet, and sup- 
 posing the pestle to be raised once by every revolution, w e must 
 tlien consider what weight, passing in one second, through a 
 space equal to the circumference of the fly, which is about 22 n 
 feet, will be equivalent to 30 pounds passing through one foot 
 in a second. I’his will be 30 divided by 22, or Were a 
 
 fly of this kind applied, and the machine set in motion, it 
 would be able to lift the pestle once after the moving power w'as 
 w ithdrawn ; but by increasing the w eight of the fly to teu, 
 twelve, or twenty pounds, the machine, when left to itself, 
 would make a considerable number of strokes, and after the 
 incumbrance of the fly at the outset was overcome, it would be 
 workc-d wdth much less labour than if no fly had been used. 
 The mode of calculation here adopted, is equally applicable to 
 the motion of pumps ; but the weight which can be most advan 
 tageously given to a fly lias never been satisfactorily determined. 
 
MECHANICS. 
 
 337 
 
 Causes obstructing the motions of machines. 
 
 Of Friction* 
 
 With tlte exception of a few incidental remarks, we iiave 
 hitherto paid no attention to the physical properties of the mate- 
 rials of which machines are composed, and of the alterations those 
 properties occasion in the effects established by theory alone. 
 This was necessary, to disentangle the theory from the constant 
 recurrence of proviso’s and limitations, which would be best un- 
 derstood if distinctly considered ; but we must now proceed to 
 point out the impediments to the perfect action of machines, and 
 the allowances to be made for them. 
 
 Among the various physical causes which occasion a difference 
 between theory and practice, with respect to machines, the two 
 following may be considered the most important and most gene- 
 ral : 1. The weight of the parts composing the machines. 2. 
 
 Friction, a term which designates the obstruction to the motion 
 of a machine, produced by the resistance of the air, as well as that 
 produced by the rubbing of one part against another. In trying 
 experiments to exemplify the theory of the lever, it has been shewn 
 that the shorter arm must be made as heavy as the longer arm, 
 otherwise they will not perfectly succeed. This shews the prin- 
 ciple on which allow ances must be made for the weight of machi- 
 nery, and liow this cause of obstruction may be obviated. Fric- 
 tion is, however, a cause of obstruction alw ays present ; it may be 
 diminished by various artifices, but never entirely removed. 
 
 Leslie, in his valuable work on the nature and propagation 
 of beat, adverts to the cause of friction in a very able manner : 
 If the two surfaces,” says he, which rub against each 
 ether, are rough and uneven, there is a necessary waste of force, 
 occasioned by the grinding and abrasion of their prominences. 
 But friction subsists after the contiguous surfaces are w'orked 
 down as regular and smooth as possible. In fact, the most ela- 
 borate polish can operate no other change than to diminish the 
 size of the natural asperities. The surface of a body being 
 moulded by its internal structure, must evidently be furrowed, 
 toothed, or serrated. Friction is, therefore, commonly ex- 
 plained on the principle of the inclined plane, from the effort 
 required to make the incumbent weight mount over a succes- 
 sion of eminences. But this explication, however currently 
 repeated, is quite insufficient. The mass which is drawn along 
 is not continually ascending ; it must alternately rise and fall : 
 for each superficial prominence will have a corresponding 
 cavity; and since the boundary of contact is supposed to be 
 horizontal, the total «levations will be equalled by their collate* 
 Vol.L 2X ^ 
 
S38 
 
 MECHANICS. ' 
 
 Leslie's investigation of the nature of friction. 
 
 ral depressions ; consequently, if the lateral force might suffer a 
 perpetual diminution iu lifting up the weight, it would, the next 
 moment, receive an equal increase by letting it down again ; and 
 those opposite effects, destroying each other, could have no influ- 
 ence whatever on the general motion. 
 
 ‘‘ Adhesion seems stijl less capable of accounting for the 
 origin of friction. A perpendicular force acting on a solid, 
 can evidently have no effect to impede its progress ; and though 
 this lateral force, owing to the unavoidable inequalities of 
 contact, may be subject to a certain irregular obliquity, the 
 balance of chances must, on the whole, have the same tendency 
 to accelerate, as to retard, the motion. If the conterminous 
 surfaces were, therefore, to remain absolutely passive, no fric- 
 tion could ever arise. Its existence demonstrates an unceasing 
 mutual change of figure, the opposite planes, during the 
 passage, continually seeking to accommodate themselves to all 
 the minute and accidental varieties of contact. The one surface 
 being pressed against the other, becomes, as it were, compactly 
 indented, by protruding some points and retracting others. 
 This adaptation is not accomplished instantaneously, but 
 requires very different periods to attain its maximum^ ac- 
 cording to the nature and relation of the substances concerned. 
 In some cases a few seconds are sufficient; in others, the full 
 effect is not produced till after the lapse of several days. 
 While the incumbent mass is drawn along, at every stage of its 
 advance, it changes its external configuration, and approaches 
 more or less towards a strict contiguity with the under surface.. 
 Hence the effort required to put it first in motion ; and hence, 
 too, the decreased measure of friction, which, if not deranged 
 by adventitious causes, attends generally an augmented rapidi- 
 ty. This appears clearly established by the curious experiments 
 of Coulomb, the most original and valuable which have been 
 made on that interesting subject. Friction consists in the force 
 expended to raise continually the surface of pressure by an oblique 
 action. The upper surface travels over a perpetual system of 
 inclined planes ; but that system is ever changing with alter- 
 nate inversion. In this act, the incumbent weight makes inces- 
 sant, yet unavailing, efforts to ascend : for the moment it has 
 gained the summits of the ' superficial prominences, these sink 
 down beneath it, and the adjoining cavities start up into eleva- 
 tions, presenting a new series of obstacles which are again to be 
 surmounted ; and thus the labours of Sisiphus are realized in the 
 phenomena of friction. 
 
 The degree of friction must evidently depend on the 
 angles of the natural protuberances, and which are determined 
 
! 
 
 MECHANICS. 
 
 ’ Quantity of friction in various instances. 
 
 ; by the elementary structure or the mutual relation of the two 
 approximate substances. The effect of polishing is only to 
 abridge those asperities and increase their number, without 
 altering in any respect their curvature or inflexions. The con- 
 I staiit or successive acclivity produced by the ever-varying adap- 
 } tation of the contiguous surfaces remains, therefore, the same, 
 
 I and consequently the expense of force will still amount to the 
 same proportion of the pressure. The intervention of a coat of 
 oil, soap, or tallow, by readily accommodating itself to the vari- 
 ations of contact, must tend to equalize it, and therefore must 
 ‘ lessen the angles, or soften the contour, of the successively emerg- 
 
 ! ing prominences, and thus diminish likewise the friction which 
 thence results.” 
 
 The friction of a single lever is very trifling. The friction of 
 
 i the wheel and axle is in proportion to the weight, velocity, and 
 the diameter of the axle ; the smaller the diameter of the axle, the 
 less will be the friction. 
 
 I Pulleys have very great friction, on account of the smallness 
 ! of their diameters in proportion to that of their axles, and their 
 I friction is greatly increased when they bear, as they are very apt 
 , to do, against their blocks, and when their centres and axles are 
 worn untrue. 
 
 j The friction of bodies is in general proportionate to their 
 ji weight, or the force with which their rubbing surfaces are 
 I pressed together ; and is for the most part equal to betw een one- 
 half and one-fourth of that force. Although friction increases with 
 an increase of surface, yet this does not take place in direct pro- 
 
 I portion to that increase. It also increases, with some exceptions, 
 in proportion to the velocity of bodies^ particularly when very dif- 
 !! ferent substances are employed without an unguent. 
 
 According to Emerson, when a cubical piece of soft wood, of 
 
 I eight pounds weight, moves upon a smooth plane of soft wood, 
 at the rate of three feet per second, its friction is about one-third 
 i of its w^eight ; but if it be rough, the friction is little less than half 
 ! the weight : on the same supposition, when both the pieces of 
 j wood are very smooth, the friction is about one-fourth of the 
 ! weight ; the friction of soft wood on hard, or of hard wood on 
 j soft, is one-fifth or one-sixth of the weight ; of hard w^ood upon 
 i hard wood, one-seventh or one-eighth ; of polished steel moving 
 » on steel or pewter, one-fourth ; moving on copper or lead, oue- 
 ! fifth of the weight. 
 
 It was generally supposed, that in the case of wood, the fric- 
 tion is greatest when the bodies are draped contrary to the course 
 of their fibres ; but the experiments of Coulomb demonstrate the 
 contrary. 
 
 3S9 
 
340 
 
 MECHANICS.’ 
 
 Quantity of friction in various instances. 
 
 The longer the rubbing surfaces remain in contact, the 
 greater is their friction. When wood was moved upon wood 
 .according to the direction of the fibres, the friction was increas- 
 ed by keeping the surfaces in contact for a few seconds ; and 
 when the time was prolonged to a minute, the friction seemed ta 
 have reached its farthest limit. But when the motion was 
 performed contrary to the course of the fibres, a greater time 
 -was necessary before the friction arrived at its maximum. When 
 W'ood was moved upon metal, the friction did not attain its maxi- 
 mum till the surfaces continued in contact for four or five days 
 and it is very remarkable, that when wooden surfaces were 
 anointed with tallow, the time requisite for producing the greatest 
 quantity of friction was increased. The increase of friction 
 w hich is generated by prolonging the time of contact is so great, 
 that a body w eighing l6‘o0 pounds was moved with a force of 64 
 pounds when first laid upon its corresponding surface. After 
 having remained in contact for the space of three seconds, it re- 
 quired 160 pounds to put it in motion, and when the time was 
 prolonged to six days, it could scarcely be moved with a force of 
 622 pounds. When the surfaces of metallic bodies were moved 
 upon one another, the time of producing the greatest effect or 
 maximum of friction was not changed by the interposition of olive 
 oil ; more time, however, was required to produce the maximum 
 W'hen swine’s grease was employed as an unguent ; and it was pro- 
 longed to five or six days when the surfaces had been besmeared 
 with tallow'. 
 
 In wood rubbing upon wood, oil, grease, or black-lead, pro- 
 perly applied, makes the friction tw’o-thirds less. Wheel naves,, 
 when gi eased, have not more than one-fourth of the friction they 
 w'ould have if only w etted. 
 
 When polished steel moves on steel or pewter, properly 
 oiled, the friction is about one-fourth of the w eight ; on copper 
 or lead, one-fifth of the w eight ; on brass, one-sixth ; and 
 metals have more friction when they move on metals of the 
 same kind, than when they move on different metals. This 
 seems principally owing to the superior strength of the attraction 
 . of cohesion between similar metals. It is aKvays desirable, 
 therefore, to make the parts of machines opposed to or working 
 in each other, of different materials ; thu.s, in clocks and 
 v’atches, the wheels are brass, and the pinions steel, and iron 
 pivots are made to w'ork in brass or bell-metal collars. The axes 
 of wheels should also be made as small as the weight they have 
 to bear w ill allow' ; because the diminution of the surfaces rub- 
 bing against each otlier, will be attended with a diminution of the 
 friction. 
 
MECHANIC.?. 
 
 341 
 
 Viuce’s experiments on friction. — Coulomb’s experiments. 
 
 According to Vince’s experiments, which were made 
 with great care, and frequently repeated, the friction of hard 
 bodies in motion is a uniformly retarding force. Experiments 
 w'ere instituted to determine whether the same law obtained 
 when the bodies were covered with cloth, woollen, &c. and it 
 was found, in all cases, that the retarding force increased with 
 the velocity ; but on covering the bodies with paper, the results 
 agreed with those before stated. By other experiments, the 
 same philosopher found, that the quantity of friction, contrary to 
 the prevailing opinion, increased in a less ratio than the quantity 
 or weight of the body ; also, that the smallest surface has the 
 least friction. It may be proper to describe the apparatus by 
 which these results were obtained : a plane was adjusted parallel 
 to the horizon ; at the extremity w as placed a pulley, which could 
 be elevated or depressed, so as to render the string which con- 
 nected the body and the moving force, parallel to the plane. A 
 divided scale was placed near the pulley, perpendicular to the ho- 
 rizon ; and the moving force descended by the side of this scale. 
 A moveable stage was placed upon the scale, which could be ad- 
 justed to the space through which the moving force descended in 
 any given time, which time was measured by a well regulated pen- 
 dulum, vibrating seconds. 
 
 According to Coulomb’s experiments, which were ^conduct- 
 ed on a large scale, and are therefore much relied on, the 
 friction of lignum-vit£e cylinders, two inches in diameter, and 
 loaded with one thousand pounds, was 18 pounds, or nearly^J!^. 
 of the weight or force of pression. In cylinders of elm, the 
 friction was greater by |, and was scarcely diminished by the 
 interposition of tallow’. From a variety of experiments on the 
 friction of the axes of pulleys, the following results w'ere ob- 
 tained ; w hen an iron axle moved in a brass bush or bed, the 
 friction was ^ of the pression ; but when the bush w as besmeared 
 with very clean tallow^, the friction w as only ; when swdne’s 
 grease was interposed, the friction was about and when 
 olive oil was employed, it was about When the axle was of 
 green oak, and the bush of lignum-vitee, the friction was wdien 
 tallow’ was interposed ; but when the tallow was removed, so 
 that a small quantity of grease only covered the surface, the 
 friction was increased to When the bush was made of 
 
 elm, the friction was, in similar circumstances, and 
 which is the least of all. When the axle w’as made of box, and 
 the bush of lignum-vitge, the friction was and circum- 
 stances being the same as before. If the axle be of boxwood, 
 and the bush of elm, the friction will be and ^ ; and if the 
 
342 
 
 MECHANICS. 
 
 Use of friction rollers. 
 
 axle be of iron, and the bush of elm, the friction will be ^ of the 
 force of pression. 
 
 In calculating the force of an engine, friction should never 
 be overlooked. Though it varies so much with circumstances, 
 that it is not yet reduced to certain rules, still the specific 
 details we have given will enable the young mechanic to come 
 tolerably near the truth, in ascertaining its amount, at each part 
 of a machine according to the pressure, surface, atid materials ; 
 and as he goes along from the power to the resistance, he must 
 consider these amounts as actual deductions from the advantage 
 of the machine. It must be understood, that the amount of 
 friction stated in this section, will apply only to machines that 
 are well made ; the loss of power that may be occasioned by 
 bad workmanship is incalculable, and as bad workmanship may 
 exist when it is not perceived, no conjectural calculation should 
 be relied on, when the real loss of power can be obtained by ex- 
 periment. 
 
 One general rule of preventing friction is to substitute, 
 whenever it is possible, the rolling for the sliding motion. 
 Every one knows that a weight which the application of a given 
 force cannot drag, may be easily drawn along by the same 
 force, if mounted upon wheels turning on their axes. On this 
 principle idepends the utility of what are called friction rollers, 
 which are small cylinders or spheres interposed so as to revolve 
 between surfaces that would otherwise rub upon each other ; or 
 small wheels so disposed that the pivots of larger wheels revolve 
 upon their circumferences, instead of turning in a bush or 
 socket. Three wheels, it is obvious, if they touch the different 
 sides of the pivot in three points equidistant from each other, will 
 support it as effectually as a cylindrical socket, which would have 
 far greater friction. When the motion to which these rollers or 
 wheels are subjected, is equal and steady, the use of them will 
 often be productive of permanent advantage ; but when, as for the 
 axletrees of carriages, they are subject to violent strains, jolts, 
 and, occasionally at least, to enormous pressure, they are, how- 
 ever excellent in principle, seldom found of much practical utility, 
 and are often positively injurious ; for they and the parts bearing 
 upon them, either do not receive from the workman the precise 
 figure they ought to have, or if well made, they wear so unequally 
 as soon to lose that correctness of figure which is inseparable from 
 their* value. 
 
MECHANICS. 
 
 S4S 
 
 — 
 
 I Variableness of the force exerted by a man at a winch. 
 
 !| Of the Application of Men and Horses, as moving Powers in 
 jj Machinery, &;c. 
 
 ] A man turning a horizontal windlass by a handle or winchj 
 I should not have to exert a greater force than 30 pounds, if lie have 
 i to work ten hours a day ; therefore the power exhausted by fric- 
 if tion, the stiffness of ropes, and the intensity of the rsistance, should 
 i not, altogether, require more than a force of 30 pounds to over- 
 
 I come them. 
 
 In the operation of turning a winch, the effect of a man’s 
 force varies in every part of the circle described by the handle. 
 The greatest force is when he pulls the handle upwards from 
 about the height of his knees ; and the least force, when, the 
 handle being at the top, he thrusts from him horizontally ; then 
 again the effect is increased as he lays on his w'eight in pushing 
 downwards ; but that action is not so great as when he pulls up, 
 
 ; because it is merely produced by the weight of his body, where- 
 
 I as, in pulling up, he can exert his whole strength. In pulling the 
 handle horizontally, when at its low'est, the force exerted is very 
 small. 
 
 The weight of a man of moderate strength, may be stated 
 at 140 pounds, and such a person may be consi^red capable of 
 ii[ exerting the following forces, viz. in the strongest point, or that 
 , position of the winch which is most favourable to him, a force 
 j) equal to l60 pounds ; in the weakest, a force equal to 27 pounds; 
 i in the next strong point, 130 pounds ; and in the last, or second 
 ^ weak point, 30 pounds. The sum of these forces is 347, which, 
 divided by 4, gives 84| pounds for the weight that a man might 
 |j lift by a winch, if he could exert his whole strength continually, 
 without stopping to take breath ; but this being impossible, the 
 ; weight must return, and overpower him at the first w^eak point, 
 I especially when the handle moves slowly, as it must if he would 
 I exert his utmost strength all round. Besides, in overcoming such 
 I a resistance, the man is in theory supposed to act always along 
 j the tangent of the circle of motion, which application of his force 
 ! is not practicable ; and there must also be such a velocity given, 
 i that the force applied at the strong points may not be spent before 
 the hand comes to the weak ones, w hich is a regulation of exertion 
 also unattainable ; hence, when no adventitious advantages are 
 superadded, the resistance ought to be no more than 30 pounds. 
 If a fly be added to the windlass, when the motion is pretty quick, 
 as about four or five feet per second, a man may exert for a short 
 time a force of 80 pounds, and work a whole day against a resist- 
 ance of 40 pounds. 
 
344 
 
 MECHANICS. 
 
 Best position -for the handles of a windlass. — Powers of men and Horses. 
 
 If the windlass be provided with two handles, one at each 
 extremity, and Vhe elbows of these handles are at right angles 
 to each other, two men will more_ easily, for the same length of 
 time, act against a constant resistance of 70 pounds, than a 
 single man against 30 pounds ; for one^ian will act at the strong- 
 est point while the other acts at the weakest point of the revolu- 
 tion, and thus they will mutually and successively help one 
 another. The utility of this disposition of the handles is now 
 generally known and attended to, but it was forinerJy little 
 thought of. 
 
 The whole art of carrying large burdens consists in keeping 
 the column of the body as directly under the weight and as up- 
 right as possible. Standing in his natural posture, a man can sup- 
 port a weight which would break the back of the strongest horse. 
 The reason is evident : the column of the man’s bones support 
 the weight directly ; but the weight is laid across the column of 
 the horse. 7'he more a man bends his body, the less weight he g 
 can support; hence two men carrying a load, can sustain much 
 more than double the weight which either of them could carry 
 separately, because they can move more upright, and with the 
 column of their bones more opposed to it. Chairmen having 
 straps from their shoulders to the poles of the chair, will wait 
 with 300 pounds (that is 130 pounds each) at the rate of four 
 miles an hour. A porter will carry upon his shoulders a load of 
 180 pounds and walk at the rate of three miles an hour; a coal- 
 heaver will carry 230 pounds, but he only goes to a short distance 
 with his load. In rowing, men exert their strength with great 
 effect; and they more usually draw the oar to them than push it 
 from them, because, in the former case, they can bring into action 
 a greater number of muscles, and experience quickly convinces 
 them of the best mode. 
 
 A horse draws with the greatest advantage, when the line of 
 draught is not level with his breast, but inclines upwards, making 
 a small angle with the horizontal plane. 
 
 A horse drawing a weight over a single pulley, can exert a 
 force of 200 pounds, w hile walking at the l ate of tw o miles and 
 a half per hour, or about three feet and a half per second. If the 
 same horse have to draw 240 pounds, he can work but six hours 
 a day, and cannot go quite so fast. To this mode of exertion 
 may be referred the working of horses, in all sorts of mills, in cal- 
 culating the probable effect of which, previous to their being 
 erected, after making the necessary allowances for all frictions and 
 hinderances, the task assigned to the horse should be carefully de- 
 termiiied. 
 
MECHANICS. 
 
 345 
 
 Power of horses. — Diftercnt kinds of mills. 
 
 I The force M’ith ^vhicli a horse acts, is compounded of his 
 weight and muscular strength. If then the weight of one horse 
 exceed that of another to which it is inferior with respect to 
 strength, the weaker horse will overcome a resistance which the 
 stronger cannot, provided the excess of his w^eight in the smallest 
 degree exceeds his deficiency in strength. 
 
 AVhen a horse draws in a mill or gin of any kind, great care 
 should be taken that the horse-w alk, or circle in which he moves, 
 be large enough in diameter, otherwise he cannot exert all his 
 strength : for in a small circle, the tangent in w'hich he draws, de- 
 viates more from the circle in which he is obliged to go than in a 
 larger circle. The diameter of the horse-w alk should never, if 
 possible, be less than forty feet. In a w^alk of nineteen feet, it has 
 been calculated that ahorse loses two-fifths of his strength. 
 
 A horse exerts his force to the greatest disadvantage in drawl- 
 ing or carrying up a hill. The human form is so much better 
 idapted for climbing than that of a horse, that if the hill be steep, 
 :hree men will do more than a horse; each man, loaded wdth 100 
 pounds, will move up faster than a horse that is loaded with 300 
 pounds. But one horse can give motion to the horizontal beam 
 in a walk of forty feet, with as much ease as five men ; in a walk 
 3f nineteen feet^ three men would exert themselves with as much 
 effect as a horse. 
 
 Of Mill Work. 
 
 The term mill, originally signified a machine for grinding 
 corn ; but at present the expression mill-w^ork is frequently 
 applied to all kinds of machinery where large wheels are em- 
 ployed. 
 
 Mills are distinguished into various kinds, either according 
 to the powers by which they are moved, or the uses to wdiich 
 they are applied ; hence we have water-mills ^ horse-mills, and 
 wind-mills ; corn-mills, fulling-mills, powder-mills, horing-mills, 
 Scc. These appellations, indefinite as they are, answ^er the pur- 
 pose of common conversation ; but it is evident, that a mill is not 
 completely named, unless its use as w'ell as its motive force, is 
 designated. 
 
 In ancient times, corn was ground only by hand-mills, con- 
 sisting of two stones, similar to those used in water-mills, but 
 much smaller, the low'er one being fixed, and the upper one 
 having a piece of wood fastened into it to move it by. Machines 
 of this description are are still used in India, and also in some se- 
 questered parts of Scotland ; in the latter country, they are called 
 querns; but in general, wherever large quantities of grain aie to 
 15. — VoL. I. 2 Y 
 
346 
 
 MECHANICS. 
 
 Different kinds of water-wheels. — Directions to obtain the velocity of a stream. 
 
 be gTOiind, they have been entirely superseded by mills not moved 
 by manual labour. 
 
 In treating of water-mills, which will here be our principal 
 object, we shall have occasion to advert to the most eligible mode 
 of forming the cogs of wheels, and other particulars applicable to 
 machinery in general, and shall conclude with a description of the 
 most simple form of the water corn-mill. 
 
 Water-mills aie of three kinds, viz. hreast-mills, undershot- 
 mills, and over shot-mills, according to the manner in which the 
 water is applied to the great-wheel. In the first, the water falls 
 down upon the wheel at right angles to the float-boards or buckets 
 placed all round the wheel to receive it. In the second, which is 
 used where there is no fall but a considerable body of water, the 
 stream strikes the float-boards at the lower part of the wheel. In 
 the third, the water is poured over the top, and is received in 
 buckets formed all round the wheel. 
 
 It was the opinion of Smeatoii, that the powers necessary to 
 produce the same effect on the undershot-wheel, a breast-wheel, 
 and an overshot-wheel, must be to each other as the numbers 
 2.4, 1.75, and 1. 
 
 The effect or momentum of water depending jointly upon 
 its velocity and its quantity, it is of importance to ascertain 
 these particulars ; Dr. Desaguliers has given the following 
 easy directions for the purpose : Observe a place w'here the 
 banks of the river are steep, and nearly parallel, so as to make a 
 kind of trough for the water to run through ; then by taking the 
 depth in various parts of the stream’s breadth, obtain a correct 
 section of the river. Stretch one line over it at right angles, and 
 another at a small distance above or below, but perfectly parallel. 
 Now thrown in some buoyant body (such as an apple, which will 
 not float so high as to be affected by the wind) immediately above 
 the upper line : observe the time it occupies in passing from one 
 to the other string. Thus you ascertain how many feet the cur- 
 rent runs in a second, or in a minute. Then having the tw'o sec- 
 tions, that is, one at each line, reduce them to a mean or average 
 depth, and compute the area of the mean section, which being 
 multiplied by the distance between the lines, will give the solid 
 contents of the intermediate volume of fluid, which in the noted 
 time passed from on string to the other. Now this way, by the 
 rule of three, is adapted to any portion of time ; the question 
 being merely, if the velocity be such in such an area, or trough, 
 wdiat would be the velocity in another of less size. It is obvious, 
 that if the area give twelve solid feet, and that the water passed 
 at the rate of four feet in a second, through a conduit of one 
 foot square, if the conduit were only six inches square, the 
 
MECHANICS. 
 
 347 
 
 Rules for the construction of mills. 
 
 velocity would be as sixteen to four ; or in other words, qua- 
 drupled. 
 
 The arch of a bridge is often an excellent station for observing 
 the force of a stream ; because the sides are there regular, and 
 the intermediate space may be correctly measured. But the 
 depth is not always to be ascertained in such places without the 
 aid of a boat, or of two intelligent assistants, who should be very 
 correct in their observations. The arch of a bridge is not a pro- 
 per station, when the velocity of the current is accelerated for 
 want of sufficient water-way. 
 
 , Practical Rules and Observations relative to the Construction of 
 
 Water-Mills. 
 
 1. Measure the perpendicular height of the fall of water, in 
 feet, above that part of the wheel on which the water begins to act, 
 and call that the height of the fall. 
 
 2. Multiply this constant number 64.2822 by the height 
 of the fall in feet, and the square root of the product will be the 
 velocity of the water at the bottom of the fall, or the number of 
 feet that the water there moves per second. 
 
 3. Divide the velocity of the water by three, and the quo- 
 tient will be the velocity of the float-boards of the wheel, or the 
 number of feet they must each go through in a second, when the 
 water acts upon them so as to have the greatest power to turn the 
 mill. 
 
 4. Divide the circumference of the wdieel in feet by the velo- 
 city of its floats in feet per second, and the quotient will be the 
 number of seconds in which the w heel turns round. 
 
 5. By this last number of seconds divide 60, and the quo- 
 tient will be the number of turns of the wheel in a minute. 
 
 , 6. Divide 120 (the number of revolutions a mill-stone four 
 
 feet and a half in diameter ought to have in a minute) by the 
 number of turns of the wheel in a minute, and the quotient wdll 
 be the number of turns the mill-stone ought to have for one turn 
 of the wheel. 
 
 7. Then, as the number of turns of the wheel in a minute is 
 to the number of turns of the mill-stone in a minute, so must the 
 number of staves in the trundle be to the number of cogs in the 
 wheel, in the nearest whole numbers that can be found. 
 
 By these rules the following table is calculated to a water- 
 wheel eighteen feet in diameter, which size has been found by ex- 
 perience to be the most eligible for general use. 
 
348 
 
 MECHANICS, 
 
 Calculations of mill-work. 
 
 The Millwrighfs Table. 
 
 ’it 
 
 Hei-ht of 
 the fall of 
 water. 
 
 Velocity 
 of the fall 
 of water 
 per se- 
 cond. 
 
 Velocity 
 of the 
 wheel per 
 second. 
 
 Revolutions 
 of the wheel 
 per minute. 
 
 Revolution 
 of the mill- 
 stone for 
 one of the 
 wheels. 
 
 Cogs in the 
 wheel, and 
 staves in the 
 trundle. 
 
 Revolutions 
 of the mill- 
 stoneper mi- 
 nute by tliese 
 staves&cogs 
 
 Feet. 
 
 'V 
 
 lOOtli parts 
 
 of a foot. 
 
 - - 
 
 Feet. I 
 
 lOOth parts 
 of a foot. 
 
 Revolu- 
 
 tions. 
 
 tooth parts 
 of a rev. 
 
 3 ai 
 o S 
 > .2 
 <u 
 
 tooth parts 
 of a rev. 
 
 Cogs. 
 
 Staves. 
 
 Revolu- 
 
 tions. 
 
 tooth parts 
 of a rev. 
 
 1 
 
 8 
 
 . 02 
 
 2 
 
 . 67 
 
 2 
 
 . 83 
 
 42 
 
 . 40 
 
 234 
 
 6 
 
 119 . 84 
 
 2 
 
 11 
 
 . 34 
 
 3 
 
 . 78 
 
 4 
 
 . 00 
 
 SO 
 
 . 00 
 
 2 lb 
 
 7 
 
 120 . OOl 
 
 3 
 
 13 
 
 . 89 
 
 4 
 
 . 63 
 
 4 
 
 . 91 
 
 24 
 
 . 44 
 
 196 
 
 8 
 
 120 . 28^ 
 
 4 
 
 16 
 
 . 04 
 
 5 
 
 . 35 
 
 5 
 
 . 67 
 
 21 
 
 . 16 
 
 190 
 
 9 
 
 119 . 74l 
 
 5 
 
 17 
 
 . 93 
 
 3 
 
 . 98 
 
 6 
 
 . 34 
 
 18 
 
 . 92 
 
 170 
 
 9 
 
 119 . 68 
 
 6 
 
 19 
 
 . 64 
 
 6 
 
 . 33 
 
 6 
 
 . 94 
 
 17 
 
 . 28 
 
 156 
 
 9 
 
 120 . 20 
 
 7 
 
 21 
 
 . 21 
 
 7 
 
 . 07 
 
 7 
 
 . 50 
 
 16 
 
 . 00 
 
 144 
 
 9 
 
 120 . 00. 
 
 8 
 
 22 
 
 . 68 
 
 7 
 
 . 36 
 
 8 
 
 . 02 
 
 14 
 
 . 96 
 
 134 
 
 9 
 
 119 . 34 
 
 9 
 
 24 
 
 . 05 
 
 8 
 
 . 02 
 
 8 
 
 . 51 
 
 14 
 
 . 10 
 
 140 
 
 10 
 
 U9 . 14 1 
 
 10 
 
 23 
 
 . 35 
 
 8 
 
 . 43 
 
 8 
 
 . 97 
 
 13 
 
 . 38 
 
 134 
 
 10 
 
 120 . ISJ 
 
 11 
 
 26 
 
 . 39 
 
 8 
 
 . 86 
 
 9 
 
 . 40 
 
 12 
 
 . 76 
 
 128 
 
 10 
 
 120 . 32 I 
 
 12 
 
 27 
 
 . 77 
 
 9 
 
 . 26 
 
 9 
 
 . 82 
 
 12 
 
 . 22 
 
 122 
 
 10 
 
 119 . 801 
 
 13 
 
 28 
 
 . 91 
 
 9 
 
 . 64 
 
 10 
 
 . 22 
 
 11 
 
 . 74 
 
 118 
 
 10 
 
 120 . 36 
 
 14 
 
 30 
 
 . 00 
 
 10 
 
 . 00 
 
 10 
 
 . 60 
 
 11 
 
 . 32 
 
 112 
 
 10 
 
 118 . 72 
 
 15 
 
 31 
 
 . 05 
 
 10 
 
 . 35 
 
 10 
 
 . 99 
 
 10 
 
 . 98 
 
 1 10 
 
 10 
 
 120 . 96 
 
 16 
 
 32 
 
 . 07 
 
 10 
 
 . 09 
 
 1 1 
 
 . 34 
 
 10 
 
 . 58 
 
 106 
 
 10 
 
 120 . 20 
 
 17 
 
 33 
 
 . 06 
 
 11 
 
 . 02 
 
 1 1 
 
 . 70 
 
 10 
 
 . 26 
 
 102 
 
 10 
 
 119 . 34 
 
 18 
 
 34 
 
 . 02 
 
 11 
 
 . 34 
 
 12 
 
 . 02 
 
 9 
 
 . 98 
 
 100 
 
 10 
 
 120 . 20 
 
 19 
 
 34 
 
 . 95 
 
 11 
 
 . 65 
 
 12 
 
 . 37 
 
 9 
 
 . 70 
 
 98 
 
 10 
 
 121 . 22 
 
 20 
 
 35 
 
 . 86 
 
 11 
 
 . 93 
 
 12 
 
 . 68 
 
 9 
 
 . 46 
 
 94 
 
 10 
 
 119 . 18 
 
 1 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 7 A 
 
 To construct a mill by this table, find the height of the fall 
 of water in the first column, and against that height in the sixth 
 column, is given the number of cogs in the wheel and staves in 
 the trundle, for causing a millstone four feet six inches in SO 
 
 diameter, to make 120 revolutions in a minute as nearly as 
 possible, when the circumference of the wheel ‘moves with one- „ 
 
 third part of the velocity of the water. And it appears by the ] 
 
 seventh column, that the number of cogs in [the wheel, and ] 
 
 staves in the trundle, are so nearlv adapted to the required pur- * 
 
MECHANICS. 
 
 349 
 
 Computations of the effects of water-wheels. 
 
 pose, that the least number of revolutions of the millstone in a 
 minute is 118, and the greatest number exceeds not 121, which 
 is according to the speed of some of the best mills. 
 
 It should be observed, that the breadth of the water-wheel 
 ought to correspond with the power necessary on the occasion, 
 supposing that a proportionate volume of water is at command ; 
 for a wheel of two feet in breadth wdll be more than doubly as 
 powerful as one only a foot broad, there being a double volume of 
 water acting upon it, w'hile the friction of the axis is by no means 
 doubled with this augmentation of breadth. 
 
 To compute the effects of water-wheels with precision, it is 
 necessary to ascertain, 1. The real velocity of the water which 
 acts upon the wheel; 2. the quantity of water expended in a 
 given time ; and 3. how much of the pow'er is lost by friction. 
 After a variety of experiments, Smeaton found that the mean 
 power of a volume of water 15 inches in height gave 8-96 feet 
 of velocity in each minute to a wheel on which it impinged. 
 The computation of the power to produce such an effect, allow- 
 ing the head of water to be 105.8 inches, gave 264.7 pounds of 
 water descending in one minute through the space of 15 inches : 
 therefore 264.7, multiplied by 15, was equal to 3.Q70. But 
 as that pow'er will raise no more than 9-375 pounds to the 
 height of 135 inches, it was manifest that the major part of the 
 pow’er was lost ; for the multiplication of these two sums only 
 amounted to 1,266; of course the friction was equal to three- 
 fourths of the power. The distinguished Engineer above-men- 
 tioned, considers this the maximum single effect of water upon an 
 undershot-wheel, wdiere the fah is fifteen inches. The remainder 
 of power, it is plain, must be equal to that of the velocity of the 
 W’heel itself, multiplied into the weight of the whaler, which in this 
 case brings the true proportion betw'een the power and the effect 
 to be as 3,849 to 1,266, or as 1 1 to 4. 
 
 Care should be taken to make the float-boards rather numer- 
 ous than few'. Smeaton found, that in undershot-mills, when 
 he reduced the number of floats from twenty-four to twelve, 
 the effect was reduced one-half, because the water escaped 
 betw een the floats without touching them ; but when he added 
 a circular sweep of such length, that before one float-board 
 quitted it, another had entered it, he found the former effect 
 nearly restored. This mode more particularly applies to breast- 
 wheels, or such as receive the w'ater immediately below the 
 level of the axis. In such the circular trough is necessary, to 
 make the water communicate the full effect desirable from the 
 joint operation of velocity and weight. In wheels of this kind, 
 the float-boards should be confined both at their sides and at 
 
350 
 
 MECHANICS. 
 
 The overshot-wheel the most powerful. 
 
 their extremities, so that the water may accompany them all 
 the way from the head down to the lowest part of the wheel, 
 whence it should pass off with sufficient readiness to allow the 
 succeeding fall to supply its place, without being in the least 
 retarded. Any quantity of water remaining in the trough, at 
 the bottom of a breast-wheel in particular, must tend to oppose 
 its motion, in the exact ratio with the disposition of the fluid to 
 become stagnant or stationary. It has been ascertained that a 
 very sensible advantage is gained by including the float- boards to 
 the radius of the wheel, so that each float-board, when lowest, 
 shall not be vertical, but have its edge turned up the stream about 
 twenty degrees. 
 
 The overshot-wheel is by far the most powerful ; both 
 because it receives the water at the very commencement of its 
 descent, and because the buckets with which it is ordinarily 
 furnished retain the power so long, the water being gradually 
 discharged, as these buckets successively become inferior parts of 
 the circumstance. It may be proper to state, in this place, that 
 much may be effected by allowing the water merely to flow upon 
 the upper part of the wheel, into the superior buckets, whereby 
 an immense auxiliary force is erected as they successively become 
 filled. Add to this, Smeaton’s discovery, that the more 
 slowly any body descends by the force of gravity while acting 
 upon any piece of machinery, the more of that force will be 
 spent upon it, and consequently the effect will be the greater.” 
 That effect is by no means increased in proportion to the velo- 
 city of the w heePs motion ; on the contrary, Smeaton found, 
 that w hen the wheel with which he experimented, and which was 
 two feet in diameter, revolved 20 times in a minute, its effect 
 was greatest: when it made only 18f turns, the effect was irre- 
 gular: and when so laden as not to make 18 turns, the W'heel 
 was overpowered by the load. He found that 30 turns in the 
 minute occasioned a loss of about one-twentieth, and that when 
 turned above 30 times in a minute, the diminution of effect was 
 nearly one-fourth of its powers. This proportion may be easily 
 estimated on any wheel of greater extent, by computing the 
 proportion of accumulated power lost by greater velocity than 
 may be sufficient to load the wheel by means of the buckets 
 being filled ; observing that the progress of a machine may be 
 so much retarded as to cause the effect to be irrevalent of the 
 purpose, although the machine may be kept in motion. Some 
 machines do their w'ork well, simply in consequence of a 
 certain celerity, as is generally the case in a grinding apparatus : 
 and every person conversant in the practice of agriculture is 
 aware, that when a plough is draw'll at a certain pace, it will 
 
MECHANICS. 
 
 351 
 
 I 
 
 Power of the breast-wheel. — Management of the supply of water. 
 
 cut the soil regularly and freely, while, on the other hand, the 
 Ijsame cattle proceeding at a very slow pace will be more fatigued 
 li though they do less work, and that diminished quantity of work 
 iby no means so neatly executed. 
 
 Tlie breast-wheel, when well constructed, will carry an effect 
 equal to half or even three-fifths of the power, while the over- 
 shot-wheel will work with a result equal to four-fifths of the 
 power ; yet in general, from inattention to the lessening of fric- 
 tion, and other imperfections of construction, the overshot-wheel 
 does not perhaps perform work beyond half the power, and the 
 effect of the breast-wheel, from similar causes, is proportionately 
 reduced. 
 
 When the stream would supply too much water, the 
 redundancy can in general be easily carried off by sluices or 
 
 I overflows, constructed for the purpose ; and when it is desirable 
 to increase the velocity of the usual current, much may be 
 done towards the complete attainment of this object, by con- 
 tracting the banks. It is also very obvious, that by giving 
 additional height to the fall, or head, whence the water flows 
 
 S upon the wheel, velocity, or at least pow'er, may be greatly aug- 
 mented. 
 
 I In situations where the supply of water, though often 
 i superabundant, is at other times liable to be greatly deficient, 
 
 ! the propriety of forming a suitable reservoir, from which the 
 supply may be derived in seasons of draught, deserves to be 
 considered. In some cases, the expense of carrying such a 
 j plan into execution, would doubtless exceed any advantage it 
 would produce ; but in others it might be adopted with the 
 happiest success ; for it w ill certainly be understood, that such 
 a reservoir is not necessarily required to be near the mill, but at 
 any part of the course of the stream where the cheapness of the 
 land combines with its suitableness in other respects for the 
 I purpose. 
 
 Attempts have been made to construct water-wheels which 
 receive the impulse obliquely, like the sails of a common 
 windmill. By this means a slow but deep river could be made 
 to drive our mills ; though much powder would be lost by the 
 obliquity. Dr. Robinson describes one that was very powerful ; 
 it W’as a long cylindrical frame, having a plate standing out 
 from it about a foot broad, and surrounding it with a very 
 oblique spiral like a cork-screw^ This w^as immersed nearly a 
 quarter of its diameter, (which was tw'elve feet,) having its axis 
 in the direction of the stream. By the work performed, it 
 seemed more powerful than a common wheel that occupied the 
 same breadth of the river. Its length w^as not less than twenty 
 
352 
 
 MECHANICS. 
 
 Toothed wheel-work- 
 
 feet ; had it been twice as long it would have nearly doubled its 
 power, without occupying more of the water-way. Perhaps such 
 a spiial continued quite to the axis, and movii>g in a suitable 
 canal, wholly tilled by the stream, might be an advantageous way 
 of employing a deep and sluggish current. 
 
 Emeison observes, that the teeth of wheels ought not to act 
 upon each other before they arrive at the line which joins their 
 centres ; and though the inner or under sides of the teeth may be 
 of any form, yet it is better to make both sides alike, that the 
 wheels may admit of being revolved either w'ay w ith equal facility. 
 The utility of making the teeth as fine as the case admits, so that 
 the greatest number possible may be in contact at once, has al- 
 ready been insisted on ; and the utmost care should be taken to 
 have them so regularly disposed that they may not interfeie w’ith 
 each other before they begin to w ork. 
 
 It is of the greatest consequence to have the teeth so formed, 
 that the pressure by which one of them urges the other round its 
 axis, may be constantly the same. This is by no means the case, 
 when the common construction of a spur-wheel, acting in the cy- 
 lindrical staves of a lantern or trundle, is used. The ends of teeth 
 should never be formed of parts of circles, unless w orking with 
 other teeth specifically adapted to them, as will be more fully ex- 
 plained hereafter. 
 
 The wheels and pinions of the best clock and w'atch-w’ork, 
 are made true with almost mathematical precision ; but in treat- 
 ing of the endless screw, we have had occasion to notice, that 
 many great impediments combine to prevent very large wdieels, 
 either in w’ood or metal, from possessing that absolute corect- 
 ness of form which is so desirable. In consequence, the trundle 
 seldom divides the wheel so exactly, as to make a given number 
 of revolutions for one of the wheel without a fraction ; but as 
 any exact number is not necessary in mill-w’ork, and the cogs 
 and rounds cannot be set in so truly as to make all the intervals 
 betw^een them precisely equal, it is a useful precaution, w'hich 
 skilful millw rights seldom fail to adopt, to give the wheel what 
 is called a hunting-cog; that is, one cog more than what will 
 answer to an exact division of the wheel by the trundle. This 
 being done, every cog, as it comes to the trundle, wull take the 
 next staff or round behind the one which it took in the former re- 
 volution ; and by this means, the parts of the cogs and rounds 
 which work together, will, in a little time, be worn equally, and 
 to equal distances from one another. 
 
MECHANICS. 
 
 S53 
 
 Setting out a spuv-w-keel — wallow«r — face-wheel — spur-nuts. 
 
 The Method of selling out ff heels. 
 
 For a sptir-wljeel and wallower, draw ftiie pkcti lines A 1, B ][, 
 A 2 , B 2, (%. I, pL IV.) then divide diesn eilo t!ae mianber <}f 
 teeth 08 ' cogs ns a h e:. Divide oite of Shese distances, 
 
 as h c, into seven equal parts, i, % 5, 4-, -5, 6, 7 - allow three 
 
 parts for the tlikkness of the cogs, as H, 2, -S, in the cog ei ; ar^d 
 four for tire diameter of the stave of the TvalioM. er, as, 1, % 3y 4, am 
 tlie stave ^ dg. £. Three parts are allowed for the cog, aaid 
 foJii' for the stave, because the vvalJower is supposed to be of les^s 
 diauieter than the wheel, therefore subject to more wear, an pro- 
 poi tion as the tmmbier of cogs exceed the number of staves ; but 
 if 5D nxiy case llie number of staves and cogs be the srame, titej 
 anaj be of equal thickness. The height of the cog is equal to four 
 parts; then divide its height bito five equal parts, as i, ‘2, 3, 4, o, 
 in die -cog c; allow three for the botta.ua to die pitch line of -the 
 cog; the otiier two parts for die curve wliicli nsuet be given it to 
 make it lit and bear 012 the stave equally.. 
 
 In common pj'actice, the anilh% sights ■gm xccs-istomed 
 put the point of a pair of compasses in the dot 3 of die cog <s, 
 and strike the line de; theii they s'cmove the pokit of die com- 
 passes to the point d^ and strike die 3 by which anean^’ 
 
 thej/ oblaiu a cm ve which they consider suSicienlly proper for 
 their puqiose. 
 
 Tor a face-wheel, die following method is ^adopted : divide the 
 pitch line AB, hg. 2, Into foe aiunibcr -af cogs intended, as nbc; 
 divide the distance b c, into seven equal parts ; fcee -of those pai-ts 
 allow for the thicknees o-f foe cogss, as I, 2, 3, an the cog c, four 
 for foe height, and four for foe width, as d c, ami four for dic! 
 foickness of foe stave m, Draw a line forough foe centre of foe 
 cog, as the line A 1, at S; and on foe point .5 sdescribe the lisie 
 de ; remove the compasses to the point A, and draw foe linej" g„ 
 %vh ich forms foe shape of the cog- 
 
 For common spur-nuts, divide foe pitch-linej. A, dg. 3, int« 
 twice as many equal parts as 4here sue intended to be feeelh„ 
 as cr, h, c, d^ £ ; w ifo a pak of com|)a 3 ses opened t-o half the 
 distance of any of these divisions, from the points .a 1, c 3, e 5^ 
 draw foe semi-circles < 3 , c, and e, i^vtiicli will forra the ends oC foe 
 teefo From foe points 2, 4^ and fi, draw die semi-circles g /e 4 
 which will form foe lower parts of foe spaces- Thoogh ispur-nuts 
 are i^uallv .set out in this manner, yet it should he remembered, 
 foat in all good worlq, foe -ends of ^lie teeth oiwst not be semi- 
 circles unless working with other ifeeefo adapted ^kem m 
 be afterwards noticed. 
 
 U- — Vo£^ L 
 
 ■f. Z 
 
354 
 
 MECHANICS. 
 
 Nature of bevel-geer. 
 
 Of Bevel-Geer. 
 
 Instead of spur-wheels and trundles, bevelled wheels, more 
 commonly called bevel-geer, are now generally used. Wheels 
 of this class, it may be shewn, are, in effect, truncated cones, 
 
 • rolling on the surface of each other. Suppose the cones A and 
 B, revolving on their centres a p, a c, tig. 4, pi. IV ; if their 
 bases are equal, they will perform their revolutions in equal 
 times, and consequently any two points equally distant from 
 the centre a of A, as ab, a c, a a e, will revolve in the same 
 time as af ag,ahj ai. In like manner if one of the cones, 
 as in fig. 5, be twice the diameter of the other at the base, and 
 they are turned upon their centres, the base of the larger will 
 only have made one revolution, \vhile that of the smaller will 
 have made two revolutions ; and all the corresponding parts of 
 the conical surfaces will observe the same proportion ; that is, 
 a by a Cj ad, a e, will turn only once round, while af, a g, 
 a h, a i, turn twice round. Hence it is obvious, that the num- 
 ber of the revolutions of all cones revolving in this manner, 
 must be to each other as their respective diameters. Now let 
 tw^o cones have teeth cut in them ; as represented by tig. 6 ; they 
 will then become bevel-geer. The teeth in an entire cone would 
 be broadest at the base, from whence they would gradually 
 taper wdth the lessening circumference of the cone, till they 
 terminated at the apex or centre a in a point ; but as such an 
 extent of teeth would be unnecessary, and if the cones w'ere 
 entire the axes at a w'ould incommode each other, the slender 
 useless part of the teeth are cut off, as at E and F ; or rather, 
 bevel-geer is composed of wlieels made at first in the form of 
 truncated cones, as shewn by fig. '7, where the upright shaft or 
 axle, AB, with the bevel-wheel CD, turns the bevel-wheel 
 EF, with its shaft GH, and the teeth-work freely in each 
 other. The teeth may be made of any dimensions, according 
 to the strength required ; and this method will enable them to 
 overcome a greater resistance, and work much more smoothly 
 than a common face-wheel and trundle ; besides, the facility with 
 which it enables us to change the direction of a motion, is of 
 great importance. 
 
 The method of conveying motion m any direction, and of 
 proportioning or shaping the wheels accordingly, is as follows : 
 let the line a b, fig. 8, represent a shaft coming from a wheel ; 
 draw the line c d to intersect the line a b, in the direction 
 intended for the motion to be conveyed, and this line c d will 
 represent the shaft of the bevel-wheel which is to receive the 
 
MECHANICS. 
 
 355 
 
 Universal joint — Cycloid and epicycloid, 
 
 motion. Suppose then the shaft c c? is to revolve three times, 
 whilst the shaft a b revolves once ; draw the parallel line ^ i at 
 any moderate distance (suppose one foot by a scale,) then draw 
 the parallel line kk, at three feet distance, after which draw the 
 dotted line zo Xy through the intersection of the shafts a h and c d, 
 and likewise through the intersection of the parallel lines i i and 
 k ky in the points x and y, which w ill be the pitch-line of the two 
 bevel-wheels, or the line where the teeth of the two wheels act on 
 each other, as may be seen by fig. 9, where it is obvioRS to inspecr 
 tioii that the motion may be conveyed in any direction. 
 
 Of Hooke's Universal Joint. 
 
 The contrivance called the universal joint, which was invented 
 by Dr. Hooke, may be applied to communicate motion instead of 
 bevel-geer, where tlie velocity is not to be changed, and w'here 
 the angle does not exceed 30 or 40 degrees. This joint may be 
 constructed as represented by fig. 10; or with four pins fastened 
 at right angles upon the circumference of a hoop, or solid ball. 
 It is useful in cotton-mills, where the tumbling shafts are conti- 
 nued to a great distance from the moving power, as the use of it 
 allows the convenience of cutting them into convenient lengths. 
 It is most proper, w hen the' irregularity of its motion, as it recedes 
 from a right line, is not disadvantageous. 
 
 Of the Cycloid and Epicycloid^ and the formation of the Teeth 
 
 of Wheels. 
 
 If on the plane CD, fig. 11, a circle B, proceeds in a 
 right line, and at the same time revolves round its centre, till 
 every part of the circumference has touched the plane, a point 
 or pencil, at «, which was lowest at the commencement of the 
 motion, will have described the curve CED, which is called a 
 cycloidy and is evidently compounded of a rectilinear and circular 
 motion. 
 
 If a circle A, fig. 12, roll from o to on the convex circum- 
 ference of another circle B, the point o will describe the curve 
 opqy which is called an exterior epicycloid; and if the circle A 
 were to roll on the concave circumference of the circle B, as from 
 r to s, the point r would describe an interior epicycloid. — In all 
 these cases, the circle by which the curve is obtained, is called the 
 generating circle. 
 
 The teeth of wheels and leaves of pinions require great 
 care and judgment in their formation, that they may neither 
 clog the machinery by unnecessary friction, nor act so kregu?» 
 
356 
 
 MECHANICS. 
 
 Mode# in which teeth act upon each other. 
 
 larly an to produce any inequalities in the motion, and the wear- 
 ing of one part before another. It has long been known that 
 one wheel will not drive another with uniform velocity, unless 
 the teeth of one or more of the wheels have their acting surfaces 
 formed into a curve generated after the manner of an epicy- 
 cloid. But in order to ensure a uniformity of pressure and 
 velocity in the action of one wheel upon another, it is not 
 absolutely necessary that th«e teeth of one or both wheels be 
 exactly epicycloids ; for if the teeth of one of them be either 
 circular or triangular, with plain sides, or like a triangle with 
 its sides converging to the centre of the wheel, or of any other 
 form, this uniformity of force and motion will be attained, 
 provided that the teeth of the other w'heel have a figure which 
 is compounded of that of an epicycloid, and the figure of the 
 teeth of the first wheel. De la Hire has shewn in a variety of 
 cases, how to find this compound curve ; bat as it is often diffi- 
 cult to describe, or even to discover «";s nature, we shall select 
 such forms for the teeth, as are better adapted to practice. There 
 are three different ways in which the teeth of wheels may act upon 
 one another ; and each mode of action requires a different foi m 
 for the teeth : 
 
 1st. When the teeth of the wheel begin to act upon the 
 leaves of the pinion just as they arrive at the line of centres ; 
 and their mutual action is carried on after they have passed 
 this line. 
 
 2nd. When the teeth of the wheel begin to act upon the 
 leaves of the pinion, before they arrive at the line of centres, and 
 conduct them either to this line or a very little beyond it. 
 
 3rd. W^hen the teeth of the wheel begin to act upon the leaves 
 of the pinion, before they arrive at the line of centres, and con- 
 tinue to act after they have passed that line. 
 
 When the first mode of action is adopted, the acting faces 
 of the leaves of the pinion should be parts of an interior epicy- 
 cloidf generated by a circle of any diameter rolling upon the 
 concave superficies of the pinion ; and the acting surfaces of the 
 teeth of the wheel should be portions of an exterior epicycloid, 
 formed by the same generating circle rolling upon the convex 
 superficies of the wheel. Now it is demonstrable, that when one 
 circle rolls w'ithiii another whose diameter is double that of the 
 rolling circle, the line generated by any point of the latter will 
 be a straight line tending to the centre of the larger circle. If 
 the generating circle, therefore, mentioned above, should be 
 taken with its diameter equal to the radius of the pinion, and be 
 
MECHANICS. 
 
 357 
 
 r 
 
 i 
 
 Formation of the teeth of wheels. 
 
 made to roll upon the concave superficies of the pinion, it will 
 ij generate a straight line tending to the pinion’s centre, which 
 I will be the form of the acting faces of its leaves ; and the teeth 
 ' of the wheel will in this case be exterior epicycloids, formed by 
 i a generating circle, whose diameter is equal to the radius of the 
 k pinion, rolling upon the convex superficies of the wheel. This 
 ^ construction of the teeth of the wheel, and leaves of the pinion, is 
 1 represented by fig. 13, pi. IV ; it is strongly recommended by 
 De la Hire and Camus, and is perhaps the most advantageous, as 
 it requires less trouble, and may be executed with greater accu- 
 racy than if the leaves of the pinion had been curved as well as 
 : the teeth of the wheel. 
 
 Lanterns or trundles, which consist of cylindrical staves 
 ij ' fixed by both ends nearly at the circumferences of two equal 
 Ij circular boards, and which are so frequently substituted by 
 ■ millwrights for pinions, may often be adopted with great pro- 
 I priety, provided the teeth of the wheels working in them, have 
 'j a proper form. The construction pointed out by Brewster, 
 1; which we shall here present to the reader, possesses the merit of 
 ij greatly diminishing the friction arising from the mutual action 
 
 ' of the staves and the teeth, and of being easily reduced to 
 
 practice. 
 
 Let Af fig. 14, pi. IV, be the centre of the small wheel or 
 r trundle, TCHQ, whose teeth are circular like ICR, having their 
 I centres in the circle PDEY. Upon B, the centre of the large 
 
 I wheel, at the distances BC, BD, describe the circles FCK, 
 
 GDO; and with PDEY, as a generating circle, form the 
 exterior epicycloid DNM, by rolling it upon the convex super- 
 
 I ficies of the circle GDO. The epicycloid DNM thus formed, 
 
 I would have been the proper form for the teeth of the large 
 wheel GDO, had the circular teeth of the small wheel been 
 infinitely small ; but as their diameter must be considerable, the 
 ’ teeth of the wheel should have another form. In order to 
 determine their proper figure, divide the epicycloid DNM into 
 i a number of equal parts, 1, 2, 3, 4, &c. as shewn in the 
 ! figure, and let these divisions be as numerous as possible. Then, 
 ; upon the points 1, 2, 3, &c. as centres, with the distance DC 
 equal to the radius of the circular tooth, describe portions of cir- 
 cles similar to those in the figure ; and the curve OPT, which 
 touches these circles, and is parallel to the epicycloid DNM, will 
 be the proper form for the teeth of the large wheel. 
 
 In order that the teeth may not act upon each other till they 
 reach the line of centres AB, the curve OP should not touch 
 the circular tooth ICR till the point O has arrived at D. The 
 -tooth OP, therefore will commence its action upon the circu- 
 
558 
 
 MECHANICS. 
 
 Formation of the teeth of ifv heels. 
 
 lar tooth at the point 1, where it is cut by the circle DRE. On 
 this account, the part ICR of the cylindrical pin being super- 
 fluous, may be cut off, and the staves of the trundle will then be 
 segments of circles similar to the shaded part of the figure. 
 
 If the teeth of wheels and the leaves of pinions consisted of 
 materials perfectly hard, and were accurately formed according 
 to these directions, they would act on each other not only with 
 uniform force, but also without friction ; because the surfaces 
 in contact would roll upon each other, and neither slide nor 
 rub so as to occasion any imperfection in the performance. But 
 as it is impossible in practice to attain the perfection which 
 theory requires, a certain quantity of friction will remain after 
 every precaution has been taken in the formation of the com- 
 municating parts. This friction may be removed, or at least 
 greatly diminished, with respect to a trundle, in the following 
 maimer. 
 
 If instead of fixing the staves as in the customary manner, 
 at the top and bottom, they are made capable of receiving 
 a rotarv motion in their frame, all tire friction will be taken 
 aw'ay except that which arises from the motion of the cylin- 
 drical tooth upon its axis. The advantages attending this 
 mode of construction are very important. The cylindrical 
 staves or teeth may be formed in the lathe with the greatest accu- 
 racy ; the curve required for the teeth of the large w heel is easily 
 traced ; the pressure and motion of the w heels will be uniform ; 
 and the teeth are very little subject to wear, because whatever 
 friction remains is almost wholly removed by the revolution of the 
 cylindrical spokes about their axis. This improvement, however, 
 can only be adopted where the machinery is large ; for small, 
 works, the acting faces of the leaves of the pinion or small wheel 
 should be rectilinear, and those of the large wheel epicycloidal, as 
 exemplified by fig. 13. 
 
 We have now' to consider the second mode of the mutual 
 action of wheels and pinions, viz. w hen the teeth of the w heel 
 begin to act upon the leaves of the pinion, before they arrive at 
 the line of centres, and conduct them either to this line or a 
 very little beyond it.” This mode of action is by no means so 
 advantageous as the former, and therefore should, if possible, be 
 always avoided. It is evident, that when the tooth of the 
 wheel acts upon the leaf of the pinion before they arrive at the 
 line of centres, and quits the leaf when they reach this line, 
 that the tooth works deeper and deeper betw'een the leaves of 
 the pinion the nearer it comes to the line of centres ; hence a 
 considerable quantity of friction arises, because the tooth does 
 BoU as before, roll upon the leaf, but slide$ upon it ; and from 
 
MECHANICS. 
 
 359 
 
 Formation of the teeth of wheels. 
 
 the same cause, the pinion soon becomes foul, as the dust which 
 lies upon the acting faces of the wheels is pushed into the hol- 
 lows between them. One advantage, however, attends this 
 mode of action, for it allows us to make the teeth of the large 
 wheel rectilineal, and thus renders the labour of the mechanic 
 less, and the accuracy of his work greater, than if they had been 
 of a curvilineal form. If the teeth therefore of the wheel are 
 made rectilineal, having their surfaces directed to the wheel’s cen- 
 tre, the acting surfaces of the leaves must be epicycloids formed 
 by a generating circle, whose diameter is equal to the sum of the 
 radius of the wheel, added to the depth of one of its teeth, rolling 
 upon the circumference of the pinion. But if the teeth of the 
 wheel and the leaves of the pinion are made curvilineal, the acting 
 surfaces of the teeth of the wheel must be portions of an interior 
 epicycloid formed by any generating circle rolling w'ithin the con- 
 cave superficies of the large circle, and the acting surfaces of the 
 pinion’s leaves must be portions of an exterior epicycloid, pro- 
 duced by rolling the same generating circle upon the convex cir- 
 cumference of the pinion. 
 
 When the teeth of the large wheel are cylindrical spindles, 
 either fixed or moveable upon their axis, an exterior epicycloid 
 must be formed like DNM, in fig. 14, pi. IV, by a generating 
 circle whose radius is AC, rolling upon the convex circumference 
 FCK ; AC being in this case the diameter of the wheel, and FCK 
 the circumference of the pinion. By means of this epicycloid, a 
 curve OPT must be formed as before described, which will be 
 the proper curvature for the acting surfaces of the leaves of the 
 pinion, when the teeth of the wheel are cylindrical. In deter- 
 mining the relative diameter of the wheel and pinion for this mode 
 of action, the radius of the wheel is reckoned from its centre to 
 the extremity of its teeth, and the radius of the pinion from its 
 centre to the bottom of its leaves. 
 
 The third mode in which one wheel may drive another, 
 viz. “ when the teeth of the wheel begin to act upon the leaves 
 of the pinion before they arrive at the line of centres, and con- 
 tinue to act after they have passed that line,” remains to be 
 considered. It is represented by fig. 1, pi. V, and as it is 
 a combination of the two first modes, it partakes both of their 
 advantages and disadvantages. It is evident from the figure, 
 that the portion e ^ of the tooth acts upon the part b c o( the 
 leaf till they reach the line of centres AB, and that the part 
 ed oi the tooth acts upon the portion 6 a of the leaf after they 
 have passed that line. It follow’s, therefore, that the acting 
 parts e h and b c must be formed according to the directions 
 given for the first mode of action, and that the remaining 
 
360 
 
 .MECHANICS. 
 
 Formation of the teeth of wheels. 
 
 parts edy b must have that curvature which the second mode 
 of action requires ; consequently e h should be part of an inte- 
 rior epicycloid formed by any generating circle rolling on the 
 coucavo circumference of the wheel, and the corresponding 
 part of the leaf should be part of an exterior epicycloid 
 formed by the same generating circle rolling upon b EO, the 
 convex circumference of the pinion ; the remaining part e d of 
 the tooth should be a portion of an exterior epicycloid, formed 
 by any generating circle roiling upon e L, the convex super- 
 ficies of the wheel ; and the corresponding part b a oi the leaf 
 should be part of an interior epicycloid described by the same 
 generating circle rolling along the concave side E o of the 
 pinion. But, as in practice, the production of this double curva- 
 ture of the acting surfaces of the teeth would be exceedingly trou- 
 blesome to the workman, who would probably never correctly ac- 
 complish his object, bis labour may be abridged by making e h 
 and b a radial lines, that is, e A a straight line tending to the centre 
 of the wheel B, and b a likewise a straight line tending to the 
 centre A, of the pinion. 
 
 fn the preceding remarks, the form assigned to the teeth has 
 been stated on the supposition that the wheel drives the pinion ; 
 but when, on the contrary, die pinion drives the wheel, the form 
 assigned to the teeth of the wheel must be given to tlie leaves of 
 die pinion, and the shape assigned to die leaves of the pinion must 
 be transferred to the teeth of tiie wheel, 
 
 A still different mode of fonning the teeth of wheels has had 
 many advocates, who have considered it well calculated to ensure 
 the uniformity of action so mucli desired. It consists in making 
 die acting faces of the teeth involutes of the wheers circumfer- 
 ence, Thus, let AB, %. 2, pi. V, be a portion of the wheel on 
 which the tooth is to be fixed, and let A p o be a thi ead wrapped 
 round its circumference, having a loop-hole at its extremity, o. 
 la tliis loop-hole Ex a pin <7, with which describe the curve or 
 inwlatCy a bed ehy by unwrapping the thread gradually from the 
 circumference A p m. The curve thus obtained w ill be the pro- 
 per form for the teeth of a wheel whose diameter is AB. it is 
 a form. v\hich admits of several teeth acting together, a circum- 
 stance attended with the advantage of diminishing the pressure 
 upon any one tooth so much as to make the wheels wear longer 
 aird more equally ; and it possesses the merit of being more easily 
 understood than the other methods directed to be observed. 
 
 This last mode of forming the teeth of wheels, is, however, 
 only a modification of the general principle, and indeed an 
 involute is sometimes reckoned, among the exterior epicycloids. 
 The propriety of this will be allowed, when it *s considered 
 
MECHANICS. 
 
 S6l 
 
 The drawing of epicycloids. 
 
 that the involute ab c dj &c. may be produced by an epicy- 
 cloidal motion. Thus, let o « be a straight ruler, at whose ex- 
 tremity is lixed the pin w, and let the point of the pin be placed 
 upon the point m of the circle, then by rolling the straight ruler 
 upon the circular base, so that the point in which it touches the 
 circle may move gradually from m towards B, the curve m n will 
 be generated exactly similar to the involute a b Cj &c. obtained by 
 the string. 
 
 The practical mechanic may wish to have more particular 
 directions tor drawing epicycloids, than he can derive from the 
 explanation of these curves at the commencement of the present 
 section. For this perpose then, let him take a piece of plain 
 wood G H, hg. 3, pi. V, and fix upon it another piece of 
 wood E, having its circumference m b of the same curvature as 
 the circular base upon which the generating circle AB is to roll. 
 When the generating circle is large, the shaded segment B will 
 be sufficient. In any part of the circumference of this segment, 
 fix a sharp-pointed steel pin a, which ought to be tempered, that 
 it may easily make a distinct mark ; and it must be driven in 
 sloping, so that the distance of its point from the centre of the 
 circle may be exactly equal to its radius. Fasten to the board 
 GH, a piece of thin brass, or copper, or tin-plate, a b. Place 
 the segment B in such a position, that the point of the steel pin 
 a may be upon the point by and roll the segment towards G, so 
 that the nail a may rise gradually, and the point of contact be- 
 tween the two circular segments may advance towards rn ; the 
 curve a by described upon the brass plate, w ill be an accurate 
 exterior epitydoid. Bemove, with a file, 'the part of the brass 
 on the left liand of the epicycloid, and the remaining concave 
 arch a b will be a pattern tooth, by means of which all the rest 
 may easily be formed. When an interior epicycloid is required, 
 the generating circle must revolve upon a concave instead of a 
 convex base as in the present instance. The cycloidy which is 
 useful in forming the teeth of rack-zcorky is generated in precisely 
 the same manner, with this difference only, that the base on which 
 tlie generating circle rolls must be a straight line. 
 
 Perhaps no part of the mechanism of mill-w^ork is executed 
 with so little attention to theory as the teeth of wheels. Almost 
 pery celebrated milhvright has his favourite construction, and 
 it is seldom indeed that the best methods are adopted. Gregory 
 describes one of the many plans in ordinary use, and we shall 
 here recite it ; from its being, as he observes, of tolerably easy 
 application, and allowing much strength to the teeth, w'hile it is 
 tolerably free from friction in comparison with other practical 
 methods. Let AB, fig. 4, pi. V, be two spur-wheels of 
 16. — VoL. I. 3 A 
 
362 
 
 MECHANICS. 
 
 Instauce of the common mode of forming cogs. — Forms of vripers. 
 
 different diameters, of which the cogs are intended to work into 
 each other at half pitch. The dotted circular arcs GH, EF, 
 touching each other between s and <7, are the centre or pitch-lines, 
 from which the teeth are formed. If the teeth of both wheels 
 are iron, as is generally the case in the first motions of works, 
 those teeth are then made nearly both of a size at the pitch-line ; 
 but if the teeth of one be wood and the other iron, then the 
 iron ones are made to have less pitch than the wooden ones, be- 
 cause they are then found to wear better. In the figure, both 
 are supposed to be of iron. Suppose the wheels to move from 
 G towards H, and from E towards F, and that the sides of the 
 teeth at b c, and d e, are in contact ; from 6 as a centre, with a 
 radius equal to b p, describe the arcs pd, Im; from <7 as a centre, 
 with the same radius, describe the arcs k i,f g, c k. Thus the 
 same opening of the compasses, and a centre chosen where the 
 wheels are in contact on the pitch-lines, will mark the contour of 
 the upper part of a tooth of one wheel, and the lower part of 
 a corresponding tooth of the other wheel ; and by taking 
 several centres on the two pitch-lines, the various teeth may be 
 formed. To prevent the cogs from bottoming^ as the workmen 
 call it, let the lower part, r e, of one tooth be made rather 
 longer than the upper part, p d, of the other w hich is to play into 
 it. The way in which cogs thus constructed will w ork into one 
 another, may be understood by considering the motion of tw'o of 
 them, )i and o for example : w hen they first come into contact, 
 they will appear as the curve x V z; when they arrive at Q, the 
 same sides will appear as in the dotted lines there represented ; 
 and when the same arrive at R S, they are in contact on their 
 middle points. 
 
 Of Wipers for raising Stampers and Hammers. 
 
 The notches which project from the circumference of a wheel 
 or an axle, for the purpose of raising stampers, pounders, or ham- 
 mers in a vertical direction, and then leaving them to fall by their 
 own weight, are usually called tcipers, though sometimes denomi- 
 nated lifting cogs. 
 
 When the wipers are only small cylinders or pins projecting 
 perpendicularly from the surface of a horizontal arbor, the force 
 with which they elevate the stampers, &c. will not act uniformly 
 during the whole time in which they are rising ; yet a uniformity 
 of force and velocity is generally desirable, and may alw^ays be 
 obtained by assigning a proper form to the communicating parts. 
 On this subject, a few directions for the use of the practical me 
 chanic will take up little room. • 
 
MECHANICS. 
 
 363 
 
 Forms of wipers. — Description of a corn-mill. 
 
 Fig. 5, pi. V, represents portions of a stamper for bruising 
 ore, beating hemp, &c, and of its shaft with lifting cogs. G is 
 the vertical arm of the stamper, sliding, when in actual work, 
 between rollers or in a groove, to keep it steadily in its proper 
 position ; a is the horizontal arm of the stamper ; H part of the 
 axle, on which the wipers or lifting cogs EF are fixed ; the 
 dotted lines at A, shew the height to which the horizontal arm 
 «, of the stamper, is elevated by each wiper. AB is a line 
 corresponding with the arm of the stamper upon which the 
 wipers first act ; CD is the pitch-line of the axis, or the bottom 
 of the curves of the wipers. The curved or acting faces of 
 tlie wipers are involutes of a circle equal in radius to the axis 
 CD, and obtained as already described in noting its application 
 to the formation of the teeth of wheels, viz. by unwrapping 
 from the circumference of the circle alluded to, a thread or 
 cord bj in the loop-hole at the extremity of which is a pencil 
 or marking point, describing the curve as it approaches towards 
 c. The arm of the stamper is flat at the part where the wiper 
 acts upon it, and should be placed in a line with the centre of 
 the shaft or axis, at the time the first wiper comes into contact 
 with it. 
 
 The wipers for a hammer, see fig. 6, may be formed on the 
 same principle. The centre upon which the hammer moves, 
 the centre b of the axis, and the flat part of the hammer-tail upon 
 which the wipers act, should be in the same line. 
 
 Description of a CoTn-mill. 
 
 The following is a description of a corn-mill of the most 
 common sort. AB, fig. 7, ph V, is. the water-wheel, which is 
 generally from eighteen to twenty-four feet in diameter, reckoning 
 from the outermost edge of any float-board at A, to that of the 
 opposite one at B. The water striking on the floats of this wheel, 
 drives it round, and gives motion to the mill. The wheel is fixed 
 upon a very strong axis or shaft C, one end of which rests on D, 
 and the other on E within the mill-house. 
 
 On the shaft or axis C, and within the mill-house, is a wheel 
 F, about eight or nine feet in diameter, having cogs all round, 
 which work in the upright staves or rounds of a trundle G. 
 This trundle is fixed upon a strong iron axis, called the spin- 
 dle, the lower end of which turns in a brass foot fixed at H, 
 in a horizontal beam H, called the bridge-tree ; and the upper 
 end of the spindle turns in a wooden bush fixed into the nether 
 mill-stone, which lies upon beams in the floor I. The top of 
 the spindle above the bush is square, and goes into a square 
 
364 
 
 MECHANICS. 
 
 Description of a corn-mill. 
 
 liole in a strong iron cross, a h c d, fig. 8, called the rynd ; under 
 which, and ciose to the bush, is a round piece of thick leather 
 upon the spindle, which it turns round at the same time it does the 
 rynd. 
 
 The rynd is let into grooves in the under surface of the upper 
 or running mill-stone, which it turns round in the same time that 
 the trundle G is turned round by the cog-wheel F. This mill- 
 stone has a large hole quite through its middle, called the eye of 
 the stone, through which the middle part of the rynd and upper 
 end of the spindle may be seen ; whilst the four ends of the rynd 
 lie below the s^one in their grooves. 
 
 One end of the bridge-tree FI, which supports the spindle, 
 rests upon the wall, and the other end is let into a beam, called 
 the brayer, LM. The brayer rests in a mortice at L, and the 
 other end M, hangs by a strong iron rod N, which goes through 
 the floor I, and has a screw and nut on its top at O ; by the turn- 
 ing of this nut, the end M of the brayer is raised or depressed 
 at pleasure, and consequently the bridge-tree and the upper 
 mill-stone. By this means the upper mill-stone may be set as 
 close to the under one, or raised as much above it as may be 
 necessary. It will of coy.rse be understood, that the nearer the 
 mill-stones are to each other, the finer the corn will be ground ; and 
 that on the contrary, the further they are separated, the coarser it 
 will be. 
 
 The upper mill-stone is inclosed in a round box, which no- 
 where touches it, and is about an inch distant from its edge 
 all round. On the top of this box stands a frame for holding 
 the hopper P, to which is hung the shoe Q, by two lines fas- 
 tended to the hinder part of it, fixed upon hooks in the hopper, 
 and by one end of the string R fastened to the fore part of it, 
 the other end being twisted round the pin S. By turning this pm 
 one way, the string draws up the shoe closer to the hopper, and 
 so lessens the aperture between them ; and as the pin is turned 
 the other way, it lets down the shoe, and enlarges the aperture. 
 If the shoe be drawn up quite to the hopper, no corn can fall from 
 the hopper into the mill ; if it be let down a little, some will fall ; 
 and the quantity will be more or less, according as the shoe is 
 more or less let down ; for the hopper is open at the bottom, and 
 there is a hole at the bottom of the shoe, not directly under the 
 bottom of the hopper, but nearer to the lowest end of the shoe, 
 over th« middle or eye of the stone. 
 
 In a square hole at the top of the spindle, is put the feeder 
 E, fig. 8. This feeder, as the spindle turns round, jogs the 
 shoe three times in each revolution, and so causes the corn to 
 run xonstantly down from the hopper through the shoe, into 
 
MECHANICS. 
 
 365 
 
 Description of a corn-mill. 
 
 the eye of the mill-stone, where it falls upon the top of the rynd, 
 and is, by the motion of the rynd, and the leather under it, 
 thrown below the upper stone, and ground between it and the 
 lower one. The rapid motion of the stone, creates a centrifugal 
 tendency in the corn going lound with it, by which means it gets 
 farther and farther from the centre, as in a spiral, in every revolu- 
 tion, until it is quite thrown out; and being then ground, it falls 
 through a spout, called the mill-eye, into a trough placed for its 
 reception. 
 
 When the mill is fed too fast, the corn bears up the stone, 
 and is ground too coarse ; besides, the mill is apt to get clogged, 
 and to go too slowly. When the corn is scantily supplied, the 
 mill goes too fast, and the stones, by their collision, are apt 
 to strike fire. Both these inconveniences are avoided, by turn- 
 ing the regulating pin S backward or forward, in order to draw 
 up or let down the shoe, as the case is observed by the miller to 
 require. 
 
 Tlie heavier the running mill-stone, and the greater the quan- 
 tity of water falling upon the wheel, the faster will the mill bear 
 to be fed, and consequently the greater the performance of the 
 mill ; and, on the contrary, the lighter the stone, and the less 
 the quantity of water, the slower must be the feeding. When 
 the stone is considerably worn, and become light, its weight 
 must either be increased by some artificial addition, or the mill 
 must necessarily be fed slowly ; otherwise the stone will be too 
 much born up by the corn under it, to grind the meal sufficiently 
 fine. 
 
 The power necessary to turn a heavy mill-stone, is but very 
 little more than what is necessary to turn a light one ; for as the 
 stone is supported upon the spindle of the bridge-tree, and the end 
 of the spindle that turns in the brass foot is but small, the differ- 
 ence arising from the weight produces only an inconsiderable ac- 
 tion against the pow'er or force of the w ater. Besides, a heavy 
 stone affords the same advantage as a heavy fiy, that is, it regulates 
 the motion much better than a light one, from its not being liable 
 to such great fluctuations of velocity. 
 
 7'he centrifugal force carrying the corn towards the circum- 
 ference of the stones, it is obvious that it will be crushed when 
 it comes to a place where the interval between the two mill- 
 stones is less than its thickness ; yet, as the upper mill-stone is 
 supported on a point which it can never quit, it may not be 
 considered equally obvious why it should produce a greater 
 efl’ect when it is heavy than when it is light; since, if it were 
 equally distant from the nether mill- stone, it could only be 
 capable of a limited impression. But as experience proves 
 
366 
 
 MECHANICS. 
 
 Form of the acting faces of mill'Stones. 
 
 that the difterence actually occurs, it may be proper to state the 
 cause. The spindle of the mill-stone being supported by a hori- 
 zontal piece of timber, about nine or ten feet long, resting only on 
 both its ends, the upper mill-stone, by the elasticity of this piece, 
 is allov/ed a vertical motion, and plays up and down ; by which 
 movement, the heavier the stones are the more forcibly is the corn 
 w^edged in betw^een them. 
 
 In order to cut and grind the corn, both the upper and 
 under mill-stones have channels or furrows cut in them, proceed- 
 ing obliquely from the centre to the circumference. These 
 furrows, in the direction of their length, are cut slantwise on 
 one side, and perpendicularly on the other, so that each of the 
 ridges which they form has a sharp edge ; and in tlie two stones, 
 these edges pass one another like the edges of a pair of scissars, 
 and so cut the corn, to make it grind the more easily, when it 
 falls upon the furrows. The furrows are cut the same way in 
 both stones, when they lie upon their back^, which makes them 
 run crosswise to each other when the upper stone is inverted by 
 turning its furrowed surface towards that of the low^er; for, 
 if the furrows of both stones laid the same way, part of the 
 corn would be driven onward in the lower furrows, and come 
 out from between the stones without being either ground or 
 bruised. 
 
 The grinding surface of the under stone is a little convex, 
 from the edge to the centre, and that of the upper stone a little 
 concave ; and they are farthest from one another in the middle, 
 but approach gradually nearer towards the edges. By this means 
 the corn, at its first entrance betw^een the stones, is only bruised ; 
 but as it goes farther on towards the circumference or edge, it is 
 cut smaller and smaller, and at last finely ground, just before it 
 comes out from between them. 
 
 When the ridges become blunt and the furrows shallow by 
 wearing, the running stone must be taken up, and both of them 
 may then be drest anew with a .chisel and mallet. Every time the 
 stone is taken up, there must be some tallow put round the spindle 
 and upon the bush ; this unguent will soon be melted by the heat 
 the spindle acquires from its turning and rubbing against the bush, 
 which it will prevent from taking fire. 
 
 The bush must embrace the spindle quite ‘dose, to prevent 
 any shake in the motion, which would cause some parts of the 
 stones to grate against each other, whilst the other parts of 
 them would be two far asunder, and by that means spoil the 
 meal. Hence, whenever the spindle has worn the bush, so as 
 to begin to shake in it, the stone must be taken up, and a chisel 
 driven into several parts of the bush ; and when it is taken out. 
 
MECHANICS. 
 
 367 
 
 American mode of raising ground corn to the top of the mill. 
 
 wooden wedges must be forced into the holes ; by which means 
 the bush \vill be made closely to embrace the spindle again all 
 round. In doing this, great care must be taken to drive equal 
 wedges into the bush on opposite sides of the spindle ; otherwise 
 it will be throw n out of the perpendicular, and so hinder the upper 
 stone from being set parallel to the under one, which is absolutely 
 necessary for making good work. When any accident of this kind 
 occurs, the perpendicular position of the spindle must be restored, 
 by adjusting the bridge-tree with proper wedges put between it 
 and the brayer. 
 
 It often happens, that the rynd is a little wrenched in lay- 
 ing down the upper stone upon it, or is made to sink a little 
 lower on one side of the spindle than on the other ; and this 
 will cause one edge of the upper stone to drag all round upon 
 the lower, while the opposite edge will not touch. This is 
 easily rectihed, by raising the stone a little with a lever, and put- 
 ting bits of paper, card, or thin chips, between the rynd and the 
 stone. 
 
 We shall mention in this place a very useful and ingenious 
 contrivance, adopted by the American millwrights for raising the 
 ground corn to the cooling boxes or place, from which it is con- 
 veyed into the bolting machine. They place a large screws hori- 
 zontally in the box which receives the dour from the mill-stones. 
 The thread or spiral line of the screw' is composed of pieces of 
 wood about two inches broad and three long, tixed into a wooden 
 cylinder seven or eight feet in length, w'hich foi ms the axis of the 
 screw'. When the screw is turned round this axis, it forces the 
 meal from one end of the trough to the other, where it falls into 
 another trough, from which it is raised to the top of the mill-house 
 by means of elevators, a piece of machinery similar to the chain 
 pump. These elevators consist of a chain of buckets, or concave 
 vessels like large tea-cups, fixed at proper distances upon a lea- 
 thern band, going round tw o wheels, one of which is placed at the 
 top of the mill-house, and the other at the bottom in the meal- 
 trough. When the wheels are put in motion, the band revolves, 
 and the buckets, dipping into the meal-trough, convey the meal to 
 the upper story, where they discharge their contents. The band 
 of buckets is inclosed in tw o square boxes, in order to keep them 
 clean, and preserve them from injury. It is obvious how much 
 more complete this contrivance is, than the mode adopted in this 
 country, of putting the meal into sacks, and then raising it up by 
 the common machinery for that purpose. 
 
368 
 
 MECHANICS. 
 
 The mechanism of a horse with respect to draught. 
 
 Of Wheel-carriages. 
 
 In considering this subject, it will be proper to advert to the 
 formation of the animal by which wheel- carriages are put in 
 motion. The horse is admirably calculated for draught, and 
 the circumstances enabling him to draw to the greatest advantage 
 are, to a certain extent, so well known to every one at all con- 
 versant wdtii mechanics, that it is not less a cause of surprise 
 than of regret, that his valuable properties should still continue 
 to be so much abused as we find them to be. But information 
 spreads slowly among the mass of the people, and it is long in 
 reaching provincial wheelwrights, among whom, as amongst other 
 classes of men, there is a disposition to follow the practice of their 
 forefathers, wilhoKt inquiring whether they are right or wrong. 
 M uch as men are attached to their interest too, the arrogance of 
 dominion, even over a brute, often renders them cruel in opposition 
 to it ; and cruelty can never bear the light of reason in her path. 
 When the broad-wheel act was passed, a great outcry was raised 
 against it by those whom it would have benefited, and who, in- 
 stead of complying wdlh it, perversely rendered its provisions worse 
 than nugatory, by bevelling their wheels. Thus, instead of reliev- 
 ing their hoises, they adopted a practice tending to oppress them 
 more than ever. With respect to the position of the line of trac- 
 tion, errors of equal moment are frequently committed, as a little 
 attention will enable any one to perceive, who shall consider for a 
 moment the form of the shoulders of a horse. It is evident (see 
 fig. 9, ph V,) that, at the place w here the neck rises from the chest 
 of the animal, the shoulder-blades form the resting place of his 
 collar or harness into a slope, a p. This slope or inclination 
 forms an angle wdth a perpendicular to the horizon, of about four-’ 
 teen or fifteen degrees; and therefore the line of traction or 
 draught should form the same angle with the horizon, because he 
 w ill then pull perpendicularly to the shape of his shoulder, and all 
 parts of that shoulder will be equally pressed by the collar. Ber 
 sides, in overcoming obstacles, the advantage of this inclined direc- 
 tion is mechanically great ; the following demonstration of it, 
 taken from Walker’s ‘‘ System of Familiar Philosophy” has not 
 perhaps been improved upon. Call cf, fig. 10, a wheel, h an ob- 
 stacle, c the axle of the w heel, d the spoke which at present sus- 
 tains the w'eight. A line draw n from the nearest part of the hori- 
 zontal line of draught c k to the fulcrum or obstacle at e, will form 
 the acting part of a lever g e ; and another line e d being drawn 
 from the fulcrum e to the nearest part of the spoke d, will form 
 the resisting part of the same lever. Now as the acting and re- 
 sisting arm® of the lever are of equal lengths the lever becomes 
 
MECHANICS. 
 
 369 
 
 Advantages of an inclined line of draught. 
 
 a scale-beam, and a draught in the line g k must be equal to tlie 
 weight of the wheel and all that it sustains, besides the friction ; 
 for i( g ed he 2 l crooked lever, a pull at g must be equal to all 
 the weight supported by d. But when a horse draws agreeably 
 to the shape of his shoulder, in the line i //, the acting part of 
 the lever h e is lengthened nearly one-fourth ; so that if it would 
 require a pull at g equal to four hundred weight, a power ap- 
 plied at h will draw the wheel over the obstacle b with three 
 hundred weight. To those unacquainted with the principles of 
 mechanics, this truth may be easily proved by an ordinary 
 scale-beam. The horse himself, considered as a lever, has in 
 this inclined draught a manifest advantage over his obstacles, in 
 comparison of a horizontal draught, as may be seen by fig. 9 » 
 When the horse is yoked to a post, or has any great obstacle to 
 overcome, he converts himself into a lever, making his hind 
 feet the fulcrum, and the centre of gravity of his body to lean 
 over it, at as great a distance as possible, by thrusting out his 
 hind feet ; by this means, acting both by his w eight and muscular 
 strength, and lengthening the acting part of the lever a by he 
 overcomes the difficulty more by his weight than by his muscular 
 strength ; for the muscles of the fore legs act upon the bones to 
 so great a mechanical disadvantage, that though he exerts them 
 with all his might, they serve, in great efforts, for little more 
 than props to the fore-part of his body, tlence we see the 
 great use of heavy horses for draught. But the great mechani- 
 cal use and advantage of the inclined line of diaught may be 
 more particularly seen, by calling the line a b the acting part 
 of the lever, and the nearest approach from the fulcrum b to the 
 inclined line of draught (that is, b c) the resisting part of the 
 lever : compare this with the resisting part of a lever touching 
 the horizontal line of draught, (that is, b d) and it will be found 
 nearly double ; in consequence, agreeably to the known properties 
 of the lever, a weight at g would require double the exertion in 
 the horse to remove it, that the same weight would require were it 
 placed at e. 
 
 From the above data, several important practical conclusions 
 may be drawn ; — one is particularly important, that single-horse 
 carts are preferable to teams, because in a team, all but the 
 shaft horse must draw horizontally, and consequently in a man- 
 ner inconsistent with their structure, and the established laws 
 of mechanics. The small horses of the north of England 
 draw more w eight of actual goods than our largest waggon 
 horses, and go longer stages. The small horses of Ireland, 
 as a common load, draw fifteen hundred weight of goods, and 
 travel farther in a day than our waggons, and over worse roads 
 
 1G.—V0L.I. SB 
 
370 'MECHANICS. 
 
 Forms of wheels. — Source of the advantage of wheels. 
 
 than ours are in general ; ten or twelve hundred weight of real 
 goods is as much as falls to the share of one waggon horse, whose 
 superior strength is wasted upon a cumberous vehicle, and by the 
 mechanical disdavantages of his draught. 
 
 Waggon-wheels are generally made with the extremities of 
 the axle inclined downwards ; thus is forfeited the advantage 
 of their being formed in a lathe, without mofe trouble than the 
 makers are inclined to bestow for ordinary purposes ; and the ends 
 are seldom, perhaps never, inclined in the same angle or exactly 
 opposite each other, consequently the tendency of the motions of 
 the wheels is in different directions, and the draught of the horses 
 is constantly exerted in twisting them out of their natural course. 
 This mischief is increased by bevelling the wheels, and the 
 horses are harassed to no purpose except that of grinding the 
 roads. Let a bevelled wheel be rolled by iteelf, it will soon be 
 seen that it will not proceed in a straight line, but in a curved 
 line, like a cone. Sir George Saville, a philosophic patriot, 
 whose memory is illustrious, calculated ho^v far a waggon was 
 rendered a sledge in a journey from Loudon to York. The dis- 
 tance is two hundred miles ; thirty of which he found the wag- 
 gon is drawn as if it were a sledge without wheels. Another 
 disadvantage of a waggon arises from the sluggishness of its 
 motion. This will be readily understood and allowed, when it is 
 considered how’ small a force will continue the motion of a heavy 
 body, moving with a certain degree of rapidity, in comparison 
 with what is required to impel it from a state of rest ; but if the 
 motion of the body be extremely slow, the force necessary to keep 
 it up, must be nearly equal to that which moved it at first. The 
 latter case is precisely that of w'aggon horses, which have, every 
 instant of their draught, to overcome nearly the whole inertia of 
 their load. 
 
 A sledge, in sliding over a plane, suffers a friction equiva- 
 lent to the distance through which it moves ; but if we apply 
 wheels, the circumference of which is eighteen feet, and they 
 turn upon axles, the circumference of which is only six inches, 
 it is plain, that while the carriage moves eighteen feet over the 
 plane, the wheels make but one revolution ; and as there is no 
 sliding of parts between the plane and the wheels, but only a 
 mere change of surface, no friction takes place there, the 
 whole being transferred to the nave acting on the axle ; so 
 that the only sliding of parts has been betwixt the inside of the 
 nave and the axle, which, if they fit one another exactly, is 
 no more than six inches ; hence the friction is reduced in the pro- 
 portion of six inches to eighteen feet, that is, as thirty-six to 
 one. In all cases, by applying wheels, the friction is thus 
 
mechanics. 
 
