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•Jr 
 
 PHILOSOPHICAL 
 
 TRANSACTIONS. 
 
 GIVING SOME 
 
 ACCOUNT 
 
 O F T H E 
 
 Trejent Undertakings, Studies > and Labours 
 
 OF THE 
 
 INGENIOUS, 
 
 IN MANY 
 
 Confiderable Parts of the World. 
 
 V o l. XXXV. For July, Attgufl, September ? Octo¬ 
 ber, November and ©ecember 172,7, and for the 
 Year 17x8, 
 
 LONDON: 
 
 Printed for W. Innys, Printer to the Royal Society , at 
 the IVeft End of St. Paul's. 
 
 M . DCC. XXIX. 
 
\ 
 
TO HIS 
 
 royal highness 
 
 FREDERICK 
 
 Prince o {WALES. 
 
 SIR, 
 
 S INCE your Royal Highness 
 has gracioufly condefcended to in- 
 fcribe Your Name in the Book of 
 Charters and Laws of the Royal Society , 
 as a Token of your Favour and Encourage¬ 
 ment of it, I prefume to offer this Thirty- 
 fifth Volume of Thilofophical T'ran facti¬ 
 ons t fome of its lateft Productions, to your 
 Royal Highness’s more immedi¬ 
 ate Protection. 
 
 This alone would be fufficient Honour 
 to the feveral Authors of the TraCts con¬ 
 tained in it: But they muft ftill have a pri¬ 
 vate Satisfaction in knowing, that they are 
 
 a fub- - 
 
DEDICATION. 
 
 fubmitted to the Obfervation of a Prince, 
 whofe Difcernment renders him a compleat 
 Judge of the Merits of their Labours; and 
 whofe Benevolence, at the fame Time, will 
 readily incline Him to think favourably 
 of them. 
 
 If, befides this, they fhould chance to 
 furnilh any Matter for your learned, or in¬ 
 genious Entertainment, I beg your Royal 
 Highness to receive it, as one In- 
 ilance of the grateful Senfe conceived of 
 the late Honour done us, and particular¬ 
 ly, of the profound Refpeft, with which 
 I am. 
 
 Tour Royal Highness’s 
 
 O ■ »ifl 
 
 Moji Devoted^ Obedient , 
 and Humble Servant , 
 
 William Rutty, R. S, Seer . 
 
/ 
 
 \ 
 
 
 
 
 ft 
 
gm &qCP. 
 
 

 
 
 
 
Numb. 406. 
 
 PHILOSOPHICAL 
 
 TRANSACTIONS 
 
 FOR THE 
 
 Month of DECEMBER, 17x8. 
 
 The CONTENTS. 
 
 L (Remarks on the Height of Mountains in general, 
 and of thofe of Swiflerland in particular, with an 
 Account of the Rife of fome of the mofl conjide- 
 rable Rivers of Europe. By J. G. Scheuchzer, 
 M. T>. See. 
 
 II. Optical Experiments made in the Beginning of 
 Auguft 1718, before the Brejident and feveral 
 Members of the Royal Society, and other Gentle¬ 
 men of feVeral Nations, upon Occafion of Signior 
 Rizzetti’r Opticks, with an Account of the faid 
 Book, By J. T. Defaguliers, L L. B. and 
 F. % S. 
 
 a ^ 
 
 III. The 
 
The CONTENTS. 
 
 III. The Method of making Tin-Plates, extracted 
 from the Memoirs of the Academy of Sciences , 
 for the Tear 1715, by William Rutty, M. T>. 
 Of S. Seer. 
 
 IV. A Letter from the (Reverend Mr. James Brad¬ 
 ley SaVilian Trofeffor of Aflronomy at Oxford, 
 andF.%S. to £)r.Edmund Halley Aftronom. 
 Reg. See. giving an Account of a new dif- 
 coVered Motion of the Fixt Stars. 
 
( 577 ) 
 
 I. Remarks on the Height of Mountains in general, 
 and of thofe of SwifTerland in particular, with an 
 Account of the Ejfe of fome of the mofl conjide- 
 rable Quivers of Europe. By J. G. Scheuchzer, 
 M. T>. &c. 
 
 I N a former Paper, I took Notice that ‘Dicaarchus 
 found Mount Telius in TheJfalia , to be 115*0 Pa¬ 
 ces high, which make 615*0 Roman , or 6811 Tar is 
 Feet, a Height which we may well pronounce too 
 great even for the abfolute Height of Mount Telius , I 
 mean its Rife above the Level of the Sea. Conform 
 to the Determination of ‘Dicaarchus , I mentioned, that 
 Tint arch fixes the Height of the higheft Mountains, 
 and the greateft Depth of the Sea to 10 Stadia, and 
 Cleomedes affirms, that they cannot exceed 16 Stadia. 
 The celebrated Galileus de Galileis is one of the moft 
 modeft among the modern Writers on this Head: For he 
 fays, * that the higheft Mountains do not rife above a 
 Mile, or 8 Stadia, or ?ooo old Roman Ve/pafian Feet, 
 which make f^VParis Feet above the Level of the Sea, 
 which we fhall find by and by to agree pretty well 
 with fome of the higheft Mountains in France , and 
 may conjecture to do fo with thofe in Italy. Kepler 
 went rather too far f when he affigned the Mountains 
 of Rhcetia (thought the higheft in Swijferland) a 
 Height of 16 Stadia, or 10000 old Roman Vefpajian 
 Feet, which make 10916 Tar is Feet. The Opinions 
 of fome other antient and modern Geographers and Ma¬ 
 thematicians, will appear better by the Table annexed. 
 
 * Nuntius Sidereus, p. 14. 
 
 t Aftronom. Optic, p. 129, 135* & Epitom. Aftronoin. lib I. pag. 2 6 . 
 
 H h h h A Table 
 
( 578 ) 
 
 A Table ihewing the Height of Mountains according to 
 feveral antient and modern Writers. 
 
 Strabo ( Lib. II. Geog .) fays, 
 that the higheft Mountain,call¬ 
 ed by him (Petra Sogdiana , is 
 of - - - 
 
 \Pererius {Lib. XII. in Ge-') 
 nejin) determines the higheft > 
 Mountains to -5 
 
 Leo Baft. Albertus (Architect, p 
 Lib . x. Cap. i.) to - * c 
 
 Ath. Kircher. (Ars magn. luc. a 
 & umbr. (P. II. Trobl. 5.) C 
 brings them to - - 3 
 
 Fromond. (Lib. I. Meteor. Cap A 
 2. Art. i. - - 3 
 
 Gilbertus de magnete. L. IV. C. i. 
 iPliny (Lib. III. Cap. lxiv.) ac¬ 
 cording to the Explanation of 
 Fortunius Licetus (de Luna} 
 Luce fubobfcura, Lib. II. p. I 
 306.) to - - -J 
 
 Ricciolus, Geophr. (Lib. VI.) is of^ 
 .Opinion, in Furfuance of what 
 he imagines to have demon- 
 ftrated of the Mountains A- 
 thos and Caucafus , that pofli- 
 bly there may be Mountains 
 of * • 
 
 Stadia.jOld Roman 
 
 Paris Feet. 
 
 
 Vefpajian 
 
 Feet. 
 
 
 30 
 
 187*0 
 
 20468 
 
 3* 
 
 20000 
 
 21832 
 
 
 22500 
 
 23661 
 
 43 
 
 26875 
 
 *9337 
 
 64 
 
 4OOOO 
 
 43664 
 
 128 
 
 80000 
 
 87328 
 
 400 
 
 25OOOO 
 
 272900 
 
 yix 
 
 320000 
 
 349312 
 
 Now, 
 
( 579 ) 
 
 Now, in Oppofition to this Table, wherein the 
 Heights muft needs, upon firft View, appear romantick 
 and unnatural, let us confider the Height of fuch Moun¬ 
 tains, as have been meafured,either by Trigonometrical 
 or Barometrical Obfervations. 
 
 In England , the Height of Snowdon-hill, one of 
 the higheft Mountains in Wales , was meafured Trigo¬ 
 nometrically, by Mr. J. Cafwell of Oxford, and found 
 to be of 1240 Yards, or 3710 Engl'tjb Feet, which 
 make 3488 Ear is Feet. At the Top of this Mountain, 
 the Mercury fubfided to 2.$" 6 ’", which being redu- 
 cedto Earis Meafure, make juft 24". Now in the 
 Tables above, the Height of the Place where the Mer¬ 
 cury fubfides to 24//, is, according to Mariotte, of 744 
 Toifes, two Foot, or 3266 Foot above the Level of the 
 Sea, according to CaJJini , 676 Toifes, or 405 6 Feet, 
 and according to my Uncle’s Calculation 559 0 2', or 
 335<5/, fo that Mariotte comes 222 Feet fhort of its 
 Height, as it was determined Trigonometrically, 
 Dr. Scheucbzer but 132', but Cajfmi exceeds this 
 Height by 568 Feet, which confirms again, as I have 
 fhewn in a former Paper, that the Mariottian Table is 
 preferable to that of Cajfmi, though pretended to have 
 been corrected upon the former, and that that of Dr. 
 Scheucbzer is an Improvement upon both. According to 
 the Obfervation made by Dr. Halley , May 26,1697, the 
 Mercury flood at the Top of, Snowden-bill, at 2 6" 
 F" Englifb , which, if reduced as above, would give 
 the Height of the Mountain fomething lefs. 
 
 In France , when the Meridian Line, firft begun in 
 1669, was continued in 1703, the Heights of feveral 
 Mountains, particularly in the South of France, were 
 determined Trigonometrically by the Members of the 
 
 H h h h 2 Royal 
 
( 5 8 ° ) 
 
 Royal Academy of Sciences: And I find up and down 
 in their Memoirs, the Heights of the following. 
 
 Height in 
 Toifes. Feet. 
 277 or 1662 
 397 — 2-382 
 
 408 — 2448 
 
 Mont Clairet in Brovence 
 La Majfane in RouJJilion - - 
 The fame according to another - ! 
 
 Obfervation ... - -y 
 
 Bugarach , a Mountain in Lan -7 
 gitedoc - - - -$ 
 
 Mountains in Auvergne. 
 
 Le Buy de Domme, near Clermont 
 La Courlande 
 La Cofie 
 
 Le Buy de Violent 
 Le Cantal ... 
 
 Le Mont d’ or • 
 
 In the County of Avignon. 
 
 Le Mont ventoux - - 1036 
 
 Pyrenean Mountains. 
 
 S. Barthelemy dans le poke? Q 
 
 a, fnix . . X II8 ? ~ 71 
 
 de foix 
 La Montagne du Mouffet 
 Le Canigou 
 
 648 — 3888 
 
 810 — 4860 
 838 — 5028 
 851 — 5106 
 
 8 f 3 —• 5118 
 984 — 5 ’ 9°4 
 1030 — 6180 
 
 6zi6 
 
 10 
 
 1158 — 7548 
 1440 — 8640 
 
 Before I proceed farther, I muft beg Leave to obferve, 
 that the Heights of thefe Mountains, in the main, feem 
 rather too great. This indeed is eafily accounted for, 
 as they were meafured byTrigonometrical Obfervations, 
 which will, as I have took Notice above,becaufe of the 
 
 Re- 
 
( J8* ) 
 
 Refraction of the Air, give the Heights greater than 
 they actually are. But what confirms it ftill more, is, 
 that according to the Tables above, the Numbers which 
 anfwer to the Heights of the Mercury, as they were 
 obferved at the Top of fome of thtpfe Mountains, are 
 confiderably lefs, and that even Monf. Cajfini's own 
 Numbers, which yet we have by fome undoubted Ex¬ 
 periments (hewn to be too great, fall often Ihort. It 
 will be enough to mention two or three Inftances. At 
 the Tower of Majfane in RouJJillon$\z Mercury flood 
 at 25 '' S 1 '' 1 and the Height of that Place was deter¬ 
 mined trigonometrically, of - - 39,7 Toifes. 
 
 Now 2.5'' <" r anfwer according? 
 