 371 
 
 Reason of low fore-wheels. — Lord Somerville’s cart. 
 
 lessened, in the proportion of the diameter of the axles to that 
 of the wheels. Another advantage is also gained, by having the 
 surfaces of friction confined to so small an extent, arising from the 
 circumstance of their being more easily made true, kept smooth, 
 and fitted to each other. The only inconvenience is the height of 
 the wheels, which must in most cases be added to that of the car- 
 riage itself. 
 
 A four-wheeled carriage may be drawn with five times as 
 much ease as one that slides upon the same surface in the con- 
 dition of a sledge. In four-wheeled carriages, the fore-wheels 
 are made of a less size than the hind ones, in order to enable 
 them to turn in less room ; and not for the purpose of bringing 
 into action any supposed pushing quality in high back-wheels. 
 
 Large wheels have the advantage over small ones in over- 
 coming obstacles, because wheels act as levers in proportion to 
 their various sizes ; but when they are so high as not to allow 
 the line of draught to have the inclination before stated, their 
 advantage as longer levers is counterbalanced by their lessening 
 the intensity of the moving power ; therefore the total advantages 
 of wheels drawn horizontally do not increase proporlionally to 
 their height. 
 
 In ascending, high wheels will be found to facilitate the 
 draught in exact ratio with the squares of their diameters ; but in 
 descending they are liable to press in the same proportion. An 
 admirable device was produced by Lord Somerville, to remedy 
 the latter evil ; it consisted in throwing the weight behind the cen- 
 tre in going down hill, by raising the fore part of the body of the 
 cart ; so that while the shaft may incline downwards, in propor- 
 tion to the line of declivity, the bottom of the cart’s body should 
 remain horizontal. This construction is now common in Devon- 
 shire, and some other counties. 
 
 As small wheels turn as much oftener round than large 
 ones as their circumferences are less, so, when the carriage is 
 loaded with an equal weight on both axles, the fore axle must 
 sustain as much more friction, and consequently wear out as 
 much sooner than the liind axle, as the fore-wheels are less than 
 the hind ones. This points out that the greatest w'eight should 
 be laid upon the large wheels ; yet it is generally the practice 
 to put the greatest load over the small wheels, which not only 
 makes the friction greatest where it ought to be the least, but 
 also presses the fore-wheels deeper into the ground than the 
 hind-wheels, notwithstanding the former are with more difficulty 
 drawn out of it than the latter. The limitation to loading the 
 hind-wheels with the greatest part of the w’eight, will consist in 
 
MECHANICS. 
 
 S72 
 
 Dished wheels. 
 
 not carrying it to such an excess as to endanger the tilting of the 
 vehicle, in going up-hill. 
 
 Wheels are commonly made with what is called a dish, that 
 is, the spokes are inserted not at right angles, but vith an incli- 
 nation towards the the axis of the nave or centre-piece; so that, 
 if the interior end of the nave were placed on the ground, the 
 spokes being higher at the outside than at their termination in the 
 nave, the wheel appears dished or hollow. Wheels are usually 
 dished about four inches in a diameter of live feet. If the wheels 
 were always to go on smooth and level ground, the best way 
 would certainly be to make the spokes perpendicular to the naves 
 and axles ; because they would then bear the load perpendicu- 
 larly, in which position wood supports the greatest weight. But 
 because roads are generally uneven, one wheel often falls into a 
 cavity, or rut, when the other does not, and then it bears much 
 more than an equal share of the load. Hence the utility of dish- 
 ing the wheels, because when a dished wheel falls into a rut, 
 the spokes become perpendicular’ in the rut, aird therefore have 
 the greatest strength when the obliquity of the load throws most 
 of its weight upon them ; whilst those on the high ground, hav- 
 ing less weight to bear, have no occasion, at that time, for their 
 utmost strength. Dished wheels, when on straight or horizontal 
 axles (and no other axles should be used,) have many other excel- 
 lencies ; they make carriages to stand oir a broader base, and 
 therefore render them less liable to be overturned ; they give more 
 room to the body of the carriage than if the spokes were perpen- 
 dicular ; they stand against side-jolts like an arch, and when the 
 carriage is going along the inclined side of a road, they render it 
 less liable to be overthrown. 
 
 If the spokes be set so far from the outer end of the nave, 
 that a perpendicular from the sole to the under side of the axle 
 may fall betvyeen an inch and two inches between the bushes, 
 the pressure will be somewhat greater on the outer than on th^ 
 inward bush, when the wheels are on a level. This will be an 
 advantage, particularly w'hen the inner part of the axle-arm is ,, 
 much larger than the outer; as it has then more friction, there- ' 
 fore the pressure should be diminished ; besides, every sinkjng of 
 one wheel more than the other, causes it to pinch the inner 
 bush. It has been proposed, as the best mode of placing the 
 spokes in the naves, to mortice them in two rows, alternately ; 
 this does not weaken the centre so much as when all the spokes 
 are in one row or band, and gives a greater power of resistance 
 outwards. 
 
 The question whether broad or narrow wheels are best, has , 
 
MECHANICS. 
 
 373 
 
 Breadth of -wheels. — Form of axle-armi. 
 
 been much contested. The popular opinion has always been 
 in favour of narrow wheels ; and accordingly, the carriers thought 
 themselves injured by the broad-wheel act, though it allowed 
 them to draw with more horses, and carry greater loads than 
 usual. This matter deserves a moment's attention. — We have 
 observed on a former occasion, that friction increases somewhat 
 w ith an increase of surface : so far then, the opinion of the car- 
 riers tallys with a general truth ; but it is at the same time true, 
 that if the moving body have so thin a resting edge as to cut into 
 the surface over which it passes, the friction will be actually in- 
 creased by the diminution of the surfaces in contact. It is the 
 want of duly considering this, that supports the prejudice in favour 
 of narrow' wheels, w'hich cut and sink into the roads, and may be 
 considered, except on the very few roads that are impervious to 
 them, as constantly going up-hill, even upon level ground. But 
 experience testifies, that instead of thus ploughing the roads, broad 
 cylindrical wheels smoath and harden them, and move witlr greater 
 facility. 
 
 If the tire or iron binding of a wheel be in separate parts, 
 and not in one single hoop, these parts should not be made 
 quite to meet each other at the first ; because when the wheel 
 has been some time in use, they will settle more closely to the 
 wheel than they can be laid, and the vacancies will then be 
 filled up. The axle-arm should be a perfect cylinder, or if 
 tapered towards the extremity, the difference of its two diameters 
 should be very trifling ; a small degree of taper is preferred by many, 
 because it gives the wheel rather a disposition to slide off, thus 
 preventing it from being apt to close inwardly, and creating exces- 
 sive friction ; but it increases the necessity for good iron washers 
 exteriorly, and of substantial linch-pins. It is not an uncommon 
 |)ractice to set the wheels, that is, to give them a slight inclina- 
 tion tow'ards each other, w hereby they are, perhaps, an inch nearer 
 at the front than at the back ; this is chiefly done to wheels that 
 are bevelled, with a view to make them run more evenly on their 
 sole or bearing part, and to prevent their gaping forward ; but it 
 is evidently a distortion, an attempt to rectify one bad thing by 
 another of tfie same stamp, as if the multiplication of mischief 
 would produce good. 
 
 The nave of a heavy wheel, as for an ordinary cart for field 
 puposes, need not be mpre than twelve or fourteen inches in 
 length; if too shprt, tlie wheel will wabble, unless fitted very 
 tightly on the axle ; while top l^g a nave is apt to catch the dirt 
 from the upper part, and to project too much beyond the outer 
 face of the fellies ; the length jusjt stated is exclusive of the pan at 
 the outer end. 
 
374 
 
 MECHANICS. 
 
 Height of carriage-wheels. — Clock-W'ork. 
 
 The proportions of wheels are often regulated as much by 
 the purposes to which the vehicles are applied, as by the facilities 
 they afford to motion; thus waggons have in general large hind- 
 wheels, while in timber-carriages the four are nearly of the same 
 height ; the London common stage- carts have large wheels, while 
 the drays used by brew'ers have very low ones. The reason is ob- 
 vious : waggons and carts load behind ; but dra\ s, and the timber- 
 carriages alluded to, load at the sides ; and therefore, for them, 
 large wLeels, however much they might favour the draught, would 
 be extremely inconvenient, indeed incompatible with their use. 
 The w'heels of single-horse carts for ordinary purposes, where 
 there is no particular necessity for having them low, may be from 
 four feet to four feet six inches in diameter, for a horse of about 
 sixteen hands high. For four-wheeled carriages, suppose four 
 feet to be the height of the fore-wheels, and the line of traction to 
 be drawn at an elevation of fourteen degrees from the centre of its 
 axle, the point w here that line cuts the circumference of the wheel 
 in its front, gives that height from the plane on wdiich the carriage 
 stands, that will determine the radius of the hind-wheels. 
 
 Wheels, whatever their size, should be made of well-seasoned, 
 tough wood, perfectly free from blemish ; the naves are generally 
 of elm, the spokes of oak, and the fellies of elm or of ash. I'he 
 bent fellies, when the w^ood has not been hurt by too much heat, 
 have greater strength with less wood, than those which are cut by 
 the saw in a curved.diiection. 
 
 It is common in many places, and deserves to be so every- 
 where, to have a contrivance for relieving the horse of a loaded 
 cart from the weight pressing on his shoulders, when it is neces- 
 sary for any purpose to stop awhile ; this useful object is attained 
 by an appendage so simple as that of a pole or staff, w'hich, turn- 
 ing on a hook-and-eye hinge, is let down from one of the shafts, 
 when the occasion requires. 
 
 Clock-Work. 
 
 The term clock-tt^rk, originally imported those wheels, pi- 
 nions, and other mechanism, which constituted the striking part, 
 or what was formerly called the clock part of a movement for 
 measuring time ; and that part of the machine which gave motion 
 to the hands for shewing the parts of time, was called the watch 
 part. But at present the term watch is appropriated to such 
 movements for measuring time as are carried in the pocket; and 
 the larger movements whether they strike or not, are called clocks. 
 
 Watches which regularly strike the hour, are called pocket- 
 clocks ; but if they only strike the hour, when some particular part 
 for that purpose is touched, they are called repeating watches* 
 

MECHANICS. ’ 
 
 375 
 
 Definitions. — Historical remarks. 
 
 The term chronometer, is chiefly used by workmen and navi- 
 gators, to denote a watch or portable machine, constructed with 
 so much care and skill, as to be fit for determining the longitude 
 at sea. The word time-keeper is a fit appellation for an astrono- 
 mical clock, but it is often employed with the same latitude as 
 time-piece, which is of more extensive signification, as a general 
 name, or a varied or compendious mode of denoting any instru- 
 ment for measuring time.” 
 
 In treatises, it is usual, for the sake of distinction, to use 
 the terms clock and watch in their ancient sense, generally with 
 the word part after them ; thus, by the expression clock part, is 
 meant the striking portion only, and by that of watch part, the 
 going portion only. 
 
 There are many documents to prove the existence of clocks, 
 with wheels and weights, in the middle of the fourteenth cen- 
 tury ; but the invention cannot be traced to an earlier date with 
 any certainty. The sphere of Archimedes, indeed, has been 
 considered as the first attempt towards the formation of a clock ; 
 it had a maintaining power, but being without any kind of 
 regulator, could only measure time, as a planetarium exhibits 
 the motion of the stars, with relative, but not positive precision. 
 The opinion of Berthoud, who has investigated the subject 
 with attention, is evidently just, when he asserts that the clock 
 is not the invention of any one man, but an assemblage of suc- 
 cessive inventions, each of which is worthy of a separate con- 
 triver. 1. Wheelwork, which was known in the time of Archi- 
 medes ; 2. the application of the weight as a maintaining power ; 
 3. the use of the fly as a regulator; 4. the ratchet wheel and 
 click ; 5. the substitution of the balance for the fly, and the 
 escapement w hich was necessarily introduced at the same time ; 
 (j. the application of the dial and hands ; and 7. the addition of 
 the striking part. 
 
 The introduction of the spiral spring, as a first mover, in- 
 stead of a weight, took place about the beginning of the sixteenth 
 century. About the year 1630, a new era in the art of clock- 
 work commenced, by the application of the pendulum as a 
 regulator; in the year 1713, a compensation for the effects of 
 change of temperature w'as applied to time-pieces, which, not 
 long after, were contrived so as to go while they were w^ound 
 up. From this time, improvements succeeded each other with 
 great rapidity. These improvements relate partly to more ac- 
 curate workmanship, but principally to the escapement, or 
 different modes of connecting the wheel-work wdth the pendulum 
 or balance. Volumes would be required to do justice to the 
 persons who have distinguished themselves in this line of art ; 
 
376 
 
 MECHANICS. 
 
 Description of a clock. 
 
 but Wfc must go little beyond the general principles of this sort of 
 mechanism, and these will perhaps be best understood, if, in the 
 first place, we describe the parts of a clock. 
 
 The profile of the watch or going part of a clock is shewn bj 
 fig. 1, pi. VI. P is a weight which keeps the clock going; it is 
 suspended by a catgut band that winds about the cylinder or bar- 
 rel C, which is fixed upon the axis aa ; the pivots bh go into 
 holes made in the brass plates SS, TT, in w'hich they turn freely. 
 These plates are connected by means of four pillars, only two of 
 which, ZZ, can be seen in the profile, and the whole together is 
 called the frame. On the circumference of barrel C is a spiral 
 groove, in which the catgut lies, and is thereby caused to wind 
 round in a regular manner. 
 
 The weight P, if not restrained, w'ould necessarily turn 
 the barrel C, with a uniformly accelerated motion, in the same 
 manner as if the weight w as falling freely from a height ; but 
 the barrel is furnished with a ratchet-wheel, KK, the right sides 
 of the teeth of which strike against the click, which is fixed 
 with a screw to the wheel DD, so that the action of the weight 
 is communicated to the wheel DD, the teeth of which act 
 upon the leaves of the pinion which turns upon the pivots c c. 
 This communication of the teeth of one wheel with another is 
 called pitching. Several things are requisite to form a good 
 pitching, which is very important in all machinery where 
 wheels and pinions are employed ; the teeth and pinion leaves 
 should be of a proper shape, and perfectly equal among them- 
 selves ; the size also of the pinion should be of a just proportion 
 to the wheel. 
 
 The w'heel EE is fixed upon the axis of the pinion d ; and 
 the motion communicated to the wheel DD by the weight, is 
 transmitted to the pinion d, consequently to the wheel EE, as 
 likewise to the pinion e, and wheel FF, which moves the pinion 
 f\ upon the axis of which, the last or swing wheel GH is fixed. 
 In a word, the motion begun by the weight is transmitted from 
 the wheel GH to the pallets IR of the escapement (fig. ^,) fixed 
 on an arbor going through the back plate of the frame, and carry- 
 ing a lever XU, (fig. 1,) which is forked at the lower part to re- 
 ceive the pendulum. The pendulum consists of a metallic rod, 
 suspended by a very slender piece of steel spring y, from a brass 
 bar A, screwed to the frame of the clock, and has a heavy weight 
 or bob at its low er end. I'he pendulum y B, if once put in 
 motion, will describe round the point of suspension y, an arc 
 of a circle, and will continue to go alternately backward and 
 forw ard till the force impressed upon it is wasted, by the usual 
 causes w hich tend to destroy all other artificial motions, viz. 
 
MECHANICS. 
 
 377 
 
 Description of a clock. 
 
 friction and gravitation. The pendulum is the time-measurer : 
 at every vibration which it makes, the teeth of the swing-wheel 
 GH act upon the pallets IR, (fig. 2,) in such a manner, that after 
 one tooth H has communicated motion to the pallet R, that tooth 
 escapes ; then the opposite tooth G acts upon the pallet I, and 
 escapes in the same manner : and thus each tooth of the wheel 
 ' escapes the pallets IR, after having communicated their motion 
 to the pallets ; so that the pendulum, instead of stopping, conti- 
 nues to move. 
 
 The wheel EE, (fig. 1,) revolves in an hour ; the pivot c ot 
 this wheel passes through the plate, and is continued to r ; upon 
 this pivot is a wheel NN, with a long socket fastened in its 
 centre; upon the extremity of this socket r the minute-hand is 
 fixed. The wheel NN acts upon the wheel O ; the pinion of 
 ' which, p, acts upon the wheel ggj fixed upon a socket which 
 turns along with the wheel N. 'i'his wheel gg makes its revo- 
 : lulion in tw elve hours, and upon the barrel of it is fixed the hour- 
 ' hand. 
 
 ' From the above description it is easy to see, 1. that the weight 
 
 * P turns all the wheels, and at the same time continues the mo- 
 tion of the pendulum ; 2. that the quickness of the motion of 
 
 • the wheels is determined by that of the pendulum ; and, 3. 
 that the wheels point out the parts of time divided by the 
 uniform motion of the pendulum. When the catgut upon which 
 
 ' the weight is suspended is entirely run down from off the barrel, 
 it is wound up again by means of a key, which goes on the 
 square end of the arbor or axis Q, by turning it in a contrary 
 direction from that in wFich the w'eight descends. For this 
 purpose, the inclined side of the teeth of the ratchet-wheel K, (fig. 
 2,) removes the click C, so that the wheel K turns with the barrel, 
 wliile the wheel D is at rest ; but as soon as the band is wound 
 up, the click falls in between the teeth of the wheel K, and the 
 right side of the teeth again act upon the end of the click, which 
 obliges the wheel D to run along with the barrel, and the spring 
 A keeps the click between the teeth of the ratchet-wheel K. Sup- 
 posing the wheel DD to turn once round in tw^elve hours, which it 
 is usually calculated to do in the best time- pieces, then will six- 
 teen turns of the catgut on the barrel C, (fig. 1,) suffice to keep the 
 clock going eight days. 
 
 It will now be proper to explain how' time is measured by 
 the motion of the pendulum ; and hovv the wheel E, upon the 
 axis of which the minute hand is fixed, makes but one precise 
 revolution in an hour. The vibrations of a pendulum are per- 
 formed in a shorter or longer time in proportion to the length 
 of the pendulum itself. A pendulum of 3Qj W length 
 
 16. — VoL. I. 3 C 
 
378 
 
 ' MECHANICS. 
 
 Description of a clock. 
 
 makes 3600 vibrations in an hour ; that is, each vibration is 
 performed in a second of time, and for that reason it is called 
 second pendulum. But a pendulum of 9yl inches makes 
 7,200 vibrations in an hour, or two vibrations in a second 
 of time, and is called a half second pendulum. Hence, in 
 constructing a wheel whose revolution must be performed in 
 a given time, the time of the vibrations of the pendulum 
 which regulate its motion must be considered. Supposing 
 then, that the pendulum y B makes 7,200 vibrations in an 
 hour, let us consider how the wheel E shall take up an 
 hour in making one revolution. This entirely depends on the 
 number of teeth in the wheels and pinions. If the swing wheel 
 contains 30 teeth, it will turn once round in the time that 
 the pendulum makes 60 vibrations ; for at every turn of the 
 wheel, the same tooth acts once on the pallet I, and once on 
 the pallet K, and at each stroke the pendulum makes a vibration ; 
 therefore, as the wheel has 30 teeth, it occasions twice 30 
 vibrations ; consequently this wheel must perform 180 revolu- 
 tions in an hour ; because 60 vibrations, which it occasions at 
 every revolution, are contained 120 times in 7,200, the num- 
 ber of vibrations performed by the pendulum in an hour. Now 
 in order to determine the nimiber of teeth for the wheels EF, 
 and their pinions ef it must be remarked, that one revolution 
 of the wheel E must turn the pinion e as many times as the 
 number of teeth in the pinion is contained in the number of 
 teeth in the wheel. Thus, if the wheel E contains 72 teeth, 
 and the pinion e contains 6, the pinion will make 12 revolutions 
 in the time that the wheel makes one ; for each tooth of the 
 wheel drives forward a tooth of the pinion; and w'hen the 6 
 teeth of the pinion are moved, a complete revolution is per- 
 formed ; but the wheel E has by that time only advanced 6 teeth, 
 and has still 66 to advance before its revolution is completed, 
 which will occasion 1 1 more revolutions of the pinion. For 
 the same reason, the wheel F having 60 teeth, and the pinion 
 f only 6, the pinion will make the 10 revolutions while the wheel 
 performs one. Now the wheel F being turned by the pinion 
 c, makes 12 revolutions for one of the wheel E; and the pinion 
 f makes 10 revolutions for one of the wheel F ; consequently, 
 the pinion f performs 10 times 12, or 120 revolutions in the 
 time the wheel E performs one. But the wheel G, which is 
 turned by the pinion f occasions 60 vibrations in the pendulum 
 each time it turns round ; consequently, the wheel G occasions 
 60 times 120, or 7,200 vibrations of the pendulum while the 
 V heel E performs one revolution; but 7,200 is the number of 
 vibrations made by the pendulum in an hour, and consequently 
 
MECHANICS. 
 
 379 
 
 Description of a clock. 
 
 the \theel E performs but one revolution in an hour; and so of 
 the rest. 
 
 From this reasoning it is easy to discover liow a clock may 
 be made to go for any length of time without being wound 
 up; 1, by increasing the number of teeth in the wheels; 2, 
 by diminishing tlie number of teeth in the pinions ; 3, by increas- 
 ing the length of the cord that suspends the w eight ; and lastly, 
 by adding to the number of wheels and pinions : but, in propor- 
 tion as the time is augmented, the weight must be increased, or 
 the force which it communicates to the last wheel GH will be di- 
 minished. 
 
 With respect to the watch part of the movement we are 
 considering, it only remains to take notice of the number of 
 teeth in the wheels which turn the hour and minute hands. — 
 The wheel E performs one revolution in an hour ; the wheel 
 NN, which is turned by the axis of the wheel E, must like- 
 wise make only one revolution in the same time ; and the minute 
 hand is fixed to the socket or tube of this wheel, which is fit-* 
 ted pretty tight upon the axis c of the wdieel E, and being thus 
 carried round only by friction, the hand may be moved round 
 without affecting the wheels between the brass plates. The 
 wheel N has 30 teeth, and acts upon the wheel O, which has 
 likewise 30 teeth, and the same diameter ; consequeatly the wheel 
 O takes an hour to a revolution ; now the wdieel O carries the 
 pinion p, which has six leaves, and which acts upon the wdieel gg 
 of 72 teeth; consequently the pinion p makes 12 revolutions 
 while the wheel gg makes one ; of course the wheel gg takes 
 twelve hours to one revolution, and upon the barrel of this wheel 
 the hour-hand is fixed. 
 
 Most of the wheels belonging to the striking part, as well as 
 most of those belonging to the going part, are included between 
 the brass plates SS, TT, seen edgeways in fig. 1 ; the reason 
 of their being omitted in that figure, is to avoid confusion ; 
 but a projection of the whole of the wheels betw'een the plates, as 
 they would be seen if the brass plate SS was removed, is shewn 
 by fig. 2. 
 
 Fig. 3. shews so much of the mechanism of the striking part as 
 is contained between the brass plate SS/and the dial-plate. The 
 numbers annexed to the different wheels denote the number of 
 teeth they contain. 
 
 The striking part now more particularly requires our at- 
 tention. In fig. 2, h is the great wheel of this part, wdth a 
 barrel and click the same as D ; it turns a pinion of eight, on 
 the same arbor with which pinion is the wheel f, turning a 
 pinion of eight on the arbor of the wheel k of 48 teeth. The 
 
380 
 
 MECHAKICS. 
 
 Description of a clock. 
 
 wheel k turns another pinion of eight, on the same arbor with 
 the wheel t of 48, and this last wheel turns a pinion of 6, 
 on the axis of which is a broad flat piece of metal, called 
 the fly, seen edgeways at s. This fly strikes the air with so 
 large a surface, that the resistance it experiences prevents the 
 train of wheels from going too fast. The wheel i has eight 
 pins projecting from it ; these pins raise the tail of the hammer 
 in succession, as the rotation of the wheel brings them to it; 
 the hammer is returned violently when the pins leave its tail, 
 by a spring z, pressing on the end of a pin through its arbor, 
 and strikes the bell x : m is a short spring which the end of a 
 pin through the arbor touches, just before the hammer strikes 
 the bell, and its use is to lift the hammer oft’ the bell the instant 
 it has struck, that it may not stop the sound. The eighth pin 
 in the w'heel i, which is called the pin-wheel, must pass by the 
 hammer-tail 78 times in striking the 12 hours; 78 being the 
 number of strokes the bell receives during that time. As the 
 Opinion of the wheel f, has eight leaves, each leaf of the pinion 
 answers to one of the pins ; and as the wheel h has 78 teeth, 
 it will turn once in 12 hours, like the great wheel D of the 
 watch part. In the pin-wheel f, eight teeth correspond to one 
 of the pins for the hammer, and as the pinion of the wheel k has 
 eight teeth, the w heel k will turn once for each stroke of the ham- 
 mer. Then, as the pinion of the wheel t turns 6 times for the 
 wheel k once, and the pinion of the fly turns 8 times for the wheel 
 t once, 6x 8=48, the number of turns made by the fly for one 
 stroke of the hammer. 
 
 In fig. 3, r is a small pinion of one tooth called the gathering 
 pallet ; it is fixed on the arbor of the wheel k, (fig. 2,) which arbor 
 comes through the brass plate SS (fig. 1,) for the purpose ; and 
 consequently, like the wheel k, the gathering pallet turns round 
 once for each stroke of the hammer ; s is a segment of a large 
 wheel, turned by the gathering pallet, and called the rack. The 
 arm a is attached to the rack, and the end of it rests against a 
 spiral plate, v, called, from its form, the snail. The snail is 
 fixed on the same tubular arbor as the hour-hand and wheel 
 72, and turns round with it once in 12 hours. Each of the 12 
 divisions or steps, as they are called, of the snail, answers to an 
 hour ; the circular arcs forming their circumference are struck 
 from the centre of the arbor w'ith a different radius, decreasing a 
 certain quantity each time, in the order of the hours ; and each 
 step is an arc equal to the twelfth part of the circumference of 
 which it is a part. 
 
 The circular part of the rack s is. cut into teeth, each of 
 w'hich is of such a length, that every step upon the snail 
 
MECHANICS ■ 
 
 581 
 
 Description of a clock. 
 
 answers to one of them. A spring w presses against the tail 
 of the rack, and its use is to throw the arm a of the rack 
 
 against the snail. The click g, is called the hawk^s bill ; it 
 
 takes into the teeth of the rack, and holds it up in opposition 
 to the spring zv. The three-armed detent, b^k, is called the 
 warning piece ; the arm k is bent at its end, and passes through 
 a hole in the plate SS of the frame, so as to catch a pin fixed 
 
 in one of the arms of the wheel t, fig. 2, and which pin describes 
 
 the dotted circle in fig. 3 ; the other arm b, stands so as to fall 
 
 in the w^ay of a pin in the wheel O, of 30 teeth. In the pre- 
 
 sent position of the figure, the wheels of the striking train are in 
 motion, and would continue turning until the gathering pallet r, 
 which, as before observed, turns once at each stroke of the 
 
 hammer, lifts the rack s, in opposition to the spring a’, one 
 
 tooth each turn, and the hawk’s bill g retains the rack, until a pin 
 in the end of the rack is brought in the way of the lever of the 
 gathering pallet r, and stops the wheels from turning any farther : 
 it is in this position, with the rack wound up, that we shall begin 
 to describe the operation of the striking of the clock. — The 
 wheel O, as just observed, turns once in an hour, and conse- 
 quently at the expiration of every hour, the pin in it takes the 
 end 6, and moves it towards the spring near it ; this depresses 
 the end /c, until it falls into the circle of the motion of the pin 
 in the wheel ty fig, 2, at the same time the short tail depresses 
 one end of the haw'k’s bill, and raises the other g, so as to clear 
 the teeth of the rack s; immediately the spring zi) throws the 
 rack back, until the end of its arm a rests against the snail. 
 When the rack falls back, the pin in it is moved dear of the 
 gathering pallet r, and the wdieels are set at liberty ; the main- 
 taining power or weight puts them in motion, but in a very 
 short time before the hammer has struck, the pin in the wheel t 
 falls against the end /c, and stops the whole : this operation hap- 
 pens a few minutes before the clock strikes, and the noise of the 
 wheels turning is called the warning. W^hen the hour is expired, 
 the wheel O has turned so far as to allow the end of the arm b 
 to slip over its pin, as in the figure ; the small spring pressing 
 against it raises the end k so as to be within the circle of the pin, 
 in the w heel ty fig. 2 : every obstacle is now removed ; the pin- 
 wheel fig. 2, raises the hammer j), and it strikes the bell ; the 
 gathering pallet r takes up the rack, a tooth at each turn; the 
 hawk’s bill g retaining it until the pin in the rack comes under the 
 gathering pallet r, and stops the motion of the train, till the pin in 
 the wheel O, at the next hour, takes the warning piece b k, and 
 the whole operation is repeated. 
 
 As the gathering pallet, turns once for each blow of the 
 
382 
 
 MECHANICS. 
 
 Description of a clock. 
 
 hammer, and it gathers up one tooth of the rack at each ^ 
 turn, it is evident that the number of teeth the rack can fall D 
 back, is the number of strokes the hammer will make. It is R 
 obvious also, from the form of the snail, a fresh step of whicli Q 
 is turned to the end of the arm a of the rack every hour, that 
 the rack must fall back differently each time at the end of that 
 period, and as each step of the snail answers to one tooth of the 
 rack, and each tooth of the rack to one stroke of the hammer, 
 the number of strokes is increased, one at a time, from one to 
 twelve. 
 
 As nothing affects the position of the snail but the motion R 
 of the wheel g, upon the axis of which it is affixed, and " 
 as the step upon which the arm <7, fig. 3, of -the rack rested, fl 
 while any given hour w'as struck, still remains to be the step , 
 upon which it rests till the next has arrived ; so, if in any part 
 of the interval between the striking of the given hour, and the 
 warning of the next, any contrivance be adopted to move the arm R 
 h of the detent, as much as it is moved in the regular period by J 
 the pin in the wheel O, the hour which w'as last struck will be | 
 struck over again, or, according to the usual expression, the hour i 
 will be repeated ; and yet all the subsequent hours will be struck 
 with the same precision, as if the mechanism had not been touched. 0 
 A slender cord, for instance, attached to the arm h, will be quite 
 sufficient to effect the purpose, and might be conveyed through 
 the side of the clock to the bedside, by which means a person may 
 easily ascertain, during the night, the last hour which the clock 
 has struck. 
 
 It will also probably be understood, that as the snail accom- 
 panies the wheel g, fig. 1, on the end of the hollow axis of 
 which the hour hand is fixed, and as g, through the medium 
 of the pinion p, and the wffieel O, derives its motion from the 
 wheel N, the hollow axis of wdiich, carrying at r the minute 
 hand, fits tightly, but is not immoveably fixed, upon the arbor 
 c ; so, if the time of the clock be rectified by pushing forward 
 the hands, the clock part will undergo corresponding changes, i 
 and strike each hour passed over, although the wheels of the I 
 watch part, between the brass plates, are not affected by the 
 operation. 
 
 Clocks intended to keep time with the greatest nicety, are 
 generally contrived so as to go while they are w'ound up. For 
 this purpose, a second larger ratchet wheel is added on the same 
 arbor with that which admits the clock to be wound up, but ^ 
 with teeth pointing the contrary way; a strong spring, usually 
 the greatest portion of a circle, connects this large ratchet-vvheel 
 with the great wheel of the clock, wffiich is on the same axis 
 
MECHANICS. 
 
 383 
 
 Division of circles into odd numbers. 
 
 with it ; one end of this spring being attached to the great wheel, 
 and the other end to the large ratchet ; and a catch proceeds 
 from the inner face of the back plate to the teeth of this rat- 
 chet, which prevents its moving back when the clock is winding 
 up, and serves as a support for the re-action of the maintaining 
 spring. When the clock is left to the operation of the weight, 
 the small ratchet turns round the large one, and contracts or coils 
 up the spring till it has strength sufficient to impel the great w heel 
 and train ; and when the action of the weight is suspended, as in 
 winding up, the spring, freed from the contracting power of the 
 w eight, expands itself, and forces round the great wheel ; its ac- 
 tion on the contrary direction on the great ratchet being prevented 
 by the catch before mentioned. 
 
 Clocks which have pendulums vibrating half seconds, fre- 
 quently have a spring instead of a weight for the maintaining 
 power. This spring consists of a long flat piece of steel, coiled 
 up in a spiral form ; it is inclosed in a cylindrical box, to which 
 its external extremity is attached, while its internal end is con- 
 nected with a fixed axis, round which the spring box revolves. 
 As the strength of the spring is greater the more it is coiled up 
 u by turning round the box, its action would be unequal in iin- 
 ^ pelling the work of the clock ; and to remedy this inconvenience 
 
 I the fusee wheel, of the same construction as in watches, is 
 adopted. The manner in which the fusee regulates the action 
 - of the spring, has been explained in page 310, and illustrated by 
 a figure. If, instead of the barrel C, fig. 1, a fusee wheel or j) al- 
 ley be supposed to be substituted, and the spiral spring, inclosed 
 in its cylindrical barrel, be added immediately below it, w ith a cat- 
 gut to connect the fusee and barrel of the spring, a good idea will 
 be obtained of a spring clock, for in other respects the work is 
 the same. 
 
 Mode of dividing the Circumference of Circles. 
 
 Very uncommon and odd numbers of teeth are sometimes 
 required for the wheels of astronomical clocks, orreries, &c. 
 such as the plate of no common engine used by clockmakers 
 for cutting the teeth of their wheels, in the common loutine 
 of their business, is calculated for; the following directions 
 are given, for the purpose of shewing how to divide a circle 
 into any given odd number of equal parts, so that the num- 
 ber may be laid down upon the dividing plate of a cutting 
 engine. 
 
 With respect to the division of a circle into any even num- 
 ber of equal parts, no difficulty can arise ; and if this be easy, 
 
384 
 
 MECHANICS. 
 
 Division of circles into odd numbers. 
 
 the division of any given portion of a circle into any number 
 of equal parts is not less so. A little consideration will shew 
 that this leads tis to a solution of our difliculty. There is no odd 
 number, but, if a certain number be subtracted from it, an even 
 number, of easy subdivision, will remain. Supposing the num- 
 ber of equal divisions wanted to be 69, subtract 9, and 60 will 
 remain ; then, as every circle is supposed to contain 360 degrees^ 
 say, as the desired number of equal parts in the circle^ which is 
 69, is to 360 degrees, so are 9 parts to the corresponding arc of 
 the circle that will contain them ; w hich arc, by the rule of three, 
 will be found to be 46.9*5. Therefore, by the line of chords on a 
 common scale, or rather wdth a sector, set off 46.95 (or 46.9) de- 
 grees with a pair of compasses, on the circumference of the circle, 
 and divide that arc, or portion of a circle into 9 equal parts, and 
 the rest of the circle into 60 ; so will the whole be divided into 69 
 equal parts. 
 
 It IS obviously not necessary to take off so many parts as 
 nine, in order to leave a convenient number for subdivision ; 
 but this advantage attends taking off a considerable number 
 of degrees from a scale, particularly when they are taken from 
 the scak of chords of a common rule, that they are in general 
 less liable to be taken off inaccurately than a few degrees. The 
 following example is proper when the scale inteiKled to be used 
 is a good one : suppose it is required to divide the circumference 
 of a circle into 83 equal parts, subtract 3, and 80 will remain ; 
 then, as 83 are to 360 degrees, so, by the rule of proportion, are 
 three parts to 13.01 degrees. The small fraction here brought 
 out may be neglected ; therefore, by the line of choids or sector, 
 as before, with a pair of finely pointed compasses, set off 13 de- 
 grees on the circumference of the circle ; divide the arc thus set 
 ofif into three parts, and the remainder of the circle into 80, and 
 the work will be done. 
 
 Again, suppose it is required to divide a given circle into 
 365 equal parts, subtract 3, and 360 will remain ; then, as 365 
 parts are to 360 degrees, so are 5 parts to 44)3 degrees ; there- 
 fore set off 4.93 (or 4.9) degrees by the scale ; divide that space 
 into 5 equal parts, and the rest of the circle into 360 ; the whole 
 will then be divided into 365, the desired number of equal 
 parts. 
 
 Any person accustomed to the use of a pair of compasses, 
 and to the scale or sector, may very easily, by a little practice, 
 take of degrees, and fractional parts of a degree, w ith great facility, 
 by the accuracy of his eye. 
 
 A few remarks, with respect to the sector, with which a 
 case of mathematical insUumeuts is always furnished, may be 
 
MECHANICS. 
 
 S85 
 
 . " Use of the sector in dividing circles. — Sizing of vheels and pinions 
 
 useful to some. The sector is made to fold in the middle, not 
 only that it may lie in a smaller compass, but to solve many pro- 
 blems by means of the references given to various tables and 
 scales that are engraved on both sides of each limb. When 
 opened to its full length, it commonly measures one foot, each 
 inch being numbered and divided into tenths. At the edge is 
 ' another scale, which divides the foot into ten equal parts, and each 
 tenth part of the foot is again subdivided into ten ; thus giving a 
 division of the 12 inches into 100 equal parts. But the first scale 
 which we have to notice as useful in dividing circles, is that next 
 ' to the inner edges, marked Pol. for polygon. By opening the 
 sector to such a width as may admit the radius of any circle to 
 measure exactly from the figure 6 on one limb, to the figure 0 on 
 ' the other, we at once ascertain the division of that circle’s cir- 
 cumference into any number of equal parts, from four to twelve; 
 
 ' because from the figure 4 to the opposite figure 4 will give a 
 chord subtendiug a quadrant of the ciix-le ; from 5 to 5 w ill give the 
 side of a regular pent;igon, or divide the cir&umferencje of the circle 
 into five parts ; from 6 to G into six parts ; and so on. Two equa. 
 lines of chords marked with tlie letter C, are inclined to each 
 other, and meet at the centre of the joint of the instrument. To 
 set off any number of degrees on the circumference of any circle, 
 the radius of which does not exceed the length of both these lines 
 together, open the sector till the distance between the 60th degree 
 on each scale of chords, is exactly equal to the radius of the circle 
 to be divided ; then from SO to 50 will give an extent of 50 de- 
 grees for the same circle ; from 30 to 30 an extent of 30 degrees ; 
 and so on for any other number. 
 
 Cy* proportioning the Diameters of Wheels and Pinions. 
 
 The due proportioning of wheels and pinions is an important 
 object in all kinds of wheel-w'ork, but especially in clock-mak- 
 ing, where, unless the respective sizes be properly adjusted, the 
 transmission of the maintaining power, and communication of 
 motion, will be unequable, and the meehanism liable to ra[)id 
 destruction. The subject has on these accounts engaged much of 
 the attention of waiters on clock-woi k ; but practical men are 
 not yet agreed in the observance of any invariable rule. The 
 usual mode of proportioning wheels and pinions, is, first to 
 make both a little too big for the proposed calliper, and then, 
 having rounded all the teeth of the pinion, and a few of the 
 teeth of the wheel, they gradually diminish the latter in the 
 lathe, until, by successive trials in the clock-frame, they are 
 found to act at a proper depth, when placed in the pivot holes 
 17.— VoL. L 3 P 
 
MECHANICS. 
 
 SV;6 
 
 Sizing of wheels and pinions. 
 
 previously made. This practice is extremely objectionable, as it J 
 loses much time, and leaves to the discretion of the workman the m 
 determination of the very matter in which he is most apt to err ; I 
 accordingly, he will at different times differ from himself, and ah " 
 most to a certainty from other workmen. We shall therefore ad- 4 
 vert to the directions of the best authorities on this subject, any of .J 
 which may be followed in preference to the common practice, 
 which has so intimate a connection with caprice. 1 
 
 If the teeth were intended to be rounded in a circular shape 
 which is, however, by no means to be recommended, the pitch - 
 line would be considered as at one-half of the breadth of the 
 tooth from the extreme edge ; but if they be rounded in an 
 epicycloidal, or, as the workmen call it, the bay-leaf form, 
 Hatton found, by numerous experiments, that the depth, or dis- 1 
 tance of the pitch-line from the circumference, will generally be i 
 three-quartei’s of the breadth of the tooth in any wheel or pinion ; .i 
 and as the epicycloidal shape is the best for the regular trans- j 
 mission of force and velocity, it is well entitled to be generally | 
 adopted in practice. If then we suppose the teeth, and the ' 
 spaces between them, to be reciprocally equal, which they 
 usually are in clock-work, we shall have the true acting diame- 
 ter of any wheel or pinion greater than the diameter to the 
 pitch-line, (which is sometimes called the geometrical, and i 
 sometimes the primitive diameter,) by ^ of a tooth or space on i 
 each side of the centre, or 1| in the whole diameter. A tooth 
 or space may be called a measure, and it is obvious that there 
 must in any wheel be twice as many measures as teeth. These 
 measures of the circumference may be reduced into measures 
 of the diameter by the usual ratio, of 3.1416; 1, and then, 
 if 1 1 be added to such geometrical measures of the diameter, 
 we shall have the proper acting diameter, which may be 
 expressed in inches and parts, when the number of measures in 
 the inch are known. For instance, let a wdieel and its pinion, 
 ^ 3 -, be taken at 12 teeth per inch at the pitch-line ; the num- 
 ber of measures of the wheel is twice 96, or 192, each 
 measuring -j^^th of an inch; then, as 3.1416: 1 :: 192:61.1; 
 therefore, if to the geometrical diameter expressed by 6 1.1 
 measures, there be added 1.5, the sum 62.6, or 62 will be 
 the acting diameter in the same denomination, which are so 
 many 24th pai ts of an inch ; but 62.6-i-24, gives 2.6 inches 
 for the full acting diameter of the wheel in question. With 
 respect to the pinion of 8, which has of course 16 similar 
 measures in its circumference, by the same proportion the 
 diameter will be 5.09 measures; to which if 1,5 be added, the 
 acting diameter will be 5.09+ 1.5 =»6.59, or with sufficient 
 
MECHANICS. 
 
 387 
 
 Sizing of wheels and pinions. 
 
 accuracy 6 - 1 ^, which divided by 24 as before, will give an 
 
 inch, or somewhat more than a quarter for the acting diameter of 
 the pinion. 
 
 The following table, which may be considered sufficiently ac 
 curate for practice, agrees very nearly with the experiments for 
 determining the proper sizes of wheels and pinions, by Berthoud, 
 an author of the first estimation on these subjects. It is calcu- 
 lated on the supposition that the teeth are epicycloidal, and 
 that the circumference is to the diameter as 3: 1, instead of 
 3.1416: 1. 
 
 Table of the Practical Sizes of Pinions. 
 
 t 
 
 Teeth in the Measures of the Wheel for a 
 
 Pinions. Diameter of the Pinion. 
 
 2 3.0 
 
 4 4.1 
 
 4.8 
 
 6 5.5 
 
 7 6.1 
 
 8 6.8 
 
 9 7.5 
 
 10 8.1 
 
 1 ] 8.8 
 
 12 9.5 
 
 13 10.1 
 
 14 10.8 
 
 15 ... 11.5 
 
 16 .... 12.1 
 
 To state the manner in which this table is constructed, will 
 [enable any person to continue it as far as he pleases. It is 
 simply this : multiply the number of the leaves in the pinion 
 I by 2, for the measures in the circumference, divide by 3 for the 
 diameter, and add thereto 1^ for the acting size. Thus, suppose 
 the diameter of a pinion of 9 leaves be required: 9x2=18, 
 and 18-r-3=6, and 6+ 1.5 = 7-5, or 74-, which last quantity, 
 taken by the callipers across the extreme edge of the wheel, (the 
 teeth of which are supposed to be cut, but not rounded) will be 
 34 ^ teeth and 4 spaces, or 4 teeth and 3i spaces. 
 
 Perhaps some mechanists, not acquainted with decimal arith- 
 j metic, may wish for still plainer directions. On this account, and 
 ^ to shew the agreement of Berthoud’s practical directions with the 
 [ rule just laid down, we shall recite the method of sizing pinions 
 , practised and recommended by him, viz. 
 
388 
 
 MECHANICS. 
 
 Sizing of wheels and pinions. 
 
 
 The full ov acting Diameter of the Pinion. T 
 
 Leaves* tMm 
 
 4 = two full teeth of the wheel, unrounded, and the space 
 between them. 
 
 5=three teeth rounded from point to point. 
 
 6= three full teeth, unrounded. 
 
 7 = three full teeth, and a quarter of a space beyond, 
 
 8 = four teeth, rounded from point to point. 
 
 9 = somewhat less than four full teeth. 
 
 10= four full teeth. 
 
 11, no measure given. 
 
 12=five full teeth. 
 
 13, no measure given. 
 
 14=six teeth, rounded from point to point. 
 
 15 = six full teeth. 
 
 It may be proper to observe, that the relative size of a well- 
 proportioned pinion must be somewhat less for a small wheel than 
 lor a large one, and also smaller when driven than when it is the 
 driver. Pennington, of Camberwell, the ingenious artist w ho con- 
 structed Mudge’s time-piece, adopts the practice of adding 2^ 
 measures of the geometrical diameter to the wheel, and to the 
 pinion, in w’atch-work, when the wheel is the driver; and to 
 each when the pinion is the driver. 
 
 When the, disLance is given between ~tlie centres of tw'o 
 wheels, unequal in the number of their teeth, but intended to 
 turn each other, their respective diameters may be determined 
 by the following rule : as the distance betw een the centres of the , 
 wheels is equal to the sum of both their geometrical radii, 
 (that is, their radii to the pitch-lines, or the radii which they 
 wduld have if they were merely two cylindrical rollers, one of;^^ 
 which turned the other,) therefore say, as the sum of the num-^JJ 
 her of teeth in both wheels is to the distance betw'cen their ^ 
 centres, taken in any kind of measure, as feet, inches, or parti 
 of an inch, so is the number of teeth in either of the wheels to 
 ' the radius or semi-diameter of that wheel, taken in the like 
 measure, from its centre to the pitch-line. Thus, suppose we 
 require tw o w heels, of such a size that the distance between their 
 centres shall be live inches, and that one of them is to have 75 
 teeth, and the, other 33; the sum of the teeth in them both is l08; 
 therefore, as 108 teeth are to live inches, so are 75 teeth to 3.47 
 inches; and as 108 are to 5, so is 33 to 1.52 inches; so that from 
 the centre of the wheel of 75 teeth to its pitch-line is 3.47 inches; 
 and from the centre of the wheel of 33 teeth to its pitch-line is 
 1.52 inches. 
 
MECHANICS. 
 
 389 
 
 Nature of the pendulum. 
 
 W ith respect to the best forms for the teeth of clock-work, 
 the subject has been discussed at length in treating of Mill-work, 
 and a reference to that article will render any further notice of 
 it here unnecessary, as the same principles are applicable to both 
 large and small machinery. W e may also observe, that the 
 sizing of wheels, which was hardly noticed under Mill-work, will 
 be completed by what has now been said with regard to that par- 
 ticular. 
 
 Of the Pendulum, 
 
 A pendulum is any heavy body so suspended that it may swung 
 backwards and forwards, about some fixed point, by the force of 
 gravity. A body thus suspended necessarily describes an arc, in 
 one half of w liich it descends, and ascends in the other. 
 
 Each swing which a pendulum makes is called a vibration, or 
 oscillation. 
 
 PC, fig. 4, pi. VI, is a pendulum consisting of a body P, 
 attached by a cord PC, which is fastened to and moveable about 
 the point C. If the body P was not retained by the cord, it 
 would descend in the vertical line PL, but as the cord prevents its 
 falling in this manner, it describes the arc PA, which is the seg- 
 ment of a circle of which PC is the radius. The velocity ac- 
 quired by the body P in falling through the arc PA, has a ten- 
 dency, when it arrives at the point A, to carry it off in the tangent 
 AD, but the cord continually drawing it towards the centre, it 
 rises and describes the arc AE. Having arrived at E, it will fall 
 back again, and the velocity acquired in thus failing back will 
 cany it tow'ards P ; and this backward and forward motion will 
 be continued till it is overcome by the joint effects of the resist- 
 ance of the air, the friction at the point of suspension, and the 
 force of gravitation, by w hich the body P is attracted to the centre 
 of the earth, in a direction perpendicular to the liorizon. These 
 causes of obstruction lessen the range of the pendulum at each 
 vibration, and therefore inevitably cause it to stop in a longer or 
 shorter time according to their intensity. 
 
 The nature of a pendulum consists in the following particu- 
 lars : 1. The times of the vibrations of a pendulum in very small 
 arches, are all equal. 2. The velocity of the ball or bob, in the 
 lowest point, will be nearly as the length of the chord of the arch 
 which It describes in th^ descent. 3. The times of vibration in 
 different pendulums, at the same part of the earth, are as the 
 square roots of the times of their vibrations. 4. The time of one 
 vibration is to the time of the descent, through half the length of 
 the pendulum, as the circumference of a circle to its diameter. 
 
3.90 
 
 MECHANICS. 
 
 ~r 
 
 Cycloidal vibrations. — Long and short vibrations. 
 
 o. Whence the length of a pendulum vibrating seconds, is found 
 to be 39.2 inches nearly; that of a half-second pendulum 9 .B 
 inches ; and one for quarter-seconds 2.45 inches. 6. A uniform 
 homogeneous body, BG, hg. 5, as a rod, staff, &c. which is one- 
 third longer than a pendulum, CP, fig. 4, will vibrate in the same 
 time with it. From these properties of the pendulum may be de- 
 duced its utility as a regulator of time, for which purpose it far 
 exceeds every contrivance yet discovered. 
 
 When pendulums were first applied to clocks, they were made 
 very short ; and the arches of the circle being large, the time of 
 vibration through different arches w'as therefore unequal. To re- 
 medy this defect, the pendulum w’as contrived to vibrate in a cy- 
 cloid, by suspending it between two cycloidal cheeks. A thread, 
 or some very pliable material, formed the upper part of the pen- 
 dulum, and folded alternately upon these cheeks. The property 
 of the cycloidal curve is, that a body vibrating in it will describe 
 all its arches, whether great or small, in equal times ; the theory is 
 therefore good, but the practical application of it to the motion 
 of the pendulum, is so imperfect, that cycloidal cheeks are entirely 
 disused. In clocks for astronomical purposes, the arc of vibration 
 must be accurately ascertained, and if it be different from that 
 described by the pendulum when the clock keeps lime, a correc- 
 tion must be applied to the time shewn by the clock. This cor- 
 rection, expressed in seconds of time, will be equal to the half of 
 three times the difference of the square of the given arc, and of ; 
 that of the arc described by the pendulum when the clock keeps 
 time, these arcs being expressed in degrees ; and so much will the 
 clock gain or lose, according as the first of these arches is less or ^ 
 greater than the second. Thus, if a clock keeps true time when 
 the pendulum vibrates in an arch of 3 degrees, it will lose 104 ^ se- 
 conds daily in an arch of 4 degrees, and 24 seconds in an arch of 
 3 degrees. 
 
 In all that has hitherto been said, the powder of gravity has 
 been supposed constantly the same. But this powder is not the 
 same in different latitudes ; and the length assigned above to the 
 second, half-second, and quarter-second pendulum, is accurately 
 adapted only to the latitude of London. A pendulum, to vibrate 
 seconds at the equator, must be somewhat shorter ; for the semi- 
 diameter of the earth’s equator is about seventeen miles longer 
 than the polar semi-diameter, consequently the force of gravity is 
 on this account less at the equator than at of tow'ards the poles ; 
 and it is further greatly diminished by the centrifugal force, arising 
 from the diurnal motion of the earth, being greatest at the equator. 
 The following Table shews the amount of the variations thui 
 arising : 
 
MECHANICS 
 
 391 
 
 Centre of oscillation. 
 
 r-- • 
 
 Length of PeJidulums to vibrate Seconds at every Fifth Degree 
 !■ of Latitude. 
 
 Degrees of La- 
 titude. 
 
 Length of Pen- 
 dulum. 
 
 ' Degrees of La- 
 1 titude. 
 
 Length of Pen- 
 dulum. 
 
 Degrees of La- 
 titude. 
 
 1 
 
 Length of Pen- 
 dulum. 
 
 
 Inches. 
 
 
 Inches. 
 
 
 Inches. 
 
 0 
 
 39.027 
 
 35 
 
 39.084 
 
 65 
 
 39.168 
 
 5 
 
 39.029 
 
 40 
 
 39.097 
 
 70 
 
 39.177 
 
 10 
 
 39.032 
 
 45 
 
 39.111 
 
 75 
 
 39.185 
 
 15 
 
 39.036 
 
 50 
 
 39.126 
 
 80 
 
 *39.191 
 
 20 
 
 39.044 
 
 55 
 
 39.142 1 
 
 85 
 
 39.195 
 
 25 
 
 30 
 
 39.057 
 
 39.070 
 
 60 
 
 39.158 
 
 90 
 
 1 
 
 39.197 
 
 * To find the length of a pendulum to. make any number of 
 ■vibrations, and vice versa : Call the pendulum making 60 vibra- 
 tions in a minute, the standard length ; then say, as the square of 
 the given number of vibrations is to the square of 60, so is the 
 length of the standard to the length sought. If the length of the 
 spend ulum be given, and the number of vibrations it makes in a 
 minute be required ; say, as the given length is to the standard 
 length, so is the square of 60, its vibrations in a minute, to the 
 square of the number required; the square root of which will be 
 [the number of vibrations made in a minute. 
 
 In considering a simple pendulum, or a ball suspended by a 
 string having no sensible weight, the whole weight of the ball 
 is supposed to be collected in its centre of gravity, and the length 
 of the {jcndulum is measured from the centre of gravity to the 
 point of suspension. But when a pendulum consists of a ball, 
 or any other solid, suspended by a metallic or wooden rod, the 
 lengtl) of the pendulum is the distance from the point of suspen- 
 sion to a point in the pendulum called the centre of oscillation, 
 which does not exactly coincide with the centre of gravity of the 
 ball. If a rod of iron, or any other substance w^ere suspended, 
 and made to vibrate, that point in which all its force vvas collect- 
 ed, and to which, if an obstacle were applied, all its motion 
 
392 
 
 MECHANICS. 
 
 Mode of obviating the effects of change of temperature. 
 
 would cease, and be received by the obstacle, is called the centre 
 of oscillation. We have shewn that a homogeneous rod or bar, 
 will perform its oscillations in the same time as a pendulum con- 
 sisting of a ball and a thread, though only two-thirds of its length. 
 Hence a point taken one-third of the length of such a bar from 
 the lower end, is its centi e of oscillation. 
 
 The greatest inconvenience attending the use of the pendu- 
 lum, is its being constantly liable to an alteration of its length, 
 from the effects of heat which expands, and of cold which 
 contracts every material of which it can be made. To remedy 
 this inconvenience, the common method is, to continue the rod 
 through and a little below the bob, wdiich will slide upwards or 
 downwards upon it. The lower part of the rod is screwed, and 
 furnished w ith a nut, upon which the bob rests ; consequently, 
 as the nut is screwed upwards or downwards, the bob, and with it 
 the centre of oscillation, is raised or lowered. This contrivance 
 is a very unscientific one, and of little use ; accordingly, the title 
 cf a compensation for temperature, is given to those inventions 
 alone in which the efficient principle of regulation is contained 
 within themselves, and constantly acting. Of this character, we 
 shall notice two or three ; to enlarge further, would exceed our 
 limits. 
 
 In the year 1721, Graham produced a compensation pendu- 
 lum, and applied it to a clock, the going of which he compared 
 with one of the best of the common sort, for three years together, 
 and found its errors to be but one-eighth part of those of the 
 latter. The following considerations will explain the principle 
 of this pendulum. If a glass or metallic tube, uniform 
 throughout, filled with quicksilver, and o8.8 inches long, were 
 applied to a clock, it would vibrate seconds, for 39-2 =|- of 
 o8.8, and such a pendulum admits of a twofold expansion and 
 contraction, viz. one of the tube and the other of the mercury, 
 and these will be at the same time contrary, and therefore will 
 correct each other. The tube will extend in length with heat, 
 and so the pendulum will vibrate slower on that account. The 
 mercury also will expand with heat, and since by this expansion 
 it must extend the length of the column upward, and consequently 
 raise the centre of oscillation, so will its distance from the point 
 of suspension be shortened, and therefore the pendulum on this 
 account W'ill vibrate quicker : hence, if the tube and the mercury 
 be skilfully adjusted, the time of the clock will, by this means, for 
 a long course of time, continue the same, without any sensible 
 gain or loss. 
 
 The gridiron pendulum, which has justly obtained a high 
 degree of celebrity, is a contrivance for the same purpose. 
 
MECHANTCS.’' 
 
 393 
 
 Gridiron pendnlura 
 
 Instead of one rod, this pendulum is composed of any convenient 
 odd number of rods, as live, seven, or nine, being so connected, 
 that the effect of one set of them counteracts that of the other 
 set; and therefore, if they are properly adjusted to eacli other, the 
 centres of suspension and oscillation will always be equidistant. 
 Fig. 6, pi. VI, represents a gridiron pendulum composed of nine 
 rods, steel and brass alternately. "J"he two outer rods, AB, CD, 
 which are of steel, are fastened to the cross pieces, AC, 13 D, by 
 means of pins. The next two rods, EF, GH, are of brass, and 
 are fastened to the lower bar BD, and to the second upper bar 
 EG. The two following rods are of steel, and are fastened to 
 the cross bars EG and IK. The two rods adjacent to the cen- 
 tral rod being of brass, are fastened to the cross pieces IK and 
 LM ; and the central rod, to which the ball of the pendulum is 
 attached, is suspended from the cross piece LM, and passes 
 Feely through a perforation in each of the cross bars IK, BD. 
 From this disposition of the rods, it is evident that, by the expan- 
 sion of the extreme rods, the cross piece, BD, and the rods at- 
 I tached to it, will descend ; but since these rods are expanded by 
 the same heat, the cross piece, EG, will consequently be raised, 
 and therefore also the two next rods; but because these rods are 
 also expanded, the cross piece IK will descend; and by the ex- 
 pansion of the two next rods, the piece LM will be raised a 
 quantity sufficient to counteract the expansion of the central rod : 
 whence it is obvious, that the effect of the steel rods is to increase 
 ffie length of tlie pendulum in hot weather, and to diminish it in 
 I cold weather, and that the brass rods have at the same time a con- 
 trary effect. I'he effect of the brass rods must, however, be 
 equivalent not only to that of the steel rods, but also to the part 
 above the frame and spring which connects it with the cock, and 
 to that part between the lower part of the frame and the centre 
 of the ball. 
 
 Another useful method of constructing the pendulum is the 
 following : a bar of the same metal with the rod of the pendulum, 
 and of the same dimens’ons, is placed against the back part of the 
 olock-case ; from the top of this a part projects, to which the 
 upper part of the pendulum is connected by two fine pliable 
 chains or silken strings, which just below pass betw een two plates 
 of brass, whose lower edges will always terminate the length of 
 the pendulum at the upper end. These plates are supported on 
 a pedestal fixed to the back of the case. The bar rests upon an 
 immoveable base at the lower part of the case, and is inserted into 
 a groove, by which means it is always retained in the same posi- 
 tion. By this construction, it is intended that the extension op 
 contraction of the bar, and of the rod of the pendulum, shall be 
 17.~Vol.I. 3E 
 
394 
 
 MECHANICS. 
 
 Compensation of pendulums. 
 
 equal and in contrary directions. Thus, suppose the rod of the 
 pendulum to be expanded by any given quantity by heat, and its 
 centre of oscillation consequently tending to a lovrer point, the 
 bar being at the same time expanded upwards, will, it is pre- 
 sumed, raise the upper end of the pendulum just as much as its 
 length is increased ; and hence its length below the plates will be 
 the same as before. Of a pendulum of this sort, somewhat im- 
 proved by Crosthwaite, a clock and watch-maker of Dublin, we 
 shall insert a further description. AB, fig. 7, pi. VI, are two 
 rods of steel, forged out of the same bar, at the same time, of the 
 same temper, and in every respect similar. On the top of B is 
 formed a gibbet C ; this rod is firmly supported by a steel bracket 
 D, fixed on a large piece of marble F, firmly set into a wall, and 
 it has liberty to move freely upwards between cross staples of 
 brass, 1, 2, 3, 4, which touch only in a point in front and rear, 
 (the staples having been carefully formed for that purpose ;) to 
 -the other rod is firmly fixed by its centre, the lens G, of twenty- 
 four pounds weight, although it should in strictness be a little be- 
 low it. This pendulum is suspended by a short steel spring on 
 the gibbet at C ; all which is entirely independent of the clock. 
 To the back of the clock-plate, I, are firmly screwed two cheeks 
 nearly cycloidal at K, exactly in a line with the centre of the 
 •verge L. The maintaining power is applied by a cylindrical 
 steel-stud, in the usual way of regulators, at M. Here the ex- 
 pansion or contraction that takes place in either of these exactly 
 similar rods, is counteracted by the other in the manner above 
 mentioned ; and this, which has been usually called a compensa- 
 tion pendulum, has been supposed to have an advantage over 
 other compensation pendulums ; because the latter being com- 
 posed of different materials, how'ever just the calculation may 
 seem to be, they will be defective,, as not only different metals, 
 but also different bars of the same metal, that are not manufac- 
 tured at the same time, and exactly in the same manner, are found 
 by a good pyrometer to differ materially in their degrees of ex 
 pansion and contraction, a very small change affecting one and 
 not the other. N o kind of steel will be so likely to answer well for 
 the tw o bars in question, as the best cast steel, no other kind pos- 
 sessing nearly so much uniformity of texture. When it is ob- 
 served that the bars should be of the same temper, it is not to be 
 understood, according to the usual import of the term temper 
 among artists, that they should undergo the regular process of 
 hardening, and then be reduced to a blue or- any other colour at 
 discretion ; on the contrary, it seems reasonable to suppose, that 
 they can be submitted to no process of preparation so suitable as 
 that of annealing, which will leave them in the softest, but, at the 
 
ME CH Allies. 
 
 TLJT 
 
 aCo^erdU. 
 
MECHANICS. 
 
 395 
 
 Rittenliouse’s Penduluni. 
 
 same time, the most uniform state the steel is capable of receiv- 
 ing. They should be annealed along side of each other, and after 
 undergoing this operation, they should not be hammered. It is, 
 however, to be observed, of the pendulum constructed with these 
 two similar bars, that the bracket D is supposed to be absolutely 
 fixed, whereas it is itself moveable by the expansion and contraction 
 of the block in w hich it is placed. The pendulum cannot therefore 
 strictly be called a compensation pendulum ; but its performance 
 is found to be much superior to that of the common construction. 
 
 The most approved mode of diminishing, in a simple pendu- 
 lum, the errors arising from change of temperature, is to make the 
 rod of straight-grained, well seasoned fir wood, of which the ex- 
 pansion or contraction lengthwise is very small. This wood is 
 best used in its natural state, without baking, varnishing, painting, 
 gilding, or any other preparation, excepting, perhaps, the mere 
 rubbing of it with a waxed cloth. A pendulum also performs 
 better when the ball is heavy, and the arc of vibration .small. 
 
 In the construction of clocks intended to measure time with the 
 utmost possible exactness, a compensation should also be esta- 
 blished for the resistance of the air, which, by its unequal density, 
 varying the weight of the pendulum, must in a small degree acce- 
 lerate or retard its motion. The celebrated David Rittenhouse, 
 who paid particular attention to this subject, estimates the extreme 
 difference of velocity, arising from this cause, at half a second per 
 day ; and he observes, that a remedy dependent-on tlie barometer 
 will not be strictly accurate, as the weight of the entire column of 
 air does not precisely correspond with the density of its base. He 
 proposes, therefore, as a very simple and easy remedy, that the 
 pendulum shall, as usual, consist of an inflexible rod carrying the 
 ball beneath, and continued above the centre of suspension to an 
 equal, (or an unequal) distance upwards. At this extremity is to 
 be fixed another ball of the same dimensions (or greater or less, 
 according as the continuation is shorter or longer,) but made as light 
 as possible. The buoyancy of this upper ball will accelerate its 
 oscillations by the same quantity as those of the lower would be re- 
 tarded ; and thus, by a proper adjustment, the two effects might be 
 made to balance and correct each other. The inventor made a 
 compound pendulum on these principles, of about one foot in its 
 whole length. This pendulum, on many trials, made in the air 57 
 vibrations in a minute. On immersing the whole in water, it made 
 59 vibrations in the same time ; shewing evidently, that, contrary 
 to what takes place with the common pendulum, its returns were 
 quicker in so dense a medium as water than in air. When the 
 lower bob or pendulum only w'as plunged in water, it made na 
 more than 44 vibrations in a minute. 
 
396 
 
 ABSTRACT OF MECHANICS. 
 
 Matter. — Space. 
 
 ABSTRACT OF MECHANICS; 
 
 OR, 
 
 A Review of the most important Definitions and Principles of 
 this Division. 
 
 The ingenious student will readily perceive the advantage he 
 may derive from being furnished with an epitome like the 
 following of the different branches of Natural Philosophy 
 successively treated of. After he has perused the respective 
 dissertations at length, the condensation of the subjects of 
 them thus presented to him, is calculated to save him the 
 trouble of repealed perusal, as a view of the leading prin- 
 ciples will often be all that he wants to remind him of the 
 reasoning and facts by which they have been supported and 
 explained. 
 
 Of Matter. 
 
 1. Every' portion of matter is possessed of the following pro- 
 perties, viz. solidity, extension, divisibility, mobility, inertia, at- 
 traction, and repulsion. 
 
 2. Soliditp is that property by which two bodies cannot oc- 
 cupy the same place at the same time. It is sometimes called the 
 impenetrability of matter. 
 
 3. I'he extensiunj like the solidity of matter, is proved by the 
 impossibility of two bodies co-existing in the same place. 
 
 4. Divisibility is that propei ty by which bodies are capable of 
 being divided into parts removeable from each other. 
 
 5 Mobility expresses the capacity of matter to be moved from 
 one position or part of space to another. 
 
 6. Inertia is the term which designates the passiveness of 
 matter, which, if at rest, will for ever remain in that state until 
 compelled by some cause to move ; and on the contrary, if in 
 motion, that motion wiil not cease, or abate, or change its direc- 
 tion, unless the body be resisted. 
 
 Space. 
 
 1. Space is either absolute or relative: 
 
 2. Absolute space is merely extension, illimitable, immoveable. 
 
ABSTRACT OF MECHANICS. 
 
 397 
 
 [f — 
 
 I Attraction. — Repulsion. — Motion. 
 
 and without parts ; yet lor the convenience of language, it is usu- 
 lally spoken of as if it had parts. Hence the expression, 
 
 I 3. Relative space, w hich signities that part of absolute space 
 I which is occupied by any body, as compared with any part occu- 
 ipied by another body. 
 
 AttractiofU 
 
 1. Attraction denotes the property which bodies have to ap- 
 *"roach each other. 
 
 2. There are five kinds of attraction — the attraction of cohe- 
 lion, of gravitation, of electricity, of magnetism, and chemical at- 
 r action. 
 
 3. '^rhe attraction of cohesion is exerted only at very small dis- 
 tances. 
 
 4. i'he strength of the attraction of cohesion being different 
 in difteient kinds of matter, is supposed to be the cause of the re- 
 lative degiees of hardness of different bodies. 
 
 ' 5. Capillary attraction is only a particular modification or 
 
 branch of the attraction of cohesion. 
 
 6. I he attraction of gravitation is exerted by every particle of 
 matter on ever» other panic ie at all distances, but by no means 
 with equal intensit\ at all distances. 
 
 7. Gravitation de< reases from the surface of the earth up- 
 wards as the square of the distance incjeases ; but from the sur- 
 face of the earth downwards, it decreases only in a direct ratio to 
 the distance from the centre. 
 
 Repulsion. 
 
 1. Repulsion is that propeity m bodies, whereby, if they are 
 placed just beyond the sphere of each other^s attraction of cohe- 
 sion, they mutually fly tiom each other. 
 
 2. Oil refuses to mix with w ater, from the repulsion between 
 the particles of the two substances ; and from the same cause, a 
 needle gently laid upon water will sw im. 
 
 Motion. 
 
 1. Absolute motion is the actual motion that bodies have, con- 
 sidered indeptndeiitly of each other, and only with regard to the 
 parts of space. 
 
 2. Relative motion is the degree and direction of the motion 
 of one body, when compared with that of another. 
 
 3. Accelerated motion is when the velocity continually increases. 
 
398 
 
 ABSTRACT OF MECHANICS. 
 
 Motion. — Central forces. 
 
 4. Retarded motion is when the velocity continually decreases; 
 and the motion is said to be urnforndy retarded^ when it decrease? 
 equally in equal times. 
 
 0. The velocity of uniform motion is estimated by the time 
 employed in moving over a certain space ; or, which amounts tQ 
 the same thing, by the space moved over in a certain time. 
 
 6. To ascertain the velocity, divide the space run over by the 
 time. 
 
 7. To ascertain the space run aver, multiply the velocity by 
 the time. 
 
 8. In accelerated motion, the space run ove? is as the square 
 of the time, instead of beuig directly as the time, a4 in uniform 
 motion. 
 
 9. A body acted upon by only one force, will always move in 
 a straight line. 
 
 10. Bodies acted upon by two single impulses, whether equal 
 or unequal, will also describe a right line. 
 
 11. But when a body is acted upon by one uniform force, or 
 single impulse, and another accelerating or retarding force, the two 
 forces will cause it to describe a curve, 
 
 12. The curve described by a body projected from the 
 earth, and drawn down by the action of gravity, would, in an un- 
 resisting medium, be that of a parabola; but frorn the resist- 
 ance of the air, which, when the velocity is very great, will often 
 amount to one hundred times the weight of the projectile, the 
 curve really described approaches more nearly to that of an hy- 
 perbola. 
 
 13. The momentum of a body is the force with which it moves, 
 and is in proportion to the weight, or quantity of matter, multi- 
 plied into its velocity. 
 
 14. The actions of bodies on each other are always equal, and 
 exerted in opposite diiections ; so that any body acting upon ano- 
 ther, loses as much force as it communicates. 
 
 Central Forces, 
 
 1. The central forces are the centrifugal and the centripetal 
 
 forces. . • 
 
 2. The centrifugal force is the tendency which bodies that re- 
 volve round a centre, have to fly from it in a tangent to the curve 
 they move in, as a stone from a sling. 
 
 3. The centripetal force is that which prevents a body from 
 flying off, by impelling it towards the centre, as the attraction of 
 gravitation. 
 
ABSTRACT OF MECHANICS. 
 
 399 
 
 Centre of gravity. — The lever. 
 
 Centre of Gravity, 
 
 1. The centre of gravity is that point in a body, about which 
 all its parts exactly balance each other in every position. 
 
 2. A vertical line passing through the centre of gravity of a 
 body, is called the line of direction. 
 
 3. When the line of direction falls within the base of a body, 
 that body cannot descend ; but if it fall without the base, the body 
 will fall. 
 
 The Lever, 
 
 1. There are three kinds of levers, the difference between 
 which is constituted by the difference in the situation of the ful- 
 crum and the power with respect to each other. In the frst kind 
 of lever, the fulcrum is placed between the power and the weight. 
 In the second kind of lever, the fulcrum is at one end, the power 
 at the other, and the weight between them. In the third kind 
 of lever, the power is applied between the fulcrum and the 
 weight. 
 
 2. In all these levers, the power is to the weight, as the dis- 
 tance of the weight from the fulcrum is to that of the power from 
 the fulcrum. 
 
 3. A bent, or hammer lever, differs only in form from a lever 
 of the first kind. 
 
 4. Scissors, pincers, snuffers, and the common iron-crom, are 
 all levers of the first kind. 
 
 5. The stater a or Roman steel-yard is a lever of the first kind, 
 with a moveable weight. 
 
 6. A balance is also a lever of the first kind with equal arms ; 
 a perfect balance should combine the following requisites : 1. 
 The arms of the beam should be exactly equal, both as to weight 
 and length, and should at the same time be as long as possible, 
 relatively to their thickness. 2. The points from which the scales 
 are suspended, should be in a right line, passing through the centre 
 of gravity of the beam. 3. The fulcrum ought to be a little 
 higher than the centre of gravity. 4. The axis of motion should 
 be formed with an edge like a knife, and with the rings and other 
 bearing parts, should be very hard and smooth. 5. The pivots, 
 which form the axis of motion, should be in a straight line, and at 
 right angles to the beam. 
 
 7. The best balances are not calculated to determine weights 
 with certainty to more than five places of figures. 
 
 8. The oars and rudders of vessels are levers of the second 
 order ; a pair of bellows, nut-crackers, &c. are composed of two 
 levers of the same kind. 
 
400 
 
 'ABSTRACT OF MECHANICS. 
 
 The pulley. — Wheel and axle. 
 
 9 . The third kind of lever is used as little as possible, 
 on account of the disadvantage to the moving power, the 
 intensity of which must always exceed the resistance ; yet in 
 some cases this disadvantage is overbalanced by the quickness of 
 its operations, and the small compass in which it is exerted ; hence 
 its fitness for the bones of the arm, and the limbs of animals ge- 
 nerally. 
 
 10. In compound levers, the power is to the weight, in a ratio 
 compounded of the several ratios which those powers that can 
 sustain the weight by the help of each lever, when used singly and 
 apart from the rest, have to the weight. 
 
 The Pulley^ 
 
 1. Pulleys are of two Vmds, fixed and moveable. 
 
 2. The fixed pulley only turns upon its axis, and aflPords no 
 mechanical advantage ; therefore when the power and the weight 
 are equal, they balance each other. It is used for the convenience 
 of changing the direction of a motion. 
 
 3. The moveable pulley not only turns upon its axis, but rises 
 arrd falls with the weight. 
 
 4. Every moveable pulley may be considered as hanging by 
 two ropes equally stretched, and which consequently bear equal 
 portions of the weight ; therefore each pulley of this sort doubles 
 the power. 
 
 5. A pulley of one spiral groove upon a truncated cone, as 
 the fusee of a watch, is calculated to maintain a constant equili- 
 brium or relation between two powers, the relative forces of which 
 are continually changing. 
 
 - Wheel and Axle. 
 
 1. The power must be to the weight, in order to produce an 
 equilibrium, as the circumference of the wheel is to the circum- 
 ference of the axle. 
 
 2. As the diameters of different circles bear the same propor- 
 tion to each other that their respective circumferences do, the 
 power is also to the w^eight as the diameter of the wheel to the 
 diameter of the axle. 
 
 3. If one wheel move another of equal circumference, no 
 power will be gained, as they will both move equally fast. 
 
 4. But if one wheel move another of different diameter, 
 whether larger or smaller, the velocities with which they move 
 will be inversely as their diameters, circumferences, or number of 
 teeth. 
 
 5. The wheel and axle may be considered as a perpetual 
 lever, from the constant renewal of the points of saspensioa 
 
ABSTRACT OF MECHANICS. 
 
 401 
 
 The inclined plane. — The wedge. — The screw. 
 
 and resistance. The fulcrum is the centre of the axis, the longer 
 arm is the radius of the wheel, and the shorter arm the radius of 
 the axis. 
 
 6. The crane, and many other machines of the first conse- 
 quence, are composed principally of the wheel and axle. 
 
 The Inclined Plane. 
 
 1. Tlie power and the weight balance each other, when the 
 former is to the latter as the height of the plane to its length. 
 
 2. In estimating the draught of a w aggon or other vehicle up- 
 hill, the draught on the level must be added ; so that if the hill 
 rises one foot in four, one-fourth part of the weight must be added 
 to the draught on level ground. 
 
 The Wedge. 
 
 1. When the resistance acts perpendicularly -to the sides, that 
 is, when the wedge does not cleave at any distance, tliere is an 
 equilibrium between the resistance and the power, when the latter 
 IS to the former as half the thickness of the back of the wedge is 
 to the length of one of its sides. 
 
 2. When the resistance on each side acts parallel to the back, 
 that is, w'hen the wedge cleaves at some distance, the power is to 
 the resistance as the whole length of the back to double its per- 
 pendicular height. 
 
 -3. The thinner the wedge, the greater its power. 
 
 4. The further a wedge is driven into any material, the greater 
 also is its power, the sides of the cleft affording it the advantage 
 of operating on two levers. 
 
 0. Axes, spades, chisels, needles, knives, and all instruments 
 which begin with edges or points, and grow gradually thicker, acf 
 on the principle of the wedge. 
 
 * The Screw. 
 
 1. Tlie screw is an inclined plane encompassing a cylinder. 
 
 2. It is generally used with a lever ; and the pow er is to the 
 w-eight, as the distance from one thread or spiral to another is to 
 tlie circumference of the circle described by the power. 
 
 2u The friction of the screw’ is very great, a circumstance that 
 occasions this machine to sustain a weight or press upon a body, 
 after the power by which it was impelled is lemoved. 
 
 4. A screw cut on an axle to serve as a pinion, is called an 
 endless screw. 
 
 6. The endless saew' Is very useful, either in converting a very 
 rapid motion into a slow one, or vice vetsUf as for each of its revo- 
 lutions the wheel moves but one tooth. 
 
 17 . — Vol-L -3 F 
 
402 
 
 ABSTRACT OF MECHANICS. 
 
 Compound maclmies. — Flv wheels. — Friction. 
 
 Compound Machines. 
 
 1. In all machines, simple as well as compound, what is 
 gained in power is lost in time ; but the loss of time is compen- 
 sated b)' convenience. 
 
 2. The mechanical power of an engine may be known by 
 measuring the space described in the same time by the power and 
 the resistance or weight ; or by multiplying into each other the se- 
 veral proportions subsisting between the power and the weight, in 
 every simple mechanical power of which it is composed. 
 
 3. The power of a machine is not altered by varying the sizes 
 of the wheels, provided this proportion produced by the multipli- 
 cation of the power of the several parts remains the same. 
 
 4. In constructing machines, simplicity of parts and unifor- 
 mity of motion should be particularly studied. 
 
 5. The teeth of wheels should ahvays be made as numerous 
 as possible ; and when great strength is required, it should be ob- 
 tained by increasing the width or thickness of the wheel. 
 
 6. The use of the crank is one of 'the best modes of convert- 
 ing a reciprocating into a rotatory motion, and vice versa. 
 
 Fl^ Wheels. 
 
 1. A fly wheel is a reservoir of power, and is employed to 
 equalize the motion of a machine. 
 
 2. This equalization of the motion is the only source of the 
 advantage of a fly, which can impart no power it has not re- 
 ceived. 
 
 3. When a fly is used merely as a regulator, it should be near 
 the first mover; if intended to accumulate force in the working 
 point, it should not be far separated from that point. 
 
 Friction. 
 
 1. Friction is occasioned by the roughness and cohesion of 
 bodies. 
 
 2. It is in general equal to between one-half and one-fourth of 
 the weight or force with which bodies are pressed together. 
 
 3. It is increased in a small degree by an increase of the sur- 
 faces in contact. 
 
 4. ft is increased to an extraordinary degree, by prolonging the 
 time of contact. 
 
 5. Two metals of the same kind have more friction than two 
 different metals. 
 
 6. Steel and brass are the two metals which have the least 
 friction upon each other. 
 
ABSTRACT OF MECHANICS. 
 
 403 
 
 First movers. — Mill-work. 
 
 7 . The general rule for lessening friction consists in substi- 
 tuting the rolling for the sliding motion. 
 
 Meu and Horses , considered as first Movers. 
 
 1. In turning a ^vinch, a man exerts his strength in different 
 proportions at different parts of the circle. The greatest force is 
 when he pulls the handle up from the height of his knee ; and the 
 least when he thrusts from him horizontally. 
 
 2. When two handles are used to an axle, one at each extre- 
 mity, they should be fixed at right angles to each other. 
 
 3. The art of carrying large burdens, consists in keeping the 
 column of he body as directly under the w'eight and as upright 
 as possible. 
 
 4. The horse exerts his force to the greatest disadvantage in 
 drawing or carrying up a hill. 
 
 5. The force wdth which a horse acts is compounded of his 
 weight and muscular strength. 
 
 6. The walk of a horse working in a mill, should never be less 
 than forty feet in diameter. 
 
 7 . A horse exerts most strength when drawing upon a plane. 
 
 Mill Work. 
 
 1. Water-wheels are of three kinds, viz. imdershot-zoheels, 
 breast-xeheels, and overshoUwkeels. The pow’ers necessary to pro- 
 duce the same eflfect on each of these, must be to each other as 
 the numbers 2.4, 1.75, and 1. 
 
 2. The under shoUs^\itc\ is used only when a fall of w ater can- 
 not be obtained. 
 
 3. A water-wheel twice as broad as another, has more than 
 double the powder. 
 
 4. An axis furnished with a very oblique spiral, and placed in 
 the direction of a stream, may be rendered a pow^erful first mover, 
 adapted to a deep and slow current. 
 
 5. A mill-stone should make 120 revolutions in a minute. 
 
 6. Bevelled-wheels are much used for changing the direction 
 of a motion in wheel-work. 
 
 7 . Hookers universal joint is sometimes used with advantage 
 for the same purpose. 
 
 8. The teeth of wheels should never, if it can be avoided, 
 act upon each other before they arrive at the line joining their 
 centres. 
 
 9* To ensure a uniformity of pressure aud velocity in the 
 action of one wheel upon another, the teelli should be formed 
 into epicycloids ; or into involutes of the circumferences of the 
 
404 
 
 ABSTRACT OF MECHANICS. 
 
 Mill-work. — Wheel carriags. 
 
 respective wheels ; or if the teeth of one of the w heels be either 
 circular or triangular, the teeth of the other wheel should have a 
 ligure compounded of an epicycloid and that of the figure of the 
 first wheel. 
 
 10. The object of thus forming the teeth, is, that they may 
 not slide but roll upon each other, by which means the friction if 
 almost annihilated. 
 
 11. It is a great improvement in machinery, where trundles 
 are employed with cylindrical staves, to make these staves move- 
 able on their axis. 
 
 12. A heavy mill-stone requites very little more power than a 
 light one ; but it performs much more work, and more effectually 
 equalizes the motion, like a heavy fly. 
 
 13. The com, as it is ground, is thrown out from between the 
 mill-stones by the centrifugal force it has acquired. 
 
 14. The manual labour of putting the ground corn into sacks, 
 in order to raise it to the top of the mill-house, may be obviated 
 by the use of a chain of buckets wrought by the machinery. 
 
 JVheel Carriages. 
 
 1. A horse draws with the greatest advantage, when the 
 line of traction or draught is inclined upwards so as to make an 
 angle of about lo degrees with the horizontal plane. 
 
 2. By this inclination, the line of traction is at right angles to 
 the shape of the horse’s shoulders, all parts of which are there- 
 fore equally pressed by the collar. 
 
 3. Single horses are preferable to teams, because in a team, 
 all but the shaft horse draws horizontally, and consequently to 
 disadvantage. 
 
 4. A horse, when part of the weight presses on his back, will 
 draw’ a weight to which he would otherwise be incompetent. 
 
 o. The fore-w heels of carriages are less than the hind- wheels, 
 for the convenience of turning in a smaller compass. 
 
 6. In ascending, high wheels facilitate the draught, in propor- 
 tion to the squares of their diameters ; but in descending, they 
 press in the same proportion. 
 
 7. In descending, the body of a cart may be advantageously 
 thrown backwards, so that the bottom of it will be horizontal, 
 while the shafts incline downwards. 
 
 8. In loading four-wheeled carriages, the greatest weight 
 should be laid upon the large wheels. 
 
 p. Dished wheels are better calculated tlian any other to sus- 
 tain the jolts and unavoidable inequalities of pressure arising from 
 tlie roughness of the roads. 
 
ABSTRACT OF MECHANICS. 
 
 405 
 
 Clock-work. 
 
 10. The extremities of the axles should be in the same hori- 
 zontal plane, and the wheels should be placed on them at right 
 angles. 
 
 11. Broad cylindrical wheels smooth and harden a road, while 
 narrow ones cut it into furrows, and conical ones grind the hardest 
 stones to powder. 
 
 Clock-work, 
 
 1. To ascertain the number of revolutions which a pinion 
 makes, for one of the wheel working in it, divide the number of 
 its leaves by the number of the teeth of the wheel; and the answer 
 is obtained. 
 
 2. By increasing the number of teeth in the wheels ; by dimi- 
 nishing the number of leaves in the pinions ; by increasing the 
 length of the cord that suspends the weight ; and lastly, by adding 
 to the number of wheels and pinions, a clock may be made to go 
 any length of time, as a month, or a year, without winding up. 
 
 3. The inconvenience of taking up more room, but princi- 
 pally the increase of friction which would be introduced, are the 
 causes of its being inexpedient to make a clock go much beyond 
 eight days. 
 
 4. Clocks intended to keep exact time, are contrived to go 
 whilst winding up. 
 
 5. Clocks which have pendulums vibrating half seconds, are 
 frequently moved by a spring instead of a w eight. 
 
 6. A spring is strongest when it is first wound up, and gradu- 
 ally decreases in strength till the movement stops ; it is therefore 
 contrived to draw the chain off a conical barrel, so that the lever 
 at which it pulls is lengthened as it grows weaker, by which means 
 its effects are equalized. 
 
 7. The plates of clock-makers’ engines, may quickly be 
 divided into odd numbers, by subtracting from the odd number 
 so much as will leave an even number of easy subdivision ; then 
 calculating the number of degrees contained in the parts sub- 
 tracted, and setting them off on the circumference of the circle 
 from a sector. 
 
 8. Tiie geometrical radius of wheels, when the teeth are epi- 
 cycloidal, is less than the acting diameter, by about |^ths of the 
 breadth of a tooth or measure. 
 
 9- The relative size of a pinion must be less for a small wheel 
 than for a large one, and also smaller when driven than when it is 
 the driver. 
 
406 
 
 ABSTRACT OF MFXHANICS. 
 
 Pendulums. 
 
 Pendulums. 
 
 1. All the vibrations of the same pendulum, whether great or 
 small, if cycloidal, are performed in equal times. 
 
 2. The longer a pendulum, the slower are its vibrations. 
 
 3. A pendulum to vibrate seconds, must be shorter at the 
 equator than at the poles. 
 
 4. Heat lengthens and cold shortens pendulums. 
 
 5. The quicksilver pendulum, the gridiron pendulum, and 
 many others, have been contrived to obviate these effects of change 
 of temperature. 
 
 6. The vibrations of pendulums are affected by differences ir 
 the density of the medium in which they are performed. 
 
 7. The merit of the only contrivance to remedy this defect is 
 due to Rittenhouse. It consists in the use of two pendulums, one 
 of which is very light, and placed in an inverted position, extend- 
 ing above the point of suspension of the other. 
 
 8. This compound pendulum may be made to vibrate quicker 
 in so dense a medium as water than in the open air. 
 
OPTICS. 
 
 407 
 
 Historical remarks. 
 
 OPTICS. 
 
 This branch of Natural Philosophy treats of the mechanical 
 properties of light, and the phenomena of vision. 
 
 Many optical appearances are of such frequent recurrence, 
 that they could not escape the notice of the earliest observers; 
 but ages appear to elapsed, before any progress was made towards 
 an explanation of them. Empedocles was the first person on re- 
 cord, who attempted to write systematically on light. A treatise 
 on optics, attributed to Euclid, who flourished about 400 years 
 before the Christian era, shews the state of knowledge on the 
 subject about that time. It adverts to the eflect of bringing into 
 view, by refraction, an object at the bottom of a vessel, by pour- 
 ing water upon it ; but chiefly treats of reflected rays, explaining 
 the effects of different kinds of mirrors, and demonstrating the 
 equality of the angles of incidence and reflection. It appears, 
 also, that the ancients were acquainted with the magnifying power' 
 of glass globes filled with water, though they probably knew no- 
 thing of the reason of this power; and it is supposed that the an- 
 cient engravers used glass globes to magnify their figures, that they 
 might work to more advantage. 
 
 Ptolemy, about the middle of the second century, wrote a 
 considerable treatise on optics. The work is now lost, but from 
 the accounts of others, it appears that his observations had even 
 enabled him to treat of astronomical refractions. Alhazen, an 
 Arabian writer, was the next author of consequence ; he wrote 
 about the year 1100, and gave the first account of the magnifying 
 power of glasses. 
 
 In 1270, Vitellio, a Polander, published a treatise on optics, 
 containing all that was valuable in Alhazen, digested in a better 
 manner, and with clearer explications of various phenomena. He 
 observes, that light is always lost by refraction,^hich makes ob- 
 jects appear less luminous. He gave a table of^nk results of his 
 experiments on the refractive powers of air, water, "4nd glass, cor- 
 responding to different angles of incidence. He ascribes the 
 twinkling of the stars to the motion of the air in which the light 
 is refracted ; and illustrates this hypothesis by observing, that they 
 twinkle still more when viewed in water put in motion. He also 
 asserted, that refraction is necessary as well as reflection, to form 
 the rainbow'; because the body which the rays fail upon is a trans- 
 parent substance, at the surface of w hich one part of the light is 
 
408 
 
 OPTICS. 
 
 Histoiical remarks. 
 
 always reflected, and another refracted. He makes some ingenious 
 attempts to explain refraction, or to ascertain the law of it; and 
 considers the foci of glass spheres, and the apparent size of objects 
 seen through them, though with but little accuracy. 
 
 The celebrated Roger Bacon was contemporary with Vitellio, 
 with whose writings, if not also with those of Alhazen, he was 
 probably acquainted. He seems to have acquired the knowledge 
 of some facts, which were unknown to them ; yet, with several 
 important truths, he blended, on this subject, much that was wild 
 and fanciful. The invention of the magic lantern is attributed to 
 him ; and he demonstrated, by actual experiment, that a small seg- 
 ment of a glass globe w ould greatly assist the sight of old persons ; 
 but concerning the actual inventor of spectacles we have no cor- 
 rect information ; this only is certain, that the use of them was ge- 
 nerally known about the beginning of the fourteenth century. 
 
 In the year 167o, Maurolycus, a teacher of mathematics at 
 Messina, published a treatise on optics, in which he demonstrates 
 that the crystaline humour of the eye is a lens, which collects the 
 rays of light from external objects, and throws them on the retina 
 or back of the eye. From this principle he assigned the reason 
 why some people are short-sighted, and others long-sighted, and 
 w'hy the former are relieved by concave, and the others by convex 
 glasses. 
 
 John Baptista Porta of Naples, was contemporary with Mau- 
 rolycus. He invented the camera obscura, and his experiments 
 with that instrument convinced him that light is a substance, by 
 the reception of which into the eye, vision is accomplished. This 
 was a great discovery, and corresponds very nearly with the ex- 
 periments and reasoning of Maurolycus ; but it must be remarked, 
 that neither of these two philosophers had any knowledge of what 
 the other had done. The importance of Porta’s discovery will be 
 evident, when it is observed, that previous to his time, vision w'as 
 supposed to be dependent upon what w ere termed visual rays, 
 proceeding from the eye. He justly considered the eye itself as a 
 camera obscura, the pupil performing the office of the hole in the 
 window shutter he remarked, also, that a defect of light is reme- 
 died by the dilatation of the pupil, which contracts involuntarily 
 when exposed to a strong light, and expands w'hen the light is too 
 faint for distinct vision. 
 
 Antonia de Dominis, whose work was published in l6ll, 
 was the first who came near the true theory of the rainbow'. 
 He describes the progress of the ray of light through each drop 
 of the falling rain ; he shews that it enters the upper part of the 
 drop, where it suffers one refraction ; that it is reflected once, and 
 diLii refracted again, so as to come directly to the eye of the 
 
OPTICS. 
 
 409 
 
 Historical remarks. 
 
 spectator ; why this refraction should produce the different co- 
 lours, was reserved for Sir Isaac Newton to explain. 
 
 ‘ Telescopes were invented towards the latter end of the six- 
 teenth century. Of thii^, as of many other memorable discoveries, 
 accident furnished the first hint : Zacharias Jansen, a spectacle 
 maker of Middleburg, trying, it is said, the effects of a con- 
 cave and convex glass united, found that, if they were placed 
 at a certain distance from each other, they caused distant 
 objects to appear nearer the eye. Other accounts transfer the 
 merit of the first discovery from Jansen himself to his children, 
 who, while playing wdth spectacle-glasses in his shop, perceived, 
 that when they held two of these glasses between their fingers, 
 at a certain distance from each other, the dial of the clock 
 appeared greatly magnified, but in an inverted position. This 
 incident suggested to their father the idea of adjusting two of 
 these glasses on a board, so as to move them at pleasure. 
 To the Jansens we are also indebted for the discovery of the 
 microscope, an instrument depending upon exactly the same 
 principles as the telescope. Galileo greatly improved the tele- 
 scope, and constructed one that magnified thirty-three times ; with 
 which he made the astronomical discoveries that have immortalized 
 his name. 
 
 Kepler paid great attention to the phenomena of light and 
 vision ; he w'as the first who demonstrated that the degree of re- 
 fraction suffered by light in passing through lenses, corresponds to 
 the diameter of the circle of which the convexity or concavity 
 is the portion of an arch. He very successfully pujsued the dis- 
 coveries of Maurolycus and Porta, The images of external ob- 
 jects, he asserted, were formed upon the optic nerve by the foci 
 of rays coming from every part of the object ; and he attributed 
 to habit the power of enabling us to see objects in their right posi- 
 tion, though their images upon the retina are inverted. He ac- 
 counted for the apparent diminution of the moon’s disk in solar 
 eclipses by observing, that the disk of the moon does not appear 
 less in consequence of being unenlightened, but that at other times 
 it appears larger than it really is, in consequence of its being en- 
 lightened : for pencils of rays from such distant objects generally 
 come to their foci before they reach the retina, which they conse- 
 quently reach in a state of divergence. Hence he concluded, that 
 different persons may imagine the lunar disk to be of different 
 magnitudes, according to the relative goodness of their sight. 
 
 In 1625, the curious discovery of Scheiner was published at 
 Rome, which placed beyond the reach of contradiction, the fact 
 that vision depends upon the images of external objects being de- 
 picted upon the retina, and that these images are inverted ; for 
 
 18 . — VoL. I. 3 G 
 
410 
 
 OPTICS. 
 Historical remarks. 
 
 taking the eye of an animal, and cutting away the coats of the back 
 part, and presenting different objects before it, he displayed their 
 images distinctly painted on the naked retina or optic nerve. 
 
 About the middle of the seventeenth century, the velocity of 
 light \\ as discovered by Roemer ; and towards the close of it pub- 
 lished by James Gregory, the first proposal for a reflecting tele- 
 scope. 
 
 At length arose Sir Isaac Newton, who discovered the cause 
 of colours, an investigation which had eluded the abilities of all 
 the philosophers who had gone before him. He applied his prin- 
 ciples to the satisfactory explanation of most of the phenomena of 
 nature w here light and colour are concerned ; and almost all that 
 we know upon these subjects was laid open by his experiments. 
 He found that the refractive powers of different substances were 
 in general proportionate to their densities, except when they con- 
 tained inflammable or oily particles. Having proved that the re- 
 fractive power of diamond was much greater than that of other 
 substances of equal density, he supposed it to contain an inflain- 
 mahle principle, — a happy conjecture, which the modern disco- 
 veries in chemistry have irrefragably proved, and which has always 
 been considered a proof of his wonderful sagacity, because it was 
 at complete variance with all that was known in his day respecting 
 the nature of the diamond. 
 
 The spleiidow' of Sir Isaac Newton’s discoveries obscures in 
 some degree the merit of both earlier and subsequent writers ; yet 
 a great variety of interesting facts and improvements have ap- 
 peared since his time, though it must be admitted that the light by 
 which many of them have been discovered was furnished by his 
 labours. The present admirable mode of constructing reflecting 
 telescopes, to which, after him, many ingenious philosophers and 
 artists have contributed, has led to the most brilliant discoveries ; 
 and Dollond’s improvement of the refracting telescope has added 
 not a little to the convenience and the value of optical instruments 
 of that description. This artist, by using three glasses, of dif- 
 ferent refractive powers, was enabled to enlarge the diameters 
 of his object-glasses, and thus to admit more light, and enlarge his 
 field of view, far beyond what the common telescope admitted. 
 
 After explaining the principal terms made use of in treating of 
 optics, w^e shall take a view of the properties of light in general, 
 and then proceed with the particular consideration of refraction, 
 reflection, inflexion, and the pheomena depending upon them. 
 
OPTICS. 
 
 411 
 
 Definitions. 
 
 Dejinitiom. 
 
 1. By a of light is meant the least particle of light that 
 can be either intercepted or separated from the rest. In the dia- 
 gramd by which the science is illustrated, rays of light are repre- 
 sented by right lines. 
 
 2. Any small parcel of rays proceeding from or to a point, 
 considered apart from the rest, is called 2l pencil of rays. 
 
 3. A beam of light is used to denote any considerable aggre- 
 gate of light, or parcel of rays. 
 
 4. Parallel rays are such as move always at the same distance 
 from each other, as represented by fig. 3, pi. 1. 
 
 o. Converging rays are such as approach nearer and nearer to 
 each other, and tend to unite in a point ; they are represented by 
 fig. 4, forming a cone, the base of which is at the place where the 
 rays began to converge. 
 
 6. Diverging rays are those which continue to recede further 
 and further from each other through their whole progress, and if 
 proceeding from a point, form, as shewn by fig. 5, a cone, the 
 base of which is the termination of their course. 
 
 7. The point at which converging rays meet/ is called the 
 focus. 
 
 8. When converging rays are prevented from meeting by some 
 obstacle, the point towards which they tend, and where they would 
 have united, if not intercepted, is called the virtual or imaginary 
 
 ^ focus. 
 
 9. Void space, and whatever substance the rays of light pass 
 through, is called by opticians, a medium. 
 
 10. Media are either dense or rare : one medium is said to be 
 more dense than another, when it is heavier, or contains more 
 matter under the same bulk ; and vice versa^ it is called more rare 
 than another, when it is lighter, or contains less matter under the 
 same bulk. Glass is more dense than water ; water is more dense 
 than air. 
 
 11. By a lens is meant a transparent body of a dififerent den- 
 sity from the surrounding medium, and terminated by two surfaces, 
 either both curved, or one plane and the other curved. As lenses 
 are commonly made of glass, it is usual to call them glasses, with 
 the addition of an epithet, designating their use, or the nature of 
 their curve, or both ; as a magnifying-glass, an object or eye-glass 
 of a telescope, a concave spectacle-glass, a convex spectacle-glass. 
 
 12. An incident ray is that which comes from any body to the 
 reflecting surface; the reflected ray is that which is sent back 
 from the reflecting surface. Children sometimes amuse them- 
 
412 
 
 ' OPTICS. 
 
 Definitions. — Opinions of the ancients respecting light. 
 
 selves with presenting a piece of looking-glass to the sun, and cast- 
 ing a vivid spot of light on any object they please. In this case, 
 the rays received directly from the sun are called incident^ and the 
 lucid spot is made by the reflected rays. 
 
 13. The angle of incidence is the angle which is formed by 
 the incident ray with a perpendicular to the reflecting surface 
 at the point of incidence ; and the angle of reflection is the angle 
 formed by the same perpendicular and the reflected ray. Thus, 
 in flg. 6, a ray of light, a, falls in the direction a d, upon the sur- 
 face efy and is reflected in the direction dh ; c d \s perpendicular 
 to e fj therefore adc 'n the angle of incidence, and cdb \s the 
 angle of reflection. 
 
 14. A mirror^ or speculunif is an opaque body, the surface of 
 which is very smooth and finely polished, so that it will reflect the 
 rays of light which fall upon it, and by this means represent the 
 images of objects opposed to it. 
 
 15. Plane mirrors are those reflecting bodies, the surfaces of 
 which are perfectly plane, such as our common looking-glasses. 
 
 16. Concave and convex mirrors are those the surfaces of which 
 are curved. If a watch-glass were silvered on the round side, it 
 would be a concave mirror ; if it were silvered on the hollow side* 
 it would be a convex mirror. 
 
 Of Light. 
 
 The nature of light has been a subject of speculation from the 
 first dawnings of philosophy. Several of the earliest philosophers 
 thought, that objects became visible by, means of something pro- 
 ceeding from the eye; while some maintained, that vision was 
 occasioned by particles continually flying oft' from the surfaces ot 
 bodies, w hich met with others proceeding from the eye ; but Py- 
 thagoras is said to have ascribed the effect solely to the particles 
 proceeding from external objects, and entering the pupil of the 
 eye. The ancients possessed much greater ingenuity to invent 
 theories, than inclination (or perhaps opportunity) to determine the 
 truth of them by experiment ; and therefore, if Pythagoras actually 
 promulgated this opinion, it seems to have been supported by no 
 facts w'hich placed it on an immutable basis. Hence it gained for 
 many ages no greater credit than other opinions, the most incon- 
 gruous and vague. It was not till about a thousand years after 
 the time of Pythagoras, that J. Baptista Porta fully satisfied him- 
 self and others of vision being performed entirely by the intromis- 
 sion of light into the eye. 
 
 Several eminent philosophers have imagined, that the sensa- 
 tion which we receive from light is to be ^ attributed entirely to 
 
OPTICS.' 
 
 413 
 
 Opinions respecting light. 
 
 the vibrations of a subtile medium or fluid, diffused throughout 
 the universe, and put in action by the impulse of the sun. 
 According to this hypothesis, light may be considered as 
 analogous to sound, which is known to depend entirely on the 
 pulsations of the air striking upon the ear. But this theory is 
 encumbered with many difficulties. If light depended altogether 
 upon the vibrations of a fluid, a quick motion of the hand, or 
 of a machine contrived for the purpose, would produce light 
 at any time ; or rather, as no solid reason can be assigned why 
 the fluid should cease to vibrate in the night, since the sun 
 must always affect some part of it, we ought to have perpetual 
 day. Again, the texture of certain bodies is actually changed 
 by exposure to light, even though they be inclosed in glass ; 
 but if covered with the thinnest plate of metal, which excludes 
 the light, no alteration takes place. Many other objections which 
 militate with equal force against this theory, might be adduced, 
 but these have never received answers which can be deemed satis- 
 factory. 
 
 A late writer is decidedly of opinion that light is dilated Jire, 
 that is, fire weakened and diffused, as ardent spirit when 
 mingled with water. It appears at first view favourable to this 
 opinion, that light may be collected and condensed by the 
 burning-glass, so as to burn like the fiercest flame ; on the 
 contrary, flame itself may be attenuated, even by artificial 
 means, to such a degree as to be perfectly innoxious. The 
 flame,” says Dr. Goldsmith, “ which hangs over burning 
 spirit of wine, we all know to scorch with great power, yet 
 these flames may be made to shine as bright as ever, yet be 
 perfectly harmless. This is done by placing them over a gentle 
 fire, and leaving them thus to evaporate in a close room without 
 a chimney; if a person should soon after enter with a candle, 
 he will find the whole room filled with innoxious flames. The 
 parts have been two minutely separated, and the fluid, perhaps, 
 has not force enough to send forth its burning rays with 
 sufficient effect.” But when we consider the remarkable disco- 
 very of Dr. Herschel, that light may be separated from the 
 caloric* which accompanies it, the identity of the two sub- 
 stances, if not fully disproved, becomes very doubtful. The 
 Doctor, w'hen employed in making observations on the sun by 
 means of telescopes, found that the coloured glasses used to 
 prevent the inconvenience arising from the heat, very soon 
 
 * Caloric is the term by which modern philosopheri have agreed to distin- 
 guish that peculiar substance which produce^ heat. 
 
414 
 
 OPTICS. 
 
 Separation of lijljt and heat. — Vtlocity of light. 
 
 cracked and broke in p'cccs when their colour was deep enough 
 to intercept the light. On prosecuting the inquirv, in a course 
 of experiments devised for the purpose, he found that the solar 
 beam contained rays which gave no light, and vet produced a 
 greater degree of heat than even those rays which produced the 
 strongest light. The two species of rays may be separated 
 from each other by a very easy experiment. If a glass mirror 
 be held before the bre, it strongly reflects the rays of light, but 
 the rays of caloric which it sends forth are very few'; a metal- 
 lic mirror, on the other hand, made of planished tin-plate, for 
 example, reflects the rays of light indiflerently, but it supplies 
 caloric rays in great abundance. The glass mirror becomes hot; 
 the metallic mirror does not alter its temperature. If a plate of 
 glass be suddenly interposed between a glowing Are and the face, 
 it intercepts completely the warming power of the Are, without 
 causing any sensible diminution of its brilliancy ; consequently it 
 intercepts the rays of caloric, but allows the rays of light to pass. 
 If the glass be allowed to remain in its station till it becomes as 
 hot as its distance from the Are will allow, it ceases to intercept 
 the rays of caloric, which then pass through it as freely as the rays 
 of light. These data lead us inevitably to tlie conclusion, that 
 light is not diluted Are or caloric. 
 
 We shall now proceed to notice the most important properties 
 of light in an optical point of view ; — a task much more delightful 
 than the investigation of its nature and essence; arid eminently 
 calculated to entrance the mind with astonishment. 
 
 In the very short space of one secondy a ray of light 
 traverses the prodigious extent of nearly two hundred thousand 
 miles. The manner in which the velocity of light is calcu- 
 lated, is not less ingenious than the discovery is surprising. 
 It w’as by observing the eclipses of Jupiter’s satellites, which ap- 
 peared to be eclipsed sooner or later than the times given by 
 the tables of them, and the observation was always before or 
 after the computed time, according as the earth was nearer to 
 or further from the planet Jupiter than the mean distance. To 
 understand this fact more fully, suppose the earth in going its 
 annual circuit round the sun, is at C, (Ag. 1, pi. I,) and that 
 an eclipse is there obser\ed of a satellite of Jupiter, which 
 regularly suffers an eclipse in forty-two hours and a half. If 
 the earth never left C, but continued there immoveable, and 
 Jupiter remained at the same distance, the eclipse of the 
 satellite would always be observable at the expected interval of 
 forty-two hours and a half; and consequently, in thirty times 
 forty- two hours and a half, the spectator would see thirty 
 eclipses. But the earth travelling onward to D, tlie spectator 
 
OPTICS. 
 
 413 
 
 Velocity and minuteness of light. 
 
 does not see thirty eclipses till some time after the stated period, 
 for the further off the earth removes, the longer the time 
 required for the light to reach the spectator. In traversing 
 from C to D, light takes up about sixteen minutes and a half, 
 therefore, as the sun is half way between C and D, it must 
 perform its journey from him in half that time, that is, in 
 eight minutes and a quarter ; or, according to the most exact 
 calculation, in eight minutes and seven seconds. No difference 
 in the velocity of light has ever been discovered, whether it is 
 original, as from the stars, or reflected only, as from the 
 planets. 
 
 Such being tlie rapidity with which the rays of light dart 
 themselves forward, it becomes a matter of easy calculation, and 
 may a little assist our conception to observe, that a journey 
 which they perform in eight minutes, could not be performed by 
 a cannon ball at its ordinary speed in less than thirty-two years. 
 That the motion of light is inexpressibly rapid, we may easily 
 convince ourselves, if we notice the firing of a cannon or a 
 musket at a considerable distance, and observe the time which 
 elapses between seeing the flash and hearing the report. Sound, 
 it has been calculated, travels at the rate of 1142 feet, or 380 
 yards, in a second ; yet the time which intervenes between seeing 
 the flash and hearing the report of the fire-arm, is a satisfactory 
 proof of the prodigious disparity between the velocity of light and 
 sound. 
 
 If the velocity of light, then, be so very great, it may be 
 inquired W'hy it does not strike against objects with a propor- 
 tionate force ? If the finest sand were thrown against our bodies 
 with a hundredth part of this velocity, each grain would be as 
 fatal as the stab of a dagger ; yet our eyes, the most exquisitely 
 sensible of all our organs of perception, receive its impressions 
 without the smallest pain. But we have suflicient evidence to 
 convince us that the minuteness of the particles of light is still 
 more extraordinary than their velocity, and that their minute- 
 ness, therefore, is the cause of their being harmless. A lighted 
 candle will fill a sphere of four miles in diameter, and may be 
 extinguished without having lost any sensible portion of its weight ; 
 yet itmust have diffused several hundreds of millions more par- 
 ticles of light than there could be grains in the whole earth, if it 
 were entirely composed of the finest sand. 
 
 It is a principle in mechanics, that the momentum of 
 moving bodies, or the force with which they strike, is propor- 
 tionate to the quantity of matter they contain multiplied by 
 their velocity ; consequently, if the particles of light were not 
 infinitely smaller than we can conceive, nothing could with- 
 
416 
 
 OPTICS. 
 
 Effect of light on the eye not instantaneous. 
 
 Stand their impulse. Light moves 2,000,000 of times faster than 
 a cannon ball ; consequently, if its particles were only equal in 
 size to the 2,000, OOOtli part of a grain of sand, they would pro- 
 duce an effect equal to that of sand from the mouth of. a cannon. 
 If, keeping this in mind, we further consider the facility with which 
 light penetrates the hardest bodies, as glass, crystal, and even the 
 diamond itself, through all which it finds an instant passage, and 
 that it does not displace the smallest atom of dust which it en- 
 counters in its progress ; we shall not hesitate to admit, that the 
 particles of light cannot be equal in size to the 4,000,000th part 
 of a grain of sand. 
 
 As the particles of light are continually passing from every 
 luminous body, in all directions, it may be inquired why 
 they do not interfere with each other in such a manner as to 
 confound all distinct perception of objects, if not quite destroy 
 the sense of seeing ? Their velocity, however, enables us to 
 answer these questions, by convincing us that they may be 
 separated at least a thousand miles, and yet be perfectly 
 efficient to the purposes of vision. It is an undoubted fact, 
 that the effect of light upon the eye is not instantaneous, but 
 that the impression remains after the light has been w ithdrawn. 
 Of this any one may satisfy himself, by shutting his eye, 
 after having looked for some time on a candle, a star, or any 
 other luminous object, when a faint momentary picture of the 
 object will remain. The same thing may be proved by w hirling 
 round a stick, the extremity of which is on fire ; if the motion be 
 quick enough, the perception of a complete circle of flame w ill be 
 impressed on the eye. The actual duration, for a certain time, 
 of the impression of light being thus proved, let it be supposed 
 to continue distinct only for the 150th part of a second ; then if 
 one lucid point of the sun's surface emit 150 particles of light in 
 a second, these will be amply sufficient to afford light to the eye 
 without any intermission; and yet the particles emitted w'ill be 
 more than 1000 miles apart. 
 
 Notwithstanding the extreme tenuity of light, the task of 
 actually measuring its momentum has not proved either too 
 bold to be conceived, or too difficult to be executed. Boerhaave 
 gave motion to the needle of a compass, by concentrating the 
 rays of the sun upon it with a pow erful burning-glass ; and 
 Mitchell, in later times, tried an experiment to the same purpose 
 in a stiU more satisfactoi 7 manner. He constructed an instru- 
 ment, in the form of a small vane or weathercock. It consisted 
 of a very thin plate of copper, of about one inch square, 
 which was attached to one of the finest harpsichord wures, 
 about ten inches long. To the middle of the wire vvas fixed an 
 
OPTICS. -4.17 
 
 Momentum of light. — Diminution of the sun by tiie emission of light. 
 
 agate cap, such as is used for the smailest mariners^ compasses, 
 after the manner of which it w'as intended to turn ; and the 
 thin plate was balanced on the other side by a grain of small 
 shot. 7’he instrument weighed ten grains ; and to prevent its 
 being affected by the vibrations of the air, it was inclosed in a 
 glass box. The rays of the sun were thrown upon the plate of 
 copper from a concave mirror of two feet in diameter ; in conse- 
 quence of which, the vane or copperplate moved, on repeated 
 trials, with a gradual motion of about one inch in a second of 
 time. The instrument weighing ten grains, and the velocity 
 with which it moved being one inch in a second, the quantity 
 of matter contained in the rays which fell upon the instrument 
 in that time, was equal to the twelve hundred millionth part of 
 a grain, the velocity of light exceeding the velocity with which 
 the instrument moved in that proportion. The mirror contain- 
 ing about three square feet of surface, and mirrors in general 
 reflecting only half the rays w'hicli fall upon them, the quantity 
 of matter contained in the rays of the sun incident upon a 
 square foot and a half of surface, is no more than one tw'elve 
 hundred millionth part of a grain. But the density of the 
 rays of light at the surface of the sun is greater than at the 
 earth, in the proportion of 45,000 to 1. If in one second, 
 therefore, one square foot of the sun’s surface emit one forty-five 
 thousandth part of a grain of matter, the supply will be adequate 
 to the consumption of light ; yet it will be little more than 
 two grains a day, or about 4,380,000 grains, or 626 pounds, in 
 6,000 years ; a loss which would have shortened the sun’s dia- 
 meter about ten feet, if it were formed of matter the density of 
 water only. 
 
 By those who adopt the opinion that light is constituted by 
 the vibrations of a subtile fluid, it has been urged, that if a flood 
 of particles w ere incessantly streaming from the sun, the diminu- 
 tion of the bulk of that luminary would have become very percep- 
 tible from the diminution of his beneflcial effects. But even if 
 the preceding calculation of the emission of light be estimated far 
 too low, when the immensity of the sun’s diameter, which amounts 
 to 883,246 English miles, is considered, we shall see little reason 
 to justify the apprehensions of its being exhausted, however ex- 
 travagant our ideas of the duration of the present system of na- 
 ture, and although we reject the great probability thabi^like our 
 atmosphere, it is provided with the means of its own perpetual 
 replenishment. 
 
 It has been objected to Mitchell’s experiment, that the air, 
 rarefied by his mirror, was sufficient to give motion to the vane, 
 although the light whigh fell QU it had uo momentum whatever ; 
 18. — VoL. L 3 H 
 
OPTICS. 
 
 
 Media not warmed by the passage of light. — Direction of light. 
 
 but it has been found that precisely the same motion is produced 
 in vacuo as when the glass case is filled with air. The fact that 
 light has momentum may therefore be considered as proved, 
 though the deductions drawn from the experiment relative to the 
 quantity of it, are liable to much uncertainty. 
 
 i\nother argument in favour of the extreme tenuity of light, 
 and of its particles following each other at an immense distance, 
 is the well known fact, that the rays collected by the strongest 
 burning-glass, will not inflame the most combustible matter 
 while they merely pass through it. A phial containing spirit 
 of wine will not be set on fire ; but if the spirit is poured into a 
 spoon or any opake vessel, w'hich stops the progress of the con- 
 centrated rays, inflammation instantly results. This familiar 
 experiment enables us to account for the cold experienced at the 
 summits of high mountains. The atmosphere is not w^armed 
 by the mere passage of the rays through it; these rays first 
 heat the earth, where they are first stopped, and the earth then 
 communicates its own temperature to the air, which being a very 
 bad conductor of heat, the lower, denser stratum of it, becomes 
 the chief residence of its warmth. Such parts, therefore, as are 
 elevated above the general surface of the earth, derive, in pro- 
 portion to their height, a dimiuisbed advantage from this cir- 
 cumstance, and though they may receive as many rays as an 
 equal extent of burning desart, yet (he suiface they contain is so 
 trifling, when compared with the stratum of air on a level with 
 them, that the heat prod»ced is abstracted as quickly as it is 
 formed. 
 
 The next property of light which demands our attention, is, 
 that it is propelled from every luminous body in right lines. This 
 is evident from an experiment which any person may make with a 
 bent tube, through which nothing can be discerned, provided it 
 be so much bent that nothing can pass through it in a right line. 
 Another proof of the same fact, is, that shadows are bounded by 
 right lines drawn from the luminous body past the contour of the 
 body casting the shadow. 
 
 It is generally supposed, according to this principle, that 
 those bodies only are transparent whose pores are such as to 
 permit the rays of light to pervade them in a right line; and 
 that those bodies are opake, which intercept the rays like the bent 
 tube. 
 
 Every ray of light carries with it the image of the point 
 from which it was emitted. If, therefore, the pencils of rays 
 from every point of an object, are united in the same order in 
 which they proceeded when first emitted, they will form a perfect 
 image or representation of that object, at the place where tliey 
 
OPTICS. 
 
 41 Q 
 
 Means by which objects become visible. — Reflections. 
 
 are thus united. The rays of light proceeding in straight lines> 
 it is obvious, that to make an object visible, at any place to which 
 a straight line can be drawn from it to the eye, they must be de- 
 tached from every physical point of it in all directions ; but only 
 those rays which enter our eyes can render them visible to us. 
 Thus the object ACB, fig. Q, pi. I, is rendered visible to an eye 
 in any part where the rays Aa, Ab, Ac, Ad, Ba, B6, Be, Be?,, Ca, 
 Cb, Cc, Cd, can come ; and these rays affect our sight with the 
 sense of different colours and shades, according to the properties 
 of the body from which the light is reflected. However confused 
 the figure may appear, those, to whom the subject is new, will ob- 
 serve that the rays are only made to proceed from three points ; 
 and hence they may be enabled to form some slight conception of 
 the numberless crossings of the rays, in order that every part of a 
 body may be visible, in whatever direction it is looked upon. 
 
 Here perhaps we may allow the young reader, before we 
 pass on with didactic philosophy, to contemplate, for a moment, 
 that luminary from which all light proceeds. The barbarous 
 and the civilized, the ignorant and the wise, have in ail ages 
 regarded the sun as the grandest object of the whole visible crea- 
 tion. With what delight does the lover of nature view, in what 
 glowing colours has the enraptured poet painted, his rising, hjs 
 meridian, and his setting glories. Allusions to the sun, and to 
 light, constitute the most beautiful expressions and figures of 
 speech in all languages ; and such are the ideas of association 
 connected with them, that frequency of repetition, or even abuse, 
 can scarcely render any of them contemptible. A few might 
 be adduced, which accompany only triteness of sentiment, and 
 feebleness of diction ; but genius has still the power of eliciting 
 new descriptions and combinations of allusion, as poetically 
 beautiful, as philosophically correct. Thomson’s apostrophe to 
 the sun is distinguished for grandeur of idea, and simplicity of ex- 
 pression : 
 
 “ Great source of day, best image here below 
 Of thy Creator, ever pouring wide. 
 
 From world to world, the vital ocean round.” 
 
4^0 
 
 OPTICS. 
 
 Oblique rays only refracted. 
 
 I Refrangihihtr/ of light. 
 
 The natural progress of the rays of light, we have already 
 shown, is in straight lines ; yet, like all other matter, light is in- 
 tliienced by attraction, which sometimes turns it out of its direct 
 course. This happens when it passes out of one medium into 
 another of different density, as from air into water or glass, or 
 from w ater or glass into air. This disposition or capability of 
 light to be bent, is called its refrangibilityy and the change of 
 direction actually assumed when the rays enter another medium, 
 is called refraction. 
 
 A very easy experiment will convince any one that light is 
 influenced by some pow er or other when coming out of a different 
 medium : put one end of a stick into water, and it will appear at 
 the surface as if it were broken. This effect is owing to the rays 
 of light being attracted or drawn out of their direct course, as may 
 be proved in a variety of ways. Place a shilling, or any other 
 conspicuous but small object at the bottom of a basin, and then 
 retire to such a distance, that the edge of the vessel just prevents 
 its being seen. Let the vessel be then tilled with water, and the 
 shilling will be perfectly visible, though neither it nor the spec- 
 tator have changed places in the slightest degree. Auothe«' expe- 
 riment of the same nature, and more easily managed by one per- 
 son, is the follow ing : take a basin, or any convenient vessel, and 
 place it in such a situation that the shadow of a lighted candle 
 will till one half of it ; hold the end of a ruler or stick exactly on 
 the place where the shadow terminates ; then pour water into the 
 basin, and the shadow will be seen immediately to withdraw from 
 the ruler. Increase the quantity of water, and the distance to 
 which the shadow will retreat, will also be increased. 
 
 It is necessary to observe, that only those rays which enter 
 another medium obliquely, suffer refraction, for a ray which 
 falls perpendicularly is equally attracted on all sides, and 
 therefore has no tendency to deviate in any direction. In the 
 experiment with the shilling, the spectator looks at it in an 
 oblique direction ; and the rays proceeding from it, by which it 
 is rendered visible after the water has been poured in, are bent 
 towards him, on entering the air. A ray of light, AC, fig. 7, 
 pi. 1, which passes obliquely from the air into water at C, 
 instead of continuing its course to B, takes the direction CH^ 
 and the reverse is equally true, a ray of light from H, reflected 
 in the direction HC, instead of continuing its rectilinear course^ 
 proceeds in the direction CA ; therefore an object at H is seen 
 by an eye at A, as if it were actually at B, every object 
 
OPTICS. 
 
 4S1 
 
 Experiment to prove refraction. 
 
 appearing to be in the direction of those rays which last ap- 
 proached the eye. 
 
 That the rays of light actually proceed in the direction 
 ascribed to them, is not merely a matter of speculation ; w« 
 may have ocular demonstration of the fact in a variety of ways 
 of which the following is one : Take an empty basin,^ and on 
 the bottom of it fix marks at a small distance from each other, 
 then take it into a dark room, and let in a ray of light, and 
 where the ray falls upon the floor, place the basin, so that its 
 marked diameter may point towards the window, and that the 
 light may fall upon the mark most distant from the window. 
 Pour water into the basin, and it will be found that the ray 
 which before fell upon the most distant mark, will, by the 
 refractive power of the water, be turned out of its straight 
 course, and fall two or three marks nearer the centre of the 
 basin. The water may now be rendered rather turbid, without 
 destroying its transparency, which may easily be done by the 
 addition of a few drops of milk, then on filling the room with 
 dust, the light will be completely visible, both in its passage 
 through the air and the water. Three directions of the ray 
 will be distinctly perceived ; the direction of incidence, or that 
 in which, from the aperture, it falls obliquely on the water ; 
 that of reflection, equal to that of incidence ; and that of 
 refraction, which commences at the surface of the w^ater, and 
 is continued in a direct line to the bottom of the basin. All 
 things remaining the same, place a small piece of looking-glass 
 at the bottom of the basin, where the refracted beam falls, and 
 it will be reflected back again through the water, and in passing 
 out of the water into the air, will be again refracted or turned out 
 of its course. 
 
 The greater the density of any medium, the greater is its re- 
 fractive power ; and of tw'o refracting media, that which is of an 
 oily or inflammable nature, will have a greater refracting power 
 than the other. 
 
 The incident angle is the angle made by a ray of light and a 
 line drawn perpendicular to the refracting surface at the point 
 where the ray enters the surface ; and the refracted angle, is the 
 angle made by the ray in the refracting medium with the same 
 perpendicular continued. The sine of the angle is a line which 
 serves to measure the angle, being drawn from a point in one leg 
 perpendicular to the other. In tig. 7, pi. 1, ACD is the incident 
 angle; HCE is the refracted angle; BCH is the angle of devia- 
 tion; AF is the sine of the angle of incidence; HG is the sine of 
 the angle of refraction. 
 
422 
 
 OPTICS. 
 
 Refrangibility of light. 
 
 If the incident ray, AC, fell in a more oblique direction 
 than is represented by lig. 7, the refraction would be still 
 greater; but in all cases of similar media, the angle of refrac- 
 tion will always be found to bear a regular and constant propor- 
 tion to the angle of incidence ; or in the language of opticians, 
 the sine of incidence is to the sine of refraction in a given ratio 
 or proportion, and this ratio is discovered by experience. In 
 the present instance, if vvith a pair of compasses we divide the 
 sine of incidence into four parts, the sine of refraction will be ex- 
 actly three of those parts. Hence, when a ray passes out of air 
 into water, the ratio is said to be as 4 to 3 ; when out of water 
 into air, the ratio is reversed, and therefore becomes as 3 to 4. 
 The ratio out of glass into air is as 2 to 3 ; consequently out of 
 air into glass, it is as 3 to 2. 
 
 It will be observed, from what has been just said, that in pass- 
 ing into a denser medium, light is refracted towards the perpen- 
 dicular, that is, the angle of refraction is less than the angle of 
 incidence ; on the contrary, when passing into a rarer medium, it 
 is refracted from the perpendicular. The inspection of fig. 7, 
 will render this evident ; the angle of incidence, ACD, being so 
 much larger than the angle of refraction, ECH, the refracted ray 
 CH, is nearer the perpendicular than if, proceeding in a right line, 
 it had gone to B. 
 
 It may seem, at first view, a little extraordinary, that light 
 should pass more directly through a dense than through a rare 
 medium ; but we have already seen that light is subject to 
 attraction, and Sir Isaac Newton discovered and demonstrated, 
 that this power is the cause of refraction. A circumstance that 
 confirms the truth of this theory, is the known fact, that the 
 change in the direction of the ray commences, not, as might be 
 supposed, when it comes in contact with the refracting medium, 
 but a little before it reaches the surface, aud the incurvation aug- 
 ments in proportion as it approaches the medium. Then as the 
 attractive power of different substances are proportionate to their 
 densities, we discover at once the reason of the ray being bent to- 
 wards the perpendicular on entering another medium of greater 
 density, and from the perpendicular, on entering a medium of less 
 density. 
 
 In passing from a dense into a rare medium, however, 
 there is a certain degree of obliquity at which the refraction is 
 changed into reflection. In other words, a ray of light will not 
 pass out of a dense into a rare medium, if the angle of incidence 
 exceeds a certain limit, but will be reflected back. Thus a ray of 
 light will not pass out of glass into air, if the angle of incidence 
 
OPTICS. 
 
 425 
 
 Effects of refraction. 
 
 exceeds 40° 1 1', or out of glass into water, if the angle of inci- 
 dence exceeds 59° 23'. 
 
 A knowledge of refraction will enable us to explain a variety 
 of curious phenomena, and on some occasions will usefully direct 
 our judgment. When a spectator stands on the bank of a 
 river, just above the level of the water, the bottom will be one- 
 third deeper than it appears. This is a necessary consequence 
 of the rays from the bottom being bent from the perpendicular, 
 and consequently inclined towards the spectator, on passing out 
 of the water to his eye. Those who shoot fish in the water, 
 soon obtain a practical knowledge of this deception, for they 
 find that they connot hit their mark, unless they aim considerably 
 below the place which it seems to occupy. A proof of the fact 
 may very easily be obtained : immerse a stick or straight rod of 
 any kind, perpendicularly in water, until the part which is im- 
 mersed appears of equal length with the part above ; upon taking 
 it out and measuring the parts, it will be found that they are to 
 one another as 4 to 3. We can now have no difficulty in account- 
 ing for the broken appearence of an oar, when partially immersed 
 in water. Let FGH, fig. 8, pi. II, repre^nt an oar, the part 
 GH being in the water, and the part GF out of it. The rays 
 sent to the eye from H, will appear to come from I, nearer to the 
 surface of the water, and as every part of GH will appear pro- 
 portionately nearer the surface, ibe part GH will appear to make 
 an angle with the part GF. 
 
 It is not merely objects that are partly out of water, which 
 lose their natural appearance from refraction; those which are 
 entirely immersed, especially when they are large, and lie at a 
 considerable depth, appear distorted in a variety of ways. A flat- 
 bottomed basin seems deeper in the middle than at the sides, and 
 a long straight leaden pipe appears to be curved. The reason is, 
 that the rays from the more distant extremities come in a more 
 oblique direction on their emergence into the air, and consequently 
 suffer a greater refraction than the rest. 
 
 The distortion of objects seen through a wrinkled or crooked 
 pane of a window, can have escaped the observation of 
 few; it arises from the unequal refraction of the rays which 
 pass through the glass. In looking through an even pane of 
 glass, about the tenth of an inch thick, objects appear nearly 
 one-thirtieth part of an inch out of their true place ; but as 
 they all maintain the same relative situation, the error is not per- 
 ceived. 
 
 A stranger to this subject would scarcely suspect, that we 
 never see the stars in the place where they really are, and that we 
 actually see the sun, and other heavenly bodies, before they are 
 
424 
 
 OPTICS. 
 
 Effects of refraction. 
 
 risen above the horizon. Our atmosphere decreases in density 
 as it increases in height, and though, at the tops of the highest 
 mountains, its pressure is so diminished, that breathing is 
 difficult, and the blood often gushes through the skin, yet, at 
 the elevation of forty-five miles, it is still capable of refracting 
 a ray of light. When, therefore, the sun is sunk so far below 
 the horizon, that his rays can only strike the upper part of the 
 atmosphere, he still remains visible, because, instead of passing 
 directly onward, the rays are attracted towards the perpendicu- 
 lar, and consequently bent towards the spectator on the earth. 
 To exhibit this effect to the eye, let fig. 9, represent the earth 
 surrounded by its atmosphere. HO is the visible horizon of 
 a spectator standing at P. S represents the sun as yet really 
 below the horizon, from which a ray of light ascends, and 
 falling on the upper part of the atmosphere, is, by the attrac- 
 tion it there experiences, bent out of its direct course towards 
 D, into the oblique one IP, by which means it falls upon the 
 eye of the spectator, who, perceiving the sun in the direction 
 of the refracted ray, will suppose it to be at R, which is higher 
 than its true place by more than a diameter. The length of 
 time that the sun appears above the horizon, while he is actually 
 below it, varies a little with a number of circumstances, but 
 chiefly with a difference of latitude. At the approach of summer, 
 near the poles, the sun becomes constantly visible for two or three 
 weeks before he actually rises above the horizon ; and at the ap- 
 proach of winter, when he has sunk below the horizon, his pre- 
 sence, from refraction, continues to cheer these dreary regions for 
 a similar space of time. The heavenly bodies, when in the zenith, 
 
 . are not subject to any refraction ; but when in the horizon, they 
 have the greatest of all : from the horizon to the zenith, the refrac- 
 tion continually decreases. 
 
 Mankind have availed themselves of the principle of refrac- 
 tion, to a most excellent purpose, in the construction of lenses ; 
 for by grinding the glass thinner at the edges than in the mid- 
 dle, those i*ays of light which would strike upon it in a straight 
 line, or perpendicularly, if it were plain, strike upon it obliquely, 
 and the refraction they suffer causes them to converge ; on the 
 contrary, by making the glass thinner in the middle than at the 
 sides, the rays are refracted the contrary w'ay, and therefore be- 
 come divergent. 
 
 The consideration of refraction through lenses may perhaps 
 be rendered more clear, if we reflect, that all curved surfaces are 
 composed of a number of straight lines, or points, infinitely 
 short, and inclining to each other like the stones in the arch of 
 a bridge. lu fig. 10, pi. II, parallel rays are represented as 
 
OPTICS. 
 
 425 
 
 Different kinds of lenses. 
 
 falling upon a surface of this sort ^ and it is evident that those 
 only which enter the middle part will go on in a straight direc- 
 tion ; those which strike the sides will strike them obliquely, 
 and will consequently be made to converge. Jf the surface, then, 
 was a perfect curve, as in fig. 11, it is plain that only the ray 
 which strikes the centre part of the curve will enter it in a straight 
 direction ; all die rest will be more or less refracted, according 
 to die degree of obliquity with which they strike the surface, and 
 the whole of the refracted rays will converge to a point called the 
 focus. 
 
 Glasses are usually ground for optical purposes into eight 
 different forms, 1. The glass may be flat on both sides, like 
 the pane of a wmidow. 2. It may be flat on one side, and 
 convex on the other. 3. It may be convex on both sides. 
 4. It may be flat on one side, and concave on the other. 5. 
 It may be concave on both sides. 6. It may be convex on 
 one side, and concave on the other, like the crystal of a watch. 
 
 7. It may have one side, wliich must be convex, ground into 
 litde facets, while the other side is plane. 8. It may have 
 some considerable length, in a triangular form.— -The sections 
 of these various forms of glasses are shewn at fig. 12, pi. II. 
 and they are distinguished by the following names: the glass 
 No. 1, is called a plane glass, as its sides are parallel; No. 2, 
 is called a plano-couvex glass; No. 3, a double-convex glass; 
 No. 4, a plano-concave glass; No. 5, a double-concave glass; 
 No. 6, a meniscus g\‘dss \ No. 7, a multiplying glass; and No. 
 
 8, a prism. The term lens is given to such glasses as either 
 magnify or diminish the apparent size of objects viewed through 
 them; Nos. 2, 3, 4, and 5, are therefore lenses; No. 6 is 
 also a lens, when its surfaces are portions of different spheres ; 
 but when they are of equal radii, it has only the eftect of a plane 
 glass. 
 
 From the view we have already given of refraction, the effects 
 of the first .seven of these glasses will be easily understood. — The 
 prism will receive a distinctive consideration. A ray entering the 
 plane glass, No. 1, will indeed be refracted, but it will suffer ano- 
 ther refraction on its emergence, which will rectify the former ; 
 the place of the object will therefore be a little altered, but its 
 figure will remain the same. Suppose AB, fig. 13, to represent 
 a solid piece of glass with two parallel surfaces, an incident ray 
 EF, will be refracted into FG, and FG will be refracted on pass- 
 ing from the second surface into GH, parallel to the original di- 
 rection EF. 
 
 If parallel rays enter the plano-convex glass, as shewn by 
 fig. 1 1, the ray E will be refracted upwards to F, the ray K 
 18. — VoL. I. 3l 
 
426 
 
 OPTICS. 
 
 Refraction throuj'h lenses. 
 
 will be refracted downwards to the same point; there they \\ill 
 cross, and then go onward in a straight line, and continue to di- 
 verge, till intercepted by some obstacle. 
 
 When parallel rays fall upon a double-convex glass, KG, they 
 will be refracted still more abruptly, and meet sooner in a point 
 or piinclpal focus at F. The distance of this focus is equal to 
 the semi-diameter of the circle which the convexity of the glass 
 continued would produce. Either this glass or the former, as 
 they collect the rays of the sun into a point, will burn at that 
 point, the whole force of the rays that pass through them being 
 ■concentrated there. 
 
 From all luminous objects, the rays of light proceed 
 in a state of divergence ; but when the distance from which 
 they come is very great, the quantity of divergence is too 
 small to require notice The sun, for example, is so immense- 
 ly distant, that his rays are always considered as parallel ; and 
 it is only parallel rays which are converged to a focus, in the 
 manner shewn above. Divergent rays, proceeding from a 
 point, as the flame of a candle, will be differently affected. 
 If, therefore, we place a candle exactly at the focal distance of 
 a single or double-convex lens, as at F, figs. 11 and 14, the rays 
 will emerge parallel to each other. If the candle is placed nearer 
 to the glass than its focal distance, the rays, after passing through 
 the glass will no longer be parallel, but separate or diverge ; if 
 the candle be placed still further off, the rays will then strike 
 the glass, more nearly parallel, and will, therefore, upon passing 
 through, converge or unite at a distance behind the glass more 
 nearly approaching the distance at which parallel rays would be 
 converged. 
 
 After the rays have united or converged to a focus, they will 
 cross each other, and form an inverted picture of the flame of 
 a candle, which may be received on a piece of paper placed at 
 the meeting of the rays behind. The cause of the inversion of 
 the image is therefore very evident ; for the upper rays, after re- 
 fraction, w’ere such as came from the under part of the luminous 
 body ; and the under rays, on the contrary, came from the upper 
 part. 
 
 The foregoing theory may be demonstrated with a common 
 reading glass. If a candle be held so near a glass of this sort, 
 that the rays passing through, fall upon a screen or w^all with 
 a bright spot just as large as the glass itself, the candle is 
 then at the focal distance, and it is clear that the rays which 
 struck the glass divergently, are refracted parallel, or the spot 
 of light could not be just the size of the glass. If the candle 
 is held nearer than the focal distance, the rays will. then fall 
 
OPTICS. 
 
 427 
 
 Refraction through lenses. 
 
 more divergently upon the glass, and will consequently, after 
 refraction, still have so much divergence as to form a very 
 broad spot of light upon the screen. If the candle be held at 
 a much greater distance than the focus, the rays will fall upon the 
 glass more nearly parallel ; and therefore, when they are n^fract- 
 ed, they tend to unite and converge behind the glass, and will 
 fojui upoji the screen a small speck of vivid light, which, if 
 closely examined, will appear a perfect image or picture of the 
 candle. 
 
 In looking through a plano-convex or double-convex lens, the 
 objects we examine will appear magnified ; consonantly with the 
 general rule, that we see every thing in the direction of that line 
 in which the rays last approached the eye ; and it will be under- 
 stood when we come to treat of the eye, that the larger the angle 
 under which any object is seen, the larger that object will appear. 
 That the convergence of the rays of the convex-lens, enlarges 
 greatly the angle of vision, and consequently enlarges the appa- 
 rent size of objects seen through it, wall be evident, if we continue 
 the lines in the direction of their convergence, thus^, lyK to L, and 
 FG to R, fig. 14. 
 
 From lenses the reverse in form to those we have hitherto 
 been considering, we shall naturally expect opposite etfects. Ac- 
 cordingly, the attractive and refractive powers of the lenses, 
 J^os. 4 and 5, fig. 12, are not towards the centre but towards 
 the circumference. Parallel rays failing upon them are made 
 to diverge, or are dispersed. Rays already divergent are ren- 
 dered more so, and convergent rays are made less convergent. 
 Hence objects, seen through one of these glasses appear smaller 
 than to the naked eye. Thus, let a 6, fig. 13, pi. 11, represent 
 an arrow, which would be seen by the eye, if no lens w'ere be- 
 fore it, by the convergent, rays ca^ib; but if. the double-con- 
 cave glass DH is interposed between the object and the eye, 
 the ray ac will be bent towards g, and the ray b i will be bent 
 towards /r, and consequently both will be useless, as they do not 
 enter the eye. The object then will be seen, by the, rays a o, 
 
 w'hich, on entering the glass, will be refracted into the lines 
 oc and ri; and according to the rule just laid down, the object 
 will be seen in the last direction of these rays ; therefore, as the 
 angle ocr, is so much smaller than the angle a c 3, the arrow ne- 
 cessarily appears diminished, and as with the diminution of its 
 apparent size, we connect the idea of its being further off, it seems 
 to be at the distance nm. 
 
 The meniscus acts lik§ a convex glass, when it is thickest 
 in the middle, that is, when its convex surface is a poiiion of 
 a less sphere than its concave one; on the contrary, when it 
 
428 
 
 OPTICS. 
 
 Refraction throiiwh lenses. 
 
 is tliiimest in the middle, or has its concave surface a portion of a 
 less sphere than the other, it has tlie effect of a concave lens. 
 Sometimes, to distinguish these different forms from each other, 
 when the glass magnilies, it is called a convexo-concave lens; 
 when it diminishes the apparent size of objects, it is called a con- 
 cavo-convex lens ; and the term meniscus is restricted to such as 
 have both their surfaces equally arched, and, therefore, like the 
 crystal of a watch, neither magnify nor diminish. 
 
 The axis of a lens, is a line supposed to be drawm through 
 the centre of its spherical surface or surfaces. When one side 
 of the lens is plane, the axis of course falls perpendicularly upon 
 that side. The axis of a lens continued, would pass exactly 
 through the centre of that sphere, of which the lens is the seg- 
 ment. I’his will be apparent from an inspection of figs. 1 1 and 
 14, and in fig. 12, a line is drawn representing the axis of the 
 lenses. When opticians mention the focal distance of a convex 
 lens, they mean the focus of parallel rays ; but when they wish to 
 be more precise, the focus of parallel rays is distinguished by the 
 term primipat focus. Mathematicians demonstrate, that the prin- 
 cipal focus of a plano-convex lens is at a distance from the con- 
 vex surface equal to the diameter of the sphere of which it is a 
 part; and that the piincipal focus of a double and eqtniily Convex 
 lens is at half the same distance. The principal focus of these 
 lenses may therefore be found, with sufficient accuracy for com- 
 mon purposes, by holding a sheet of paper behind the glass, when 
 exposed to the ravs of the sun, and observing when the spot is 
 smallest, and when the paper most readily bums. Or when the 
 focal distance does not exceed tw o or three feet, it may he found 
 by holding the glass at such a distance from the wall opposite a 
 wdndow', that the image of the sash may appear distinct upon the 
 wall. When accuracy is desirable, and the focal distance exceeds 
 two or three feet, the following method may be pursued : cover 
 the lens w ith a piece of paper or paseteboard, in which tw’o round 
 holes have been made ; each hole being at an equal distance from 
 the edge of the lens, and in one of its diameters. Direct the axis 
 of the lens thus covered to the sun, and receive upon a piece of 
 paper the ci.*'cles or white spots produced by the two holes, which 
 will be observed gradually to approach each other as the paper is 
 moved farther off ; at last they will coincide ; and if the distance 
 of the lens from the paper be increased, they will again separate. 
 The distance of the paper from the glass, when the circles coin- 
 cide, is the focal distance required. 
 
 When a lens is more convex on one side than the other, 
 and both radii are known, the following rule wdll determine 
 the focus; -as the sum of the radii of both convexities, is to 
 
OPTICS. 
 
 429 
 
 . Refraction through lenses. 
 
 the radius of either, so is double the semi-diameter of the other 
 to the distance of the focus; or divide the double product of 
 the radii by their sums, and the quotient will be the distance 
 sought. 
 
 As convex lenses cause many rays to enter the eye which 
 would otherwise have been scattered and dispersed, the objects 
 seen through them appear clearer and more splendid than when 
 viewed by the naked eye. They should always be made as thin 
 as their curvature will admit, otherwise the brilliancy of the image 
 will be lessened without necessity, by the rays which, though they 
 enter the glass, are reflected or sent back. 
 
 A large object, seen through a lens which is very convex, 
 appears more or less distorted ; this proceeds from the refraction 
 not being equal at all points, and the degree of it is proportionate 
 to the size of the object compared with the focus of the glass. 
 When the object is not much too large, its extremities only appear 
 confused. 
 
 To ascertain the focal distance of a globular glass vessel 
 containing water, such as a decanter, make a round hole, 
 about an inch diameter, in a piece of browm paper, which must 
 then be pasted on one side of the body of the decanter ; hold the 
 covered side of the decanter to the sun, that the perpendicular 
 rays may pass through the middle of the water, and the emergent 
 rays will be collected to a focus, whose nearest distance from the 
 decanter will be nearly equal to the radius of the body of it, as 
 will appear by receiving the rays upon a paper held at that dis- 
 tance. That this eftect is owing to the water, and not to the glass, 
 may be proved by emptying the decanter; for the light that then 
 passes through the hole, will be as broad as the hole itself, at all 
 the distances of the paper from the decanter, a circumstance of 
 which we might also be assured, from considering, that the sides 
 of the empty decanter can only act like two meniscus glasses 
 placed at a little distance from each other. If a solid globe or 
 ball of glass, be covered and tried like the decanter, the distance 
 of the focus from the nearest part of the ball, will be one quarter 
 of its diameter. 
 
 Parallel rays, on passing through a single or double-con- 
 cave lens, immediately diverge ; hence the term focus can- 
 not be applied to a concave lens, in the same sense as to 
 a convex lens. A concave lens has only what is termed 
 a virtual focuSj the distance of which from its surface, is 
 the same as that of a convex lens of equal radius, but it 
 is before the glass, because it denotes that point from which 
 the rays, after their last refraction, appear to diverge. To 
 And the focal distance of a concave lens, let the lens be 
 
430 
 
 OPTICS.. 
 
 Refraction thronph lenses. — Nature of the multiplying glass. 
 
 covered Mitli a piece of paper, containing two small circular 
 holes ; and on the paper for receiving the light, describe also 
 two small circles, but wiih their centres twice as far apart as 
 the centres of the circular holes Move the paper backwards 
 and forwards, till the middle of the sun’s light, coming through 
 the holes, fails exactly on the middle of the circles, and the dis- 
 tance of the paper from the lens will then be the focal length re- 
 quired. 
 
 To find the vertex or centre of a lens, hold it at some dis- 
 tance from the eye, and observe the two reflected images of 
 a candle made by the two surfaces. Move the lens till these 
 images coincide, and the point of coincidence is the vertex, 
 which, if in the middle of the surface, the glass is truly, 
 centred. 
 
 To understand the effect of the multiplying- glass. No. 7, 
 fig. 12, pi. II, it is only necessary again to revert to the general 
 principle, that objects appear in the direction of the line last 
 described by the rays that render them visible. Hence, if the ob- 
 ject B, fig. 1, pi. HI, is seen through the glass EH, by the ray 
 BA, that passes through the surface FG, the object, by the eye at 
 A, w ill be seen at B ; the ray BG passes through the surface 
 GH, and after refraction, comes to the eye in the direction AD, 
 as if it proceeded from D, and therefore the object appears at D ; 
 and for the same reason, through the surface FE, it appears at C; 
 consequently, there will be the appearance of as many objects as 
 there are flat surfaces on the glass, for each of them shews the 
 same object in a different place.. 
 
 To become familiar with the laws of refraction, and the pro- 
 perties of lense'j, the student should make experiments with lenses 
 of different foci, diameters, atid colours. The room should be 
 darkened, and the sun’s rays admitted through an aperture cut in 
 the shutter, or through a tube placed in the shutter, and moveable 
 in any direction. "I lie lenses should be fitted into cells connected 
 with a sliding board, or adapted to convenient frames. By these 
 iTieans, the lenses may be combined in any way required. It 
 would be proper, also, to have lenses ground to the figure of a, 
 meniscus, or watch-glass, and fftted up so that two or more of 
 them might be connected in one frame, with a view to include 
 fluids between them, and thus exemplify the refractive powers of 
 fluid lenses. Tlie dust usually in molion, will, in the darkened 
 room, give the various and natural figures of the converging and. 
 diveiging pencils of ra)S. 
 
OPTICS. 
 
 431 
 
 Reflection the effect of repulsion. 
 
 Of the Reflexibility of Light. 
 
 The disposition or capability of the rays of light to be turned 
 back into the medium from whence they came, is called their re- 
 fiexibility : the change of direction produced by their being actu* 
 ally turned back, is called refection. 
 
 The property which bodies possess of reflecting light, is attri- 
 buted by Sir Isaac Newton to the principle of repulsion. In 
 confirmation of this theory, it is justly remarked by him, that those 
 surfaces, which to our senses appear smooth and polished, are 
 found, when viewed through a microscope, to abound with ine- 
 qualities ; if, therefore, the power which produces reflection, did 
 not act at some distance from the reflecting surface, these inequa- 
 lities would prevent the rays from being reflected with so much 
 regularity as we find they are. Other theories have been pro- 
 posed, to account for reflection, but none of them appears more 
 probable in itself, or so reconcileable with the facts connected 
 with this property of light. But however well founded the opi- 
 nion, that light is reflected by the power of repulsion, and con- 
 sequently before it comes into absolute contact with the reflecting 
 surface ; it will not be necessary to introduce the circumlocu- 
 tion required to express this idea, in treating of the general phe- 
 nomena ; the incident rays will always be spoken of as ac- 
 tually impinging on the reflecting body, from which they re- 
 bound, as a perfectly elastic ball would rebound from a marble 
 slab. 
 
 All objects which are not themselves luminous, are rendered 
 visible by reflection. Even glass, crystal, water, and the most 
 pellucid media, reflect a part of the rays of light which fall on 
 them, or their forms and substance could not be distinguished ; 
 on the contrary, the whole of the incident light is not reflected 
 from any surface, however bright, smooth and opake. It is calcu- 
 lated that the best mirrors reflect little more than half the light 
 they receive. 
 
 From the account we have given of refraction and other 
 properties of light, the student is already fully apprized, that it 
 is the rays emanating from any given point, which assure our 
 sight of the existence of a body at that point; and that, as in the 
 instance of a fish in the water, if the direction of the rays be 
 changed in any manner, the sense of sight, always referring the 
 place of an object to the point in the direction of the last course 
 .of the rays, is easily deceived, and will suppose an object to be 
 where it is not. This it will be necessary to bear constantly in 
 mind while we pursue our present subject; for in co«inexion with 
 
432 
 
 OPTICS. 
 
 Universal law of reflection, — Reflection from plane mirrors. 
 
 another law of nature, it forms the key by which all the pheno- 
 mena of reflection are explained. The other law to which we 
 allude, is, that the angle of reflection is always efpial to the angle 
 of incidence.. In fig. (), pi. I, a d is an incident ray, falling up"un 
 the surface ef, from which it is reflected in the direction i A; 
 hence the angle of incidence, a dc/is exactly equal to the angle 
 of reflection b dc, from on either side of the perpendicular c d, the 
 obliquity of the ray is the same, and consequently when the re- 
 flecting surface is a plane, the obliquity of the line of incidence 
 and reflection are also equal, when measured from it. It is only 
 the rays which fall obliquely upon a reflecting surface, that 
 are reflected in an angle to their direction of incidence ; those 
 rays which fall perpendicularly, return in precisely the same di- 
 jectiou. 
 
 To pursue the consequences deducible from these principles, 
 or their application to particular effects, will soon shew how 
 much depends upon them. Let NO, fig. 2, pi. Ill, be con- 
 sidered as a ray of light striking perpendicularly on the surface 
 of the mirror AB, and it is reflected back in the same line. 
 The ray DO, coming from the luminous body D, strikes the 
 mirror obliquely, and is reflected to the eye in the line OE, thus 
 making the angle of reflection, NOE, equal to the angle of 
 incidence DON, and the object is seen at S, in the direction of 
 the reflected ray. To be a little more explicit, with regard to 
 the last particular, it must be recollected, that an object is 
 rendered visible, not by single rays proceeding from every 
 point of its surface, but by pencils of rays, or collections of 
 divergent rays issuing from every point. These pencils of rays 
 are afterwards, by the refractive powers of the eye, converged 
 again to points upon the bottom of the eye or retina, and 
 these points of convergent rays in the eye, are correspondent 
 to the points of the objects, from which the rays diverged. 
 Hence, as the pencils of rays strike the mirror while they are in 
 .heir divergent state, from the equality of the angles of incidence 
 and reflection, they are reflected in the same state, and con- 
 verge exactly as they would have done if they had not been 
 intercepted by the mirror, but had gone on to G, a distance 
 equal to the incident and reflected course taken together. In 
 consequence of this identity in the convergence of the rays, 
 and the general rule that the place we assign to objects is 
 in the last direction of the rays, the two lines, namely, that from 
 the object to the mirror, and that from the mirror to the eye, 
 are united in the mind of the spectator, and the object is conse- 
 quently seen at S, just as far behind the mirror as the object is 
 before it. Another figure, will, perhaps, render this more 
 
OPTICS 
 
 JRZ.I. 
 
 J JLiy€n’^oo7 Ap” -3(7 IS7S. 
 
OPTIC S . 
 
 PL. II. 
 
 / 
 
 Zis^urr- SdixcH. In cl 7i/'4. 
 
OPTICS. 
 
 433 
 
 Reflection of light. 
 
 evident: the lines DC, fig. 3. pi. Ill, are the lines of inci- 
 dence, CB are the lines of rellection, and these form equal 
 angles on the surface of the mirror, so that all the rays coming 
 from the object, and falling upon the mirror at C, will strike 
 the eye at B, and the reflected image will thus become visible. 
 Now every object appears to be in a straight line from the eye, 
 and as no alteration is made in the relative position of the rays, 
 but only in their direction, the body DD, when it comes reflected 
 to the eye, will appear to be at A A, because rays from thence 
 would, at the mirror, exactly coincide with the rays DC, DC, and 
 would, by the time they arrived at BB, be converged to an equal 
 focus. 
 
 Hence we shall not find it difficult to understand why a man 
 may see his whole figure in a plane looking-glass only half as long 
 and half as broad as himself ; for the image is seen under an angle 
 as large as the life : the mirror is exactly half way between the 
 image and the eye, and therefore the divergent rays, by which the 
 image*" is seen, only cover at the glass, a space equal to half the 
 size of the man himself. 
 
 When light is reflected from convex or concave minors, it 
 obeys precisely the same law as from plane mirrors, however 
 extraordinary the effects they produce may at first sight appear. 
 To understand the manner in which they act, it will be proper 
 to recur to an observation formerly made respecting spherical 
 surfaces: all curves or arches are to be considered as com- 
 posed of a multitude of infinitely short lines or points, lying 
 obliquely to one another. Parallel rays, therefore, fall upon 
 them more or less obliquely at every point of incidence except 
 the centre. This part of the subject may perhaps be best eluci- 
 dated, if we consider, in the first place, the direction given to 
 rays which fall upon two plane mirrors inclined to each other 
 in opposite directions. Thus, in fig. 4. pi. Ill, the rays AB, 
 CD, which would fall perpendicularly on a flat surface in the 
 direction IK, strike obliquely upon that which is opposed to 
 them, and instead of being reflected parallel, are reflected diver- 
 gently. For the same reason, convergent rays would be re- 
 flected with less convergence by such a surface as this, and di- 
 vergent ravs would be rendered still more divergent towards E 
 and H. 
 
 Fig 5, which is the reverse of the preceding, will serve to 
 shew us the nature of reflection from concave surfaces. Here 
 the parallel rays AB, CD, which would have been reflected pa- 
 rallel by a plane mirror, are made to converge, and meet at L, 
 because, instead of striking this mirror in a direct line, they strike 
 it obliquely, and therefore convergent rays will be reflected with 
 19.— VoL. I. 3 K 
 
434 
 
 OPTICS. 
 
 Reflection of light. 
 
 greater convergence, and divergent rays will have their divergency 
 lessened. 
 
 As a convex mirror diminishes the convergency of the rays 
 which fall upon it, the effect it produces is, to diminish the ap- 
 parent size of objects ; because the visual angle is diminished, that 
 is, there is not so large a picture formed in the eye, as when the 
 object itself is seen without the mirror. But a concave mirror, on 
 the contrary, enlarges the visual angle, or picture of an object on 
 the eye, and consequently its effects are the reverse of the former. 
 Reflection, then, from a convex mirror, corresponds to refraction 
 through a concave lens ; and upon the same principle, reflection 
 from a concave surface corresponds to refraction through a convex 
 lens. 
 
 More particularly to exemplify this theory, let AB, fig. 6,' 
 pi. Ill, represent a dart, which is seen in the convex mirror 
 CD. Though rays issue from the object AB in all directions, 
 yet it is seen only by means of those which are included within 
 the space ON, because no other can be reflected to the eye at 
 R. If the rays had gone forw ard in their original direction, they 
 w ould have united at P, and the object would have been seen of 
 its full size; but as, from the nature of a convex curve, the angle 
 of reflection cannot be equal to the angle of incidence w ithout di- 
 minishing their convergence, the angle SRT is less than the angle 
 APB, and the image at L appears smaller than the object, and 
 nearer to*the surface of the mirror. The reason of this last effect 
 has been already explained, when it was observed, that objects 
 are rendered visible, not by a single ray, but by pencils of diver- 
 gent rays proceeding from every point of them ; and that the eye 
 transfers the place of the object to the place where these pencils 
 of rays unite in points. Suppose, then, a radiant point G, fig. 7, 
 sends forth a pencil of divergent rays which falls on the convex 
 mirror AB ; the rays of this pencil will diverge still more after 
 reflection, and therefore they will appear to come from a point 
 nearer the eye than if their divergency had not suffered any 
 change. 
 
 For these reasons, a person looking at his face in a convex 
 mirror will see it diminished. This is shewn by fig. 8, pi. Ill ; 
 though rays proceed from every part of the face, it is only the rays 
 that fall on the mirror between C and R, that can be reflected to 
 the eye ; the rays CR being therefore rendered less convergent, he 
 will see his chin along the line OS, and the forehead along the line 
 ON ; the angle of vision being thus reduced, all the rest of the 
 features will be proportionately reduced. 
 
 When large objects are seen in a convex mirror, they not only 
 appear, according to its curvature, smaller than they leally are, 
 
OPTICS. 
 
 435 
 
 Reflection of light. 
 
 but somewhat distorted. The reason of this is, that the different 
 points of the object are not at an equal distance from the mirror, 
 and consequently they are reflected [to the eye under different an- 
 gles. Glass globes, lined, like looking-glasses, with an amalgam 
 that converts them into mirrors, are sometimes suspended in apart- 
 ments as an ornamental piece of furniture. In these, the com- 
 pany seated in a room, or round a table, are represented by very 
 minute images, which appear very near their surface, and are al- 
 ways, though beautiful, in some degree distorted. Latterly, con- 
 vex mirrors, fitted in a frame, have almost superseded the globes, 
 over which they have several advantages : they can be placed 
 against a wall as conveniently as a picture ; they can be made the 
 segment of a larger sphere than it would be convenient to have 
 entire ; and in particular, they can be ground to a regular figure, 
 whereas the globes are only blown glass, and therefore can never 
 be true. 
 
 The effects of concave mirrors may be gathered from the c n- 
 siderations already premised; but though it may introduce some 
 repetition, it will be proper to consider them separately. Their 
 general effect is to render convergent rays more convergent, and 
 consequently they have the power of magnifying. By fig. 9, ph 
 III, is represented a face looking at itself in the concave mirror 
 IK, and the extreme of those rays which can be reflected to the 
 eye are exhibited, one from the forehead, and one from the chin. 
 These lines, a c and m n, are reflected to the eye at O, which sees 
 the image in the line of reflection, and in the angle DOQ, and 
 therefore evidently magnified, aJid at a small distance behind the 
 minor. 
 
 The magnifying effect of a concave mirror, is, however, only 
 perceived when the object is nearer to it than its centre of con- 
 cavity, or centre of the sphere to which the curve of the 
 mirror belongs. When we come to the description of the 
 
 telescope, the reader wdll better understand how the image is 
 formed by the large concave mirror of that instrument, if we here 
 refer to another diagram. Let A c B, fig. 10, pi. Ill, be the 
 reflecting surface of a mirror, whose centre of concavity is at 
 C ; and let the upright object DE, be placed beyond the centre 
 C, and send out a conical pencil of divergent rays from its 
 upper extremity D, to every point of the concave surface of 
 the mirror, A c B. But to avoid confusion, we only draw 
 three rays of that pencil, as DA, Dc, DB. From the centre of 
 concavity, C, draw the three right lines, CA, Cc, CB, touching 
 the mirror in the same points as the three rays of the pencil 
 from D, and all these lines will be perpendicular to the surface 
 of the mirror Make the angle CA equal to the angle D AC, 
 
436 
 
 OPTICS. 
 
 Reflection of light. 
 
 and draw the right line A for the course of the reflected ray 
 DA : make the angle Q c d, equal to the angle D c C, and draw 
 the right line c r/, for the course of the reflected ray Dc; make 
 also the angle CB^?, equal to the angle DBC, and draw the 
 riglit line Br/, for the course of the reflected ray DB. All these 
 reflected rays will meet in the point dj where they will form the 
 extremity, d, of the inverted image, e d. similar to the extre- 
 mity, D, of the upright object DE. If the pencil of rays, 
 
 E E gy E hy be also continued to the mirror, and their 
 
 angles of reflection from it be made equal to their angles of 
 incidence upon it, as in the former pencil from D, they will 
 meet at the point e, by reflection, and form the extremity c, of 
 the’ image, e d, similar to the extremity, E, of the object, DE. 
 As each intermediate point of the object betw een D and E, sends 
 out a pencil of rays in like manner to every part of the mirror, 
 the rays of each pencil will be reflected back from it, and meet 
 in all the intermediate points between the extremities, e and dy 
 of the image; and hence the whole image will not be at f, that 
 is, at half the distance of the mirror from its centre of conca- 
 vity C, but at a greater distance between i and the object DE; 
 and the image will be inverted with respect to the object. Thus 
 it is, that when the object is more remote from the mirror than 
 its centre of concavity C, the image appears less than the 
 object, and l)etween the object and the mirror ; contrary to w hat 
 is observed when the object is nearer than the centre of conca- 
 vity, as in the example of the face looking at itself, fig. 9, ph 
 III. If DE, fig. 10, be the object, c will be its image; for 
 as the object recedes from the mirror, the image approaches 
 nearer to it, and as the object approaches nearer to the 
 mirror, the image recedes further from it, on account of the 
 less or greater divergency of the pencils of the rays which 
 proceed from the object ; for the less they diverge, the sooner 
 they are converged to points by reflection; and the reverse 
 necessarily follows, that the more they diverge, the further 
 they must be reflected before they meet. If the radius of the 
 mirror’s concavity, and the distance of the object be known, 
 the distance of the image from the mirror is found by this rule : 
 Divide the product of the distance and radius by double the 
 distance made less by the radius, and the quotient is the distance 
 required. If the object be in the centre of the mirror’s con- 
 cavity, the image and object will be coincident, and equal in 
 bulk. 
 
 If a man place himself directly before a large concave mirror, 
 but further from it than its centre of concavity, he will see an in- 
 verted image of himself in the air, between him and the mirror, of 
 
OPTICS. 
 
 437 
 
 Experiments with concave mirrors. 
 
 a less size than his own person. If he hold out his hand towards 
 the mirror, the hand of the image will come out towards his hand, 
 and coincide with it, of an equal bulk when his hand is in the cen- 
 tre of concavity; and he will imagine he may shake hands with his 
 image. If he reach his hand further, the hand of the image will 
 pass by his hand, and come between it and his body ; and if he 
 move his hand towards either side, the hand of the image will move 
 towards the other; so that whatever way the object moves, the 
 image will move the contrary w'ay. A by-stander will see nothing 
 of the image, because none of the reflected rays that form it enter 
 his eyes. 
 
 From this remarkable property of a concave mirror to form 
 an image in the air, mirrors of this sort are used to produce a 
 variety of singular appearances, to amuse the curious, or to 
 impose upon the ignorant and superstitious. To a few we shall 
 give a place. If a lire be made in a large room, and a smooth 
 mahogany table be placed at a considerable distance near the 
 wall, before a large concave mirror, so situated that the light of 
 the lire may be reflected from the mirror to its focus upon the 
 table ; if a person stand by the table, he will see nothing upon 
 it but a longish beam of light ; but if he stand at a distance 
 towards the lire, not directly between llie fire and the mirro!:, 
 he will see an image of the fire upon the table, large and erect. 
 If another person, who knows nothing of the experiment before- 
 hand, should chance to come into the room, and should look 
 from the fire towards the table, he w'ould be startled at the appear- 
 ance; for the table would seem to be on fire. In this experiment, 
 there should be no light in the room, but what proceeds from 
 the fire; and the mirror ought to be at least fifteen inches in dia- 
 meter. 
 
 If the fire used in the last experiment be extinguished or covered 
 by a screen, and a large candle be placed in a similar position, a 
 person standing by the candle will see the appearance of a star, or 
 rather planet, upon the table, as brilliant as Venus or Jupiter in 
 a cloudless sky. If a slender wax taper be placed near the candle, 
 a satellite to the planet will appear on the table ; and if the taper 
 be moved round the candle, the mimic satellite will go round the 
 planet. 
 
 Another experiment is the following : Take a glass bottle, 
 partly fill it with water, and cork it in the common manner ; place 
 this bottle opposite a concave mirror, and beyond its centre of 
 concavity, that it may appear reversed; let the spectator retire to 
 a still greater distance than the bottle, which he w ill then see in tlie 
 air inverted, and the water which is actually in the lower part of 
 the bottle, will in the image appear uppermost. Invert the bottle 
 
438 
 
 OPTICS. 
 
 Experiments with concave mirrors.— Foci of spherical mirrors. 
 
 before the mirror, and in the image the water will appear 
 in the lower part of the bottle : when it is in this inverted slate, 
 uncork the bottle, and whilst the water is running out, the image 
 will appear to be filling ; but as soon as the bottle is empty, the 
 illusion ceases. 
 
 As the image formed by a concave mirror may be thrown through 
 a hole into an adjoining room, where a spectator will in certain 
 situations perceive the image of any object the concealed manager 
 may chuse, it is evident how much the credulous may be imposed 
 upon. Birds, angels, &c. may be represented flying, and may 
 be instantly changed for other objects. A nosegay may be sliewn, 
 and when the beholder attempts to grasp it, a dagger, with all the 
 appearance of reality, may be presented to his breast, or a death’s 
 head snap at his hand. 
 
 . To find the focal distance of a convex mirror or speculum, 
 cover it with paper containing two pin-holes, one near each edge 
 of the mirror; expose it to the sun, holding another paper be- 
 fore it that has a hole large enough to let the solar rays pass 
 through to the two pin-holes. Tw’O white spots of reflected 
 light will be observed on each side of the hole; move the paper 
 backwards and forw^ards, till the distance of the spots be twice the 
 distance of the holes in the cover ; and that distance of the paper 
 from the cover is its focus. 
 
 To find the focal distance of a concave speculum, place it so 
 that its axis may be directed to the centre of the sun. Find the 
 burning point, or receive the image upon a white piece of paper, 
 and the distance thus found between the paper and the centre 
 of the speculum, is the focal length. Or, cover the mirror 
 w ith paper, in which make two or more holes, and observe where 
 the beams of , light reflected from these 'holes unite, and this 
 will be the focal distance. Or, lastly, place the speculum at 
 the end of a long table, in a vertical position ; place a candle 
 at the opposite end of the table, so that its flame may be op- 
 posite to the axis of the speculum; then take a piece of 
 white paper, and having fixed it to stick, place the stick in the 
 socket of a candlestick, so that the paper may be supported 
 at about the same height with the candle; then move the paper 
 or the candle forwards or backwards, till the image of the candle 
 on the paper is exactly over the candle itself, and the point 
 of coincidence is the centre of the speculum. 
 
 For purposes of amusement, mirrors are frequently made 
 in the form of entire or half cylinders or cones. These are 
 called mixed mirrors, as they produce, at the same instant, the 
 effects of plain and spherical mirrors. The properties of cylin- 
 drical mirrors arc, 1. The dimensions of objects corresponding 
 
oi’L'irs. 
 
 ri..in 
 
 
 
 Vuj. /. 
 
 2'ki. 
 
 J'if(>/tjM Ov Siiff.iU.FhluTkTU.w'ri.T.iVi'rpccl.Ar: - 'Sa. 
 
OPTICS. 
 
 439 
 
 Cylindrical and conical mirrors. — Prismatic dissection ot' light. 
 
 icngthwise to the mirror, are not much changed ; but those 
 corresponding breadtlnvise liave their bgure altered, and their 
 dimensions lessened, the further they are from the mirror, 
 whence arises a very great distortion. 2. If the plane of the 
 reflection cut the cylindrical mirror through the axis, the reflec- 
 tion is performed in the same manner as in a plane mirror; and if 
 parallel to the base, the reflection is the same as in a spherical 
 mirror; if it cut it obliquely, the reflection is the same as in an 
 elliptic mirror. Hence, as the plane of reflection never passes 
 through the axis of the mirror, except when the eye and the object 
 are in the same plane ; nor parallel to the base, except when the 
 radiant point and the eye are at the same height ; the reflection 
 is therefore usually the same as in an elliptic one. 3. If a 
 hollow cylindric mirror be directly opposed to the sun, instead of 
 a focus of a point, the rays will be reflected into a lucid line 
 parallel to its axis, at the distance of about one-fourth of its dia- 
 meter. 
 
 Conical mirrors produce a still more extraordinary distortion of 
 the flgures seen in them than cylindrical ones, from the breadth of 
 the image being gradually reduced as it approaches the apex, where 
 it becomes a mere point. The effect of a cylindrical mirror in di- 
 minishing the breadth of objects facing it, being the same as that 
 of a convex mirror of the same radius, and the effect of the conical 
 mirror, at any given height, being regulated by the same principle, 
 it becomes easy to draw anamorphoses for a cylindrical or conical 
 mirror of any size. The pictures called by this name, and which 
 are sold with mixed mirtors by the opticians, appear excessively 
 wild and deformed to look at, but when placed before the mixed 
 mirror for which they are intended, their images are well propor- 
 tioned and beautiful. 
 
 Of the different Refrangibility of the Rays of Light. 
 
 The different refrangibility of the rays of lights, which we have 
 now to consider, is demonstrated by means of the prism, a name 
 which opticians give to a solid piece of glass, of any length at 
 pleasure, but of a wedge-like or triangular form. As any section 
 of a prism, passing through its axis, is a triangle, it is always re- 
 presented by a triangle, which may be considered as shewing cor- 
 rectly one of its ends, see fl^. 12, No. 8, pi. II. 
 
 If a beam of light from the sun be let into a darkened room, 
 and be received upon a white screen or opposite wall, it will 
 form a circular image, and will be of one uniform whiteness. 
 If a prism be interposed, so that the light must pass through it 
 before it reaches the wall, the image is no longer circular and 
 
440 
 
 OPTICS. 
 
 Prismatic dissection of light. 
 
 no longer white. It assumes an oblong shape, terminated by 
 semi-circular arches, and exhibits seven different colours. This 
 oblong image is called the spectrum, and from its being pro- 
 duced by the prism, the prismatic spectrum. In the compass 
 of philosophical experiment, a more beautiful appearance cannot 
 be presented to the eye ; and its instructive nature will appear not 
 less extraordinary than its beauty, when it is considered, that the 
 investigation of the cause of it, led Sir Isaac Newton to form 
 the first rational theory of the cause of colours. The manner in 
 which this illustrious philosopher tried the experiment of the 
 spectrum, is described by himself in the following words, which 
 remain to this day, perhaps, the most suitable introduction to this 
 subject : 
 
 ‘^In a very dark chamber, at a round hole, F, fig. 1. pi. IV, 
 about one-third of an inch broad, made in the shutter of a 
 window, L placed a glass prism, ABC, whereby the beam of the 
 sun's light, SF, which came in at that hole, might be refracted 
 upwards, toward the opposite wall of the^chamber, and there 
 form a coloured image of the sun, represented at PT. The axis 
 of the prism (that is, the line passing through the middle of the 
 prism, from one end of it to the other end, parallel to the edge 
 of the refracting angle) was in this and the following experiments, 
 perpendicular to the incident rays. About this axis 1 turned 
 the prism slowly, and saw the refracted light on the wall, or 
 coloured image of the sun, first to descend, and then to ascend. 
 Between the descent and ascent, when the image seemed station- 
 ary, I stopped the prism, and fixed it in that posture. Then 
 I let the refracted light fall perpendicularly upon a sheet of 
 white paper, MN, placed at the opposite wall of the chamber, 
 and observed the figure and dimensions of the solar image, PT, 
 formed on the paper by that light. This image was oblong, 
 and not oval, but terminated by two rectilinear and parallel 
 sides and two semi-circular ends. On its sides it was bounded 
 pretty distinctly, but on its ends very confusedly and indis- 
 tinctly, the light there decaying and vanishing by degrees. 
 At the distance of 184- from the prism, the breadth of the 
 image was about 2|- inches, but its length was about 10| inches, 
 and the length of its rectilinear sides about eight inches; and 
 ACB, the refracting angle of the prism, whereby so great a 
 length was made, was 64 degrees. With a less angle, the 
 length of the image was less, the breadth remaining the same. 
 It is farther to be observed, that the rays went on in straight 
 lines from the prism to the image, and therefore, at their going 
 out of the prism, had all that inclination to one another from 
 which the length of the image proceeded. This image PT w'as 
 
OPTICS. 
 
 441 
 
 Prismatic dissection oflight. 
 
 coloured, and the more eminent colours lay in this order from the 
 bottom at T to the top at P ; red, orange, yellow, green, blue, in- 
 digo, violet, together with all their intermediate degrees, in a con- 
 tinual succession, perpetually varying.” 
 
 If the spectrum be divided into 100 parts, the red part of it 
 is found to occupy 11 of these parts, the orange 8, the yellow 14, 
 the green 17, the blue 17, the indigo 1 1, and the violet 22. The 
 red part of the spectrum, it will be observed, is nearest the prism^ 
 and the violet at the greatest distance. It is clear, therefore, that 
 light is not homogeneous, because the attractive power of the 
 prism has been greater upon some parts of it than upon other 
 parts. Accordingly, it is universally concluded, that the solar 
 beam, or white light, is composed of particles differing in size 
 or density ; that this difference of their size or density, is the 
 cause of their being differently refrangible, and that the sepa- 
 ration of the rays of one or more sizes from the rest, by various 
 means, produces all the diversity of colours which affect our 
 
 \Ve find the red part of light capable of struggling through thick 
 and resisting mediums, when all the other colours are stopped. 
 Thus the sun appears red when seen through a fog; the light of 
 distant lamps seen through the smoke of a long street, appears 
 red, while the light of those at hand is white. Dr. Halley’s hand 
 appeared red in the w^ater, when he was in a diving-bell at the bot- 
 tom of the sea, and red light always makes the strongest impres- 
 sion on the eye. These are all proofs, that red light consists of 
 particles which are larger than those of other colours, and which 
 having therefore the greatest momentum, are the least disturb- 
 ed by the action of a given force, so that they necessarily take 
 a shorter course than other rays in passing through the prism. 
 The particles which compose orange light, are next in size and 
 refrangibility to the red, and so on to the violet, which consists of 
 the smallest sixed particles, and they are therefore the most turned 
 out of their course. 
 
 The seven colours of the spectrum, are called the original 
 or primary colours. White is composed of them all mixed 
 together in their due proportions; for if a solar ray, separated 
 by the prism into its component parts, be collected and con- 
 verged into a focus by a lens or a concave mirror, a paper 
 placed perpendicularly in this point will exhibit a white spot. 
 The same conclusion may be drawn from the experiment of 
 mixing together paints of the same colours as the parts of the 
 spectrum, and in the same proportion ; the mixture will be 
 white, though not a resplendent whiteness, because the colours 
 mixed are less bright than the primary ones, and beeause they 
 19.— VoL. I. 3 L 
 
44 £ 
 
 OPTICS. 
 
 Permanency of the primary colours. 
 
 cannot be intimately blended without exercising a chemical ac- 
 tion on each other, which in part ^changes their properties. When 
 this action is prevented, the i esult of the experiment is much more 
 satisfactor> : the nin of a wheel may either be painted or covered 
 with pieces of cloth, &c. of all the seven colours in due propor- 
 tion, and if it be then swiftly turned upon its axis, it will appear 
 to be white. Supposing the wheel to be divided into 360 equal 
 parts, the red should occupy 43 of these parts, the orange 27, 
 the yellow 48, the green 60, the blue 60, the indigo 40, and the 
 violet 80. 
 
 It may be supposed that the seven primary colours may be re- 
 duced to three, viz. red, blue, and yellow, because of these three 
 all the rest might be made. It is necessary therefore to explain 
 the sense in which it is understood that there are seven primary 
 colours. None of the colours thus named can be changed by any 
 art, or whatever number of refractions or reflections they undergo. 
 Let a hole be made in the paper or screen receiving the prismatic 
 spectrum, at the place covered by the green rays ; if these rays be 
 refracted through fifty prisms, and as often reflected from mirror 
 to mirror, they still retain their greenness : hence we are com- 
 pelled to consider the green rays as composed of simple homoge- 
 neous particles, consequently one of the primary colours ; and the 
 same may be said of any other colour, which might be supposed 
 to be compound. 
 
 When any coloured body is placed in homogeneous light, 
 it always appears of the colour of the light in which it i* 
 placed, whatever may be its natural hue ; thus, if prussian 
 blue and vermillion be placed in a red light, both will appear 
 red; in a green light, green; in a blue light, blue; &c. It is, 
 however, to be allowed, that a body appears brighter when in a 
 light of its own colour than in another ; and from this we may 
 see that the colours of natural bodies arise from an aptitude in 
 them to reflect some rays more copiously and strongly than 
 others. But lest this phenomenon should produce a doubt of 
 the constancy of the primary colours, it is proper to assign the 
 reason of it, which is this : that when placed in its own 
 coloured light, the body reflects the rays of the predominant 
 colour more strongly than any of those intermixed with it; 
 therefore the proportion of the rays of the predominant colour 
 to those of the others, in the reflected light, will be greater than 
 in the incident light ; but when the body is placed in a light of 
 a different colour from its own, for a similar reason, the contrary 
 effect will follow, that is, the proportion of the predominant 
 colour to the others will be less in the reflected than in the 
 incident light ; and therefore as its splendour 'itould be greater 
 
OPTICS. 
 
 443 
 
 Rays of light not actually coloured. — Cause of the rainbow. 
 
 in the former case, and would be less in the latter, than if all 
 the rays were equally reflected, the splendour of the predomi- 
 nant colour will be much greater in the former case than in the 
 latter. 
 
 When bodies either reflect all the rays of light which they 
 receive, or reflect them in the proportion in which they exist in 
 the solar beam, they appear white ; when they reflect none of the 
 rays, they appear black. Betw een perfect w hite and perfect black, 
 innumerable species of colours may be formed by different com- 
 binations of the rays, under different influences of reflection or 
 transmission. 
 
 When the expressions, red rays, yellow rays, &c. are used, 
 it is not to be understood that the rays themselves possess any pro- 
 perty analogous to what we call colour, but merely that they have 
 the power of exciting in us the sensations which we call redness, 
 yellowness, &c. They would, therefore, with more correctness, 
 be called red-making or red-causing rays, yellozc-making or yellow- 
 causing rays, &c. but the frequent recurrence of this precision 
 would be as aw kward as it is unnecessary. 
 
 Of the Rainbow and other natural Phenomena dependent upon 
 the Refrangibility of Light. 
 
 Of the natural phenomena produced occasionally by the sepa- 
 ration of the primary colours, the rainbow* is one of the most beau- 
 tiful. This meteor, which in poetical language is called the his, 
 never makes its appearance, except when the spectator is situated 
 between the sun and a shower of rain ; hence we conclude that it 
 is occasioned by the sun and the drops of rain; and that this con- 
 clusion is just, any one may satisfy himself by the following expe- 
 riment : Fill a hollow glass globe with water, and suspend it in 
 the sun, in such a manner that it may be raised or lowered at plea- 
 sure; at a certain height above the eye of the spectator, who looks 
 at it with his^ back to the sun, the globe will appear to be red; let 
 it then be slowly low'ered, and it will appear to be orange, and 
 afterwards in succession, as it descends, it will appear yellow, 
 green, blue, indigo, and violet. Hence the same drop of rain, 
 which must be considered as a little globe, supplies all the seven 
 colours to the eye, just as they would be supplied if, in trying the 
 experiment of the spectrum, the operator were to remove the pa- 
 per MN, fig. 1, pi. IV, and to place his eye so that it would re- 
 ceive the red rays, when, if he gradually raise himself, or lower 
 the aperture for the light and the prism together, he would per- 
 ceive all the other colours in succession. 
 
 There are sometimes two rainbows seen at the same time, 
 
444 
 
 OPTICS. 
 
 T''iffercnce between the exterior and interior rainbow. 
 
 one wiLliin the othti, and, what may stefii remarkable, the order 
 of the colours of the eximor bow is the reverse of that of the in- 
 terior one. Wlien two bows are seen, the exterior one is compa- 
 ratively faint, and it is therefore sometimes called the false or se- 
 condary bow, while the greater distinctness of the interior one has 
 obtained for it the appellation of the primary bow. To trace the 
 progress of a ray of light through a drop of rain in each of these 
 bows wiW explain the cause of their dilfering in brightness. 
 
 In the true or primary bow, the rays of light arrive at the 
 spectator’s eye after two refractions and one reflection. Thus, 
 let A, tig. 2, pi. IV, be a drop of rain, and Sc? a ray from 
 the sun falling on the upper part of the drop at d. It will 
 suffer a refraction, and instead of going forw'ard to c, it will 
 be bent to n ; at n, part of it will emerge, but the remainder 
 will be reflected to g ; at g it w ill be again refracted on passing 
 into the air tow ards the eye at h ; by being thus twice refracted 
 and once reflected, the ray is separated into its primitive 
 colours ; the red part, w hich is i.:.‘^st throw n out of its course, 
 makes an angle, at its emergence, with the incident solar 
 ray of forty degrees two minutes, as ^ f h; and the violet, 
 being the most easily thrown out of it course, makes with the 
 solar light an angle of forty degrees seventeen minutes, as S c g. 
 The difleient colours, therefore, at the distance of the spectator, 
 are considerably separated, and affect the eye in succession with 
 the seven colours ; but the succession is so quick, and so many 
 drops fall through the same circuit in the same time, that the mind 
 loses the idea oi succession, and the bow seems permanent as long 
 as the shower continues in a proper direction from the eye. 
 