 Mariotte, to " * S ° 
 
 According to CaJJini , - - 392 4 
 
 According to Dr. Scheuchzer - - 3 70 o 
 
 At the Top of the Mountain called la Cofle in Au¬ 
 vergne, the Mercury flood, 0 £l. 9,1700, at 2 
 and the Height of this Mountain was determined Tri¬ 
 gonometrically of 
 
 Now 23 // 4"' anfwer ac-~) 
 cording to Mariotte,to j 
 Caffint 
 
 Dr. Sbeuchzer - - 
 
 The Difference is ftill more - 
 confiderable withRegardto 
 the high MoontamMontdl l rp - r 
 
 or toAuvergne,th.e\He\g\\t I ~ ~ I 0 -l° 01 ‘ es 
 
 whereof was determin- / 
 ed Trigonometrically to J 
 
 Toifes. 
 
 differ. 
 
 206° 
 
 At 
 
< 581 ) 
 
 At the Top of this Mountain the Mercury fell, ac¬ 
 cording to an Obfervation made by F. Sebajlien 
 Truchet, June 8,1705-, to 22^ uwhich anfwer 
 according to 
 
 Mariotte ,to • 707° 5^ C3 32Q i> 
 
 Cajfmi to - - 9x5 1 ^differ .< 114 5 
 
 Dr. Scheuchzer - 717 3 j £311 3 
 
 I come now to the Mountains of Swijferland. The 
 Barometrical Obfervations made by my Father upon 
 ieveral of the higheft will convince us, that they rife 
 aloft, above all the neighbouring ones in France,Spain, 
 Italy and Germany. And that it mull be fo appears 
 farther, becaufefrom their elevated Tops, they difpenfe 
 their Waters to all the European Kingdoms and Pro¬ 
 vinces around them. Nay, I doubt not, but that they may 
 vye in Height with the moft confiderable Mountains in 
 any other Part of the known Globe. Swijferland it 
 felf, I mean its Valleys and lower Parts, as they are 
 confiderably remote from the Sea, rife alfo in Propor¬ 
 tion above the Level of it. ’Tis true, the Afcent thi¬ 
 ther is but gradual, in Proportion to the Remotenefs. 
 At Zur'tc, for Inftance,which lies towards the Northern 
 Borders of Swijferland, the mean Height of the Baro¬ 
 meter hath been obferved of 16" p», which give the 
 Elevation of that Town, above the Level of the Sea, 
 according to Mariotte , 105- Toifes, 4 Foot, or 1234', 
 according to Dr. Scheuchzer, zio° 4L or 1164', and 
 according to CaJJini, 211° 4', or 1330'. This Town 
 is diftant from the Mouth of the Rhine, which is the 
 neareft Part of the Ocean, at leaft 3 75- Englijk Miles, 
 or an hundred marine French Leagues, and from Ge¬ 
 noa which is neareft upon the Mediterranean, 225 En- 
 1 glijh 
 
( 58 1 ) 
 
 glijb Mile?, or 6z French marine Leagues. So that go¬ 
 ing down from Zuric Northwards towards the Sea, the 
 Defcent, or Fall, is but fomething more than iz Foor, 
 .’for a marine League of France , if we fuppofea ftreight 
 Line to be drawn from Z uric to the Sea-fhore in Hol¬ 
 land \ but it is much greater going Southward towards 
 the Mediterranean, where it comes at leaft to zo Foot 
 for one League. Nay, if we confider that the high- 
 eft Mountains of Swijferland lie almoft dire&ly be¬ 
 tween Zuric and the Mediterranean Shores, we muft 
 allow fo much more in Proportion, as thofe Mountains 
 are elevated above the Horizon of Zuric , and how 
 great and fudden this Elevation be, will appear by the 
 following Obfervations. 
 
 At Ennen Sewen gen Aweren in the Afcent 
 of the high Mountain Freyberg, in the Canton of 
 Glarus , which lies South Eaft of Zuric , the Mercu¬ 
 ry was obferved Sept, it, *710, at 23" 10'", which 
 gives the Height of that Place above the Level of the 
 Sea, according to 
 
 Mariotte - - 569° 2' or 3416' 
 
 Dr. Scheuchzer - 584 4 — 35-08 
 
 Cajfmi - - - l\z 3 — 4275 
 
 Upon Scherf, one of the Branches of the Freyberg, 
 the Mercury fell Sept. 12,1710, to 21" 8'", which gives 
 the Height of that Part of the Mountain according to 
 Mariotte - - 906° 1' or 543 7' 
 
 Dr. Scheuchz>er - - 931 2 or 5588 
 
 CaJJini • - - . 1247 4 or 7486 
 
 Still 
 
C 5*4 ) 
 
 Still higher upon Blattenjlock, another Part of 
 the fame Mountain, the Mercury fell on the fame 
 Day to 21" 6 '", which anfwer according to 
 
 Mariotte, to - - 933 0 2' or $600' 
 
 Dr. Scheuchzer - - 95-9 2 or 575-6 
 
 CaJJini - - 1293 3 or 7761 
 
 Hence from Zuric to the Blattenjlock near the 
 Top of the Freyberg, there is, in lefs than three Days 
 Journey, a Rife of 4366 Feet, according to Mariotte , 
 and 4492, according to Dr. Scheuchzer, that is, more 
 than three times the Elevation of Zuric above the Le¬ 
 vel of the Sea. 
 
 At Gujpen ob Schwanden, in the fame Canton of 
 Glarus, the Mercury was obferved, Augujl 5, 1705-, 
 at 23" 4"', which give, according to 
 
 Mariotte - - 644° 1' or 3865' 
 
 Dr. Scheuchzer - - 661 5 or 3971 
 
 (I omit giving the Numbers according to the Tables 
 of Mr. Cajfini , having already {hewn, that they are 
 too great) The Height of this Mountain is nearly the 
 fame with the celebrated Buy de Domme, where Monf. 
 Terier obferved the Mercury, Sejt. 19, 1648, at 
 23 " 2'". 
 
 Upon Joch , a high Mountain in the Territory of En- 
 gelberg , where it confines upon the Canton of Bern , full 
 South of Zuric, the Mercury flood, June 23, 1706, at 
 21" 4"', which gives the Height of that Mountain 
 according to 
 
 Mariotte - - 961° o' or $766 
 
 Dr. Sheuchzer, - - 987 4 or 5-926 
 
 This Mountain, though very high, is far from being 
 the higheft in that Neighbourhood, for next to it 
 
 there 
 
( 5 8 * ) 
 
 there rifes another called the Titlisberg, , covered with 
 everlading Snow, which we may, upon a moderate 
 Computation, pronounce at lead 1000 Foot higher 
 than the Top of the Joch , and confequently one of 
 the higheft in the Country, 
 
 Upon the Avicula , by the Italians called Monte 
 del ’ Uccello , and by fame S. Bernhard's Mountain, 
 from a Chappel built in Honour of that Saint, a high 
 Mountain in Rhoetia , towards Italy , the Mercury was 
 obferved, July 30, 1707, at 22" n'", which give ac¬ 
 cording to 
 
 Mariotte - - 707° 5' or 4247' 
 
 Dr .Scheuchzer - - 727 3 4365 s 
 
 This Height mud be underdood only of that Part of 
 the Mountain which is paffed over by Travellers, the 
 Mountain it felf riling confiderably above it, andthe^- 
 dula^o r of Strabo>Gz og. L. III. of which 
 
 the Avicula is only a Part, being dill higher. The 
 Rhenus pofterior , or Hinder Rhein , and the Moiifs , 
 which at lad lofes itfelf into the Tejin , near Beilin - 
 not much above the Entry of the Tejin into 
 the Lake of Locarno , arife upon this Mountain. 
 
 At Santa Maria , upon the Luckmannier Berg , by 
 fome S. BarnabJ s Mountain, which is likewife a 
 Branch of the Adula^ the Mercury dood^/£. 9, 1725 s , 
 as upon the Avicula, , at 22" n'", which diews the 
 Height of thefe two Places to be equal. 
 
 In the Alp jPtfr/^,near the Source of the Hinter 
 
 Rhein ,, Rhenus pojlerior , five Hours and a half from 
 Speluga^ Splugen in Rhoetia , the Mercury was ob¬ 
 ferved, July 29, 1707, at 21" 4'", where it dood like¬ 
 wife upon the above-mentioned Mountain whi¬ 
 
 ther the Reader is referred for the Height of this Alp. 
 
 I i i i - At 
 
( ) 
 
 At Splugen itfelf, the Mercury flood the fame Morn¬ 
 ing early, at X3" 4'", which give the Elevation of 
 Spliigen according to Mariotte 644° P or 3865, and 
 according to Dr. Scheuchzer , 66 1° 5' or 3971'. So 
 that the Fall of the Rhine from the Aip aforefaid to 
 Splugen , in five Hours and a half, comes, according to 
 Mariotte\ to 1901, and according to Dr. Scheuchzer , 
 to 1955 Faris Feet perpendicular. 
 
 At the Capuchins, upon the high Mountain S. 
 Gothardj a celebrated PaiTage out of Swifferland into 
 Italy , the Mercury flood, 30, 1705, at n" o, 
 
 which gives the Height of that Paflage, which with 
 Regard to the higheft Tops of S. Gothard , lies but 
 as it were at the Foot of a high Mountain, according 
 t o Mariotte 85^°, or 5 nx', and according to Dr. 
 Scheuchzer , 875° 5', or 5x55, above the Level of the 
 Sea. 
 
 Upon the Furca , a high Mountain between the 
 Urferen Thai , Urfaria Vallis , and the upper 
 /e/K?, and one of the Branches of the S. Gothard , the 
 Height of the Mercury in the Barometer was obferved, 
 July , 31, 1707, atxP' 5'", which give the Height of 
 this Mountain above the Level of the Sea, according 
 to Mariotte , 947 0 Per 5683', and according to Dr. 
 Scheuchzer , 973 0 3' Or 5841. Near this Mountain 
 there are others, which cannot be lefs than 800 or 900 
 Foot higher. 
 
 Thefe Mountains, I mean the Avicula> the Luck - 
 mannier Berg , the S. Gothard , and the Furca , toge¬ 
 ther with the Grimfula , the Crifpalt , the Sempronier , 
 or Sempronius Mons , the Adula , and a Chain of others, 
 are the Lepont'ue Alpes of Fliny * and the Summa 
 
 t * Lib. iii. c. xx* 
 
 Alpes 
 
( 587 ) 
 
 Alpes of Cafar * They begin in the upper Valle/ia , 
 traverfe the Canton of ri t and fo run on Ea ft wards, 
 a-crofs the Country of the Grifoons , towards Tirol. 
 Their greateft Height above the Level of the Sea, may 
 be fixed in round Numbers to 7500, or 8000 Faris 
 Feet. 
 
 5 f is upon thefe very Mountains, that fome of the 
 moft confiderable Rivers of Europe take their firft 
 Rife, within very fmall Diftances of each other. The 
 Rhojhefov Inftance, Rhodanus , by Marc Minus call¬ 
 ed, maximi nominis flumen , and by Varro , Fluvius 
 inter tres Europe maximus , arifes from two Gletch* 
 ers , as we call them, or Montes glaciates^ huge Moun¬ 
 tains of Ice , near the Furca, whofe Height hath been 
 above determined, and thence runs with great Impetu- 
 ofity down Vallefia , the Walii(ferland, forming a 
 long Valley, furrounded on both Sides with huge 
 Mountains, till it loofes its Waters and Name in the La - 
 cus Lemannus , or Lake of Geneva, , but refumes it 
 again near the Town of Geneva, whence it flows with 
 a more gentle Defcent through fome Provinces of France 
 into the Mediterranean Sea. 
 
 The The fin, Ticinus , by Claudian , in his Pane¬ 
 gyric upon the Confulate of the Emperor Honorius , 
 called Fulcher , the handfom , takes its firft Rife from 
 two fmall Lakes upon the S. Gothard , and fome late¬ 
 ral Sources from the Lago fopra la Cima di Fettine , 
 upon a Mountain called Fettine, the Lago della Sella, 
 the Lake of Rottom upon the Luckmannier Berg, the 
 Lake of Tom , and the Lake of Bedretto, upon a Moun¬ 
 tain of this Name. It defcends the Lavinia Vallis , 
 or Liviner Valley , and in its Way to the Lake of 
 
 * De Eello Gallico 1. iii. 
 