 The exterior or secondary bow is formed by two reflections and 
 tw o refractions. Let B represent one of the drops of rain forming 
 this bow ; a ray, T, from the sun, falling upon it at r, is refracted, 
 and falls upon the back of the drop at s ; at s, from the transpa- 
 rency of the drop, a portion of it passes through towards Wy but 
 the remainder of it is reflected towards t; here again, for the same 
 reason as before, part of it emerges from the drop, in the direction 
 Xy but the portion still left is reflected to w, where it is refracted 
 towards the spectator, with the red rays uppermost. I'he great 
 quantity of light lost at each reflection, is the cause of the indis- 
 tinctness of this bow, and therefore w e cannot be surprised that w'e 
 rarely, if ever, see bows formed by a still greater number of re- 
 fltrctions within the drops ; for though they may exist, they are too 
 faint to be seen. Tlie secondary bow' cannot be seen when the 
 elevation of the sun is above fifty-four degrees seven minutp ; it is 
 broader than the interior bow, because the rays are more dispersed 
 before they reach the eye. 
 
OPTICS. 
 
 445 
 
 Rainbow. — Colours of the sky. 
 
 When the spectator is upon a plain, and the sun is close to the 
 horizon, the rainbow is a semi-circle; but according as the sun 
 is higher above the horizon, so the rainbow is a smaller part of a 
 circle. When the spectator is upon an eminence, and the sun is 
 near the horizon, then the rainbow may exceed a semi-circle ; and 
 from a great elevation, a complete circle maybe perceived. In 
 all cases, the centre of the bow, the spectator, and the sun, must 
 be ill the same straight line, which is called the line of aspect. 
 The sun may therefore be considered as the vertex of a cone, 
 whose base is the rainbow, and whose axis constitutes the line of 
 aspect, if several people are standing together, they each see 
 a different rainbow, because they are each in the axis of a 
 different cone. Hence the boNv moves as the spectator moves, 
 but other drops in another circle producing the same effect, the 
 change is not perceived. The bow assumes a circular form, 
 because it can only be seen by rays which form the same angle 
 with the line of aspect. The primary bow can never be seen, if 
 the sun be elevated more than forty-two degrees two minutes above 
 the horizon. 
 
 Lunar rainbows are sometimes observed, but the faintness of 
 the moon’s light, compared with that of the sun, is the reason that 
 their colours can very rarely be distinguished. 
 
 When the sun and the eye of the spectator are properly situ- 
 ated, drops of water produce the phenomenon under a variety of 
 other circumstances. A bow with its convex side towards the spec- 
 tator, is sometimes seen on the ground, when the sun shines on a 
 very thick dew ; also at waterfall’s, and at sea, when part of the 
 agitated water is dispersed in drops, the bow is seen in its usual 
 form. VV’^ater blown violently from, the mouth of an observer, will 
 also produce a miniature iris, and by means of a small fountain, an 
 artificial one may be shown by candle-light. 
 
 The diversified colours of the sky under different circum- 
 stances, are also accounted for on the principle of the different 
 refrangibility of the rays of light. When the sun is near the 
 horizon, as in the morning and evening, the vast tract of dense 
 and vaporous air in a direct line from him to the earth, produces 
 the absorption of the weaker rays, and therefore the clouds 
 reflect only the vivid yellow, the orange, and the red. As the 
 quantity of the vapours, and even their nature, are not similar 
 for two days together, this irregularity produces corresponding 
 differences in their effects. Terrestrial objects, and even the sun 
 himself, are necessarily tinged like the light by which they are 
 seen. W hen the light at these times is received by a prism, it 
 is found that the rays actually absorbed are such as we should 
 expect to be so from the colouration of the moment. In con- 
 
446 
 
 OPTICS. 
 
 Halo. — Parhelia. — Anecdote. 
 
 sequence of the successive increase and density of the vapours tra- 
 versed by the light, clouds differently placed, are at the same in- 
 .stant tinged of different colours. The highest may even be white, 
 while the rest, at a less elevation, will be yellow, and others still 
 lower will be red, or approach nearly to red. At an equal eleva- 
 tion, the most distant from the point where the sun sets, will in- 
 cline to red, and the nearest to yellow'. 
 
 A luminous circle called a halo or corona, is frequently observ- 
 ed to surround the sun, moon, and even the planets and stars. It 
 is sometimes coloured, but generally white. It is attributed to the 
 effects produced on the light of these bodies, in passing through 
 dense clouds, or a frozen medium of hail or snow', and it is w'hite 
 or coloured, according as the rays have undergone a prismatic se- 
 paration or not. The parhelia, or mock suns, w hich are sometimes 
 observed, are referable to a similar origin. 
 
 To the thinking mind, it cannot but be interesting to observe, 
 in how many instances the causes of phenomena which were 
 formerly considered as altogether inexplicable, and regarded 
 with superstitious dread, are now' familiar to children, and 
 allusions to them assimilated with the language of poetry. 
 They are no longer considered as the ominous harbingers of 
 evil; they are produced for amusement by experiment, and 
 when they occur under natural circumstances, are contem- 
 plated with delight, because they are known to be the result 
 of those immutable laws which the Author of nature has assign- 
 ed for the government of his works. Optical science furnishes 
 several illustrations of this subject, and we shall mention one 
 which is not the least remarkable, from its connection with histori- 
 cal events. A day or two before the massacre of St. Bartholomew', 
 Henry IV. of France (then a prince at the court of Charles IX, 
 and not implicated in the guilt of that massacre,) sat down to dice 
 with the Duke of Guise, and just as they were going to play, he 
 perceived what he supposed to be drops of blood. Attempts 
 were made to wipe these drops aw'ay, but they still returned, 
 or new ones were seen in their places. The strictest examina- 
 tion was made among the spectators, yet it could not be dis- 
 covered whence the drops proceeded. This event made so 
 strong an impression upon the mind of Henry, that he alluded to 
 it in the memorable speech he delivered to his parliament, not 
 less than twenty-seven years afterw ards : I,” says he, looked 
 
 upon it as a dismal omen, and rising up from play, I turned 
 aside, and said to some of my particular friends, that I fore- 
 saw much blood would be shed.’’ From the best evidence 
 that history affords us of this event, we can collect but little 
 information of the circumstances of the moment; but black. 
 
OPTICS. 
 
 447 
 
 Black changed into red. — Inflection. — Deflection. 
 
 as the black spots of the dice, may be so completely changed 
 into scarlet, that we cannot at this day consider the appearance 
 of the blood-spots as any thing more than the accidental occurrence 
 ef a natural phenomenon. Let any person take a book, and 
 standing in the strong light of the sun, which must fall upon his 
 eyelids but not upon the book, let him look steadily at the printing, 
 and he will soon see the blackness of the letters changed to the 
 hue of Vermillion. 
 
 This appearance is of easy explanation ; it is well known, 
 that when we turn our eyes to the sun, with our eyelids closed, we 
 have the sensation of a lively red. Now the eyes, which are covered 
 with respect to the sun by the eyelids, are open to the book, and 
 consequently the wliife of the paper, which reflects so much light, 
 makes upon the retina an impression sufficient to erase the impres- 
 sion of the red rays in all the points of the retina upon which the 
 white rays fall. The black letters, on the contrary, which are 
 printed on the w hite paper, send back fe\v or no rays to the bottom 
 of the eye, and consequently all the points of the retina, on which, 
 in other cases, they are depicted black, preserve in all their 
 vigour, the first impression of red which had been made by the 
 sun upon the whole retina. The circumstances, essential to the 
 production of this illusion, are, 1. thfe sun’s shining upon the eye- 
 lids ; 2. the rays of the sun not falling upon any of the black 
 spots; 3. the sun's having shone for at least two minutes upon the 
 eyelids. 
 
 Of the Inflection and Deflection of Light. 
 
 The refraction of light, we have seen, is attributed to a power 
 of attraction appertaining to all bodies, and exerted at a little dis- 
 tance from their surfaces; reflection, on the contrary, is produced 
 by a power of repulsion, and also takes effect before the light ac- 
 tually strikes the reflecting surface. If these attractive and repul- 
 sive energies have any real existence, the rays of light, under cer- 
 tain circumstances, will be bent, in a manner that cannot be 
 classed under either refraction or reflection. Accordingly, we find, 
 that if a ray of light pass very near a body, without impinging upon 
 it, it is bent inwards, or towards the body; this change in the 
 direction of the ray, is called inflection. If the ray pass at a 
 greater distance, it is bent outwards, or from the body ; this change 
 in the direction of the ray, is called deflection. 
 
 When a beam of the sun’s light passes through a hole in 
 a wfindow shutter, the image thrown upon a screen or an opposite 
 wall, is larger than it would be if the rays, crossing at the aper- 
 Ittre, proceeded in straight lines from the circumference of the 
 
448 
 
 OPTICS. 
 
 Effects of inflection. 
 
 sun’s disk to the uall. It becomes therefore an object of inquiry, 
 to determine the cause by which it has been expanded; and we tind 
 on close examination, that the side of the apei lure has inflected or 
 caused to diverge from the axis of the beam, the rays which have 
 passed near it all around. 
 
 Inflection was first discovered by Grimaldi, who made many 
 curious experiments and observations relative to it; but the fol- 
 lowing experiments of Sir Isaac Newton, will be better adapted 
 than Grimaldi’s, to explain the nature of this property of 
 light. At the distance of two or three feet from the window of a 
 darkened room, in w hich there was a hole three-fourths of an inch 
 broad, to admit the light, he placed a black sheet of pasteboard, 
 having in the middle a hole about a quarter of an inch square, 
 and behind the hole the blade of a sharp knife, to intercept a 
 small part of the light which would otherwise have passed through 
 the hole. The planes of the pasteboard and blade were paral- 
 lel to each other, and when the pasteboard was removed to 
 such a distance from the window, that ail the light coming into 
 the room must pass through this hole in the pasteboard, he re- 
 ceived what came through the hole on a piece of paper tw o or 
 three feet beyond the knife, and perceived two streams of faint 
 light shooting out both ways from the beam of light into the sha- 
 dow\ As the brightness of the direct rays obscured the fainter 
 light, by making a hole in his paper, he let them pass through, 
 and had thus an opportunity of attending closely to the two streams, 
 which were nearly equal in length, breadth, and quantity of light. 
 That part which was nearest to the sun’s direct light, was pretty 
 strong for the space of about a quarter of an inch, decreas- 
 ing gradually till it became imperceptible, and at the edge of the 
 knife it subtended an angle of about twelve or at most of fourteen 
 degrees. 
 
 Another knife was then placed opposite to the former, and he 
 observed, that when the distance of their edges was about the 
 four-hundredth part of an inch, the stream divided in the middle, 
 and left a shadow between the two parts which was so dark, 
 that all the light passing between the knives seemed to be bent 
 aside to one knife or the other; as the knives were brought nearer 
 to each other, this shadow grew broader, till upon the contact 
 of the knives the whole light disappeared. He observed also, 
 fringes of different coloured light, three made on one side by the 
 edge of one knife, and three on the other side by the edge of the 
 other. 
 
 Grimaldi, Dr. Hooke, and all the philosophers who made expe- 
 riments on inflection before the time of Newton, ascribed the 
 broad shadows, and even, the fringes which he mention* to th§ 
 
OPTICS. 
 
 449 
 
 Brougham’s experiments on inflection. 
 
 ordinary refraction of the air; but the investigation upon which he 
 entered to discover llieir cause, afforded him satisfactory evidence 
 to conclude that bodies have the power of acting upon light at a 
 distance. 
 
 The Philosophical Transactions for 1796, contain a paper 
 by Henry Brougham, F. R. S., detailing one of the most valu- 
 able series of experiments on the mechanical properties of 
 light, which has appeared since the time of Newton. It had 
 been generally supposed that the parts of wliich light consists, 
 have all the same disposition to be acted upon by bodies which 
 inflect, deflect, and reflect them ; but he soon proved that this 
 opinion was erroneous. Having admitted into a darkened 
 chamber, a beam of the sun’s light, through a hole in a metal 
 plate (flxed in the window shutter) of ^th of an inch in diame- 
 ter; and all other light being absorbed by black cloth hung 
 before the window and in the room, at the hole he placed a 
 prism of glass, whose refracting angle was 45 degrees, and, 
 which was covered all over with black paper, except a small 
 part on each side, which was free from impurities, and through 
 which the light was refracted, so as to form a distinct and 
 tolerably homogeneous spectrum on a chart at six feet from 
 the window. In the rays, at two feet from the prism, he 
 placed a black unpolished pin, one-tenth of an inch in diame- 
 ter, parallel to the chart, and in a vertical position. Its sha- 
 dow was formed in the spectrum on the chart, and had a con- 
 siderable penumbra, especially in the brightest red. It was 
 by no means of the same thickness in all its parts; in the violet 
 it was broadest and most distinct ; in the red it w'as narrow^est 
 and most confused, and in the intermediate colours it was of 
 an intermediate thickness and degree of distinctness. It- was 
 not bounded by straight, but by curvilinear sides, which were 
 concave outw'ardly. This figure of the shadow was not owing 
 to any irregularity in the pin, for the same thing happened with 
 all sorts of bodies that were used; and also if the prism was 
 moved on its axis, so that the colours might ascend and descend 
 on these bodies, still, wherever the red fell it made the least, 
 and the violet the greatest shadows In the next place, he 
 fixed a screen, having in it a large hole on which was a brass 
 plate, pierced with a small hole of ^ of an inch in diameter ; 
 then causing an assistant to move the prism slowly on its axis, 
 he observed the round image made by the different rays passing 
 through the hole to the chart; that made by the red w'as greatest, 
 by the violet, least, and by the intermediate rays of an intermediate 
 size. Also, when at the back of the hole he held a sharp blade of 
 a knife, so as to produce the fringes mentioned by Grimaldi and 
 VoL. L 3 M 
 
450 
 
 OPTICS. 
 
 Brougham’s experiments on inflection. 
 
 Newton, those fringes in the red were broadest, and most moved 
 inwards to the shadow, and most dilated when the knife was moved 
 over the hole ; and the hole itself on the chart was more dilated 
 during the motion when illuminated by the red, than when illumi- 
 nated by any other of the rays, and least of all when illuminated by 
 the violet. From these, and a great variety of other experiments, 
 well devised and often repeated with rigorous care, he infers, that 
 the rays of light are se|>arable into their primitive colours, by in- 
 flection, deflection, and reflection, as well as by refraction; that 
 the flexibilities and reflexibilities of the rays are inversely as their 
 refrangibilities ; that is, those which deviate the least by refraction, 
 deviate the most by flection, and are reflected the furthest from 
 the perpendicular ; and that the different flexibilities, reflexibilities, 
 and refrangibilities of the rays, are all produced by the differences 
 in the magnitude of the particles. He calculates that the size of 
 the red particles are to those of the violet as 1275 to 1253. This 
 he extends to all the other colours by similar calculations, their 
 sizes lying between 1275 and 1253; which are the extreme red 
 and extreme violet; the red therefore is from 1275 to 1272|-; the 
 orange from 12724- to 1270; the yellow from 1270 to 1267; 
 the green from 1267 to 1264; the blue from 1264 to 1260; 
 the indigo from 1260 to 1258, and the violet from 1258 to 
 1253. The whole paper is highly curious and deserving of at- 
 tention, but it would be impossible to do it justice by analysis in 
 our limits. 
 
 From the separation of light into its component parts, by 
 inflection, we readily perceive how the coloured fringes of 
 light thus influenced are produced ; but other appearances de- 
 rive their origin from inflection, where colour is not always observ- 
 able. On looking at the printing of a book through a very small 
 hole, such as the hole made by a needle in a piece of paper, the 
 letters appear considerably magnified. In this case, the rays^ 
 which form the image pass so near the circumference of the hole, 
 as to be inflected by it, and they therefore form a larger picture on 
 the retina than the direct rays would have done. When we look 
 at a distant steeple, or any similar object, and intercept the direct 
 rays from it by crossing the line of sight with a wire of rather less 
 diameter than the pupil, held very near the eye, the object will be 
 seen by rays bent inwards or inflected, which would otherwise not 
 have entered the eye, and as they make a much larger angle, the 
 object appears magnified. 
 
 When we look at a candle, or any other luminous body, 
 with our eyes almost closed, streaks of light appear to dart 
 upwards and downwards; if the head be reclined, the change in 
 the direction of the streaks is correspondent. This appearance 
 
OPTICS. 
 
 451 
 
 Description of the eye. 
 
 has been most commonly attributed to the inflection of light pass- 
 ing between the eye-lashes; but it is most probable that inflection 
 only occasions a very small part of it; and that the rest is produced 
 by the refraction of light through the humours adhering to the eye- 
 lids, because, as Brougham observes, the streaks which dart from 
 the top of the luminous body are formed by the under eye-lid ; or 
 at least by the moisture adhering to the under ciliary process, and 
 those which appear from the bottom of the body, by the upper eye- 
 lid ; which could not be, either if they were formed, as some have 
 supposed, by reflection from the processes, or by inflection through 
 the lashes. 
 
 Of the Ei/e and the general Phenomena of Vision, 
 
 The eye is an organ, not less admirable in its mechanical pro- 
 perties and structure, than invaluable for its use. To expatiate on 
 its utility would be idle, to treat of its expression belongs to the 
 physiognomist; it is our duty to explain merely the manner in 
 which it performs its office. To this end we must first desciihe 
 its several parts : 
 
 The eye is of a globular figure, rather protuberant in front, 
 and is composed of three membranes called coats, and three pel- 
 lucid substances called humours. Each coat and each humour lias 
 a different name. By fig. 3, pi. IV, is represented a section of 
 the human eye. The part DHHG, of the outer coat, is called the 
 sclerotica, the exterior part or continuation of it, DEFG, is called 
 the cornea, from its resembling horn. The sclerotica is stroiig and 
 unelastic, and the muscles which move the eye are attached to it. 
 What is called the white of the eye, is a part of this coat. The 
 cornea bulges out a little from the eye-ball; it is circular, and ex- 
 ceedingly transparent. 
 
 The next coat to the sclerotica is called the choroides, which 
 serves as a lining to it. It is of a dark colour in the human 
 eye, but white in cats and owls, and green in animals that live 
 on grass and vegetables. Its texture is soft and pulpy, and 
 too weak to be susceptible of muscular motion, except at its 
 extremities towards the front of the eye. Like the sclerotica, 
 it is distinguished into two parts; the fore-part being called 
 the iris, while the hinder part retains the name of the choroides. 
 The iris commences immediately under the commencement 
 of the cornea. it here attaches itself more strongly to the 
 sclerotica by a cellular substance, forming a kind of white, 
 nafrow, circular rim, called the ciliary circle. The iris is 
 that remarkable circle, which gives the eye its character as 
 to colour ; it is composed of two sets of muscular fibres ; the 
 
452 
 
 OPTICS. 
 
 Description of the eye. 
 
 one tending, like radii, towards the centre of the circle, and 
 the other forming a number of concentric circles round the 
 same centre. The centi al part of the iris is perforated, and the 
 aperture, which is called the pupil, is always round, but varied 
 in diameter by the action of the two sets of muscular libres com-' 
 posing the iris. When a very luminous object is viewed, the 
 circular fibres contract, the radial are relaxed, and thus the 
 size of the pupil is diminished; on the other hand, when the ob- 
 jects are dark and obscure, the radial fibres of the iris contract, 
 the circular are relaxed, and the pupil is enlarged, so that it ad- 
 mits a greater quantity of light. By candle-light, the contraction 
 and dilation of the pupil may be very distinctly observed, with 
 the assistance of a looking-glass. If, with our eyes directed 
 to the mirror, we bring the candle close to our face, we shall 
 find the pupil become very small; if the candle be removed 
 and completely shaded for about a minute, and then brought 
 to its former place, it will be found that the pupil has greatly 
 dilated, and that it again contracts as the light draws nearer: 
 if the light shine much more strongly on one eye than the other, 
 the pupil of the shaded eye will not contract so much as the 
 other. 
 
 The whole of the choroide membrane is opake, by which means 
 no light can enter the eye but what passes through the pupil, but 
 to render the chamber of the eye still darker, the posterior surface 
 of this membrane is covered with a black mucus called the pig- 
 mentum nigrum. 
 
 Under the iris, there is a prolongation, n n, of the choroidesr, 
 which forms a circular band of radial libres, turning inwards to- 
 wards the centre of the eye, and filled up between with a black 
 mucus, giving it the appearance of a membrane. This circular 
 band is called the ligamentum ciliare, or ciliary ligament. 
 
 The third and last coat of the eye is called the retina. This 
 is a find and delicate membrane, being an expansion of the 
 optic nerve L, which proceeds from the brain. Its course is re- 
 presented by dots in the figure; it is spread like a net of exquisite 
 delicacy, all over the concave surface of the choroides, and termi- 
 nates at the ciliary ligament n n. It receives ihe images of 
 objects, which are depicted upon it by the rays of light that enter 
 at the pupil. It is of itself transparent, and of an ash-coloured 
 white, but appears black on account of the piginentum nigrum 
 behind it. The optic nerve, L, which passes through a small 
 hole in tlie bony cavity containing the eye, and conveys to the 
 sensorium the impressions made on the retina, is not in the 
 centre of the eye, but a little on one side, inclining towards ih^ 
 nose. 
 
OPTICS. 
 
 453 
 
 Description of the eye. 
 
 The three transparent substances inclosed by the coats of 
 the eye are called the aqueous, crystaline, and vitreous humours. 
 The first of these, or aqueous humour, resembles water, whence 
 its name. It gives a protuberant figure to the cornea, fills 
 the two cavities m m, n n, between the cornea and ciliary liga- 
 ment, which cavities communicate by the pupil P. The part 
 m m, is called the anterior, and n n the posterior portion of the 
 aqueous humour, the iris, which swims in it, constituting the divi- 
 sion. The refractive power of the aqueous humour is like that of 
 water. 
 
 The second, or crystaline humour, R, is, like the former, 
 eminent for its transparency, which exceeds that of the purest 
 crystal ; it has the consistence of a hard jelly, growing some- 
 what softer from the middle towards the edges. Its form is 
 that of a double convex lens, but more convex on the interior 
 than the exterior surface. Resembling a lens in its form, 
 it also resembles one in its use : it converges the rays which 
 
 pass through it from every visible object to its focus on the 
 retina. It is inclosed in a fine transparent cover, or membrane, 
 called the arachnoides, which is attached to the ligamentum 
 filiare before mentioned ; and by that means it is suspended. 
 The radial fibres of the ligamentum ciliare, have the power 
 of contracting and dilating occasionally, by which means they 
 alter the shape or convexity of this natural lens, and shift it 
 a little backwards and forwards in the eye, so as to adapt its 
 focal distance from the retina to the different distances of ob- 
 jects. Without this or some equivalent arrangements, we should 
 only see those objects distinctly that were at one distance from the 
 eye. 
 
 At the back of the crystaline, lies the third or'vitreous humour 
 KK. It receives its name from its supposed resemblance to melted 
 glass. It is not so hard as the crystaline, nor so liquid as the 
 aqueous humour. It is by far the largest of all the humours in 
 quantity, and gives the eye its globular shape. In its refractive 
 pow er, it very little exceeds water ; in consistence it is like the 
 white of an egg. 
 
 As every point of an object, ZVX, sends out pencils of rays 
 in all directions, some rays from every point on the side next the 
 eye, will fall upon the cornea between EF, and by passing on 
 through the pupil and humours of the eye, they will be converged 
 to as many points on the retina, and wdll there form a distinct 
 inverted picture BYA, of the object: for the pencil of rays 
 flowing from the point Z of the object, will be converged to the 
 point A of the retina; .those from the point V will be converged 
 to the point Y j those from the point X will be converged to the 
 
454 
 
 OPTICS. 
 
 Phenomena of vision. 
 
 point B, and rays from all the intermediate points being converged 
 in like manner, the whole image BYA is formed, and the object 
 made visible. 
 
 With two eyes we not only see objects about one-thirteenth 
 part brighter than when we use only one, but we see them in 
 a dilferent situation. Those who have lost the use of one eye 
 are apt to mistake the distances of objects even within arm’s 
 length ; and in such actions as the threading of a needle, or the 
 snuffing of a candle, they do not succeed without considerable 
 experience. 
 
 It has often been a subject of inquiry, why we see objects 
 in their true position, though the image on the retina is inverted, 
 but no satisfactory solution of the difficulty has ever been given. 
 And we should be as little likely to receive an answer, if we 
 were to ask, why w'e do not perceive every object bent, because 
 the image of it is depicted upon a concave surface. It is cer- 
 tain that unless distinct images are painted on the retina, 
 objects cannot be clearly perceived. If from too little light, 
 remoteness, or any other cause, a picture is indistinctly painted 
 on the retina, an obscure or indistinct idea of the object is 
 conveyed to the mind. The picture on the retina is therefore 
 so far the cause of vision, that our ideas of visible objects vary 
 as it varies, and when it is not formed, nothing is seen. Yet 
 w e may fairly conclude, that the mind does not look upon the 
 image on the retina; for in cases of the gutta serena, a disorder 
 which affects only the optic nerve, the pictures on tlie retina 
 are as perfectly formed as in the best eyes, although the pa- 
 tient is afflicted with incurable blindness. It is the optic nerve, 
 therefore, which conveys the impressions made on the retina 
 to the brain, but how they are there communicated to the mind, 
 is screened from the view of man. It has been supposed that 
 we acquire by experience the habit of seeing objects erect, 
 but there are many striking facts to prove the contrary; persons 
 who have been blind from infancy, and who have been sud- 
 denly restored to sight by a surgical operation, have not been 
 led into the smallest mistake. In fact, no reason can be given 
 why the mind should not perceive as accurately the position 
 of bodies, when the rays reflected from the upper parts of those 
 bodies fall upon the lower parts of the eye, as if the contrary took 
 place. 
 
 To see an object distinctly, besides a sufficient quantity of 
 light, it is necessary that the pencils of divergent rays which en- 
 ter the eye, should converge to points on the retina. If the pen- 
 cils fall in an unconverged state on the retina, the scattered state 
 ©f the light causes it to make an indistinct impression, which mqy 
 
OPTICS. 
 
 455 
 
 Phenomena of vision. 
 
 arise from two causes: 1. the rays may require a greater distance 
 
 than the size of the eye admits of, before they can be converg- 
 ed ; and, 2. they may converge before they reach the retina, 
 in consequence of which they will fall upon it in a dispersed 
 state, because they have crossed and begun to diverge. Persons 
 having the former defect, are called long-sighted, as they can 
 frequently see objects at a great distance better than those near 
 at hand. The reason is, that from age and infirmity, the cornea 
 and crystaline lens become flatter, and consequently incapable of 
 converging the rays so soon to points; and the defect is further 
 increased, and may sometimes perhaps be wholly occasioned, 
 by the ciliary ligament becoming either too flaccid or two rigid, 
 to produce the various adjustments necessary to view objects 
 at different distances. When the pencils of rays are converged 
 to points before they reach the retina, the persons subject to the 
 visual defect thence arising are said to be short-sighted, because 
 they can only see those objects distinctly which are very near 
 their eyes. The subject of long and short sightedness will be 
 resumed, when we treat of spectacles ; here we shall only further 
 observe, that the eye is as much under the dominion of habit as 
 any of our senses, and when long inured to one class of objects, 
 becomes less fit for all others. The engraver, who is constantly 
 employed in viewing objects near at hand with great attention, 
 loses much of the facility of discerning objects at a distance; but 
 the sailor, who never, perhaps, strains his eyes, except at distant 
 objects, becomes less capable of seeing distinctly those which are 
 near at hand. 
 
 The advantages of having two eyes, even so far as we are ac- 
 quainted with them, are not confined merely to improving the 
 brightness of objects, and showing them in their true places. In 
 each eye there is a spot where no vision takes place, and this spot, 
 which is about the fortieth of an inch in diameter, lies exactly upon 
 the insertion of the optic nerve, so that we cannot perceive the 
 image of any object that falls upon it at the hinder part of the eye, 
 provided the other eye be shut; but as the insensible parts of the 
 two eyes are on the sides next each other, that part which is in- 
 visible to one eye, is visible to the other, and therefore the whole 
 is seen. To be satisfied of the existence of such a spot, the 
 following experiment may be resorted to. Let three pieces of 
 paper be fastened upon the side of a room, about two feet 
 asunder; and let the person place himself opposite to the 
 middle paper, and beginning near to it, retire gradually back- 
 wards, all the while keeping one of his eyes shut, and the other 
 turned obliquely towards that outside paper which is towards 
 the covered eye, and he will find a situation, (which is gene- 
 
456 
 
 ■ OPTICS. 
 
 Experiments on vision. 
 
 rally about five times the distance at which the papers are 
 placed from one another,) where the middle paper will entirely 
 disappear, while the two outermost continue plainly visible ; 
 because the rays which come from the middle paper will fall 
 upon that part of the retina where the optic nerve enters. Hence 
 it is evident, that if the optic nerve had not been inserted on 
 one side, the centre of our field of view would have been invi- 
 sible. 
 
 We have previously had occasion to mention the visual 
 angle, and to intimate that the larger it \Vas, the greater the 
 apparent magnitude of any object beheld. The meaning of this 
 ex,pression will now be apparent, fjom the description we have 
 given of the eye ; but to render it still more evident, we shall 
 refer to a figure. Let AB, fig. 4, pi. IV, be an object viewed by 
 the eye QR. From each extremity draw the lines AN and BM, 
 intersecting each other in the chrystaline humour at I. Then 
 draw the line IK, in the direction in which the eye is supposed 
 to look at the object. The angle AIB is then the optical or 
 visual angle, and the line IK is called the optical axisy because 
 it is the axis of the lens or crystaliiie humour continued to the 
 object. The apparent magnitude of objects, then, depending 
 thus on the angle under which they are seen, must evidently 
 vary according to their distances. Thus difierent objects, as 
 AB, CD, EF, the real magnitudes of which are very une<|ual, 
 may be situated at such distances from the eye as to have their 
 apparent magnitudes all equal.; for if they are situated at such 
 distances, that the rays AN, BM, shall touch the extremities 
 of each, they will tlien all appear under the same optical angle, 
 and tho diameter MN, of each image on the retina will conse- 
 quently be equal. In the same manner, objects of equal mag- 
 nitude, situated at unequal distances, will appear unequal. 
 For *let AB and GH, be two objects of equal size, placed 
 before the eye at different distances, IK and IS ; draw the lines 
 GP and HO, crossing each other in I; then OP, the image 
 formed by the object GH on the retina, is evidently of a 
 greater diameter than the image M N, wiiicli represents the object 
 AB ; in other words, the object GH will appear as large as an 
 object of the diameter TV, situated at the same place as the ob- 
 ject AB. 
 
 When we look from one end towards the other of a long 
 and straight row of houses or trees, they appear gradually to 
 diminish as they are further removed from the eye, though 
 upon a nearer inspection they are all found to be of equal size. 
 It will be evident, from the observations we have just made 
 respecting the v’sual angle, that this must be the case ; for the 
 
OPTICS. 
 
 457 
 
 Phenomena of vision. 
 
 angle under which similar objects are seen, and consequently the 
 evidence which sight affords us of their magnitude, is in an inverse 
 proportion to the distance of those objects. The apparent excep- 
 tions to this rule apply to objects where the evidence ot sight is 
 corrected by the judgment. When objects are near, we do not 
 judge of their magnitude according to the visual angle. Though 
 a man six feet high is seen, at the distance of six feet, under the 
 very same angle as a dwarf only two feet high, at the distance of 
 two feet, still the dwarf does not appear as large as the man, be- 
 cause we are instantly able to make the requisite allowance for the 
 difference of distance. 
 
 But when the distance is considerable, and we have no opportu- 
 nity of comparing one object with another, we soon perceive that the 
 rule just laid down has its foundation in nature. When Denon first 
 drew near the gigantic pyramids of Gizeh, he was not particularly 
 struck with their magnitude, principally because there were no ob- 
 jects in the vicinity by which a comparison could be made; but 
 this impression was speedily effaced, when he had observed a hun- 
 dred people who had preceded him assembled at the base of one 
 of them, the deception instantly vanished, a comparison was 
 formed and the stupendous pile assumed all its appropriate ma- 
 jesty. • 
 
 As an image of every visible object is painted on the retina 
 of each of our eyes, we may be inclined to inquire, why 
 we do not see every object doubled ? Of the various opinions 
 which have been advanced in explanation of this difficulty, 
 the most satisfactory is, that in the two eyes there are corre- 
 sponding parts of the retinas, which are probably susceptible 
 of the same impression in equal degree, and convey it to the 
 sensorium in that equal degree ; hence, as long as similar points 
 of the images fall upon the corresponding points of the retinas, 
 the perception of the same object is single, otherwise it is double. 
 It is a confirmation of this theory, that when a person, whose 
 sight is perfect, looks with both eyes at an object straight before 
 him, the axes of both eyes are inclined towards each other in 
 equal degrees, aud directed to the same point. In this case the 
 images are formed upon the corresponding parts of the retinas, 
 and in all other cases, the eyes move in unison, to produce the 
 same effect; but while the same object still continues to be. re- 
 garded, let the position of one eye be varied a little by a slight 
 pressure of the finger, and the object will instantly appear 
 double. Now the aberration of the axis of one eye, which 
 is thus effected for a moment by design, is often produced by 
 disease or habit, so that the power of directing both eyes to the 
 same point is permanently lost : when this is the case, the person 
 20.— Vo L. I. 3N 
 
458 
 
 OPTICS. 
 
 Phenomena of vision. 
 
 is is said to squint, and the squint-eyed always see objects double, 
 unless they have acquired the habit of entirely disusing the eye of 
 whose motion they have lost the natural and perfect command. 
 No method of curing this defect, when it has not been absolutely 
 irremediable, has been more successful than that of binding up for 
 a time the sound eye, by which means the other is obliged to per- 
 form its office. 
 
 The nearer any object is, when viewed with two eyes, the 
 greater the inclination of the axes of the eyes to each other, and the 
 converse necessarily follows, when we take a distant view. It is 
 by this adjustment that we are materially assisted in judging of the 
 distances of objects not very remote, and that we know whether 
 a person is looking at us or not. 
 
 We acquire also the habit of almost involuntarily taking into 
 consideration a variety of other circmnstances, in judging of dis- 
 tances. When objects appear obscure or confused, w e judge them 
 to be remote; and when they appear distinct, we form a contrary 
 opinion : of these principles painters sedulously study to avail them- 
 selves. Rooms, the walls of which are whitened, appear smaller 
 than when of a dark colour ; fields covered with snow, or white 
 flowers, appear less than w hen clothed wdth grass ; mountains 
 covered with snow^ appear nearer than at other times; and in the 
 evening, when it is nearly dark, an upright post half white and half 
 black, may be taken fcr a body of considerable size in horizontal 
 extension. 
 
 An Englishman, w hen he first views an Italian landscape, makes 
 the most egregious mistakes, in estimating the distances of places 
 and objects by the eye. He has no conception of the clearness of 
 the air, in that delightful climate,^ by which he is enabled to per- 
 ceive objects at the distance of twenty miles, with so much distinct- 
 ness, that he supposes himself to be within half an hour’s walk of 
 them. Italian painters, true to the characteristics cf their country, 
 have made their most distant mountains well defined at their sum- 
 mits, and all other objects proportionately distinct; and we are apt 
 to think they have deviated from nature, because the scenery of our 
 own country is never clothed with such fascinating splendour. 
 
 The eye can only see a very small part of an object distinctly 
 at once; for the lateral parts of an object are not represented dis- 
 tinctly in the eye ; and therefore the eye is obliged to turn itself 
 successively to the several parts of the object it wants to view^, that 
 they may fall on or near the axis of the eye, w here alone distinct 
 vision is performed. 
 
 When the eye is placed above a horizontal plane, the different 
 parts of this plane will appear elevated in proportion to their dis- 
 tance, till at length they will appear on a level with it. For in 
 
OPTICS. 
 
 459 
 
 Phenomena of vision. 
 
 proportion as the different parts are more distant, the rays which 
 proceed from them, form angles w ith the optical axis, IK, fig. 4, 
 pi. IV, more and more acute, and at length become almost paral- 
 lel. This is the reason why, if we stand on the sea-shore, those 
 parts of the ocean wdiich are at a great distance appear elevated ; 
 for the globular form of the earth is not perceptible to the eye; and 
 if it was, the apparent elevation of the sea is far greater than the 
 arch which a segment of the globe would form, within any dis- 
 tance that our eyes are capable of reaching. 
 
 The best eye can hardly distinguish any object that subtends at 
 the eye an angle less than half a minute of a degree; and very few 
 can distinguish it when it subtends a minute. If the distance of 
 tw'o stars be not greater than this, they will appear as one. 
 
 Though men see distinctly at different distances, by the alterations 
 of the position and figure of the chrystaline lens, yet they can only 
 see distinctly beyond a certain extent. This extent is not the same 
 in different people, but in general it is between six and ten inches. 
 A good eye can see distinctly when the rays fall parallel upon it ; 
 and then the principal focus is at the bottom of the eye. 
 
 The following is a summary of the laws of vision, wdth regard 
 to the figure of visible objects: 1. If the centre of the eye be ex- 
 
 actly in the direction of a right line, the line will appear only as a 
 point. 2. If the eye be placed in the direction of a surface, it will 
 appear only as a line. 3. If a body be opposed directly towards 
 the eye, so that only one plane of the surface can radiate on it, the 
 body will appear as a surface. 4. A remote arch, viewed by an 
 eye in the same plane with it, will appear as a right line. 5, A 
 sphere, viewed at a distance, appears a circle. 6. Angular figures, 
 at a distance appear round. 7. If the eye look obliquely on the 
 centre of a regular figure, or a circle, the true figure will not be 
 seen, but the circle will appear an ellipse, &c. 
 
460 
 
 OPTICo. 
 
 Utility of convex and concave glasses to improve vision. 
 
 Of Optical Instruments and Machines. 
 
 Of Spectacles. 
 
 By the observations illustrating fig. 4. pi. IV, we hope it is 
 clearly understood, that the nearer any object can be brought to 
 the eye, the larger will be the angle under which it appears, and 
 the more it will be magnified. But objects, we find by experience, 
 may be brought so near the eye, that the advantage of their form- 
 ing a large visual angle, is more than counter-balanced by the in- 
 distinctness that ensues. The generality of people see small ob- 
 jects best, at the distance of eight inches; when such objects are 
 brought nearer than eight inches, they become indistinct, and if 
 to four, or three, they will scarcely be seen at all. The reason of 
 this indistinctness is, that the pencils of rays from objects brought 
 within the limits where distinct vision commences to the naked eye, 
 are in so divergent a state, that they are not converged to points 
 exactly upon the retina. But we have already seen, that a 
 convex glass will lessen the divergence of the most divergent 
 rays passing through it, and that if its curvature be sufficient, it 
 will refract them parallel or even convergent. If, then, an ob- 
 ject be viewed at the distance of two or three inches, with a glass 
 of suitable convexity interposed, it immediately becomes distinctly 
 visible at that distance, with the advantage of appearing larger 
 and more enlightened than it would under any circumstances ap- 
 pear to the naked eye. By an obvious parity of reasoning it is 
 evident, that if an object can be rendered distinctly visible by a 
 convex glass, at a less distance than that at which common 
 vision is effected, a person who does not see objects but at ah 
 unusual and inconvenient distance, may be made to see them at 
 the common and most eligible distance. Hence the use of convex 
 glasses, which are an invaluable remedy for the visual defect of the 
 long-sighted. Concave glasses, on the contrary, which are directly 
 opposite in their effects to the convex, prove a remedy of equal 
 value to the short-sighted, or those who cannot see distant objects 
 distinctly, and are obliged to bring near objects almost close to 
 their eyes. 
 
 By fig. 3, pi. IV, is represented an eye in its perfect state, 
 the image of the object being exactly upon the retina, or as 
 nearly so as could be shown by an engraving; for we need 
 not inform the most ignorant reader, that of the picture formed 
 by light, no part projects, in the manner shown by the plate. 
 Fig. 1, pi. V, is the eye of a long-sighted person, where, 
 from the flatness of the crystaline humour, and of the cornea, 
 
OPTICS. 
 
 46.1 
 
 Spectacles. 
 
 the foci of the pencils of rays from objects are not at d, where they 
 ought to be, in order to render vision distinct, but tend to a point 
 at F beyond the eye. Hence the rays which How from the object 
 C, and pass through the humours of the eye, instead of forming an 
 image of a point, form a large speck of light, and when the rays 
 from all parts of an object do the same, the wdiole image is con- 
 fused. But if a convex glass, AB, of a proper focus, be inter- 
 posed, the rays converge sooner, they meet in a point at d, on the 
 retina, and distinct vision is obtained. 
 
 In fig. 2, from the great convexity of the cornea and crystaline 
 humour, the rays that enter from the object C, converge to a fo- 
 cus in the vitreous humour, as at F, and by diverging from thence, 
 fall as in the example of the long-sighted, obtusely on the retina, 
 and vision is as indistinct as if the eye had been too flat. But by 
 placing a concave glass AB, before the eye, the rays are spread 
 out a little, or fall with greater divergency upon the eye than be- 
 fore, and they do not unite till they fall on the retina, consequently 
 the defect disappears. 
 
 When glasses are put in frames for spectacles, their frames 
 ought not to be straight, but bent a little in the middle, so that 
 the axes of both glasses may be directed to one point, at the dis- 
 tance most proper for reading or examining objects in general. 
 By this means the axes of the eyes will fall perpendicularly upon 
 the glasses, and vision will be more distinct. 
 
 Spectacles are much better for the eyes than those glasses \\hich 
 are held in the hand. By hand-glasses, indeed, more harm than 
 benefit may eventually result, for the distance at which they are 
 held from the eye is perpetually varied, and the eye is thus perpe- 
 tually strained, that it may accommodate itself to such changes. 
 
 When the eyes of persons first begin to be affected by age, 
 the opticians furnish them with spectacles, the glasses of which 
 are about 40 inches focus, which are therefore called No. 1, or 
 glasses of the first sight. When the focal length is about 16 
 inches, the lenses are called No. 2. About 12 inches is the 
 focal length of No. 3. Ten inches is what they call No. 4. 
 Nine inches is that of No. 5. Eight inches is the focal length 
 of No. 6. Seven inches is the focal length of No. 7. Six 
 inches is the focal length of No. 8, and sometimes they make 
 spectacles of a focus shorter still. Concave spectacles are dis- 
 tinguished by numbers in the same way. But there is a consi- 
 derable difference in the focal ^distances of glasses made by dif- 
 ferent opticians, though tjiey give them the same number. In 
 chusing spectacles, therefore, actual trial alone can be depended 
 on; and when an actual trial is made, perhaps the best direction 
 
462 
 
 OPTICS. 
 
 Spectacles. — Preservation of the sight. 
 
 that can be given to make a proper choice, is to prefer those 
 spectacles which, when near the eye, show objects nearest their 
 natural state, neither enlarged nor diminished, and that give a 
 blackness and distinctness to the letters of a book, without strain- 
 ing the eye, or causing any extraordinary exertion of the pupil. 
 No spectacles can be relied on as properly accommodated to the 
 eyes, which do not procure them ease and rest ; if they fatigue the 
 eyes, we may safely conclude, either that we have no occasion 
 for them, or that they are ill-made, or not proportioned to our 
 sight. 
 
 In the choice of glasses for the short-sighted, no rules can be 
 laid down; it is a state of the eye which has no connection with 
 age; no stated progression that can be a guide to the optician, in 
 order that he may recommend one glass in preference to another; 
 but the short-sighted themselves, by trying different glasses, will 
 soon discover which are most advantageous. 
 
 We shall probably not be charged with taking up too irrelevant 
 a subject, if we introduce a few observations on the means which 
 may be employed to render spectacles unnecessary, or at least to 
 lengthen the period of visual enjoyment w ithout them. The long- 
 sighted should accustom themselves to read and examine objects 
 at a less distance than is entirely agreeable to them; while the 
 short-sighted should attentively pursue a contrary practice. Nothing 
 is more conducive to the preservation of the sight, than the con- 
 stant use, both in reading and writing, of a moderate degree of 
 light. If in the apartment we commonly use, there are two 
 windows, it is better to sit at an equal distance from both, than 
 to let the principal light come from one side only. In all cases, 
 it should be our object to afford each eye an equal quantity of 
 light. 
 
 To flaming -colours and white objects, the eyes should not 
 be often or long exposed. The poor, untutored Indian,’^ 
 when he traverses his native wilderness, while it is every-where 
 covered with snow, fixes before his eyes a wooden frame, which 
 only permits the rays of light to pass through a very minute 
 aperture. His view is thus confined, and his light is smal., 
 but he preserves his sight from certain injury. Long or frequent 
 excursions over snow, especially when the sun shines, it will 
 easily perhaps be admitted, must have an injurious tendency to 
 the eyes; but it may be asserted in return, that none of the 
 circumstances attending common life in this country, can 
 occasion an effect so much to be deprecated. The following 
 relation will evince the fallacy of this supposition. A student at 
 Cambridge, who sat daily for several hours in an apartment, 
 
OPTICS. 
 
 463 
 
 Preservation of the sight. — Burning lenses and mirrors. 
 
 the walls of M'liich were white- w ashed, felt himself in a short time 
 affected with dimness of sight. A fellow^ student had the like oc- 
 casion for complaint. Suspecting the strong light reflected from 
 the w^alls to be the cause, they had their apartments coloured 
 green; and their eyes then gradually regained their former 
 strength. 
 
 After the long interval from exertion occasioned by a night's 
 rest, the sudden exposure of the eyes to a strong light, or the 
 intense exertion of them, has an unfavourable effect. Scarlet 
 window and bed-curtains are, for this reason, the worst that can 
 be chosen. The cataract, a complaint which frequently follows 
 an inflammation of the eyes, and is often irremediable, may be oc- 
 casioned by looking very frequently at a lire, or any very glaring 
 object. 
 
 Green is of all colours the most agreeable to the eyes, and 
 scarlet the most offensive, or endurable for the shortest space of 
 time. White is the next to scarlet. Hats and other coverings for 
 the head, the undersides of which are w hite, certainly tend to im- 
 pair the sight. 
 
 The affusion of the eyes in clean, soft, cold water, contri- 
 butes greatly to strengthen them, and of all applications is the 
 most strongly to be recommended. But with respect to the most 
 proper time of performing it, an erroneous opinion is prevalent. 
 Morning is commonly thought to be the most proper time. The 
 middle of the day is certainly to be preferred. Morning is an in- 
 eligible time, because the eyes are then well refreshed, and amply 
 replenished with the moisture which is most suitable for them ; but 
 when the affusion is postponed till mid-day, it becomes seasonable 
 and beneficial. 
 
 Of Burning Lenses and Mirrors, 
 
 A burning lens must be convex, a burning mirror must be cof> 
 cave; because both produce their effect by concentrating, into a 
 very small compass, the rays of light and heat, incident upon a 
 large surface. 
 
 As the rays which pass through a convex lens, or are 
 reflected from ,a concave mirror, are united at its focus, their 
 effect is so much the greater, as the surface of the lens or mirror, 
 exceeds that of the focus. Thus, if a lens four inches broad 
 collect the sun's rays into a focus at the distance of one foot, • 
 the image will not be more than one-tenth of an inch broad. 
 The surface of this little circle is l600 times less than the 
 surface of the lens, and consequently the density of the sun’s 
 rays within it is proportionately increased. It is not therefore 
 
464 
 
 OPTICS. 
 
 Burning lenses and mirrors. 
 
 surprising, that large lenses and mirrors burn with irresistible in- 
 tensity. 
 
 The most remarkable burning lens which has ever been 
 constructed, was made by Parker, of Fleet-street, London, 
 at an expence of upwards of 700/. It was undertaken with 
 a view to fuse and vitrify such substances as resist the fires of 
 ordinary furnaces, and more especially of applying heat in 
 vacuo, and in other circumstances in which it cannot be applied 
 by any other means. After directing his attention to this object 
 for several years, and performing a great variety of experiments 
 in the prosecution of it, he at last succeeded. His lens was of 
 flint-glass, three feet in diameter, and when fixed in its frame, 
 exposed a surface of two feet eight inches and a half in diame- 
 ter, without any other material imperfection besides a disfigure- 
 ment of one of the edges, occasioned by a piece of scoria, 
 which had found its way into its substance. Its weight was 212 
 pounds; its focal length six feet eight inches; and the diameter 
 of the focus, one inch. To concentrate the rays still further, 
 a second lens was used, and reduced the diameter of the focus 
 to half an inch. Some of the principal effects of this lens are the 
 following : 
 
 1. Every kind of wood took fire in an instant, whether hard or 
 green, or soaked in water. 
 
 2. Thin iron plates grew' hot in a moment, and then melted. 
 
 3. Tiles, slates, and all kinds of earth, were almost instantly 
 converted into glass. 
 
 4. Sulphur, pitch, and all resinous bodies, melted under 
 water. 
 
 6. Fir-w ood, exposed to the focus under w’ater, did not seem 
 changed, but when broken, the inside was burnt to a coal. 
 
 6. If a cavity were made in a piece of charcoal, and the sub- 
 stances to be acted upon were put in it, the effect of the lens was 
 much increased. 
 
 7. Any metal whatever, thus inclosed in the charcoal, melted 
 in a moment, the fire sparkling like that of a forge. 
 
 8. The ashes of wood, paper, linen, and all vegetable substances, 
 were instantly turned into a transparent glass. 
 
 9. The substances most difficult to be wrought upon, were 
 those of a white colour. 
 
 10. All metals vitrified on a China plate, when it was so 
 thick as not to melt, and the heat w^as gradually communi- 
 cated. 
 
 11. ‘When copper was thus melted, and throwm quickly 
 into cold w'ater, it produced so violent a shock as to break the 
 strongest earthen vessels, and the copper was entirely dissipated 
 
r 
 
OPTICS. 
 
 465 
 
 Burning lenses. 
 
 Though the heat of the focus was so intense as to melt gold in 
 a few seconds? yet there was so little heat at a short distance from 
 the focus, that the finger might be placed about an inch from it 
 without injury. The proprietor had the curiosity to try what the 
 sensation was at the focus, and having put his finger there for that 
 purpose, he described the sensation, not as resembling that pro- 
 duced by a fire or lighted candle, but like that of a sharp cut with 
 a lancet. 
 
 If a piece of w'ood be placed in a decanter of water, and the 
 focus of a large burning glass be thrown upon it, the wood will be 
 completely charred, though the sides of the decanter through 
 which the rays pass will not be cracked, nor any way affected, nor 
 the water -perceptibly w armed. If the w^ood be taken out, and 
 the rays be thrown on the water, neither the vessel or its contents 
 will be in the least affected ; but if a piece of metal be put into 
 the water, it soon becomes too hot to be touched, and the water 
 will presently boil. Though pure watei alone, contained in a 
 transparent vessel, cannot be heated, yet it by a little ink, or any 
 other addition, it be made of a dark colour, or the vessel itself be 
 blackened, the effect speedily takes place. 
 
 As all transparent substances, denser than air, when they are 
 spherically convex, or approaching to that form, will converge the 
 rays of light, they may, under particular circumstances, produce 
 effects which few would suspect. A very globular decanter full of 
 water, standing in an apartment where it is exposed to the fervid 
 action of a summer’s noon-tide sun, will converge the incident 
 light with sufficient regularity and intensity to inflame any very com- 
 bustible body that may happen to be at the proper distance. We 
 have the recollection of a serious fire -having aiisen from such a 
 cause. 
 
 Upon the same principle, we may explain another appearance 
 sometimes observable. If in summer, after much dry weather, a 
 shower of rain falls, and the sun quickly after shines with full splen- 
 dour, an accurate observer will detect a very curious phenomenon. 
 Many of the leaves and flow ers of plants which were entire before 
 the shower, are found to be perforated with small holes. It might 
 be supposed that the caterpillar has renew ed his depredations with 
 new vigour, but it is found upon closer examination, that the leaves 
 he never touches are no more exempt from these perforations than 
 others. Perhaps the following considerations will point out to us 
 the truth. If water be thrown upon a dusty floor, it is well known, 
 that it collects itself into small drops or globules. If then heavy 
 drops of rain be thus collected on the dusty leaves, they w ill in effect 
 be little burning-lenses, and when steadily exposed to the direct 
 lays of a hot sun, they may produce the perforations in question. 
 -20. — VoL. L 3 0 
 
466 
 
 OPTICS. 
 
 Burning mirrors. — Camera obscura. 
 
 Burning mirrors were made in very remote times; the most fa- 
 mous were those of Archimedes and Proclus ; by the former of 
 which the Roman ships, besieging Syracuse, according to the tes- 
 timony of several writers, and by the latter, the navy of Vitalian, 
 besieging Byzantium, were reduced to ashes. Among the mo- 
 derns, the burning mirror contrived by BufFon is the most remark- 
 able. It is a polyhedron, six feet broad, and as many high, con- 
 sisting of 160 small mirrors, or flat pieces of looking-glass, each 
 six inches Square; by means of this compound mirror, with the 
 faint rays of the sun in the month of March, he set on fire boards 
 of beech-wood at the distance of 150 feet. It maybe used to 
 burn downwards, or horizontally, at pleasure; for each of the 
 pieces that compose it is moveable by three screws, so that it may 
 be set to a proper inclination for directing the rays towards any 
 given point; and it turns either in its greater focus, or in any nearer 
 interval. Buffbn, at another time, burnt wood at the distance of 
 above 200 feet, and melted silver at 50. 
 
 A metallic mirror, of the same size as one of glass, is much 
 superior to the latter in its power of burning: and those metals 
 answer perfectly well which reflect images but indifferently; a 
 piece of tin-plate hammered into the form of a parabola, and well 
 polished, makes an excellent burning mirror. 
 
 Of the Camera Obscura. 
 
 If a hole be made in a window shutter or side of a darkened 
 room, the inverted images of all external objects, from which rays 
 of light can enter at the hole, may be observed upon the opposite 
 wall of the room. This is the camera obscura in its most imper- 
 fect state. Whether the hole be small or large, the rays are so 
 much scattered, and partly too inflected by the sides of the aper- 
 ture, that the picture is indistinct, and little interesting. But if a 
 convex glass be applied to the hole, the pencils of divergent rays 
 proceeding from the illuminated objects without, are converged to 
 their proper foci, and if a screen be placed there to receive them, 
 a picture is formed by them, incomparably superior to the happiest 
 efforts of the painter^s skill. A large lens, with a considerable 
 focal distance, forms the best image, which is also the most beau- 
 tiful, when the external objects are aJl nearly at the same distance. 
 When they are very unequally distant, some confusion arises, be- 
 cause the foci of the pencils of rays proceeding from them require 
 the glass to be at different distances lioni the screen, and no ar- 
 rangement can be made to admit of this adjustment. The eye is 
 a natural camera obscura, and we find that we cannot see a near 
 and a distant object distinctly at the same moment; if we look at 
 
OPTICS. 
 
 467 
 
 Camera obscura. 
 
 the near one, the crystaline lens adjusts itself so that the rays from 
 it are duly converged upon the retina; if we look at the further^ 
 another adjustment ensues, or the rays, from their less divergence, 
 would not form a distinct picture on the retina. Hence as we 
 cannot hope to exceed the works of nature, we must be satisfied 
 with altering the position of the glass or the screen, to suit the 
 objects we most wish to have clear and well defined. 
 
 It is necessary, in this experiment of the camera obscura, 
 that the window should not be opposite the sun, otherwise we shall 
 have no image but that of his brightness; and yet it is necessary 
 also, that the sun should illuminate strongly the objects which are 
 to be depicted within, or the rays will be sent so feebly from every 
 part, that the picture will not be brilliant. The inverted position 
 of the image shown by this camera obscura, is rather an imperfec- 
 tion; but if you take a looking-glass, and hold it before you, with 
 the face towards the picture, and inclining downwards, the image 
 will be seen erect in the glass, and appear with greater brilliancy 
 than upon the screen. The colouring of the picture is exquisitely 
 soft and delLcate, every part is ii. due proportion, the light and 
 shade are distributed with the most accurate propriety, and the 
 motions of all objects perfectly expressed. Thus, in faithful mi- 
 niature. 
 
 Hills, dales, and woods appear. 
 
 Flocks graze the fields, birds wing the silent air. 
 
 In darkened rooms, where light can only pass 
 Through the small surface of a convex glass : 
 
 On the white sheet the moving figures rise, 
 
 The forest waves, clouds float along the skies.” 
 
 The camera obscura is frequently made of a portable size, and 
 sometimes so small that it may be carried in the pocket. The 
 construction of them is sometimes a little varied, but they are all 
 essentially the same in principle. The section of the one we shall 
 describe, is shewn by fig. 3, pi. V. It consists of a rectangular 
 box, ABCD, in the front of which slides a tube F. At the ex- 
 tremity of this tube is a double convex lens, EH. A plane mirror, 
 ST, is contained within the box, and set at an angle of 43 degrees. 
 The pencils of rays flowing from external objects through the con- 
 vex glass, are reflected upwards by this mirror, and meet in points 
 on IK, which is either a piece of oiled paper, or what is still bet- 
 ter, a piece of glass, of which the polish has been removed from 
 one side, by rubbing it with sandstone and water. To an eye 
 looking from the top downward upon the dass IK, a distinct pic- 
 ture of external objects in their proper cmours will be perceived, 
 
468 
 
 OPTICS. 
 
 Camera obscura. 
 
 and with a black-lead pencil their outlines may be traced upon the 
 semi-transparent glass, the rough side of which should always be 
 uppermost. The view will be inverted witli respect to right and 
 left, but when the outline has been finished, if the glass be taken 
 and turned down upon a piece of damp paper, and subjected to a 
 gentle pressure for a short time, the pencil-m,arks on the glass will 
 be transferred to the paper, and the right and left side of the draw- 
 ing will correspond to those of the objects they are meant to re- 
 present. The picture formed upon the glass IK, should be freed 
 as much as possible from ail extraneous light: it should therefore 
 have a thin board in the position BY, and from Y should be sus- 
 pended a curtain of black silk, w’hich should extend over the ends. 
 The inner surface of the board YB, and all the internal parts of 
 the instrument, should likewise be blackened. 
 
 The tube in which the convex glass EH is fixed, slides in the 
 forepart of the box, in order that the distance of the glass may be 
 adjusted to the distance of outward objects, and the proper dis- 
 tance is soon discovered, because the picture is then the most dis- 
 tinct. This portable camera obscura may be made in the form of 
 a book, which will contain within itself every part of the instru- 
 ment except the tube. 
 
 The mirror of a camera obscura is commonly made of glass, 
 which contributes not a little to render the instiument imper- 
 fect, for the first surface of the glass reflects a part of the in- 
 cident light, and forms an image which the reflection from 
 
 the silvered side does not wholly efface, because the rays from 
 the two surfaces do not coincide; and though the whole pic- 
 ture appears exquisitely beautiful to the eye, at a little dis- 
 
 tance, yet it is found upon trial, that the double reflection de- 
 feats the attempt to make a correct copy. The occasional 
 use of the instrument will, how'ever, furnish the young stu- 
 dent of perspective with many useful hints. A metallic mir- 
 ror, of a composition similar to that employed for telescopes, 
 is a remedy for the defect in question, but adds exorbitantly to 
 the expense- 
 
 Dr. Wollaston endeavours to prove that the image formed by 
 the camera obscura is much improved by the use of a convexo- 
 concave lens, instead of the double convex one usually employed. 
 The tendency of the improvement is to make the sides of the 
 picture as distinct as the central part of it. For the Doctor’s 
 paper, see the Philos. Trans, for 1812 , part II; it contains the 
 necessary data for deriving the greatest advantage from bis disco- 
 very. 
 
OPTICS- 
 
 46*9 
 
 Magic lantern. 
 
 The Magic Lantern aiid Phantasmagoria, 
 
 The magic lantern is a machine employed to throw a magnified 
 image of paintings upon glass or any transparent substance, on a 
 white screen in a darkened chamber. It has generally been de- 
 voted to the amusement of children, paintings of a ludicrous de- 
 scription being its usual accompaniments; but it may be employed 
 \^ith propriety to illustrate the principles of the sciences, by a se- 
 lection of suitable diagrams. 
 
 A section of this machine is snown by fig. 4, pi. V, where 
 ABCD is a tin lantern, from the side of which proceeds a square 
 tube, h nklm c, consisting of two parts ; the outermost of which, 
 n k I m, slides over the other, so that the whole tube may readily 
 be lengthened or shortened. In the end of the arm, nklm, is 
 fixed a convex glass k 1; about d e there is a contrivance for ad- 
 mitting and placing an object d e, painted in transparent colours, 
 on a plane thin piece of glass. A single or double convex glass, 
 h h c, is employed to cast the light from the flame of the candle, 
 a, strongly on the picture d e, painted on the plane thin glass. 
 From the shortness of its focus, and consequently great convexity, 
 it is usually called the bull's eye. If the object d e, be placed 
 further from the glass k I than its focus, a distinct image of the ob- 
 ject will be projected by the glass k I, upon the opposite white 
 wall or screen FH, at f gy and it will be in an erect posture, 
 provided care be taken to slide the transparent painting invertedly 
 into its receptacle. If the tube b n k I m c, he contracted^ 
 and thereby the glass, k I, brought nearer the object, d e, the 
 representation, f g, will be projected so much the larger, and so 
 much the more distant from the glass k 1; the image may also be 
 enlarged by drawing back the lantern to a greater distance from 
 the screen; but as the image is enlarged, the same quantity of 
 light i.^ spread over a greater surface, and consequently it diminishes 
 in distinctness. 
 
 Tlie apartment in which the exhibition is made, should be com- 
 pletely darkened, and no light should escape from the lantern ex- 
 cept Wiiat passes through the glass b h c. To increase the light, 
 a concave refiector, ST, is frequently used, of such a curvature, 
 that the candle is in its focus, so that the rays proceeding from it 
 fall parallel upon the glass b h c. The glass upon which the pic- 
 tures are made, is generally of sufficient length to contain several 
 sets of figures; so that when the spectators are satisfied with the 
 first set, by sliding the same glass a little further on, another figure 
 is exhibited. 
 
 The exhibition called the Phantasmagoria, which has been 
 
470 
 
 OPTICS'. 
 
 Phantasmagoria. — Microscopes. 
 
 SO much admired, is performed by means of a magic lantern, 
 generally one of extraordinary dimensions, but in other respects 
 not much varied in its construction. * In the common lantern, the 
 figures are painted on the glass, and all the rest of the glass is left 
 transparent, consequently the image on the screen is a circle of 
 light with the figures in the midst of it ; but in the Phantasma- 
 goria, the whole of the glass is made opaque, except the space 
 taken up by the figures painted with the transparent colours, hence 
 this difference in the effect is produced, that no light falls upon 
 the screen but what passes through the figures themselves, conse- 
 quently there is no circle of light, or any thing but the figures 
 upon the screen. To complete the deception which this change 
 may be made to produce, the following, or some equivalent ar- 
 rangement must be resorted to. Let the door of a darkened room, 
 in which the exhibition is to be seen, be set wide open, and let its 
 place be supplied with a screen of thin silk, or fine linen, or of 
 paper rendered transpaient. From the outside of the room, let 
 the pictures, painted as above described, be thrown upon the screen, 
 of a very minute size. They will immediately be seen within the 
 room, and though remarkably brilliant, they will be supposed to be 
 distant by the spectators, because they see nothing but the light 
 which comes from them. Let the lantern be drawn back to a 
 greater distance from the screen, and as the images are gradually 
 enlarged, the spectators will suppose them to be actually approach- 
 ing towards them, and pendant in the air. The chief defect in 
 this exhibition, is, that the images decrease in distinctness as they 
 increase in size ; but this might be remedied, if a contrivance w ere 
 added to the machine, so that the mirror should be wholly covered, 
 and the bull’s eye covered in part, at the commencement, and 
 gradually uncovered, as more light was required to keep the en- 
 larged figure as bright or somewhat brighter, than when it was 
 small. 
 
 Of Microscopes. 
 
 The microscope is an instrument for magnifying small objects. 
 This effect is produced by means of convex lenses. When only 
 one convex glass or lens is used, the instrument is called a single 
 microscope ; but if two or more are employed conjointly, it is 
 called a double or compound microscope* 
 
 The Single Microscope* 
 
 The apparent magnitude of objects is measured by the angle 
 under which they are seen by the eye j and those angles are recf-^ 
 
OPTICS. 
 
 471 
 
 spherule microscoprs. 
 
 procally as the distances from the eye. If eight inches be the 
 nearest limit of distinct vision to the naked eye, and by interposing 
 a lens or other body, we can see with equal distinctness at a nearer 
 distance, the object will appear to be as much larger through the 
 lens than to the naked eye, as its distance from the eye is less than 
 the distance of unassisted vision. If the focal distance ot a convex 
 lens be one quarter of an inch, then will that be but one thirty- 
 second part of the common limit of vision or eight inches, so that 
 the lineal dimensions of an object examined with it will be magni- 
 fied 32 times; the surface 1024 times; and the solidity 1,048,676 
 times ; for these two last numbers are the square and cube of 32. 
 For a lens or spherule of any other focus, the magnifying power is 
 easily found by the same rule. 
 
 The simplest microscope which can be employed to any use- 
 ful purpose, is perhaps that which is made with a drop of water, 
 suspended in a very small hole in a thin slip of brass, or any simi- 
 lar material. This may easily be constructed where no other mi- 
 croscope can be obtained, and its performance will afford not a 
 little pleasure. A spherule of water, it must be observed, of the 
 same size as one of glass, will not magnify so much as the latter, 
 because, as its density is not so great, it has a longer focus. A 
 small drop or spherule of water, held to the eye by candlelight or 
 moonlight, without any other apparatus, magnifies in a very sur- 
 prising manner the animalcula contained in it. The reason is, that 
 the rays coming from the interior surface of the first hemisphere, 
 are reflected so as to fall under the same angle on the surface of 
 the hind hemisphere, to which the eye is applied, as if they came 
 from the focus of the spherule ; whence they are propagated to 
 the eye in the same manner as if the objects were placed vyithouL 
 the spherule in its focus. 
 
 These water microscopes have given rise to the use of other 
 fluids, with several varieties of construction. Brewster de- 
 scribes one in the Appendix to his edition of Ferguson’s lectures. 
 Instead of water, he makes use of very pure and viscid turpen 
 tine, which he takes up by the point of a piece of wood, and 
 drops successively upon a thin and well-polished glass: different 
 quantities being thus taken up and dropped in a similar 
 manner, form four or more plano-convex lenses of turpentine 
 varnish, which may be made of any focal length, by taking 
 up a greater or less quantity of the fluid. The lower surface 
 of the glass having been first smoked with a candle, the black 
 pigment below the lenses is then to be removed, so that no light 
 may pass by their circumference. The piece of glass is then 
 to be perforated, and surrounded with a toothed wheel, which 
 
472 
 
 OPTICS. 
 
 Spherule microscopes. 
 
 can be moved round the Imle as a centre b}^ an endless screw. 
 The apparatus is then placed in a circular case, and this case 
 fixed to a horizontal arm by means of a brass pin, which passes 
 through its upper and under surfaces, and through the hole 
 already mentioned, which does not embrace the pin very 
 tightly, in order that the toothed wheel may revolve with 
 facility. On the upper surface of the circular case is an aper- 
 ture directly above the line described by the centres of the fluid 
 lenses, when moving round the central hole ; and in this 
 aperture is inserted a small cap, with a little hole at its top, to 
 which the eye is applied. A moveable stage carries the slider, 
 on which microscopic objects are laid, and is brought nearer 
 or removed from the lenses by a vertical screw'. The objects 
 on the slider are illuminated by a plane mirror, which has both 
 a vertical and horizontal motion for this purpose. When tbe 
 microscope is thus constructed, the object to be viewed is placed 
 upon the slider, and the endless screw' is tmned till one of the 
 lenses be directly under the aperture; and the slider is thus 
 raised or depressed by the vertical screw, till the object be 
 brought into the focus of the lens. In this manner, by turning 
 the endless screw, and bringing all the lenses, one after anothei-, 
 directly below the aperture, the object may be successively examined 
 with a variety of magnifying pow ers. These fluid lenses 'have been 
 employed as the object-glasses of compound microscopes. 
 
 Minute glass spherules make very excellent microscopes, to 
 those who have a little patience in using such instruments; for 
 the foci of the smallest sort are so short, that it requires consider- 
 able attention to employ them well. F. Di Torre, of Naples, in 
 1765, sent several glass globules to the Royal Society. The 
 largest of them was only two Paris points in diameter, and is said 
 to magnify the diameter of an object 640 times ; another was the 
 size of one Paris point, magnifying the diameter 1280 times; and 
 the smallest no more than one-half of a Paris point, or the 144th 
 part of an inch in diameter, and is said to magnify the diameter or 
 an object 2560 times, and consequently the square of that diameter 
 6,553,600 times. Globules so exceedingly minute as these, were 
 at one time highly prized, but spherule microscopes are not 
 now' made so small, to avoid straining the eyes. The third or 
 smallest globule above-mentioned, could only be the 576tb 
 part of an inch distant from the object, because the focus or 
 a glass globe is at the distance of one-fourth of its diameter; 
 it is obvious therefore that it could admit very little light, jpid 
 could not be used without pain and difficulty even by practised 
 observers. 
 
OPTICS. 
 
 473 
 
 INIode of making glass spherules. 
 
 Of the various methods which have been recommended for 
 making glass spherules, the following by Nicholson is perhaps 
 the best. It is observed, by this valuable practical writer, 
 that the usual method has been to draw out a line thread of the 
 soft white glass called crystal, and to convert the extremity of 
 this into a spherule by melting it at the flame of a candle. But 
 this glass contains lead, which is disposed to become opaque by 
 partial reduction, unless the management be very carefully at- 
 tended to. He found that the hard glass used for windows sel- 
 dom fails to afford excellent spherules. This glass is of a clear 
 bright green when seen edgeways. A thin piece, less than one- 
 tenth of an inch broad, was cut from the edge of a pane of glass. 
 This was held perpendicularly by the upper end, and the flame 
 of a candle was directed upon it by the blow-pipe, at the 
 distance of about an inch from the lower end. The glass 
 became soft, and the lower piece descended by its own weight 
 to the distance of about two feet, where it remained suspended, 
 by a thin thread of glass, about -j^-^th of an inch in diameter, 
 A part of this thread was applied endways to the lower blue 
 flame of the candle, without the use of the blow-pipe. The 
 extremity immediately became white, and formed a globule. 
 The glass was then gradually and regularly thrust towards the 
 flame, but never into it, until the globule was sufiiciently large. 
 A number of these were made, and being afterwards examined bj 
 viewing their focal images with a deep magnifier, proved very 
 bright, round, and perfect. 
 
 Spherules are mounted for use, by placing them betw'een two 
 very thin plates of brass, each containing a small hole rather less 
 than themselves. If any imperfection in the globule is discover- 
 able, it is placed on one side, so that it may be covered by the 
 plates. The objects may be placed on the point of a needle, the 
 direction of which should be at right angles to the axis of the eye, 
 to prevent accidents. 
 
 In using these spherule microscopes, the objects are to be 
 placed in one focus, and the eye in the other; and from the 
 shortness of the focus, it becomes nearly impossible to view any 
 but pellucid objects, because the nearness of the eye obscures the 
 Jight. 
 
 At page 273, w^e have adverted to the discoveries of Le- 
 wenhoeck; these were all made with single microscopes, which, 
 though spherules in appearance, were in reality double convex: 
 lenses. He bequeathed to the Royal Society a cabinet of micro- 
 scopes made with his own hands. The focus of the greatest mag- 
 nifier was at the distance of of an inch from its centre, conse- 
 quently it magnifies the diameter of an object I6O times, the su- 
 
 20>ol. I. 3P 
 
474 
 
 OPTICS. 
 
 Single microscope. — Wollaston’s improvement of the microscope. 
 
 perficies 25,600, and the solidity 4,096,000, on the supposition 
 that distinct vision is not naturally effected nearer than eight 
 inches, which is an average distance. 
 
 In all microscopes, it is desirable to have the means of 
 viewing objects with ease and steadiness. The following form 
 of a single microscope is very convenient. AB, fig. 5, plate V. 
 is a tablet of wood, ivory, &c. to which is fitted a small handle 
 of the same material. Upon the top of it is fixed a small cylin- 
 drical stem, G, and this stem receives the frame containing a lens 
 C, at the focal distance of which a small pair of pliers holding 
 the object, may be placed by means of a slide and adjusting 
 screw L. The pliers are opened by means of two little studs, 
 a e. The eye is at the other focus of the lens, and the object is 
 of course seen magnified according to the power of the lens em- 
 ployed. This instrument, inclosed in a case, may be carried in 
 the pocket without encumbrance. Lenses, the focal lengths of 
 which are from three to five-tenths of an inch, are the most suit- 
 able for ordinary use. 
 
 Dr. Wollaston has proposed an improvement of microscopes, 
 which he thinks highly advantageous. The great desideratum, he 
 observes, in employing high magnifiers, is sufficifency of light ; 
 and it is accordingly expedient to make the aperture of the little 
 lens as large as is consistent with distinct vision. But if the ob- 
 ject to be viewed is of such magnitude as to appear under an 
 angle of several degrees on each side of the centre, the requisite 
 distinctness cannot be given to the whole surface by a common 
 lens, in consequence of the confusion occasioned by the oblique 
 incidence of the lateral rays, excepting by means of a very small 
 aperture, and proportionable diminution of light. In order to 
 remedy this inconvenience, he used two plano-convex lenses 
 ground to the same radius, and applied their plane surfaces on 
 opposite sides of the same aperture in a thin piece of metal. 
 Thus he virtually obtained a double convex lens, with this advan- 
 tage, that the passage of oblique rays was at right angles with the 
 surfaces, as well as the central pencil. With a lens so construct- 
 ed, the perforation that appeared to give the most perfect distinct- 
 ness, was about one-fifth part of the focal length in diameter, and 
 when such an aperture is well centred, the visible field is at least 
 as much as twenty degrees in diameter. It is true, that a portion 
 of light is lost by doubling the number of surfaces, but this is 
 more than compensated by the greater aperture which, under these 
 circumstances, is compatible with distinct vision. 
 
OPTICS. 
 
 475 
 
 Ellis’s microscope. 
 
 Ellis s single aquatic Microscope, 
 
 This instrument takes its name from John Ellis, the author 
 of an essay towards a natural history of corallines, and many 
 curious zoophytes. It enabled him to pursue his investigations, 
 relative to the structure and economy of these wonderful pro- 
 ductions of Nature, with considerable success; and it is w’ell 
 adapted to show botanical subjects. Jt is simple in its construc- 
 tion, very portable, and commodious in use. It is represented by 
 hg. 6, pi. V. 
 
 The whole apparatus is contained in a small box or case, K, 
 which is generally covered with tish-skin. On the top of the 
 box there is a socket, for receiving the screw which is at the 
 bottom of the brass pillar. A, when the instrument is prepared 
 for use. D is a cylindrical brass pin, which exactly fills and 
 slides up and down in a hole drilled in the middle of the pillar 
 A. The pin D is moveable, in order to adjust the lenses to 
 their focal or proper distances from the object. It may be fast- 
 ened at any height by the screw Z, which presses against it ; at 
 the top of it is a socket, to receive the arm or bar E, which car- 
 ries the magnifiers. The arm E may be moved backwards and 
 forwards in the socket X, and sideways by turning it with the pin 
 D ; so that the magnifier, which is screwed into the ring at the 
 end of the bar E, may be easily made to traverse over any part of 
 the object lying on the stage or plate B. 
 
 F is a polished silver speculum, with a magnifying lens 
 placed at the centre of it, which is perforated for the purpose. 
 The silver speculum screws into the arm E, as at F. G, 
 another speculum, with its lens, which is of a different magni- 
 fying power to the former. H is a brass semi-circle, which 
 supports the mirror I ; the pin R of this semi-circle, passes 
 through a hole towards the bottom of the pillar A. B, the stage 
 on which the objects are to be^ placed; it fits into a small dove- 
 tailed ^arm, which is at the upper end of the pillar A. C, a 
 plane round glass, witli a small piece of black silk stuck on it, 
 is used to lie in a circular groove made in the stage B. A 
 hollow glass, like a watch-glass, is occasionally laid on the stage 
 instead of the plane glass. L, a pair of nippers. I’hese are 
 fixed to the stage by the pin at the bottom ; the steel wire of 
 these nippers slides backward and forward in the socket, and 
 this socket is moveable upwards and downwards by means of 
 the joint, so that the position of the object may be varied at 
 pleasure. The object may be fixed in the nippers, stuck on 
 the point, or affixed by a little gum-water, 8cc. to the ivory 
 
476 
 
 OPTICS. 
 
 Ellis’s microscope. — Hand megalscope. 
 
 cylinder N, which screws, when occasion requires, to the point of 
 the nippers. 
 
 To use this microscope, after taking* all the parts of the appa- 
 ratus out of the box, begin by screwing the pillar A to the cover ; 
 pass the pin R, of the semi-circle which carries the mirror, 
 through the hole which is near the bottom of the pillar A ; push 
 the stage into the dove-tail at B, slide the pin D into the pillar ; 
 then pass the bar E through the socket X, which is at the top of 
 the pin D, and screw one of the magnifying lenses into the ring 
 at F. These operations, which are performed in a very short 
 time, in a less time, indeed, than is necessary to describe them, 
 prepare the microscope for use. Let the object be now placed 
 either upon one of the glasses of the stage, or in the nippers L, 
 and in such a manner that it may be as nearly as possible over 
 the centre of the stage. Bring the speculum F over the part you 
 mean to observe ; then throw as much light as possible on the 
 speculum, by means of the mirror I, which, it will be perceived, 
 admits of a double motion — one horizontally, the other to set it 
 in any required angle ; the light received by the speculum is re- 
 flected by it on the object. The distance of the lens F from the 
 object, is regulated by moving the pin D, up and down, until a 
 distinct view of it is obtained. The best rule is, to place the lens 
 beyond its focal distance from the object, and then gradually to 
 slide it down till the object appears clear and well defined. The 
 adjustment of the lenses to their foci, and the distribution of 
 the light on the object, are what require the most attention; but 
 the management of the instrument is easily acquired by a little 
 practice. 
 
 This microscope is sometimes furnished with a rack and 
 pinion, for more steadily elevating or depressing the pin D, in ad- 
 justing the lens to its focus. 
 
 The Hand Megalscope. 
 
 This instrument was contrived by Martin, and is well 
 adapted for viewing the larger sort of small objects expedi- 
 tiously. A, fig. 7, plate V, is the case of brass, tortoise shell, 
 &c. with its three lenses D, E, F, each surrounded by a rim or 
 frame of the same material as the case. H is the handle. The 
 lenses are commonly of 1, 1|, and 2 inches focus ; as they 
 all move on the same pin, they turn over each other, and can 
 be used either conjointly or separately. The three lenses 
 singly, afford, of course, three magnifying powers, and by 
 combining tw'o and two, we obtain three more : for D with E 
 makes one, D with F another, and E with F a third; and all 
 
OPTICS. 
 
 2'L. V. 
 
 
 BCooptfdtl, • ^ TDixon jc 
 
 JPnbiyh’d by Wnftall .Tislifr kUixcTt. Zivef'P^cl I8l’t. 
 
OPTICS,' 
 
 477 
 
 Compound microscope. 
 
 three together make another ; so that by this simple apparatus 
 we have seven different magnifying powers. When the three 
 lenses are combined, it is better to turn them in, and look 
 through them by the small aperture in the sides of the case, 
 the eye will not then be incommoded by external light; the dis- 
 tortion of objects by the sides of the glasses, will be prevented, 
 and the eye will coincide more exactly with the common axes of 
 the lenses. 
 
 Double or Compound Microscope, 
 
 Compound microscopes present to the eye, not the object it- 
 self, like the single microscope, but its image. 
 
 In treating of these instruments and of telescopes, the lens 
 which is next to the object, is called the object-glass, and all the 
 other lenses are called eye-glasses. 
 
 Fig. 1, pi. VI. represents the two lenses of a compound mi- 
 croscope : a b C IS Vi small object, placed at a little greater distance 
 from the object glass d ej\ than its principal focus : pencils of rays 
 proceeding from the object, pass through the glass, by which they 
 are converged, and united in points at ABC, where an image is 
 formed, which is larger than the object, in proportion as the dis- 
 tance B e exceeds the distance e c. The eye-glass, DEF, is so 
 placed, that its focus is at B, and the eye, to view the image, must 
 be about the same distance on the other side. The rays of each 
 pencil will be parallel after going out of the eye-glass, but they will 
 be again converged by the refractive powers of the eye, and will 
 form on the retina, a large inverted image of the object ; for it is 
 evident that it is seen, by the interposition of the glasses, under the 
 angle IFD, instead of the angle which to the naked eye would be 
 contained between b L a. 
 
 The magnifying power of this microscope is easily comput- 
 ed. In the first place, the image AC is to the object, as the 
 distance Be is to the distance e c ; and secondly, the image 
 AC will be seen by the eye at I, under the angle DIF, which 
 is equal to the angle AEC ; and therefore the image will appear 
 as much longer than to the naked eye, as the distance BE is 
 shorter than eight inches, (or the limit of distinct vision to the 
 naked eye ;) so that, if the distance e c be one inch, e B six inches 
 and EB two inches, then the image AC is six times longer than 
 the object a and that image is magnified four times by the lens 
 DF. Hence if four be multiplied be six, we shall have the total 
 power of the microscope, which magnifies the diameter of objects 
 24 limes, their superficies 24 times 24, or 376 times, and their so- 
 lidity 13,824 times. 
 
478 
 
 OPTICS. 
 
 Compoiiiid microscope. — Solar microscope. 
 
 This microscope has a larger field of view than a simple 
 microscope of the same power, and its field of view may be 
 further enlarged by the addition of one or two more lenses 
 instead of the single lens DF. The magnifying power of the 
 instrument with more than two lenses, must be computed from 
 the effect of all the lenses ; or it may be ascertained experi- 
 mentally in the following manner : Place part of a divided 
 ruler before the microscope, so that, looking through the instru- 
 ment, you may see one of its divisions magnified ; then open 
 the other eye also, and looking with it at the ruler out of the 
 microscope, the image of the magnified division will seem to 
 be projected on the ruler ; and you may easily see how’ many 
 divisions, of the unmagnified ruler, measure or are equal to the 
 single magnified division, and that number is the magnifying 
 power of the microscope. Thus, if the ruler be divided after 
 the common way, into inches and tenths, and one-tenth is mag- 
 nified, so as to appear equal to three inches, you may conclude 
 that the microscope magnifies the diameter or length of objects 
 thirty times. 
 
 The Solar Microscope. 
 
 This microscope is sometimes called the camera obscura mi- 
 croscope, but it still more nearly resembles the magic lantern in 
 its effect. '^Fhe exhibition it affords is made in a darkened room, 
 and it can only be used w hen the sun shines. 
 
 This instrument consists of one plane mirror, and two lenses ; 
 the mirroi, 5 o, fig. 2, pi. VI. must be without the window shutter 
 d u ; the lens a b in the shutter ; and the lens n w ithin the dark 
 room. The lens a h is inclosed in a brass tube, and the lens n in 
 another smaller tube, N^liich slides in the former, for the purpose 
 of adjusting it to the proper distance from the object. The mir- 
 ror can be so turned by adjusting screws, that however obliquely 
 the incident rays EF fall upon it, they can be reflected horizon- 
 tally into the dark room, through the illuminating lens a h. This 
 lens collects those rays into a focus near the object eg ; and pass- 
 ing on through the object, they are met by the magnifier ?i: here 
 the rays cross, and proceed divergently to a vertical white screen 
 prepared to^ receive them, on which screen the image or shadow^, 
 q ?■, of the object will appear. The magnifying power of this in- 
 strument depends on the distance of the white screen ; and in 
 general bears a proportion to the distance of the object eg from 
 the magnifier n; that is, if the screen be ten times that distance 
 from the lens ?/, the image will be ten times as long, and ten 
 times as broad, as the object. About ten or twelve feet is the 
 
OPTICS. 
 
 479 
 
 Opaque microscope. — Management of microscopic objects. 
 
 best distance, for, if furllieF olf, tlie image, though larger, will be 
 obscure, and ill -defined. 
 
 A telescope may be converted into a microscope, by removing 
 the object-glass to a greater distance from the eye-glass. And 
 since the distance of the image is various, according to the dis- 
 tance of the object from the focus, (and it is magnified the more, 
 as its distance from the object is greater,) the same' telescope may 
 be successively converted into microscopes, which magnify in dif- 
 ferent degrees. 
 
 Opaque Microscope. 
 
 The solar microscope above described, is calculated only to 
 exhibit transparent objects, or at least such as permit a part of 
 the incident light to pass through them ; to view opaque objects, 
 another mirror must be used, in order that they may be seen by 
 the light they reflect. In fig. 3, pi. VI, the mirror <7, and the 
 lens c, are the same as in the common solar microscope ; but the 
 converging rays from c are met by a mirror e n, placed diagonally, 
 and which throws up the rays much condensed upon the opaque 
 object SR ; from the object they are reflected to the magnifier O, 
 from which they proceed diverging to the screen p </, where the 
 object will be painted and greatly magnified. The objects are 
 generally stuck by a wafer, or gum-water, to a thin slider of wood, 
 and not placed in the focus of the lens C, to prevent their being 
 burnt. 
 
 Of Microscopic Objects. 
 
 Whatever object offers itself as the subject of our examina- 
 tion with the microscope, the size, contexture, and nature of it, 
 are first to be considered, in order to examine it with such glasses, 
 and in such a manner, as may show it best. The first step should 
 always be to view the w hole of it together, with a magnifier that 
 will take the whole of it in at once ; and after this, the several 
 parts of it may the more fitly be examined, w hether remaining on 
 the object or separated from it. The smaller the parts are, the 
 more powerful ought the magnifiers to be which are employed. 
 The transparency or opacity of the object must also be consi- 
 dered, and the glasses employed must be selected accordingly ; 
 for a transparent object will bear a much greater magnifier than 
 one which is opaque, since the nearness of the eye, when a high 
 magnifier is used, unavoidably darkens an object, in its own nature 
 opaque, and renders it very difficult to be seen, unless by the help 
 of an apparatus contrived for that purpose, like Ellis’s micro- 
 7 
 
480 
 
 OPTICS. 
 
 Management of microscopic objects. 
 
 scope, with a silver speculum. Most objects, however become 
 transparent by being divided into extremely thin parts. 
 
 The nature of the object also, whether it be alive or dead, 
 a solid or a fluid, an animal, a vegetable, or a mineral substance, 
 must likewise be considered, and all the circumstances of it 
 attended to, that we may proceed in the most advantageous man- 
 ner. If it be a living object, care must be taken not to squeeze 
 or injure it, that we may see it in its natural state and full perfec- 
 tion. If it be a fluid, and that too thick, it must be diluted with 
 water ; and if too thin, we should let some of its watery parts eva- 
 porate. Some substances are fittest for observation when dry, 
 others when moistened, and others after they have been kept for 
 some time. 
 
 Light is the next thing to be taken care of, for on this the 
 truth of all our observations depends ; a little experience will 
 show how very differently objects appear in one degree of it to 
 what they do in another. Hence, every new object should be 
 viewed in all degrees of light, from the greatest glare of bright- 
 ness to perfect obscurity ; and in all positions to each degree, 
 till we obtain all the evidence we can of its form and figure. 
 In many objects it is very difficult to distinguish between a 
 prominency and a depression, a black shadow and a black 
 stain ; and in colour, betw een a bright reflection and w hiteness. 
 The eye of a fly in one kind of light appears like a lattice 
 drilled full of holes, in the sun-shine like a solid substance co- 
 vered with golden nails ; in one position like a surface covered 
 W'ith pyramids, in another with cones, and in others w'ith still dif- 
 ferent shapes. 
 
 The degree of light must always be suited to the object. If 
 that be dark, it must be seen in a full and strong light ; but if 
 transparent, the light should be proportionably weak ; for 
 which reason, there is a contrivance, both in the single and 
 double microscope, to cut off the superabundant rays, when 
 such transparent objects are to be examined with the largest 
 magnifiers. The light of a candle, is, for many objects, and 
 especially for such as are very bright, transparent, and minute, 
 preferable to day-light ; for others, a serene day-light is best : 
 but sun-shine is the w'orst light of all ; for it is reflected from 
 objects with so much glare, and exhibits such gaudy colours, 
 that nothing can be determined from it with any certainty. 
 This remark is not to be extended to the solar microscope, for 
 which nothing but sun-shine will do, and the brighter it is, the 
 better ; w ilh this instrument, however, we do not see the object 
 itself on which the sun-shine is cast, but only the image or 
 shadow of it greatly extended on a screen; and therefore no 
 
OPTICS. 
 
 481 
 
 Management of microscopic objects. 
 
 confusion can arise from the glaring reflection of the sun’s ravs 
 from the object to the eye, as with other microscopes. And with 
 the solar microscope, we must rest contented with viewing the true 
 figure of an object, without expecting to find its natural colour ; 
 since no shadow can possibly exhibit the colour of the body which 
 it represents. 
 