 I i i i a Lo- 
 
( J88 ) 
 
 Locarno , receives many Brooks and Rivulets from the 
 adjoining Mountains : It unites its Waters with the 
 *p0) near Ravia , and loofes itfelf jointly with that 
 River into the Adriatick Gulf. 
 
 The Rhine, Rhenus , by Cafar de Bello Galileo 
 termed, latijfimus atque altijjimus , arifes in three fe- 
 veral Branches, which are called Rhenus anterior , 
 fofterior , & medius , the further , the hinder , and 
 middle Rhine. The hinder Rhine takes its Rife upon 
 the high Mountain Avicula^ Colmen del Occello , 
 Part of the Adula, in the Alp San Rorta , from a 
 Gletcher , or Ice-mountain, which extends in Length 
 full two Hours. The middle Rhine , Rhenus medius , 
 arifes upon the Luckmannier Berg , which is likewife 
 Part of the Adula , in the upper Part of a Valley called 
 San Maria , oppofite to one of the Sources of the 
 Thefin . The furthermojl Rhine, Rhenus anterior , 
 arifes upon that Branch of the Crifpalt , which is call¬ 
 ed del Badut , Baduz , and foon receives feve- 
 
 ral lateral Branches from the Mugels and Come- 
 ra. My prefent Purpofe will not fuffer me to purfue 
 the Courfe of this River in its feveral Branches. Near 
 the Monaftery of Difentis , the farther and middle 
 Rhine join together, and the united Stream falls into 
 the hinder Rhine, near Reichenau . Below Rheineck , 
 the Rhine falls into the Lacus Bodamicus 3 or Boden 
 Sea y and comes out of it near Stein ^ whence walking 
 for fometime the Borders of Swijferland\ it then tra- 
 verfes great Part of Germany in a very irregular 
 Courfe, till at laft, in Holland , it loofes itfelf into the 
 great Ocean. 
 
 The Reufs , Rufa y arifes from a fmall Lake called 
 ,Lago di LuzendrOj upon the S-. Gothard , but foon 
 receives a confiderable Inforcement from the Furca 9 
 
 and 
 
( S8p ) 
 
 and near ^rfelen , another from a mountainous Lake 
 in Oberalp. Near Fiuelen , not far from Vry 9 it en¬ 
 ters the IV. Waldfietten Sea, Lacus quatuor Civi- 
 tatum Sylveftrium, but refumes its Courfe and Name 
 at Lucern , and at laft falls into the Aar below Win- 
 dijb , Vindonijfa. 
 
 The Aar^Arola, Arula. , arifes upon the high Moun. 
 tain Grimfula , in the upper Vallefia. About three 
 Hours below that, it falls into the Lake of Brientz , 
 and out of that, not far from the Monaftery Interla- 
 chen , into the Lake of Thun , which it leaves near the 
 Town of Thun , and thence running by Bern , Solo- 
 thurn , andfo down, falls at laft, after many Windings 
 and Turnings into the Rhine near Coblentz, Confluen - 
 tia , probably fo called from the uniting of thefe two 
 confiderable Rivers. But to proceed. 
 
 Gemmius Mons , the Gemmi , is a very high and 
 fteep Mountain in Vallefia , over which there is a Paf- 
 fage, but only in Summer-time, from the Fruttinger 
 Valley , in the Canton of Bern, to the Mineral Waters 
 at Leiik in Vallefia. The Defcent, on the South-fide 
 of this Mountain, is fteep and frightful, even to the 
 Afped, beyond what can be imagined, being a narrow 
 Path, cut on the Side of almoft perpendicular Precipices, 
 fometimes with trembling woodenBridges,or Planks over 
 the Clefts in the Mountain, and here and there fupported 
 with low Walls. Having been geometrically meafu- 
 red, it was found of ioiio Feet in Length, or rather 
 Height, its many Windings and Turnings included. 
 At a fmall Cottage, called Zur TDauben, a poor refting 
 Place for weary Travellers, being the higheft Part of 
 the Mountain which is paflable, the Mercury fub- 
 fided July i, 1709* to 21" 3"' which gives the Height 
 
 o 
 
( 59 o 
 
 of that Place, according to 
 Mariotte 
 
 And Dr. Scheuchzer 
 
 974 ° 5 ' or 5-849' 
 1002 o or6012 
 
 Not far from this Cottage, is a fmall mountainous 
 Lake, called the ‘Dauben Sea , or the Fidgeons Lake , 
 encompaffed on all Sides with high Mountains, the 
 Tops whereof, for their Steepnefs, it would be impof- 
 fible to reach. At Kandelftag , the firft Village in the 
 Frutinger Valley , in the Territory of Bern , going up 
 to the Gemmi^ the Mercury rofe on the fame Day to 
 24" 2'", which give according to 
 
 Mariotte - - 5*20° 1' or 3121' 
 
 Dr. Scheuchzer - - 534 1 or 3205' 
 
 And at Mullenen , at the Foot of the Gemmi , it flood 
 at 25" 7" / , which anfwer according to 
 
 Mariotte to - - 318° 5' or 1913* 
 
 D r.Sheuchzer - - ' 327 o or 1962 
 
 On the other Side of the Gemmi. t at Leuck , a celebra¬ 
 ted Place for its Mineral Waters,the Mercury was obferv- 
 ed July 2, and July 1709, at 23" 9'", which anfwers 
 according to Mariotte , to ^81° 4', or 3490', and ac¬ 
 cording to Dr. Sheuchzer, to 5*97° 3', or 35%$'. So 
 that the Cottage ! Dauben , rifes above Leuck, ac¬ 
 
 cording to 
 
 Mariotte , - - - 2.3 5^9 7 
 
 Dr. Scheuchzer - 2427 
 
 Above Mullenen , in the Frutinger Valley , accord¬ 
 
 ing to 
 
 Mariotte 
 Dr. Scheuchzer 
 
 3936' 
 
 4050 
 
 And 
 
( 5 JM ) 
 
 And the perpendicular Height of the Gemtni , above the 
 Level of the Sea, confiderably exceeds 6000 Taris 
 Feet. 
 
 But high above all the Mountains of Swijferland 
 rifes the Stella , *Piz Stall , a fteep Mountain in the 
 Schamfer Valley, in Rhcetia,or the Grifoons ? the Height 
 whereof was by my Uncle Dr. John Scheuchzer , by 
 Lome Obfervations made in the Year 1709, determin¬ 
 ed to 9585* Raris Foot, above the Level of the Sea, 
 according to his own Calculation, or 9441 according 
 to Mariotte, and 11196 according to Cajjini : A Height, 
 which the Ruficafra, or Shamoys themfelves fcarce 
 venture toafcend. And his to theie only, and the like 
 Heights the following Verfes of Stilus It aliens ought 
 to be applied. 
 
 Cundta gelu , canaque aternum grandine tetta, 
 Atque aviglaciem cohibent: riget ardua mentis 
 Ait herei facies, furgentique obvia Rhoebo 
 ' T>uratas nefeit jlammis mollire Rruinas. 
 
 Nullum ver ufquam , nullique aflat is honor es , 
 
 Sola jugis habitat diris,fedefque tuetur 
 R erf etnas deformis hyems - 
 
 Having thus determined the Heights of the Moun¬ 
 tains of Swijferland from Barometrical Obfervations, I 
 will now clofe thefe Remarks, which are infenfibly 
 grown to a much greater Length, than I at firft intended 
 they Should, with a few general Obfervations on the na¬ 
 tural Hiftory of that Country, arifing from the Height 
 of its Mountains. 
 
 The firft Observation (hall regard the Lakes, of 
 which there are feveral, and very remarkable ones, in 
 
 and 
 
( 55 >* ) 
 
 and upon the Borders of Swifferland* , which will af¬ 
 ford roe a Angular Inftance of Divine Providence. The 
 Afcent of the Mountains of Swijferland being fo very 
 fudden and quick, that as I have above ftiewn, the Ele¬ 
 vation of the Mountains in the Canton of Glarus a- 
 bove the Horizon of Zuric , though not quite three 
 Days diftant, is more than three Tiroes as great, as the 
 Elevation of Zuric itfelf above the Level of the Ocean, 
 of which it is upwards of 375 Englijh Miles diftant 
 in a ftreight Line ^ and fo in Proportion of others; and 
 the Rivers, which arife in thefe Mountains, rulhing 
 down, in Confequence of fo quick a Defcent, with great 
 Force and Impetuofity, it was to be feared, they would 
 often overflow their Banks, and caufe frequent Inunda¬ 
 tions in the flat Countries, (of which there are too many 
 Inftances in our own Vallies and Plains,) if this Force 
 and Impetuofity was not in great Meafure broke, and 
 their Waters difpofed to a more gentle Defcent. And 
 this is efteftually done by thofe great Receptacles of 
 Water, the Lakes, which are befides of infinite Ufe to 
 the Inhabitants around them, fupplying them with 
 Plenty of Fifh for their Suftenance, and enriching them 
 by the Facility with which Commerce may be carried 
 on over them. Thus the Rhine falls into the Lacus 
 Bodamkus , Boden-Sea , the Rhofne into the Lacus 
 Lemannus, or Lake of Geneva , the Muefa and The - 
 Jin into the Lake of Locarno, the Reus into the Lake 
 of Lucern, the Adda and Maira into the Lake of Co- 
 mo, the Lint, or Limat , into the Lake of Zuric , the 
 Aar, into the Lakes of Brientz, and Thun. And it 
 feems, that the more confiderable the Rivers are, and 
 the more impetuous their Courfe, fo much the greater 
 muft the Receptacles be, wherein they are to loofe their 
 
 Force 
 
< 59? ) 
 
 ‘Force and Rapidity. The Lake of Geneva, and the 
 ' Boden-Sea, the two largeft in Svuijferland , evidently 
 evince what I hete aflert, and the others above-named 
 gradually decreafe in Largenefs, in proportion as the 
 Rivers, which fall into them, are lefs and lefs rapid. 
 
 The extream Smallnefs of the Alpine Plants is ano¬ 
 ther Obfervatidn, 1 intend to make. They become lefs 
 and lefs, in proportion as the Mountains, upon which 
 they grow, rife higher. Whether this be outing to the 
 Sharpnefs and Purity of the Alpine Air, or the decreaf- 
 ing Preffure of the Atmofphere, which is far lefs upon 
 Mountains than in Valleys and lower Countries, or to 
 a Want of a fufficient Quantity of fubterraneous Heat, 
 to pulh the Nourilhment into the Roots and Velfcls of 
 the Plants, or rather to a joint Concurrence of thefe 
 a:nd other Caufes, would require a more leifurely Con- 
 fideration. The Thing itfelf is an indifputable Matter 
 of Fad, and it extends alfo to Trees and Shrubs, which 
 become fmaller, as they grow higher. Nay, what is 
 Rill more remarkable, no Trees will grow beyond a 
 certain Height, which is the Reafon why the Tops of 
 Mountains appear fo bare and naked, if viewed at a 
 Diftance, though a curious Traveller fhall not fail 
 meeting upon their rich Paftures with an agreeable Vari¬ 
 ety of beautiful Plants. The Height, where the Trees 
 ccafe to grow, hath been, found, by Barometrical Obfer- 
 vations, nearly to be the fame in divers Parts of Swijfer- 
 land. Otherwife, the Smallnefs of the Alpine Plants is a- 
 bundantly compenfated by the Richnefs of their Virtues, 
 which are, as it were, purpofely centred there into fo 
 narrow a Compafs. 
 