 Most objects require also some management, in order to 
 bring them properly before the glasses. If they are flat and 
 transparent, and such as will not be injured by pressure, the 
 best way is to inclose them between two thin pieces of Mus- 
 covy talc, fastened by brass rings in a hole made through a slip 
 of metal or ivory, called a slider. By this means, we may very 
 conveniently preserve the feathers of butterflies, the scales of 
 fishes, and the farina of flowers, as .also the parts of insects, 
 the whole bodies of minute ones, and a great number of other 
 objects. Each slider should contain three, four, or more, 
 holes, which should not be filled promiscuously ; but all the 
 objects preserved in one slider should be such as require the 
 same magnifying power to view^ them, that there may not be a 
 necessity of changing the glasses for every object ; and the sliders 
 should be marked with the number of the magnifier to be used in 
 viewing what it contains. In placing the objects in the sliders, it 
 is proper to have at hand a small magnifier, of about an inch focUvS, 
 to examine and adjust them by, before they are fixed down with 
 the rings. 
 
 Small living objects, such as lice, fleas, bugs, mites, minute 
 spiders, &c. n*ay be placed between the talcs witiiout injuring 
 them, if care be taken to lay on the brass rings without pressing 
 them down ; but if they are too large to be treated thus, they 
 should be either preserved between two concave glasses, or else 
 viewed immediately, by holding them in the pliers, or sticking them 
 on the point, with which the other end of that instrument is usually 
 furnished. 
 
 The circulation of the blood may be most easily seen in the 
 tails and fins of fishes, in the fine membranes between the toes of 
 a frog, or best of all in the tail of a water-newt. Such parts of 
 objects of this description as are intended to be particularly view ed^ 
 must be expanded within a glass tube ; several sizes of glass tubes 
 usually accompany a microscope, and such a one should be select- 
 ed as will just admit the object, wdiich w ill then remain more quiet 
 to be examined. 
 
 If fluids come under examination, to discover the animal- 
 cula they contain, a small drop is to be taken with a hair 
 pencil, or on the nib of a clean peiq and placed on a plate of 
 21.— VoL. I. 3Q 
 
482 
 
 OPTICS. 
 
 
 Management of microscopic objects. 
 
 glass. If they are too numerous to be thus seen distinctly, some 
 lukewarm water must be added to the drop; they will then sepa- 
 rale, and may be seen extremely well. But if we are to see the 
 salts in a fluid, the contrary method must be observed, and the 
 plate of glass must be held gently over the fire, till part of the 
 liquor is evaporated. 
 
 'I’he dissection of minute animals, requires considerable pa- 
 tience and care ; but may be done very accurately by means of a 
 needle and a line lancet, placing the subject in a drop ot water, 
 for then the parts will readil) unfold themselves, and will be very 
 distinctly seen. 
 
 Such may be the methods resorted to for preserving transparent 
 objects ; but the opaque ones, such as seeds, woods, &c. require a 
 very different treatment, and are best preserved and viewed in the 
 foiiowing manner : 
 
 Cut card-paper into small slips, about half an inch long, 
 and the tenth of an inch broad ; wet these half way of their 
 length in gum-water, and with that fasten on several parcels 
 of the object. These are very convenient for viewing by the 
 microscope made for opaque objects w ith the silver speculum ; 
 but they are proper for any microscope adapted for opaqu« 
 bodies. 
 
 A small box should be contrived for these slips, with little 
 shallow^ holes for the reception of each : and this is conveniently 
 done, by cutting pieces of pasteboard, such as the covers of 
 books are made of, to the size of the box, so that they will just 
 go into it ; and then cutting holes through them w ilh a small 
 chisel, of the shape of the slips of card ; these pasteboards 
 having then a paper pasted over their bottom, are cells very 
 proper for the reception of these slips, which may easily be 
 taken out by means of a pair of pliers, and will be always ready 
 for use. 
 
 Great caution is necessary, in forming a judgment on what 
 is seen by the microscope, if the objects are extended or con- 
 tracted by force or dryness. Nothing can be determined re- 
 specting them, without making the proper allowances, and dif- 
 ferent lights and positions will often show the same object as very 
 dilferenl? from itself. There is no advantage in any greater mag- 
 nifier than such as is capable of showing distinctly the object 
 viewed ; and the less the glass magnifies, the more pleasantly the 
 object is seen. 
 
 The colours of objects are very little to be depended on, 
 as seen by the microscope ; for their several component parti- 
 <tles being by this means removed to greater distances from one 
 
OPTICS. 
 
 483 
 
 Nature of telescopes. 
 
 another, may give reflections very dilferent from what they would 
 if seen by the naked eye. 
 
 I'he motions of living creatures, also, or of the fluids contained 
 in their bodies, are by no means to be hastily judged of from what 
 we see by the microscope, without due consideration ; for as the 
 moving body, and the space in which it moves, are both magnifled, 
 the motion must be so too ; and therefore that rapidity with w hich 
 the blood seems to pass through the vessels of small animals must 
 be judged of accordingly: suppose, for instance, that a horse and 
 a mouse move their limbs exactly at the same time, if the horse 
 runs a mile while the mouse runs fifty yards ; though the number 
 of steps are the same in both, the motion of the horse must not- 
 withstanding be allow’ed to be the swiftest, and the motion of a 
 mite, as viewed by the naked eye, or through the microscope, is 
 perhaps not less different. 
 
 Of Telescopes. 
 
 Telescopes are instruments employed for discovering and view- 
 ing distant objects. They are of two kinds — refracting and re- 
 fecting ; the former consist of different lenses, through which the 
 objects are seen by rays refracted by them to the eye ; but the 
 latter consist of specula, by which the rays are reflected, with 
 lenses to magnify the image formed by the specula. 
 
 The principal effects of telescopes depend upon this maxim, 
 that objects appear larger in proportion to the angles which 
 they subtend at the eye ; and the effect is the same, w hether the 
 pencils of rays, by which objects are visible to us, come 
 directly from the objects themselves, or from any place nearer 
 to the eye, where they may have been united so as to form an 
 image of the object, because they issue again, in certain di- 
 rections, from those points where there is nothing to intercept 
 them, in the same manner as they did from the corresponding 
 points in the objects themselves. In fact, therefore, all that is 
 effected by a telescope, is, first to make such an image of a 
 distant object, by means of a lens or mirror ; and then to give 
 the eye some assistance for view ing that image as near as possi- 
 ble ; so that the angle which it shall subtend at the eye, may 
 be very large^ compared with the angle which the object itself 
 would subtend in the same situation. This is accomplished by 
 means of an eye-glass, which so refracts the pencils of rays 
 that they may afterwards be brought to their several foci by the 
 humours of the eye. But if the eye was so formed as to be able 
 to see the image with sufficient distinctness at the same distance 
 without an eye-glass, it would appear as much magnified as it 
 
484 
 
 OPTICS. 
 
 Astronomical telescope. 
 
 does when a glass is used for that purpose, though it would not in 
 all cases have so large a held of view\ 
 
 Although no image be actually formed by the foci of the 
 pencils without the eye, yet if, by the help of any eye-glass> 
 the pencils of rays shall enter the pupil, just as they would 
 have done from any place without the eye-glass, the visual angle 
 will be the same as if an image had been actually formed in that 
 place. 
 
 The forms of refracting and reflecting telescopes have been 
 frequently varied, and sometimes they are distinguished by the 
 name of their inventor, as the Galilean and Newtonian telescopes; 
 sometimes by the particular use for which they are most proper, 
 as the land telescope,” ‘‘ the night-glass, or astroiK)mical tele- 
 scope;” and sometimes by one of their remarkable properties, as 
 the achromatic telescope.” We shall in the first place notice 
 refracting telescopes. 
 
 T/fe Astronomical Telescope. 
 
 This telescope consists of two convex lenses, AB, KM, fig, 4, 
 pi. \T. each fixed at the extremity of a different tube. One of 
 the tubes is very short, as its use is merely to adjust the focus in 
 proportion to the distance of the object viewed ; and it slides 
 within the other. The tubes are not represented in the figure, as 
 the external form of telescopes are familiar almost to all. Con- 
 trary to the arrangement which takes place in the microscope, the 
 glass which has the longest focus is presented to the object, and 
 therefore constitutes the object-glass. 
 
 PR represents a very distant object, from every point of which 
 rays come so very little diverging to the object lens KiNI of the 
 telescope, as to be nearly parallel : pr \s the picture of the object 
 PR, which would be formed upon a screen situated at that place. 
 Beyond that place, the rays of every single radiant point proceed 
 divergently to the eye-glass, AB, of greater convexity, and which 
 causes the rays of each pencil to become parallel; in which direc- 
 tion they enter the eye of the observer at O. 
 
 The axes of the two lenses are coincident, in the direction 
 QLO ; L 9 is the focal distance of the object-glass, and E ^ is 
 the focal distance of the eye-glass ; consequently the distance 
 between the two glasses is equal to the sum of their focal dis- 
 tances. An object viewed through this telescope, by an eye 
 situated at O, will appear magnified and distinct, but inverted. 
 The object seen without the telescope, will be, to its appearance 
 through the telescope, as E to ^ L ; that is, as the focal dis- 
 tance of the eye-glass to the focal distance of the object-glass* 
 
OPTICS. 
 
 485 
 
 Astronomical telescope. 
 
 For the pencils of rays, which, after their crossing 2i\. r q p pro- 
 ceed divergently, fall upon the lens AB, in the same manner as 
 if a real object were situated at r q p, and consequently, after 
 passing through that lens, the rays of each pencil proceed paral- 
 lel. Now to the eye at O, the apparent magnitude of the ob- 
 ject PK, is measured by the angle BOA, or by its equal p 
 but to the naked eye at L, when the glass is removed, the appa- 
 rent magnitude of the object is measured by the angle PLR, or 
 by its equal r p; therefore the apparent magnitude to the naked 
 eye, is to the apparent magnitude through the telescope, as the 
 angle r L is to the angle p E r ; or as the distance 5- E is to the 
 distance q L. 
 
 If the angles r L y? and p E 7’ were equal to each other, the 
 telescope would not magnify, and they would be equal, if the 
 lenses were of equal focal distance. Hence, as the magnifying 
 power of the telescope is produced by making the focal distance 
 of the eye-glass less than that of the object-glass, it will easily be 
 perceived, that the greater the difference of the focal lengths, the 
 greater will be the magnifying power. In practice, however, it is 
 found that they may be so disproportionate, that the increased 
 inagHifying power is overbalanced by the indistinctness which 
 ensues. In order, therefore, to obtain a very great magnifying 
 power, with the preservation of just proportion, these telescopes 
 have sometimes been made one hundred feet or upwards in length; 
 and as they were used for astronomical purposes, or mostly in the 
 night time, they were frequently used without a tube; viz. the ob- 
 ject lens was fixed on the top of a pole, in a frame capable of 
 being moved by a cord or wire in any required direction, and the 
 eye-glass, fixed in a short tube, was held in the hand, or fitted to 
 another frame about the height of the observer, so as to be sus- 
 ceptible of a simultaneous movement. A telescope of this de- 
 scription was called an a'eriai telescope. Its use is evidently in- 
 commodious ; but such was the incredible pains taken by philoso- 
 phers in exploring the wonders which even the imperfect tele- 
 scopes at first constructed promised to lay open, tllat with such 
 an instrument the five satellites of Saturn, and many other remark- 
 able objects, were discovered. 
 
 The length of common refracting telescopes must be increased 
 in no less a proportion than the square of the increase of their 
 magnifying power ; so that in order to magnify twice as much as 
 betoie, with the same light and distinctness, the telescope must be 
 lengthened four times, and to magnify three times as much, nine 
 times. On this account, their unwieldy length, when great powers 
 are desired, is unavoidable. 
 
 The breadth of an object-glass adds nothing to the magnify- 
 
486 
 
 OPTICS. 
 
 Astronomical telescope. — Night-glass. 
 
 ing power; for whatever it may be, the image will be equally 
 formed at the distance of its focal length ; but the brilliancy of 
 the image will be increased by the breadth, as a greater number of 
 rays will then diverge from every point of the image. 
 
 The magnifying power, and the field of view, in this telescope, 
 may be increased by using two plano-convex lenses, combined so 
 as to act like one glass, and such a combination is now generally 
 employed. The disproportion above alluded to, which may sub- 
 sist between an object-glass and an eye-glass, arises from this 
 circumstance, that a lens does not converge the rays of the same 
 radiant point, exactly to the same refracted focus, and the indis 
 tinctness, which is thus occasioned in the image, increases with 
 the thickness of the lens and the increase of its curvature. Hence 
 arises the limitation to the use of highly magnifying-eye-glasses. 
 The field of view is enlarged by enlarging the eye-glass : but if a 
 lens of very short focus were made large, the irregularity of its 
 refraction would throw the view into confusion; if it were made 
 .sufficiently small to prevent this, it would be equally useless from 
 its contracted field of view, and the want of light. But if two 
 plano-convex lenses be used, the curvature of both conjointly uill 
 be less than the curvature of a single lens of equal magnifying 
 power ; such a combination therefore will improve the eye-glass 
 .of a telescope, because the aberration of the rays passing through 
 it will be less than through a single lens of the same focus. Let 
 IK, fig. 8. plate V, be a plano-convex lens, of which the focus is 
 at F, so that an object placed at F would be seen magnified 
 through it. If another lens LM be placed between the first 
 lens and its focus, the focus of the rays passing through both will 
 be shortened, and will fall at about the distance f, so that, when 
 thus combined, they will act like a single lens of much greater 
 curvature. 
 
 The slight Glass. 
 
 The telescope called a 7iight-glass is nothing more than the 
 common astronomical telescope, with tubes, and made of a short 
 length, with a small magnifying power. Its length is usually 
 about two feet, and it is generally made to magnify from six to 
 fen times. It is much used by navigators at night, for the pur- 
 pose of discovering objects that are not very distant, but which 
 cannot otherwise be seen for want of sufficient light, such as ves- 
 sels, coasts, rocks, &c. From the smallness of its magnifying 
 power, and the obscurity of the objects upon which it is employed, 
 it admits of large glasses being used, and consequently has au 
 extensive and well-enlightened field of view. 
 
OPTICS. 
 
 487 
 
 Terrestrial telescope. — Galilean telescope. 
 
 The Terrestrial Telescope j or Perspective Glass, 
 
 I The astrononiical telescope, by the use of two additional eye- 
 ‘ glasses, shows objects in their right position, and becomes a ter- 
 ; restrial telescope, that is, adapted for looking at objects on the 
 i earth. — It is still more frequently called, a perspective glass. 
 
 I This telescope is shown by fig. 5, plate VI. The rays of each 
 pencil coming from the image LM of the object IK, emerge pa- 
 If rallel from the lens AB, and having crossed at its focus, Oj they 
 ] continue in that direction to the lens EF, in consequence of which 
 they form an image ST at the focus of the second lens, and again 
 I diverging, they fall upon the third lens, CD, in the same manner 
 ‘ as tiiey did upon the lens AB ; therefore, after their emergence 
 I from this last lens, they fall parallel upon the eye at G. But as 
 ! the last image ST, is not inverted as at LM, but in the same po- 
 ^ sition as the object IK, the eye sees a true or upright picture, as 
 .i if the rays had come directly from the object, 
 fj The last lens, or the one nearest to the eye, in this telescope, 
 f is mostly now made double, that is, two plano-convex lenses are 
 
 8 substituted for the double convex one, for the purpose of enlarg- 
 ing the field of view, as pointed out for the eye-piece of the as- 
 tronomical telescope. By this means all the best terrestrial tele- 
 scopes contain four lenses in the tube next the eye. 
 
 II 
 
 I The Galilean Telescope. 
 
 A report having reached the illustrious Galileo, that an optical 
 I instrument had been constructed, by means of which distant ob- 
 I jects appeared as if they were near; that philosopher, forcibly 
 struck with the advantage which might be derived from the con- 
 trivance, instantly directed his attention to the discovery of its na- 
 ture. The night after lie heard the account, the construction of 
 such an instrument occurred to him ; the day following he put the 
 several parts of it together, and the telescope which he thus in- 
 I vented is still called by his name. ^V illi that noble frankness for 
 which he was remarkable, and which ought always to distinguish 
 the true philosopher from the empiric, he forthwith explained 
 publicly the structure and wonderful uses that might be made of 
 the instrument ; and shortly afterw ards, as an acknowledgment 
 of his merit, the Republic of Venice more than tripled the salary 
 of his professorship. 
 
 Galileo’s telescope consists of a convex object-glass, and a 
 concave eye-glass, as represented by fjg. G, plate VI. The dis- 
 tance between the two lenses is less than the focal distance of 
 the object-glass ; but the concave glass is situated so as to make 
 
488 
 
 OPTICS. 
 
 Galilean telescope. — Achromatic object-glass. 
 
 the rays of each pencil fall parallel upon the eye, as is evident by 
 conceiving the rays to go back again through the eye-glass, to- 
 wards O ; *EO being the focal length of tlie eye-glass. 
 
 When the sphere of concavity in the eye-glass of a Galileati 
 telescope is equal to the sphere of convexity in the eye-glass ol 
 another telescope, their magnifying power is the same ; but the 
 concave glass being placed between the object-glass and its focus, 
 the Galilean telescope will be shorter than the other, by twice the 
 focal length of the eye-glass. Hence, if the lengths of the tele- 
 scopes be the same, the Galilean will have the greatest magnifying 
 power. 
 
 The held of view^, or quantity of objects taken in by the Gali- 
 lean telescope, does not depend, as in those with convex glasses, 
 upon the size of the eye-glass, but upon the breadth of the pupil 
 of the eye ; because the lateral pencils of rays diverge from the 
 axis of the eye-glass at their emergence from it. Upon this ac- 
 count, the eye should be placed as near to the eye-glass as possible, 
 in order that it may receive the greatest number of pencils. Yet 
 no nearness of the eye will wholly prevent the field of view from 
 being more confined than with convex eye-glasses of equal curva- 
 ture ; but this disadvantage is counterbalanced by the valuable rpro- 
 perty of superior distinctness. 
 
 The Achromatic and Aplanatic Object-glasses. 
 
 The ordinary telescopes which we have hitherto described, 
 will only bear a small aperture, without exhibiting circular 
 prismatic rings of colours, which are subversive of their utility. 
 Two causes contribute to this effect, 1, Spherical surfaces do 
 not refract the rays of light accurately to a point ; and ^2, The 
 rays of compounded light, being differently refrangible, come 
 to their respective foci at different distances from the glass, the 
 more refrangible rays converging, of course, sooner than the 
 less refrangible. This may be proved by an easy experiment.; 
 for if the image of a paper painted entirely red, and properly 
 illuminated, be cast by means of a lens upon a screen, it will 
 be formed at a greater distance than the image of a blue paper 
 cast by the same lens. Hence the image of a white object is 
 composed of an indefinite number of coloured images, the 
 violet being nearest, and the red furthest from the lens, and the 
 images of intermediate colours at intermediate distances. The 
 whole image is therefore in some degree confused, though its 
 extremities are most perceptibly so ; and this confusion being 
 very much increased, not only by the magnifying pow'er ot 
 the eye-glass, but also by the dispersive power which it has in 
 common with the object-glass, the necessity for that limitatiou 
 
 5 
 

 J^ibhj7i'd T^' JVjitt/iH. I'isher'kJ)ixoii.I.i\’Ci'f:>ool.diifif 
 
OPTICS. 
 
 489 
 
 Achromatic object-glass. 
 
 which has been already intimated, for a certain moderate pro- 
 portion between the object and eye-glass, becomes indispensable. 
 Under these circumstances, to increase the length of refracting 
 telescopes seemed to constitute the only means of obtaining great 
 magnifying powers ; and though a method of converging to the 
 same point, all the rays of light which is composed, still remained 
 an object of speculation, the attainment of it seemed, in the 
 days of Newton, to be attended with insuperable difficulties. 
 But however great they were, they have been nearly overcome, 
 and the signal merit of this success is due to our countryman 
 John Dollond. By making a compound lens of three different 
 substances, of differently refrangible powers, the rays of light, 
 which were too much dispersed by one convex lens, are brought 
 nearer to a union with each otlier ; the telescopes made with an 
 object-glass of this kind are now commonly used, and are distin- 
 guished by the name of acln'omatic telescopes, a prefix which sig- 
 nifies colourless. 
 
 The object-glasses of Dollond’s telescopes are composed of 
 three distinct lenses, two of which are convex and the other 
 concave. The concave one is, by British Artists placed in 
 
 the middle, as represented in fig. 1, pi. VI I, where a and c 
 
 show the two convex lenses, and b h the concave one. The 
 
 two convex ones are made of London crown glass, and the 
 middle one of white flint-glass, and they are all ground to 
 spheres of different radii, according to the refractive powers of 
 the different kinds of glass, and the intended focal distance of 
 the object-glass of the telescope. According to Boscovich, the 
 focal distance of the parallel rays for the concave glass is one- 
 half, and for the convex glass one-third of the combined 
 
 focus. When put together, they refract the rays in the follow- 
 ing manner: Let ah, ab, fig. 2, pi. VI f, be two red rays 
 of the sun’s light falling parallel on the first convex lens c. 
 Supposing there was no other lens present but one, they would 
 then be converged into the lines he, be, and at last meet in the 
 focus q. Let the lines g h, g li, represent two violet rays 
 falling on the surface of the lens. I’hese are also refracted, 
 and will meet in a focus ; but as they have a greater degree of 
 refrangibility than the red rays, they must of consequence con- 
 verge more by the same power of refraction in the glass, and 
 meet sooner in a focus, suppose at r . — Let now the concave 
 lens, d d, be placed in sTich a manner as to intercept all the rays 
 before they come to their focus. If this lens was made of the 
 same materials, and ground to the same radius as the convex 
 one, it would have the same power to cause the rays to diverge, 
 that the former had to make them converge. In this case, the 
 
 2 I.—V 0 L. I, 3 R 
 
490 
 
 OFTICS. 
 
 Achromatic object-glass. 
 
 red rays would become parallel, and move on in the lines o o, oo; 
 but the concave lens being made of f^int-glass, and upon a 
 shorter radius, has a greater refractive power, and therefore 
 they diverge a little alter they come out of it ; and if no third 
 lens was interposed, they would proceed diverging in the lines 
 optf opty but, by the interposition of the third lens oro, 
 they are again made to converge, and meet in a focus somewhat 
 more distant than the former, as at .r. By the concave lens, 
 the violet rays are also refracted, and made to diverge ; but 
 having a greater degree of refrangibility, the same power of 
 refraction makes them diverge somewhat more than the red 
 ones ; and thus, if no third lens was interposed, they would 
 proceed in such lines as Imjiy Imn. Now as the dilferently 
 coloured rays fall upon the third lens with different degrees of 
 divergence, it is plain, that the same power of refraction in 
 that lens will operate upon them in such a manner as to bring 
 them all together to a focus very nearly at the same point. The 
 red rays, it is true, require the greatest power of refraction to 
 bring them to a focus, but they fall upon the lens with the 
 least degree of divergence. The violet rays, though they re- 
 quire the least power of refraction, yet have the greatest degree 
 of divergence, and thus all meet together at the point x, or very 
 nearly so. 
 
 The achromatic effect may be produced by the union of one 
 convex and one concave lens, but not so perfectly as with three 
 lenses. 
 
 To proportion accurately the densities of the glasses to 
 each other, requires much professional practice and attention. 
 Experiments with the lenses after they are perfectly finished 
 can alone be depended on. The essential parts of telescopes 
 being few and cheap, the manufacture of them is frequently 
 attempted by individuals for the purpose of amusement ; and in 
 every kind of telescope, but the achromatic, they have, with a 
 due degree of perseverance, a fair hope of success. But in 
 attempting to form an achromatic object-glass, however well 
 they may think they have selected their glass, and proportioned 
 the curvatures of the surfaces, they will be - almost certain to 
 find that a single set of lenses, when combined, produce an 
 effect that disappoints their expectations. To succeed perfect- 
 ly, they must therefore n»ake, from different parcels of glass, 
 a considerable number of lenses, with slight differences of cur- 
 vature, and those must be selected which will bear the largest 
 aperture and magnifying power. But this would render the 
 undertaking, for the sake of one or two instruments, an Her- 
 culean labour, which would not bring to any private individual 
 
 6 
 
OPTICS. 
 
 491 
 
 Aplanatic object-glass. 
 
 ail adequate recompense, and which, from the number of 
 impertect telescopes which are manufactured by those in the 
 most extensive line of business, it would appear that opticians 
 themselves do not fully enter into the spirit of. It has been 
 very generally said and believed, that Dollond made his original 
 experiments, and constructed those excellent three-foot glasses, 
 (which at present bear so high a price, and are considered as ini- 
 mitable,) with one single parcel of glass, which accidentally proved 
 superior to any that has since been produced. — Nicholson has 
 rendered it extremely probable that this is a vulgar error ; the 
 proprietor of the' glass-house having assured him, that the ori- 
 ginal receipts and practice are still followed in the making of 
 optical glass : that the principal opticians always complain of the 
 bad quality of the glass, but never fail to take the whole quantity 
 he makes at their request ; and that, when they renews their orders, 
 they aUvays desire it may be exactly the same as the last. Jt 
 seems therefore reasonable to conclude, that though different par- 
 cels of glass made according to the same formula, may differ a 
 little, yet that as good glass for optical uses may be obtained now 
 as formerly, and consequently as good telescopes, if the same great 
 skill and disregard of expense which Dollond evinced, in adapting 
 the curvatures of his .lenses to each other awd to the glass, were 
 again brought into action. 
 
 The impossibility of obtaining perfectly homogeneous glass, 
 and the consequent failure of producing that complete correction 
 of the aberration of the rays of light in the telescopes called achro- 
 matic, induced Dr. Robert Biair to try the effects of fluid me- 
 diums ; and his success was such as to induce him to give the 
 term aplanatic^ or, free from error,’’ to ilie object-glasses he 
 thus constructed. He made a compound lens, consisting of a 
 plano-convex of crown-glass, with its flat side towards the object, 
 and a meniscus of the same materials, with its convex side iu the 
 same direction, and its flatter concave next the eye, and the inter- 
 val between these lenses he filled with a solution ot antimony in a 
 certain proportion of muriatic acid. The lens thus constructed, 
 did not exhibit the slightest vestige of any extraneous colour ; 
 still, the invention, after a lapse of more than twenty years, has not 
 come into general use, probably from the difficulty of preserving 
 anv fluid from growing turbid iu the course ot time. 
 
 The Rejitetwg Telescope. 
 
 Mathematicians had demonstrated, that a pencil of rays 
 -could not be collected in a single point by a spherical lens ; and 
 .also that the image transmiUed by such a lens, would be ia 
 
492 
 
 OPTICS. 
 
 Reflecting telescope. 
 
 some degree incurvated. Gregory, a young man of great abili- 
 ties, was induced to believe that these inconveniences, and also the 
 great length of refracting telescopes, would be obviated by substi- 
 tuting for the object-glass of the common telescope, a metallic 
 speculum, of a parabolic figure, to receive the image, and to re- 
 flect it towards a small speculum of the same metal. t This again 
 was to return the image to an eye-glass placed behind the great 
 speculum, which for that purpose was to be perforated in its cen- 
 tre. These ideas were published by Gregory in 1663, but as he 
 possessed no mechanical dexterity himself, and could find no work- 
 man capable of realizing his invention, it might have sunk into ob- 
 livion, had not Sir Isaac Newton, in the course of his discoveries, 
 found that the errors arising from the different refrangibility of 
 light, were incomparably greater than those arising from reflection, 
 and being himself as remarkable for manual skill, as geometrical 
 knowledge, he was independent of others for the execution of his 
 design. He therefore determined to try what could be done, and 
 executed two reflecting telescopes, in 1672, on a plan somewhat 
 different to what Gregory had proposed. Although a good metal 
 for the speculum was not then known, although he had to con- 
 trive for himself the method of polishing it, and although his tele- 
 scopes were but six inches long, yet in power, they were compa- 
 rable to a six-feet refractor, and were capable of showing Jupiter’s 
 satellites. 
 
 One great advantage of the reflecting telescope is, that it will 
 admit of an eye-glass of a much shorter focal distance than a re- 
 fracting telescope of the same length, consequently it w ill magnify 
 so much the more. This advantage arises principally from the 
 rays of light not being dispersed by reflection as they are by re- 
 fraction, and partly from the practicability of giving the large re- 
 flectors a form either parabolical, or at least such as answers better 
 than the spherical figure. 
 
 Gregorian Telescope. 
 
 This construction of the reflecting telescope, which is more 
 common than any other, is represented by fig. 3, pi. VII. At 
 the bottom of the tube A BCD, is placed the large concave 
 reflector k /, with a circular hole through the middle of it in the 
 direction of its axis. Within the tube of the telescope, and 
 directly facing the perforation, is placed the small concave 
 speculum, g A, supported by the arm i. Two lenses, tt and 
 a n. are coijtaiued in the eye-tube LMNO, and the observer 
 applieL bis ^'ye to a sniali hole at /, in order to view^ the magni- 
 fitid image of the disunt object YZ. The iaige reflector k 
 
OPTICS. 
 
 493 
 
 Gregorian telescope. — Newtonian telescope. 
 
 receives the rays from the distant object, and reflects them to 
 its focus, where they form an inverted image, EF, in the manner 
 already mentioned as one of the peculiar properties of a concave 
 mirror. Diverging from the points of union, the pencils of 
 rays proceed onwards, and cross each other a little before they 
 reach the small mirror g h, the focus of which is at //, or a 
 little further from the large speculum than its principal focus. 
 From the small mirror, the rays are reflected somewhat con- 
 vergently, and in that state are received before they meet by a 
 plano-convex lens 1 1. By the action of this glass, their conver- 
 gency is increased, and they form a second image, a by which is 
 erect like the object. This second image is magnified by the lens 
 q qy through w hich the rays of each pencil pass nearly in a paral- 
 lel direction to the eye ; to exclude all extraneous light, the eye 
 is applied to a very small hole, and sees the image under the angle 
 c f d. 
 
 If the lens t ty were removed, the image would be formed some- 
 what larger at r, but the area or held of view w’ould be smaller ami 
 less pleasant ; for which reason it is not usual to omit the second 
 lens. Sometimes the lens q qy is made double, for the reason as- 
 signed, in p. 486 in illustrating flg. 8. pi. V. 
 
 In this and other reflecting telescopes, containing two curved 
 reflectors, it is necessary to have the power of altering the distance 
 betw^een the tw'o mirrors. This is usually done by a w ire, e s,. 
 passing along the outside of the tube, and w ith a screw at the end 
 of it, w hich works in an external projection zCy of the arm z, w ithin 
 the tube. The other end of the wire, passes through a small stud 
 affixed to the tube of the telescope at m ;• and the observer, while 
 looking through the hole aty', turns the milled head p, of the wdre^ 
 which is very near him, and thus regulates the distance of the small 
 speculum as he flnds requisite. 
 
 Nezetonian Telescope. 
 
 A section of this telescope, is shown by fig. 4, pi. VIL 
 ABCD is the tube, which is open at the end AB, opposite 
 the large speculum, n o. The large concave speculum, ?i Oy i& 
 not perforated as in the last- mentioned instrument, but the small 
 speculum, qy is set aslant, so as to direct the rays received 
 from the large speculum through an aperture, g, at the side of 
 the tube, where they are received and refracted to the eye by a 
 lens or lenses in a tube //. The speculum q, is suspended within 
 the tube, by an arm, py w ith its centre opposite the centre of the 
 speculum wo; it is not curved, but plane, and has therefore no 
 other effect than that of changing the direction of the rays. 
 
494 
 
 OPTICS. 
 
 Newtonian telescope. — Heiscliel’s telescope. 
 
 Without the small reflector, the rays from the large speculum 
 would be converged at R, and the observer might have an eye- 
 glass placed to view the image formed there, with his face to- 
 Nvards the speculum n o; but in this case his head would inter- 
 cept the greatest part of the rays, unless the instrument was very 
 large. 
 
 The Newtonian telescope, as first described, is very conve- 
 nient for viewing objects in the zenith ; as they may be seen while 
 the observer retains his ordinary position of looking forward hori- 
 nzontally. 
 
 TlerscheVs Telescope. 
 
 The best and most powerful telescopes ever constructed, have 
 been made by -Dr. Hersdiel, who is so well knowm by his labours, 
 as one of the most eminent astronomers of the present day. The 
 astronomers of the Continent have adopted a phraseology, which 
 contains a very extraordinary acknowledgment. of the value of his 
 telescopes; when they wish to convey the highest possible idea of 
 ■the excellence of a. telescope, they assert that it is equal to one of 
 Herschers. 
 
 I'he largest reflecting telescope now' existing was made by Dr. 
 Herschel. It is forty feet in length, and the polished surface of 
 the large speculum is four feet in diameter. It has no second re- 
 flector, a circumstance that adds much to the brightness of the 
 objects viewed in it. The observer, who looks through an eye- 
 glass, as in other telescopes, has of course his back to the objects; 
 but it is so contrived that little or no light is intercepted by this 
 means. We may use the diagram of the Newtonian telescope, 
 fig. 4, pi. VII, todllustrate the position of the observer more par- 
 ticularly. Supposing the speculum q and its support to be re- 
 moved, the rays n o, as before observed, would be converged to R ; 
 but if the observer were placed there, he would intercept a large 
 portion of tlie light, even when facing this gigantic telescope ; 
 suppose then the upper part n, of the speculum, to incline down- 
 wards, that is, to be set at an angle to the axis of the tube, tlie rays 
 may be directed to S, or any other point nearer the tube, where 
 the spectator may be placed, and will occasion no sensible dim- 
 ness of the image thrown by a large mirror. In Herschel’s forty- 
 feet telescope, the converging rays reflected by the mirror, pass the 
 extremity of the tube at the distance of four inches, and come into 
 the air ; so that the observer scarcely at all interferes with the in- 
 cident light, as the diameter of the tube exceeds that of the mirror 
 about ten inches. It will bear a magnifying pow er of six thousand 
 atimes the diameter of an object. 
 
■I 
 
 OPTICS. 495 
 
 Cassegrainian telescope. — jMagiiifying power of telescopes. 
 
 Cassegrainian Telescope, 
 
 This telescope differs from those of the Gregorian and New- 
 tonian construction, in having the small reflector convex instead of 
 concave ; in consequence of which the small reflector must be 
 placed nearer to the large reflector than the focus of the latter. It 
 shows objects inverted, because the rays do not cross, and no 
 image is formed between the two reflectors ; but with the same 
 magnifying power, it is shorter than the Gregorian, by twice the 
 focal length of the small mirror. 
 
 , Of the Magnifying Poucer of Telescopes. 
 
 The magnifying power of a telescope is constituted by the dif- 
 ference in size between the object viewed by the naked eye, and 
 the image seen in the held of view of the instrument ; and this dif- 
 ference may be expressed in three different ways: 1, we express 
 the effect of the telescope on the apparent increase of the diame- 
 ter of objects : 2, on the superfcies, and 3, on the solidity or cube 
 of the objects. But it has been generally agreed to express the 
 magnifying power of telescopes by the effect they produce on the 
 diameter only of objects ; this mode of expression conveys clearer 
 ideas of the power of the instrument than any other, and is always 
 understood to be meant when the contrary is not expressed. 
 Hence, if a telescope show s an object at the distance of 100 yards, 
 as large as that object would appear to the naked eye at the dis- 
 tance of one yard, it is understood to magnify one hundred times. 
 
 The magnifying power of the astrononaical telescope, or the 
 night-glass, is as the focal distance of the eye-glass to the focal 
 distance of the object-glass ; therefore, if the latter be divided by 
 the foimer, the quotient will express the magnifying power. Tlie 
 pow'er of the perspective-glass may be estimated in the same way; 
 for the two additional eye-glasses are not employed to change the 
 magnifying power, being equal in focal distance to the third, but 
 only to produce the erect position of objects; by dividing the 
 focal distance of one eye-glass by the focal distance of the object- 
 glass, the true power is therefore obtained. 
 
 la the Galilean telescope, the virtual focus of the concave 
 or eye-glass, must be divided by the real focus of the object- 
 glass. 
 
 To find the magnifying power of the Gregorian telescope, 
 multiply the focal distance of the great mirror by the distance 
 of the small mirror from the image next the eye ; and multiply 
 the focal distance of the small mirror by the focal distance of 
 
496 
 
 OPTICS. 
 
 Magnifying power of telescopes, — Binocular telescope. 
 
 the eye-glass ; then divide the product of the former multiplica- 
 tion by the product of the latter, and the quotient will express the 
 magnifying power. 
 
 The magnifying power of the Newtonian telescope, or of 
 Herschers, is found by dividing the focal distance^ of the large 
 speculum by the focal distance of the eye-glass. 
 
 The magnifying power of the Cassegrainian reflector is the 
 same as that of the Gregorian, when their respective specula are 
 of equal curvature. 
 
 As it is often inconvenient, and sometimes impossible without 
 injuring the instrument, to ascertain with precision the foci of the 
 lenses and mirrors, it is advantageous to possess the means of as- 
 certaining the magnifying power of a telescope experimentally. 
 This may be done in several ways, of which the following is the 
 one recommended by Edwards : 
 
 At the distance of 100 or 200 yards from the telescope, put 
 up a small circle of paper, of any determinate diameter, an inch 
 for instance. Upon a card, or any piece of strong paper, through 
 which the light cannot be easily transmitted, draw two black pa- 
 rallel lines, the distance of which from each other is exactly equal 
 to the diameter of the small circle. Adjust the telescope to dis- 
 tinct vision, and through it view the small circle with one eye, and 
 with the other eye, open also, view at the same time the two pa- 
 rallel lines. Let the parallel lines be then moved nearer to, or 
 farther from the eye, till you see them exactly to cover the small 
 circle viewed in the telescope. Measure now the distance of the 
 small circle, and also of the parallel lines, from your eye ; then 
 divide the distance of the former by that of the latter, and you will 
 have the magnifying power of the telescope. 
 
 Miscellaneous Remarks, relative to Telescopes. 
 
 In treating of vision, we observed, that objects seen with 
 both eyes at once, appear more vivid than they do to either eye 
 singly. This fact gave rise to what is called the binocular or 
 double telescope, in the use of which the phenomenon is very 
 obvious. This instrument consists of two equal telescopes, 
 fastened together, and made to point to the same object. They 
 must be at the same distance from each other as the pupils of the 
 eyes of the observer, who must look through both the telescopes 
 at once, by which means objects will appear brighter, and some- 
 what larger, than when only one telescope is used, though the 
 magnifying power is the same in both cases. The inconvenience 
 of managing the binocular telescope, seems, however, to be the 
 cause of its being disused. 
 
OPTICS. 
 
 497 
 
 Miscellaneous remarks relative to telescopes. 
 
 Valuable telescopes are usually furnished with what is called 
 a finder, which is nothing more than a short telescope, and is 
 generally affixed to the tube of the large telescope, as at G, lig. 
 3, pi. VII. It is used for the purpose of bringing an object ex- 
 peditiously into the field of the large telescope. It magnifies but 
 little, seldom more than six or eight times ; but it has a very ex- 
 tensive field of view, so that any given object is easily compre- 
 hended within it. In the inside of its tube, and exactly at the 
 focus of the eye-glass, there are two slender wires, which cross 
 each other in the axis of the telescope. Now the finder is so ad- 
 justed, that when an object seen through it appears to be near the 
 crossing of its wires, it is at the same time visible through the 
 great telescope, and by this means much time is saved, especially 
 to unpractised observeis. 
 
 Both refracting and reflecting telescopes of the best sort, are 
 furnished with two or more different eye-pieces, in order that a 
 greater or less power may be employed, as the occasion appears 
 to require. 
 
 In every telescope, the distance between the eye and the 
 object-glass must be alterable, in order to suit different eyes, or 
 render objects, at different distances, distinctly visible to the same 
 eye. 
 
 The following observations on another adjustment, of which 
 telescopes ought to be susceptible, are from the first volume of 
 Nicholson’s Journal, 4to series : “ Every attentive observer 
 must have taken notice, that light is of as much consequence to 
 artificial vision as magnifying power. It may therefore afford 
 matter of surprise, that the most variable of all adjustments of 
 the eye, viz. that of aperture, should never be introduced into our 
 artificial combinations. Distant woods, and other land objects, 
 are invisible to a high magnifying power, for want of light, 
 when the same objects may be distinctly seen with a lower. By 
 means of an artificial iris, which an ingenious artist will find 
 little difficulty in contriving, this disadvantage in telescopes 
 might be obviated. Suppose a brass ring to surround the object 
 
 end of the telescope, and upon this let eight or more triangular 
 slips of brass be fixed, so as to revolve on equi-distant pins pass- 
 ing through each triangle near one of its corners. If the trian- 
 gles be slidden in upon each other, it may readily be apprehended 
 that they will close the aperture; and if they be all made to 
 revolve or slide backwards alike, it is clear that their edge will 
 leave an octagonal aperture, greater or less according to cir- 
 cumstances. The equable motion of all the triangles may be 
 produced either by pinions and one toothed wheel, or by what 
 is called snail-work. Another kind of iris, more compact, 
 £1. — VoL. I. 3S 
 
498 
 
 OPTICS. 
 
 Miscellaneous remarks relative to telescopes. 
 
 may be made by causing thin elastic slips of brass to slide along 
 parallel to the tube, and be conducted through a slit in a brass 
 cap, w'hich^ will lead them across the aperture in a radial direc- 
 tion.’’ 
 
 To stop the wandering rays which would tend to confuse the 
 image of a telescope, a plate of metal, which has a small round 
 hole in the middle of it, is placed in the tube containing the eye- 
 glasses, parallel with them. It is called the diaphragm, and is 
 generally fixed within the telescope, at the focal distance of the 
 glass nearest the eye. By this means, the field of view' is lessen- 
 ed, but the advantage of greater distinctness is obtained. 
 
 To ascertain the goodness of object-glasses, the best expedient 
 is, to place them successively in a tube, and examine some distant 
 object v^ith different eye-glasses; that glass which represents 
 objects the brightest and most distinct, and which bears the 
 greatest aperture with the shortest eye-glass, without colouring, is 
 the best. 
 
 In double convex lenses, the most perfect is that whose radius 
 of curvature of the first surface, is to that of the second as one to 
 six ; its aberration being the least possible, viz. of its thickness : 
 but if this glass is turned with its other side to the rays, the aber- 
 ration will be and therefore would be much worse. The 
 same ratio holds with respect to concave lenses. 
 
 The lenses of a good telescope should all be perfectly well 
 polished, and should be as thin as possible for their size and 
 curvature. When a lens is exactly circular, and yet is found to 
 be thicker on one side than another, it is a proof that the axes 
 of the two surfaces are not coincident, and it ought to be re- 
 jected. 
 
 Of reflecting telescopes, the Gregorian is almost the only 
 one made for common use. This preference seems to be owing 
 to the neatness of its appearance, and to its being pointed 
 towards objects in the same manner as refracting telescopes, 
 rather than to any really intrinsic advantages which it has over 
 the Newtonian construction. In all the reflecting telescopes w'e 
 have noticed, the diameter of the large mirror, on account of the 
 interposition of the small one, at the aperture, must be greater 
 than would otherwise be required, in order to enlighten the field 
 of view sufiiciently ; and it may also be observed, that as 
 specula have probably never been ground to a truly parabolic 
 figure, the central part, the benefit of which is lost, is the most 
 valuable portion of the whole, because it would reflect the light 
 with the greatest regularity. To this it may be added, that 
 the difficulty of grinding a speculum increases perhaps in so 
 great a proportion as the square of the diameter, and it is 
 
 7 
 
OPTICS. 
 
 499 
 
 Miscellaneous remarks relative to telescopes. 
 
 therefore of some importance to those who may undertake the 
 construction of these instruments for themselves, to make the 
 large speculum as small as possible, consistently with a given 
 power and quantity of light. We shall therefore mention the 
 form of a reflecting telescope, which we recollect to have been 
 sliown many years ago, by a self-taught optician, who, so far as w'e 
 can recollect, on the question being put to him, declared himself 
 to be the first who had adopted it. It is represented by fig. 5, pi. 
 VII, where the great mirror is as usual at the bottom of the prin- 
 cipal tube, but instead of having its axis coincident w ith that of the 
 tube, it is inclined, so as to throw the image upwards, through an 
 aperture for that purpose, to n o, whence the rays diverge, and 
 falling upon the small speculum, are reflected by it to the eye, as 
 in the Gregorian telescope. Here we have supposed the small 
 mirror to be concave, but we are not certain whether it was so or 
 not ; it is clear, the principle of it is preserved, if the whole dia- 
 meter of the mirror is employed to reflect light ; and therefore it 
 may have the small mirror convex like Cassegrain^s reflector, or 
 plane, like Newton’s. We can speak with confidence of the ex- 
 cellence of the instrument we allude to, and we gladly bear this 
 testimony to the merit of an obscure individual, w'ho, we have rea- 
 son to believe, is now deceased. He was tenacious of his optical 
 knowledge, and reserved in speaking of it ; yet he W'as far from an 
 illiberal character. His mirrors were cast by himself in his ow'n 
 house, and every other part of the workmanship of his instru- 
 ments, and the tools with which they were made, were con- 
 structed by himself : in solitude and silence, by day and by night, 
 though surrounded by the chilling difficulties of poverty, he seems 
 to have pursued his studies with an assiduity seldom paralleled ; 
 and the knowledge which it cost him so much to acquire, he 
 might feel it difficult to part with to a casual acquaintance. 
 Perhaps the notice we have taken of him in this place, may be 
 more than is called for by any proof we have given of his 
 abilities, in the new arrangement of a telescope ; but the exam- 
 ple may be an encouragement to others, and ought not to be 
 lost. In another place, we may have an opportunity of dis- 
 closing his name, and of investigating his particular merits ; 
 here we will only add, that he had taken no scanty draught of 
 the Pierian spring,” and that his poetical effusions were as ele- 
 gant and impassioned, as the work of his hands was correct and 
 effectual. 
 
 The two mirrors of a telescope should not have the same 
 curve. Maskelyne has observed, that when the two specula 
 are parabolas, they cause a very considerable aberration, which 
 is negative, that is to say, the focus of the extreme ray is 
 
500 
 
 OPTICS. 
 
 Miscellaneous remarks relative to telescopes. — Micrometers. 
 
 longer than those of the middle ones. If the large speculum is 
 a parabola, the small one ought to be an ellipse ; but when the 
 small speculum is spherical, which is generally the case in prac- 
 tice, il concave, the ligure of the large speculum ought to be an 
 hyperbola; if convex, the large speculum ought to be an ellipse, 
 to free the telescope from aberration. This will be easier under- 
 stood, by attending to the positions of the first and second images; 
 when a curve is of such a form that lines drawn from each image, 
 and meeting in any point of the curve, make equal angles with the 
 tangent to the curve at that point, it is evident that such curve 
 will be free from aberration. 
 
 From a property of reflecting telescopes, that the apertures of 
 the two specula are to each other very nearly in the proportion of 
 their focal lengths, it follows that their aberrations will be to each 
 other in the same proportion ; and these aberrations are in the 
 same direction, if the two specula are both concave ; or in con- 
 trary directions, if one speculum is concave, and the other convex. 
 
 In the Gregorian telescope, both specula being concave, the 
 aberration at the second image will be the sum of the aberrations 
 of the two mirrors ; but in Cassegrain’s construction, one mirror 
 being concave, and the other convex, the aberration at the second 
 image will be the difference between their aberrations. By as- 
 suming such proportions for the foci of the specula as are gene- 
 rally used in the reflecting telescope, which is about as 1 to 4, the 
 aberration in the Cassegrainian construction will be to that in the 
 Gregorian as 3 to 5. Hence appears the service which might be 
 rendered to artists by a complete mathematical investigation of the 
 curves adapted to optical purposes. 
 
 Of Micrometers. 
 
 A micrometer is an instrument by which the apparent magni- 
 tudes of objects viewed through microscopes or telescopes are 
 measured. 
 
 A great variety of micrometers have been contrived, some 
 of which are very complicated and expensive, others possess great 
 simplicity. The accuracy of their performance has not always 
 been proportionate to their cost. We shall notice only one or two 
 of the most simple micrometers, as our limits will not permit us 
 to enlarge much beyond what may prove interesting to the gene- 
 rality of our readers. 
 
 Lewenhoeck’s method of estimating the size of small objects 
 was by comparing them with grains of sand, of which one 
 hundred placed in a line took up an inch. These grains he 
 laid on the same plate with his objects, and viewed them at the 
 
r 
 
 OTT U'S. 
 
 lu.ni. 
 
 r dU . 
 
 TDtceCnJC. 
 
 J’lihlijb'J bv XulhiH. Finhcr ir.JSib. 
 
OPTICS. 
 
 501 
 
 Dr. Hook’s micrometer. — Cavallo’s. 
 
 same time. Dr. Jurin^s method was similar to this, for he 
 wrapped a piece of line silver wire very closely round a pin, 
 and observed how many rings made an inch ; and he used this 
 wire as Lewenhoeck used his sand. Dr. Hook used to look upon 
 the magnified object with one eye, while, at the same time, he 
 viewed other objects placed at the same distance, with the other 
 eye. In this manner he was able, by the help of a ruler, divided 
 into inches and small parts, and laid on the pedestal of the mi- 
 croscope, to cast the magnified appearance of the object upon the 
 ruler, and thus exactly to measure the diameter which it appeared 
 to have through the glass ; ^hich being compared with the dia- 
 meter as it appeared to the naked eye, easily shewed the degree in 
 which it was magnified. A little practice renders this method 
 very easy. 
 
 Cavallo’s micrometer, which he has described in the third 
 volume of his Elements of Natural and Experimental Phi- 
 losophy,” is simple and valuable. It consists of a small semi- 
 transparent scale or slip of mother-of-pearl, about the twentieth 
 part of an inch broad, and of the thickness of common writing 
 paper. It is divided into a number of equal parts by means of 
 parallel lines, every fifth and tenth of which divisions is a little 
 longer than the rest. 
 
 This micrometer, or divided scale, is situated wdthin the tube, 
 at the focus of the eye-lens of the telescope, where the image of 
 the object is formed, and with its divided edge passing through the 
 centre of the field of view ; though this is not absolutely necessary. 
 It is immaterial whether the telescope be a refractor or a reflector, 
 provided the eye-lens be convex, and not concave, as in the Gali- 
 lean telescope. 
 
 The simplest way of fixing it, is to stick it upon the diaphragm, 
 which generally stands w ithin the tube, at the focal distance of the 
 eye-lens. 
 
 By looking through the telescope, the image of the object 
 and the micrometer will appear to coincide : hence the observer 
 may easily see how many divisions of the latter measure the length 
 or breadth of the former ; and know ing the value of the divisions 
 of the micrometer, he may easily determine the angle which is 
 subtended by the object. 
 
 There are several methods of ascertaining the value of the 
 divisions of a micrometer in a given telescope. The following 
 is one of the easiest : Direct the telescope to the sun, and 
 
 observe how many divisions of the micrometer measure its 
 diameter exactly; then take out of the Nautical Almanack the 
 diameter of the sun for the day in which the observation is 
 made ; divide it by the above-mentioned number of divisions, 
 
502 
 
 ABSTRACT OF OPTICS. 
 
 Cavallo’s micrometer. 
 
 and the quotient is the value of one division of the micrometer. 
 Thus, suppose that 26J divisions of the micrometer measure 
 the diameter of the sun, and the Nautical Almanack gives for 
 the measure of the angle which is subtended by the same diame- 
 ter, 31 minutes, 22 seconds; or (by reducing the whole into 
 seconds) 1882 seconds. Divide 1882 by 26.5, and the quo- 
 tient, neglecting a small remainder, is 71 seconds, or 1 minute 11 
 seconds ; which is the value of one division of the micrometer ; 
 and therefore the value of any greater number of divisions is easily 
 ascertained. 
 
 The mother-of-pearl micrometer may be applied to a micro- 
 scope, and it will thus serve to measure the lineal dimensions of 
 the object. The value of its divisions may be ascertained by 
 placing an object of a known dimension before the microscope, 
 and by observing how many divisions of the micrometer mea- 
 sure its magnified image. For instance, place a piece of paper 
 which is exactly one-tenth of an inch long, before the micro- 
 scope, and if you find that 50 divisions of the micrometer measure 
 its magnified image, you may conclude that each division denotes 
 an extension of the 500dth part of an inch in the object ; for if 50 
 divisions measure one-tenth, 500 divisions must measure the whole 
 inch. 
 
 Mother-of-pearl was found, by Cavallo, after many trials, to be 
 a much more convenient substance than either glass, ivory, horn, 
 or wood, as it is a very steady substance, the divisions are easily 
 marked upon it, and when made as thin as common writing paper, 
 it has a very useful degree of transparency. 
 
 Abstract of Optics. 
 
 1. The particles of light, which are inconceivably small, pro- 
 ceed from luminous bodies in right lines. 
 
 2. Consequently the density of light is inversely as the square 
 of the distance from the luminous centre : 
 
 3. Light moves at the rate of nearly 200,000 miles in one se- 
 cond of time. 
 
 4. Its impression on the retina is not instantaneous ; hence 
 though its particles may be separately projected, so as to be, in 
 their progress, at the rate of 1000 miles apart, its velocity is suffi- 
 cient to produce distinct vision. 
 
 5. Every ray of light carries with it the image of the point 
 from which it was emitted ; when, therefore, pencils of rays 
 
ABSTRACT OF OPTICS. 
 
 503 
 
 Light. — Lenses. — Mirrors. 
 
 from every point of an object are united in the same order in 
 uliicli they were emitted, they form an image or representation of 
 that object, at the place where they are thus united. 
 
 6. All the rays of light, which enter another medium obliquely, 
 suffer refraction ; that is, they either move farther from, or nearer 
 to, the perpendicular, as the medium into which they enter is rarer 
 or denser than the other medium. 
 
 7. On the refrangibility of light depends the properties of 
 lenses. 
 
 8. Convex lenses collect the rays of light, and make them con- 
 verge to a centre or focus. 
 
 9. Concave lenses disperse the rays of light, their power of 
 refraction not being towards their centre, but tow ai ds their circum- 
 ference. 
 
 10. When light strikes upon a surface, it is reflected so that 
 the angle of reflection is equal to the angle of incidence ; on this 
 the properties of mirrors depend. 
 
 11. Plane mirrors have no other effect than that of changing 
 the direction of the incident rays. 
 
 12. Convex mirrors cause parallel rays to diverge. 
 
 13. Concave mirrors collect parallel rays, or cause them to 
 converge to a focus. 
 
 14. Mixed mirrors exhibit distorted images, because they in- 
 crease or lessen the divergence or convergence of the rays in one 
 or two directions only. 
 
 15. The solar beam is composed of rays possessed of different 
 degrees of refrangibility, and these differences of refrangibility, 
 which are dependent on the size of their particles, produce all the 
 phenomena of colours. 
 
 16. The solar beam, or white light, contains rays of .seven dif- 
 ferent colours, viz. red, orange, yellow, green, blue, indigo, and 
 violet. These are called the primitive colours, because they are 
 immutable, except by intermixture. 
 
 17. It is inferred that red light is composed of particles of the 
 largest size, because it is found to be capable of struggling through 
 thick and resisting mediums, which stop every other colour. 
 
 18. The size of the particles of other colours are iu the order 
 of their enumeration, the violet being the smallest. 
 
 19. The rainbow is owing to the separation of the light into 
 its primitive colours, by the drops of falling rain, which act like a 
 prism. 
 
 20. The rays of light are inflected when they pass very near a 
 body, and deflected when they pass at a greater distance. 
 
 21. Those rays which deviate the least by refraction, deviate 
 the most by flection. 
 
504 
 
 ABSTRACT OF OPTICS. 
 
 Vision. — Microscopes. — Telescopes. 
 
 22. The images of all visible objects are depicted on the re- 
 tina, in an inverted position. 
 
 23. With two eyes, vision is not only more distinct, but more 
 accurate than with one. 
 
 24. A good eye can see most distinctly when the rays fall 
 parallel upon it, because the foci of such rays fall exactly on the 
 retina. 
 
 25. The best eye can hardly distinguish any object that sub- 
 tends an angle of less than half a minute. 
 
 26. The apparent magnitude of objects is dependent on the 
 angle under which they are seen, or on the size of their images 
 depicted on the retina. 
 
 27- The long-sighted require convex spectacles, the short- 
 sighted, concave ones. 
 
 28. Burning lenses must be convex, and burning mirrors con- 
 cave, as the effects of both these instruments are dependent on the 
 condensation of the incident light. 
 
 29* Microscopes are instruments for viewing small objects. 
 They appear to magnify objects, because they enable us to see 
 them with distinctness, nearer than the natural limits of vision. 
 
 30. Refracting telescopes are formed by lenses only; when 
 manufactured in the best manner, they are either furnished with 
 an achromatic object-glass, which corrects the defect arising from 
 the unequal refraction of the different rays, by a combination of 
 one or two convex lenses with a concave one of a different sort of 
 glass ; or, though more rarely, they have an aplanatic object-glass, 
 which corrects the same defect by the combination of a plano- 
 convex and meniscus glass, with a fluid between them that acts 
 like a third lens. 
 
 31. Reflecting telescopes consist of lenses and at least of one 
 speculum. When there is more than one speculum, the second is 
 only about one-fourth of the size of the other, and may be either 
 convex, concave, or plane. 
 
 32. Reflecting telescopes admit of a much greater magnifying 
 power in a given length, than refracting telescopes. 
 
 33. The binocular telescope consists of two telescopes so 
 combined, that both eyes may be employed in looking at the same 
 object. 
 
ASTRONOMY. 
 
 505 
 
 Historical remarks. 
 
 ASTRONOMY, 
 
 Astronomy is the science which treats of the motions, eclipses 
 magnitudes, periods, and other phenomena of the heavenly bodies. 
 
 The early history of astronomy, as of all other ancient sciences, 
 is enveloped in much obscurity. It may be presumed, that some 
 knowledge of it would be nearly coeval with the human race, be- 
 cause it would excite attention, as well from motives of curiosity, 
 as from its connection with the common concerns of life ; but in 
 what age or country the united observations of many, were first so 
 far methodized, as to raise it to the dignity of a science, is now be- 
 yond the reach of unquestionable discovery, and it would be use- 
 less to multiply conjectures on the subject. 
 
 Those delightful regions of Asia, which were the first abodes 
 of mankind, w^ere peculiarly calculated to favour the growth of as- 
 tronomical knowledge. Accordingly it appears that astronomy 
 was much cultivated among the Chaldeans. The level and exten- 
 sive plains of that country, the nights which they passed in the 
 open air, an unbroken horizon, a pure and serene sky, all con- 
 spired to engage that people to contemplate the motions of the 
 stars, and to lead them to conjecture on the laws by which they 
 were governed. 
 
 From Chaldea, astronomy passed into Egypt, and was soon 
 afterwards carried into Phoenicia, where the people began to apply 
 the observations which had been made to the uses of navigation, 
 and thus rendered themselves the masters of the sea and of com- 
 merce. Their guide in steering their ships, when far from land, 
 was one of the stars in the constellation called the Little Bear, 
 wdiich, unlike other stars, appeared always to retain the same situ- 
 ation. Other nations, less skilful in astronomy, observed only the 
 Great Bear in their voyages, a guide too imperfect to enable them 
 to lose sight of land with safety. 
 
 Thales, the Milesian, who flourished about 700 years before 
 the Christian era, brought the science of the stars from Phoeni- 
 cia into Greece, where he taught the utility of the constellation of 
 the Little Bear in navigation. He also taught the theory of the 
 motion of the sun and moon, by which he accounted for the length 
 and shortness of the days, determined the number of the days of 
 the solar year, and not only explained the cause of eclipses, but 
 the art of predicting them, which he even reduced to practice, 
 foretelling an eclipse which took place soon after, and was ren- 
 dered memorable by its happening on a day of battle between the 
 Medes and Lydians. 
 
 22.— VoL. I. 3 T 
 
506 
 
 ASTRONOMY. 
 
 Historical remarks. 
 
 To Anaximander, one of the disciples of Thales, is ascrib- 
 ed the invention of the terrestrial globe, and of a gnomon which 
 he erected at Sparta, by means of which he observed the equi- 
 noxes and solstices, and determined the obliquity of the ecliptic 
 more exactly than had ever been done before. The Greeks, 
 assisted by the instructions they had received from Thales and 
 Anaximander^ ventured to make considerable voyages, and planted 
 several colonies in remote countries ; yet the latter philosopher 
 and his children, were proscribed by the Athenians, and their 
 lives would have been sacrificed but for Pericles, through whose 
 influence the sentence was commuted for banishment. The 
 charge against him was the discovery of truth ; for it was thought 
 impious to suppose the works of the gods subject to immutable 
 laws ! 
 
 Pythagoras, another of the disciples of Thales, taught 
 many sublime astronomical truths. To him is attributed the 
 discovery of the true system of the world, which, after the lapse 
 of many centuries, was revived by Copernicus, and which is 
 now settled on the basis of so many proofs^ that it can never be 
 overthrown. It w'as even thought, in his school, that the 
 planets were inhabited like the earth, and that the stars which 
 are disseminated through infinite space, are suns, and the centres 
 of other planetary systems. He is also said to have considered 
 the comets as permanent bodies, moving round the sun ; and not 
 as perishing meteors, formed in the atmosphere, as they were 
 thought to be in after times. Pythagoras died about the year 497 
 before Christ. 
 
 Pythias w as the first who taught the method of distinguish- 
 ing climates by the length of the days and nights, and about 
 bis time, a remarkable emulation to excel in astronomy pre- 
 vailed among the Greeks. Eudoxus, a disciple of Plato, not 
 satisfied with what he could learn at Athens, repaired to Egypt, 
 to cultivate astronomy at its source. At his return he compiled 
 several books on astronomy, and among others, a description 
 of the constellations. Eudoxus attempted to explain the cele- 
 brated cycle of nineteen years, which had been imagined by 
 Meton, in order to conciliate the solar and lunar motions. This 
 is the most accurate period, for a short interval of time, that 
 could have been devised, for embracing an exact number of revo- 
 lutions of the sun and moon; and is so simple and useful, that 
 when Meton proposed it to the Greeksj assembled at the Olympic 
 games, as the basis of their calendar, it was received with great 
 approbation, and unanimously adopted by all their colonies. The 
 year of the cycle is called the golden number^ and is still placed 
 in the calendar. 
 
ASTRONOMY. 
 
 5or 
 
 Historical remarks. 
 
 Aristotle, the disciple of Plato, and the contemporary of 
 Eudoxus, made use of astronomy for improving physics and 
 geography. He attempted to determine, by astronomical ob- 
 servations, both the figure and the magnitude of the earth; 
 He demonstrated that it was of a spherical form, by the circur 
 lar appearance of its shadow on the disc of the moon in 
 eclipses ; and from the inequality of the meridian altitudes of 
 the sun, which are different in different latitudes. Callisthenes, 
 who attended Alexander the Great, having been sent to Baby- 
 lon, found there astronomical observations made by the Babylo- 
 nians during the space of 1Q03 years, and sent them to Aris- 
 totle. 
 
 But of all the schools of antiquity in which astronomy was 
 taught, that of Alexandria has justly attained the highest 
 celebrity. Here we have the first record of a combined series of 
 observations, made with instruments proper for measuring angles, 
 and calculated trigonometrically. Here the position of the 
 stars began to be minutely determined ; the course of the planets 
 to be traced with care ; and the inequalities of the solar and 
 lunar motions to be better known. The theory adopted, aimed 
 at the explanation of all the celestial motions; and though 
 inferior to that of Pythagoras, yet the numerous observations 
 which were made, furnished the means of detecting its fallacy, 
 and of enabling succeeding astronomers to discover the true 
 system of nature. — Hipparchus of Bithynia, who flourished at 
 Alexandria about the year 162 before Christ, is particularly 
 famous for the excellence of his observations ; and he determin- 
 ed the length of the tropical year with a precision never attained 
 before, his result not varying more than minutes from the 
 truth. 
 
 Ptolemy, an Egyptian, who has always been considered the 
 prince of astronomers among the ancients, flourished in the se- 
 cond century of the Christian era. He has preserved and trans-» 
 mitted to us the observations and principal discoveries of the an- 
 cients, much enriched and enlarged by his own labours, in a trea- 
 tise called The Great Syntaxis,” in which he gave the theory 
 and tables of the motion of the sun and mpon, the planets and 
 the fixed stars. He adopted the most ancient system, which sup- 
 posed the earth to be in the centre of the universe, and this sysr 
 tern, to distinguish it from others, has been, called the Ptolemaic 
 System. The defects of his system did not, however, 'prevent him 
 from calculating all the eclipses that were to happen for 600 years 
 to come. 
 
 About the year 8^6, Ptolemy’s great work was translated by 
 the Arabians into their language, in which it wgs called thp 
 
508 
 
 ASTRONOMY. 
 
 Historical remarks. 
 
 Almagest. About the year 1230, it was translated from the Ara- 
 bic into Latin, under the auspices of the emperor Frederic II. 
 who was willing that the Christians should understand astronomy 
 as well as those whom they styled Barbarians. Alphonso, king of 
 Castile, went further, for he assembled the most able astronomers 
 from all parts, who composed new tables, called after him the Al- 
 phonsine tables. 
 
 By these means the attention of the learned men in Europe 
 became directed to a science, the knowledge of w hich promised 
 so much gratification, utility, and fame. Calculations and 
 treatises became very frequent, as well as the invention of 
 instruments to facilitate observations. The most memorable 
 event of this period was the revival of the ancient or Pythago- 
 rean system of the world, wdiich had been set aside ever since 
 the time of Ptolemy. This was done by Nicholas Copernicus, 
 a native of Thorn, in Prussia, born in 147^. The Ptolemaic 
 system, which supposes the earth to be fixed in the centre of the 
 universe, and the Sun and Moon, with Mercury, Venus, and 
 the other planets, to revolve about it in concentric circles, he 
 perceived to be inconsistent with the phenomena, and encum- 
 bered with may absurdities which did not affect the hypothesis 
 which considered the Sun to be in the centre, and the Earth a 
 planet revolving annually w'ith the rest about the Sun, and 
 daily on its own axis. He established his theory by such 
 incontrovertible reasoning, that it gradually prevailed from that 
 time, and continuing to spread with the lapse of ages, it is now 
 the only one received in Europe, and wherever true science is 
 known. But Copernicus had not himself the satisfaction of see- 
 ing the triumph of the theory he defended : threatened with perse- 
 cution by the religious bigots on one side, and with a perverse 
 opposition from those who called themselves philosophers on the 
 other, he could not be prevailed upon to publish the work which 
 contained the result of his observations, till long after it had been 
 finished. His consent to the publication of it w’as wrung from 
 him by the importunities of his friends, and a copy was brought to 
 him only a few' hours before his death, which happened in his se- 
 venty-first year. In this age, or rather in this country, we regard 
 with astonishment the degrading thraldom to which the human 
 mind was in these times subjected. 
 
 The only opposition of any consequence which the theory 
 of Copernicus ever met with from science and argument, pro- 
 ceeded from Tycho Brahe, a celebrated Danish astronomer, 
 who attempted to set up against it a theory of his ow’u. — 
 His system is not very different from the Ptolemaic, but is 
 generaJly called by his name. He supposed the earth to be 
 
ASTRONOMY. 
 
 509 
 
 Historical remarks. 
 
 immoveable in the centre of the universe, and the Sun to revolve 
 about it every twenty-four hours : the planets he thought went 
 round the Sun in their periodical times, Mercury being nearest 
 to the Sun, then Venus, Mars, Jupiter, and Saturn, and of course 
 to revolve also about the earth. But some of Brahe’s disciples 
 supposed the earth to have a diurnal motion round its axis, and the 
 sun, with all the planets, to move round the earth in one year. 
 The inconsistencies with which this hypothesis abounded, will be 
 most obvious when we have explained the Copernican or true sys- 
 tem. In defence of Brahe’s just fame, it is proper to add, that 
 though he adopted an erroneous theory, he was actuated by pious 
 motives, and that he rendered very important services to astronomy, 
 by the correctness and number of his observations. 
 
 Kepler was one of the pupils of Tycho Brahe, and a man of a 
 truly original and admirable genius. Hipparchus, Ptolemy, Tycho 
 Brahe, and even Copernicus himself, were indebted for a great 
 part of their knowledge to the Egyptians, Chaldeans, and Indians ; 
 pursuing paths already pointed out, they did little more than sepa- 
 rate fancy from fact, with more or less success ; but Kepler, by his 
 own talents and industry, has made discoveries of which no traces 
 are to be found in the annals of antiquity. Galileo was contem- 
 porary with Kepler, and while the latter was tracing the orbits of 
 the planets, and settling the laws of their motions, he was investi- 
 gating the doctrine of motion in general, which had been neglected 
 for 2000 years; and from the results of their united labours, New- 
 ton and Huygens were afterwards enabled to establish the most 
 complete theories of all the planetary motions. Though Galileo 
 proved, in the most incontestable manner, the annual and diurnal 
 motion of the earth, his doctrine w as declared heretical by a con- 
 gregation of cardinals assembled for the purpose, and though not 
 only venerable for his years, but also one of the most virtuous and 
 enlightened men of his age, he was condemned to perpetual im- 
 prisonment, for believing and promulgating truths which accorded 
 with the Older of nature, and which he therelore believed to be 
 written with the finger of God. Through the solicitations of his 
 i^great patron, the Grand Duke of Tuscany, he partially regained 
 his liberty at the end of a year, and in 1642 he died, regretted by 
 the learned and liberal of all Europe. 
 
 From the time of Newton, who carried the theoretical part of 
 the science to perfection, astronomy has never been without an il- 
 lustrious phalanx of supporters, whose particular merits and disco- 
 veries it would be impossible to mention in this concise historical 
 abstract. We shall now therefore proceed with the elementary 
 details of the science. 
 
510 
 
 ASTRONOMY. 
 
 Definitions. 
 
 Definitions and preliminary Explanations. 
 
 All the visible bodies which are dispersed throughout the whole 
 heavens, receive in ordinary discourse the appellation of stars ; but 
 they are divided by astronomers into several classes, according to 
 certain differences which distinguish them. 
 
 The fixed stars, or simple stars, are so called because they ab 
 W'ays appear to be at the same distance from each other. The sun 
 Is a fixed star, its apparent motion being occasioned only by that 
 of the earth, and all the other fixed stars are supposed to be suns, 
 which to us appear small, on account of the immensity of their 
 distances from us. Another property supposed to be peculiar to 
 the fixed stars, is, that they shine by their own light. 
 
 The term planet is derived from a Greek word signifying wan- 
 derer, and is given to those stars which are continually changing 
 their position with respect to each other, as well as with respect 
 to the fixed stars. 
 
 Dr. Herschel more particularly defines planets to be celestial 
 bodies of a considerable size and small eccentricity of orbit, moving 
 in planes that do not deviate many degrees from that of the earth, 
 in a direct course, and in orbits at considerable distances from each 
 other, with atmospheres of considerable extent, but bearing hardly 
 any sensible proportion to their diameters, and having satellites or 
 rings. 
 
 The planets are further distinguished into primaty and seconds 
 ary. The primary ones, called by way of eminence, planets, are 
 those which revolve round the sun as a centre ; and the secondary 
 planets, more usually called satellites or moons, are those which 
 revolve round a primary planet as a centre, and constantly attend 
 it in its revolution round the sun. 
 
 The primary planets are again distinguished into superior and 
 hifeiior. The superior planets are those farther from the sun than 
 our Earth, as Mars, Jupiter, Saturn, and the Herschel; and the 
 inferior planets are those nearer the sun than our Earth, as Venuz 
 and Mercury. 
 
 The newly discovered planets, Ceres Ferdinandea, Juno, Pallas, 
 and Vesta, and all such as may hereafter be discovered, and found 
 to resemble them. Dr* Herschel proposes to call by the name of 
 asteroids, which he defines as celestial bodies, moving in orbits 
 either of little or of considerable eccentricity round the sun, the 
 plane of which may be inclined to the ecliptic in any angle what- 
 soever. This motion may be direct or retrograde ; and they may 
 or may not have considerable atmospheres, very small comas, discs, 
 or nuclei. 
 
 Comets are celestial bodies moving in directions wholhr 
 
 16 
 
ASTRONOMY. 
 
 5U 
 
 Definitions. 
 
 undetermined, in very eccentric orbits, situated in every variety of‘ 
 position, and having very extensive atmospheres. 
 
 Planets, satellites, asteroids, and comets, all shine by reflected 
 light. 
 
 The planets, for the convenience of expressing them on globes, 
 or in tables, &c. are often denoted by peculiar characters, as fol- 
 lows : Mercury 5 ; Venus 9 ; the Earth 0 ; Mars ^ ; Jupiter % ; 
 Saturn ; and the Georgium Sidus, or Georgium Planet, which 
 the foreign astronomers usually call Uranus, or Herschel The 
 four lately discovered planets, or asteroids, have not yet any pecu- 
 liar characters assigned to them. 
 
 The orbit of a planet or comet, is its path, or the curve it de- 
 scribes in its revolution round its central body. The orbits of all 
 the planets are elliptical^ yet they deviate but very little from cir- 
 cles ; while the orbits of comets deviate widely from circles, and 
 are therefore said to have great eccentricity. 
 
 The direct motion of a planet, is when it appears to move 
 from west to east, its retrograde motion^ is when it appears to 
 move the contrary way, that is, from east to west. When a planet 
 seems to remain a certain time in the same place, it is said to be 
 stationary. 
 
 The ecliptic is that path or way which the earth appears to de- 
 scribe among the fixed stars, to an eye placed in the sun ; or, 
 which amounts to the same thing, it is the path which the sun ap- 
 pears to describe among the stars, to an eye on the earth. 
 
 The zodiac is a zone extending eight degrees on each side of 
 the ecliptic all round the heavens. It is divided into 12 equal 
 parts, called signs ; and as every circle, w hether great or small, is 
 supposed to be divided into 360 degrees, each sign of the zodiac 
 contains SO degrees. 
 
 Each sign of the zodiac is distinguished by a particular name, 
 and each name is denoted by a particular symbol, which is some- 
 times used alone. These names and symbols are as follow's : 
 Aries 7 "; Taurus Gemini £[ ; Cancer 35 ; Leo $ 1 ; Virgo I 5 J ; 
 Libra ; Scorpio TT^ ; Sagittarius j ; Capricornus ; Aquarius 
 XXT, and Pisces^. The signs of the zodiac are situated in the 
 order in w'hich they are here enumerated, reckoning from west to 
 east, which is called the order of the signs. 
 
 The fixed stars are divided into groups, called constellations, 
 which comprise a number of stars that appear to be near each 
 other. The ancients distinguished the constellations by the names 
 of men, birds, fishes, &c. such a name being selected, that the 
 stars of the constellation it is given to, could be contained within a 
 figure of the original. Then, in order to specify any particular 
 star, they spoke, for instance, of the star on the shoulder of Orion, 
 
512 
 
 ASTROKOMY. 
 
 Definitions. 
 
 or on the tail of the Fish, &c. The assistance these imaginary 
 figures afford to the memory, has induced the moderns to continue 
 the use of them, with some improvements. They use these con- 
 stellations to indicate the general assemblage of stars in a certain 
 portion of the heavens ; but they distinguish each particular star 
 by a Greek letter or by figures, as 1, 2, 3, &c. and mark its true 
 place by mentioning its distances from particular points. 
 
 The horizon is either sensible or rational : the sensible horizon 
 is that circle which limits our view ; the rational or true horizon is 
 parallel to the former, and is a circle in the heavens supposed to 
 be formed by the continuation of a plane which passes through the 
 centre of the earth. The planes of these two horizons, are sepa- 
 rated by the earth’s semi-diameter, but with respect to the heavens, 
 they may be considered as coincident, the distance of the fixed 
 stars being so prodigiously great, that the error of considering the 
 earth’s semi-diameter as a point, is insignificant. 
 
 The whole concave orb or expanse which invests our globe, 
 and in which the heavenly bodies are seen, is called the sphere. 
 
 The sphere appears to turn round two points which are oppo- 
 site each other, and are called the poles of the world, one of them 
 being called the arctic, or north pole ; and the other the antarctic, 
 or south pole. The axis itself, or that imaginary line which joins 
 the tw o poles, is called the axis of the world. 
 
 The highest point of the heavens, or that directly over our 
 heads, is called the zenith. 
 
 The opposite, or low'est point of the heavens, which is directly 
 under our feet, is called the nadir. 
 
 The nodes are the two points in which the orbit of a planet in- 
 tersects the ecliptic. That node from which the planet ascends 
 northward, above the plane of the ecliptic, is called the ascending 
 node ; the other node, from which the planet descends southward, 
 is called the descending node. A line passing from one node to 
 the other, is called the line of nodes. 
 
 The orbit of each planet being an ellipse, in one focus* of 
 
 * The nature of the foei of an ellipse is well shown by the common mecha- 
 nical method of describing that figure : two pins or nails are driven far 
 enough into a board to bear a slight pressure ; a cord with its two ends fasten- 
 ed together, so as to form what is, in the arts, called an endless cord, is then 
 put over them ; and a pencil or any marking instrument is employed to stretch 
 one point of the cord parallel to the board, and at the same time to mark out 
 its own eourse, while the hand carries it round ; the line produced falls into 
 itself, and forms an ellipse, of which the nails occupy the places called the foci. 
 Those who have once tried the experiment, immediately perceive, tliat the 
 longer the endless cord, while the nails retain the same distance, the nearer the 
 ellipse approaches to a circle ; and on the contrary, the shorter it is, the nearer 
 the foci are to the sides. The distance between either of the foci and the cen* 
 tre of an ellipse, is called its eccentricity. 
 
ASTRONOMY. 
 
 513 
 
 Apparent motions of the heavenly bodies. 
 
 which the sun is situated, it is obvious that the distances of the pla- 
 nets from the sun is different at different times. The points of the 
 greatest and least distance, when spoken of indifferently, are called 
 the apsides of the planet ; but when contradistinction is necessary, 
 the point of greatest distance is called the higher apsis, or aphelion; 
 the point of least distance is called the lozver apsis, or perihelion. 
 The line connecting these two points, is called the line of the ap^ 
 sides, and is supposed to pass through the centre of the sun. 
 
 When the sun or moon is at its least distance from the earth, it 
 is said to be in per/gee.— When either is at its greatest distance 
 from the earth, it is said to be in apogee. 
 
 To those who live exactly under the equator, the poles of the 
 world appear in the horizon; hence that situation is called the 
 right position of the sphere. 
 
 To those who have either of the poles of the world in their 
 zenith, the horizon coincides with, or is parallel to the equator; 
 hence that situation is called a parallel sphere. 
 
 To all the other inhabitants of the earth, the sphere is said to 
 be in an ohliqiie position, because the equator is neither perpendi- 
 cular nor parallel, but oblique to the horizon. 
 
 These definitions and explanations are brought together, for the 
 more easy reference of the reader ; but a variety of others with 
 more enlarged explanations of some already enumerated, will be 
 necessary as we proceed. 
 
 Of the apparent Motions of the hea'oenly Bodies. 
 
 As the true motions of the heavenly bodies can only be obtained 
 by a careful observation of their apparent ones, it is absolutely ne- 
 cessary, for those who wish to become acquainted with them, to 
 know' perfectly the different changes which take place in the hea- 
 vens as seen from the earth, the only place from which man can 
 make any observation. By carefully attending to these, a little 
 knowledge of optics will enable us to understand with great cer- 
 tainty, not only the true system of nature, but also w hat appearance 
 the heavens would make to a spectator placed in any part of the 
 visible creation. 
 
 When we cast our eyes tow ards the heavens, w^e perceive a vast 
 concave hemisphere, at an unknown distance, and of which the 
 eye seems to constitute the centre. The earth, or our sensible 
 horizon, stretching on every side like an immense plain, appears to 
 meet and to bound the heavenly expanse. The sun we observe to 
 rise in the east, and to set in the w est ; after which the moon and 
 stars appear, still keeping the same westerly course, till we lose 
 sight of them altogether. We further discover, upon attentive and 
 22. — VoL. I. 3 U 
 
514 
 
 ASTRONOMY. 
 
 Apparent motions of the heavenly bodies. 
 
 repeated observation, that neither the sun nor the moon always rise 
 exactly in the same point of the heavens. If we begin to observe 
 the sun, for instance, in the beginning of March, we find that he 
 seems to rise almost every day more to the northward than he did 
 the day before, to continue longer above the horizon, and to be 
 nearer the zenith at mid-day. This continues nearly to the end of 
 June, when he is observed to move backward in the same manner; 
 and his retrograde motion continues till nearly the end of Decem- 
 ber, after which he gradually attains the point from which he set 
 out, and ag^iiii renews the phenomenon. 
 
 The motion of the moon through the heavens, as well as her 
 appearance at different times, are still more remarkable than those 
 of the sun. When she first becomes visible, at the time she is 
 called the new moon, she is in the western parts of the heavens, 
 and appears to be at no great distance from the sun himself. 
 Every night she not only appears larger but at a greater distance 
 from the sun, till at last she rises from the eastern part of the 
 horizon, just at the time the sun disappears in the western, and 
 presents, at the same time, a completely circular face. She then 
 gradually moves farther eastward, diminishing in size, and rising 
 later and later every night, till at last she seems to approach the 
 sun as nearly in the east as she did in the west, and rises only a little 
 before him in the morning, as in the first part of her course she set 
 in the west not long after him. All these different appearances are 
 completed in the space of a month, after which they begin in the 
 same order as before. They are not, however, at all times regular ; 
 for at some seasons of the year, particularly in harvest, the moon 
 appears for several days to recede no farther from the sun, and to 
 rise for several nights nearly at the same hour. 
 
 In contemplating the stars, on a clear evening, we observe con- 
 tinual changes. JSew stars keep rising in the east, while others 
 are setting in the west. Looking towards the south, some stars 
 just appear on the horizon, skim its boundary for a little while 
 without rising above it, and then vanish ; others, a little farther 
 from the south, rise above the horizon, make a small arc, and then 
 go down ; while some again describe a large arc, and take a long 
 time before they set. If we look toward the north, we find 
 that some just skim the horizon, then mount to the middle of 
 the heavens; then again descend and remount, without ever 
 disappearing. Others describe complete circles, without de- 
 scending to the horizon, and these circles diminish, till at last 
 we arrive at a star that the eye cannot perceive to have any 
 motion, but which seems to be in that particular point round 
 which the whole hemisphere appears to turn. This star, is 
 called i\\^ jpoh-star. 
 
ASTRONOMY. 
 
 515 
 
 Apparent motions of the heavenly bodies. 
 
 Upon considering these phenomena, it is easy to conceive, that 
 as there is a hemisphere above us, there is also its counterpart be- 
 low us, though invisible; and that, of course, the earth, with all 
 its inhabitants, is suspended in the middle of this heavenly sphere, 
 which the horizon divides into two parts. Consonantly with this 
 idea, we find, that the remarkable star called the pole-star is more 
 or less elevated, according to the different parts of the earth from 
 which we take our view. The inhabitants of Lapland, for instance, 
 see it much nearer the zenith than we do*; we see it nearer the 
 zenith than the inhabitants of France and Spain ; and they again 
 see it nearer the zenith than the inhabitants of Barbary. By con- 
 tinually travelling south, this star would be seen nearer and nearer 
 to the horizon, and would at length become invisible. Another 
 point would then appear in the south part of the horizon, round 
 which the stars in that quarter would seem to turn. In the south- 
 ern hemisphere, however, where we should now be arrived, there 
 is no star so near the pole as in the northern, nor is the number of 
 stars so great. 
 
 Hence we know that if the opacity of the earth did not limit 
 our view, the general appearance of the heavens would be that of 
 a vast concave sphere, turning round two points fixed in the north 
 and south parts of it once in twenty-four hours. 
 
 On further observation of the stars, w'e find the greatest 
 number of them to keep their places wdtli respect to one another, 
 that is, if we observe two stars having a certain apparent dis- 
 tance from each other this night, we find that they have the 
 same to-morrow, and every succeeding night; but we by no 
 means observe them to have the same places with respect to the 
 sun and moon. The stars that do not appear to be of this fixed 
 kind, are very few' in number ; the naked eye can see but seven, 
 and astronomers, with the use of the best telescopes, have not 
 added more than four to that number. These eleven stars are the 
 planets and asteroids defined in the preceding section, and they 
 change their places very remarkably with regard to the stars, 
 and w ith regard to each other. Sometimes they seem to be 
 moving to the w'estward, sometimes to the eastward, and some- 
 times they appear stationary for a considerable time. There are 
 other bodies, which appear only occasionally, move for some time 
 with prodigious velocity, and then recede from us so far, that they 
 cease to be visible. These are comets, of which the number 
 is unknowm. 
 
 To obtain correct views of the motions of the heavenly bodies, 
 it is necessary to possess the means of indicating with precision, 
 the places they occupy. This is done by several imaginary lines 
 or circles supposed to be described upon the surface of the sphere. 
 
516 
 
 ASTRONOMY. 
 
 Arti6cial divisions of the sphere. 
 
 These circles are divided into degrees, minutes, and seconds, a 
 degree, it has been already mentioned, is the 360th part of the cir- 
 cumference of a circle ; a minute is the 60th part of a degree ; and 
 a second the 60th part of a minute. When these divisions are not 
 expressed by words, degrees are marked by a small circle ° placed 
 near the upper part of the figures; minutes by a small line ' in the 
 same situation, and seconds by two lines " of the like sort. This 
 being premised, we shall first illustrate by a diagram, the position 
 of the axis of the world, to which the imaginary circles of the 
 sphere have a reference. Let HO, fig. 1, plate II, represent the 
 circle of the horizon, seen edgeways, in which case it will appear 
 as a straight line: let HMON represent the complete sphere of 
 the heavens, of which, as only half of it can be seen at once, HMO 
 may be considered the visible hemisphere, and HNO the invisible 
 hemisphere : then P will be the pole or fixed point among the 
 stars, visible to us, round which the sphere will appear to turn, 
 and as the opposite point will be at R, the line PR will be the 
 axis of the sphere. 
 
 J f through the centre of the earth c, there be drawn a line QE, 
 it will represent the edge of a great circle, which will be a quarter 
 of a circle, or 90 degrees distant from each pole. It is called the 
 equator, because it divides the heavens into two equal parts, tl^e 
 north and south. 
 
 The equator having reference to the pole, is always the same, 
 but this is not the case with the horizon, the position of which a 
 spectator changes at every remove, consequently also the hemi- 
 sphere he beholds, and the position of his zenith and nadir; these 
 two points being always QO degrees distant from his horizon. If 
 HO be the horizon, M will be the zenith, and the opposite point 
 N the nadir. 
 
 The horizon of a spectator can never be coincident with the 
 equator, nor his zenith and nadir with the axis of the sphere, un- 
 less he be situated exactly at the pole. The zenith and nadir are 
 often called tlie foles of the horizon; the young student must there- 
 fore be careful to distinguish these poles, which vary with the place 
 of the spectator, from those of the world, which are constantly the 
 same. 
 
 All circles drawn through the zenith and nadir, are perpen- 
 dicular to the horizon, and are therefore called 'vertical circles, 
 or azimuths. 
 
 Two of these vertical circles or azimuths, are particularly 
 important, one of them is called the meridian, and passes through 
 the poles, through the zenith and nadir, and cuts the equator 
 perpendicularly. As this circle passes through the poles, it is 
 obvious that if a spectator travels exactly north or south, he 
 
ASTRONOMY. 
 
 517 
 
 Artificial divisions of the sphere. 
 
 may go round the globe without changing his meridian ; but if 
 he travel east or west, he changes it every instant. The number 
 of meridians may therefore be supposed to be infinite, but a very 
 small number, seldom more than thirty-six, are drawn upon maps 
 and globes. At mid-day, when the sun has attained his greatest 
 elevation, his centre is precisely on the meridian of tlie place where 
 he is observed, and the moment he has passed the meridian he de- 
 clines tovxards the west. The meridian divides the circles de- 
 scribed by the stars into two equal parts, and those stars which 
 never sink below the horizon, may be observed to cross it twice in 
 twenty-four hours. All the rest of the stars, as well as the sun, do 
 the same, but w hen they pass the meridian below our horizon, they 
 are of course invisible to us. 
 
 The second remarkable azimuth is called the prime vertical. 
 It divides the eastern and western sides of the horizon into two 
 equal parts, and the points of intersection are called the true east 
 and west points ; so that the meridian and prime vertical divide the 
 horizon into four equal parts, and the points of division, viz. the 
 north j the eastj the south, and the west, are called the principal 
 points of the horizon, or cardinal points. 
 
 These three great circles, viz. the equator, the meridian, and 
 prime vertical, form the basis of reference in observations on the 
 heavenly bodies, and it is therefore necessary to determine their 
 relative situations. If the pole-star had been accurately at the 
 pole of the heavens, nothing more would be necessary, in order to 
 obtain the altitude of the pole, but to take the altitude of this star; 
 but as it is at the distance of 2 degrees from the pole, 2 degrees 
 must be added to this altitude, to find that of the pole. The ele- 
 vation of the pole being know n, it is easy to find that of the equa- 
 tor. Thus, m fig. t, pi. II, HMO, or the visible part of the hea- 
 vens, contains 180 degrees; and it is ninety degrees from the pole 
 P, to E the equator. If, therefore, we take away PE from the 
 semi-circle HMO, there remains 90 degrees for the other two arcs, 
 PH, and EG, that is, the elevation of the pole and the equator are 
 together equal to 90 degrees ; so that the one being known, and 
 subtracted from 90, it will give the other. Hence the elevation of 
 the pole, at any place, is equal to what the elevation of the equator 
 wants of 90 degrees : and the elevation of the equator is equal to 
 the distance from the pole P to the zenith M. 
 
 We cannot, under ordinary circumstances, observe the sun’s 
 motion among the fixed stars, because his splendour prevents their 
 being seen ; but we can observe the instant of his coming to the 
 meridian, and his meridional altitude. We can also compute what 
 point of the starry heavens comes to the same meridian at the same 
 time, and with the same altitude. Or we can observe that point 
 
518 
 
 ASTKOSOMV. 
 
 Artificial divisions of llie sphere. 
 
 in the heavens, which comes to the meridian at midnight, with a 
 declination as far from the equator at one side, as the sun’s is on 
 the other side ; and it is evident that the sun must be in that part 
 of the heavens which is diametrically opposite to this point. By 
 either of these methods, we obtain a series of points in the hea- 
 vens, through which the sun passes in a year, and a line supposed 
 to be drawn through them all, forms that circle called the ecliptic. 
 This appellation is derived from the circumstance, that all the 
 eclipses of the sun and moon are performed either actually in or 
 very near it. 
 
 The ecliptic or annual path of the sun coincides with the equa- 
 tor only in two points ; for the sun rises above the equator in sum- 
 mer, and does not rise so high in winter. Those two points of the 
 ecliptic, where the sun is situated, when he is most distant from the 
 equator, are called the solstitial points, and the points of intersec- 
 tion of the ecliptic and equator, are called the equinoctial points. 
 
 The equinoctial colure is the great circle which passes at right 
 angles to the equator, through the equinoctial points. The solsti- 
 tial colure, is another great circle cutting the equator at right 
 angles, but passing through the solstitial points, and through the 
 poles of the ecliptic. 
 
 The angle which the plane of the ecliptic makes with that of 
 the equator, is called the obliquity of the ecliptic. The obliquity 
 of the ecliptic is equal to the elevation of the sun above the equator 
 when in either solstitial point. It amounts nearly to 
 
 The smaller circles of the sphere are all those which have their 
 centre in the axis, but not in the centre of the sphere ; two of them 
 cut the solstitial points, and their planes are at right angles to the 
 axis of the world, as AC, BD, hg. 1, pi. II ; these are called tro- 
 pics, of which that on the south side of the equator is called the 
 tropic of Capricorn, and that on the north side of the equator, the 
 tropic of Cancer. The iwo polar circles, FG, IK, are at the same 
 distance from the two poles that the tropics are from the equator, 
 namely 
 
 The distance of the heavenly bodies from the equator is called 
 their declination, which is used w ith the addition of the word north 
 or south, to denote on which side of the equator they are. The 
 declination of the sun can never exceed the obliquity of the eclip- 
 tic ; but stars have all degrees of declination, because they have all 
 degrees of altitude. Great circles, drawn through the pole.s of the 
 equator, are called circles of declination, or meridians, because 
 upon them declination is measured. Twenty-four of these circles 
 of declination, are called hour-circles ; because each of them con- 
 tains 15 degrees, which space the sun passes over every hour. 
 
 The rU^ht ascension of the heavenly bodies is their distance 
 
ASTRONOMY. 
 
 519 
 
 Artificial divisions of the sphere. 
 
 from the first point of Aries Y', estimated in time on the equator, 
 where cut by a declination circle passing through the body, reckon- 
 ing, as above stated, 15 degrees for every hour. 
 
 The olique ascension of a celestial body is an arc of the equator, 
 extending, according to the order of the signs, from Aries to that 
 point of the equator that rises w ith the star in an oblique sphere ; 
 and the difference between the right and the oblique ascension of 
 the body, is called the ascensional difference. 
 
 The latitude of a star is its distance from the ecliptic, measured 
 on a circle of latitude^ w hich is a circle perpendicular to the eclip- 
 tic, and is either north or south, as the star is situated either on the 
 north or south side of the ecliptic. 
 
 The longitude of a star is estimated from the first point of Aries 
 to the place where a line from the star w ould cut the ecliptic per- 
 pendicularly, or would cut the star’s circle of longitude. Circles 
 of longitude are lesser circles parallel to the ecliptic, and which 
 diminish as they recede from it. The longitude of a star, is of 
 course, like its latitude, either north or south. 
 
 When the latitude or longitude of a celestial body is spoken of 
 as if that body w ere seen from the centre of the sun, it is said to be 
 heliocentric ; but when the body is supposed to be seen from the 
 earth, its latitude or longitude is said to be geocentric. 
 
 Of the Figure, Motion, and Magnitude of the Earth — Magni- 
 tilde and Distance of the Sun and Moon — and of planetary 
 Motion. 
 
 Having now taken a superficial view' of the general pheno- 
 mena of the heavens, and of the principal lines or circles which 
 constitute the artificial means that have been adopted to give 
 precision to our ideas in speaking of these phenomena ; — w'e 
 may now proceed to inquire whetiier the evidence of vision 
 does not require correction from our judgment. This examina- 
 tion of our first impressions, will lead us to ascertain, if w'e can, 
 the real figure and size of the earth, and when we have satisfied 
 ourselves on these points, we may then proceed to consider whe- 
 ther the earth is in mol ion or at rest, for till w e have done this, it 
 is clear we can form no rational judgment of the real motions of 
 the stars. 
 
 Mankind at first considered the earth to be an illimitable plain, 
 and we have every reason to suppose, that they found it difficult 
 to divest themselves of a prejudice so natural. But when obser- 
 vations of certain appearances on the earth, and ot the motions of 
 the heavenly bodies, became considerably multiplied, and com- 
 pared with each other, the more penetrating of our forefathers 
 
520 
 
 ASTRONOMY. 
 
 Figure of the earth. 
 
 would perceive that the earth could not possibly be a plain, and 
 that if it was not a plain, it was in all probability not boundless. 
 In the flat countries of the East, on approaching any elevated and 
 very distant object, it would be perceived that the top of that 
 object first became visible, that the lower parts appeared in regu- 
 lar succession, and that the- base was seen the last of all. I'his 
 would be found a constant phenomenon, depending upon no 
 accidental circumstance, but observable in every direction ; and 
 the clearer the atmosphere, and the more distinct the difference 
 between the base and the summit, the more obvious it would 
 appear. If the earth were a plain, the surface of water might 
 certainly be expected to exhibit perfect flatness ; the local ine- 
 qualities of the land were easily accounted for, but water could 
 rest only at a perfect level. When, therefore, in sailing from 
 land, it was found that the base of all elevated objects disap- 
 peared first, and all the other parts in succession, in a still more 
 regular and remarkable manner than upon land, a doubt could 
 hardly remain of the curvature of the earth’s general surface ; 
 the progress of knowledge gradually rendered the fact more 
 evident, and at last it received the most ample confirmation, 
 from a circumstance within the comprehension of every one. 
 Magellan, Sir Francis Drake, Lord Anson, Captain Cooke, 
 and others, have all at different times, sailed round the earth. 
 They set out from European ports, and by steering their course 
 W'estward, arrived at length at the very place they departed 
 from; which could not have happened, had the earth been of 
 any other than a globular figure. These, as well as all other 
 navigators and travellers, whatever the direction or length of 
 their course, still find themselves surrounded, as much as those 
 who never change their residence, by the same appearance of 
 an immense vault, in which the stars are beheld. It is clear, 
 therefore, that the earth has nothing analogous to what men would 
 call a support, but that it exists by itself, perfectly detached 
 from all other bodies, in the unfathomable expanse of the Uni- 
 verse.” 
 
 From the spherical form of the earth, it follows, that all its in- 
 habitants, being directed, with their feet towards the centre of it, 
 must be variously inclined to one another, in a manner easily re- 
 presented by drawing lines from the circumference of a circle so 
 as to point to its centre, or still better by sticking pins into a ball 
 in the same direction. The inhabitants of countries diametrically 
 opposite, are called the antipodes of each other. 
 
 To the mind just entering the confines of philosophy, it seems 
 a strange and inconceivable thing, that people can stand on every 
 side of a globe, and that there can be those who have their 
 
 4 
 
ASTRONOMY. 
 
 521 
 
 Figure and size of the earth. 
 
 feet opposite to ours ; but when they have considered a little the 
 nature of the attraction of gravitation, for the general doctrine of 
 which we must refer to the pages we have devoted to the sub- 
 ject, they will soon perceive that the greatest absurdity would 
 be to suppose the contrary. They will perceive also, that the 
 terms up and down, with respect to the universe, have no absolute 
 or constant meaning ; for the region which we call uppermost, is 
 opposite t)ie feet of our antipodes ; and the region opposite our 
 feet, is consequently above their heads. But with respect to the 
 earth, the words up or down, above or below, mean the situations 
 nearer to, or farther from its centre, to which all bodies on its sur- 
 face are unceasingly drawn, in a right line, by the inliuence ol gra- 
 vitation. 
 
 When the globular form of the earth became clearly understood, 
 a tolerable idea of its size could not be long a secret. We have 
 the record of the attempt to measure it being made 550 years be- 
 fore the Christian era ; but the first time that great accuracy was 
 shown, was in l635, by Richard Norwood, who measured the dis- 
 tance between London and York with a chain; then having ascer- 
 tained the sun’s meridian altitude, for the same moment, at each 
 extreuiity of this known base, he deduced that a degree, or the 
 3f)0th part of t’ne earth’s* circumference, amounted to 69 miles, 
 one half, and 14 poles, from which he found that the circuit of the 
 earth was about 25,036 miles. 
 
 AVhen the size of the earth was supposed to be known, the next 
 object of astronomical investigation might very properly be, to dis- 
 cover the size and distance of the sun, moon, and stars. To offer 
 conjectures respecting these bodies, before something like adequate 
 ideas of their distances w'ere obtained, w'ould be folly ; and to do it 
 before the figure and size of the earth w ere demonstrated, could 
 not lead to any satisfactory conclusions. 
 
 When the effects of the sun, in giving light and heat, were 
 compared with the effects of any artificial fire, it would be 
 admitted by the most ignorant, that the size of that luminary 
 must be very great ; but the ideas of the earliest ages, on such 
 a subject, could never be commensurate to the truth. We 
 must, however, leave the speculations of the ancients, with 
 observing, that every method which ingenuity could devise, 
 has been tried to resolve the interesting questions of the sun 
 and moon’s distances in particular ; but the only one which can 
 satisfy the mind, is that which depends upon mathematical 
 principles, and which is similar to that employed in the mea- 
 surement of distant terrestrial objects. From the two extremities 
 of a base, whose length is known, the angles which the visual 
 rays from the object, whose distance is to be measured, make 
 
 3X 
 
522 
 
 ASTRONOMY. 
 
 Method of discovering the distance of the moon. 
 
 ^vith the base, are measured by means of a quadrant ; their 
 sum, subtracted from 180°, gives the angle Mhich these rays 
 form at the object where they intersect. This angle is called the 
 parallax, and when it is once known, mathematicians find it 
 easy, by means of trigonometry, to ascertain the distance of the 
 object. Let AB, in fig. 2, pi. II, be the given base, and C 
 the object whose distance we wish to ascertain. The angles 
 CAB and CBA/’^ formed by the rays CA and CB with the base, 
 may be ascertained by observation ; and their sum, subtracted 
 from 180°, leaves the angle ACB, which is the parallax of the 
 object C. It gives us the apparent size of the base AB, as seen 
 from C. When this method is applied to the heavenly bodies, 
 it is necessary to have the largest possible base, and in the usual 
 astronomical sense, the parallax of a star is the difference 
 between its true and apparent distance from the zenith. In 
 
 explanation of this subject we may observe, that when a star 
 is in the zenith, it is seen by the spectator on the earth’s sur- 
 face, where it would appear if it could be seen from the earth’s 
 centre ; it is therefore seen in what is called its true place ; but 
 when the star is not in the zenith, a spectator does not see it in 
 the same place that he would, could he view^ it from the centre of 
 the earth ; but, in all cases, the difference betw^een the true and 
 apparent place of an object is less in proportion as its distance is 
 greater. Of the popular illustrations of the method of taking 
 a parallax, the following by Walker is perhaps one of the 
 plainest : Suppose the distance of the moon w ere our first 
 object, and that w'e had a sea horizon, and were situated so that 
 the moon passed over our zenith; then let A, fig. 3, pi. II, be 
 the earth, and a the place of the observer, who must be sup- 
 posed to have a quadrant, b c, by which he can note the 
 moment the moon comes to the zenith. Novv as the moon 
 apparently passes from a meridian to that meridian again in 
 twenty-four hours and forty-eight minutes, she will perform a 
 quarter of that circuit, viz. from d to c, in a quarter of that 
 time, or in six hours and tw'elve minutes. But the observer 
 
 will find that she will set before the six hours and twelve minutes 
 are expired, wdiich he must note ; for when she comes to his 
 sensible horizon, a c she sets to him. Now as the sensible 
 horizon is parallel to the rational horizon /c g, a diagonal a e, 
 will make the angle c z equal to its opposite angle. To find 
 what the angle c z is, say, by the rule of three, if six hours 
 
 * When an angle is referred to, the middle letter indicates its vertex, or the 
 centre of the circle upon which it is measured. 
 
ASTRONOMY. 
 
 523 
 
 Method of discovering the distance of the snn. 
 
 and twelve minutes be required for a quarter of the moon’s 
 circuit, or 90°, vie. from d \o e ; how many degrees of the ninety 
 would she pass through in the time of her going from the zenitli 
 to the sensible horizon, or from d to g, which being taken from 
 90°, or the quadrant de^ will leave the quantity of the arch ge, or 
 the angle c z. Now as the angle 7i is equal to c z, we are in pos- 
 session of a right-angled triangle a k e, with a side and an angle 
 known ; for a kj the semi-diameter of our globe, is nearly SQ60 
 miles : it is the property of a right-angled triangle to have its sides 
 proportional to the sides of its opposite angles ; therefore as the 
 angle ?i is to its opposite side a k, so is the angle o to its opposite 
 side k e, or the mean distance of the earth’s centre from that of the 
 moon, viz. 240,000 miles. The parallax or angle ??, is on a me- 
 dium about 57". 
 
 The sun is so distant, that his parallax or angle g « S is too 
 small to be measured with any certainty by a horizontal paral- 
 lax. The radius of the earth is indeed found to be an almost 
 * insignificant space compared with the sun’s distance. Dr. 
 Halley therefore recommended the transits of Venus, which 
 were to happen in the years 1761 and 1769, as affording the best 
 means of ascertaining the distance of the sun. The transit of a 
 planet takes place when the planet comes exactly between the 
 earth and the sun, whose disc it traverses in the form of a small 
 round, dark spot. Transit^ can only be exhibited by the planets 
 Mercury and Venus, because no other have their orbits within that 
 of the earth. Only two transits of Venus occurred during the last 
 century, and there will not be another till 1874. By these tran- 
 sits, which were very carefully observed, almost the whole diame- 
 ter of the earth formed a parallax, instead of the above semi-dia- 
 meter, by which a commensurate angle was thus procured : 
 Venus moves in her orbit in the direction z w, fig. 4, pi. II, and 
 from the centre of the earth c, would be seen to move over 
 the sun’s disc from s to i;; an observer, therefore, at r/, would 
 ^.see the contact at s, at the same instant that one at b would see 
 the planet at u; and one at d would see it at its egress at v, 
 along the line V This would be the case were the earth at 
 rest; but it is turning on its axis in the direction a h d. Now if 
 the planet stood still at V, while the earth turned from a to d, 
 that time would be easily turned into the parallactic angle a V d, 
 and might be treated like that of the moon ; but the motion of 
 Venus, as well as that of the earth, was to be taken into this 
 calculation, as well as observations made in different latitudes ; 
 after taking every possible precaution, and making every requi- 
 site allowance, the sun’s parallax was found to be about 8'^ ; the 
 angle, though so very small, admits of calculation, and the sun’.s 
 
524 
 
 ASTRONOMY. 
 
 Magnitudes of the sun and moon. 
 
 distance was from it deduced to be about ninety-six millions of 
 miles. 
 
 The distances of the sun and moon being thus determined, 
 the facility of ascertaining their size is comparatively great ; be- 
 cause it may be appioximated by very easy calculations, according 
 to the principles of optics. The angle subtended at the eye by 
 an object is easily measured, and is inversely as the distance of 
 that object. Now the distance gives the radius of a circle, the 
 centre of which is at the pupil of the spectator’s eye, and on the 
 circumference of which is the object, which we will suppose to 
 be the sun. Having the radius, the circumference of this im- 
 mense circle is obtained by the common arithmetical method ; 
 and when we have the circumference, the value or extent of its 
 degrees and minutes may also be found. Therefore, if the num- 
 ber of minutes subtended by the sun, be multiplied by the value 
 of a minute, we have the diameter of that luminary. In ascertain- 
 ing the distances as well as the magnitudes of the sun and moon, ^ 
 a variety of mathematical and mechanical means are resorted to, in 
 order to secure the most accurate results ; but it would be useless 
 on the present occasion, to enter upon the consideration of the de- 
 tails. it is enough to give with their results, a general idea of the 
 processes employed, and to observe, that by the latest and most 
 careful calculations, the moon has been found to be 2180 miles in 
 diameter; and the diameter of the sun 883,246 miles. The evi- 
 dence in favour of these dimensions being very nearly correct, is 
 so conclusive, as to enforce conviction in every mind open to the 
 investigation of truth. 
 
 When the sphericity of the earth, its insulated existence, 
 and its magnitude, have become familiar to the young student ; 
 and, keeping in mind the general doctrine of attraction above 
 referred to, when he has further compared these three circum- 
 stances with the correspondent circumstances of the sun and 
 moon, he will then perhaps begin to inquire, whether the real 
 motions of the sun and moon are such as they appear to be, 
 and whether the earth revolves round them, or they round the 
 earth. With respect to the moon, a very momentary attention 
 will give satisfaction. This satellite always presents to us the 
 same face or side ; therefore she revolves round the earth, other- 
 wise this would be impossible. But with respect to the sun, in 
 thus consulting his judgment, he will have much reason for 
 doubt, and to believe that the evidence of sense is fallacious. 
 The sun is a million of times larger than the earth, and if he 
 revolves round the earth in twenty-four hours, he must each 
 hour advance over a space of more than 24,000,000 of miles. 
 That a body of such prodigious size sliould revolve at such a 
 
ASTRONOMY. 
 
 525 
 
 Absurd to suppose the earth at rest. 
 
 rate round another body, which (like the earth to the sun) is a 
 mere point when compared with it, is as absurd as to suppose 
 that a grain of sand should command the motion of a mill- 
 stone, or a pepper-corn that of a mountain. It would be totally 
 irreconcileable with all that we know of the simplicity of the 
 laws of nature, which always accomplish the grandest objects 
 by the simplest means ; and when we advanced to the consi- 
 deration of other celestial phenomena, we should find our diffi- 
 culties increase with our progress, till at last inextricable con- 
 fusion beset us. It is absolutely necessary to relinquish the idea 
 that the earth is immoveable, and philosophers after the most 
 minute investigation of the phenomena, have completely shown, 
 that the earth has at least two motions, one on its own axis called 
 its diurnal motion, which causes it to revolve from east to west in 
 twenty-four hours, and the other a progressive motion, called its 
 annual motion, which carries it round the sun once in a year. 
 The diurnal motion produces the regular return of day and night, 
 and the apparent revolution of the sphere ; the annual motion pro- 
 duces the vicissitudes of the seasons, summer, winter, spring, and 
 autumn. 
 
 It is no reason against the rotation of the earth, that we are 
 unable to perceive it. For as the motion of a ship at sea, when 
 swiftly sailing over the smooth surface of the water, is not 
 perceptible to the company on board ; certainly we may 
 expect that so large a body as the earth, which has no impedi- 
 ment in its way, or resistance to overcome, will in this respect 
 deceive us effectually. It has been asserted again, that if the 
 earth moved, a stone dropped from the top of a tower, or any 
 other high building, would not fall just at the bottom of it, as 
 the building must have advanced considerably forward, during 
 the time of the fall. But this is evidently a mistake; for it is 
 well known, by repeated experiments, that if a body be projected 
 from another body in motion, it will always partake of the mo- 
 tion of that other body. Thus, a stone dropped from the top of 
 a mast, while the ship is under sail, is not left by the vessel, but 
 falls exactly at the foot of the mast. And if a bottle of water be 
 hung up in the cabin, with its neck downwards, it will empty it- 
 self, drop by drop, into another vessel placed exactly underneath it, 
 though the ship shall have run many feet whilst each drop was in 
 the air. 
 
 The motion of the eartff, in the manner stated by astrono- 
 mers, being admitted, as also the magnitudes and distances of 
 die sun and moon, the remaining w'onders of astronomy, though 
 they may dverw'helm with the idea of immensity, will scarcely 
 excite dis-belief. It is ^ound by data as indisputable as those 
 
 15 
 
526 
 
 ASTRONOMY. 
 
 Origin of Sir Isaac Newton’s theory of nature. 
 
 which establish the facts hitherto ineiitioned, that the planets or 
 wandering stais, and the comets, all revolve round the sun in 
 diffeient orbits, and that these bodies are in reality all of them 
 worlds. We shall therefore make a few remarks on that power 
 which preserves them in the order we observe, and afterwards pro- 
 ceed to the separate consideration of them as parts of the solar 
 system. 
 
 When Sir Isaac Newton undertook the reformation of phi- 
 losophy, he proceeded upon a very different plan to those who 
 had gone before him ; he proposed to assume nothing as an 
 hypothesis which was not deduced from data acknowledged and 
 obvious to our eyes, and thus, by arguing from those things 
 which are within our reach, he thought we might come to know 
 with certainty what must happen in the celestial regions. The 
 manner in which he was first led to form his system of gravita- 
 tion, which is now universally received, is said to have been as 
 follow s : He w as sitting alone in a garden, when some apples 
 falling from a tree, directed his thoughts to the subject of gravity, 
 or the cause of their fall ; and reflecting on that principle, he 
 began to consider, that, as its pow'er is not found to be sensibly 
 diminished at the most remote distance from the centre of the 
 earth to which we can rise, neither at the tops of the loftiest build- 
 ings, nor on the summits of the highest mountains, it was reason- 
 able to conclude that its influence must extend much further than 
 was usually thought. Then a train of thought arose in his mind. 
 Might it not extend as far as the moon ; and if so, must not her 
 motion be influenced by it ? perhaps it retains her in her orbit : 
 however, though ihe power of gravity is not sensibly weakened in 
 the little change of distance, at which we can place ourselves 
 from the centre of the earth, yet it is very possible, that, as high 
 as the moon, this power may differ much in strength from what 
 it is here.’’ To make an estimate of the degree of this diminution, 
 he considered, that if the moon be retained in her orbit by the 
 force of gravity, no doubt the primary planets were retained in 
 their orbits by a similar gravitation towards the sun ; and by com- 
 paring the periods of the several planets with their distances from 
 the sun, he found, that if any power like gravity held them in their 
 courses, its strength must decrease in the duplicate proportion of 
 the increase of distance. This was concluded from a supposition 
 that these bodies moved in perfect circles round the sun ; which, 
 though they are not found to do exactly, yet the error was but of 
 little consequence. 
 
 To account then for the perpetual motions of the planets and 
 comets in their orbits, Newton had recourse to the force of 
 gravity, and a projectile force compounded wdth it. These two 
 
ASTRONOMY. 
 
 527 
 
 Explanation of planetary motion. 
 
 forces had indeed been proposed by fJoirox, before his time, 
 with the same view’; and the power of gravity, as existing in 
 the celestial regions, had been intimated by several philoso- 
 phers ; but their ideas appeared as little better than vague 
 speculations, and were almost unnoticed by the world. As 
 Newton was ignorant of any natural power by which the pla- 
 nets could be impelled in the direction of a tangent line to any 
 part of their orbits, he had recourse, for one of the forces, to 
 the immediate action of the Deity. According to him, God 
 having created this world, impressed the universal law of at- 
 traction or gravitation upon matter, and impelled each of the 
 planets in the direction of a right line touching their orbits. 
 Being immediately acted upon by the attraction of the sun, 
 their courses were bent from a straight line into a curve ; and 
 the same causes still continuing to act, the original rectilinear 
 direction was changed into one nearly circular, which has con- 
 tinued ever since. The same mode of reasoning is applicable 
 to the continued motion of the secondary planets round their 
 primaries. The manner in which Sir Isaac Newton demon- 
 strates the operation of the projectile and gravitating forces upon 
 the planets, so as to direct them in circles round the sun, is by 
 supposing the orbits they describe divided into infinitely small 
 parts, each of which will not differ from a right line, and conse- 
 quently the whole curve may be considered as consisting of the 
 diagonals of parallelograms infinitely small, one of whose sides is 
 represented by the space which the planet would have moved 
 through by the projectile force alone, and the other by that which 
 it would have moved through by the force of gravity alone in the 
 same time. To those who would enter deeply into the subject, 
 Newton’s Principia will afford ample gratification, but it will bet- 
 ter suit our purpose of popular illustration to adopt the following 
 explanation of Ferguson : 
 
 From the uniform projectile motion of bodies in straight 
 lines, and the universal power of attraction which draws them 
 off from these lines, the curvilineal motions of all the planets 
 arise. If the body A be projected along the right line ABX, 
 in open space, where it meets with no resistance, and is not 
 drawn aside by any other power, it will for ever go on with the 
 same velocity and in the same direction. For the force which 
 moves it from A to B in any given time, will carry it from B to 
 X in as much more time, and so on; there being nothing to 
 obstruct or alter its motion. But if, when this projectile force 
 has carried it, suppose to B, the body S begins to attract it, 
 with a power duly adjusted, and perpendicular to its motion at 
 B, it will then be draw n from the straight line ABX, and forced 
 
528 
 
 ASTRONOMY. 
 
 Explanation of planetary motion. 
 
 to revolve about S in the circle BYTU. When the body A 
 comes to U, or any other part of its orbit, if the small body ?/, 
 within the sphere of U’s attraction, be projected as in the right 
 line Z, with a force perpendicular to the attraction of U, then n 
 will go round U in the orbit W, and accompany it in its whole 
 course round the body S. Here, S may represent the sun, U the 
 earth, and u the moon. 
 
 If a planet at B gravitates, or is attracted towards the sun, 
 so as to fall from B to y, in the time that the projectile force 
 would have carried it from B to X, it will describe the curve-BY, 
 by the combined action of these two forces, in the same time that 
 the projectile force singly would have carried it from B to X, or 
 the gravitating powder singly have caused it to descend from B 
 to y; and these two forces iDeing duly proportioned and perpen- 
 dicular to each other, the planet obeying them both, will move in 
 the circle BYTU. To make the projectile force balance the gra- 
 vitating power so exactly, as that the body may move in a circle, 
 the projectile velocity of the body must be such as it would have 
 acquired by gravity alone in falling through half the radius of the 
 circle. 
 
 But if, whilst the projectile force would carry the planet 
 from B to b, the sun’s attraction (which constitutes the planet’s 
 gravitation) should bring it down from B to 1, the gravitating 
 power would then be too strong for the projectile force, and 
 W'ould cause the planet to describe the curve BC. When the 
 planet comes to C, the gravitating power (which always in- 
 creases as the square of the distance from the sun, S, diminishes) 
 will be yet stronger for the projectile force, and by conspiring 
 in some degree therewith, will accelerate the planet’s motion all 
 the w'ay from C to K ; causing it to describe the arcs BC, CD, 
 DE, EF, &c. all in equal times. Having its motion thus acce- 
 lerated, it thereby gains so much centrifugal force, or tendency 
 to fly off at K, in the line K k, as overcomes the sun’s attrac- 
 tion ; and the centrifugal force being too great to allow' the 
 planet to be brought nearer the sun, or even to move round him 
 in the circle K / w it goes oft', and ascends in the curve KL 
 MN, &c. its motion decreasing gradually from K to B, as it 
 increased from B to K, because the sun’s attraction now' acts 
 against the planet’s projectile motion just as much as it acted with 
 it before. VVhen the planet has got round to B, its projectile 
 force is as much diminished from its mean state about F or M, 
 as it w'as augmented at K ; and so the sun’s attraction being 
 more than sufficient to keep the planet from going off at B, it de- 
 scribes the same orbit over again, by virtue of the same forces or 
 powers. 
 
ASTRONOMY. 
 
 629 
 
 Explanation of planetary motion. 
 
 ‘‘ A double projectile force will always balance a quadruple 
 power of gravity. Let the planet at B have twice as great an 
 impulse from thence towards X, as it had before ; that is, in 
 the same length of time that it was projected from B to b, as in 
 the last example, let it now be projected from B to c ; and it 
 will require four times as much gravity to retain it in its orbit : 
 that is, it must fall as far as from B to 4, in the time that the pro- 
 jectile force would carry it from B to c ; otherwise it would not 
 describe the curve BD, as is evident by the figure. But in as 
 much time as the planet moves from B to C, in the higher part of 
 its orbit, it moves from 1 to K, or from K to L, in the lower part 
 thereof; because, from the joint action of these two forces, it 
 must always describe equal areas in equal times throughout its 
 annual course. These areas are represented by the triangles BSC, 
 CSD, DSE, ESF, &c. whose contents are equal to one another 
 quite round the figure. 
 
 As the planets approach nearer the sun, and recede farther 
 from him, in every revolution ; there may be some difficulty in 
 conceiving the reason why the pow’er of gravity, when it once 
 gets the better of the projectile force, does not bring the planets 
 nearer and nearer the sun in every revolution, till they fall upon 
 and unite w ith him ; or why the projectile force, when it once gets 
 the better of gravity, does not carry tlie planets farther and farther 
 from the sun, till it removes them quite out of the sphere of his at- 
 traction, and causes them to go on in straight lines for ever after- 
 w'ard. But by considering the effects of these powers, as already 
 described, this difficulty will be removed. Suj)pose a planet at B 
 to be carried by the projectile force as far as from B to b, in the 
 time that gravity would have brought it down from B to 1 : by 
 these tw'o forces it will describe the curve BC. fVhen the planet 
 comes down to K, it will be but half as far from the sun, S, as it 
 was at B ; and therefore, by gravitating four times as strongly to- 
 wards him, it would fall from K to V in the same length of time 
 that it would have fallen from B to 1 in the higher part of its 
 orbit, that is, through four limes as much space ; but its projec- 
 tile force is then so much increased at K, as would carry it from K 
 to k in the same time, being double of what it was at B; and is 
 therefore too strong for the gravitating power, either to draw' the 
 planet to the sun, or cause it to go round in the circle 
 which would require its falling from K to rc’, through a gieater 
 space than gravity can draw' it, while the projectile force is such 
 as would carry it from K to k: and therefore the planet ascends in 
 its oibit KLMN, decreasing in its velocity, for the causes already 
 assigned. 
 
 “ By the above-mentioned law, bodies will move in all kinds 
 
 (23.— VoL. 1. ' ’ 3 Y 
 
5S0 
 
 ASTRONOMY. 
 
 Planetary motion. — Kepler’s laws. 
 
 of ellipses, whether long or short, if the spaces they move in 
 be void of resistance. Only, those which move in the longer 
 ellipses, have so much the less projectile force impressed upon 
 them in the higher parts of their orbits ; and their velocities, in 
 coming down tow'ards the sun, are so prodigiously increased by 
 his attraction, that their centrifugal forces, in the lower parts of 
 their orbits, are so great as to overcome the sun’s attraction there, 
 and cause them to ascend again towards the higher parts of iheit* 
 orbits ; during which time the sun’s attraction acting so contrary 
 to the motion of those bodies, causes them to move slower and 
 i^ower, until the projectile forces are diminished almost to no- 
 thing ; and then they are brought back again by the sun’s attrac- 
 tion^ as before.” 
 
 The celebrated Kepler discovered by assiduous observation, 
 the two remarkable laws of planetary motion ; and in honour of 
 him they are called Kepler’s laws. Sir Isaac Newton demon- 
 strated them according to the above theory of attraction, and they 
 show, in a remarkable manner, the harmony of the celestial mo- 
 tions : 
 
 I. If a straight line be dravvn from a planet to the sun, and 
 this line be supposed to be carried along by the periodical 
 motion of the planet, then the areas which are described by this 
 right line and the path of the planet, are proportional to the 
 times of the planet’s motion ; for instance, the area thus de- 
 scribed in two hours, is the double of that which is described in 
 one hour, and a third part of that which is described in six hours ; 
 though the arc which is described by the planet itself in two 
 hours, is not the double of the arc which is described by the same 
 in one hour ; nor the third part of that w hich is described in six 
 hours. 
 
 II. The planets are at different distances from the sun, and 
 perform their periodical revolutions in different times ; but the 
 cubes of their distances, or of the principal axes of their ellip- 
 tical orbits, are constantly as the squares of their periodical 
 times, viz. of the times of their performing their periodical revo- 
 lutions. 
 
 The sun forms the centre of attraction, round which all 
 the planets move ; but all the planets and comets, according to 
 the quantities of matter they contain, and their distances, exert 
 their power of attraction upon the sun. The sun, therefore, is 
 not at rest; but as his magnitude so greatly exceeds that of all the 
 planets and comets put together, and as these exert their attrac- 
 tive powers in opposite directions, from being on opposite sides, 
 the centre of gravity of the whole system, which is the point 
 round which the sun performs a drcuit, is always near him, 
 
F/ff./. 
 
 //. 
 
 4^x 
 
 
 A 
 
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ASTRONOMY. 
 
 531 
 
 Effects of the mutual attractions of the heavenly bodies. — Solar system. 
 
 and generalJy within his body. From the grand principle of 
 universal attraction, also, it follows that the planets must attract 
 each other ; and in fact it has not escaped the observation of 
 philosophers, that they sensibly disturb each other’s motions. 
 When any two planets are in conjunction, their mutual attrac- 
 tions, which tend to bring them nearer to one another, draws the 
 inferior one a little farther from the sun, and the superior one a 
 little nearer to him ; by which means the figure of their orbits is 
 somewhat altered, though the alteration is too small to be disco- 
 vered in several ages. 
 
 The orbits of such planets as have one or more moons, are 
 still more remarkably disfigured by the attraction of these atte«id- 
 ants ; for example, the point which describes the orbit of the earth 
 round the sun, is not the centre of the earth, but the common cen- 
 tre of gravity of the earth and moon, and which is found bv cal- 
 culation, on comparing their quantities of matter, to be about 
 2000 miles from the earth’s surface. But these irregularities, 
 while they show the truth of the theory that accounts for them all, 
 are too inconsequential to require much notice in a popular view 
 of the solar system. 
 
 Of the Solar System, 
 
 The plate, marked Solar System, is intended to give an idea 
 of the appearance of that system, could a spectator view it from 
 a considerable distance above the sun, in a line perpendicular 
 to the earth’s orbiu In every plan of this kind, however, some* 
 sacrifices of strict propriety are made for the sake of conve- 
 nience. The sun is only supposed to be seen by the radiance he 
 throw's around; the planets and their satellites, with their distances 
 from the centre of the sun, are not exhibited in due proportion ; 
 and the orbits of all the planets are represented by circles, from 
 which the reader will recollect they differ so little, that though in 
 reality ellipses, they could scarcely on so small a scale be distin- 
 guished from circles. Of the comets, but four are noticed, each 
 moving in an orbit of different eccentricity, but the actual number 
 of these bodies is known, to be very considerable, though by no 
 means determined. 
 
 To a spectator, situated where w'e have supposed, above the 
 sun, the planetary motions, would appear extremely regular ; 
 but it is easy to conceive, that tp one situated sideways, or 
 viewing them, for example, as from the earth, they will exhi- 
 bit great anomalies. Accordingly, the planets sometimes 
 appear to be going forwards, sometimes backwards, and some- 
 times ^pear to be atationary. These irregularities aie such as 
 
552 
 
 ASTKONOWV. 
 
 Number of the planets and their satellites. — The Sun. 
 
 must take place, if the Copeniican be tlie true system of nature, 
 and therefore the observation of them is a proof of the correct- 
 ness of that system. 
 
 Idle planetary bodies of the Solar System, so far as at pre- 
 sent known, and commencing with the one nearest the sun, are the 
 following : 
 
 1. Mercury, 
 
 2. Venus, 
 
 3. The Earth, with one Satellite or Moon, 
 
 4. Mars, 
 
 5. Vesta, 
 
 6. Juno, 
 
 7. Ceres, 
 
 8. Pallas, 
 
 9. Jupiter, with four Satellites, 
 
 U). Saturn, with seven Satellites, 
 
 ] 1. Herschel, with six Satellites. 
 
 The fifth, the sixth, the seventh, and eighth, are the four which 
 Dr. Herschel calls asteroids. After treating of the sun, we shall 
 notice all the planetary bodies in their order. 
 
 The Sun. o. 
 
 The sun is an immense globe, the mean diameter of which 
 amounts to 883,246 miles. When viewed through a telescope, 
 dark spots of various sizes and duration, are perceived on its sur- 
 face ; and fi om observations on the motions of these spots, it has 
 been found that the sun revolves on its axis, from east to- west, 
 once in 23 days, 14 hours, 8 minutes, and his axis deviates about 
 8° from a perpendicular to the plane of the ecliptic. 
 
 The dark spots of the sun’s surface, are often called maculoe^ 
 and those parts which are brighter than the rest, are called /hew 
 
 A variety of very different and heterogeneous conjectures 
 have been offered to account for the maculae. As the constant 
 emanation of light and heat, induced the opinion that the sun 
 was nothing else than one prodigious mass of fire, so it was 
 interw'oven with the idea that so vast a combustion required 
 adequate accessions of matter to keep it up ; on this account, 
 it was supposed that the maculae might be only the scoria or 
 unconsumed portions- of the additional fuel, swimming on the 
 liquid surface of the sun. Faculae, on the contrary, have been 
 called clouds of light, and luminous vapours ; and because 
 maculae have sometimes been observed to change into faculae, 
 it has been conjectured, that the latter were the dark fnnies of 
 
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ASTRONOMY. 
 
 o33 
 
 Solar system reviewed. — The Sun. 
 
 volcanoes rapidly blazing out, after the dark smoky matter, 
 which produced the inaculaj, became dissipated by combustion. 
 But these ideas partook very much of the vagaries of imagina- 
 tion ; they possessed nothing admirable in themselves, and no 
 attempt was made to render them, in a satisfactory manner, con- 
 sistent with all the phenomena. In the year 1788, however, ap- 
 peared a dissertation by Edward King, F. R. S. wliich contained 
 many original remarks : this philosopher asserted, that tlie real 
 body of the sun is less than its apparent diameter ; that we never 
 discover the real body of the sun itself, except when we behold its 
 spots, (maculje,) that the sun is inhabited as well as our earth, and 
 is not necessarily subject to burning heat ; and that there is in re- 
 ality no violent elementary heat existing in the rays of the sun 
 themselves essentially. 
 
 Several years after the publication of these opinions by 
 King, Dr. Herschel began to publish in the Philosophical Tran- 
 sactions, his theory concerning the nature of the sun, and to 
 which he was led by his numerous observations, made with his 
 admirable instruments, and the most persevering industry. This 
 theory has received a degree of acceptance proportionate to the 
 skill with which it was established, and we subjoin the following 
 outline of it : 
 
 '1 he Doctor considers the sun to be a magnificent habitable 
 globe, surrounded by a double set of clouds. Those which are 
 nearest its opaque body, are less bright and more closely 
 connected together than those of the upper stratum, which form 
 the luminous apparent globe we behold. This luminous exter- 
 nal matter, he observes, is neither a liquid nor an elastic fluid 
 of an atmospheric nature ; for in either of these two cases, it 
 would not admit of any chasms or openings. Therefore, it 
 must be concluded that this shining matter exists in the manner 
 of empyreal, luminous, or phosphoric clouds, residing in the 
 higher regions of the solar atmosphere. The Doctor then is of 
 opinion that the spots are only accidental openings between the 
 luminous clouds, through which we behold the opake body of the 
 sun, or the inferior less luminous clouds ; hence the spots appear 
 of different shades. The Doctor rejects the terms macula?, fa- 
 culae, luculi, and others previously in use, and substitutes the fol- 
 lov\ing, as better adapted to express what he considers the true 
 phenomena of the sun. 
 
 Openings are those places where, by the accidental removal of 
 the luminous clouds of the sun, its own solid body may be seen ; 
 and this not being lucid, the openings through which we see it 
 may, by a common telescope, be mistaken for mere black spots or 
 their nuclei. 
 
534 
 
 ASTRONOMY. 
 
 Solar Ssystem reviewed. — The Sun. 
 
 Shallows are extensive and level depressions of the luminous 
 %olar clouds, generally surrounding the openings to .a considerable 
 distance. As they are less luminous than the rest of the sun, they 
 seem to have some distant, though very imperfect, resemblance 
 to penumbrse ; which might occasion their having been called so 
 formerly. 
 
 Judges are bright elevations of luminous matter, extended in 
 rows of an irregular arrangement. 
 
 Nodules are also bright elevations of luminous matter, but 
 confined to a small space. These nodules and ridges correspond 
 to what have hitherto been called faculag and luculi. 
 
 Corrugations, he applies to that very particular and remark- 
 able unevenness, ruggedness, or asperity, which is peculiar to the 
 luminous solar clouds, and extends all over the surface of the 
 globe of the sun. As the depressed parts of the corrugations are 
 less luminous than the elevated ones, the disc of the sun has an 
 appearance which may be called mottled. 
 
 Indentations are the depressed or low parts of the corruga- 
 tions; they also extend over the whole surface of the luminous 
 solar clouds. 
 
 Pores are very snaall holes or openings, about the middle of 
 the indentations. 
 
 As the Doctor considers the solar clouds to consist rather of 
 a flame than a liquid substance, he attributes the spots to the 
 emission of an aeriform fluid, not yet in combustion, which 
 displaces the general luminous atmosphere, and which is after- 
 wards to serve as fuel for supporting the process ; hence he 
 supposes the appearance of copious spots to be indicative of the 
 approach of warm seasons on the surface of the earth, a theory 
 which he has attempted to maintain by historical evidence. 
 The shallows he considers as parts of an inferior stratum of 
 opaque clouds, capable of protecting the immediate surface of 
 the sun from the excessive heat produced by combustion in the 
 superior stratum, and probably of rendering this stupendous 
 world fit for animated existence. In general, more or less of the 
 real body of the sun is supposed to be visible through its lucid 
 atmosphere, where its substance is not very intense, or where it 
 is removed by the varying circumstances of the combustion. 
 As some of the spots appear below the surface of the shining 
 fluid, it may be presumed that they show us the lower parts of 
 the sun’s surface ; and as other spots appear above the shining 
 fluid, it is equally reasonable to conclude, that they are the 
 solar mountains. The former kind of spots are very variable 
 in situation, and it is clear, that if we have the true theory of 
 the sun before us, they may never occur twice precisely at thft 
 
ASTRONOMY. 
 
 535 
 
 Solar system reviewed. — The Sun. 
 
 same place. The spots attributed to the projection of the solar 
 mountains, are iixed with respect to the sun’s surface, and are 
 those by which the sun’s rotation on his axis has been determin- 
 ed. Dr. Herschel, taking into consideration the great attraction 
 exerted by the sun upon bodies placed at its surface, and the 
 slow^ revolution it has about its axis, thinks that the solar moun- 
 tains may be more than 300 miles high, and yet stand very 
 firmly. 
 
 Dr. Young, in opposition to the theory of Dr. Herschel, 
 argues, that if we inquire into the intensity of the heat which 
 must necessarily exist wherever this combustion is performed, 
 we shall soon be convinced that no clouds, however dense, could 
 impede its rapid transmission to the parts below. Besides, the 
 diameter of the sun is 111 times as great as that of the earth ; 
 and at its surface, a heavy body would fall through no less than 
 450 feet in a single second ; so that if every other circumstance 
 permitted human beings to reside on it, their owm weight would 
 present an insuperable difficulty, since it would become thirty 
 times as great as upon the surface of the earth, and a nian of 
 moderate size would w^eigh above two tons. This hypothesis is 
 not so satisfactory as Dr. Herschel’s. If the sun be not inhabited, 
 the only adequate purpose of its existence which we can con- 
 ceive, is that of maintaining the existence of the other parts 
 of the solar system, and that this is the only or chief use of it 
 is inconsistent with that frugality of means which appears the 
 more eminent, the more closely we examine the works of nature. 
 As the sun is so much larger than the aggregate mass of the 
 planets, if it had been endued with no more inherent heat than 
 we can easily reconcile to our gross ideas of organization, 
 it w'ould have supported a greater number of inhabitants than 
 all the planets, supposing each of them to be the residence of 
 living beings. It seems absurd, then, to suppose that the sun is 
 created solely for the use of the comparatively mean and dimi- 
 nutive w'orlds that surround him ; and a strange, unphilosophical 
 limitation of the Deity’s power and beneficence, to conclude that 
 he cannot call into existence beings whose organs are as admirably 
 suited to their peculiar circumstances on the sun, as we are to 
 our ow'ii situation. With the knowledge we possess of modes 
 of existence infinitely diversified, we cannot deny the existence 
 of the Power to produce such beings ; and when we consider the 
 fulness of creation within the sphere of our observation, it 
 seems as difficult to deny the presence of the will to create 
 them, rather than leave a mighty blank of that possible happi- 
 ness, which myriads of beings might enjoy. The arguments 
 against the inhabitable condition of the sun, drawn from the 
 
536 
 
 ASTRONOMY. 
 
 Solar system reviewed. — The Sun. 
 
 circumstance of the excessive weight of bodies on its surface, 
 are inconclusive, unless inhabitants could be formed onlv of 
 the same kind of matter as ourselves, which is not certainly 
 the most probable conjecture. It may also be observed, that 
 the strength or muscular energy of ordinary men is not more 
 inferior to the muscular energy of some individuals that have 
 occasionally appeared in the world, than the muscular energy 
 of these remarkable men is inferior to what an inhabitant of the 
 sun would require, provided greater muscular energy were the 
 only means he possessed to overcome the disadvantage of his 
 weight. 
 
 With respect to the excessive heat at the surface of the sun, 
 there are many facts which show that heat is produced by the 
 sun's rays only when they act on a calorific medium ; they pro- 
 duce heat by uniting with the matter of fire which is contained 
 in the substances that are heated. The collision of a flint and 
 steel will inflame a magazine of gunpowder, by putting all the 
 latent fire which it contains into action; while the same spark 
 upon a heap of sand would not have the slightest effect. Now if 
 we suppose the matter on the surface of the sun, not to admit of 
 any extensive chemical combination with its rays, it is easy to per-' 
 ceive, that the heat may not be such as to be inconsistent with or- 
 ganized existence. 
 
 Walker’s speculations on the sun are very ingenious. He 
 supposes the particles of light, which constitute the solar atmo- 
 sphere, strongly to repel each other ; that this property of 
 repulsion, is one cause of the light being dispersed or projected 
 in every direction through space ; but that, at the equatorial 
 regions of the sun, the centrifugal force created by his rotation, 
 co-operating with repulsion, produces a more copious emission 
 of the vital ocean, than at the polar regions. The sun turns 
 on an axis inclined but 8° from the plane of the ecliptic ; his 
 equatorial, or greatest centrifugal discharge of particles, will 
 therefore be nearly confined to the zodiac or track of the planets, 
 where the greatest supply is required. Taking for granted, 
 (and we shall afterwards see the reasonableness of the belief,) 
 that the fixed stars are suns, he supposes that these suns lecruit 
 each other, by imbibing at and near their poles, most of the 
 light that reaches them : for tow'ards their poles, the strong 
 attraction of the sun’s body, overcomes the repulsion of light, 
 when that repulsion is but little assisted by the centrifugal force, 
 and the light thus imbibed, sinks down for the supply of the 
 equatorial regions. But a still more beautiful part of this hypo- 
 thesis, is, that it explains the cause of the diurnal motion of 
 tlie earth and other planets, which Newton left unaccounted for. 
 
ASTRONOMY. 
 
 537 
 
 Solar system reviewed. — The Sun. 
 
 Light has been shown (see Optics) to possess momentum ; 
 suppose then, S, fig. 6, plate II, to be the sun, and E the earth, 
 and that the lines «, h, c, c?, 8cc. represent rays of light issuing 
 centrifugally from the sun. A line from the sun’s centre of 
 gravity, s, to that of the earth, o, will show’ the direction in 
 which the two bodies mutually attract each other, or the line 
 in which they would fall together, if no counteracting prin- 
 ciple prevented it. The equal distances of the lines which 
 represent the rays, being agreeable to the equal distribution of 
 light through the zodiac, it may be seen that twice the quantity 
 falls on one side of the line of direction, s o, as on the other ; 
 thus ah c d are on one side, w hile e and f only fall on the opposite 
 side of the line of direction. Though this is an exaggerated 
 proportion, yet in all the positions in which the earth stands to 
 the sun, during its annual revolution round him, it will be 
 found that more rays fall on one side of its axis than on the other; 
 and the consequence of such an inequality of impulse must be its 
 rotation on its axis, if not also of its annual revolution. The 
 author further inquires, whether the repulsion of light may not 
 be a balance to the gravitation of the planets towards the sun, 
 and whether their several distances may not be produced by 
 differences in their densities, because the larger the surface of a 
 given quantity of matter, the more it will be repelled by the 
 action of light? To put these ideas to the test of experiment, 
 he had recourse to the following means : He took a circular 
 
 wooden box, BB, fig. 1, plate HI, and made oblique holes, re- 
 presented by the tubes, in its side. This box was close, both at 
 top and bottom, w ith the exception of an aperture, rt, to admit the 
 mouth of a double pair of bellow s, to make the current of air 
 equal. A very thin glass globe was hung by a very long thread, 
 exactly over the centre a. This gave it a constant tendency to- 
 wards that centre, and represented the gravitation of the planets 
 towards the sun. The ball being now drawn to the side of the 
 box, and the bellows blown, the box instantly began to turn on its 
 axis, and to revolve in an ellipse. This experiment is ingeniously 
 contrived, and will gratify those who may not be disposed to give 
 full credence to the theory it was instituted to support ; yet the 
 theory itself is certainly more entitled to respectful attention than 
 many that have been proposed ; the present state of our knowledge 
 does not enable us to decide against it ; and it therefore remains 
 for future investigations to establish or subvert. 
 
538 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Mercury. 
 
 Mercury. ^ 
 
 Mercury is the nearest planet to the sun, yet his distance from 
 that luminary is not less than 37,000,000 of miles. He emits a 
 very bright while light, always to us appears near the sun, and as 
 he is generally lost m the splendour of the solar beams, astrono- 
 mers have but scanty opportunities of making observation upon 
 him. His diameter is 3224 miles; and his revolution round the 
 sun is performed in 87 days, 23 hours, 14 minutes, 33 seconds; 
 his summers and winters cannot therefore be more than 44 days 
 each. i\s no spots have yet been discovered on his disc, the time 
 of his diurnal rotation is not known with certainty, though Schroeter 
 is induced by some of his observations, to believe that it is per- 
 formed in 24 hours and 5 minutes. The position of his axis is 
 also undetermined. 
 
 Mercury never goes to a greater distance from the sun than 
 about 27° 3'; so that he is never longer in setting after the sun 
 than an hour and fifty minutes ; nor does he ever rise sooner than 
 the same space of time before the sun. When he begins to make 
 his appearance in the evening after sun-set, he can scarcely at first 
 be distinguished on account of the twilight; but he is seen at a 
 greater distance from the sun every successive evening, till he has 
 attained the distance of about 22° 3', when he begins to return 
 again. During this interval, his motion, referred to the stars, is 
 direct; but when he approaches within 18° of the sun, he appears 
 for some time stationary, and then his motion begins to be retro- 
 grade, till at last his proximity to the sun prevents his being seen. 
 Soon after, he may be perceived in the inornmg before sun-rise, 
 receding further and further from that luminary, as before he dis- 
 appeared. Having attained the distance of 18°, he again becomes 
 stationary, then assumes a direct motion, but continues to separate 
 till his distance is 22° 3', after which he again returns to the sun, 
 plunges into his rays, and appears soon after in the evening, after 
 sun-set, to repeat the same career. 
 
 The inclination of Mercury’s orbit, to the plane of the 
 ecliptic, is about 7°. He may be much better observed near 
 our equator than in high latititudes. When examined with 
 a telescope magnifying 200 or 300 times, he exhibits the same 
 ditferences of phases as the moon, being sometimes horned, 
 sometimes gibbous, and sometimes shining almost with a round 
 fa(^e — it is not entirely full, because his enlightened side is never 
 turned directly towards us ; but he is at all times well defined, 
 without any ragged edge, and perfectly bright; like the moon, 
 the crescent is always turned towards the sun. The apparent 
 9 
 
ASTRONOMY. 
 
 539 
 
 Solar system reviewed. — Venus. 
 
 diameter of Mercury varies with his position; it is least when 
 the planet plunges into the solar rays in the morning, or when it 
 disengages itself from them ; it is greatest when the planet 
 plunges into the solar rays in the evening, or when it disen- 
 gages itself from them in the evening ; that is, when tlie planet 
 passes the sun in its retrograde motion, its diameter appears the 
 greatest possible ; and when it passes the sun in its direct mo- 
 tion, it is the smallest possible. Sometimes, when the planet 
 disappears during its retrograde motion, that is, when it plunges 
 into the sun’s rays in the evening, it may be seen crossing tlie 
 
 sun, in a right line, under the form of a black spot. This 
 
 black spot is recognized to be the planet, by its position, its 
 apparent diameter, and its retrograde motion. These transits, 
 as they are termed, are miniature eclipses ; they demonstrate 
 
 that the planet is itself an opaque body, and shines only by the 
 
 light it borrows from the sun. A transit of Mercury does not 
 occur at every one of his revolutions, because his orbit is 
 inclined to the ecliptic, and coincides with it only at the two 
 nodes; hence the planet cannot be seen to pass over the disc of 
 the sun, unless its nodes are in or very near the line which joins 
 the sun and the earth. Transits of Mercury are expected in the 
 years 1815, 18^2, 1832, 1835, 1845, and 1848. 
 
 Mercury moves in his orbit about 1 10,000 miles per hour; the 
 sun to him appears three times as large as to us, and affords him, 
 at a medium, seven times the heat felt at our torrid zone. Hence 
 it appears that if the materials of which the planet is composed 
 were as susceptible of being heated as these of our earth, they 
 would be either melted or vitrified, and the planet would have 
 acquired such a brightness, as to prevent its appearing like a black 
 spot at its transit. Why then may we not suppose that he has 
 inhabitants as different from us as his matterf 
 
 Venus, 9 
 
 Venus is the most beautiful star of the heavens. She is 
 called the Morning or Evening Star, according as she precedes 
 or follows the apparent course of the sun, and retains each title 
 about 290 days. Her light is remarkable for its brightness and 
 whiteness, casting a sensible shadow, and so vivid, at her 
 greatest elongations, that she is visible to the naked eye in full 
 day-light; and the light of the moon is frequently observed to 
 be comparatively dull. She is the second planet from the sun, 
 round which she performs lier annual revolution, at the medium 
 distance of 68,000,000 of miles, in 224 days, 16 hours, 41 
 minutes, 27 seconds. The time ©f her diurnal revolution, 
 
540 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Venus. 
 
 or the length of her day, is supposed to be 23 hours, 21 
 minutes. It is necessary to remark, that with respect to this 
 point, and conclusions drawn from it, there is some uncertainty. 
 Dr. Herschel is among those observers who have not satisfied 
 themselves as to the existence of spots on this planet, and 
 therefore do not consider the position of her axis, and the period 
 of her diurnal rotation, as fully determined. Her diameter is 
 7687 miles, and she is therefore very nearly the size of the 
 Earth. The • inclination of her orbit to the ecliptic, is about 
 3° 23' 35". Her greatest distance from the sun vaiies from 45® 
 to nearly 48°. 
 
 The transits of Venus are very rare phenomena; only two, 
 we have before observed, occurred during the last century, and 
 another will not happen until the year 1874. During a transit, the 
 black spot, under the form of which she appears, has an apparent 
 diameter of 59 ^ A few days after this has been observed, she is 
 seen in the morning, west of the sun,' and appears through a good 
 telescope in the form of a fine crescent, with the convexity 
 turned towards the sun. She moves gradually westward with a 
 retarded motion, and the crescent becomes more full. In about 
 ten weeks, she has moved about 46° west of the sun, and is now 
 a semi-circle, and her diameter is 26'. She is now stationary. 
 She then moves eastward wdth a motion gradually accelerated, 
 and overtakes the sun in 91 months after having been seen on his 
 disc. Sometime after, she is seen in the evening, east of the sun, 
 round, but very small. She moves eastward, and increases in 
 diameter, but loses of her roundness, till she gets about 46° east 
 of the sun, when she is again a semi-circle. She now moves 
 westward, increasing in diameter, but becoming a crescent, like 
 the waning moon ; and, at last, after a period of nearly 584 days, 
 comes again into conjunction with the sun, wfith an apparent 
 diameter of 59"* It may perhaps, excite the surprise of some, 
 that Venus should keep longer on the east or w^est of the sun, than 
 the whole time of her period round him. But the difficulty 
 vanishes when we consider that the earth is all the while going 
 round the sun the same way, though not so quickly, as Venus; 
 and therefore her relative motion to the earth must in every 
 period be as much slower than her absolute motion in her orbit, 
 as the earth during that time advances forward in the ecliptic, 
 which is 220 degrees. 
 
 To the inhabitants of Venus, the sun will appear nearly twice 
 as large as to us, and Mercury will be a morning and an evening 
 star to them, as she is to us. Her atmosphere has been calcu- 
 lated at fifty miles in height, from a shade appealing on the 
 «aun’s face, about five seconds before the dark body of Venus 
 
ASTRONOMY. 
 
 541 
 
 Solar system reviewed. — The Earth. 
 
 appeared to touch his edge, at the time of her transit. During the 
 transit, also, the greatest attention was exerted to discover whether 
 she was attended by any satellite, but none was observed, and she 
 is therefore not known to have one. 
 
 The axis of Venus is generally supposed to be inclined 75 
 degrees from the axis of her orbit, which is 514 degrees more 
 than our earth’s axis is inclined from the axis of the ecliptic ; and 
 therefore the variation of her seasons is proportionately greater. 
 The north pole of her axis inclines towards the 20th degree of 
 i\quarius, our earth’s to the beginning of Cancer : consequently 
 the northern regions of Venus have summer in the signs where 
 those of our earth have winter, and vice versa. As the sun’s 
 greatest declination on each side of her equator amounts to 75 
 degrees, her tropics are only 15 degrees from her poles, and her 
 polar circles as far from the equator. She has at her equator 
 two summers and tw^o winters, in each of her annual revolu- 
 tions. 
 
 The Earth. 0 
 
 The Earth is the next planet above Venus in the Solar Sys- 
 tem. Her distance from the sun, is at a mean 95,000,000 of 
 miles, and her annual revolution is performed in 365 days, 5 
 hours, 48 minutes, 49 seconds; this is called her tropical year ; 
 but the time she takes to perform an annual revolution from any 
 fixed star to the same again, as seen from the sun, is 365 days, 
 6 hours, 9 minutes, 12 seconds, which is caWed 2 i sidereal year. 
 Her rotation on her axis is performed in 24 hours, which is 
 called the length of a natural day. Her diameter is 791 1 miles. 
 Though she travels along her orbit at the rate of 68,000 miles 
 per hour, a motion about 140 times swifter than that of a cannon 
 ball, yet her motion is little more than half as swift as that of 
 Mercury in his orbit. Her diurnal rotation being from west to 
 east, the diurnal motion of all the heavenly bodies appears to be 
 from east to west. Besides the 68,000 miles of progressive ad- 
 vance, to which the inhabitants of every part are subjected alike, 
 the inhabitants of the equator are carried 1030 miles every hour by 
 the earth’s motion on its axis ; but from the equator to the poles, 
 this motion diminishes, and at the latitude of London, it amounts 
 only to about 580 miles per hour. 
 
 If the axis of the earth were perpendicular to the ecliptic, 
 or plane of its orbit, the days and nights would be of equal 
 length in every part of it ; but its axis is inclined 234 degrees 
 to the ecliptic, and this produces the seasons ; the excessive lieat 
 ^'hich would be felt at the equator, and the excessive cold 
 
542 
 
 ASTRONOMY. 
 
 Solar system reviewed. — The Earth. 
 
 which would be felt at the poles, being thereby both moderated. 
 The earth’s axis keeps parallel with itself, that is, constantly pre- 
 serves the same oblique direction, during the whole of its annual 
 journey, with a very small exception to be afterwards noticed. 
 
 The following experiment will give a plain idea of the diur- 
 nal and annual motions of the earth, and the phenomena of the 
 seasons : Take about seven feet of strong wire, and bend it 
 
 circularly, 2 ls ab cd, fig. % pi. Ill, which being viewed obliquely, 
 appears elliptical as in the figure. Place a lighted candle on a 
 table ; and having fixed one end of a silk thread, K, to the 
 north pole of a small terrestrial globe H, about three inches 
 diameter, cause another person to hold the wire circle, so that 
 it may be parallel to the table, and as high as the flame of the 
 candle, 1, which should be in or near the centre. Then having 
 twisted the thread as towards the left hand, that by untwisting, 
 it may turn the globe round eastward, or contrary to the way in 
 which the hands of a watch move, hang the globe by the thread 
 within this circle, almost contiguous to it; and, as the thread 
 untwists, the globe (which is enlightened half round by the 
 candle, as the earth is by the sun) will turn round its axis, and 
 the different places upon it will be carried through the light 
 and dark hemispheres, and have the appearance of a regular 
 succession of days and nights, as our earth has in reality by 
 such motion. As the globe turns, move your hand slowly, so 
 as to carry the globe round the candle, according to the order 
 of the letters abed, keeping its ceiitre even with the wire 
 circle, and you will perceive, that the candle, being still perpen- 
 dicular to the equator, v^ill enlighten the globe from pole to 
 pole in its whole motion round the circle ; and that every place 
 on the globe goes equally through the light and dark, as it 
 turns round by the untwisting of the thread, and therefore has 
 a perpetual equinox. The globe thus turning round, represents 
 the earth turning round its axis ; and the motion of the globe 
 round the candle represents the earth’s annual motion round the 
 sun ; and shows, that if the earth’s axis had no inclination to 
 the plane of its orbit, all the days and nights of the year 
 would be equally long, consequently there would be no differ- 
 ence of seasons. This peculiarity distinguishes the planet 
 Jupiter, whose inhabitants have a perpetual equinox, namely, 
 his axis is perpendicular to the plane of his orbit, as the 
 thread, round which the globe turns in this experiment, is per- 
 pendicular to the plane of the area inclosed by the w'ire. — But 
 now let the person who holds the wire, hold it obliquely in the 
 position ABCD, raising the side 25 just as much as he depresses 
 
ASTRONOMY. 
 
 543 
 
 Solar system reviewed. — The Earth. 
 
 the side VJ, that the flame may be still in the plane of the 
 circle ; and twisting the thread as before, that the globe may 
 turn round its axis in the same way it is carried round the 
 candle, that is, from west to east ; let the globe down into the 
 lowermost part of the wire circle at VJ; and if the circle be pro- 
 perly inclined, the candle will shine perpendicularly on the 
 tropic of Cancer ; and the frigid zone, lying within the arctic 
 or north polar circle, will be enlightened as in the figure ; and 
 will keep in the light, however often the globe turns round its 
 axis. From the equator to the north polar circle, all the 
 places have longer days and shorter nights ; but from the equa- 
 tor to the south polar circle, just the reverse. The sun does 
 not set to any part of the north frigid zone, as shown by the 
 candle’s shining on it, so that the motion of the globe can carry 
 no place of that zone into the dark ; and at the same time the 
 south frigid zone is involved in darkness, and the turning of the 
 globe brings none of its places into the light. If the earth were 
 to continue in the like part of its orbit, the sun would never set 
 to the inhabitants of the north frigid zone, nor rise to those of 
 the south. At the equator, it would be always equal day and 
 night ; and as places were gradually more and more distant 
 towards the arctic circle, they would have longer days and shorter 
 nights ; w hilst those on the south side of the equator would have 
 their nights longer than their days. In this case there would 
 ke a continual summer on the north side of the equator, and 
 continual winter on the south side of it. But as the globe 
 turns round its axis, move your hand slowly forward, so as to 
 carry the globe from H towards E, and the boundary of light 
 and darkness will approach towards the north pole, and recede 
 towards the south pole : the northern places will go through 
 less and less of the light, and the southern places through more 
 and more of it ; showing how the northern days decrease in 
 length, and the southern days increase, whilst the globe proceeds 
 from H to E. When the globe is at E, it is at a mean state be- 
 tween the low'est and highest parts of its orbit ; the candle is di- 
 rectly over the equator ; the boundary of light and darkness just 
 reaches to both the poles, and all places on the globe go equally 
 through the light and dark hemispheres, showing that the days and 
 nights are then equal at all places of the earth, the poles only ex- 
 cepted ; for the sun is then setting to the north pole, and rising to 
 the south pole. 
 
 Continue moving the globe forward, and as it goes through 
 the quarter A, the north pole recedes still farther into the dark 
 hemisphere, and the south advances more into the light, as the 
 
544 
 
 ASTRONOMY. 
 
 Solar system reviewed. — The Earth. 
 
 globe comes nearer to 05 ; and when it comes there at F, the 
 candle is directly over the tropic of Capricorn ; the days are at 
 the shortest, and the nights at the longest in the northern hemi- 
 sphere, all the way from the equator to the arctic circle ; and 
 the reverse in the southern hemisphere, from the equator to the 
 antarctic circle ; within which circle it is for many weeks conti- 
 nual daylight. 
 
 Continue both motions, and as the globe moves through the 
 quarter B, the north pole advances towards the light, and the 
 south pole recedes towards the dark ; the days lengthen in the 
 northern hemisphere, and shorten in the southern ; and when 
 the globe comes to G, the candle will be again over the equator, 
 (as when the globe was at E,) the days and nights will therefore 
 again be equal, and the north pole will be just commencing her 
 long day, while the south is left for an equal space of time in 
 darkness. 
 
 Thus we see the reason why the days lengthen and shorten 
 from the equator to the polar circles every year ; why there is 
 periodically no day or night for many turnings of the earth 
 within the polar circles ; why there is but one day and one night 
 in the whole year at the poles ; and why the days and nights 
 are equally long all the year round at the equator, which is al- 
 ways equally cut by the circle bounding light and darkness. The 
 inclination of an axis or of the plane of an orbit, is merely rela- 
 tive, arising out of the comparison we make with some other axis 
 or orbit, which we do not consider as inclined at all. If the axis 
 of the earth be inclined 23^ degrees to the axis of his orbit, the 
 axis of this orbit must be inclined 23^- degrees to the axis of the 
 earth ; and it amounts to the same thing, whether we use the for- 
 mer expression or the latter. 
 
 Though we say that the inhabitants within the arctic and 
 antarctic circles are at opposite times of the year deprived 
 of the sight of the sun, yet they are not altogether deprived of 
 his light ; for the atmosphere reflecting and refracting the sun’s 
 light, forms a twilight at the distance of even 18 degrees. 
 Hence the day breaks to us when the sun is still 18 degrees 
 below the eastern horizon ; and we have his light in the evening 
 till he sinks 18 degrees below the western. But as the sun in 
 summer rises considerably to the. north of the east, and sets 
 also to the north of the west with us, he rises and sets very, 
 obliquely to the horizon, and the twilight is of long duration. 
 At midsummer, Great Britain has no night, for the whole 
 island is within 18 degrees of the horizon, or boundary of ihe 
 sun’s light. In high southern latitudes, they enjoy in thei? 
 
ASTRONOMY. 
 
 545 
 
 Solar system reviewed. — The Earth. 
 
 turn, the same advantage; but at the equator, the sun sinks 
 abruptly from the horizon, because his patli is at right angles to 
 it, or very nearly so; and the quick transition from the glare of 
 day, to utter darkness, cuts off’ the enjoyment of the twilight 
 hour. 
 
 From the revolution of the earth round its axis, Sir Isaac 
 Newton was led to suppose, that its shape was not a perfect 
 sphere, but that of an oblate spheroid, flattened at the poles ; 
 because in revolving its parts have a tendency to recede from 
 the axis, from the equator particularly, consequently the poles 
 press internally, and raise the equatorial parts, till an equilibrium 
 occurs. Sir Isaac Newton calculated that the equatorial exceeds 
 the polar diameter 34 miles and one-fifth ; and this instance of 
 his penetration w'as afterwards fully confirmed by the admea- 
 surements and observations of two deputations of mathemati- 
 cians, wdio visited the vicinity of the northern and southern 
 poles in 1735, and agreed in pronouncing them flattened, 
 making the difference betw'een the diameters as £66 to £65, or 
 as 179 to 178. Another proof of the flatness of the poles has 
 been mentioned, at page 277, where we have observed, that a 
 pendulum, calculated to swing seconds at the poles, or in a 
 high latitude, will not swing seconds at the equator unless it be 
 shortened ; therefore, as a pendulum swings by the power of 
 gravity, that power is rather less at the equator than at the 
 poles ; partly on account of the greater distance of those parts 
 from the' equator, and partly from their greater centrifugal force, 
 which tends to throw every thing in a tangent from its surface. 
 If the diurnal motion of the earth were to cease, its centrifugal 
 force would of course be destroyed at the same time, in which 
 case, the water it supports would immediately retire, in order 
 to restore the perfectly globular form of the earth, and would 
 overwhelm the polar regions. An experiment may easily be con- 
 trived to show the tendency of a swift whirling motion to produce 
 an oblate spheroid like the earth. Let two slips of pasteboard, 
 or any tolerably flexible material, be bent into a circular form, 
 and fitted upon an axis, as shown by fig. 3, plate III ; so as to 
 turn with that axis. When they are made to revolve very 
 slowly by turning the winch G, the change in their form will 
 not be perceptible; but if the wunch be turned with rapidity, the 
 poles of the hoops are depressed, and their sides bulge out. 
 
 That the apparent diameter of the sun is greater in winter 
 than in summer, is obvious to common observation. The rea- 
 son is, the earth’s elliptical orbit, in the lower focus of which 
 the sun is situated, and as this point is 1,6 17, 941 miles from the 
 centre of the orbit, the earth comes twice that distance, or 
 24. — VoL. I. 4 A 
 
546 
 
 ASTRONOMY. 
 
 Solar system reviewed. — The Eartli. 
 
 0,235,882 miles nearer the sun at one time of the year tlian 
 another, and its nearest approach is in our winter, for it is then 
 the sun appears under tiie largest angle. Hence it may be 
 inquired, w’hy have we not the hottest weather when the earth 
 is nearest the sun ^ In the first place, it must be observed in 
 answer, that the eccentricity of the earth’s orbit, or l,f) 17,941 
 miles, bears no greater proportion to the earth’s mean distance 
 from the sun, than 17 does to 1,000; and therefore this small 
 
 difference of distance is in itself of little consequence. Bat 
 
 the principal cause of the difference between winter and summer 
 is, that in the former season, the sun’s rays fall so obliquely 
 upon us, that any given number of them is spread over a much 
 greater surface in the latitude in which we live. Another cause 
 contributing greatly to the winter’s cold, is the length of the 
 nights, which carry off more beat ^than the sun imparts during 
 the day. In summer, on the contrary, the sun attaining a con- 
 siderable altitude, his rays dart down upon us more nearly 
 
 perpendicular, a greater nunjber of them fall upon the same 
 space ; and by their long continuance, more heat is imparted 
 by day than can fly off by night. The atmosphere is heated, not 
 by the passage of the sun’s rays through it, but from the earth, 
 which absorbs the rays, and when the earth is once well heated, 
 it does not lose its temperature very readily, because the air 
 carries off heat slowly. We have therefore a greater number 
 of warm days, and all our hottest weather, after the longest day, 
 though the sun’s meridian altitude from that day begins to 
 decline ; and this is only stating wdth regard to a season, a fact 
 of which we have diurnally a proof, viz. that it is hotter at two 
 or three in the afternoon, when the sun has gone towards the 
 w'est, than at noon when he is on the meridian. By parity of 
 reasoning, it is equally evident, that w’e shall have the greatest 
 number of cold days, and the coldest weather, after the shortest 
 day, though after that time, the sun is actually higher every 
 ■^ay ; but the heat received by the .Earth is so trilling, that the 
 long night is not only adequate to counteract its influence, but has 
 power enough to reduce it for some time lower and lower. 
 
 The Muon. ^ 
 
 Next to the sun, the Moon appears to us the most splendid 
 of ail the heavenly bodies. Her orbit is an ellipse, with the 
 Earth in one of its foci ; her revolution i ound the earth, is per- 
 formed in 29 days, 17 hours, 44 minutes, 3 seconds, and with 
 the earth she revolves round the sun in a yeac. Her diame- 
 ter is 2180 miles, and her distance 240,000 miles. It is there- 
 
ASTRONOMY. 
 
 547 
 
 Solar system reviewed. — The Moon. 
 
 fore her comparative nearness to us, that makes her appear so 
 large, and alford so much light. Her rotation on her axis is per- 
 formed in the same time as her revolution round the earth, hence 
 she always keeps tlie same side towards us, except that we some- 
 times see a little more of one side, and at other times a little more 
 of another side. I'his is called her librcdioH. This libration is 
 produced by her unequal motion in her orbit, and from the 
 difference in its direction, it is distinguished into tw'o kinds; 1, 
 Libration in latitude ; 2, Libration in longitude. Her libra- 
 tion in longitude, is her seeming motion to and fro, so as to 
 show sometimes more of her eastern edge, and sometimes more 
 of her western edge. Her libration in latitude, is when either 
 of her poles appears to dip a little tow'ards the earth. 
 
 The moon is an opaque globe like the earth, and shines 
 only by the light she receivesy from the sun, and reflects to us. 
 She therefore disappears when she comes between us and the 
 sun, because her dark side is tiien towards us. Thus when she 
 is at A, flg. 4, plate HI, in conjunction with the sun S, her 
 dark half is tow^ards the earth, and she disappears as at «, there 
 being no light on that half to render it visible. On the innermost 
 circle, the moon is delineated as she would appear to a spectator 
 in the sun ; on the outermost circle, is showm her appearance to a 
 spectator on the earth at T. Wlien she comes to her first octant 
 at B, or has gone an eighth part of her orbit from her conjunc- 
 tion, a quarter of her enlightened side is towards the earth, and 
 she appears horned as at h. When she has gone a quarter of 
 her orbit from between the earth and sun to C, she shows us 
 one half of her enlightened side, as at c, and we say she is a 
 quarter old. At D she is in lier second octant ; and by show- 
 ing us more of her enlightened side, she appears gibbous as at 
 d. At E her whole enlightened side is towards the eai th ; and 
 therefore she appears round, as at c, and we say it is full moon. 
 In her third octant at F, part of her dark side being towards, 
 the earth, she again appears gibbous, and is on the decrease as 
 at /'. At G w'e see just one half of her enlightened side; and 
 she appears half decreased, or in lier third quarter, as at g. 
 At H we only see a quarter of her enlightened side, being m 
 her fourth octant, where she appears horned, as at //. And at 
 A, having completed her course from the sun to the sun again, 
 she disappears; and we say it is new moon. Thus in going 
 from A to E, the moon seems continually to increase ; and in 
 going from E to A, to decrease in the same proportion ; having 
 like phases at equal distances from A lo E, but as seen from the 
 sun S, she is always full. 
 
 13 
 
548 
 
 ASTRONOMY. 
 
 Solar system reviewed. — The Moon. 
 
 The moon appears not perfectly round \vhen she is full in 
 the highest or lowest part of her orbit, because we have not u 
 full view of her enlightened side at that lime. AYhen full in 
 the highest part of her orbit, a small deficiency appears on her 
 lowest edge ; and the contrary when full in the lowest part of 
 her orbit. 
 
 It is evident from the figure, that when the moon changes 
 to the earth, the earth appears full to the moon, and vice versa. 
 For when the moon is at A, new to the earth, the whole enlight- 
 ened side of the earth is towards tlie moon ; and when the 
 moon is at E, full to the earth, its dark side is towards her. 
 Hence a new moon answers to a full earth, and a full moon to a 
 new earth. The quarters are also reversed to each other. 
 
 Between the third quarter and change, the moon is fre- 
 quently visible in the forenoon, even when the sun shines ; and 
 then she affords us an opportunity of trying an amusing experi- 
 ment, if we can find a globular stone above the level of the 
 eye, as suppose on the top of a gate. For if the sun shine on 
 the stone, and we place ourselves so that the upper part of the 
 stone may just seem to touch the point of the moon’s lowermost 
 horn, we shall then see the enlightened part of the stone exactly 
 of the same shape with the moon, horned as she is, and inclined 
 the same way to the horizon. The reason is plain ; for the sun 
 enlightens the stone the same way that he does the moon ; and 
 both being globes, when we put ourselves into the above situa- 
 tion, the moon and the stone have the same position to our 
 eyes, and therefore we can see no more of the illuminated side 
 of the one than of the other. 
 
 The position of the moon’s cusps, or a right line touching 
 the points of her horns, is very differently inclined to the 
 horizon, at different hours of the same days of her age. Some- 
 times this line is perpendicular to the horizon ; when this happens, 
 .she is in what astronomers call the nonagesirnal degree^ which 
 is the highest point of the ecliptic above the horizon at that 
 time, and is QO degrees from both sides of the horizon where it 
 is then cut by the ecliptic. But this never happens when the 
 moon is on the meridian, except when she is at the very begin- 
 ning of Cancer or Capricorn. 
 
 It is apparent to the naked eye, that the surface of the moon 
 is irregular, from the different shades of colour which it exhi- 
 bits ; and when viewed ihrongh a good telescope, it appears 
 diversified with every variety of hill and dale. In every 
 situation of the moon, the elevated parts of her surface cast a 
 shadow on the acyoining lower parts in the direction from the 
 
ASTRONOMY. 
 
 549 
 
 Solar system reviewed. — The Moon. 
 
 sun, and the cavities are always dark on the side next the sun. 
 The darker parts were at one time very generally believed to be 
 water, but it is now as generally admitted that they are cavities, 
 and that the disc of the moon is such as our earth would 
 have to her inhabitants, if the ocean was dried up. ' The 
 heights of the lunar mountains were formerly supposed to be 
 much greater than those of the earth ; but J3r. Herschei has 
 demonstrated that very few are more than half a mile high, 
 and the highest little more than a mile. Before or after the 
 moon is at full, the boundary of the light and dark part of the 
 side presented to us, in consequence of the multiplicity of 
 her caverns and mountains, appears exceedingly jagged or 
 uneven; while, contrary to what we sliould expect, the real 
 boundary of her disc, or the boundary between the hemisphere 
 which we see, and that which is turned away from us, appears 
 smooth and well defined, though examined with a considerable 
 magnifying power. In explanation of this difficulty, it has 
 been observed, that as all the parts adjoining to the real boun- 
 dary or edge of the moon are full of irregularities, the elevations 
 of some parts may stand before the hollow's of others, so as to 
 form upon the wdioie, the appearance of a smooth surface, and 
 perhaps the atmosphere of the moon may likewise contribute to 
 this appearance. 
 
 The ruggedness of the moon’s surface adds greatly to her 
 utility as an attendant of the earth; for if her disc W'ere free 
 from all asperities, or covered with w'ater, her light would only 
 in certain situations show us the image of the sun, apparently 
 nothing more than a brilliant point. But her inequalities 
 .scattering the light on every side, her disc becomes entirely re- 
 splendent. 
 
 Bright specks have been observed on the dark parts of the 
 moon’s disc, and so far from the illuminated portion as not 
 to depend on the sun’s rays. These spots, becoming extinct 
 after a certain time, have been supposed to be volcanoes. Dr. 
 Herschei, in 1787, saw threo of these volcanoes at once in 
 the dark ])art of the moon ; two of them were barely visible ; 
 but the third was more vivid, and exhibited an elongation like 
 an eriq'ition or lava of luminous matter, resembling a small 
 piece of burning charcoal, covered by a very thin coat of 
 ashes. If volcanoes or lire exist in the moon, it seems reason- 
 able to conclude, that the moon has an atmospnere, and that this 
 is the case, there is good reason to believe, inongh it has been 
 much controverted, and even denied till lately. Some astrgno- 
 
550 
 
 ASTRONOMY. 
 
 Solar system reviewed. — The Moon. 
 
 mers think that they can attribute the variable brightness of the 
 moon, at certain times, to no other cause, than the variable state 
 of her atmosphere, more or less loaded with vapours. Cassini ob- 
 served, that Saturn, Jupiter, and the fixed stars, had their circular 
 ligures changed into elliptical ones, when they approached either 
 the dark or the illuminated edge of the moon; and to these indi- 
 <*ations of a lunar atmosphere, Schroeter adds the following; that 
 the two cusps or apices of the luminous horns, in a new moon, 
 appear tapering in a very sharp and faint prolongation. He also 
 observed tliat, when once Jupiter came very near the moon, two 
 of 1ms satellites appeared indistinct for a short time before they 
 went quite behind the body of the moon. If the moon have really 
 an atmosphere, still we need not be surprised at the difiiculty with 
 which it is distinguished, for it probably is extremely attenuated ; 
 L^iplace indeed calculates that it must be more rare than what we 
 call the vacuum of our best air-pumps. 
 
 Excepting the occasional changes attributed to the volcanoes 
 a«d to the atmosphere of the moon, the general appearance of 
 this luminary is constantly the same ; and selenographia, or maps 
 of the face of the moon, have frequently been published ; the most 
 remarkable spots in these maps, have been sometimes distinguished 
 by tlie names of places on the earth, but mostly by the names of 
 eminent persons, as Plato, Archimedes, &c. The most accurate 
 transcript of the moon's disc is perhaps that executed by John 
 Russel, R. A. 
 
 The moon has scarcely any difference of seasons, her axis 
 being almost perpendicular to the ecliptic. What is very siiv- 
 guJar, one half of her has no darkness at all; the earth constantly 
 affording it a strong light in the sun’s absence; while the other 
 half has a fortnight’s darkness and a fortnight’s light by turns. 
 The earth, as before observed, acting as a moon (o the moon, will 
 from her be seen to wax ami wane regularly, but will appear about 
 
 times as large, and afford about 3 3 times as much light as she 
 does to us. 
 
 From the moon, one half of the earth is never seen ; from the 
 middle of the other half of ,the moon, the earth is always seen 
 over head; turning round almost 30 times as quickly as the 
 moon does. From the circle which limits our view of the 
 moon, only one half of the earth’s side next her is seen ; the 
 other half being hid below the horizon of all the places on that 
 Circle. 
 
 As the earth turns round its axis, the several continents, 
 seas and islands, appear to the moon’s inhabitants like so many 
 
ASTRONOMY. 
 
 551 
 
 Solar system reviewed. — The Moon. 
 
 spots of dlftereiit forms and brightness, moving over its surface ; 
 but much fainter at some times than otliers, as our clouds cover 
 them or leave them. By these spots the lunarians can determine 
 the time of the earth’s diurnal motion, just as we do the motion 
 of the sun ; and their observations will at the same time furnish 
 them with an accurate measure of time. 
 
 The moon’s axis being nearly perpendicular to the ecliptic, 
 the sun never removes sensibly from her equator; and the 
 obliquity of her orbit, which is next to nothing as seen from the 
 sun, cannot cause the sun to decline sensibly from her equator. 
 Yet her inhabitants are not destitute of means for ascertaining 
 the length of their year, though their method and ours must 
 differ. We can know the length of our year by the return- of 
 our equinoxes; but the lunarians, having always equal day 
 and night, must have recourse to another method ; and we may 
 suppose, they measure their year by observing when either of 
 the poles of our earth begins to be enlightened, and the other to 
 disappear, which is always at our equinoxes ; they being conve- 
 niently situated for observing great tracts of land about the poles 
 of our earth, wliich are entirely unknown to us. Hence we con- 
 clude, that the year is of the same absolute length both to the 
 earth and moon, though very different as to the number of days ; 
 we having 365^ natural days, and the lunarians only 
 day and niglit in the moon being together as long as on the 
 «artb. 
 
 The lunar rays excite no perceptible warmHi, not even 
 when concentrated upon a thermometer, by the largest burning 
 mirror or lens. This is accounted for on the supposition of 
 their extreme diffusion, so that we are unable to collect a suffi- 
 cient quantity of them to evidence their heating effects. The 
 calculations which have been made of the tenuity of the moon’s 
 light, differ so widely from each other, that we know not which to 
 credit ; some having asserted that it is ninety thousand, and others 
 that it is three hundred thousand times less than that of common 
 day-light. But all calculations founded on the supposed fact 
 that the reflected light and heat are diminished in equal propor- 
 tions, are probably fallacious : she may reflect little or no heat, yet 
 her light may reach us in a far greater proportion than has yet 
 been allowed. 
 
 The moon has not received more attention from the astrono- 
 mer than the poet ; but while the recondite speculations of the 
 one please but few, and often almost bewilder the keenest 
 understanding, the other depicts only those benignant effects 
 which every eye can behold, and every heart enjoy. TIio 
 following night-piece from Homer, is esteemed by Pope, its 
 
552 
 
 ASTRONOMY. 
 
 Solar sj'stem reviewed. — Mars. 
 
 translator, to be siipereminent in the original, and it will not be 
 denied the tribute of admiration in its English dress : 
 
 So when the Moon, refulgent lamp of night. 
 
 O’er heaven’s clear aznre spreads her sacred light, 
 
 Wlien not a breath disturbs the deep seroie, 
 
 And not a cloud o’ercasts the solemn scene ; 
 
 Around her throne the vivid planets roll, 
 
 And stars unnumbered gild the glowing pole. 
 
 O’er the dark trees a yellower verdure shed, 
 
 And tip with silver every mountain’s head ; 
 
 Then shine the vales; the rocks in prospect rise;. 
 
 A flood of glory bursts from all the skies ; 
 
 The conscious swains, rejoicing in the sight, 
 
 ^ Eye the blue vault, and bless the useful light.” 
 
 Mars. ^ 
 
 The next planet beyond the earth in the solar system, i.s 
 Mars, whose distance from the sun is 144,000,000 of miles, and 
 who requires nearly 687 days to complete his annual revolution. 
 The colour of this planet is a dusky red, an appearance attributed 
 to the great density of his atmosphere, which permits only the 
 red rays to be reflected to us. From the spots which have been 
 observed on his disc, his rotation on his axis has been ascertained 
 to be performed in 24 hours, 39 minutes, 22 seconds, and the 
 inclination of his axis to his orbit is 39° 22'. His equatorial is 
 to his polar diameter as l6 to 15, and his mean diameter is 4189 
 miles. 
 
 Mars is not subject to the same limitation in his motion as 
 Mercury or Venus, but appears sometimes very near the suii, and 
 at other times at a great distance from him ; sometimes rising when 
 the sun sets, or setting when the sun rises. He sometimes appears 
 gibbous, but never horned, like the moon, a proof that his orbit 
 includes that of the earth, and that he shines by a borrowed ii<»-ht. 
 When he is opposite to the sun, or when we see him on the meri- 
 dian at midnight, he is much more brilliant than in another situa- 
 tion, being five times nearer to us than at a conjunction. 
 
 The sun affords to Mars only about a third of the light he 
 affords to the earth, and it has therefore been thought singular, 
 that he has not yet been discovered to have a moon : but perhaps 
 he is compensated for this apparent want, and the light he re- 
 ceives may be prolonged, by the height and density of his atmo- 
 sphere, which is so remarkable, that when lie approaches any of 
 the fixed stars, they change their colour, grow dim, and often be- 
 come nearly invisible, though at some little distance from the body 
 of the planet. 
 
ASTRONOMY. 
 
 553 
 
 Solar system reviewed. — Mars. 
 
 Mars appears to move from west to east round the earth, but 
 his apparent motion is very unequal. When we first perceive 
 him in the morning, separating from the sun, his motion is the 
 most rapid ; but this rapidity diminishes gradually, and ceases 
 altogether w’hen the planet is about 137° distant from the sun; 
 then his retrograde motion commences, and increases in rapidity 
 till he comes into opposition with the sun. It tlien gradually 
 diminishes again, till the planet is within 137° of the sun; it 
 then becomes direct, till at last the planet is lost for a time in 
 the evening rays of that luminary. As his distance from us is 
 so different at different times, so is his apparent diameter, w hich 
 at a mean is 275 when in opposition, his apparent diameter 
 is 81". 
 
 Besides the dark spots, which serve to determine the diurnal 
 revolution of Mars, several early astronomers took notice that 
 a segment of his globe about the south pole exceeded the rest of 
 his disc so much in brightness, as to appear as if it were the seg- 
 ment of a larger globe. Maraldi informs us, that this bright 
 spot had been taken notice of for sixty years, and was more 
 permanent than the other spots on the planet. One part of it 
 is brighter than the rest, and the least bright part is subject to 
 great changes, and has sometimes disappeared. A similar 
 brightness about the north pole was also sometimes observed, 
 and these observations are now confirmed by Dr. Herschel, who 
 has viewed the planet with much better instruments, and much' 
 higher magnifying powders, than any other astronomer. The 
 analogy,” says the Doctor, between Mars and the earth, is 
 by far the greatest in the whole solar system. Their diurnal 
 motion is nearly the same ; the obliquity of their respective 
 ecliptics not very different. Of all the superior planets, the 
 distance of Mars from the sun is by far the nearest alike to that 
 of the earth; nor will the length of the Martial year appear 
 very different from what we enjoy, when compared to the sur- 
 prising duration of the years of Jupiter, Saturn, and the 
 Georgium Sidus. If then we find that the globe we inhabit 
 has its polar region frozen and covered with mountains of ice 
 and snow% that only partly melt when alternately exposed to 
 the sun, I may well be permitted to surmise, that the same 
 causes may probably have the same effect on the globe of Mars ; 
 that the bright polar spots are owing to the vivid reflection of 
 light from frozen regions, and that the reduction of those spots 
 is” to be ascribed to their being exposed to the sun. In the year 
 1781, the south polar spot was extremely large, w-hich we 
 might w'ell expect, as that pole had but lately been involved in 
 a whole twelvemonth’s darkness and absence from the S!in ; but 
 
 24. — VoL. I. 4 B 
 
554 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Vesta. — Juno. — Ceres. 
 
 in 17S3, I found it considerably smaller than before, and it de- 
 creased continually from the ^20th of May, till about the middle 
 of September, when it seemed to be at a stand. During this last 
 period, the south pole had already been above eight months en- 
 joying the benefit of summer, and still continued to receive the 
 sun-beams, though, towards the latter end, in such an oblique 
 direction as to be but little benefited by them. On the other 
 hand, in the year 1781, the north polar spot, w hich had been its 
 tw^elvemonth in the sunshine, and was but lately returning into 
 darkness, appeared small, though undoubtedly increasing in size. 
 Its not being visible in the year 1783, is no objection to these phe- 
 nomena, being owing to the position of the axis, by which it was 
 removed out of sight."’ 
 