 But to another ObferVation. The Mountains are 
 
 l rv *'x • i - 
 
 much more abrupt, and fteep, and the Precipices greater 
 Kkkk to 
 
( 594 ) 
 
 to the South, than to the North, and Weftwards than 
 Eaftwards. Many Inftances of this might be given 
 in particular Mountains in Swijferland, as the Gemmi , 
 the Mons fraElus and fo forth j but it is alfo evident¬ 
 ly true with Regard to the whole. Thofe are the 
 higheft Mountain?, which feparate Vallejia , tile Can* 
 ton of Dri, and the feveral Leagues of the Grifons, 
 from Savoy , Piemont , and the Tirol, which lie to the 
 South, or South-Eaft. Thofe very Countries are, . 
 as it were, one continued Set of high Mountains, quite 
 to the Mediterranean Sea, and the like Strufture feems 
 to be continued farther on into that Sea itfelf. The 
 EPyrenean Mountains alfo are but a Continuation of 
 that vaft Chain, which begins in the Lepontia Alpes, 
 or the Mountains in the upper Vallejia, the Canton 
 of ‘Dry and Rhoetia, and from thence fpreads itfelf 
 chiefly Weft and South. On the contrary to the Eaft 
 and North they break off by Degrees into gentle Plains, 
 which appears evidently by the vaft Trafts of Ground, 
 which the Rhine for Inftance, and the ^Danube com- 
 pafs, before they lofe themfelves, the one into the Ger¬ 
 man Ocean, the other into the black Sea, whereas the 
 Rhofne , on the other Side, quickly and with a pro* 
 portionable Velocity reaches the Mediterranean. The 
 fame Obfervation, with Regard to the abrupt Steepnefs 
 of Mountains to the South and Weft, holds true in other 
 Parts of Europe , remarkably in EnglandNorway , 
 more or lefs in other Countries. And fo far as our 
 Maps, and the Accounts of Travellers go, the fame 
 Thing is obfervable in other Parts of the World, but 
 moft evidently in the high Mountains of Peru and Chi¬ 
 li in South America , which, terminate very abruptly 
 Weft wards into the Pacifick Sea, buf gradually decline 
 
 to 
 
( 595 ) 
 
 to the Eaft into immenfe Plains, watered by fome of 
 the moft confiderable Rivers in the known World, oar. 
 ticularly the River of Amazons and the Rio della 
 Rlata, which arife in the faid Mountains. 
 
 To conclude, from what hath been hitherto faid, it 
 appears evidently, that the Mountains of Swijfer- 
 land are the higheft of Europe, and the great. Store- 
 houfe, whence all the Countries around them are fup- 
 plied with Water j conform to what the learned Lo- 
 ritus Glareanus hath long fince elegantly expreffed in 
 the following Verfes. 
 
 jP raterea caput Europes hanc ejfe probabufit: 
 Aiternis Alpes niv'tbus, juga Olympica, quorum 
 5 Porgitur in cxlum caput, & fub Tartar a venter : 
 Et quod ad Auroram, Boream,Solemque cadentem 
 Flumina perpetuo non deficientia curfu 
 ‘Parturit, ilia volant & in omnia membra re dun* 
 
 ( dant, 
 
 Ad Xephyrum & Libyen Rhodanus, Rhenana fu - 
 
 ( rent-em 
 
 ‘Vnda citat Boream, gelidus rotat Ifier ad Eurum 
 \Dirus aquas , Getico novus Hofpes €£ advena 
 
 ( Tonto. 
 
 Aflalios fileo, fileo quos Italia accipit amneis 
 Alpibus a nofir is, quaque alto a vertice montes 
 Agmina difparibus fundunt latijfima Sulcis. 
 
 Whether the Depths of the Sea correfpond with the 
 Heights of Mountains, muft be left to future Obfer- 
 vations. 
 
 K k k k 2 
 
 II. Opti - 
 
('■ 59 & )> 
 
 II. Optical Experiments made in the Beginning of 
 Auguft 1728, before the Brefidenl and federal 
 Members of the Bpyal Society, and other Gentle¬ 
 men of federal Nationsupon Occaflon of Signior 
 Rizzecti’-f Opticks, with an Account of the faid 
 Book,, By J. T. Delaguliers, L L. D.. and 
 F. (!{. S. 
 
 S Oine Time in the Year 1722, Signior Gizlanzoni; 
 
 an Italian Gentleman, fhew’d me a Paper of Signi¬ 
 or John Riszzetti, wherein he denied the different 
 Refrangibiiity of the Rays of Light, becaufe an Ex¬ 
 periment mentioned in Sir Ifaac Newton’s Opticks (B.i. 
 Trop. 1. Exp. 2.) concerning an oblong Paper paint¬ 
 ed half blue and half red (whofe Image projefted 
 by a Lens upon a white Paper at a confiderable Di- 
 ftance became diftinct in its blue half, nearer the Lens 
 than in its red half) had not fucceeded with him, tho’ 
 after many Trials. Sir Ifaac Newton being acquaint¬ 
 ed with this, defired me to repeat his Experiment above- 
 mentioned, which I did at my own Houfe, before him 
 and Signior Gizlanzoni and fome other Perfons, who 
 were fatisfied with the Succefs of it, according to 
 Sir Ifaac Newton's Affertion. Afterwards on the 13th 
 of ‘December 1722, I again repeated the Experiment 
 before the Royal Society, with the fame Succefs, the 
 full Account of which is printed in the Fhilofophical 
 Tran fall ions. Number 3741 After this, Signior 
 Gizlanzoni read me a Letter from Signior Rizzetti, 
 wherein he faid “ He wanted to know whether the 
 
 <e Expe- 
 
( 597 > 
 
 K Experiment would fucceed, if the’Paper was turned 
 “ fo as to bring the red Half in the Place of the blue 
 “ one j and that if it fucceeded then, yet he would 
 ct not acquiefce, but he (hould have fomething (fill to 
 c objed agaiiift it. Arid further, that he wanted to 
 H know what could be faid to feveral other Objections 
 “ (1 think there were twelve in the Paper (hewed me) 
 “ againft many more of Sir Ifaac Newton's Optical 
 Experiments, the greatefl Part of which he faid* 
 c * he had found to fucceed differently from what 
 u Sir Ifaac had related * 5 and would not allow the 
 cC Confequences to be juft, which were drawn from 
 c; the other Experiments which he had found to agree 
 a with his TrialsUpon this I acquainted Signior 
 “ Gizlanzoni in a Letter, which Idefired him to com- 
 u municate to Signior Rizzetti ; “ that as" Signior 
 <c Rizzetti had put the Ilfue of the Difpute upon the 
 Ci Succefs of an Experiment, which after repeated Trials 
 had fucceeded contrary to his Opinion, he ought to 
 “ acknowledge his Miftake ^ and then I (hould willingly 
 tC repeat all the other Experiments which he had called 
 <c in Queftion, and endeavour to remove his other Diffi- 
 66 culties. That if it was Truth and not Vidory which 
 * c he contended for, I did not doubt but he, would, com*. 
 « ply with me, in what I infilled upon ; and then I (hould 
 “ be ready to make any Experiment, or clear up any Dif- 
 “ ficulty relating to the Dodrine of Colours, in the beft 
 manner I could”. But I never heard any more from 
 Signior Rizzetti } but was told by others that he was 
 very angry at Signior Gizlanzoni ? and faid he was 
 got into Sir Ifaac Newton's Party. 
 
 Now at lad Signior Rizzetti haspublilhed a Book, 
 entituled ©e Luminis Aff'eHionibus Specimen Thyfico* 
 
 mathematicum , 
 
( 59 8 ) 
 
 mathematicum , dedicated to Cardinal 3 P olignac , and 
 printed at Trevife and Venice , 1717, which being 
 prefented to the Royal Society , and by the Society re¬ 
 commended to me to give an Account of it, I hope no 
 Body will blame me for making a faithful Report. 
 
 The Author in his Preface, and throughout the 
 whole Book, in a moft arrogant manner, hasinfulted the 
 greateft Philofopher that this or any other Age ever 
 bred, triumphing in what he thinks, the Miftakes of 
 Sir Ifaac Newton and his own Difcoveries. Had he 
 modeftly related the Fads as they appeared to him, and 
 his Reafons for drawing Confequences different from 
 thofe of Sir Ifaac Newton , the World might have 
 thought him urged on by the Love of Truth in his 
 ten Tears Labours 5 * and Ills Errors might have been 
 excufed according to what he fays in his Preface Si 
 forte decifior , baud turf is eft in re Fhyfico-mathe - 
 matica error , & magnis fe quifque tuetur exemflis * 
 Neither would his Fame have been the lefs (if he had 
 been right in his Experiments and Reafonings) for 
 treating his Adverfaries in a civil Manner, and really 
 doing what he fays at the End of his Preface « Rlacita 
 “ quidem authorum lacejfo ; at ipfos tamen authores 
 “ obfequio & veneratione frofequor For ill Man¬ 
 ners can never be excufed by what he calls Thilofo • 
 fhica Libertas . Now nothing lefs than owning, that 
 a greedy Defire of Fame , and an Obftinacy to main¬ 
 tain what he once laid down as his Opinion , mifted 
 him fo far , can excufe him to the learned IForld. 
 Wehear indeed in a Letter from Sir ThomasDereham 
 to Sir Hans Sloane , Prefident of the Royal Society , 
 that now Signior Rizzetti alledges, “ That he was 
 
 * See Preface, p. 38. 
 
 deceived 
 
( 5 99 ) 
 
 « deceived in his Experiments, by Reafon of the Rad- 
 <* nefsof his Prifms which he had from Venice' 1 ; but 
 this is but a partial Acknowledgment of his Error, and 
 only fatisfadory to fuch as have not read his Book, and 
 are likewife unacquainted with Sir Ifaac Newton's 
 Opticks. For Signior Rizzetti in the 37th and 38th 
 
 Pages of his Preface has thefe Words-“ Ut fuofmet 
 
 « oculos confulanty omnes quidem pojfunt mea Ex - 
 “ perimenta iter are ; at fiprimo intuitu deficit , qui 
 H expettatur , eventus ^ flatim me falfitatis non ar - 
 « guant {ut vir Celebris dicitur nimia fefiinatione 
 “ f ec ilf e ) ? fedprius omnes meorum experimentorum 
 tC limit at 'tones notent: Si ad has enim attenderint ^ 
 « non pingui Minerva , non trigonis imperfect is [_ut 
 ct alii di£litant~\ fed accurata diligent ia, ® inflru- 
 “ mentis idoneis me in hujufimodi res incubuijfe etiam 
 u adverfarii , vel inviti fatebuntur As to Sir Ifiaac 
 Newton's Experiments, a great many of thofe which 
 Signior Rizzetti calls in Queftion may be performed 
 with very ordinary Prifms: And of that Sort are thofe 
 which I madefince the Publication of the Book in the 
 Beginning of laft Augufi , at myownHoufe, before the 
 Prefident and feveral Members of the Royal Society , and 
 fome foreign Gentlemen. But before I give an Account 
 of the Experiment?, I beg Leave to quote fome of 
 Signior Rizzetti' s Expreflions againft Sir Jfaac New¬ 
 ton , otherwife I may be thought guilty of the very 
 Fault which I have laid to his Charge. 
 