 Vesta, 
 
 This small planet w as discovered by Dr. Olbers, of Bremen, 
 on the 29 th of March, 1807. Bs diameter is about 238 miles. 
 Its distance from the sun is almost 215,000,000 of miles, and its 
 annual revolution round the sun is performed in 1335 days. The 
 inclination of its orbit to the ecliptic is 7°. Examined by Dr. 
 Herschel, with an excellent fifteen-feet reflector, and a pow'er of 
 300, it exhibited no appearance of a disc, but appeared like a 
 fixed star of the sixth magnitude ; that is, it w’as nearly a point, 
 with an intense radiating light. When the sky was clear, it might 
 be discerned with the naked eye. 
 
 Juno, 
 
 This is another small planet, w'hich w^as discovered by Hard- 
 ing, of Lilienthal, on the 1st of September 1803. Its diameter 
 is 1425 miles, its distance from the sun 243,000,000 of miles; 
 and its annual period 1590 days. The inclination of its orbit to 
 the ecliptic is 14°. 
 
 Ceres, 
 
 This planet was discovered on the 1st of January, 1801, by 
 Piazzi, an Italian astronomer, whose name it not unfrequently re- 
 ceives. Its distance from the sun is nearly 263,000,000 of miles. 
 Its diameter is only about l63 miles. The time of its diurnal 
 motion is unknown ; but its annual sidereal revolution is calculated, 
 by Laplace, to be performed in I 68 I days, 17 hourSj 57 seconds. 
 The inclination of its orbit to the ecliptic is about 11° 48'. It 
 appeared to Herschel of a ruddy colour, but not very deep ; and 
 
ASTRONOMY. 
 
 555 
 
 Solar system reviewed. — Pallas. 
 
 he is of opinion that he has an atmosphere. Its disc seldom ap- 
 pears well defined, apparently because a very slight degree of. an 
 unfavourable state in our atmosphere, for viewing it, affects its 
 feeble light, 
 
 Fallas. 
 
 This planet was also discovered by Dr. Olbers, of Bremen, on 
 the 28th of March, 1802. Its distance from the sun is nearly 
 264,000,000 miles; its sidereal period is 1681 days, 17 hours, 57 
 seconds ; and its diameter is only 80 miles, so that it is the small- 
 est known planet of the Solar System. The inclination of its 
 orbit to the ecliptic amounts to about 38°. It is of a dusky 
 whitish colour, and appears very indistinct with a power of 500, 
 unless the atmosphere be remarkably clear. 
 
 Dr. Olbers received, from the French National Institute, La- 
 lande’s prize of 10,000 francs (£417) a sum appropriated to the 
 person whose labours most contributed to the promotion of astro- 
 nomical knowledge, in the year of its discovery. 
 
 The orbits of the Ceres and Pallas would have been most pro- 
 perly represented in the plate of the Solar System, by circles of 
 nearly equal size, but with their centres a little on opposite sides 
 of that of the sun. 
 
 Soon after the Ceres and Pallas had been discovered. Dr. 
 Herschel published some observations upon them ; and taking 
 into consideration their great deviation from the zodiac, or track 
 of the planets previously known, he observed, that if they were 
 admitted into the order of planets, we must give up the zodiac; 
 for by extending it to them, should a few more be discovered, still 
 farther deviating from the path of the earth, we might soon be 
 obliged to convert the whole firmament into a zodiac, that is, we 
 should have none at all. Hence he proposed the name of asteroids, 
 for planetary bodies of the description in question ; and made the 
 definition of that term so comprehensive, as to include all that 
 were likely to be discovered : and he distinctly stated his belief that 
 more planetary bodies deviating from the zodiac would be disco- 
 vered : a conjecture which was realized in a short lime by the dis- 
 covery of the Juno and Vesta. Dr. Thompson, in his History of 
 the Royal Society, of which society Dr. Herschel is an illustrious 
 member, intimates his disapprobation of the term, asteroids, ob- 
 serving that he can see no reason why the stars it is intended to 
 designate ought not to be called planets, imless it were to prevent 
 their discoverers from ranking so high as the discoverer of the 
 Georgium Sidus. This insinuation appears too gratuitous to be 
 considered either generous or just. 
 
556 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Jupiter and his four Satellites. 
 
 Jupiter. 1/ 
 
 Jupiter is the largest of all the planetary bodies, and next 
 to Venus, appears to us the brightest. His prodigious distance, 
 which is 490,000,000 of miles from the sun, is the reason of his 
 apparent size being less than that of Venus, though his real 
 magnitude is above 1500 times greater, his diameter being 
 89,170 miles. The length of his year is 4330 days, 14 hours, 
 39 minutes, 2 seconds. His diurnal rotation is performed in 
 the short space of 9 hours, 55 minutes, 37 seconds. This 
 rotation is so amazingly rapid, that it carries his equatorial 
 inhabitants 28,800 miles every hour, which is nearly 4000 miles 
 more than the inhabitants of our earth’s equator are carried by 
 the same motion in 24 hours, and almost equals the hourly 
 advance in his orbit. The remarkable spot, by the motion of 
 ■which Jupiter’s rotation on his axis was determined, appeared 
 in 1694, and was lost till the year 1708, when it re-appeared, 
 on the same part of his face, and has been occasionally seen ever 
 since. 
 
 From the greater rapidity of his diurnal rotation, his figure 
 is that of a much flatter oblate spheroid than the earth, his equa- 
 torial being to his polar diameter as 13 to 12, a proportion which 
 makes the former measure 6000 miles more than the latter. His 
 axis is very nearly perpendicular to the plane of his orbit, so that 
 his inhabitants have no perceptible change of seasons. If his axis 
 had been inclined any considerable number of degrees, just so 
 many degrees round each pole would alternately be involved in 
 darkness for almost six of our years together ; and as each degree 
 of a great circle of Jupiter contains at the least 800 of our miles, 
 the tracts of land thus deprived of the sun, would be immense. 
 The sun appears to him only a fifth of the size he does to us, and 
 his light and heat are only a twenty-fifth of ours. But his night 
 is on no part of his surface five hours long; at his polar regions, 
 the sun never sets, he enjoys a perpetual spring, and he has the 
 splendid attendance of four moons, one or more of which are 
 always visible to enliven his short night. It is easy, from all 
 these circumstances, to conceive, that this spacious orb may be 
 the residence of a race of beings very little dissimilar to our- 
 selves. 
 
 When Jupiter is viewed through a good telescope, we per- 
 ceive a number of zones or belts, of a darker colour than the 
 rest of his disc. They are generally parallel to his equator, 
 
 which is very nearly parallel to the ecliptic, but in other 
 respects are subject to great variations. Sometimes only one 
 
ASTRONOMY. 
 
 557 
 
 Solar system reviewed. — Jupiter and his four Satellites. 
 
 can be perceived, at other times eiglit. They are sometimes not 
 parallel to one another, and their breadth is very variable, one 
 belt having been observed to grow narrow, while another in its 
 neighbourhood has increased in breadth, as if the one had 
 flowed into the other; and in this case Dr. Long observes, that 
 a part of an oblique belt lay between them, as if to form a com- 
 munication for this purpose. The time of their continuance is 
 very uncertain ; sometimes remaining unchanged for three months ; 
 at others, new belts have been formed in an hour or two. The 
 continuity of the belts is sometimes interrupted, so as to give them 
 a broken appearance. The spots and belts seen on the 7th of 
 April, 1792 , are represented by flg. 5, pi. 111. The belts and 
 spots are considered to be the body of the planet, and the light parts 
 the clouds, transported by the winds with ditferent velocities, and 
 in different directions. 
 
 The four satellites of Jupiter are distinguished by their situa- 
 tion, that being called the first which is nearest to its primary. 
 Their periods and distances are as follows : 
 
 Day. h. m. s. Miles. 
 
 1 St satellite revolves in 1 18 27 33 at the distance of 259,170 
 2nd „ „ 3 13 13 42 „ 412,352 
 
 3rd „ „ 73 42 33 „ 657,735 
 
 4th „ „ ^ 16 16 32 8 „ 1,156,640 
 
 These satellites move round Jupiter from west to east, and 
 their orbits are supposed to be ellipses, though the eccentri- 
 city of all of them is too small to be measured, except that of 
 the fourth ; and even this amounts to more than 0,007 of its 
 mean distance from the primary. The motions ol the first 
 
 three satellites, Laplace has observed, are related to each other by 
 a most singular analogy. The mean sidereal or synodical motion 
 of the first, added to twice that of the third, is constantly equal 
 to three times the mean motion of the second. And the mean 
 sidereal or synodical longitude of the first, minus three times that 
 of the second, plus twice that of the third, is always equal to two 
 right angles. 
 
 When the satellites fall into the shadow of the primary, we 
 lose sight of them for a time, and they are said to be eclipsed. 
 The three nearest are eclipsed every revolution ; but the orbit of 
 the fourth is so much inclined, that it passes by its opposition to 
 Jupiter, without falling into his shadow, two yrars in six. 
 From the singular analogy, above eluded to,, it tollows that 
 (for a great number of years at least) the first three satellites 
 cannot be eclipsed at the same, time i for in the simultaneous 
 
558 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Jupiter’s Satellites. 
 
 eclipses of the second and third, the first will always be in con- 
 junction with Jupiter, and vice versa. J3y these eclipses, has 
 been discovered the velocity of light, (see Optics,) and they serve 
 a still more important purpose, — that of determining the longitude 
 of places on the earth, with great facility and certainty. The lon- 
 gitude of a place is its distance east or west from another, reckon- 
 ed in degrees of the equator, and it can always be ascertained with 
 certainty, provided we know the time of the day at the place from 
 which we reckon, as well as that of the place where we make the 
 observation. Since by the motion of the earth round its axis, 
 every point on its surface describes the circumference of a circle, 
 or 360°, in twenty-fours, it must consequently describe 13° in one 
 hour, because 13 is a 24th part of 360. Hence the difference of 
 longitude may be converted into time, by allowing one hour for 
 every 13°, and proportionately for minutes and seconds; or con- 
 versely, with equal ease, difference of time may be converted 
 into longitude. Voyagers, therefore, if provided with a faultless 
 timekeeper, set at their departure exactly to the time of the 
 place from which they sailed, may at any time ascertain their lon- 
 gitude, or distance east or west from that place, with great fa- 
 cility. Timekeepers are, however, subject to so many causes of 
 imperfection, that they cannot be depended on, for any long 
 period; but as the eclipses of Jupiter’s satellites are more nume- 
 rous than the days of the year, and Jupiter can be seen for 
 eleven months in the year, and as there is generally one, and 
 often two or three in one night, the observation of them may 
 be made to answer the same purpose as a timekeeper. For 
 example, the exact times at which the eclipses of Jupiter’s 
 satellites will occur, at the meridian of Greenwich, are given 
 for several years to come, in the nautical ephemeris ; let a person 
 then, at any distance east or w'est from Greenwich, notice the 
 eclipse of a satellite, and compare it with the time given for the 
 same eclipse in his tables ; he then perceives at once the difference 
 of time, and consequently his longitude. To observe these eclipses 
 w ith precision, does not require a telescope of great power ; but 
 the principal disadvantage of the plan at sea arises from the mo- 
 tion of the ship. 
 
 Saturn. Tp 
 
 Saturn is next to Jupiter in order from the sun, and the 
 next to that planet in magnitude. His distance from the sun 
 is 900 millions of mdes, and therefore we may readily account 
 for the paleness and feebleness of the light he reflects to us, and 
 that he can hardly be distinguished from a fixed star by the 
 
ASTRONOMY. 
 
 5o9 
 
 Solar system reviewed. — Saturn. 
 
 naked eye. His diameter is 79,042 miles, and he employs 
 10,746 days, 19 hours, 16 minutes, \o seconds, in revolving 
 once round the sun, so that the length of his year is almost 
 thirty of ours. His diurnal rotation is believed to be performed 
 in 10 hours, l6 minutes, 2 seconds; but the spots, the motion 
 of Nvhich determine this point, are not well delined. The 
 inclination of his orbit to the ecliptic is about degrees ; but 
 the inclination of his axis to his orbit is supposed to be 60°. 
 
 Saturn, when viewed through a good telescope, makes a 
 more remarkable appearance than any of the other planets. 
 He has belts, somewhat like those of Jupiter, though fainter; 
 but his distinguishing characteristic is a broad, double, lumi- 
 nous ring, entirely surrounding him. The appearance of the 
 planet through a good telescope, is shown by fig. 6, plate HI. 
 The ring is detached from his body in such a manner, that 
 the distance between the innermost part of the ring and the 
 body of the planet is equal to its breadth. Both the outward 
 and inward rim of the ring is projected into an ellipsis, more 
 or less oblong, according to the different degrees of obliquity 
 with which it is viewed. Sometimes our eye is in the plane of 
 the ring, and then it becomes invisible even when sought for 
 with very good telescopes, probably because it is in that posi- 
 tion too thin to subtend a sufficient angle, at such a distance. 
 But sometimes it may be detected w ith a telescope of peculiar 
 excellence, and a favourable night, and it then appears like 
 a very slender ray of light across the disc of the planet. As 
 the plane of this ring is always parallel to itself, that is, its 
 situation in one part of the orbit is always parallel to that in 
 any other part, it disappears about every 15 years, or twice in 
 every revolution of the planet, which sometimes appears quite 
 round for nine months together. At other times, the distance 
 between the body of the planet and the ring is very perceptible, 
 so that stars may be seen through it. 
 
 When Saturn appears round, if our eye be elevated above 
 the plane of the ring, a shadowy belt will be visible, caused by 
 the shadow' of the ring, as w'ell as by the interposition of part 
 of it between the eye and the planet. The shadow of the ring 
 is broadest when the sun is most elevated, but its obscure parts 
 appear broadest when our eye is most elevated above the plane 
 of it. When it is seen double, the ring next the body of the 
 planet appears brightest. When the ring appears of an ellipti- 
 cal form, the parts about the ends of the largest axis are called 
 the ama. These, a little before and after the disappearing of 
 the ring, are of an unequal magnitude ; the largest ansa is 
 
560 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Saturn’s Ring. 
 
 longer visible before the planet’s round phase, and appears 
 again sooner than the other. Herschel demonstrates, that the 
 ring revolves in its own plane, in lOh. 32' 15.4". This philo- 
 sopher has paid great attention to Saturn’s ring. According to 
 him, there is one single, dark, considerably broad line, belt, 
 or zone, which he has constantly found on the north side of the 
 ring. As this dark belt is subject to no change whatever, it is 
 probably owing to some permanent construction of the surface 
 of the ring : this construction cannot be owing to the shadow of 
 a chain of mountains, since it is visible all round on the ring ; 
 for there could be no shade at the ends of the ring ; and a 
 similar argument will apply against the opinion of very extended 
 caverns. It is pretty evident that this dark zone is contained 
 between two concentric circles, for all the phenomena corre- 
 spond with the projection of such a zone. The nature of the 
 ring the Doctor thinks no less solid than that of the planet 
 itself, and it is observed, as above mentioned, to cast a strong 
 shadow on the planet. The light of the ring is also generally 
 brighter than that of the planet, for the ring appears sufficiently 
 bright, with a telescope that affords scarcely light enough to see 
 Saturn. He concludes that the edge of the ring is not flat, but 
 spherical, or spheroidal. The dimensions of the ring, or of 
 the two rings with the space between them, he estimates as 
 follows : 
 
 Miles. 
 
 Inner diameter of the small ring 146,345 
 
 Outside diameter of ditto 184,393 
 
 Inner diameter of the larger ring 190,248 
 
 Outside diameter of ditto 204,883 
 
 Breadth of the inner ring 20,000 
 
 Breadth of the outer ring 7,200 
 
 Breadth of the vacant space or dark zone .... 2,839 
 
 If this ring be opaque, as the sun shines fifteen years on its 
 north side, and the same time on its south side, it will have 
 equal day and night, each fifteen years long. 
 
 The swiftness of Saturn’s motion on his axis produces an 
 oblate figure ; and his equatorial diameter is calculated to be to 
 his polar diameter, as 11 to 10. 
 
 The sun scarcely affords to Saturn an eightieth part of the 
 direct light that we receive from him ; but this planet, besides 
 his magnificent ring, is accompanied by not less than seven 
 satellites, which revolve round him beyond his ring. The 
 
 15 
 
ri.w. 
 
 Toy.J. 
 
 Tig. 6. 
 
 J.ifftaru 
 
 rnl>1jjh.1 hij XntinU. fisher tc ۥ T. iverpocUTaij ti. 1814 . 
 
ASTRONOMY. 
 
 56 1 
 
 Solar system reviewed. — Saturn’s satellites.' 
 
 periods of these satellites, and their distances from their primary, 
 are as follows : 
 
 
 
 D. 
 
 h. 
 
 m. 
 
 s. 
 
 1st satellite 
 
 revolves 
 
 in 1 
 
 21 
 
 18 
 
 27 
 
 2nd „ 
 
 o 
 
 2 
 
 17 
 
 41 
 
 22 
 
 od ,, 
 
 
 4 
 
 12 
 
 25 
 
 12 
 
 4th „ 
 
 
 15 
 
 22 
 
 41 
 
 13 
 
 5 th „ 
 
 
 79 
 
 7 
 
 48 
 
 0 
 
 6th „ 
 
 jf 
 
 1 
 
 8 
 
 53 
 
 9 
 
 7 th „ 
 
 
 0 
 
 22 
 
 40 
 
 46 
 
 Miles. 
 
 at the distance of 170,000 
 „ 217,000 
 
 „ 303,000 
 
 „ 704,000 
 
 „ 205,000 
 
 „ 135,000 
 
 „ 107,000 
 
 The satellites of Jupiter, we have observed, are enumerated 
 and distinguished in a regular manner, beginning at the one 
 nearest the primary ; so that the second satellite is the second in 
 size and distance, and so of the rest. This, it appears above, 
 is not exactly the case with the satellites of Saturn, and the 
 reason is, that the first five satellites were discovered in 1685 by 
 Cassini and Huygens; no more were supposed to exist, and 
 they were called the 1st, 2nd, 3d, &c. reckoning from the 
 planet; but, about a century afterwards. Dr. Herschel discover- 
 ed two others, which were smaller and nearer to the planet ; 
 these ought therefore to have been called the first and second, 
 and the numerical order of the others changed, so that the fifth 
 would have become the seventh. But to avoid the confusion 
 that might arise, in reading astronomical works, written previ- 
 ous to the Doctor’s discovery, the anomaJy of their enumeration 
 was allowed, and they are constantly reckoned the sixth and 
 seventh. 
 
 The inclinations of the orbits of the first, second, third, 
 and fourth satellites, to the ecliptic, are from to 31°. That 
 
 of the fifth, is from 17® to 18°. This satellite is further remark- 
 able for being the only one of the solar system, except our 
 own, that has been observed to have any spots. From these 
 spots, its rotation on its axis has been determined ; and it is 
 singular, that, like the moon, it revolves round its axis in the 
 same time that it revolves round its primary. 
 
 The satellites of Saturn are frequently eclipsed, and these 
 eclipses would be of the same use as those of Jupiter, for dis'r 
 covering the longitude of places on the earth, but their vast 
 distance renders them so much less susceptible of accurate 
 observation. 
 
 4 C 
 
 Vdl. I. — 24. 
 
562 
 
 ASTRONOMV. 
 
 Solar system reviewed. — Herschel. 
 
 The Herschel. 
 
 At the distance of 1800 millions of miles from the sun, the 
 planet Herschel, the last of the system, advances in his course. 
 This planet was discovered by Dr. Herschel, on the 13th of 
 March, 1781, and though commonly distinguished by his name, it 
 is also known by other appellations ; the discoverer himself called 
 it the Georgium Sidus, or Georgian Planet, in honour of his 
 sovereign ; Lalande, and the rest of the French philosophers after 
 him, called it the Herschel; while the Germans, preferring the 
 ancient mode of following the heathen mythology, called it 
 Uranus, who, in fabulous history, was the father of Saturn. The 
 Herschel seems the name most likely to content all parties ; the 
 public voice, where the evidence is clear, is always in favour of 
 those who have the strongest claim to distinction : the English 
 will not relinquish both the names w hich may indicate the country 
 where the discovery was made ; and foreign philosophers, while 
 they willingly give the honour to one of their own republic, are 
 not disposed to give it to any particular king. Galileo, who dis- 
 covered the four satellites of Jupiter, called them Mediceon stars, 
 in honour of the family of the Medici, his patrons ; but the title 
 is now almost forgotten, and never used. 
 
 The Herschel is 35,112 miles in diameter, and performs its 
 annual revolution in 30,637 days, 4 hours, that is, very nearly 
 eighty-four of our years. The period of its diurnal rotation has 
 not been determined. On a clear evening, it may be seen by the 
 naked eye, in the absence of the moon ; when view'ed through 
 a good telescope, it is of a bluish white colour, and its disc is 
 well defined. The light and heat which it receives directly from 
 the sun is about the 362nd part of what we enjoy. 
 
 When the discoverer of the Herschel first observed it, he 
 supposed it to be a comet ; but it was soon proved to be a 
 planet, the suspicion of which w'as naturally suggested by its 
 nearness to the ecliptic. It had always been considered a fixed 
 star, on account of the slow ness of its motion ; and it was no 
 sooner known to be a planet, than it was perceived to be the 
 star marked 34 in Flamsteed’s catalogue, and 9^4 in Meyer’s. 
 By these means, astronomers were in possession of a whole cen- 
 tury of observations respecting it. 
 
 Dr. Herschel had the singular merit of not only discovering this 
 star to be a planet, but of discovering all the satellites it is supposed 
 to have, and w'hich are six in number. These satellites cannot be 
 seen without a powerful telescope. Their periods of revolution 
 round their primary, and their distances from it, are as follows : 
 
ASTRONOMY. 
 
 56t} 
 
 Solar system reviewed. — Satellites of Herschcl. — Synopsis. 
 
 
 
 
 D. 
 
 h. 
 
 ni. 
 
 
 Miles. 
 
 1st satellite 
 
 revolves 
 
 in 5 
 
 21 
 
 
 at the distance 
 
 of 230,334 
 
 2nd 
 
 
 yy 
 
 8 
 
 IS 
 
 0 
 
 yy 
 
 298,838 
 
 3d 
 
 yy 
 
 yy 
 
 10 
 
 23 
 
 4 
 
 yy 
 
 348,398 
 
 4th 
 
 yy 
 
 yy 
 
 13 
 
 12 
 
 0 
 
 yy 
 
 399,434 
 
 5th 
 
 yy 
 
 yy 
 
 38 
 
 1 
 
 49 
 
 yy 
 
 798,920 
 
 6th 
 
 yy 
 
 yy 
 
 107 
 
 16 
 
 40 
 
 yy 
 
 1,597,736 
 
 These satellites move in a plane nearly perpendicular to the 
 plane of the planet’s orbit, and contrary to the order of tlie 
 signs. 
 
 The following Synopsis of the planetary system, will enable 
 the reader to compare the corresponding circumstances of the 
 planets with each other, and to refer to them with facility : 
 
 Distances of the Flanetsf rom the Sun in English miles. 
 
 Mercury 37,000,000 
 
 Venus 08,000,000 
 
 Earth 05,000,000 
 
 Mars 144,000,000 
 
 Vesta 215,000,000 
 
 Juno 243,000,000 
 
 (jeres 263,000,000 
 
 Pallas 264,000,000 
 
 Jupiter 490,000,000 
 
 Saturn 900,000,000 
 
 Herschel 1,800,000,000 
 
 Diameter of the Sun and Planets in English miles. 
 
 Sun • • • 
 Mercury 
 Venus • 
 Earth . 
 Moon 
 Mars 
 Vesta 
 Juno 
 Ceres • 
 Pallas • 
 Jupiter • 
 Saturn • 
 t{ers(?hel 
 
 883,246 
 
 3,224 
 
 7,687 
 
 7,911 
 
 2,180 
 
 4,189 
 
 238 
 
 1,425 
 
 163 
 
 80 
 
 89,170 
 
 79,042 
 
 35,112 
 
564 
 
 ASTRONOMY, 
 
 Solar system reviewed. — Synopsis. 
 
 Density of the Sun and Planets, that of Water being one. 
 
 Sun 
 
 jNIercury 
 
 Venus 
 
 Earth 
 
 Mars • • 
 
 Jupiter 
 
 Saturn 
 
 Kerscliel 
 
 14 - 
 
 Quantity of Matter in the Planetary bodies, the Earth being one. 
 
 Sun 
 
 Mercury . . . 
 
 Venus 
 
 Earth 
 
 Moon 
 
 Jupiter , . . , 
 
 Saturn 
 
 Herschel , » 
 
 329,630 
 * . . 0.133 
 
 . . . . 0.135 
 
 1.000 
 
 0.025 
 
 ,...330.600 
 , . . .103.950 
 16.840 
 
 dumber of feet per second through zichich a heavy body zcould 
 fall at the surface of each of these Planets. 
 
 Sun * 450 
 
 Mercury * 12 
 
 Venus . . 18 
 
 Earth 16 -f- 
 
 Moon 3 
 
 Jupiter 42 
 
 Saturn 15 
 
 Herschel * 4.2 
 
 Potation of the Planets round their axes. 
 
 
 D. 
 
 h. 
 
 m. 
 
 s. 
 
 Sun 
 
 25 
 
 .10' 
 
 0 
 
 0 
 
 Venus * 
 
 0 
 
 23 
 
 21 
 
 0 
 
 Earth 
 
 0 
 
 24 
 
 0 
 
 0 
 
 IMars 
 
 0 
 
 24 
 
 39 
 
 22 
 
 Jupiter 
 
 0 
 
 9 
 
 55 
 
 37 
 
 Saturn . . . ........ .... . . 
 
 0 
 
 10 
 
 16 
 
 2 
 
 Saturn’s King 
 
 0 
 
 10 
 
 32 
 
 15 
 
 Moon, a lunation, at a mean , . 
 
 29 
 
 12 
 
 44 
 
 3 
 
Astronomy. 
 
 565 
 
 Solar system reviewed. — Comets. 
 
 Tropical Revolutions of the Planets round the Sun. 
 
 D. h. m. s. 
 
 Mercury 87 23 14 33 
 
 Venus 224 16* 41 27 
 
 Earth . . . . 365 5 48 4Ji 
 
 Mars 686 22 18 27 
 
 Jupiter 4,330 14 30 2 
 
 Saturn 10,74(; 10 16 15 
 
 Herschel 30,637 4 0 0 
 
 Moon {round the earth), , 27 7 43 5 
 
 Of Comets. 
 
 We have yet to mention a numerous class of bodies, mIiIcIi 
 frequently appear within the limits of the planetary orbits, 
 and some or all of which certainly belong to the solar system. 
 We allude to comets, respecting the nature, number, and use 
 of which the opinions of philosophers have been exceedingly 
 divided, and very little has yet been determined in a satisfac- 
 tory manner, unless it be the manifest truth, that they are not 
 to be considered, as formerly, the portentous heralds of calami- 
 ties to mankind, lliey are, doubtless, noble parts of the creation, 
 traversing their appointed course, without any connexion with 
 the earth, except as parts of the same whole ; and the appear- 
 ances they present to us arise only from peculiarities of their 
 constitution, indispensable probably to the comforts of their inha- 
 bitants. 
 
 The curves in which comets move are generally considered 
 to be very eccentric ellipses, so that comets can only be seen dur- 
 ino- their vicinity to the sun, and are invisible to us during the 
 greater part of their course. Their periodical times are so very 
 long, and so difficult to be ascertained, that very few of them have 
 been observed twice, and when their appearance agrees with the 
 time foretold, it is almost impossible to identify them. Their ap- 
 pearances are very different. Some of them resemble a faint va- 
 pour ; others have a perceptible nucleus or solid part in the mid- 
 dle, When they approach near the sun, they pul forth the ap- 
 pearance of a beard or tail of luminous mutter, which is differently 
 denominated according to their position. When the pimet is 
 eastward of the sun, and moves from the sun, it is said to be 
 bearded, h^C2iWSG Xhe luminous elongation goes before it. When 
 the comet is westward of the sun, and sets after him, it is then 
 said to have a tail, because the luminous elongation follows it ; 
 
S66 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Comets. 
 
 and when the earth happens to be directly between the sun and 
 the comet, then the train of light is so much hidden by the body 
 of the comet, that only a little of it can be seen on each side of 
 the comet, which is therefore said to have a co?na or hairy appear- 
 ance. 
 
 The more conspicuous comets have been long known to the 
 vulgar under the appellation of blazing stars ; but many of them 
 are only to be seen by the help of telescopes, and are discovered 
 by accident, because they appear and disappear in a few nights, 
 and traverse the heavens in every direction. The actual motion 
 of some of them is from east to west, while others, like the planets, 
 move from west to east. Some of them move in the plane of the 
 ecliptic, or within the zodiac ; whilst others go in different direc- 
 tions, even perpendicular to the ecliptic. 
 
 Kepler and some other philosophers were of opinion that 
 comets were in reality nothing more than a congeries of vapours, 
 or exhalations from the sun and planets; but Sir Isaac Newton 
 proved the absurdity of this hypothesis, by showing that no vapours 
 or exhalations could subsist in the regions where they were found 
 to be the brightest ; that is, very near the sun. The heat of the 
 sun is as the density of his rays, or reciprocally as the squares of 
 the distances of places from the sun. Therefore, as the distance 
 of the comet of 1680, in its perihelion, December the 8th, was 
 observed to be to the distance of the earth from the sun nearly as 
 6 to 1000 ; the sun’s heat at the comet, was to his heat with us 
 at midsummer as 1,000,000 to 36; or 28,000 to 1. Now the 
 beat of boiling water is little more than three times the heat of our 
 dry earth, \^hen exposed to the midsummer’s sun; assuming then 
 the heat of red-hot iron to be about three or four times as great as 
 boiling water, Newton concludes, that the heat of the dried earth, 
 or body of the comet in its perihelion, must be nearly 2000 times 
 as great as that of red-hot iron. He computes, that a globe of 
 red-hot iron, of the dimensions of our earth, would scarcely be 
 cool in 50,000 years. If then the comet be supposed to cool 100 
 times as fast as red-hot iron, yet, since its heat was 2000 times 
 greater, supposing its magnitude to be equal to that of the earth, 
 it would not be cool in a million of years. Upon the whole, there- 
 fore, he concluded that the comets were compact, solid, fixed, and 
 durable bodies, in one word, a kind of planets, which move in very 
 oblique orbits, every way with the greatest freedom ; persevering 
 in their motions, even against the course and direction of the 
 planets ; that their tail is a very thin vapour, emitted by the head 
 or nucleus of the comet, ignited or heated by the sun ; and he 
 eutei laiiied and announced the grand idea that they might re-appear 
 at every revolution. 
 
ASTRONOMY. 
 
 567 
 
 Solar system reviewed. — Comets. 
 
 Frequently a nucleus cannot be discerned ; of sixteen comets 
 observed by Dr. Herschel, fourteen were of this description ; and 
 the other two had only a very ill-delined central liglit, which per- 
 haps might be called a nucleus, but not a disc. It is this want of 
 a well-detined disc or nucleus, that renders many of the observa- 
 tions on comets uncertain. 
 
 From observations of the comet of 16S0, Sir Isaac Newtou 
 found, that the vapour in the extremity of the tail, Januaiy 
 2oth, began to ascend from the head before December lltli, and 
 had therefore spent more than forty-live days in its ascent ; but 
 that all the tail which appeared December 10th, ascended in 
 the space of those two days, then just past since its perihelion. 
 The vapour, therefore, at the beginning, when the comet was 
 near the sun, ascended wdth a prodigious swiftness ; and after- 
 wards continued to ascend with a motion retarded by the 
 gravity of its particles, and by that ascent increased the length 
 of the tail; but the tail, notwithstanding its length, consisted 
 almost w’hoJly of vapours, which had ascended from the time 
 of its perihelion ; and the vapour which ascended first, and 
 composed the extreme of the tail, did not vanish till it was too 
 far from the sun to be illuminated by him, and from us to be 
 visible. Hence, also, the tails of comets that are shorter, do 
 not ascend with a quick and continual motion from the head, 
 and then presently disappear ; but are permanent columns of 
 vapours and exhalations, gathered from the head, by a very gentle 
 motion, and a great space of time ; which yet, by participating 
 of that motion of their heads they had at the beginning, conti- 
 nue easily to move along with their heads through the celestial 
 regions. 
 
 The ascent of the vapours from a comet, will be promoted 
 by their circular motion round the sun, and will endeavour to 
 recede from the sun, as smoke recedes from a lire ; and there- 
 fore, the more the comets advance into the dense atmosphere of 
 the sun, their tails will be the greater. The vapours being 
 thus dilated, raritied, and diffused throughout all the celestial 
 regions, Newdon thought they might probably, by little and 
 little, through their own gravity, be attracted down to the 
 planets, and become intermingled with their atmospheres, to 
 which they w'ould furnish fresh accessions of pure particles, 
 and supply the loss of air 'and moisture sustained by tlie 
 processes of life and vegetation. Another use he supposed 
 comets may be designed to serve, is that of supplying the suii 
 with fresh fuel. This may happen from their suftering a dimi- 
 nution of their projectile force, when in perihelion, fiom tlie 
 resistance of the solar atmosphere ; so that by degrees, their 
 
ASTRONOMY. 
 
 5t)8 
 
 Solar system reviewed. — Comets. 
 
 gravitation towards the sun may be so far increased as to preci- 
 pitate their fall into his body. Thus, also, fixed stars which 
 have been gradually wasted, may be supplied with fresh fuel, 
 acquire new splendour, and be taken for new stars : of tliis 
 kind may be supposed those fixed stars which appear on a 
 sudden, or shine at first with a surprising brightness that 
 gradually decays, till they disappear. If this theory were true, 
 the comets might be regarded as the conservators of the solar 
 system, while it was sliown, at the same time, how little cause 
 there was for that superstitious dread of their appearance, 
 which had prevailed so generally among the ignorant of all 
 countries. The ideas winch Sir Isaac Newton entertained of 
 comets, were certainly far removed from the current opinions 
 even of philosophers in his day, and we cannot say that the 
 ' kivestigations of the century since they were published, have 
 carried us much beyond them, though substantial objections 
 may be offered to some parts of his hypothesis. The idea of the 
 periodical revolutions of the comets is extremely likely to be 
 confirmed, but the supposition that the planets are supplied 
 with pure air and moisture diffused by them, the late disco- 
 veries in chemistry render exceedingly doubtful, by showing 
 
 abundant other means of nature’s repletion in these respects. 
 jNlodern chemistry indisputably proves to us, that water is com- 
 posed of two invisible gases ; and that by the operations con- 
 stantly going on in nature, it is produced, resolved into its 
 
 elements, and reproduced : on the one hand, water produces 
 air ; on the other, air produces water : under certain circum- 
 stances, water unites in a solid form with various substances ; 
 under other circumstances, it is liberated from its solid form, 
 and assumes that of vapour or air. These operations, there is 
 
 reason to believe, balance each other ; the atmosphere and the 
 
 water, though constantly changing, remain constantly the same 
 in quantity, oi' as nearly so as the existence of mankind requires ; 
 and we find it impossible to point out the source of a decay or 
 the need of a foreign supply. With respect to the supply of 
 combustible matter which comets may afford to the sun, and the 
 cause which Sir Isaac Newton, as above stated, has assigned for 
 their falling into it, we may consider, that if on the one hand 
 the density of the solar atmosphere has a tendency to lessen the 
 projectile force of a comet passing through it, that tendency, it 
 is reasonable to suppose, will be simultaneously checked and over- 
 come, by the greater momentum with which the rays fall upon 
 the comet. It is further to be considered, that if the comets 
 are to be regarded merely as fuel for the sun, it were inconsist- 
 ent to suppose them at the same time to be habitable worlds. 
 
•ASTRONOMY. 
 
 5()9 
 
 Solar system reviewed. — Comets. 
 
 Is it not then at least reasonable to adopt the opinion, that ihe^ 
 comets are co-existent with the planets ; and that the sun's waste 
 of matter is supplied by other means than their destruction 
 
 Rowning, who is not satisfied with Sir Isaac Newton’s 
 theory of comets, accounts for their tails in the following 
 manner : It is well known, that when the light of the sun 
 passes through the atmosphere of any body, as the earth, that 
 which passes on one side, is by the refraction thereof made to 
 converge toward that which passes on the opposite one ; and 
 the convergency is not wholly effected either at the entrance of 
 the light into the atmosphere, or at its going out ; but beginning 
 at its entrance, it increases in eveiy point of its jnogress. Jt 
 •is also agreed, that the atmospljeres of comets arc very large 
 and dense. He therefore supposes that by such time as the 
 light of the sun has passed through a considerable pai t of the 
 atmosphere of a comet, tl'.e rays are so far refracted tovvaid each 
 other, that they then begin sensibly to illuminate it, or rather the 
 vapours floating in it ; and so render lliat part they have yet to 
 pass through visible to us : and that this portion of the atmo- 
 sphere of a comet, thus illuminated, appears to us in the form 
 of a beam of the sun’s light, and passes under the denomination 
 of a comet’s tail. Some comets have had tails extending 80 
 millions of miles. 
 
 Tiiis opinion, that the cornet’s tail is merely an optical 
 appearance, produced by the light streaming ihrongh a dense 
 and very extensive atmosphere, is at the least exceedingly spe- 
 cious. "if the tail be nothing but light, we can readily account 
 for its not distorting the rays from the smallest star .seen ihrougli 
 it; and it is not difficult to admit that such an atmosphere mu.st 
 render the solid opaque body of the comet very indistinct, if not 
 altoiyether invisible. This theory of comets it has been pro- 
 posed to elucidate by experiment : Take a glass globe, suspend a 
 small opaque globe" in the middle of it, and then expose it to a 
 strono- stream of light, as from a lire or a number of candles, 
 while^it is so placed in an aperture of a wall or door, that the 
 lierht which passes through it may be seen in a darkened room : 
 but the imitation of the cometary phenomena will not be good, 
 from the envelope of the glass, nor equal to what it would be 
 if the water were in a state of vapour. 
 
 Kepler’s law^ of the analogy between the periodical times of 
 the planets, and their distances" from the sun, is applicable akso 
 to the comets. Hence the mean distance of a comet fiom the 
 sun may be found by comparing its period with the time of the 
 earth’s ‘revolution round the sun : thus the period of the comet 
 that appeared in 1531, 1607, 1682, and 1759, being about 76 
 24. — VoL. 1. 4 D 
 
57(3 
 
 ASTRONOMY. 
 
 Solar system reviewed. — Comets. 
 
 years, its mean distance is found by ibis proportion : as 1 the 
 square of one year, the earth’s periodical time, is to 5776, the 
 sipiare of 76, the comet’s periodical time, so is 1,000,000 the 
 cube of 100, the earth’s mean distance from the sun, to 
 5,776,000,000 the cube of the comet’s mean distance; the 
 cube root of which is 1794, the mean distance itself, in such 
 parts as the mean distance of the earth contains 100. If the 
 perihelion distance of this comet, 58, be taken from 3588, double 
 the mean distance, we shall have the aphelion distance 3530 of 
 such parts as the distance of the earth contains 100 ; and this 
 is a little more than 35 times the distance of the earth from the 
 sun. By a like method, the aphelion distance of the comet of 
 1680, comes out 138 times the mean distance of the earth from 
 the sun, supposing its period to be 575 years ; so that this comet, 
 in its aphelion, goes to more than fourteen times the distance of 
 Saturn from the sun. 
 
 Dr. Halley compiled a table of the elements of comets, 
 according to the best information he could obtain ; the end for 
 which he compiled this table, he observes, was this, that when- 
 ever a new' comet shall appear we may be able to know, by com- 
 paring together the elements, wdiether it be any of those which 
 have appeared before, and consequently to determine its period, 
 and the axis of its orbit, and to foretell its return. He asserted 
 his belief that the comet which Appian observed in the year 
 1531, was the same with that which Kepler and Longomon- 
 tanus more accurately described in the year l607 ; and which 
 he himself observed in the year 1682. All the elements agreed, 
 and nothing appeared to contradict his conclusion but a trilling 
 inequality of the periodic revolution. This, he remarked, 
 might arise from physical causes ; for the motion of Saturn is 
 so disturbed by the rest of the planets, especially Jupiter, that 
 the periodic time of the planet is uncertain for some whole days 
 together. How' much more therefore will a comet be subject to 
 such like errors, which rises almost four times higher than 
 Saturn, and whose velocity, though increased but very little, 
 would be sufficient to change its orbit from an ellipsis to a para- 
 bola. On looking over the histories of comets, he found that 
 one had been seen in the year 1456, and also in the year 1305, 
 which latter year he considered to be at the distance of a 
 double period from the former. From all these considerations, 
 he ventured to foretell that the return of the comet would take 
 place about the year 1758. The effect of Jupiter he computed 
 would increase its periodic time above a year, in consequence 
 of which he predicted its return at the end of the year 1758 or 
 the beginning of 1759. But Clairaut computed the effects both 
 
ASTRONOMY. 
 
 571 
 
 Comets. — Fixetl stars. 
 
 of Satiuii and Jupiter, and found that the former would retard 
 its return in the last period 100 days, and the latter oil days; 
 and he determined the time when the comet would come to its 
 perihelion to be April lo, 17o9, observing that he might err 
 a month, from neglecting small quantities in the computation. It 
 passed the perihelion on March 13, within thirty-three days of the 
 time computed. This is the first comet the period of which was 
 foretold, and the prediction was verified. It did not, however, 
 make any considerable appearance, by reason of the unfavourable 
 situation of the earth, all the time its tail might otherwise have 
 been conspicuous, the comet being then too near the sun to be 
 seen by us. Let us remark,” says Laplace, for the honour of 
 the human understanding, that this comet, which in this century 
 only excited the curiosity of astronomers and mathematicians, had 
 been, regarded in a very different manner, four revolutions before, 
 when it appeared in 1456. Its long tail spread consternation over 
 all Europe, already terrified by the rapid success of the l\irkisli 
 arms, which had just destroyed the great empire. Pope Callixtus, 
 on this occasion, ordered a prayer, in which both the comet and 
 the Turks were included in one anathema !” 
 
 Five hundred comets are supposed to have appeared since 
 the commencement of the Christian era. Of tliese probably 
 a considerable number are the different appearances of the same 
 comet ; but our knowledge of tliese bodies is so much in its 
 infancy, the period of accurate observations has so lately com- 
 menced, that we have as yet but slender means of determining 
 any question respecting them. 
 
 Of the Fixed Stars. 
 
 When we have separated the planets and comets from our 
 consideration, we find that all tiie other bodies of the sphere, con- 
 stantly preserve the same situation with respect lo each oliier for 
 this reason they are called fixed stars. They obviously differ 
 again from the planets, by the twinkling of their light, however 
 clear the atmosphere; to the unpliilosophic eye they appear to be 
 innumerable, but this is only in consequence of theii being scat- 
 tered over the expanse of heaven apparently without order, and 
 our not being able to see them all at once. 01 this deception of 
 sight, we may soon convince our.'^elves, by selecting any parti- 
 cular portion of the heavens, and counting the stars it contains ; 
 the number of which will he found very small. The whole 
 number of stars which can be seen at once by the naked eye, 
 is not above 500; but soon after the invention of the telescope, 
 the number actually noted down in catalogues amounted to 
 
572 
 
 ASTKONOMY. 
 
 Fixed stars destitute of parallax. 
 
 r>000. The further telescopes have been improved, the more 
 have been discovered, and JJr. Herschel has, in our own time, 
 added at least 30,000 to those formerly known. 
 
 Before we venture upon any speculations with respect to the 
 nature of the fixed stars, it will be proper to obtain, if possible, 
 some idea of their distances. For this purpose, we must 
 remind the reader of the nature of a parallax, which signifies 
 the angle subtended by the visual ra}s coming from an object 
 viewed at two different situations. The moon, it has been 
 shown at page 522, is so near us that its distance may be ascer- 
 tained tolerably wtH by a horizontal parallax, w'hich is equivalent 
 to viewing it from two situations separated by the semi-diameter 
 of the earth, or nearly 4000 miles. But the distance of the 
 sun is found to be so great, that a horizontal parallax will not 
 give a satisfactory result ; philosophers therefore, it has been 
 shown, turned their attention, for ascertaining the distance of 
 this luminary, to the transits of Venus, by which nearly the 
 whole diameter of the earth formed a parallax, and enabled 
 them to solve the problem. Two stations, how'ever, separated 
 by the whole diameter of the earth, are utterly incapable of 
 showing us the distance of a fixed star. What resource then 
 has human ingenuity left t one which might surely be thought 
 sufheient, that of viewing and taking the parallax of these bodies 
 from two opposite parts of the earth’s orbit. This is called the 
 gj-eat paraliax, or annual parallax. By means of the zenith 
 sector, Flook, Flamsteed, and Bradley observed, for some 
 time, at the spring and autumnal equinox, the transit of y 
 Draconis over this perpendicular telescope, hoping that the 
 diameter of the earth’s orbit might make an angle or parallax 
 with it. d'hey were disappointed. The star was seen so near 
 the same place, at each of the earth’s two stations, which are 
 nearly 200 millions of miles distant from each other, that no 
 estimate coidd be made ! Bradley guessed there might be an 
 angle of about two seconds, which would make the star 
 400,000 times further from us than the sun. Cassini supposed 
 the annual parallax of Sirius, which of all the fixed stars is 
 considered the nearest, to be six seconds, from which it was 
 calculated to be 18,000 times further from us than the sun. 
 Nothing, how'ever, on this subject, has bee.n determined with 
 certainty, except the simple but truly astonishing truth, that 
 the space of COO millions of miles is an insensible point, when 
 compared with the distance of the nearest fixed star. 
 
 d'he next telescopic peculiarity which we must notice, is, 
 that the fixed stars have no commensurable magnitude. This 
 is the more remarkable, as they differ from each other so much 
 
ASTRONOMY. 
 
 513 
 
 Fixed stars shine by native light. 
 
 in brilliancy ; but when examined with a telescope, however 
 great the magnifying power, the most sjtarkling appear only to be 
 lucid points, intensely bright, indeed, but indivisible. Their 
 twinkling is occasioned by the motion of particles in our atmo- 
 sphere, intercepting their light ; and their sensible magnitude 
 to th.e naked eye arises from adventitious rays cut off by the tele- 
 scope. 
 
 From the two circumstances which have been just mentioned, 
 to wit, the indefinite distances and magnitudes of the fixed stars, 
 we may justly infer, that they shine by native light, otherwise tl)ey 
 would be invisible to the naked eye : for the satellites of Jupiter 
 and Saturn, though we can measure their distances, and they also 
 appear of a very distinguishable magnitude through a telescope, 
 reflect too little light to be visible without that instrument. We 
 may calculate the distance at which GOO millions of miles would 
 appear to our sight as a point ; and supposing we assume that 
 distance to be the distance of the nearest fixed star, we shall be 
 satisfied, that our sun, if equally far off’, would only appear like a 
 star. Are we not then compelled to draw the conclusions, that 
 every fixed star is itself a sun ; that each of them is the centre of 
 a system of planetary worlds ; that it directs their motions, and 
 supplies them with light and heat, in the same manner that the 
 sun governs the several bodies of which our solar system is com- 
 posed ; that these planets of other systems, arc furnished like ours 
 with satellites, though these as well as their primaries are altoge- 
 ther invisible to us ; and finally that the fixed stars are, generally 
 speaking, as distant from each other as the nearest of them is from 
 the earth. 
 
 The difference of the apparent magnitudes of the fixed stars is 
 very great ; yet it is found to answ er sufficiently w ell the purposes 
 of description and discrimination, to distinguish them into six 
 ‘ orders, calling the largest of them stars of the Jirst mag^ahude ; 
 the next, stars of the second magnitude, ami so on. d'he stars of 
 the sixth magnitude are those which can barely be distinguished^ 
 by the naked eye. Those which can only be seen by the help of 
 the telescope, are usually called telescop'C stars. ^ 
 
 A betteV method than the above of distinguishing the bright- 
 ness and magnitude of the stars, is that adopted by Dr. fier- 
 schel, in his catalogues of the comparative brightness of the 
 fixed stars. It consists in referring a given star to two otlier 
 stars, one of which is somewhat brighter, and the other rather 
 less bright, than the one to be designated. “ I place,” says he, 
 “ eacli star, instead of giving its magnitude, into a short scries, 
 constructed upon the order of brightness of the nearest propei 
 
574 
 
 ASTRONOMY. 
 
 Classification of the fixed stars. 
 
 stars. For instance, to express the lustre of D, I say CDE. 
 By this short notation, instead of referring the star D to an ima- 
 ginary, uncertain standard, I refer it to a precise and determinate 
 existing one. C is a star that has a greater lustre than T); and E 
 is another of less brightness than D. Both C and E are neigh- 
 bouring stars, chosen in such a manner that I may see them at the 
 same time with D, and therefore may be able to compare them 
 properly. The lustre of C is in the same manner ascertained by 
 BCD ; that of B by ABC ; and also the brightness of E by 
 DEE: and that of E by EFG.’^ 
 
 Astronomers divide the heavens into three regions ; a northern 
 and a southern hemisphere, and the zodiac. Stars of various mag- 
 nitudes are seen in all these regions, and are classed into what are 
 called constellations, or systems of stars, according as they lie near 
 one another, so as to occupy those spaces which the ligures of 
 different sorts of animals, &c. would take up, if they were deli- 
 neated on w'hat appears to be the concave surface of the heavens. 
 Those stars which the ancients could not bring into any particular 
 constellation, they called unformed stars, but most of these are 
 comprehended in the new constellations of the moderns. 
 
 This division of the stars into different constellations, or as- 
 terims, serves to distinguish them in such a manner, that any par- 
 ticular star may be readily found in the heavens, by means of a 
 celestial globe, on which the constellations are so delineated as to 
 put the most remarkable stars into those parts of the figures, 
 which may be most easily pointed out. A great improvement of 
 this ancient mode of pointing out the stars, consists in annexing 
 a Greek or a Roman letter, or a number, to each star, the bright- 
 est star having the first letter in the alphabet, or the low'est num- 
 ber, and the rest following in regular order. By these means, 
 every star may be pointed out with as much ease, as if it had a 
 distinct name. Thus if we see a in Orion, and cc in the Twins, 
 the former is called Alpha Orionis, and the latter Alpiia Gemino- 
 rum, 8>cc. But before this arrangement prevailed, remarkable stars 
 in some of the constellations obtained a particular name, which 
 they still retain, as Aldebaran, Castor and Pollux, &c. 
 
 To the foliow'ing list of the constellations, and of the number 
 of stars observed in each of them, as far as those of tlie sixth mag- 
 nitude, is subjoined the names and magnitudes of those stars to 
 which independent names have been given : 
 
ASTRONOMY. 
 
 575 
 
 List of the constellations. 
 
 ConsteIIafio?is of the Zodiac. 
 
 No. Principal Stars and 
 Constellations. of Stars, their Mag;nitudes. 
 
 Aries ....... 
 
 T'aurus • • • • « 
 
 . . The Bull 
 
 ... GO 
 
 1 
 
 Gemini . • • • < 
 Cancer 
 
 
 
 1.2 
 
 Leo . . . . • 
 
 
 
 1 
 
 Virgo 
 
 
 
 1 
 
 Libra 
 
 
 
 2 
 
 Scorpio • • • • 
 Sagittarius • • 
 Capricornus • • 
 Aquarius •••• 
 Pisces 
 
 
 .... 44 Antares 
 
 1 
 
 ‘ The Water-carrier 108 Scheat 
 
 3 
 
 Coiistellations on the North Side of the Zodiac. 
 
 Constellations. of 
 
 V'rsa Minor The Little Bear 
 
 Ur.sa ^lajor The Great Bear • * 
 
 Cassiopeia • • Lady in her Chair 
 
 Perseus Perseus 
 
 Auriga Tlie Waggoner 
 
 Bootes The Bear Driver • • 
 
 Draco Tlie Dragon 
 
 Cepheus 
 
 Canes Veiiatlci^^ viz. As- ^he Grcvliounds .. 
 terian and Chara • • 
 
 Cor. Caroli diaries’ Crotvn 
 
 Triangulum The Triangle • • • • 
 
 Triangulum Minus • • The Lesser Triangle 
 
 Musca The Bee 
 
 Lvnx 
 
 Leo Elinor The Little Lion • • • • 
 
 Coma Bercnicis Berenice’s Hair 
 
 Camelopardalus • • • • The Camelopard • • • • 
 
 Mons Menelaus 
 
 Corona Borealis Northern Crown • • 
 
 Serpens ...*•. The Serpent .»•••• 
 
 Scutym Sobieski • • • • Sobieski s Shield • • 
 
 No. Principal Stars and 
 
 Stars. their Magnitudes. 
 
 24 Stella Poloris • • 2 
 
 B7 Dubhe 1 
 
 55 
 
 59 Algenib 2 
 
 50 Capella 1 
 
 54 Arcturus 1 
 
 00 Ilastaber 3 
 
 35 Aldcramin • • • • 3 
 
 25 
 
 3 ^ 
 
 10 
 
 5 
 
 0 
 
 44 
 
 24 
 
 40 
 
 58 
 
 11 
 
 21 
 
 50 
 
 8 
 
57(5 
 
 ASTRONOMY. 
 
 List of the constellations. 
 
 Constellations on the North Side of the Zodiac ( continued.) 
 
 No. 
 
 of Stars, 
 
 Hercules kneeling 113 
 
 07 
 
 Ras Algiatha 
 
 Ras Alhagus 
 
 Vega 
 
 Constellations. 
 
 Kerculus, cum Ramol 
 
 et Cerbero j 
 
 Serpeutarius sive \ „tarius 
 
 Opliiuchus 3 ^ 
 
 Taurus Poniatowski 7 
 
 Lyra The Harp 22 
 
 Vulpecula et Anser, . Tlie Fox and Goose 37 
 
 Sagitta The Arrow 3 8 
 
 Aquila The Eagle 40 Altair 
 
 Delphinus The Dolphin 18 
 
 Cygnus The Swan 73 
 
 Equuleus The Horse’s Head 10 
 
 Lacerta The Lizard 10 
 
 Pegasus The Fhing Horse 85 Markab 
 
 Andromeda 66 Aimaac. 
 
 Principal Stars and 
 their Magnitudes. 
 
 Deneb Adige 
 
 Constellations on the South Side of the Zodiac. 
 
 Principal Stars and 
 their magnitudes. 
 
 No. 
 
 Constellations. of Stars. 
 
 Phoenix Pheuix 13 
 
 Othcina sculptoria 12 
 
 Eridanus The River 76 Achernar 1 
 
 Hydrus The Hydra 10 
 
 Cetus The Whale 80 IMenekar 2 
 
 Fornax Cheraica 14 
 
 Horologium 12 
 
 ReticulusRhomboidalis 10 
 
 Xiphias The Swoid-tish .. 7 
 
 Celapraxilellis 10 
 
 Lepus The Hare 19 
 
 Columba Noachi. . . , Noah’s Dove .... 10 
 
 Orion 78 Betelguese 1 
 
 Argo Navis The Ship 
 
 50 Canopus 
 30 Sirius . . 
 
 Canis Major The Great Dog 
 
 Equuleus Pictorius 8 
 
 Monoceros Tlie Ehiicorn .... 31 
 
 Canis Minor The Little Dog . . 14 Procyon 1 
 
 Chamoeleou Chameleon 10 
 
 Pixis Nautica 4 
 
 Piscis Volans ^ The Flying Fish .. 8 
 
 
ASTRONOMY. 
 
 ■577 
 
 List of the constellations. 
 
 ConsteUations on the South Side of the Zodiac (continued.) 
 
 . No. Principal stars and 
 
 Constellations of stars, their Magnitudes. 
 
 Hydra 60 Cor Hydra .... 1 
 
 Sextans The Sextant 4 
 
 Robur Carolinum The Royal Oak .... 12 
 
 ^lachina Pneuraatica 3 
 
 Crater The Cup 11 Alkes 3 
 
 Corvus The Crow 9 Algorab 3 
 
 Crux ...... ........ The Cross (3 Crucis •••••••• 1 
 
 Muca The Bee 4 
 
 A pus The Bird of Paradise 11 
 
 Circiiius 4 
 
 Centaurus The Centaur 36 
 
 Lupus The Wolf 24 
 
 Quadra Euclidis .12 
 
 Triauirulum Australe< 
 
 Southern Triangle 
 Ara • • • The Altar 
 
 9 
 
 9 
 
 12 
 
 14 
 
 Telescopiuni The Telescope • 
 
 Corona Australis • • • • Southern Crown 
 
 Pavo The Peacock • . • 
 
 Indus The Indian 12 
 
 Microscopium The Microscope • • 10 
 
 Octans Hadleianus • • Hadley’s Quadrant 43 
 
 Grus The Crane ...... 14 
 
 Toucan American Goose . • 9 
 
 Piscis Australis The Southern Fish 20 
 
 Tomalhaut 
 
 The whole number of stars reckoned in the preceding 
 lists, amounts to 619C, but the number which may be discerned 
 with the assistance of telescopes is incalculable. Dr. Hook, 
 with a telescope of twelve feet, saw 78 stars among the 
 Pleiades ; and with a longer telescope still more ; and in the 
 constellation Orion, which is usually reckoned to contain 78 
 stars, there have been seen 2000. Dr. Herschel, with a telescope 
 that magnifies from five to six thousand times, has counted forty- 
 four thousand in a space of sky eight degrees in length, and 
 three in width; and supposing this quantity to be the same in 
 all portions of the sky, of equal dimensions, the whole number 
 of stars, that may be rendered visible by such a telescope, can- 
 not be fewer than seventy-four millions. 
 
 Kepler has made a very ingenious observation upon the 
 magnitudes and distances of the fixed stars. . He observes, that 
 there can be only, thirteen points upon the surface of a spheie, 
 25. — VoL. I, 4 E 
 
578 
 
 ASTRONOMY. 
 
 Changes among the fixed stars. 
 
 as far distant from each other as from the centre, and suppos- 
 ing the nearest fixed stars to be as far distant from each other 
 as from the sun, lie concludes that there can strictly speaking 
 be only 13 stars of the first magnitude. Hence at twice* that 
 distance from the sun there may be placed four times as many, 
 a4id so on and this mode of calculation gives us pretty nearly 
 the number of stars of the first, second, third, &c. magnitudes. 
 
 It is a fact no less singular than well ascertained, that sta?» 
 vrhkh were observed by the ancients are now no longer to be 
 seen, and new ones have appeared in different places, which 
 were unknown to tlie ancients, and some of these have also 
 disappeared and again- become visible. Hipparchus, the 
 ancient astronomer, having observed the disappearance of a 
 star, was iisduced to make a catalogue of the fixed stars, that 
 posterity might judge whether any other change took place 
 among them. Many ages afterwards, a new star having 
 been observed by Tycho Brahe and his contemporaries, this 
 astronomer also determined to make a catalogue with the same 
 view as Hipparchus. Of- the new star seen by Brahe, and on 
 the subject of changes among the stars in general, we have the 
 following interesting account by Dr. Halley : The first new 
 
 star in the Chair of Cassiopeia was not seen by Cornelius Gemma 
 on the 8th of November, 1372, who says, he, that night, con- 
 sidered thatt part of the heaven in a very serene sky, and saw k 
 not; but that the next night, November 9> it appeared, w’itlt 
 a splendour surpassing all the fixed stars, and scarcely less 
 bright than Venus. This was not seen by Tycho Brahe before 
 the Ilth of the same month; but from thence he assures us that 
 it gradually decreased and died away; so as in March, 1374, 
 after months, to be no longer visible ; and at this day no 
 signs of it remain. Its place in* the sphere of fixed stars, by 
 the accurate observations of the same Tycho, was 0*9° 17' a 
 >vith 33° 43' north latitude. Such another star was 
 seen and observed by the scholars of Kepler, to begin to appear 
 on September 30, St. Vet. anno. 1004, which was not to be 
 seen the day before ; but it broke out at once with a lustre sur- 
 passing that of Jupiter f and like the former, it died away gra- 
 dually, and in much abotit the same time disappeared totally, 
 there remaining no footsteps thereof in January, 1603-6. 
 This was near the ecliptic, following the right leg of Serpenta- 
 rius ; and by the observations of Kepler and others, was in 7^ 
 28° 0' a ln‘a * y, with north latitude 1° 36'. I'hcse two seem 
 to be of a distinct species from the rest, aud nothing like them 
 has appeared since. • But between them, viz. in the year 1390v 
 v^e have the first account of the wonderful star in Collo Ceti, seea 
 
ASTRONOMY. 
 
 579 
 
 Changes among the fixed stars. 
 
 by David Fabricius on the 14th of August, as bright as a star 
 of the third magnitude, which has been since found to appear 
 and disappear periodically; its period being precisely enough 
 ^even revolutions in six years, though it returns not always 
 with the same lustre. Nor is it ever totally extinguished, btit 
 may at all times be seen with a six feet tube, [telescope.] Tins 
 was singular in its kind, till that in Collo Cygni was discover- 
 ed. It precedes the first star of Aries 1° 40', with 
 south latitude- Another new star was first discovered by 
 William Jansonius, in the year 1600, in Pectore, or lather in 
 Eductione Colli Cygni, which exceeded not tlie third inagni> 
 inde. This having continued some years, became at length so 
 «mall, as to be thought by some to have disappeared entirely ; 
 but in the years l6o7, 1658, and 1659, it again arose to the 
 
 ihird magnitude ; though soon after it decayed by degrees to 
 ihe fifth or sixth magnitude, and xit this day is to be seen as 
 such in Qs 18° 38' a I^a * y', with 55° 29' north latitude, A 
 fifth new star was first seen by Hevelius in the year I67O, on 
 July 15, St. Vet. as a star . of the third magnitude; but by .the 
 beginning of October was scarcely to be perceived by tlie 
 
 naked eye. In April following, it was again as bright as 
 
 before, or rather greater than of the third magnitude, yet 
 w holly disappeared about the middle of August. J he next 
 year, in March, 1672, it was seen again, but not exceeding 
 
 the sixth magnitude : since when, it has been no further visible, 
 though we have frequently sought for its return 5 Us place is 
 3 ° 17' a I "'a ^ and has lat. north 47° •28'. The sixth 
 and last is that discovered by G. Kirch in the year and 
 
 its period determined to be of days, and though U rarely 
 
 exceeds the fifth magnitude, yet it is very regular in its returns, 
 as w.e found in the year 1714. Since then we have wutcJie.d, as 
 the absence of the moon and the clearness of the weather would 
 permit, the first beginning of its appearance in a six feet 
 tube, that, bearing a very great aperture, cUscov.ers most ininute 
 stars. And on June l^th last, it was first perceif/ed like one 
 of the first telescopical stars ; but in the rest of that month 
 and July, it gradually increased, so as to become in August 
 visible to tlie naked eye, and so continued till the month of 
 September. After thaUt again died a*vay by degrees : and on 
 the 8th of December, at night, was scarcely disceinable by the 
 tube ; and as near as could be guessed, equal to what it was 
 at its first appearance on June 15, so that this year it has been 
 seen in all nearly six months, which is but little less than half 
 its period ; and the middle, and consequently the great, eit 
 brightness^ falls about the 10th of September. 
 
580 
 
 ASTRONOMY. 
 
 Changes among the fixed stars. ‘ 
 
 In the 76lh volume of the Philosophical Transactions, a 
 paper, by Edward Pigot, gives a dissertation on the stars sus- 
 pected by the astronomers of the last century to be changeable. 
 The author divides them into two classes ; one containing those 
 which are undoubtedly changeable, and the other those which 
 are only suspected to be so. The first class contains a list of 
 12 stars fiom the first to the fourth magnitude, including the 
 new one which appeared in Cassiopeia, in 1572, and that in 
 Serpentarius in 1604: the second contains the names of 30 
 stars of different magnitudes, from the first to the seventh. He 
 is of opinion, that the celebrated new star in Cassiopeia is a 
 periodical one, and that it returns once in 130 years. Keill is of 
 the same opinion ; and Pigot thinks that its not being observed 
 at the expiration of each period, is no argument against the 
 truth of that opinion, remarking, that, “ like most of the 
 variables, it may at ditferent periods have different degrees of 
 lustre, so as sometimes only to increase to the ninth magnitude ; 
 and if this should be the case, its period is probably much shorter. 
 For this reason, in September, 1782, he took a plan of the 
 small stars near the place where it formerly appeared, but in 
 four years had observed no alteration. He also examined the 
 star in the neck of the Whale, from the end of 1782 to 1786, 
 but he never found it exceed the sixth magnitude, though it 
 bad been observed by others at different times to be of the third 
 magnitude. He deduced its period, from its apparent equality 
 with a smaller star in the neighbourhood, and thence found it 
 to be 320, 328, and 337 days. Another changeable star, and 
 one of the most remarkable of the .class, is that called Algol, 
 in the head of Medusa ; it had long been known to be variable, 
 but its period was first ascertained by Goodricke, of York, 
 who began to .observe it in the beginning of 1783. It seems 
 to have a period of about 2 days, 21 hours, during which it 
 varies gradually from the size of the second to that of the fourth 
 magnitude in about 3| hours; after which time it gradually 
 recovers its greatest lustre, which it preserves to the end of its 
 period, and then again begins t,o diminish. 
 
 These changes among the fixed stars have given rise to a 
 great variety of hypotheses to account for them. The suggestion 
 of Sir Isaac Newton, that the transitory blaze of some of them, 
 might be owing to the addition of fuel they have received from 
 the influx of a comet, has already been noticed. Maupertius 
 thought 5 ome stars, by their prodigiously quick rotations on 
 their axis, may not only assume the figures of oblate spheroids, 
 but that, from the great centrifugal force arising from such 
 rotations, they may become of the figurefc of mill-stones; or be 
 
ASTRONOMY. 
 
 581 
 
 Possible causes of the changes among the fixed stars. — Galaxy and lu bcdai. 
 
 reduced to flat circular planes, so thin as to be quite invisible 
 when their edges are turned towards us, as Salurn’s ring is in 
 such positions. But when very eccentric planets or comets go 
 round any flat star, in orbits much inclined to its equator, the 
 attraction of the planets or comets in their perihelions, must 
 alter the inclination of the axis of that star; on \>hich account, 
 it will appear more or less large and luminous, as its broad side 
 is more or less turned towards us. A third opinion is, tliat the 
 changes in question may be owing to spots on the surfaces of 
 the stars, so that when, by their periodical rotations on their 
 axis, these spots are turned towards us, the star is either not 
 seen at all, or appears less bright than at other times. Lastly, 
 it is conjectured that they revolve in very considerable orbits, 
 and that we only see them when they are nearest to us. It pro- 
 bably is not one uniform cause that occasions the phenomena, 
 which may be owing sometimes to one and sometimes to another 
 of the causes assigned, or to various others of which we liavc 
 not the least apprehension ; indeed, ages of assiduous observation 
 may elapse before any rational conjecture can be ofi’ered on the 
 subject. 
 
 On almost any evening which is clear enough for the stars to 
 be seen with tolerable distinctness, we may observe in the heavens, 
 a broad tract, of a whitish colour. This remarkable tract is called 
 from its w’hiteness, the galaxy, via lactca, or milky zcay. On 
 more minute inspection, w'e may observe a number of separate 
 whitish places, affording the same kind of light as the milky way, 
 but not so bright; these are called 
 
 The milky w'ay encircles the celestial concave; it is irregu- 
 larly extended, sometimes double, but for the most part single, 
 and varies in breadth from 4° to 20°. It passes through Cassio- 
 peia, Perseus, Auriga, the foot of Gemini, Orion’s club, part 
 of Monoceros, the tail of Canis Major, through Argo Navis, 
 Robur Carolinum, Crux, and the feet of the Centaur; here it 
 divides into two parts ; its eastern branch passes through Ara, 
 the tail of Scorpio, the eastern foot of Serpentarius, the bow 
 of Sa<yittarius, Scutum Sobiescianum, the feet of Antinous and 
 Cygnus. Its western branch passes through the upper part of 
 the tail of Scorpio, the right of Serpentarius and Cygnus, and 
 ends in Cassiopeia. With a pow-erful telescope. Dr. Herschel 
 first began to survey the via lactea, and found that it completely 
 resolved the whitish appearance into stars, which the telescope 
 he formerly used had not power enough to do. The portion he 
 first observed, w'as that about the hand and club of Orion ; 
 he perceived in it an astonishing multitude of stars, the number 
 of which he endeavoured to estimate, by counting* many 
 
582 
 
 ASTRONOMY. 
 
 Galaxy and nebulae. 
 
 different telescopic fields of view, and computing from a mean 
 of these how many might be contained in a given portion of 
 the milky way. In the most vacant place to be met with in 
 that neighbourhood, he found CiS stars; other six fields contained 
 110, 60, 70, 70, and 74 stars; a mean of all which gave 
 
 76 for the number of stars to each field ; and thus he found, 
 that by allowing fifteen minutes for the diameter of his field of 
 view, a belt of 15 degrees long, and two broad, which he had 
 often seen pass before his telescope in an hour’s time, could not 
 contain less than 50,000 stars, large enough to be distinctly num- 
 bered ; besides which, he suspected, twice as many more, which 
 could be seen only now and then by faint glimpses, for w'ant of 
 Sufficient light. 
 
 Dr. Herschel’s success in examining the milky way, induced 
 him to turn iiis telescope to the nebulous parts of the heavens. 
 Most of these yielded to a Newtonian reflector, of 20 feet focal 
 distance, and 12 inches aperture; which plainly discovered them 
 to be composed of stars, or at least to contain stars, and to afford 
 very strong indications of their consisting of them entirely. The 
 nebulae,” says he, are arranged into stiata, and run on to a great 
 length, and some of them I have been able to pursue, and to guess 
 pretty well at their form and direction. It is probable enough 
 thait they may sui round the w hole starry sphere of the heavens, 
 not unlike the milky w’ay, which undoubtedly is nothing but a 
 stratum of fixed stars ; and as this immense starry bed is not of 
 equal lustre in every part, nor runs on in one straight direction, 
 but is curved, and even divided into two streams along a very con- 
 siderable portion of it; w’e may likewise expect the greatest variety 
 in the strata of the cluster of the stars and nebulae. One of these 
 nebulous beds is so rich, that, in passing through a section of it in 
 the time of only 36 minutes, 1 have delected no less than SI 
 jiebulae, all distinctly visible upon a fine blue sky. Their situation 
 and shape, as well as condition, seem to denote the greatest va- 
 riety imaginable. In another stratum, or perhaps a different 
 branch of llie former, I have often seen double and treble nebulae 
 variously arranged ; large ones, with small seeming attendants ; 
 narrow', but much extended lucid nebulae or bright dashes; 
 some of the shape of a fan, resembling an electric brush issuing 
 from a lucid point; others of the cometic shape, with a seeming 
 nucleus in the centre, or like cloudy stars, surrounded with a 
 nebulous atmosphere: a different sort of orb again, contain a 
 nebulosity of the milky kind, like that wonderful, inexplicably 
 phenomena about Oiionis ; while others shine with a fainter 
 mottled Icind of light, which denotes their being resolvable inu» 
 itars. 
 
ASTRONOMY. 
 
 583 
 
 Galaxy and nebulse. 
 
 ** It is very probable that the great stratum called the milky 
 way, is that in which the sun is placed, though perhaps not in 
 the very centre of its thickness. We gather this from the 
 appearance of the galaxy, which seems to encompass the whole 
 heavens, as it certainly must do if the sun is within the same : 
 for, suppose a number of stars arranged between two parallel 
 planes, indefinitely extended every way, but at a given consi- 
 derable distance from one another, and calling this a sidereal 
 stratum, an eye placed somewhere within it, will see all the 
 stars in the direction of the planes of the stratum projected into 
 a great circle, which will appear lucid, on account of the 
 accumulation of the stars, while the rest of the heavens, at the 
 sides, wdll only seem to be scattered over with constellations, 
 more or Jess crowded, according to the distance of the planes, 
 or number of stars contained in the thickness or sides of the 
 stratum.’’ 
 
 The nebulae have been divided into three kinds. The first 
 kind comprises those which consist of a great number of. stars 
 crow'ded together, and which are seen to be distinct through a 
 telescope. Among these, is the famous nebulae of Cancer, or 
 the pr(£sepe caticri, forming a collection of 2o or 30 stars, and 
 many similar groups in various parts of the heavens. — The 
 second kind consist of one or more stars, surrounded by a 
 wdiitish spot, through which they seem to shine. There are 
 several of this sort, but one of the most remarkable is that in 
 Orion, which, through a telescope, appears as a whitish spot 
 nearly triangular : it contains seven stars, one of which is itself 
 surrounded by a small cloud brighter than the rest of the spot. 
 — ^Nebulae of the third kind are white spots in which no stars 
 are seen when viewed with a telescope. — Fourteen of these 
 have been observed in the austral hemisphere, among which the 
 celebrated spots, near the south pole, called by sailors the 
 Magellanic clouds, hold the first rank. — Dr. Herschel has given 
 catalogues of £000 nebulae and clusters of stars discovered by 
 himself. 
 
 We will now, says the Doctor, in one of his papers on this 
 subject, retreat to our own retired situation in one of the 
 planets, attending a star in the great combination, with num- 
 berless others ; and in order to investigate what will be the 
 appearances from this contracted situation, let us begin with 
 the naked eye. The stars of the first magnitude being in all 
 probability the nearest, will furnish us vyilh a step to begin our 
 scale ; setting olf therefore w ith the distance of Sirius, or A re- 
 turns, for instance, as unity, we will at present suppose, that 
 those of the second magnitude are at double, and those of the 
 
584 
 
 ASTRONOMY. 
 
 Galaxy.— Power of Herschet’s telescope. 
 
 third at treble the distance, and so forth. Taking it then, for 
 granted, that a star of the seventh magnitude is about seven 
 limes as far from us as one of the first, it follows that an obser- 
 ver, who is enclosed in a globular cluster of stars, and not far 
 from the centre, will never be able, with the naked eye, to see 
 the end of it : for since, according to the above estimations, he 
 can only extend hi.s view about seven times the distance of 
 Sirius, It cannot be expected that his eyes should reach the bor- 
 ders of a cluster, which has perhaps fifty stars in depth every 
 where around him. The whole universe, therefore, to him, 
 will be comprised in a set of constellations, richly ornamented 
 with scattered stars of all sizes. Or if the united brightness of 
 a neighbouring cluster of stars should, in a remarkably clear 
 night, reach his sight, it will put on the appearance of a small, 
 faint, nebulous cloud, not to be perceived without the greatest at- 
 tention. Allowing him the use of a common telescope, he begins 
 to suspect that all the milkiness of the bright path which surrounds 
 the sphere may be owing to stars. By increasing his power of 
 vision, he becomes certain that the milky way is, indeed, no other 
 than a collection of very small stars, .and the nebulae nothing but 
 clusters of stars. 
 
 Dr. Herschel then solves a general problem for computing 
 the length of the visual ray : that of the telescope which he 
 uses, will reach to stars 497 times the distance of Sirius. Now^ 
 according to his reasoning, Sirius cannot be nearer than 100,000 
 X 194 , 000,000 of miles, therefore his telescope will, at least, 
 reach to 100,000x 194,000,000x497 miles. And he observes, 
 that in the most crowded part of the milky way, he has had fields 
 of view that contained no less than 58B stars, and these were con- 
 tinued for many minutes ; so that, in a quarter of an hour, he has 
 seen 116,000 stars pass through the field of view of a telescope 
 of only 15' aperture; and at another time in 41 minutes he saw 
 £58,000 stars pass through the field of his telescope. Every im- 
 provement in his telescopes has discovered stars not seen before, 
 so that their appears no bound.s to their number, or to the extent 
 of the universe. 
 
 Tlie sun, like many oilier .stars, has probably a progressive 
 motion directed towards the constellation Hercules, carrying all 
 its attendant planets along with it ; ajui w'ith respect to this 
 motion, Dr. Herschel observes, that the apparent proper mo- 
 tions of 44 stars out of 56, are iieaily in the direction which 
 Would be the result of such a real motion of the solar system ; 
 and that the bright stars Arcturus and Sirius, which are proba- 
 bly the nearest to us, have, as they ought, according to this 
 theory, the greatest apparent motions* Again, the star Castor, 
 
ASTRONOMY. 
 
 585 
 
 Fixed stars. — Concluding observations. 
 
 appears when viewed with a telescope, to consist of two 4 ;tars, of 
 nearly equal magnitude ; and though they have an apparent mo- 
 tion, they have never been found to change their distance with re- 
 spect to one another a single second, a circumslance easily under- 
 stood, if both their apparent motions are supposed to arise from 
 the real motion of the sun. 
 
 In the infancy of science, when the distances of the sun 
 and planets were unknown, man, unable to draw any just con- 
 clusion respecting objects so much beyond his reach, never 
 doubted that he was the only being of his kind in the uni\erse, 
 or that the earth was the most consequential aggregate of matter 
 in existence. Pleased with what exalts himself, it was not with- 
 out many a struggle, and by slow degrees, that he withdrew from 
 the dominion of so contemptible a belief. At length, when the 
 evidence, continually accumulating, triumjdied over all contra- 
 diction ; when the magnificence of the planetary orbs became 
 perfectly evident ; when it was proved that they certainly expe- 
 rienced the regular return of day and night, spring, summer, au- 
 tumn, and winter ; it became difficult to deny the plurality of in- 
 habited worlds, with any hope of rationally explaining the design 
 of that exertion of Creative Power which called those orbs into 
 existence, and subjected them to the same laws as the earth ; and 
 man was constrained to admit, that a grain of sand was not 
 more completely an atom to the earth, than the earth itself was 
 an atom to the solar system. These ideas had scarcely obtained 
 general acceptance, when it was found that the distances of all 
 the fixed stars is immeasurably great ; then it became obvious that 
 they shine by their own light : that they are each of them com- 
 parable in real splendour and magnitude to the sun ; that they are 
 as distant from each other as any one of them must be from us ; 
 that therefore, instead of being merely as ornaments of the sky, 
 the most reasonable conclusions of analogy inculcated, that they 
 w^ere the centres of so many systems, dispersed throughout 
 the universe, and dispensing light and heat to planetary worlds 
 around them ; and the use of the telescope, at the same time • 
 that it imparted the power of counting millions of stars which 
 the naked eye never beheld, yielded a convincing evidence, that 
 millions of millions exist, at distances too great for them to be 
 separately distinguished. Man had now advanced another step, 
 but not in proving his own importance. On the contrary, from 
 the insignificance of the earth to the W'hole solar system, he 
 now advanced to the perception of the insignificance of the 
 25. VoL 1 . 4 F 
 
586 
 
 ASTRONOMY. 
 
 Gravitation the cause of the tides. 
 
 whole solar system, to the remaining immensity of creation ; so 
 that the total annihilation of the system that appears so vast to 
 him, would be like the abstraction of nothing from the universe. 
 On the wings of conjecture, indeed, but still with analogy by our 
 side, we may take a still more eminent if not expansive view. If 
 there are reasons to believe that every orb of the solar system is 
 inhabited, there are reasons scarcely less strong for believing that 
 every star, and every orb attending every star, is created and exists 
 only for the same great purpose. What incommunicable feelings 
 do these views excite in the thinking mind ! Perhaps all the stars 
 that we can discover, and all, of the existence of which we can ob- 
 tain any evidence, form a very finite part of the total number ; yet, 
 is it possible by any other means, to obtain so admirable a view of 
 Infinite Power and Infinite Wisdom, as may be derived from the 
 contemplation of the attributes of that Being who has created, and 
 maintains, even that portion which we can actually discern of the 
 stupendous fabric of Universal Nature? 
 
 Of the Tides. 
 
 Having now ranged over the mighty field of astronomical re- 
 search, we must next proceed to consider a variety of phenomena, 
 resulting from the general laws of nature, which are particularly 
 and immediately interesting to us as inhabitants of the earth, but 
 which would have too much interrupted our general view, if we 
 had entered sufficiently into the details of them in the course of 
 our progress. Here then we shall in the first place advert to those 
 remarkable fluctuations of the ocean called Tides. 
 
 The apparent connexion subsisting betw^een the movements of 
 the ocean and those of the moon, has been observed from the 
 earliest periods of antiquity ; but it was Kepler who first asserted 
 that the moon’s attraction was the real cause : If,” says he, the 
 
 earth ceased to attract its waters towards itself, all the water in 
 the ocean would rise and flow to the moon. The sphere of the 
 moon’s attraction extends to our earth, and draws up the water.” 
 Sir Isaac Newton afterwards demonstrated the consonance of the 
 cause assigned by Kepler, with his theory of universal gravitation, 
 and explained at the same time the cause of the tides on the side 
 of the earth opposite to the moon, and from his time to the pre- 
 sent, not a doubt has been entertained on the subject. 
 
 The principal phenomena of the tides are as follows: 
 
 1. The sea is observed to flow for about six hours from south 
 to north, gradually^ swelling ; after this it seems to rest for a quar- 
 ter of an hour ; and then to ebb or retire back again from north 
 
 15 
 
ASTRONOMY. 
 
 587 
 
 General phenomena of the tides. 
 
 to south, for six hours more. Then after anotlier pause of about 
 a quarter of an hour, the sea again begins to flow ; and so on»al- 
 ternately. 
 
 2. The time of a flux and reflux, is on an average about 
 12 hours 25 minutes, and twice this time, or 24 liours 50 
 minutes, is the period of a lunar day, or the time between the 
 moon’s passing a meridian, and coming to the same point of it 
 again. So that the sea flows as often as the moon passes the me- 
 ridian, that is, as well when she comes to the arch above the ho- 
 rizon, as when she comes to that below the horizon ; and ebbs as 
 often as the moon passes the horizon, both on the eastern and 
 western side. 
 
 3. The elevation of the waters on that side of the earth imme- 
 diately under the moon, somewhat exceeds the elevation of the 
 opposite side, and in all cases the elevation diminishes from the 
 equator to the poles. 
 
 4. The sun raises and depresses the sea twice every day, 
 in the same manner as the moon ; but the solar influence in this 
 respect is less than that of the moon, in the proportion of 1 
 to 3. 
 
 5. The tides which depend on the actions of the sun and 
 moon, are not distinguished but compounded ; and thus they form, 
 to appearance, one united tide, which, increasing and decreasing, 
 produces Neap and Spring tides. 
 
 6. In the syzygies, that is, when the moon is either new or 
 full, the action of both luminaries concur, and the tides are 
 highest ; but they are least in the quadratures, or when the lines 
 of their action are 90 degrees apart, for where the water is elevated 
 by the moon, it is depressed by the sun, and vice versa. There- 
 fore while the moon passes from the syzygy to the quadrature, the 
 daily elevations are continually diminished : on the contrary, they 
 are increased, while the moon passes from the quadrature to the 
 syzygy. At the new moon, also, the tides are the greatest, be- 
 cause the sun and moon are not only in the same line but on the 
 same side of the earth ; and at this time the tides of the same day 
 are more different than those at full moon. 
 
 7. The greatest elevations and depressions take place on the 
 second or third day after the new or full moon ; and they are the 
 greater the nearer the luminaries are to the plane of the equator ; 
 they are therefore greatest in the syzygies at the time of the equi- 
 
 110X6S# 
 
 8. The actions of the sun and moon are greater, the nearer 
 these bodies are to the earth ; and the greatest tides happen when 
 the sun is a little to the south of the equator ; but this does not 
 happen regularly every year, because some variation may arise 
 
588 
 
 ASTRONOMY. 
 
 Theory of the tides. 
 
 from the situation of the moon’s orbit^ and the distance of the 
 svzjgy from the equinox. 
 
 9 . As the mean force of the moon to move the sea, is to that 
 of the sun nearly as 3 to 1 ; if the action of the sun alone would 
 produce a tide of two feet, which it is said to do, then that of the 
 moon will be six feet; hence the spring- tides will be eight feet, 
 and the neap tides four feet. 
 
 These phenomena take place where the ocean is sufficiently 
 extended, to admit of those motions which the actions of the 
 sun and moon have a tendency to impart ; but they are va- 
 riously modified by the obstacles which the water meets with 
 in its course ; from the direction of the w ind ; from straits and 
 gulphs, from capes, bays, and other peculiarities of the shores 
 of different countries ; so that on some shores, little or no tide 
 is observed, and on others they rise far beyond the amount de- 
 termined, without taking into consideration the modifying circum- 
 stances. 
 
 The tides are propagated to great distances in large rivers; 
 and at the strait of Pauxis, in the river of the Amazons, they are 
 sensible at 600 miles from the sea. 
 
 In explaining the general principles on which the tides 
 depend, it must be observed and remembered, that if the action of 
 the moon were equal upon every part of the ocean, we should 
 have no tides ; it is the inequality of its action that produces them. 
 The action of the moon is greatest in the direction of a line that 
 would join its centre of gravity w ith that of the earth ; to this line 
 as an axis, the ocean rushes up from every side, and as this line 
 moves, the water which flowed up to it at the first moment of the 
 moon’s action, continues to move along with it. The inequality 
 of the moon’s action is not so great as to counteract the cohesion 
 which the solid parts of the earth have to each other, and there- 
 fore they are unaffected by it ; but the particles of fluids move 
 among each other with such facility that they receive it readily. 
 To understand more fully w hy the moon exerts an unequal action 
 on the earth, it is necessary to recollect that the power of gravity 
 diminishes as the square of the distance increases ; and therefore 
 the waters of the earth ABCDEFGH, fig. 1, pi. IV, must be 
 more attracted at Z, on the side next the moon, M, than at the 
 central parts of the earth, O ; and the central parts are more at- 
 tracted by the same power than the w'aters on the opposite side 
 of the earth at N ; and therefore the distance from the earth’s 
 centre and the waters on its surface under and opposite to the 
 moon will be increased. For, let there be three bodies at H, 
 O, and D ; if they are all equally attracted by the body at M, 
 they will all move equally fast towards it, their mutual distances 
 
ASTRONOMY. 
 
 589 
 
 Theory of the tides. 
 
 from each other continuing the same. If the attraction of M is 
 unequal, then that body which is most strongly attracted will 
 move fastest, and this will increase its distance from the other 
 body. Therefore, by the law of gravitation, M will attract H 
 more strongly than it will O, by which the distance between H 
 and O will be increased; and a spectator at O will perceive II 
 rising higher towards Z. In like manner, O being more 
 strongly attracted than D, it will move faster towards M than 
 D ; consequently the distance between O and D will be increas- 
 ed ; and a spectator at O, not perceiving his own motion, will 
 see D receding farther from him towards N ; all effects and 
 appearances being the same, whether D recedes from O, or O 
 from D. 
 
 Suppose no\v there are a number of bodies, as A, B, C, 
 D, E, F, G, FI, or in one word, a fluid ring, placed about 
 O ; then as the whole is attracted towards M, the parts at H 
 and D will have their distance from O increased ; whilst the 
 parts at B and F, being nearly at the same distance from M as 
 O is, these parts will not recede from one another ; but rather, 
 by the oblique attraction of M, they will approach nearer to O. 
 Hence, the fluid ring will form itself into an ellipse ZIBENK 
 FRZ, whose longer axis NOZ produced, will pass through M, 
 and its shorter axis BOF, will terminate in B and F. Let the 
 ring be filled with fluid particles, so as to form a sphere round 
 O ; then as the w’hole moves towards M, the fluid sphere being 
 lengthened at Z and N, will assume an oblong or spheroidal 
 form. If M is the moon, O the earth’s centre, A BC13EFGH 
 the sea covering the earth’s surface, it is evident, by the above 
 reasoning, that whilst the earth, by its gravity, falls toward the 
 moon, the w'ater directly below the moon at B, will swell and 
 rise gradually tow^ards her ; also the water at D will recede from 
 the centre, and rise on the opposite side of tlie earth, whilst 
 the water at B and F is depressed, and falls below the former 
 level. Hence as the earth turns round its axis from the moon 
 to the moon again in 24 hours oO', there are two tides of flood, 
 and two of ebb, in that time. By the motion of the earth on 
 its axis, the most elevated part of the water is carried beyond 
 the moon, in the direction of the rotation ; but the water, from 
 the accumulation of impulse, still continues to rise after it has 
 passed directly under the moon, though the greatest immediate 
 action of the moon has then begun to decrease. The greatest 
 elevation is not attained, till the moon has for the space of an 
 hour or more ceased to be vertical to the place wheie it occurs. 
 Thus, in open seas, where the water flows freely, the moon M 
 is past the north and south meridian, as at /?, when it is high 
 
590 
 
 ASTRONOMY. 
 
 Theory of the tides. 
 
 water at Z and N. For it will be understood, that though the 
 moon’s attraction were to cease altogether when she was past the 
 meridian, yet the motion of ascent previously communicated to 
 the water, would make it continue to rise for some time after ^ 
 much more must it do so when the attraction is only dimi- 
 nished ; this is agreeable to the ordinary phenomenon that the 
 heat is greater at two in the afternoon, than when the sun is on 
 the meridian. The tides do not always answer to the same dis- 
 tance of the moon from the meridian at the same places ; but are 
 variously affected by the action of the sun, which brings them on 
 sooner when the moon is in her first and third quarters, and keeps 
 them back w hen she is in her second and fourth ; because, in the 
 former case, the tides raised by the sun alone would be earlier 
 than the tides raised by the moon ; and in the latter case they 
 would be later. 
 
 It is not difficult for the novice to admit, that the moon 
 should have the stated effect on the waters of the hemisphere 
 immediately beneath it ; but it excites surprise, and is not so 
 easily understood, that it should produce the like effect on the 
 opposite hemisphere, and that it should be flood or ebb 
 tide with our antipodes at the same time that it is flood-tide 
 or ebb-tide with us. Here then we must remark again, that it 
 is not the absolute or total action of the moon, but the difference 
 between its action on one part and its action on another, that 
 constitutes the force which occasions any tide. The point Z is 
 nearer to the moon than any other part of the hemisphere BZF ; 
 the waters therefore about that point are more strongly attracted 
 by the moon ^l, than at others more remote ; and since this 
 attraction acts in a contrary direction to that of the earth, the 
 waters of all parts from BF to Z must have their gravity or ten- 
 dency towards the centre, O, diminished, and as this tendency 
 to the centre is the least at the point Z, they will consequently 
 stand higher there than in any other part of the hemisphere. 
 Now on the opposite hemisphere, BFN, the moon’s attraction 
 is also unequally exerted, and the necessary consequence of that 
 unequal action produces tides. From B and F to N, the force 
 of the moon’s attraction is diminished, agreeably to the 
 general rule, as the square of the distance increases. At N it 
 is the least of all, and as the attraction of the waters to the 
 centre of the earth is also the least there, because in the hemi- 
 sphere BFN, the moon’s attraction in proportion to the near- 
 ness of the parts to B and F, favours the earth’s, the water 
 will be higher there than at any other part of the hemisphere. 
 Hence the moon raises the waters, both at that point of the 
 earth which is nearest to her, and at that which is furthest from 
 
ASTRONOMY. 
 
 591 
 
 Theory of tlie tides. 
 
 her at the same time. At any port or harbour, therefore, 
 which lies open to the ocean, the action of the moon will tend 
 to elevate the waters there, when she is on the meridian of that 
 place, whether she be on the arch of the meridian above, or on 
 that below the horizon. But the water cannot be raised at one 
 place, without flowing from and being depressed at another, 
 and these elevations and depressions will obviously be the 
 greatest at opposite points of the earth’s surface. When the 
 moon raises the waters at Z and N, they will be depressed at 
 B and F ; and when by the earth’s rotation they are raised by 
 her at B and F, they will be depressed at Z and N. And as 
 the moon passes over the meridian, and is in the horizon twice 
 every day, there must therefore, as we find by experience, be 
 two tides of flood, and two of ebb in that time, at the interval 
 of about six hours and twelve minutes. 
 
 The whole attractive force exerted by the sun on the earth, 
 is far superior to that of the^moon, but as his distance from 
 the earth is nearly 400 times greater, the intensity of the forces 
 with which he acts upon different parts of it, aie much nearer 
 to equality than those of the moon ; and as it is the difference 
 of intensity that is alone exerted in producing tides, his action 
 in this respect is inferior to that of the moon. Newton calcu- 
 lated the effect of the sun’s influence to be three times less than 
 that of the moon. The action of the sun is therefore sufficient 
 to produce a flux and reflux of the sea, and in fact there are 
 two tides, a solar one and a lunar one; which have a joint or 
 opposite effect, according to the situation of these bodies. At 
 fig. 2, plate IV, they conspire together, on the same side as the 
 earth, the new moon at A, and the sun at S, their centres being 
 nearly in the same line ; and they raise the water at Z and N 
 higher than either of them could do alone. When the moon 
 is full, as at B, directly opposite the sun, and on the opposite 
 side of the earth, it again acts in the same line as the sun, and 
 the effect produced on the ocean is nearly the same as before ; 
 and in both cases they occasion what are called spring tides. 
 Now, as shown by fig. 3, let us suppose the sun to be at S, 
 while the moon is at Q, in her first quarter. Here the sun and 
 
 moon tend to counteract each others eflect ; for the one would 
 
 raise the ocean highest where the other would make it least. It 
 the forces then were equal, there would be no tide, but as that 
 of the moon is strongest in the proportion of 3 to 1, the diffe- 
 rence between the forces is the moon’s effect, and the water is 
 
 therefore raised twice as high under the moon as under the sun. 
 When the moon has arrived at her third quarter at R, her 
 situation relatively to the sun is the same as when she was in 
 
592 
 
 ASTRONOMY. 
 
 Theory of the tides. 
 
 her first, her force and that of the sun act in lines perpendicular 
 to each other as before, and therefore the effect is correspondent. 
 
 The effects of the-siin and moon, on the waters of the ocean, 
 are sensibly varied by the different distances of those bodies, pro- 
 duced by the elliptical orbit of the earth round the sun, and of 
 that of the moon round the earth. Sir Isaac Newton shows that 
 their actions increase as the cubes of the distances decrease ; so 
 that the moon at half its mean distance would produce a tide 
 eight times greater than at present. Other variations likewise take 
 place in consequence of the different declinations of the sun and 
 moon at different times ; for if either of these luminaries were 
 at the pole, it would occasion a constant elevation at both poles, 
 and a constant depression at the equator. Hence by the decli- 
 nation or departure of these bodies either northward or south- 
 ward from the equator, their effect is lessened, and the tides are 
 less. The highest tides therefore occur when both the sun and 
 moon are in the equator, where the centrifugal force is the 
 greatest; and the moon is not only either new or full at the same 
 time, but also in her perigree, or at her least distance from the 
 earth. The effect would be still greater if the sun could be 
 nearest the earth at the same time that he is in the equator ; but 
 as this is impossible, because he is nearest to us in winter, when 
 be is southward of the equator, therefore the spring tides are 
 highest and the neap tides lowest about the time of the equinoxes 
 in jMarch and September, for at these two seasons the circum- 
 stances upon which the elevation of the waters depends, conspire 
 to produce the greatest effect. The highest tides happen a little 
 before the vernal equinox, when the sun is retiring from the earth ; 
 and a little after the autumnal equinox, when the sun is approach- 
 ing nearer to the earth. 
 
 From the foregoing theory it follows, that when the earth’s 
 axis inclines to the moon, the northern tides, if not retarded in 
 their passage through shoals and channels, nor affected by the 
 winds, ought to be greatest when the njoon is above the horizon, 
 and least when she is below it; and quite the reverse w'hen the 
 earth’s axis declines from her; but in those cases they return at 
 equal intervals of time. When the earth’s axis inclines side- 
 wise to the moon, both tides are equally high ; but they happen 
 at unequal intervals of time. In summer, the earth’s axis 
 inclines towards the moon when new ; and therefore the day- 
 tides ought to be highest, and night-tides lowest about the 
 change: at the full, the reverse. At the quarter, they ought to 
 be equally high, but unequal in their returns-; because the 
 earth’s axis then inclines sidewise to the moon. In winter the 
 phenomena are the same at full moon, as in summer at new. In 
 
ASTRONOMY. 
 
 593 
 
 Theory of the tides. 
 
 autumn, the earth’s axis inclines sideways to the moon, when 
 new and full ; therefore the tides ought to be equally high, and 
 unequal in their returns at these times. At the first quarter, the 
 tides of fioad should be least when the moon is above the hori- 
 zon, greatest when slie is below it ; and the reverse at her third 
 quarter. In spring, the phenomena of the first quarter answer 
 to those of the third quarter in autumn; and vice versa. The 
 nearer any time is to either of these seasons, the more the tides 
 partake of the phenomena of the nearest season; and in the middle 
 between them, the tides are at a mean state between those of 
 both. 
 
 The tides cannot have their full motion, unless the ocean to 
 which the moon is vertical, be of a uniform depth, and ex- 
 tended from east to west, over the whole globe. They are so 
 retarded in their passage through different shoals and chan- 
 nels, and otherwise so variously affected by striking against 
 capes and headlands, that to different places they happen at all 
 distances of the moon from the meridian ; consequently at all 
 hours of the lunar day. The tide propagated by the moon in 
 the German ocean, when she is three hours past the meridian, 
 takes twelve hours to come from thence to London bridge, 
 where it arrives by the time that a new tide is raised in the 
 ocean ; and therefore when the moon has north declination, and 
 we should expect the tide at London to be greatest when the moon 
 is above the horizon, we find it is least ; and the contrary when 
 she has south declination. At several places it is high water three 
 hours before the moon comes to the meridian; but the tide that 
 appears to precede the moon, is only the tide opposite to that 
 which was raised by her when she was nine hours past the oppo- 
 site meridian. 
 
 There are no tides in lakes, because they are generally so 
 .small, that when the moon is vertical she attracts every part of 
 them almost alike, and therefore no part of the water is raised 
 perceptibly higher than tha rest. She passes over them, likewise, 
 so quickly, that the equilibrium of the w ater can at most be dis- 
 turbed for a very transient space of time. The Mediterranean 
 and Baltic seas have very small elevations, because the inlets by 
 which they communicate with the ocean are so narrow', that they 
 cannot, in so short a time, receive or discharge enough to raise or 
 sink their surfaces sensibly. 
 
 Among the islands in the West Indies, the tides seldom 
 rise higher than twelve or fourteen inches, which appears 
 remarkable, because, from their proximity to the equator, these 
 islands are more under the influence of the moon than some 
 other places where the fluctuation is twenty or thirty times as 
 VoL, I.—25. 4G 
 
594 
 
 ASTRONOMY. 
 
 Theory of the tides. 
 
 great. But it must be observed, that as the Atlantic ocean turns’ 
 from the moon, from west to east, her influence on its water 
 must be from east to west ; this tide, like a prodigious wave, is 
 stopped by America, and reflected back as the moon passes 
 over the Pacific ocean. But in the same direction that the 
 moon drags the waters of the Atlantic, the trade winds con- 
 stantly blow' ; and so great is the power of wind, that tides 
 every-where are made greater or less according as the wind is 
 with them or against them. Now the gulph of Mexico is a 
 cavity between North and South America, into which the 
 winds and tides are perpetually pouring water ; so that the 
 first tide that ever flowed into it, may be said to be kept up in 
 it by this unremitting influx, and, of course, the tides cannot 
 rise and fall as in places less in the way of these causes. But 
 water raised above the general level, always endeavours to fall 
 back to that level ; the trade winds, in some measure, prevent 
 this, by blowing perpetually from the east. As water so accu- 
 mulated, cannot return in opposition to the trade winds, it 
 turns round the west end of Cuba, and meeting with the Baha- 
 ma Islands, is turned northward along the coast of America; 
 forming that remarkable stream, the strong current of the 
 Gulph of Florida. To show that this accumulation actually 
 takes place in the Gulph of Mexico, a survey was made across 
 the isthmus of Darien, by which it was found that the water on 
 the Atlantic side was fourteen feet higher than the water on the 
 Pacific side. 
 
 From the lightness of the air, its freedom of motion, and 
 its greater nearness to the moon than the ocean, it may be sup- 
 posed that there are aerial tides of great elevation ; but it seems 
 to contradict this supposition, when we find that there are no 
 changes of the barometer indicating this to be extensively the 
 case. A little consideration, however, will show, that such a 
 change of the barometer is not to be expected. By the preced- 
 ing theory of the tides, it appears that water is higher immedi- 
 ately under the moon, and at the same time to the antipodes of 
 the point immediately under her, that is, at those two places on 
 the earth where her attraction is greatest and least. Now though 
 the water is higher at these two points than elsewhere, yet it 
 does not follow that the water, from its increased quantity, 
 presses with a proportionately greater weight on the bottom of 
 the ocean than before it was thus raised up, because the moon’s 
 attraction has diminished that tendency to the centre of the 
 earth which constitutes the weight of bodies. Hence we may 
 suppose, that though the moon draws up the atmosphere to a 
 great height, yet a higher column of mercury is not supported 
 
ASTROX05IY. 
 
 595 
 
 Harvest moon. and hunter’s moon. 
 
 through this circumstance, because the actual weight of the atmo- 
 sphere remains but little affected. 
 
 Of the Harvest MooUy and Hiuitei 's Moon, 
 
 When the earth, in turning on its axis from west to east, 
 has revolved from one meridian to the same meridian again, the 
 moon, which also revolves from west to east, has advanced 
 over little more than a thirtieth part of her orbit, or 12° and 
 some minutes ; hence the tides do not occur till after the earth 
 has overtaken her, and therefore are generally observed and 
 supposed to be about 50 minutes later every day. This is not, 
 liowever, constantly the case, except at or near the equator ; for 
 on account of the different angles made by the horizon and 
 different parts of the moon’s orbit, this retardation differs consi- 
 derably in places of high latitude. Twice in the year she rises 
 for a week together nearly at the same time, and these pheno- 
 mena happening successively in autumn, the earlier is called the 
 harvest moony and the later the hunter^s moon. We shall 
 chietly be indebted to Ferguson for the explanation of these 
 problems. 
 
 The plane of the equinoctial is perpendicular to the earth’s 
 axis ; and therefore as the earth turns round its axis, all parts 
 of the equinoctial make equal angles with the horizon, both at 
 rising and at setting ; so that equal portions of it always rise or 
 «et in equal times. Consequently, if the moon’s motion were 
 equable, and in the equinoctial, at the rate of 12° 1 1' from the 
 sun every day, as it is in her orbit, she would rise and set 50 
 minutes later every day than on the preceding day; for 12° 11' 
 of the equinoctial rise or set in 50' of time in all latitudes. But 
 the moon’s orbit is so nearly in the same plane as the ecliptic, 
 that we may consider her at present as moving in the ecliptic, 
 Now the different parts of the ecliptic, on account of its obli- 
 quity to the earth’s axis, make very different angles with the 
 horizon as they rise or set. Those parts or signs which rise 
 with the smallest angles, set with the greatest, and vice versa. 
 In equal times, whenever this angle is least, a greater portion 
 of the ecliptic rises than when the angle is larger, as may be 
 easily perceived by referring to a globe. Thus in fig. 4 and 5, 
 pi. IV, L represents the latitude of London, AB the horizon 
 to that place, FP the axis of the world, E e the equator K k 
 the ecliptic. On account of the oblique position of the sphere 
 in the latitude of London, the ecliptic has a high elevation 
 above the horizon, making the angle AVK, in fig. 4, about 
 ^2^ degrees, wlien the sign of Cancer is upon the meridian, at 
 
596 
 
 ASTRONOMY. 
 
 Harvest moon and hunter’s moon. 
 
 which time Libra rises in the east. But when the other part of 
 the ecKptic is above the horizon, that is, when the sign of Ca- 
 pricorn is upon the meridian, and Aries rises in the east, then 
 the ecliptic will make with the horizon the much smaller angle 
 k VA, as represented by fig. 5, which angle is only about 15 de- 
 grees, that is, 47| degrees smaller than the former angle. Thus 
 it may be conceived, that as the celestial sphere appears to turn 
 round the axis FP, a greater portion of the ecliptic will rise in a 
 given portion of time, as for instance, three or four hours, when 
 the ecliptic is in the situation of fig. 5, than when it is in the situa- 
 tion of fig. 4. 
 
 In northern latitudes, the smallest angle made by the eclip- 
 tic and horizon, is w hen Aries rises, at which time Libra sets ; 
 the greatest when Libra rises, at which time Aries sets. From 
 the rising of Aries, to the rising of Libra (which is twelve side- 
 real hours) the angle increases ; and from the rising of Libra to 
 the rising of Aries, it decreases in the same proportion ; hence it 
 appears that the ecliptic rises fastest about Aries, and slowest 
 about Libra. 
 
 On the parallel of London, as much of the ecliptic rises 
 about Pisces and Aries in two hours, as the moon goes through 
 in six days ; therefore, w hilst the moon is in these signs, she 
 differs but two hours in rising for six days together ; that is, 
 about twenty minutes later every day or night than on ,the pre- 
 ceding, at a mean rate. But in 14 days afterwards, the moon 
 comes to Virgo and Libra, which are the opposite signs to 
 Pisces and Aries ; and then she dift'ers almost four times as much 
 in rising; namely, one hour and about 15' later every day or 
 night than the preceding, whilst she is in these signs. As the 
 signs Taurus, Gemini, Cancer, Leo, Virgo, and Libra, rise suc- 
 cessively, the angle of the ecliptic with the horizon increases gra- 
 dually ; and decreases in the same proportion as they set ; and for 
 that reason, the moon dift'ers gradually more in the time of her 
 rising every day whilst’ she is in these signs, and less in her setting : 
 after which, through the other six signs, viz. Scorpio, Sagittary, 
 Capricorn, Aquarius, Pisces, and Aries, the rising dift’erence be- 
 comes less every day, until it be at the least of all, namely, in 
 Pisces and Aries. 
 
 The moon goes round the ecliptic in about 27 days and 8 
 liours ; but not from change to change in less than about 294- 
 days : so that she is in Pisces and Aries at least once in every 
 lunation, and in some lunations twice. 
 
 If the earth had no annual motion, the sun would never 
 appear to shift his place in the ecliptic ; and then every new’ 
 moon would fall in tho; same sign and degree of the ecliptic^ 
 
 8 
 
ASTRONOMY. 
 
 597 
 
 Harvest moon and hunter’s moon. 
 
 and every full moon in the opposite; for the moon would go 
 precisely round the ecliptic from change to change. So that if 
 the moon was once full in Pisces or Aries, she would always be 
 full when she came round to the same sign and degree again. 
 And as the full moon rises at sun-set, (because when any point 
 of the ecliptic sets, the opposite point rises,) she would con- 
 stantly rise within two hours of sun-set, on the parallel of 
 London, during the week in which she was full. But in the 
 lime that the moon goes round the ecliptic from any conjunctio?i 
 or opposition, the earth goes almost a sign forw ard ; and there- 
 fore the sun will seem to go as far forward in that time, 
 namely 271°; so that the moon must go 271° tiiote than round, 
 and as much farther as the sun advances in that interval, which 
 is 2^y°, before she can be in conjunction with, or opposite to, 
 the sun again. Hence it is evident, that there can be but one 
 conjunction or opposition of tbe sun and moon in a year in any 
 particular part of the ecliptic. This may be familiarly exem- 
 plified by the hour and minute hands of a watch, which in 
 twelve hours are never in conjunction or opposition in that part 
 of the dial-plate where they were so last before. 
 
 As the moon can never be full but when she is opposite to 
 the sun, and the sun is never in Virgo and Libra but in our 
 autumnal months, it is plain that the moon is never full in the 
 opposite signs, Pisces and Aries, but in these two months ; and 
 therefore we can have only two full moons in the year, which 
 rise nearly at the time of sun-set for a week together, as has been 
 mentioned above. 
 
 When the moon is in Pisces and Aries, she must rise with 
 nearly the same difference of time in every revolution through 
 her orbit, which is exactly the phenomenon of the harvest 
 moon ; but it passes unobserved, because in w inter tliose signs 
 rise at noon, and, being then only a quarter of a circle distant 
 from the sun, the moon in them is in her first quarter, and 
 rises about noon, at which time her rising is not noticed. In 
 spring those signs rise with the sun, for the sun is in them, 
 consequently the moon being in them too, is in conjunction 
 with the sun, and therefore her rising is invisible. In summer, 
 those signs rise about midnight ; and the sun is three signs, or 
 about 90° before them ; therefore the moon in them must be in 
 her third quarter, when she gives little light, and rises late, on 
 which accounts, the phenomenon of her rising for some nights 
 with little difference of time, passes unnoticed. In autumn, 
 however, the case is different ; for the signs of Pisces and Aries 
 then rise about sun-set, and therefore the moon being in 
 them, is in opposition to the sun, consequently full, and rises 
 
o96 
 
 ASTROXO.MV. 
 
 Harvest moon and banter’s moon. 
 
 .in great splendour when the sun sets, and seems to prolong the 
 day, tor the advantage of the husbandman at the time of 
 harvest. 
 
 In northern latitudes, the autumnal full moons are in Pisces 
 and Aries; and the vernal full moons in Virgo and Libra. In 
 southern latitudes, just the reverse, because the seasons are con- 
 trary. But Virgo and Libra rise at as small angles with the hori- 
 zon in southern latitudes, as Pisces and Aries do in the northern ; 
 and therefore the harvest moons are just as regular on one side of 
 the equator as on the other. 
 
 As those signs, which rise with the least angle, set with the 
 greatest, the vernal full moons diifer as much in their times of rising 
 every night, as the autumnal full moons differ in their times of 
 setting; and set Mith as little difference as the autumnal full moons 
 rise ; the one being in all cases the reverse of the other. 
 
 Hitherto, to avoid the complication of the subject, the 
 moon’s orbit has been supposed to coincide with the ecliptic ; but 
 since her orbit makes an angle with the ecliptic ; varying from 5° 
 to 5° 18', one half of it is on one side of the ecliptic, and the 
 other on the other side, and she only coincides with it when at 
 the points of intersection, called her nodes, which coincidence 
 cannot happen less than twice, but sometimes happens thrice, 
 between change and change. For as the moon goes almost a 
 whole sign more than round her orbit from change to change, if 
 she passes through either node at or a little before the time of 
 change, she will pass by the other about fourteen days after- 
 wards, and come round to the former node before the next 
 change. When the moon is northward of the ecliptic, she 
 rises sooner and sets later than if she moved in the ecliptic : and 
 when she is southw’ard of the ecliptic, she rises later and sets 
 sooner. This difference is variable, even in the same signs, 
 because the nodes shift backwards about 19 |° in the ecliptic 
 every year ; and so go round it contrary to the order of the 
 signs in 18 years 228 days. 
 
 When the ascending node is in Aries, the southern half of 
 the moon’s orbit makes an angle of less with the horizon 
 than the ecliptic does, w hen xVries rises in northern latitudes : 
 for which reason the moon rises with less difference of time when 
 she is in Pisces and Aries, than if she moved in the ecliptic, 
 and will differ only 1 hour and 40' for the whole of seven days. 
 Butin 9 years 114 days afterwards, the descending node comes 
 to Aries, and then the moon’s orbit makes an angle of 5j° 
 greater with the horizon when Aries rises, than the ecliptic 
 does at that time ; which causes the moon to rise with greater 
 difference of time in Pisces and Aries than if she moved in the 
 
ASTRONOMY. 59.9 
 
 Harvest moon and hunter’s moon.— Less moonlight in summer than winter. 
 
 ecliptic ; this difference, in the course of a week, will amount to 
 full 3| hours. Hence, though we observe tlie phenomenon of the 
 harvest moon every year, yet it is not every year equally remark- 
 able, but alternately for a period of nearly nine years and a half, it 
 is greatest and least ; from 1813 to 1813 is the remainder of a 
 period during which the harvest moon varies most in the time 
 of rising; from 1816 to 1825 includes a period during which the 
 difference is the least. 
 
 At the polar circles, when the sun touches the summer 
 tropic, he continues 24 hours above the horizon ; and 24 hours 
 below it when he touches the winter tropic. For the same rea- 
 son, the full moon neither rises in summer, nor sets in winter, con- 
 sidering her as moving in the ecliptic. For the winter fidl moon 
 being as high in the ecliptic as the summer sun, must therefore 
 continue as long above the horizon ; and the summer full moon 
 being as low in the ecliptic as the winter sun, can no more rise 
 than he does. But these are only the two full moons which hap- 
 pen about the tropics; for all the others rise and set. In summer, 
 the full moons are low, and their stay is short above the horizon, 
 W'hen the nights are short, and we have least occasion for moon- 
 light : in winter the full moons rise high, and stay long above the 
 horizon, when the nights are long, and we want the greatest quan- 
 tity of moonlight. 
 
 At the poles, one half of the ecliptic never sets, and the other 
 half never rises : and therefore, as the sun is always half a year in 
 describing one half of the ecliptic, it is natural to imagine that 
 the sun continues half a year together above the horizon of each 
 pole in its turn, and as long below it ; rising to one pole when he 
 sets to the other. This would be exactly llie case if there were 
 no refraction ; but by the atmosphere’s refracting the sun’s rays, 
 he becomes visible some days sooner, and continues some days 
 longer in sight, than he would otherwise do : so that he appears 
 above the horizon of either pole before he has got below the 
 horizon of the other. And, as he never goes more than 23^° 
 below the horizon of the pc^es, they have very little dark night ; 
 it being twilight there, as well as at other places, till the sun is 
 18° below the horizon. The full moon being always opposite to 
 the sun, can never be seen while the sun is above the horizon, 
 except when she is in the northern half of her orbit; for whenever 
 any point of the ecliptic rises, the opposite point sets. There- 
 fore, as the sun is above the horizon of the north pole, from the 
 20th of March till the 23rd of September, it is plain that tlie 
 moon, when full, being opposite to the sun, must be below tlie 
 horizon that half of the year. But w hen the sun is in the southern 
 half of the ecliptic, he never rises to the norUi pole ; during which 
 
600 
 
 ASTRONOMY. 
 
 Horizontal moon. 
 
 half of the year, every full moon happens in some part of the 
 northern half of the ecliptic which never sets. Consequently, as 
 the polar inhabitants never see*the full moon in summer, they have 
 her always in the winter, before, at, and after the full, shining 
 for fourteen of our days and nights. Thus the poles are sup- 
 plied, during one half the winter, with constant moonlight in the 
 sun’s absence ; and only lose sight of the moon from her third 
 to her first quarter, while she gives but little light, and would be 
 of the least benefit. 
 
 The Horizontal Moon, 
 
 When the moon is near the horizon, she appears of a shape 
 somewhat elliptical, and larger than when she is in the zenith, 
 or on tlie meridian. This phenomenon is called the horizontal 
 moon. 
 
 The longest axis of the elliptical disc of the horizontal 
 moon is parallel wdth the , horizon, but the circular shape is 
 gradually attained, as the moon rises higher. To account for 
 these appearances has greatly exercised the ingenuity of several 
 philosophers, though no solution of the problem has yet been 
 offered, which has received general assent. As the moon when 
 in the horizon is further from us by the semi-diameter of the earth 
 than when in the zenith, the real angle it subtends must at that 
 time be least; but the difference is so small, amounting only to 
 half a minute of a degree, that it would scarcely be noticed by 
 the sharpest eye. Gassendus thought, as the moon w'as less 
 bright in the horizon than in the meridian, we looked at it, in 
 the former situation, with a greater pupil of the eye, and there- 
 fore it appeared larger. An explication of this kind has been 
 lately offered, and experiments are given which appear to 
 show that a difference of aperture will make a difference in the 
 magnitude of the image of a lens; but more careful experiments 
 show this conclusion, so opposite to the principles of optics, to 
 be erroneous, and therefore that the variations in the aperture 
 of the pupil of the eye do not occasion variations in the 
 magnitude of the image on the retina. Some have supposed 
 that the image of the horizontal moon on the retina, is not 
 actually larger than at other times, but that we observe such an 
 extent of intermediate objects in that situation, as to give it a 
 greater apparent distance than at other times, and consequently 
 we consider it to be larger, although it subtends only the same 
 angle ; the dimness with which it appears, from the fewness of 
 the rays which reach us, contributing at the same time to the 
 deception. The circumstance of faintness, does not however, 
 
ASTRONOMY. 
 
 601 
 
 Horizontal moon. 
 
 seem essential to the phenomenon, otherwise, the moon on the 
 meridian would appear as large as w'hen at the horizon, or nearly 
 so, while it happens to be obscured by clouds ; and it may also 
 be remarked, that the lower part of a rainbow appears broader 
 than the upper part, at the same time that it often appears brighter, 
 and the breadth of the moon and of the rainbow in this case are 
 doubtless collateral phenomena. It is evident to the slightest con- 
 sideration, that the rays of light from the moon, pass through 
 the greatest tract of air, when the moon is in the horizon ; and 
 through the least when that luminary is in the zenith; and at all 
 intermediate distances, the quantity of air passed through is of an 
 intermediate extent, and proportionate to the elevation. This is 
 shown by fig. 6, pi. IV, where AB represents an arc of the top 
 of the atmosphere, and CD an arc of the surface of the earth. 
 \\ hen the moon is at H, she is seen by a spectator at E, through 
 the tracty’g E, w’hich is about twice as long as the tract traversed 
 when she has arrived at the altitude I, and three times the extent 
 of that passed through when she is in the zenith K. May we not 
 then suppose, that the difference in the refraction, produced by 
 the difference in the length of the tract of air passed through by 
 the rays from the moon, is the chief cause of the apparent enlarge- 
 ment of the lunar disc, particularly as we find by experiment, that 
 an object appears larger in proportion as it is viewed at a greater 
 depth in w ater, and that, though a thin piece of flat glass has no 
 perceptible magnifying power, yet a thick piece certainly has, and 
 ought to have, according to the principles of optics. The varia- 
 tions of the density of the vapours through which the rays pass, 
 account for the variations of the horizontal moon, which at equal 
 altitudes sometimes appear larger than at other times. With re- 
 spect to the elliptical figure of the moon, in the situation men- 
 tioned, it must be observed, that the rays from the extremities of 
 its horizontal diameter, strike the convex surface of the atmosphere, 
 and come to the eye with an equal angle, but this is not the case 
 with the rays in the direction of the 'vertical diameter, and there- 
 fore a distortion takes place. A watch-glass fastened upon the 
 surface of a piece of plain thin glass, and the inclosed space filled 
 with water, will illustrate this account of the horizontal moon ; for 
 if a small circle be made on a slip of paper, and the circle be 
 held upright nearly on a level with the upper surface, while it is 
 viewed tlirough the under surface of the plain glass, and at the 
 same time through at least half the diameter of the water, tlie cir- 
 cle will appear elliptical, enlarged in its horizontal and lessened in 
 its vertical diameter ; but the distortion w ill diminish in proportion 
 as it is placed and viewed in a zenith direction. 
 
 The sun and stars also appear larger in the horizon, and any 
 
 26. — VoL. I. 4 H 
 
602 
 
 ASTRONOMY. 
 
 Aberration of light. 
 
 two stars appear further apart, than at a greater elevation. The 
 causes of these appearances must be considered the same as that 
 producing the horizontal moon, whether the above explication of 
 this appearance be accurate or not. 
 
 Aberration of light. 
 
 The fixed stars, and other heavenly bodies, are not seen in 
 their real directions. This circumstance is independent of the 
 refractive power of our atmosphere, and is always present, whe- 
 ther we observe them in the zenith, or any other situation ; it is a 
 deviation which arises from the progressive motion of^ light, and 
 the annual motion of the earth ; for the light by which a star is 
 seen, at the time it sets out from the star, is not directed to the 
 spectator, but to a point beyond him, at which, or into the same 
 line with which, his eye will arrive, by the progression of the 
 earth, m exactly the same time that the light requires to reach the 
 earth. This plienomenon is called the aberration of Ught, or 
 aberration of the fxed stars. 
 
 The aberration of the fixed stars was discovered by Dr, 
 Bradley, in his attempts to ascertain their annual parallax, and 
 he explained it in the following manner: He imagined CA, 
 llg- 7, pi. IV, to be a ray of light falling perpendicularly upon 
 the line BD-; that, if the eye is at rest at A, the object must 
 appear in the direction AC, whether light be propagated in time 
 or in an instant. But if the eye is moving from B towards A, 
 and light is propagated in time, with a velocity that is to the 
 velocity of the eye, as CA to BA ; then light moving from C to 
 A, whilst the eye moves from B to A, that particle of it by 
 which the object will be discerned when the eye comes to it, 
 is at C when the eye is at B. Joining the points BC, he 
 supposed the line CB to be a tube, inclined to the line BD in 
 the angle DBC, of such diameter as to admit but one particle of 
 light. Then it was easy to conceive, that the particle of light at 
 C, by which the object must be seen, when the eye, as it moves 
 along, arrives at A, would pass through the tube BC, if it is 
 inclined to BD in the angle DBC, and accompanies the eye in 
 its motion from B to A ; and that it could not come to the eye 
 placed beliind such a tube, if it had any other inclination to the 
 line BD. If, instead of supposing CB so small a tube, we 
 imagine it to be the axis of a larger, then, for the same reason, 
 the particle of light at C would not pass through the axis, unless 
 it is inclined to BD in the angle CBD. In like manner, if the 
 eye moved the contrary way, from D towards A, with the same 
 velocity, then the tube must be inclined in the angle BDC. 
 
ASTRONOMY. 
 
 603 
 
 Aberration of light* 
 
 Although, therefore, the true or real place of an object is 
 perpendicular to the line in which the eye is moving, yet the 
 visible place will not be so ; since that no doubt must be in the 
 direction of the tube ; but the difference between the true and 
 apparent place will be greater or less, according to the different 
 proportion between the velocity of light and that of the eye ; 
 so that if we could suppose that light was propagated in an 
 instant, then there would be no difference between the real and 
 visible place of an object, although the eye was in motion ; for 
 in that case, AC being infinite with respect to AB, the angle AC B, 
 the difference between the true and visible place, vanishes. But 
 if light be propagated in time, it is evident from the foregoing con- 
 siderations, that there will be always a difference between the real 
 and visible place of an object, unless the eye is moving either di- 
 rectly from or to the object. And in all cases, the sine of the dif- 
 ference between the real and visible place of the object, will be 
 to the sine of the visible inclination of the object to the line in 
 which the eye is moving, as the velocity of the eye is to the velo- 
 city of light. 
 
 Dr. Bradley then shows, that if the earth revolve round the 
 sun annually, and the velocity of light be to the velocity of the 
 earth’s motion in its orbit, as 1000 to 1, that a star really placed 
 in the very pole of the ecliptic, would, to an eye carried along 
 with the earth, seem to change its place continually; and neg- 
 lecting the small difference arising from the earth’s diurnal revolu- 
 tion on its axis, would seem to describe a circle round that pole, 
 every way distant from it 3 ^' ; so that its longitude would be varied 
 through all the points of the ecliptic every year, but its latitude 
 would always remain the same. Its right ascension would also 
 change, and its declination, according to the different situation of 
 the sun with respect to the equinoctial points, and its apparent 
 distance from the north pole of the equator, would be 7' less at 
 the autumnal than the vernal equinox. 
 
 The greatest alteration of the place of a star in the pole 
 of the ecliptic, or which, in effect amounts to the same thing, 
 the proportion between the velocity of light and the earth’s 
 motion in its orbit being knowm, it will not be difficidt, he ob- 
 serves, to find what would be the difference, upon this account, 
 between the true and apparent place of any other star at any 
 time, and on the contrary, the difference between the true and 
 apparent place being given, the proportion between the velocity 
 of light, and the earth’s motion in its orbit, may be found. 
 —Therefore, since the apparent declination of the star y 
 Draconis, on account of the successive propagation of light, 
 would be to the diameter of the little circle which a star w’ould 
 
ao4 
 
 ASTRONOMY. 
 
 Aberration of light. 
 
 seem to describe about the pole of the ecliptic, as 39' to 40.4'; 
 the half of this is the angle ACB. I'his, therefore, being 20.^", 
 AC will be to AB, that is, the velocity of light will be to the ve- 
 locity of the eye, (or the velocity of the earth in its annual motion) 
 as 10,210 to 1 ; from which it follows, that light moves as far as 
 from the sun to the earth in 8' 12". By the near agreement of 
 multiplied observations, Dr. Bradley proved this statement to be 
 very near the truth. 
 
 The aberration of the sun and planets is inconsiderable, because 
 while the light is coming from them, the earth has moved but a 
 little way ; yet for accurate calculations, it is always taken into 
 account ; the sun’s aberration is about 20", that being the space 
 passed over by the earth while his light is coming to us. 
 
 Dr. Bradley’s explanation of the aberration of the lixed stars, 
 was no sooner proposed than acceded to by all philosophers, and 
 has never since been questioned. The phenomenon is a strong 
 proof of the annual motion of the earth round the sun, and as the 
 discovery of it so justly gives celebrity to this astronomer’s name, 
 the incident which enabled him to explain it so well, and which 
 is given by Dr. Thompson, in his History of the Royal Society, 
 deserves our notice. It shows how much we may improve by 
 habits of observation, and that the most common occurrences may 
 frequently be made subservient to the illustration of the most ab- 
 struse points of philosophy. When Dr. Bradley had discovered 
 what is now called the aberration of the fixed stars, and had even 
 completed a whole year’s observations, he perplexed himself con- 
 tinually without success to find out the cause of the phenomenon. 
 At last, a satisfactory explanation of it at once occurred to him, 
 when he w'as not in search of it. He accompanied a pleasure- 
 party in a sail upon the river Thames. The boat in which they 
 were, was provided with a mast, that had a vane at the top of it. 
 It blew a moderate wind, and the party sailed up and dowai the 
 river for a considerable time. Dr. Bradley remarked, that every 
 time the boat put about, the vane at the top of the boat’s mast 
 shifted a little, as if there had been a slight change in the direction 
 of the wind. He observed this three or four times without speak- 
 ing ; at last he mentioned it to the sailors, and expressed his sur- 
 prise that the wind should shift so regularly every time they put 
 about. The sailors told him the wind had not shifted, but that 
 the apparent change was owing to the change in the direction of 
 the boat, and assured him that the same thing invariably happened 
 in all cases. This accidental observation, led him justly to con- 
 clude, that the aberration of the stars was owing to the combined 
 motion of light and the earth. 
 
A.STTiOJVOMY. 
 
 i 
 
 rublijliU by Snttalt.JLisbcr.Jh.x'cn ktUbscn.r/n-erpool. ,ln^ 
 
ASTRONOMY. 
 
 605 
 
 Effects of the spheroidal figure of the earth. 
 
 Nutation of the Earth’s Axis. 
 
 The discovery of the aberration of the fixed stars, was 
 
 followed up by Dr. Bradley with an almost incessant assiduity 
 of observation. All his subsequent investigations afforded con- 
 firmations of what he had advanced on the subject ; and his 
 
 perseverance was further rew^arded, in his own opinion, no 
 
 doubt, and certainly in that of all other philosophers, by 
 
 another brilliant discovery, which is called the nutation of the 
 earth’s axis. This is a result and a proof of the spheroidal 
 figure of the earth, The moon, exerting a greater power of 
 attraction on the equatorial regions of the earth, than on the 
 polar regions, because the mass of matter at the equator is greater 
 than at the poles, causes a libratory motion of the earth’s axis, 
 the inclination of which to the ecliptic is, though in a very small 
 degree, continually varying backwards and forwards, so that 
 its extremity describes a small ellipse, the greater diameter of 
 which is about IQ.l" and the less 14.2". This ellipse is described 
 in the same time as a cycle of the moon, or about 18 years and 
 7 months. 
 
 Precession of the Equinoxes. 
 
 When we examine a celestial globe, we find that all the 
 symbols of the constellations are 30 degrees out of what we 
 should consider their true place, or from the constellations 
 themselves. This apparent irregularity has attained its present 
 amount, by the very slow accumulation of 50" annually. The 
 reason of it is, that the sun does not cut the equator at the 
 same point every year. If on a certain day, he cuts the equa- 
 tor in a particular point, he will on the same day of the following 
 year cut it in a point 50.25" w'estw'ard of the former, and 
 thereby comes to the equinox 20' 23" of time before he has 
 completed his circuit of the heavens, or gone from fixed star to 
 fixed star. Tiiis phenomenon is called the precession of the 
 equinoxes. It makes the tropical year, or true year of the 
 seasons, shorter than the revolution of the sun, or sidereal year, 
 because it is a deviation contrary to the order of the signs. 
 But the intersections of the equator and the ecliptic are still 
 called the beginning of beginning of =2:, though 
 
 the constellations (being always at the same place) are actually 
 at the distance above stated, from those intersections. ^ The 
 precession of the equinoxes, like the nutation of the earth s axis, 
 is produced by the protuberance of the equator; for when the 
 
606 
 
 ASTRONOMY. 
 
 Precession of the equinoxes. 
 
 sun is on either side of the equator, his return to it is hastened by 
 his attraction for the greater quantity of matter in that direction. 
 Receding at the rate of 50.25" westward annually, the equinoxes 
 make an entire revolution in 25,791 years, when the variation 
 commences another period. In consequence of it, those stars 
 which in the infancy of astronomy were in Aries, are now got into 
 Taurus; those of Taurus into Gemini, &c. Hence, likewise, it 
 is, that the stars which rose at any particular season, in the times 
 of the ancients, by no means answer at this time to the description 
 then given of them. 
 
 In consequence of tire precession, or retrograde motion of the 
 equinoctial points in the heavens, the earth’s axis does not pre- 
 serve an undeviating parallelism, but, with a conical motion de- 
 scribes a small circle, the diameter of which is equal to twice its 
 inclination to the ecliptic, or 47° • This motion may be ex- 
 
 plained by a diagram : let NZSVL, (fig. 1, pi. V,) be the earth; 
 SONA its axis, produced to the starry heavens, and terminating 
 in A, the present north pole of the heavens, which is vertical 
 to N, the north pole of the earth. Let EOQ be the equator,. 
 T 25 Z the tropic of Cancer, and VT VJ the tropic of Capricorn ; 
 VOZ the ecliptic, and BO its axis, which must be considered 
 immoveable, because the ecliptic always passes over the same 
 stars. But as the equinoctial points recede in the ecliptic, the 
 earth’s axis, SON, is in motion upon the earth’s centre O, in 
 such a manner as to describe the double cone NO w and SO s, 
 round the axis of the ecliptic BO, in the time that the equinoc- 
 tial points move round the ecliptic, that is, in 25,791 years; and 
 in that length of time, the north pole of the earth’s axis pro- 
 duced, describes the circle ABCDA in the starry heavens, 
 round the pole of the ecliptic, which keeps immoveable in the 
 centre of that circle. The earth’s axis being nearly 23|° 
 inclined to the axis of the ecliptic, the circle ABCDA, described 
 by the north pole of the earth’s axis produced to A, will there- 
 fore be almost 47° in diameter, or double the inclination of the 
 earth’s axis. In consequence of this, the point A, which is at 
 present the north pole of the heavens, and near to a star of the 
 second magnitude, in the end of the Little Bear’s tail, must be 
 deserted by the earth’s axis ; which, moving back one degree in 
 every 7 If- years nearly, will be directed tow^ards the star or 
 point B in 6447|r years hence ; and in double that time, or 
 12 , 8954 ^ years, it will be directed towards the star or point C ; 
 which will then be the north pole of the heavens, although it is 
 at present 84:° south of the zenith of London L. The present 
 position of the equator, EOQ will then be changed into e O q ; 
 the tropic of Cancer, T 25, into V^25; and the tropic of 
 
 7 
 
ASTRONOMY. 
 
 607 
 
 Ellipses of the moon. 
 
 Capricorn, N T into t Zj j and the sun, in the same part 
 of the heavens where he is now over the earthly tropic of 
 Capricorn, and makes the shortest days and longest nights in the 
 northern hemisphere, will then be over the earthly tropic of Cancer, 
 and make the days longest and nights shortest. So that it will 
 require 12,893^ years yet more, or 25,791 years from that time, to 
 bring the north pole N quite round, so as to be directed towards 
 that point of the heavens which is vertical to it at present. And 
 then, but not before, the same stars which at present describe the 
 equator, tropics, polar circles, &c. by the earth’s diurnal motion, 
 will describe them over again. 
 
 Of Eclipses. 
 
 The words, transit, occultation, and eclipse, are all employed 
 to designate the same general phenomenon, of one heavenly body 
 being lost sight of, either wholly or in part, by the interposition of 
 another; but they are not used indiscriminately. The word transit 
 denotes the passage of the inferior planets, Venus and Mercury, 
 over the sun’s disc ; occultation the disappearance of the stars or 
 planets by the interposition of the moon; and eclipse, 1st, to the 
 obscuration of the moon, when this luminary falls into the earth’s 
 shadowy called an eclipse of the moon ; 2nd, to the obscuration of 
 the siin, when the earth falls into the moon’s shadow, called an 
 eclipse of the sun; and 3d, to the obscuration of the satellites of 
 any of the planets, by their coming w ithin the shadows of their 
 respective primaries. One of the two first of these three pheno- 
 mena is always meant, when the word eclipse is used alone. 
 
 Eclipses, like comets, were formerly the objects of popular 
 dread, but the age of these vain terrors is now gone by, because it 
 has become a part of popular belief, that they happen according 
 to the common course of nature, which belief is enforced by the 
 notoriety of the fact, that they may be predicted with precision, 
 like the alternations of day and night. 
 
 Eclipses of the Moon. 
 
 As the earth, like all the rest of the celestial bodies, is globu- 
 lar, the sun can only enlighten one half of it at once ; and the 
 hemisphere turned from the sun, will cause a shadow in the re- 
 gions of space, on account of the light intercepted by its opacity. 
 If the sun and earth were of the same size, this shadow’ would be 
 almost cylindrical, and of infinite extent ; it would not be perfectly 
 cylindrical, because the rays proceed from the sun divergently ; 
 but as the earth is very considerably less than the sun, the sha- 
 
608 
 
 ASTRONOMY. 
 
 Eclipses of the moon. 
 
 dovv is proportionately conical, and though sufficiently long to 
 reach and envelope the moon, it does not reach to the planet Mars. 
 In plate V, fig. 2, S is the sun, E the earth, and M the moon. 
 The dark conical shadow terminates aty’ just where rays from the 
 upper and lower limbs of the sun, after passing the contour of the 
 earth, meet in a point : on the sides of this conical shadow, there 
 is a diverging shadow abed, the density of which decreases in 
 proportion as it recedes from the sides of the former conical sha- 
 dow ; this is called the penumbra. 
 
 When the moon passes entirely through the earth’s shadow, 
 the eclipse total; but when only, a part of it passes through 
 the shadow, the eclipse is partial. 
 
 The quantity of the moon’s disc which is eclipsed, is 
 expressed by twelve parts, called digits; that is, the disc is 
 supposed to be divided by twelve parallel lines ; then if half the 
 disc is eclipsed, the quantity of the eclipse is said to be six 
 digits, and so of other proportions. 
 
 When the diameter of the shadow through which the moon 
 must pass, is greater than the diameter of the moon, the quantity 
 of the eclipse is said to be more than twelve digits ; thus, if the 
 diameter of the moon is to that of the shadow as four to five, then 
 the eclipse is said to be 15 digits. 
 
 The general phenomena of lunar eclipses may be classed as 
 follows: — 1. All lunar eclipses are universal, or visible in all 
 parts of the earth which have the moon above the horizon ; and 
 are every .where of the same magnitude, with the same begin- 
 ning and end. — 2. In all lunar eclipses, the eastern side is that 
 which first immerges and emerges again; that is, the leftside of 
 the moon as we look toward her from the north ; for the proper 
 motion of the moon being swifter than that of the earth’s 
 shadow^, the moon approaches it from the west, overtakes it, and 
 passes through it, with her east side foremost, leaving the 
 shadow behind, or to the westw'ard. — 3. Total eclipses, and 
 those of the longest duration, happen in the very nodes of the 
 ecliptic, because the section of the earth’s shadow then falling 
 on the moon, is considerably larger than her disc, and the 
 eclipse is more than 12 digits. There may, however, be total 
 eclipses within a small distance of the nodes; but their dura- 
 tion is the less as they are farther from them, till they become 
 only partial ones, and at last the moon escapes the shadow^ — 
 4. The moon, even in the middle of a total eclipse, is not 
 invisible, when the sky is clear, but has an appearance resem- 
 bling tarnished copper; this appearance is attributed to the 
 rays refracted by the earth’s atmosphere into the shadow, of 
 which it consequently diminishes the intensity. — 5. The moon 
 
ASTRONOPTY. 
 
 609 
 
 Eclipses of the moon. — Of the sun. 
 
 becomes sensibly paler and dimmer, before entering into the 
 real shadow ; this is owing to the penumbra. 
 
 If the moon’s orbit were in the same plane as the ecliptic, 
 or plane of the earth’s orbit, site would go through the middle 
 of the earth’s shadow, and sufl'er a total eclipse at every full, 
 and would be totally darkened for above an hour and a lialf. 
 But one-half of the moon’s orbit is elevated live degrees above 
 the ecliptic, and the other half as much depressed below it ; 
 consequently the moon’s orbit intersects the ecliptic only iii the 
 two points before mentioned, called the nodes. When either of 
 these points is in a right line with the centre of the sun, at new 
 or full moon, the sun, moon, and earth, are all in a right line ; 
 and if the moon be then full, she falls into the earth’s shadow. 
 When the moon is more than 1^2° from either of the nodes at 
 the time of full moon, she is then generally too high or too 
 low^ in her orbit to go through any part of the earth’s shadow, 
 and no eclipse occurs. But when she is less than from 
 either node at the lime of opposition or full, she goes through a 
 greater or less portion of this shadow as she is more or less 
 within this limit. Her orbit contains 3G0°, of which 12°, the 
 limit of lunar eclipses on either side of the nodes, are but small 
 portions, and therefore as the sun commonly passes the nodes 
 but twice iw a year, it is not extraordinary that the eclipses are so 
 few in number. 
 
 The eclipses of the moon, as observed above, are visible 
 from all parts of the earth that have the moon above the horizon 
 at the time, and wherever observed, they are of equal extent ; but 
 the time at w hich they are seen, differs with the difference of Jon- 
 <yitude, in the same manner as the observations of the satellites of 
 Jupiter, mentioned in page 55S, and therefore they are of similar 
 use in determining the longitude of places on the earth. The 
 lunar eclipses are very variable in duration, but never exceed 
 two hours. 
 
 • Eclipses of the Sun. 
 
 As the light of the sun, in its passage to the earth, can only be 
 intercepted by the moon, when the moon is in conjunction with 
 the sun, therefore an eclipse of the sun, or the falling of the earth 
 into the moon’s shadow', can only occur at new moon. The moon 
 being a body similar to die earth, its shadow and penumbra are 
 like those occasioned by the earth. This is shown by fig. 3, plate 
 V; but as the moon is less than the earth, it could not, however 
 near, intercept the whole of the sun’s light from the earth, tliere- 
 fore,’ at the distance of 240,000 miles, and casting a conical sha- 
 
 26. — VoL. I. 4 1 
 
610 
 
 ASTRONOMY. 
 
 Eclipses of the sun. 
 
 dow, a total eclipse affects but a small part of the earth at once, 
 and even the penumbra is far from being broad enough to cover 
 the whole terrestrial disc. 
 
 The quantity of a solar eclipse is estimated by digits, like an 
 eclipse of the moon. 
 
 The general circumstances of solar eclipses are the following : 
 1. None of them are universal; that is, none of them are seen 
 throughout the whole hemisphere which the sun is then above. 
 Commonly the moon’s dark shadow covers only a spot on the 
 earth’s surface, about 180 miles broad, as indicated by fig. 3, when 
 the sun’s distance is greatest, and the moon’s least ; but her partial 
 shadow or penumbra, may then cover a circular space of 4900 
 miles in diameter, within which the sun is more or less eclipsed, 
 as tlie places are nearer to or further from the centre of the 
 penumbra. In this case, the axis of the shade passes through the 
 centre of the earth, or the new moon happens exactly in the node, 
 and then it is evident that the section of the shadow is circular ; 
 but in every other case, the conical shadow is cut obliquely by the 
 surface of the earth, and the section will be elliptical. — 2. A solar 
 eclipse does not appear the same in all parts of the earth, where it 
 is seen, but when in one place it is total, in another it is only 
 partial. Also, when the moon appears much less than the sun, 
 as is chiefly the case when she is in apogee and he in perigee,' the 
 vertex of the lunar shadow is then too short to reach the earth, 
 and though she be in a central conjunction with the sun, is yet not 
 large enough to cover his whole disc ; the portion we see of him 
 resembles a lucid ring or bracelet, an appearance constituting 
 what is called an annular eclipse. — 3. A solar eclipse does not 
 happen at the same time in all places wdiere it is seen ; but ap- 
 pears more early to the western parts than the eastern, as the mo- 
 tion of the moon, and consequently of her sliadow, is from west to 
 east. — 4. In most solar eclipses, the moon’s disc is covered with 
 a faint light, which is attributed to the reflection of the light from 
 the illuminated part of the earth. — 5. In total eclipses of the sun, 
 the moon’s limb is seen surrounded by a pale circle of light ; 
 which some astronomers consider as an indication of a solar 
 atmosphere, because it has been observed to move equally with 
 the sun, and not w'ith the moon. 
 
 Solar eclipses do not happen at every new^ moon, for the 
 same reason that lunar eclipses do not happen at every full 
 moon, that is, because the orbit of the moon is not coincident 
 with the ecliptic, except in the two nodes ; but solar eclipses 
 may happen at the distance of 17° from the nodes, which is a 
 > range of 5° on either side more than the arc tJiat comprehends 
 the lunar eclipses. 
 
ASTRONOMY. 
 
 6ll 
 
 Eclipses of the sun. 
 
 The moon's apparent diameter when largest, exceeds the sun's 
 when least, only 1' 38"; and in the greatest eclipse of the snn that 
 can happen at any time and place, the total darkness continues no 
 longer than whilst the moon is going 1' 38" of a degree in her 
 orbit, which is about 3' 13" of an hour. 
 
 When the penumbra (a h, fig. 3) first touches the earth, the 
 general eclipse begins : when the penumbra leaves the earth, 
 the general eclipse ends : from the beginning to the end, the 
 sun appears eclipsed to some part of the earth or other. When 
 the j>enunibra touches any place, the eclipse begins at that 
 place, and ends when the penumbra leaves it. When the moon 
 changes in the node, the penumbra goes over the centre of the 
 earth’s disc as seen from the moon ; and consequently, by 
 describing the longest line possible upon the earth, continues the 
 longest upon it, namely, at a mean rate, 5 hours 50’; longer, if the 
 moon be at her greatest distance from the earth, because she then 
 moves slowest; not so long, if she be at her least distance, because 
 of her quicker motion. 
 
 The general phenomena of eclipses will perhaps be made 
 still plainer by reference to the diagram, fig. 4, plate V. Let 
 S be the sun, L the earth, M the moon, and AMP the moon’s 
 orbit. Draw the right line We from the western side of the sun 
 at W, touching the western side of the moon at c, and the 
 earth at e : draw also the right line Vr/, from the eastern side 
 of the sun at V, touching the eastern side of the moon at r/, 
 and the earth at e ; the dark space c e cl, included between those 
 
 lines, is the moon’s shadow', ending in a point at e, w'here it 
 
 touches the earth ; because in this case the moon is supposed to 
 change at M, in the middle, between A, the apogee, or farthest 
 point of her orbit from the earth, and P, the perigee, or nearest 
 point of it. But as any point of the moon’s orbit may he 
 
 directed to the sun, if the point P had been at ^I, the moon 
 
 would of course have been nearer the earth, and her dark sha- 
 dow at e would have covered a space upon it about 180 miles 
 broad, and the sun would for a time be totally darkened ; but 
 had the point A been at M, the moon would have been farther 
 from the earth, and her shadow' would have ended in a point a 
 little above e, and therefore the sun would have appeared like 
 a luminous ring ail round the moon. Diaw the right lines WX 
 d Iiy and VX eg, touching the contrary sides of the sun and 
 moon, and ending on the earth at a and b ; draw also the right 
 line SXM, from the centre of the sun’s disc, through the 
 moon’s centre, to the earth ; and sup}) 0 se the two former lines 
 WX d h and VX c g to revolve on the line SXM as an axis, 
 and their points a and b will describe the limits of the penumbia 
 
612 
 
 ASTRONOMY. 
 
 Eclipses of tlie sun. 
 
 TT on the earth’s surface, including the large space a b\ within 
 which penumbra the, sun appears more or less eclipsed, as the 
 places are more or less distant from its verge. 
 
 Draw the right line y 12 across the sun’s disc, perpendicular 
 to SXM, the axis of the penumbra; then divide the line y 12 
 into twelve equal parts, as in the ligure, for the twelve digits or 
 equal parts of the sun’s diameter ; and, at equal distances from 
 the centre of the penumbra at c, on the earth’s surface YY, to 
 its edge a by draw 12 concentric circles, marked with the nume- 
 ral figures 1 to 12, and remember that the moon’s motion in lier 
 orbit AMP, is from west to east, as from M to P; then, to an 
 observer on the earth at by the eastern limb of the moon at d, 
 seems to touch the western limb of the sun at W, when the 
 moon is at M; and the sun’s eclipse begins at b; but at the 
 same moment of absolute time, to an observer at o, the western 
 edge of the moon at c leaves the eastern edge of the sun at V, 
 At the very same instant, to all those who live on the circle 
 marked 1 on the earth, the moon M cuts off or darkens a 
 tw'elfth part of the sun S, and eclipses him one digit ; to those 
 who live on the second circle, the moon cuts off tw'o digits; 
 to those on the circle 3, three digits; and so on to the centre at 
 12, where the sun is centrally eclipsed. It is evident by the 
 figure, that the sun is totally or centrally eclipsed but to a 
 small part of the earth at any time, because the dark conical 
 shadow at e of the moon M, falls but on a small part of the 
 earth, even when it covers the largest space possible; and that 
 the partial eclipse is confined at that time to the space included 
 by the circle a by of which only one half can be projected in 
 the figure, tire other half being supposed to be hidden by the 
 convexity of the earth E; and likewise, that no part of the 
 sun is eclipsed to the large space Y Y of the earth, because the 
 inoon is not between the sun and any of that part of the earth; 
 and therefore to all that part the eclipse is invisible. The earth 
 turns eastw'ard on its axis, as from g to hy which is the same 
 way that the moon’s shadow^ moves ; but the moon’s motion in 
 her orbit is much sw after tlian the earth’s on its axis ; and there- 
 fore, although eclipses of the sun are of longer duration, on 
 account of the earth’s motion on its axis, than they w^ould be 
 if that motion were stopped, yet, in four minutes of time at the 
 most, the moon’s swifter motion carries her dark shadow quite 
 over any place that its centre touches at the time of greatest 
 obscuration. The motion of the shadow^ on the earth’s disc, is 
 equal to the moon’s motion from the sun, which is about oO^' 
 of a degree every hpur at a mean rate; but so much of the 
 moon’s orbit is equal to 30^ degrees of a great circle on the 
 
ASTJJUI'COMV. 
 
 rj.x. 
 
 « 
 
 '■D. 
 
 Ify Xiillifn.L'isbei: Jiixon t Oibscn. Liverpcol.A^i^ 
 
ASTRONOMY. 
 
 Cl3 
 
 Scenery of nature, (hirinj; a solar eclipse, dclineatcJ. 
 
 earth ; and therefore the moon’s shadow goes vSO| degree s, or 
 -1830 geographical miles, on the eav<h in an liour, or 30^ miles in 
 a minute, which is about four times as swift as the motion of a 
 cannon-ball. 
 
 Description of a total Eclipse of the Sun. 
 
 1 As total eclipses of the sun are very rare' phenomena, we 
 shall here give the excellent description of one sent by Ur. 
 Stukely, to his friend Dr. Edmund Halley, Its general interest 
 is much increased by the circumstance that the author, from 
 being unprovided with instruments, describes the spectacle rather 
 as a general, though intelligent, observer of nature, than an astro- 
 nomer. 
 
 “ According to my promise, I send you what I observed 
 of the solar eclipse, though I fear it will not be of any great use 
 to you. I was not prepared with any instruments for measuring 
 lime or the like, and proposed to myself only to watch all the 
 appearances that nature would present to the naked eye upon so 
 remarkable an occasion, and which generally are overlooked, or 
 but grossly regarded. 1 chbse for my station a place called Hara- 
 don Hill, two miles eastward from Amsbury, and full east from 
 the opening of Stonehenge avenue, to which it is as the point of 
 view. Before me lay the vast plain where that celebrated work 
 stands, and I knew that the eclipse would appear directly over it ; 
 besides, 1 had the advantage of a very extensive prospect every 
 way, this being the highest hill hereabouts, and nearest the middle 
 of the shadow ; full west of me, and beyond Stonehenge, is a 
 pretty copped hill, like the top of a cone, lifting itself above the 
 horizon ; this is Clay-hill, near Warminster, 20 nnles distant, and 
 near the central line of darkness, which must come from thence, 
 so that I could have notice enough before-hand of its approach. 
 Abraiiam Sturgis and Stephen Ewens, both of this place, and 
 sensible men, were with me. Though it was very cloudy, yet 
 now and then we had gleams of sunshine, rather more than I 
 could perceive at any other place around us. These two per- 
 . son’s looking through smoked glasses, while 1 was taking some 
 bearings of the country with a circumferentor, both confi- 
 dently affirmed the eclipse was begun, when by my watch I 
 found it just half an hour after 5 ; and accordingly, from thence 
 the progress of it was visible, and very often to the naked eye ; 
 the thin clouds doing the office of glasses. From the time of 
 the sun’s body being half covered, there was a very conspicuous 
 circular iris round the sun with perfect colours. On all sides, 
 we beheld the shepherds hurrying their flocks into fold, the 
 
614 
 
 ASTRONOMY . 
 
 Scenery of nature, during a solar eclipse, delineated. 
 
 darkness corning on ; for they expected nothing less than a total 
 eclipse for an hour and a quarter. 
 
 “ When the sun looked very sharp like a new moon, the 
 sky was pretty clear in that spot ; but soon after a thicker cloud 
 covered it, at which time the iris vanished, the copped hill 
 before mentioned grew very dark, together with the horizon on 
 both sides, that is, to the north and south, and looked blue ; 
 just as it appears in the east at the declension of day. We had 
 scarce time to tell ten, when Salisbury steeple, six miles off south- 
 ward, became very black : the copped hill quite lost, and a most 
 gloomy night with full career came upon us. At this instant we 
 lost sight of the sun, whose place among the clouds was hitherto 
 sufficiently distinguishable, but now not the least trace of it to be 
 found, no more than if really absent : then I saw by my watch, 
 though with difficulty, and only by help of some light from the 
 northern quarter, that it was hours 35 minutes. Just before this, 
 the whole compass of the heavens and earth looked of a lurid com- 
 plexion, properly speaking, for it was black and blue, only on the 
 earth, upon the horizon, the blue prevailed ; there was likewise in 
 the heavens, among the clouds much green interspersed, so that 
 the whole appearance was really very dreadful, and as symptoms 
 ©f sickening nature. 
 
 “ Now 1 perceived us involved in total darkness and palpa- 
 ble, as I may aptly call it ; though it came quick, yet I was so 
 intent, that I could perceive its steps, and feel it as it were drop 
 upon us, and fall on the right shoulder (we looking westward) 
 like a great dark mantle or coverlet of a bed thrown over us, or 
 like the drawing of a curtain on that side ; and the horses we 
 held in our hands were very sensible of it, and crowded close 
 to us, startling with great surprise ; as much as I could see of 
 the men’s faces that stood by me, had a horrible aspect. At 
 this instant I looked around me, not without exclamations of ad- 
 miration, and could discern colours in the heavens, but the earth 
 had lost its blue, and was wholly black ; for some time among the 
 clouds, there w’ere visible streaks of rays tending to the place of 
 the sun as their centre ; but immediately after, the whole appear* 
 ance of earth and sky was entirely black : of all things I ever saw 
 in my life, or can by imagination fancy, it was a sight the most 
 tremendous. 
 
 “ Toward the north-west, whence the eclipse came, I could 
 not in the least find any distinction in the horizon between 
 heaven and earth, for a good breadth of about sixty degrees or 
 more ; nor the town of Amsbury underneath us, nor scarce the 
 ground we trod on. 1 turned myself round several times during 
 this total darkness, and remarked at a good distance from ffie 
 
ASTRONOMY. 
 
 615 
 
 Scenery ot nature, during a solar eclipse, delineated. 
 
 west on both sides, that is to tlie north and south, the horizon 
 very perfect; the earth being black, the lo\\er part of tlie hea- 
 vens light ; for the darkness above hung over us like a canopy, 
 almost reaching the horizon in those parts, or as if made with 
 skirts of a lighter colour ; so that the upper edges of all the hills 
 were as a black line, and I knew tliem very distinctly by their 
 shape or profile ; and northward I saw perfectly, that the inter- 
 val of light and darkness in the horizon was between Martlnsal- 
 hill and St. Ann’s hill ; but southward it was more indefinite. 
 I do not mean that the verge of the shadow passed between 
 those hills, which were but 12 miles distant from us; but, so 
 far I could distinguish the horizon, beyond it not at all : the 
 reason of it is this : the elevation of ground 1 was upon, gave 
 me an opportunity of seeing the light of the heavens beyond 
 the shadow ; nevertheless, this verge of light looked of a dead, 
 yellow ish, and greenish colour ; it w as broader to the north 
 than south, but the southern was of a tawny colour. At this 
 time behind us, or eastward toward London, it was dark too, 
 where otherwise 1 could see the hills beyond Andover ; for the 
 foremost end of the shadow' was past thither ; so that the whole 
 horizon was now' divided into four parts of unequal bulk, and 
 degrees of light and dark : the part to the north-west broadest 
 and blackest; to the south-west lightest and longest. All the 
 change I could perceive during the totality, was that the horizon 
 by degrees drew' into two parts, light and dark ; the northern hemi- 
 sphere growing still longer, lighter, and broader ; and the two op- 
 posite dark parts uniting into one, and swallowing up the southern 
 enlightened part. 
 
 As at the beginning, the shade came feelingly upon oui 
 right shoulders, so now the light from the north, where it opened 
 as it were ; though 1 could discern no defined light or shade 
 upon the earth that way, which I earnestly watched for, yet it 
 was manifestly by degrees, and with oscillations, going back a 
 little, and quickly advancing further, till at length, upon the 
 first lucid point appearing in the heavens, where the sun was, I 
 could distinguish pretty plainly a rim of light running along 
 side of us a good while together, or sw'eeping by at our elbows 
 from west to east ; just then having good reason to suppose the 
 totality ended with us, I looked on my watch, and found it to- 
 be full three minutes and a half more. Now the hill-tops 
 changed their black into blue again, and I could distinguish an 
 horizon where the centre of darkness was before ; the men cried 
 out, they saw the copped hill again, which they had eagerly 
 looked for; but still it continued dark to the south-east, jet I 
 cannot say that ever the horizon that way was undistinguishable ; 
 
6l6 
 
 ASTRONOMY. 
 
 Scenery of nature, during a solar eclipse, delineated. 
 
 immediately \ve heard the larks chirping, and .singing very briskly, 
 for joy of the restored luminary, after all things had been hushed 
 into a most profound and universal silence. The heavens and 
 earth now appeared exactly like morning before sun-rise, of a 
 grayish cast, but rather more bjue interspersed ; and the earth, so 
 far as the verge of the hill reached, was of a dark green or russet 
 colour. 
 
 ‘‘ As soon as the sun emerged, the clouds grew thicker, and 
 the light was very little amended for a minute or more, like a 
 cloudy morning slowly advancing. After about the middle of 
 the totality, and so after the emersion of the sun, we saw Venus 
 very plainly, but no other star. Salisbury steeple now appeared. 
 The clouds never removed, so that vve could take no account of 
 it afterwards, but in the evening it lightened very much. 1 
 hasted home to write this letter; and the impression was so 
 vivid upon my mind, that I am sure, I could for some days 
 after have wrote the same account of it, and very precisely. 
 After supper I made a drawing of it from my imagination, 
 upon the same paper I had taken a prospect of the country 
 before. 
 
 I must confess to you, that I was (I believe) the only 
 person in England that regretted not the cloudiness of the day, 
 which added so much to the solemnity of the sight, and which 
 incomparably exceeded, in my apprehension, that of 1715, 
 which I saw very perfectly from the top of Boston steeple, in 
 Lincolnshire, where the air was very clear; but the night of 
 this was more complete and dreadful : there, indeed, 1 saw 
 both sides of the shadow come from a great distance, and pass 
 beyond us to a great distance ; but this eclipse had much more 
 of variety and majestic terror; so that I cannot but felicitate 
 myself upon the opportunity of seeing these two rare accidents 
 of nature, in so different a manner : yet 1 should willingly 
 have lost this pleasure for your more valuable advantage of 
 perfecting the noble theory of the celestial bodies, which last 
 time you gave the world so nice a calculation of ; and wish the 
 skv had now' as much favoured us, for an addition to your ho- 
 nour and great skill, which I doubt not to be as exact in this as 
 before.*' 
 
ASTRONOMY. 
 
 617 
 
 The astronomical and civil day. 
 
 Division of Time. 
 
 Time is distinguished into absolute and relative. 
 
 Absolute time, is time considered in itself, and without any 
 relation to bodies or their motions. 
 
 Relative time, is the time measured by means of motion, as by 
 the motion of the heavenly bodies, timekeepers, &c. Motion 
 affords us the only means we possess, of rendering the flow^ of ab- 
 solute time perceptible. 
 
 Astronomical time, is that measured by the motion of the hea- 
 venly bodies. 
 
 Civil time, is astronomical time accommodated to civil uses, 
 and distinguished into days, months, years, &c. 
 
 Of the Day, and smaller portions of Time. ' 
 
 In common language, the day is the interval of time which 
 elapses from the rising to the setting of the sun ; this is called the 
 artificial day, the night is the interval that the sun continues be- 
 low the horizon. 
 
 The time which elapses during an entire revolution of the sun, 
 from a meridian to the same meridian again, is called an astrono- 
 mical or natural day. It commences at 12 o’clock at noon, when 
 the sun’s centre is on the meridian, and terminates at the next 
 noon, when the sun’s centre is again on the meridian. It is divided 
 into 24 hours, reckoning in a numerical succession from I to 24: 
 the first 12 are sometimes distinguished by the mark P. M. signi- 
 fying post meridiem, or afternoon ; and the latter twelve are mark- 
 ed A. M. signifying ante meriaiem, or before noon. But astrono- 
 mers generally reckon through the 24 hours, from noon to noon ; 
 and those hours which are by the civil or common way of reckon- 
 ing called morning hours, they reckon in succession from 12, or 
 midnit^ht, to 24 hours. Thus 10 o’clock in the morning of June 
 the 1 fth,’ they would call the 22nd hour of June the 1 1th. 
 
 The length both of the astronomical and the civil day is con- 
 stantly changing. This variation arises from two causes : 1 . The 
 unequal motioifof the earth in its orbit. 2. The obliquity of that 
 orbit to the plane of the equator. The mean and apparent solar 
 days are never equal, except when the sun s daily motion in right 
 ascension is 59' 8". This is the case about April 16, June 16, Sep- 
 tember 1 and December 25 : on these days the difference vanishes, 
 or nearly'so. It is at its greatest about November 1, when it is 
 
 16' 16". 
 
 26. — VoL. I. 
 
 4K 
 
 I 
 
618 
 
 ASTRONOMY. 
 
 Division of time. — The day. 
 
 ' An astronomical day is somewhat greater than a sidereal day, 
 which is the time employed by the earth in revolving on its axis, 
 and is at all times of the year immutably the same. It will easily 
 be understood, that at every complete rotation of the earth, the 
 same fixed stars will be on the meridian ; but that this cannot be 
 the case with the sun, because, during the earth’s rotation, the sun* 
 has advanced eastward and left the star, and therefore the earth has 
 to make somewhat more than one turn before the sun comes a 
 second time to the same meridian. If the mean astronomical or 
 civil day be taken equal to 24 hours, the duration of the sidereal 
 day will be 23h. 56' 4.1". Hence a star which w'as on the meridian 
 with the sun, will be on the meridian S' 56" before the next noon ; 
 S' 56" being the difference between a solar and sidereal day. 
 
 The civil day, or astronomical day accommodated to civil 
 purposes, begins with different nations at different times : The 
 Egyptians began their day at midnight; the same method prevails 
 in Great Britain, France, Spain, and most parts of Europe. The 
 Babylonians began their day at siimrising, reckoning the hour im- 
 mediately before its rising again, the 24th hour of the day, 
 whence the hours reckoned in this way are called the Babylonic. 
 In several parts of Germany and Italy, they begin their day at sun- 
 setting, and reckon on till it sets next day, calling that the 24th 
 hour : these are generally termed Italian hours. The Jews also 
 began their day at sun-setting, but then, like us, they divided it 
 into parts of 12 hours each. The Romans also reckoned their 
 hours in this manner, and the Turks do the same at this day. 
 
 An hour may be considered either as an equal or unequal 
 portion of time. Equal hours, or those in common use, are the 
 twenty-fourth part of a day and night precisely. They are 
 likewise called equinoctial hours, because they are measured on 
 the equinoctial ; and astronomical, because used by astronomers. 
 Unequal hours are those by which the artificial day is divided 
 
 * Nothing is more common, in astronomical treatises, than to speak of the 
 sun’s motion, as if that luminary had a real motion in the ecliptic or through 
 the zodiac. This mode of speech is used, because it is easily explained, that 
 the sun’s motion, like that of the shore to a person in a ship, is only apparent, 
 tlie real motion belonging to the earth, which in different situations of its orbit, 
 necessarily induces the terrestrial spectator to transfer the situation of the sun 
 to a different position in the sphere. It is an accommodation to the ordinary 
 testimony of our senses, that introduces no confusion of ideas ; but it shows at 
 the same time, that even in this day, a mode of speech is thought expedient, 
 that in early ages was certainly indispensable, when the sun W'as commanded 
 to stand still, and virtually obeyed. 
 
ASTRONOMY. 
 
 619 
 
 Division of time. — 'fhe week. — The month. 
 
 into 12 parts, and the night into the same number. As it is only 
 at the time of the equinoxes that the aitilicial day is equal to the 
 night, they will, except at these times, be always either increasing 
 or decreasing. 
 
 The hour is divided into 60 equal parts called minutes; each 
 minute into 60 equal parts called seconds ; and each second into 
 60 equal parts called thirds. The Jews, Chaldeans, and Ara- 
 bians, divided the hour into 1080 equal parts, called scruples ; 
 which number containing 18 times 60, each minute contains 18 
 scruples. 
 
 Of the Week. 
 
 Various divisions of this sort have obtained in different coun- 
 tries, and at different times. The early Greeks divided their 
 month into three portions of ten days each ; the Northern Chinese 
 had a w^eek of fifteen days, and the Mexicans one of thirteen ; but 
 the most general division of the month was that of the Jews, into 
 periods of 7 days. This period was adopted by the Chaldeans, 
 and most other Oriental nations. It is at this day in use w herever 
 Christianity has been introduced. The French abandoned it for 
 the decade during the season of revolutionary frenzy, but their late 
 Dictator brought it back to its former standard. The Maho- 
 metans also use the week of 7 days. 
 
 Of the Month. 
 
 The periodical changes of the phases of the moon, it seems 
 very apparent, gave rise to the division of time called a month ; 
 but the difficulty of adjusting the lunar monlh to the annual revo- 
 lution of the earth, led to the introduction of other divisions of lime 
 under the same name. 
 
 Months are now divided into astronomical and civil. 
 
 The astronomical months, with which chronology is concerned, 
 are measured by the revolutions of the moon, and are either peri- 
 odical or synodical. . . , . , , 
 
 The periodical lunar month, or the exact time in which the 
 moon passes through the zodiac, from any point of her orbit to the 
 same point again, consists of 27 days, 7 liour.^?, 43' o'. 
 
 The synodical lunar month, called a lunation, is reckoned 
 from one conjunction of the sun with the moon to another. It is 
 available period, being subject to the variation of the sun east- 
 ward on the ecliptic ; at a mean rate, a lunation consists of 29 
 days, 12 hours, 44' 3". In ancient times, this was the month m 
 general use. 
 
620 
 
 ASTRONOMY. 
 
 Division of time. — The month. — The year. 
 
 The solar month is the time in which the sun passes through 
 one sign of the zodiac ; its length is variable, because the motion 
 of the sun is so ; at a mean rate, it consists of 30 days, 10 hours, 
 
 29' o". 
 
 A civil or political month is a portion of time set out by the 
 custom of particular nations, without any precise regard to the 
 celestial motions. The British and most European nations make 
 twelve months in the year. 
 
 Civil lunar months consist alternately of 29 and 30 days. 
 Thus two of these months are nearly equal to two astronomical 
 months, and the new moon will be kept to the first day of such 
 civil months for a longer time together. This was the month in 
 common use till the time of Julius Cesar. 
 
 The civil solar month is the one at present in use. It was 
 introduced by Julius Cesar. It consisted at first alternately of 
 30 and 31 days, excepting one month of the twelve, which in 
 every fourth year consisted of thirty, and for the other three 
 years of 29 days. This arrangement was altered a little by 
 Augustus, whose name was given to the month previously 
 called Sextilis, and to make it equal to any of the rest, the 
 number of the days it contained w'as increased from 30 to 31. 
 The day thus added to it, was taken from February, which 
 month, from that time had only £8 days, except in every 
 fourth year, when it received the intercalary day. The civil 
 months thus determined in the reign of Augustus, have 
 remained in use to the present time, and are common to all 
 Europe ; they are the same with what we usually call calendar 
 months. 
 
 Of the Year, 
 
 The smaller divisions of time, under the same name, differing 
 from each other according to the motion by which they are mea- 
 sured, it will necessarily follow that the large divisions will admit 
 variations, according to the value of their integral parts. 
 
 The solar year, or tropical year, is the time that the sun takes 
 to pass through the tw^elve signs of the zodiac, or from one equinox 
 to the same again, and constitutes a natural division of time, be- 
 cause the seasons always fall in the same months. Its length is 
 equal to 363 days, 5 hours, 48' 49". 
 
 The tropical year is divided by astronomers into four parts, 
 determined by the two equinoxes and solstices. The interval 
 between the vernal and autumnal equinoxes is (on account of 
 the eccentricity of the earth’s orbit, and its unequal velocity 
 therein) nearly 8 days longer than the interval between the 
 
ASTRONOMY. 
 
 621 
 
 Division of time. — The year. 
 
 autumnal and vernal equinoxes. These intervals are at present 
 nearly as follows : 
 
 From the Spring equinox to the suniincr solstice 
 Summer solstice to the autumnal equinox 
 Autumnal equinox to the winter solstice 
 Winter solstice to the spring equinox 
 
 D. 
 
 h. 
 
 m. 
 
 D. 
 
 h. 
 
 ni. 
 
 92 
 
 21 
 
 45? 
 
 
 
 
 93 
 
 13 
 
 sbS 
 
 186 
 
 17 
 
 20 
 
 89 
 
 16 
 
 47 7 
 
 
 
 
 89 
 
 1 
 
 42 5 
 
 178 
 
 20 
 
 29 
 
 
 
 
 7 
 
 16 
 
 5J 
 
 The Julian year, so named by Julius Cesar, who established 
 it, consists of 365 days, 6 hours ; but the 6 hours were not reckon- 
 ed till the expiration of every fourth year, when, amounting just to 
 one day, they were added to the end of February, and the year in 
 which they were thus reckoned was called bissextile or leap year. 
 The Julian year exceeding the true solar year more than 11 
 minutes, the excess amounts to a whole day in 131 years; in con- 
 sequence of which, the vernal equinox, which in the first Julian 
 year fell on the 25th of Mar ch, had in the year of our Lor d 325, 
 at the time of the council of Nice, gone back to the 21st, and in 
 1582 to the 11th of March. To correct this growing error', the 
 calendar was again amended, under the auspices of Pope Gregory 
 XIII. by taking 10 entire days out of it. Accordingly, in the 
 year 1582, the day following the 4th of October, instead of being 
 called the 5th was called the 15th; by means of w'hich the real 
 equinox was restored to the 21st of March. A provisiorr was at 
 the same time made, to prevent the recurrence of so important 
 an error again. The intercalary or bissextile day, which had 
 been regularly added to February every fourth year, was order- 
 ed to be suppressed at the end of every century not divisible 
 by 4. The last year of the 17th, 18th, and 19th centuries are 
 not leap-years, because 4 will not divide these rr umbers without 
 a remainder; but the last year of the 20th century, or year 
 2000, will be a leap-year. This pope’s correction of the calen- 
 der is so near the truth, that it will not err a day in 3000 years; 
 it is called the Gregorian or the ~New Style, and obtains irr almost 
 all Christian nations. It was not used in England before 1752, 
 when it was adopted pursuant to act of parliament, and the 
 third of September was reckoned the fourteenth, the error of the 
 Julian calendar having at that time increased to 11 days. 
 
 The lunar year is the space of 12 lunar months, and is 
 either astronomical or civil. 
 
 The lunar astronomical year consists of twelve lunar synodi- 
 
622 
 
 ASTRONOMY. 
 
 Division of time. — The year. — Cycles and indiction. 
 
 cal months; and is, therefore, 354 days, 8 hours, 48', 38", being 10 
 days, 21 hours, 11" shorter than the solar year. 
 
 The lunar civil year is either common or embolimic. The 
 common lunar year consists of 12 lunar civil months, and contains 
 354 days. The embolimic lunar year, consists of 13 lunar civil 
 months, and contains 384 days. 
 
 The ancient Roman year, as first settled by the Romans, con- 
 tained only 10 months, and in all 304 days. 
 
 The Egyptian year, called also the year of Nabonassar, con- 
 tains only 365 days, divided into twelve months of 30 days each, 
 with five intercalary days added at the end. Thus the Egyptian 
 year loses a whole day of the Julian year' every four years, and 
 after the space of 1460 years, it begins with the Julian year, 
 which length of time is called the Lothic period. 
 
 The ancient Greek year consisted of 12 months, which at 
 first were divided into 30 days each ; but afterwards each month 
 contained 29 and 30 days alternately ; and this year was computed 
 from the first appearance of the new moon, with the addition of 
 an embolimic month of 30 days, every 3d, 5th, 8th, 11th, 14th, 
 l6th and IQth year, in order to keep the new and full moons to 
 the same seasons of the year. 
 
 Not only the length of the year, but the time of its com- 
 mencement, has been difl'erently reckoned among different nations. 
 The Chaldeans and Egyptians commenced their year at the 
 autumnal equinox. The jews reckoned their civil year from the 
 same period, but began their ecclesiastical year in the spring. 
 Some of the Grecian states commenced their year at the vernal 
 equinox, others at the autumnal equinox, and some at the summer 
 solstice. The Roman year at one time began in March, but 
 afterwards was made to commence in January. The new year’s 
 day of the church of Rome is fixed on the Sunday nearest the full 
 moon of the vernal equinox. In England, the year began in 
 March, till 1752, when the style was altered, and the year was at 
 the same time settled to commence on the 1st of January. The 
 period between the 1st of January and the 25th of March, was, 
 for some time subsequently to the alteration of the style, expressed 
 thus: 1735-6, or 173|, forms of writing which have now become 
 obscure. 
 
 Of the Lunar and Solar Cycles and Indiction. 
 
 A cycle is a perpetual circulation of a certain fixed and 
 determinate space of lime. 
 
 The cycle of the sun consists of 28 years ; at the end of 
 which time, the days of the months return again to the same 
 
ASTRONOMY. 
 
 623 
 
 Division of time. — Cycles. — Epochs and eras. 
 
 days of the week, and the sun is in the same sign and degree 
 •of the ecliptic, which he was in at its commencement; at least, 
 this is the case within a degree for 100 years. The leap-years, 
 also, at the expiration of the solar cycle, begin the same course 
 over again with respect to the days of the week on \n Inch the days 
 of the months fall. 
 
 The c^cle of the moon^ the year of which is called the golden 
 number, is a period of 19 years, at the end of which time, the sun 
 and moon return to the same situation in the heavens, or very 
 nearly so : the conjunctions, oppositions, &c. of these tw'O lumi- 
 naries being, w'ithin about an hour and a half, the same that they 
 were on the same days of the months at the commencement of the 
 period. 
 
 The cycle of mdictio?iy or Roman indiction, has no reference 
 to any natural period. It consisted of 15 years, and was in use 
 only among the Romans, for some civil purposes which cannot 
 now be completely ascertained. 
 
 •The year of our Saviour’s birth, according to the usual com- 
 putation, w'as the ninth year of the solar cycle ; the first year of 
 the lunar cycle; and the 312th year after his birth was the first 
 year of the Roman indiction. With this data, the year of any 
 subsequent cycle or indiction may be found. 
 
 Of Epochs and Eras. 
 
 Any remarkable event or period from which time is reckoned, 
 is called an epoch ; the years computed from an epoch, or from 
 such event or period, are called an cro, whatever be their number. 
 
 The earliest and most memorable epoch connected with mun- 
 dane affairs, is that of the creation of the world ; but tlie era re- 
 ferring to it is only continued to the birth of Christ. According 
 to Archbishop Usher, w'hose chronology is adopted in the current 
 translation of the Bible, it began in the year ^04 before Christ. 
 Playfair places it in 4007. The next epoch was the Deluge, and 
 like the former it was used by the Jews. 
 
 The epoch adopted by all Christians is that of the birth of 
 Christ; the common estimation of which, jmaking the present 
 year of the Christian era 1814, is supposed to be 4 years too 
 late. The reason of this uncertainty is, that the Christian era 
 was not used till the sixth century after the birth of Christ, when 
 it was too late to fix its commencement with indisputable accuracy. 
 But as all the nations who use it, begin at the same time, and the 
 year 1814, for example, is with all of them the year 1814, the 
 error introduces no confusion. 
 
624 
 
 ASTRONOMY. 
 
 Division of time. — Olympiads. — Julian period. — Abstract of astronomy. 
 
 The mode of reckoning by oh/mpiadsj or periods of four 
 years, was used by the Greeks. The first olympiad began 775 
 years before the birth of Christ. 
 
 The Romans reckoned from the building of Rome ; the date 
 of that event is, however, not well ascertained, from the want 
 of authentic records in those early times ; but the computation 
 adopted by the Romans themselves, places it in the year 753 
 before Christ. 
 
 The Hegira, or Mahometan era, takes date from the flight 
 of Mahomet to Medina, 622 years after the commencement of 
 the Christian era. 
 
 To obt.dn a common standard in the comparison of dates 
 and eras, Joseph Scaliger adopted a very ingenious method : 
 By multiplying into each other, the cycle of the sun, which is 
 28 years; the cycle of the moon, which is IQ years; and cycle 
 of indiction, which is 15 years, he obtained the number 7980, 
 which is called a Julian period. This period is supposed to 
 commence 706 years antecedently to the creation, and till the 
 expiration of it, or till the world is 7274 years old, the first years 
 of each of these cycles will not come together — The year of the 
 Julian period corresponding with any given year before or since 
 the commencement of the Christian era may easily be found. If 
 the year required be since the Christian era, add to it 4713, the 
 number of years elapsed before the Christian era, and the sum 
 will be the year required. _ If the year required precede the Chris- 
 tian era, subtract the years before Christ from 4713, and the 
 difference will be the answer. 
 
 Abstract of Astronomy. 
 
 1. The solar system, comprises the sun and all the bodies 
 that revolve round him, viz. the comets, the planets with their 
 respective satellites, and the asteroids. 
 
 2. The number of the comets is unknown; that of the 
 planets, so far as yet discovered, is seven, the satellites eighteen ; 
 and the asteroids four. 
 
 3. The figure of the earth is not that of a perfect globe, but 
 an oblate spheroid, flattened a little at the poles, by its revolution 
 on its axis. 
 
 4. The planets Jupiter and Saturn are also observed to be 
 flattened at the poles like the earth, but in a greater degree, 
 evidently because their diurnal revolution is swifter. 
 
ASTRONOMY. 
 
 625 
 
 Abstract of astronomy. 
 
 o. The orbits of all tlie planets, asteroids and comets, arc 
 ellipses, having the sun in one of their foci; but the orbits of 
 the two former classes of bodies are very nearly circles, while 
 the orbits of the comets arc all very eccentric. 
 
 b. The orbits of the satellites are also ellipses, in one of the 
 foci of which is situated the primary planet round which they 
 move. 
 
 7. The periods, distances, and magnitudes of the planets, 
 liave all been determined with very considerable exactness; the 
 same circumstances respecting the asteroids, are also evidently 
 determinable, though the results yet laid down, have not, from 
 the recent date of their discovery, been so amply confirmed, as 
 to be fully relied on ; but the comets recede to such immense 
 distances, and there is so much uncertainty in identifying them, 
 that their elements are hypothetical. 
 
 8. The planets, comets and asteroids, are preserved in their 
 orbits, by the joint eficcts of the ])ower of attraction, which 
 acts in a "right line from them to the sun, and a projectile or 
 centrifugal force, which would carry them otf in a tangent to 
 the curve of revolution. 
 
 9. The powers which preserve the satellites in their orbits, 
 are the same as those that act upon the planets and comets, but 
 the centripetal force is exercised by the primary. 
 
 10. The body of the sun is supposed to be opake, and to 
 be surrounded with a double set of clouds, the upper stratum 
 of which, forms the luminous globe we behold. 
 
 JJ. The planets revolve round an imaginary line or axis 
 within themselves, and the time in which they perform this 
 rotation, constitutes their da^ and tiighl. 
 
 12. The time in which a planet revolves round the sun, 
 forms its ?/ear. 
 
 13. The diversity of seasons is occasioned by the inclina- 
 tion of the axis of a planet to the plane of its orbit. 
 
 14. The annual and diurnal revolutions of the planets are 
 all performed from west to east. 
 
 15. The satellites also revolve from west to east, with the 
 exception of the satellites of the Herschel, which appear to 
 
 move in a contrary direction. , , i- r.i 
 
 16. The fixed stars are distinguished from the bodies ot the 
 solar system, by the twinkling light they afibrd, by their having 
 no parallax, and by their having, even through the best 
 telescopes, no sensible magnitude. 
 
 17. The naked eye cannot behold above five hundred stars 
 in the whole hemisphere; but the number discovered with the 
 assistance of a telescope exceeds all calculation. 
 
 27Yol. L ^ 
 
626 
 
 ASTRONOMY. 
 
 Abstract of astronomy. 
 
 18. Every fixed star is supposed to be a sun, sbining by 
 its own light, and surrounded by planetary worlds like those 
 of the solar system. 
 
 19. The tides are an effect of the attraction of the sun and 
 moon upon the ocean. When these luminaries act together, 
 or in the same line, they occasion spring tides; when they 
 counteract each others attraction, neap tides take place. 
 
 20. Eclipses of the moon are owing to the shadow of the 
 earth falling upon the moon. 
 
 21. Eclipses of the sun occur, when the moon, coming 
 between the earth and sun, throws a shadow on the earth. 
 
 22. Motion is the measure of time, and the motions of the 
 heavenly bodies are the basis by which all other motions are 
 measured. 
 
 23. The day is a natural division of time, that is, it com- 
 prises a portion of time measured out by the completion of 
 certain phenomena, successive according to regular laws. 
 
 24. The periodical and synodical lunar months are also 
 natural divisions of time, but no other ; the year, and lunar and 
 solar cycles, are of the same character as the lunar months; 
 the cycle of indiction, and the olympiad, are examples of the 
 artificial division of time. 
 
 • END OF VOLUME FIRST. 
 
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