 Preface, Page 13. “ Mir at us fum quod acutijfimus 
 « Newtonus ignoraverit Lumen album aliter a 
 «« Trigono , & aliter a lente differgif Who ever 
 read Sir Ifaac Newton's Opticks, that can be of,this 
 Opinion, except our Author ? Preface, Page 8i.~—• 
 
 “ Theoria 
 
( 6oo ) 
 
 a Theoria in qua tot Hypothefes funt, quot expli- 
 “ canda Thanomena”. When it is well known that 
 Sir Ifaac Newton did not make Hypothefes to ex¬ 
 plain,but deduced plain Confequencesfrom Rhanomena. 
 In the Book, Page 57, he fays of Richterus his Ad- 
 verfary——“ Si diverfam color urn refrangibilitatem 
 garriendo tueri deliberat and of Sir Ifaac —- 
 
 a Si Newt onus hallucinatus eft , quia in lino experi- 
 ' a mentor urn genere fallacies unici ac difficillimi in- 
 “ dulfit ; nos antequam confequentias eliciamus, 
 fi dijiimus omnia ejufdem generis experimental 
 “ peragrare , a facilioribus incipiendoP Page 76. 
 fpeaking of Sir Ifaac—Nunc igitur confidentius 
 €t afirmo, quod cquidem plures, quam debuerat con - 
 ce fequentias ex Idhanomenis anil or collegitS Page 
 '■90. “ Quis aufiffet canonem tanti viri impugnare , 
 cum cejferunt omnes , qui haffenus eum veluti 
 u trutina expenderunt , &c.” Page 91. “ This Ex- 
 4£ preffion is particularly remarkable —“ Newtonus 
 46 hoc argumentandi modovidetur fecijfe m Lesbii , 
 “ gw, *7/^ ad Regulam minime accomoda - 
 rentur , Regulam ad ades accomodarunf\ 
 
 I. cannot pafs by what he fays of Richterus in p. r 00, 
 “ becaufe it is fo applicable to himfelf, Richterum 
 admonui, ut marte fuo , quod omiferunt alii per- 
 cc ficeret ; is autem , fpreta admonitione , videre 
 a quod a natura agatur , /£*/ pot ins agere vult , 
 
 gw*/ videtur agendumP Page 127. “ CWw& 
 ^ auBor in medium ferat , parum in iis exper - 
 <£ ejfe videtur, qua in poliendis vitris occurrunt.” 
 
 Not to be tedious in fuch unpleafant Quotations, I 
 proceed to give a fhort Account of the Book it felf, to 
 fhew how much Pains fome People can take to be in the 
 
 Wrong, 
 
( 6oi ) 
 
 Wrong, there being no Experiment of Sir Ifaac's called 
 in Queftionbut what is true, and no Confequence dif¬ 
 ferent from Sir Ifaac's in thofe Experiments which he 
 found tofucceed, but what is falfe j as may be evident 
 to any one who reads Sir Ifaac Newton's Opticks with 
 Attention enough to underftand them ; and has proper 
 Inftruments and fufficient Dexterity and Accuracy to 
 perform the Experiments. v 
 
 Our Author (1110* he profeffes himfelf an Enemy to 
 Hypothefes) begins his very firft Propofition with a 
 Demonftration drawn from a falfe Hypothefis ; for he 
 fuppofes every Beam of Light to be as a Parallelo¬ 
 gram of fome Breadth like a Ribbon, as if the Rays 
 cohered together like the longitudinal Threads of the 
 Ribbon • then coniidering a narrow Side of the Paral¬ 
 lelogram (which he calls the Seftion of the Beam)as an 
 inflexible Line, he takes a great deal of Pains to draw a 
 Confequence from it, which Experiments ftiew to be falfe, 
 namely, that Light palfes with more Difficulty through 
 a denfe, than through a rare Medium. He affirms, “ That 
 white Light never affords Colours by Reflexion.” 
 
 That the Union of all Kind of Rays do not make 
 
 (C 
 
 a 
 
 u 
 
 white." 
 
 ■“ That Light reflefted from a white Gbjed, 
 and feen through a dark Medium, becomes yel¬ 
 low or red\ as the Medium is ftronger or weaker j 
 that black feen through a lucid Medium appears blue 
 violet ; and green he fays is made from a 
 
 or 
 
 black, 
 white 1 
 a 
 
 Obje& feen 
 firft thro’ a 
 
 lucid 
 1 dark 
 
 idl 
 
 ;kj 
 
 then thro 5 a 
 
 Medium. 
 
 That fome Light paffing through a refra&ing 
 “ Medium is difperfed, which he calls more than re 
 « framed, and fo produces Colours.” 
 
 L 1 11 In. 
 
( 6oz ) 
 
 In order to fupport his Hypothefes of Mediums like 
 Veils to alter the Colours of Objects looked at y he 
 <c confiders (Preface, Page 31.) the Images in the Eye 
 u as an Objed looked at, which would be fuppofing 
 “ other Organs of Vifion to look into the Eyes”— 
 whereas the Pidures of external Objeds {hewn upon 
 the back Part of an Eye placed in an Hole in the Win¬ 
 dow of a dark Room, are only fuch to thofe who fee 
 the Experiment^ but in the Animal who fees, thofe 
 Pidures are a great Number of fmaii Blows or Impref- 
 fions made upon the Fibres of the Retina by the Im- 
 pulfe of many Rays colleded in the Vertices of the 
 Cones of Light within the Eye, correfponding withfo 
 many other Cones which proceed from the vifible 
 Points of external Objeds, and make, what Opticians 
 call Pencils of Rays. 
 
 <c That the Refiftance of Water from its Tenacity is 
 greater than from its Denfity. 
 w That fince a fmall Thread, half blue and half red\ 
 a is feen diftindly by the naked Eye, that Phaenome- 
 “ non overthrows the Dodrine of different Refran - 
 « gibility ” But here the Author does not confider, that 
 the Focus of the Eye is fo fhort that the Diftance of the 
 diftind Bafe of the blue and the red Image of fuch 
 Threads is not equal to the Thicknefsof the Retina . 
 
 “ That the Experiment of the two-coloured Paper 
 iC projeding its Image through a Lens (which I repeat- 
 « ed in the Manner above-mentioned, in the Year 
 u 1712) did fometimes Tucceed with him, andfome- 
 “ times not} and therefore that it did not prove the dif- 
 “ ferent Refrangibility of Rays : But the different 
 16 Place of the diftind Bafe of the blue and the red 
 Image, was to be afcribed to the different Inclina- 
 
 tion 
 
( ) 
 
 « tion of the Parts of the painted Paper to the Sur- 
 « face of the Lens.” But in my Account of the Ex¬ 
 periment in the c Philofophical Tranfatlions , I men¬ 
 tioned particularly that the Axis of the Lens was per¬ 
 pendicular to the Image of the Card, and therefore 
 there could be no different Obliquity, as is objeded. 
 <c That though he found the SpeElrum of Colours 
 u produced by the Prifm in a dark Room tofhorten by 
 i( Degrees, and at laft become round and colourlefs 
 “ (that is white) when viewed by another Prifm, in 
 €t the fame manner that Sir Ifaac Newton had made 
 “ the Experiment; yet it did not convince him of the 
 * different Refrangibility of the Rays:, becaufe when 
 a he had caufed an Image to be painted upon a Paper 
 « like the Speltrum from the Prifm, and enlightned it 
 cc by the dired Light of the Sun, it did not become 
 <c round and white when viewed through a-Prifm as 
 “ the other SpeCtrum did.” But he did not confider 
 the Imperfedion of Painters Colours, nor remember 
 that the Surfaces of Bodies, whether of a natural, or a 
 painted or died Colour (fuch as he calls permanent Co¬ 
 lours) when expofed to any coloured Light, will re¬ 
 fled that Colour which falls upon them, and appear to 
 be of no other, only that they will feem mofl vivid in 
 that Colour which they bear in openDay ; an d 
 therefore that if the Sun’s Light confifts of Rays dif¬ 
 ferently refrangible and producing different Colours 
 (according to Sir Ifaac Newton) the Prifm mud fepa- 
 rate the Light refleded from every one of the painted 
 Colours, and could not bring them together, becauie 
 they were by no means Ample Colours. If therefore 
 he had reafoned right, the firft Experiment would have 
 proved Sir Ifaac' s Dodrine, and the laft would have 
 
 L 1 1 1 z confirm’d 
 
( <W ) 
 
 confirm’d it. And if in his own Experiment he had 
 looked at the painted SpeElrum fucceffively holding 
 the refrading Angle of the Prifm upwards, and then 
 downwards with the fame Inclination (or, what is 
 eafier, turned the SpeElrum upfide down, the Prifm 
 remaining fix’d) he would have feen his SpeElrum 
 fhorter in one Cafe than in the other. 
 
 “ That Sir IfaacNewton" s 8th Experiment of P. I. 
 “ B. II. (in which the prifmatick red and blue falling 
 <c fucceffively on the fame Place of a Book, have 
 61 a different Focus in projcding their Image through a 
 CL Lens ) is inconel ufive ; and rejeds Richterus y s 
 “ Anfwer, viz. That the Colours refleEled from the 
 u Book , as it has a rough Surface , fall always with 
 u the fame Inclination upon the Lens , in what- 
 “ ever Dire Elion they came from theBrifm to the 
 
 “ Book ---adding, That he had acquainted 
 
 <£ Ricbterus 9 that permanent Colours enlighten’d by 
 dired Light in any different Inclination, always fell 
 ts upon the Lens with the fame Inclination ; but ap- 
 parent Colours, which were produced by Refradion 
 “ with a Prifm, differed from permanent Colours in 
 « their Incidence : But that Richterus had purpofely 
 64 concealed it.” 
 
 N.B . It is not likely he would have concealed it, ifit 
 had been true. 
 
 £& That Monfieur Marriott? s Experiments difprove 
 “ the different Refrangibility of Colours”; though, if 
 he had read with Attention and Impartiality the Ac¬ 
 count of the Experiments which I made before the 
 Royal Society by their Order on that Occafion (fPhil. 
 Tranf Numb. 348.) he might have been convinced 
 as well as feveral Gentlemen of the French Royal 
 % Academy , 
 
( 605 ) 
 
 Academy , who had Teen Monfieur Marriotte make his 
 Experiments, and acknowledged themfelves fatisfied 
 when they faw me repeat thofe of Sir Ifaac in the 
 Year 171 
 
 44 That in the firft Experiment of Sir Ifaac (Book T. 
 44 Part II.) He never could deftroy any one Colour, the 
 44 reft remaining ; and that with a larger Obftacle 
 44 he could deftroy the yellow and blue , but not the 
 44 yellow and green — could bring the green , not the 
 
 41 yellow , to be next to the Shadow-and could leave 
 
 44 the green only remaining, but not the yellow." This 
 is an ealie Experiment ^ but Inaccuracy and a very bad 
 Prifm, and Prejudice towards an Hypothefis, or againft 
 an Adverfary, may miflead a Man ftrangely. 
 
 <c That when the Colours produced by the Prifm, 
 and afterwards united by the Lens, produce white 
 44 upon a Paper in the Focus, no Inclination of the 
 44 Paper will tinge the white Spot with Colours”. 
 
 44 That a yellow Paper in the blue Light appears 
 44 green , as does alfo a blue Paper in the yellow 
 44 Light.”—— But not when the Room is well darken’d, 
 and the Light homogeneal. 
 
 44 That Sir Ifaac Newton afferts falfly, that Light 
 44 immerging into a Parallelipiped, and then emerging 
 44 out of it, produces no Colours.” 
 
 44 That the 6th Experiment of Part L Book I. of 
 44 Sir Ifaac is true ^ but no different Refrangibility of 
 44 Rays is prov’d by it, though the Colours coming 
 44 fucceffively from the firft Prifm to the fecond with 
 44 the fame Incidence, are carried to different Heights 
 44 by the laft Prifm.” Now the Confequence is fo 
 plain here, that this is, after my Lord Bacon's Man¬ 
 ner, called an Experimentum Cruets . 
 
 I fliould 
 
( <>07 ) 
 
 I fhould go on with my Account of this Book, if it 
 could be of any Ufe either to vindicate Sir Ifaac 
 Newton, or to convince the Author and his Adherenrs, 
 if he has any. But Sir Ifaac *s Opticks need neither 
 Defence nor Explanation. And when the Author is in 
 the Humour to be convinced, ten Months well em¬ 
 ployed in reading Sir Ifaac *s Book, will make him 
 amends for his ten Tears prejudiced Examination. 
 
 I beg Leave now to give the particular Defcnption of 
 a few Experiments I made upon this Gccafion, fome 
 of which are exactly as Sir Ifaac Newton made them: 
 Some are his, but made fomething differently, and 
 fome altogether my own. 
 
 * 
 
 Experiment!. Big. I. 
 
 I prepared a Box of about three Foot high, and one 
 Foot wide within (whofe Shape was a truncated Pyra¬ 
 mid) in the following Manner. I painted the Infide of 
 it black, and in the back Part, one Foot above the Bafe, 
 made a fquare Hole of three Inches in Width (whofe 
 Section is rr) to receive a Piece R {hutting clofe with 
 a Rabbet or Shoulder, whofe Surface coming through 
 the Hole was wholly covered with the painted Paper, 
 on which the Experiment was to be made. Over againft 
 r r , in the fore Part of the Box, was a Door to open 
 with a Tube in it, four Inches wide and five Inches 
 long, whofe Sedion is e,fg^ h , that two Candles fet on 
 the Places /, k , to enlighten the Paper at r,r, might 
 throw no dired Light out of the Box, whofe Sedion is 
 reprefented at a Ir e d . Then having made the Room 
 perfedly dark, I fixed the Box upon a Table, that it 
 might remain in one , Place j at the Diftance of 
 
( 6o6 ) 
 
 eight Foot from rr, I fixed the Lens L L, of four 
 Foot Focus, in a Frame upon another Table, with its 
 Axis going through the Middle of rr: At the Diftance 
 of about eight Foot beyond the Lens, I fet up the 
 Skreen or Square of white PaperS. Having put into 
 the Hole r r a ftiff Paper, painted with Vermillion, 
 and wrapped four Times and an half with black Silk 
 (as represented by R), that Paper enlighten’d by the 
 Candles at /, k, the Image of the red Paper was pro¬ 
 jected upon the Skreen at p, and whenthe molt diftind 
 Place was found, the Skreen was fixed : Then a Paper 
 painted with Ultramarine being fixed in the Hole r r, 
 the Image of it was fo indiftind at p, that the Images 
 of the black Silks could not be feen; but holding a 
 Piece of Paper clofe to the Skreen, and bringing it 
 forward, at about \ of an Inch from the Skreen, the Re- 
 prefentation of the Silks began to appear on the blue 
 Image ; but it was tnoft diftind at an Inch and f, or at 
 Z Z j fo that there was i* Inch between the diftind 
 Bafe of the red, and that of the blue Paper. But 
 what has led feveral People into an Error in making 
 this nice Experiment, is the Depth of the Focus of the 
 Rays in both Cafes; for though the red Image was mod 
 diftind at p, yet the Reprefentation of the black Silks 
 might juft be perceived by a good Eye when the Skreen 
 was moved backwards or forwards | of an Inch: The 
 blue Image which was ftronger had its Silks vifible an 
 Inch on either Side of Z Z; fo that in a Paper half 
 red and half blue, painted with thefe Colours, one 
 might have feen the Silks (though faintly) upon the 
 two Images at once, and have been thereby deceived: 
 But 1 of an Inch beyond the Place common to both, 
 the red alone would have appeared diftind; and an 
 
 Inch 
 
( 6 08 ) 
 
 Inch fhort of the faid Place, the blue Image moft di- 
 ftind, and diftind alone ; that is an Inch and | nearer 
 the Glafs. Inftead of Vermilion the red Paper may 
 be painted with Carmine or Lake , but it will not do fo 
 well, as was then tried j nor does TruJJian Blue do fo 
 well ^Ultramarine, The belt Way is to heighten 
 the Vermillion with a little Carmine , and the "Ultra- 
 marine (which has too much white) with Indigo \ 
 and then there will be a Space between the two di¬ 
 ftind Bafes where both the Images will be indiflind. 
 N. B. I made the Experiment with fuch Colours , in 
 the Tear but now I ufid no Mixtures,that any 
 
 Body elfe might repeat the Experiment . 
 
 The fecond Figure reprefents the Box with one Side out, 
 whofe Place is S' d b B j eg is the Hole far theTube in the 
 Door of the Forefide, x ^ c d rr the Hole in the 
 Back to receive the Piece R with its painted Paper. 
 
 The third Figure is the Box open before, with the 
 Candles and Paper in it, the fame Parts being marked 
 with the fame Letters as in the other Figures. 
 
 N. B. I made the Experiment in this Manner, be- 
 caufe Signior Rizzetti attributed the different Foci of 
 the Colours to different Inclinations, which could not 
 be alledged here the red and blue being, as he had 
 defired, fucceffively fixed in the very fame Place: And 
 he fays (Page 64.) addidi permanentes colores a lumine 
 direElo diverfa inclinatione illujlratos conjiante In- 
 clinatione in lentem incidere. Nay, more than this 
 was performed in the Experiment • for as the Candles 
 were fixed, the Light fell upon the painted Paper always 
 with the fame Incidence. 
 
 EX PE 
 
( <Sop ) 
 
 Experiment II. 
 
 Inftead of the red or blue Paper at r r (Fig. i, 2, and 
 3,) I fixed upon the Piece R, a Paper half red, and 
 half blue , as R B (Fig. 4.) then over the Hole in the 
 fore Part of the Box reprefented by e g (Fig. 2.) I fixed 
 a fquare Plate v.$ c d (Fig.4.) with an oblong Hole 
 in it four Inches long in its Horizontal Pofition, and 
 one Inch deep, through which one might fee the parti¬ 
 coloured Paper, as if it was only of the Bignefs and 
 Figure of this Aperture, and ftrongly enlighten’d by 
 the Candles hid in the Boxj the reft of the Room 
 being very dark. N. B. I made this ‘Preparation, be¬ 
 cause Rizzetti objebls to Sir Ifaac Newton’s firft Expe¬ 
 riment of the firft Book, that the black Cloth beyond 
 the parti-coloured Paper, was not colourlefs , and 
 therefore the Experiment was not decifive as parti¬ 
 cularly relating to the Paper. 
 
 RB (Fig. si) is the Paper contraded in Length and 
 Breadth by the Aperture of the Plate, which Paper 
 being looked at, at the Diftance of five Foot, by the 
 Prifm 1, appeared as drawn at rb. ThePrifm being 
 removed to 2, at the Diftance of ten Foot, fhewed 
 the Paper as at r b. And when it was at 3 (at 
 the Diftance of fifteen Foot) the Paper appeared as p( 2 . 
 In thefe three Cafes the blue b, b, and /3 appeared 
 lower than the red r, r, p, the refrading Angle of the* 
 Prifm being downwards. When the refrading Angle 
 was held upwards, as at y, then the Blue B was 
 raifed higher than the red R ^ but if due Care be not 
 taken,in turning the Prifm,aRefledionmaybemiftaken 
 for a Refradion, as at 4', and then indeed the Red 
 
 M m m in and 
 
( dio ) 
 
 and Blue will be equally raifed as at T. This muft have 
 been Signior Rizzett'i s Miftake, when (in Fage^Z.) 
 lie fays that one Colour was raifed higher than the 
 other by two Lines, at ten Foot Diftance, but not at 
 all at five Foot ^ for feveral of the Perfons prefent at 
 my Experiments, made the fame Miftake at firft before 
 they could perform the Experiment in manner above- 
 mentioned ; which they at laft did, and found the Co¬ 
 lours feparated moft at the greateft, and leaft at the leall 
 Diftance. This miftaking a Reflection for a Refradion, 
 has been the Occafion of feveral more Errors, and Dif¬ 
 ficulties to be met with in Signior Rizzetti's Book. 
 
 Experiment III. Fig.VI. 
 
 A Candle K, refleded from the Surface A B of the 
 Prifm ABC, appeared very faintly to the Eye at E, as 
 a weak Image at k; becaufe the Rays incident at I, pafs 
 moft of them through the Prifm, and go on to R, fe- 
 parating from one another according to their different 
 Degrees of Refrangibility; whilft a few of them are 
 refleded to the Eye in the Diredion I E. 
 
 But if the Prifm be in the Pofition ACB (Fig. 7.) 
 moft of the Rays of the Candle K, incident at I, on the 
 Plane A B (after having parted perpendicularly through 
 the Plane BC) are refleded, and parting perpendicularly 
 through AC, go into the Eye atE, which fees a very 
 ftrong Image of the Candle at k, whilft very few Rays 
 go down to R to produce Colours. 
 
 This fhews that the Rays of Light pafs with more 
 Facility through Glafs (a denfe) than through the 
 Air (a rare) Medium j contrary to Rizzetti’s Af- 
 fertion. 
 
 E x- 
 
( <$M ) 
 
 ExperimentIV. Fig.VIII. 
 
 To make this more evident, and compare together the 
 Facilities with which Light pafles through the two 
 Mediums, I took a Cube of Glafs of three Inches the 
 Side, A ab B d DC, whole Seftion is A BCD, and 
 looking upon it from E to fee by Refle&ion the Candle 
 K, I faw two Images of it} one at k very faint, and re* 
 Hefted from the upper Surface A B, and the other at * 
 very ftrong, and reflected from the lower Surface C D. 
 Now it is evident, that the Vividnefs (or Brightnefs) 
 of the Image x, is to theVividnefs of the Image k ; as the 
 Facility with which the Rays, in thefe Circumftances 
 pafs through the Glafs, or through the Air: and thofe 
 are eafily compared, becaufe both the Images are feen 
 at once. 
 
 Experiment V. Fig. IX. 
 
 The Line PI being perpendicular to the reflecting 
 Plane A B of the Triangle ACB, I brought the Can- 
 die K by Degrees fo near to P, as to diminilh very much 
 the Angle of Incidence KIP, which made the Image or 
 Appearance of the Candle at k, become fainter by 
 Degrees, and at laft as faint as in Fig. 6 . 
 
 Experiment VI. F i g. X. 
 
 Having made the Experiment as at Fig. 7,1 prefled 
 another Prifm DFG, clofe to the Prifm ABC, and 
 when I fqueezed them together but gently, fome of 
 
 M m m m z the 
 
( 612 ) 
 
 the Rays from the Candle R, palled through the lower 
 Prifm, and falling upon a Paper at R, made a reddilh 
 Spot ; but when I fqueezed them very hard, the Spot 
 became much wider, white in the Middle, and only 
 tinged with Red about the Edges: At the fame Time 
 the Eye faw a black Spot in the Image of the Candle 
 at k ; and a Stander-by looking obliquely at the 
 Place I (where the Glaffes touched) faw, as it were, a 
 little Hole through the Prifms as big as the Spot k. 
 But if the Prifms be preffed together but gently, then 
 all the other Phenomena difappear, except the firft 
 little Spot at R, as in Fig. u. 
 
 When the Candle is feen by Reflexion from the 
 lower Surface of a Prifm, as in the 7th, 9th and 
 10th Figures, the Rays pafs quite through that Sur¬ 
 face, and are turned up again by the Attradion 
 of it in Curve Lines fo as to re-enter the Prifm, 
 and then (going out again through the Surface A C) 
 go up to the Eye at E. In this Cafe the moft refran¬ 
 gible Rays* being the moft eafily infleded, make the 
 leaft Curves, whofe Vertices are nearer the Glafs than 
 thofe of the greater Curves made by the leaft refrangible 
 Rays. This is proved by Experiment 6 , where the 
 under Prifm only attrads down from the Refledion of 
 the upper Prifm, the Red making Rays as in Fig. 11. 
 where the Plate of Air between the Prifms is of fome 
 fmall Thicknefs. But when the Prifms, whofe Sur¬ 
 faces are a little convex, are preffed hard together, the 
 lower Prifm is near enough to attrad Rays of a great - 
 Degree of Refrangibility; and therefore the Spot then 
 becomes white in the Middle} and only red about the 
 Edges, which are produced by fuch Parts of the lower 
 Prifm as are not fo near the upper Prifm. 
 
 There 
 
( 6il ) 
 
 There are two Circumftances in the 6th Ex¬ 
 periment, which difprove Rizzetti s Aflertion 
 (Page 11 s) vtz ' That there is a fenfible Reflexion even 
 where Glades touch ^ for when the Prifms touch at I 
 Fig. io, the black Spot appearing in the Image of the 
 Candle k (hews that there is at I a Deficiency of thofe 
 Rays, which, coming from the Middle of the Candle, 
 ufed to be reflected up to the Eye at E, and therefore 
 that A B the refleding Surface of the upper Prifm 
 ceafes to refled in a little Space round about I where 
 the upper Surface D F of the under Prifm touches it • 
 the Rays, which before were reflected, now going down 
 to make the Spot at R. The other Circumftance is 
 this •, that whereas a Paper at k is invifible to an Eye 
 at E by the Interpofition of the Prifm D F G j when 
 another Prifm A C B is laid over it and preffed hard, 
 there appears to be an Hole of about *. of an Inch (more 
 or lefs in Diameter as the prifmatical Surfaces are more 
 or lefs flat) thro’ which the Paper at k becomes vifible r 
 this being the Place of Contad where the Refledion 
 downwards (of the Surface D F) ceafes. 
 
 - •• . .-7/ -V vj ViU?'V*V ' Wi'Vi’Vv h !v 
 
 This happens becaufe thofe Rays, which (coming 
 from the Candle K) were bent in Curves under the Sur¬ 
 face A B of the upper Prifm about feveral Points near 
 I, are by the Nearnefs of the Surface D F of the lower 
 Prifm brought down to R, inftead of being turned up 
 again to the Eye at E; whilft thofe Rays, which (com¬ 
 ing from the Paper at k thro’ the Surface G F of the 
 lower Prifm, and palling thro’ the upper Surface of it 
 FD) were bent in Curves about feveral Points near I, 
 are prevented from turning down again to R, and are 
 
 brought 
 
( 6*4 ) 
 
 brought up to the Eye at E, which confequently muft 
 fee a round Part of the Paper at k, juft as big as the 
 Place of Contad, which appears like an Hole j or as if 
 the two Prifms being changed to a Parallelopiped, were 
 covered with a dark Paper that had only afmallHole 
 in it. 
 
 But to make this more evident, efpecially to fuch as 
 are not well acquainted with Sir Ifaac Newton's Op- 
 ticks, I beg Leave to explain the Manner of the bend¬ 
 ing of the Rays where they are refracted or refleded. 
 
 Of the Bending of the Rays in their Refraction. 
 
 Let D D (Fig iz) reprefent a denfe Medium (as 
 Glafs) whofe Surface is GG, and A A a rare 
 Medium (as Air). Now let us fuppofe a Power to extend 
 all over the Surface G G, ading from A A towards 
 D D in Lines perpendicular to the Surface GG, 
 very ftrong in Contad, but infenfible at a very 
 fmall Diftance from the faid Surface, which we will 
 call five Attraction of the Surface G G, without conli* 
 dering whether it be any real Virtue in the faid Surface, 
 or the Adion of a Medium impelling towards it. Let 
 Lines ii, %z, 3 3,fuch as exprefsthe Lines in which the 
 Attradion exerts it felf, and the Line M M (extremely 
 near to G G) the Limits of the Attradion, beyond which 
 it cannot affect a Ray of Light. Let the Ray of Light 
 R a moving from a rare Medium into a Denfe in the 
 Diredion R r, come towards the Surface G G in fuch 
 an Angle that it may be refraded. When the Ray 
 comes to a, by the Attradion at a it will be aded upon 
 
 in 
 
( ) 
 
 in the Line a b, and (by the known Laws of Mecha- 
 nicks) be turned out of the Way into the Diredion a a, 
 inftead of a r : When it is got to b , being afted upon 
 in the Diredion b 4, its new Diredion will become 
 b b :.At r,by the Power ading in the Line c 5*, it will 
 change its Diredion tocc; and laftly, at d it will go 
 into the Giafs in the Line ^d, continuing in that {freight 
 Line whilft: it moves in that Medium. 
 
 Now if the Lines 11,22, 33* G b, a, be infinitely 
 near (as they muft be fuppofed to be) the Ray, in¬ 
 ftead of being broken into the feveral ftreight Lines 
 ab> b c, and c d , will be bent into the Curve abc d ; and 
 the emergent Ray d d will make the fame Angle with 
 the Incident Ray R r as if the Refradionhad been made 
 at once at the Point n , which Point may be confidered 
 as in the Surface G G, becaufe MM has been fuppofed 
 extremely near that Surface: Then alfo may Refradi- 
 ons be confidered in grofs, and Rays trac’d, in all Op¬ 
 tical Propofitions, as if there were no fuch Curve as 
 what we have been defcribing. 
 
 Again, let D ( Fig, 13) reprefent the denfe Me¬ 
 dium or Giafs, and A the rare Medium or Air ; R a 
 a Ray of Light coming out of the denfe Medium in¬ 
 to the Rare, in the Diredion Rr, in which it may 
 be refraded (as for Example, in an Angle of 30 De¬ 
 grees with the Perpendicular P a). Let M M be the 
 Line which limits the Attradion of the Surface G G, 
 which Attradion is exerted in Lines tending perpen¬ 
 dicularly from MM to G G. As foon as the Ray of 
 Light has emerged at a , it is attraded in the Di¬ 
 redion a P, and therefore diverted from the Line 
 a r, into the new Diredion a ; a t b, it is turned 
 into the Line b b ; at r, into the Line c c ; and at 
 
( <*'« ) 
 
 d, into the Line d d; fo that the emergent Ray will 
 be dd, as if the Refraction had been performed in 
 the Point n, and that Point was in the Surface GG, 
 without any Curve at a b c d ; and all the reft as we 
 confidered it before, with this Difference only, viz. 
 That the Ray is bent juft as it comes out ( or rather 
 when it is come out) of the denle Medium ; where¬ 
 as before we confidered its Bending before it came 
 into it. 
 
 Of the Bending of Rays in Reflection. 
 
 But if the Ray R a {Fig. 14) coming out of 
 Glaft into Air, fliould come in fiich a Direction 
 as to be wholly reflected, as it will do when the An¬ 
 gle R a P is of 45 Degrees; I fay the Reflection will 
 not be made at the Surface GG, nor above it in 
 the Glafs; but under the faid Surface, in the Air, 
 or even in a Vacuum, or any Medium lefs denle, or 
 rather left refractive than Glaft. 
 
 MM reprefents the Limits of the Attraction of 
 the Glaft exerted in a Direction from MM to GG 
 perpendicularly, as we faid before. 
 
 The Ray R a , moving in the Direction R r, at 
 its Emerfion at a, is, for the Reafons before given, 
 turned into the Direction a a; then at b, into the 
 Direction b b j at e, into the Direction c c ; at d, in¬ 
 to the Direction d d; at f, into the Direction e e ; 
 and at f into the Direction f f parallel to GG ; 
 then at g, the Ray is again turned towards the 
 Glaft, by whofe Attraction changing fucceflively 
 into all the Directions g g, i i, k k, and l \; at laft 
 it re-enters the Glafs in the Direction m m making 
 
 the 
 
( 6\7 ) 
 
 the fame Angle with the Perpendicular m p that R a 
 mada with a P. Now as the Lines perpendicular 
 to GG drawn from MM are infinitely near, the 
 Line abcdefgh'tk l m mull: be a Curve ; 
 and as M M and G G are extremely near, the 
 Vertex of the Curve ( whole Tangent is ff parallel 
 to G G ) will be fo near the Point I, as to be con- 
 fidered as co-inciding with it, when we compare the 
 Angle of Incidence with that of Reflection ; then 
 alfo will the Space between the Parallels p m and P a 
 be fo far diminifhed, that thofe two Lines may be 
 looked upon as co-inciding, the Angles m»p and 
 R a P being equal, whether the three Points m, I, a, 
 co-incide or not. 
 
 For thefe Reafons, for common Ufe, one may 
 confider the Reflexion from the under Surface of 
 the Glafs as made at once in that Surface at the 
 Point I. But when we examine Things ItriCtly, Ex¬ 
 periments as well as the above Reafoning, will lhew, 
 that there is fuch a Curve as we have mentioned. 
 See Experiment VI. Fig. xoandii; and we fhall 
 mention others. 
 
 N. B. If any Point of the Curve a b c, Sec. be¬ 
 tween a and f, fall below ( or beyond the Line M M, 
 the Ray will then go on in a ftraight Line Tangent 
 to the Curve in that Point where it leaves the Line 
 MM. 
 
 Now let us fuppole MedcbarM (in the fame 
 Fig. ) to be Glals, or any other denfe Medium, and 
 mp PR Air, or any other rare Medium, and Ra 
 Ray of Light moving in the rare Medium towards 
 the denfe Medium in the Direction R a towards r; if 
 inftead of an Attraction at tjie Surface of the Glals 
 
 N n n n MM, 
 
( 6il ) 
 
 MM, there be luppofed a repellent Force, whofe 
 Limits are G G ; then will the Ray by the Repul- 
 fion of the Surface M M be bent into the Curve 
 abed efg hi klm in the fame Manner as we ihewed 
 it would be under the Surface G G, when G p P G was 
 confidered as a denfe Medium. Hence it follows 
 that a Ray moving in the Air, is reflected from a 
 lpecular Surface of Glafs, or any other Mirrour, 
 opake or diaphanous, without touching the faid 
 Surface. 
 
 N. B. That the fame Tower may , under different 
 Circumftances , attraB to and repel from the 
 fame Surface, Jhall be made out in the remaining 
 Tart of this Taper ; but now taking fuch a Tower 
 for granted, we will proceed in confidering the 
 Flexure of Rays of Light. 
 
 Let us fuppofe a Prifm A C B (Fig. if) to have the 
 attracting Power of its inferior Surface extend as far as 
 the Line *0 m ; if another Prifm G D F (the attracting 
 Force of whofe upper Surface extends as far as n n) be 
 brought very near to the firft Prifm ; where the at¬ 
 tracting Powers of the Prifms interfere, they will 
 deftroy one another, becaufe they act in contrary Di¬ 
 rections; and thereby the Limits of Attraction of 
 each of the Surfaces will be contracted ; the Power 
 of A B extending no farther than n n, and that of D F 
 no farther than mm, whilft the Space nn mm lofes 
 all the Force that it had ( and would have upon the 
 Removal of either Prifm ) to turn a Ray of Lighr, 
 moving obliquely, out of its Direction. 
 
 Now in this Situation of the Prifins, a Ray of 
 Light entering the Surface C B at right Angles, will 
 go through the fecond Prifm alfo at Right Angles 
 
 (not 
 
( 6 \9 ) 
 
 (not exactly in the fame Line, but) in a Line 
 parallel to the Direction of the incident Ray: For 
 Example, let the Ray R a ( not refracted at, becaufe 
 perpendicular to, the Surface C B) emerge from the 
 firfl Prifm at a, in the Direction a r ; its changed 
 Direction at a will become a a, and at b , b b, or ra¬ 
 ther the Ray will be inflexed in the Curve ab ; and 
 at b getting out of the Power of the Attraction of the 
 Surface AB, it will (for the Reafons before given) 
 move in a ftraightLine from b to c , where it will be 
 bent again the contrary Way in the Curve c d of the 
 lame kind as a b , and laftly emerge in the Direction d d 
 parallel to the firfl: Direction R r. From hence it fol¬ 
 lows, that when the Prifrns are brought fo near as to 
 touch, their mutual Attractions deftroying each other, 
 the Rays of Light will not be bent, but pafs through 
 the two Prifrns ( which in this Cafe perform the Of¬ 
 fice of a Parallelopiped ) in the fame Direction with 
 which they came into the firfl Prifm, and confe- 
 quently produce no Colours ; contrary to what is 
 affirmed by Rizzetti (Tage 78, 79, &c.) and 
 when the Rays R a fall obliquely upon the Surface 
 C B, the EffeCt of their RefraCtion at their Immer- 
 fion at S to produce Colours, is taken off by the 
 RefraCtion which they fuffer at their Emerfion 
 at z. 
 
 Experiment VII. 
 
 I took the Cube of Fig. 8, and looking oblique¬ 
 ly through it at the Hole of the Window of my dark 
 Chamber (the Sun filming or not fhining) the Hole 
 appeared entirely colourlefs, as did alio a Candle, 
 
 JSJ n n n % both 
 
{ 6io ) 
 
 both appearing fringed with Colours when fecti 
 through the Priftn. Then holding two Prifms to* 
 gather, as in Fig. io, if the Hole of the dark Cham* 
 ber be at k, it appears white to the Eye at E; but 
 if the Angles of the Prifms at B F be a little fepa- 
 rated, whilft the Points A D touch, the Hole will 
 appear coloured : When the Surfaces are feparated 
 at A D, and touch at B F, the Colours appear in an 
 inverted Order; but if the Surfaces A D and B F are 
 parallel, whether they touch or not, the Hole will 
 ippear white. 
 
 N. B. In this Cafe the Trifms muft be fimilar y 
 that the Surface F G may be parallel to AC; other- 
 wife A B and D F muft be fo inclined to one another 
 as to render A C and F G parallel. Indeed if one 
 of the Prifms be very far removed from the other y 
 the heterogeneous Light which entered in at F G, 
 may be fo far Jpread by the Separation of the dif¬ 
 ferently refrangible Rays , that the Frifm ABC 
 will not take it all in then the Eye behind the fe- 
 cond Frifm may fee Colours , as I fuppofe Riz* 
 zeti did . See Tage 79 of his Book. 
 
 If the Ray of Light R a b c d d ( Fig . 15. ) 
 changing its Direction in the Manner above-men¬ 
 tioned, makes an Angle of about 45 Degres with 
 the Perpendicular P a ^ upon the removal of the 
 lower Prifm, the Ray will be turned up again, as 
 in Fig . 14. But if the Angle P^R be greater, the 
 Ray will (till be turned up again in a Curve, as 
 a b c d e f ( Fig. 16) notwithftanding the lower 
 Prifm is at DFG; but if that Prifm be brought up- 
 clofer to the Surface A B, the Curves will be deftroyed 
 where the Prifms touch, and all the Rays in the Place 
 of Contact brought down through the lower Prifm. 
 
 THE 
 
( 6n ) 
 
 THE mod refrangible Rays confid of fmaller Par¬ 
 ticles than the lead refrangible Rays, and therefore 
 mud have lead Momentum , the Velocity of all the 
 Rays being the fame ; and confequently are more 
 eafilyturned out of the Way by Attraction or Re- 
 pulfion, which makes the Curves made by the pur¬ 
 ple and violet Rays under the Surface A B, to be left 
 and nearer the faid Surface than the Curves made by 
 red and orange Rays. 
 
 Suppofe a Violet R a moving in the Direction R r 
 ( Fig. 1 6 .) to be fo bent under the Surface A B, that 
 at the Vertex of the Curve, or where its Tangent c c 
 is parallel to A B, there dill remains afinall Space be¬ 
 tween the Curve and the Line n //, where the Limits 
 of Attraction ( contracted by the Proximity of the 
 undermod Prifm D F G end) that Ray will be turn¬ 
 ed up again in the Curve d e /, and fo reflected in 
 the Line/f, the Directions having been fucceflively 
 changed, as in Fig . 14. But a red Ray with the 
 fame Inclination, would pals on into the lower 
 Prifm, as was explained in Fig. iy. Becaufe the 
 Momentum of the red Ray being greater than that of 
 the Violet, the fame Degree of Attraction could not 
 give it the fame Flexure. 
 
 This is confirmed by Experiment, for when the 
 lower Prifm is not prefled hard againd the upper 
 (as in Fig . 11.) the Rays brought down to R make 
 a Spot of a Colour made up chiefly of red and orange 
 Rays ; but when the Prifms are prefled clofer, the 
 Spot grows bigger and perfectly white in its Middle, 
 becaufe all Sorts of Rays are brought down to the 
 Spot; but it is inclofed round with a reddiili Border, 
 occafioned by the Parts of the Prilrn which are very. 
 
 near, 
 
( 61% ) 
 
 near, but not in Contact, or at lead: not near enough 
 to bring down the green, blue, purple and violet 
 Rays. This lhews that the Refledion is not made 
 from the interior folid Parts of the Glafs, nor from 
 the Parts in the Surface, as Rizzetti affirms. But this 
 is made more evident by 
 
 Experiment VIII. Fig. 17. 
 
 A Candle being in the Pofition K, the Eye at E, 
 and the Prifm at A B C; a ftrong Image of the Can¬ 
 dle was feen at k as in Fig.j. But lifting up a Veflel 
 of Water VS S V till the Surface of the Water V V 
 touched A B the lower Surface of the Prilm, the I- 
 mage of the Candle became almoft infenfible, as the 
 Eye loft all thofe Rays which now were attraded in¬ 
 to the Water. And for a farther Proof, that the Re¬ 
 fledion is made under the Surface and not in it, 
 when the Prifm was taken out of the Water, being 
 wet at its lower Surface, or having a Stratum of 
 Water (whofe Surface was VV Fig. 18) under 
 A B, the Image of the Candle did again become vivid, 
 the Rays being turned up again under V V. Indeed 
 the Image, in this Cafe, though ftrong, did not ap¬ 
 pear well defined, by Reafon of the Unevennefs of 
 the watry Surface VV Fig. 18. 
 
 I am very well aware that Rizzetti may anfwer 
 here, that what I have faid above, does in fome Mea- 
 fure favour his Notions; and that the Rays which (in 
 Fig. 7, having pafted through AB, the lower Surface 
 of the Prifm) are turned up again to the Eye at E, 
 do not fuffer a Refledion but a new Immerfion ; for 
 he fays, in Page ixy. — ‘‘ Anglus (meaning Sir 
 
 Ifaac 
 
( 6i% ) 
 
 a Ifaac Newton ) fecundo fubjungit, quod fi lumen 
 “ in tranftu e vitro in aerem obliquius incidat r 
 cc quam in angulo graduum 40, illud. in totum re - 
 c< flettitur . Ego vero refpondeo, quod ex iis, quae 
 c{ docui in Prop. 4. Cap, 1. elicitur hanc non efie 
 “ veram luminis reflecftionem, led potius novam Im- 
 “ merfionem ; & ideo nego quod ex ifto Phamome- 
 
 no fequatur lumen a partibus corporum folidis, 
 “ aliquo interjecfto intervallo, refle6l:i/ > And a little 
 lower, having quoted what Sir Ifaac Newton lays* 
 concerning the blue Light, which, coming from one 
 Prifm obliquely upon the farther Surface of another, 
 is wholly reflected, at the lame Inclination that the 
 red Light is wholly tranfmitted. — He fays, “ Satis 
 “ fit iterum refpondere, quod in hoc etiam cafu eft 
 “ nova luminis immerfio, quae dicitur ab au<ftore 
 “ reflecftio. 
 
 But this is only cavilling about Words ; for if the 
 Ray of Light, which moving in a denfe Medium 
 falls obliquely on the Surface common to that and a 
 rarer Medium, be turned back again in the denfe 
 Medium, lo as to make the Angle in which it returns 
 from the faid Surface equal to that in which it came 
 to it; this Return of the Ray may properly be call¬ 
 ed a Reflection , whether the Ray be turned back at 
 the Point of the Incidence in the Surface, or be car¬ 
 ried about the Point of Incidence in a lmall Curve, 
 whofe Confideration may be omitted in tracing the 
 Way of a Ray of Light in its Paflage, for making of 
 optical Machines. Whoever reads the 8th Prop, of 
 the 2d Part, Book II. of Sir Ifaac’§ Opticks, may very 
 eafily find that he was not ignorant of the turning 
 back of the Ray under the Surface of the Glafs be¬ 
 fore 
 
( ) 
 
 fore it returned into it: And though the Reflection in 
 that Cafe be not made by impinging on the folid Parts 
 of the Glafs, yet it is owing to them that the Light 
 (aCted upon at a Diftance) is turned up again, as has 
 been fhewn by feveral of the Experiments above men¬ 
 tioned. 
 
 Now let us fee how Bizzetti s Account of the new 
 Immerfion agrees with Fhanomena. 
 
 Let all above the Line P p {Fig. 19) be a denfe Medi¬ 
 um, as Glafs:, and all below it a rare Medium, as Air^ 
 ABCD is a Beam of Light infenfible in Thicknefs, but 
 of fome Breadth, whofe Rays cohere to one another, 
 and whofe SeCtion or firft Line is B C. If the Medium in 
 which B C is, did not change, B C would move parallel to 
 it felf in the Lines B a and C d 5 but as the End C of 
 the Line B C comes out into a rare Medium, which be¬ 
 ing of lefs Refiftance to Light (for fo he fuppofes) the 
 Point C moving with more Facility than the Point B 
 defcribes the Curve C F H, whilft B moving in the 
 denfe Medium with more Difficulty, defcribes the let 
 fer Curve BEG ; then the Point C being got to H is 
 re-immerfed, and the Line BC being got to HG goes 
 on in the Direction HK GL parallel to itfelf, draw¬ 
 ing the Beam after it in a rectilinear Direction, after 
 Part of it has been bent within the Glafs and Part of 
 it without* 
 
 Now if this be true, and P p tt be a Prilm, I beg 
 to know what becomes of the Line at E F which u- 
 nites the Rays of the Beam about the Point of Inci¬ 
 dence I, when Water is brought to touch the Surface 
 Pp, as at A B Fig. 17 ? If it be faid that Water mak¬ 
 ing a great Refinance, though nor fo great as Glafs, the 
 Curve BE G deviates fo little from the Line B a that 
 
 the 
 
( ) 
 
 the Point E comes below I, and the Beam is wholly 
 refradted:, I ask whence comes the faint Image at k? If 
 it be anfwered, that fome Part E I of the Line EF 
 (Fig . 19 ) is turned up to the Eye at E ( Fig. 17,) 
 what becomes of the lateral Cohefion of Light on 
 'which Rizzetti founds his chief Propofition, and 
 from which he draws his Confequences } 
 
 It would be tedious as well as ufelefs to be particu¬ 
 lar in Ihewing all Rizzettis Miftakes* therefore I 
 fhall only mention one more Experiment from Sir Ifaac 
 Newton , which I repeated on Account of what is 
 faid in Rizzetti' s Preface, Rage 16, viz. that if 
 0 according to Sir Ifaac) Rays were differently reflex - 
 ible , Colours mufl be produced by Reflection from a 
 plane Surface ; but this , fays our Author, is contrary 
 to Experience. Now this his Affertion is difproved by 
 
 Experiment IX. 
 
 As this Experiment was made exaftly in Sir Ifaac 
 Newton's Manner, and with the fame Succefs, I re¬ 
 peat the Account of it in his own Words. 
 
 “ Let H F G [in Figure 20] reprefent a Prifm in 
 “ the open Air, and S the Eye of the Spectator, view- 
 £C ing the Clouds by their Light coming into the Prifm 
 at the plane Side FIGK 3 and reflefted in it by its 
 Ci Bafe HEIG, and thence going out through its plane 
 u Side H E F K to the Eye : And when the Prifm and 
 K Eye are conveniently placed, fo that the Angles of 
 “ Incidence and Reflection at the Safe may be of about 
 * 40 Degrees, the Spe&ator will fee a Bow M N, of a 
 blue Colour, running from one End of the Bafe to 
 cs the other, with the concave Side towards him, and 
 
 O 0 0 o a the 
 
 * 
 
r <m ) 
 
 “ the Part of the Bafe IMNG beyond this Bow will 
 “ be brighter than the other Part EMNH on the o- 
 “ ther Side of it. Now for underftanding the Reafon 
 “ of it, fuppofe the Plane A BC to cut the plane Sides 
 “ and the Bafe of the Prifm perpendicularly. From 
 “ the Eye to the Line B C wherein that Plane cuts the 
 <s Bafe, draw the Lines S p and S t in the Angles Sp C 
 “ 5-0 Degrees £ and S / C 49 Degrees *, and the 
 “ Point p will be the Limit beyond which none of 
 c£ the mod refrangible Rays can pafs through the 
 “ Bafe of the Prifm, and be refraCted, whofe Inci- 
 “ dence is fuch that they may be reflected to the- 
 “ Eye y and the Point t will be the like Limit for 
 “ the dealt refrangible Rays, that is, beyond which 
 “ none of them can pafs through the Bafe, whofe In- 
 “ cidence is fuch, that by Reflection they may come 
 “ to the Eye. And the Point r taken in the middle 
 « Way between p and t, will be the like Limit for 
 “ the meanly refrangible Rays. And therefore all 
 “ the leaft refrangible Rays which fall upon the Bafe 
 « beyond t, that is, between t and B, and can come 
 “ from thence to the Eye will be reflected thither : 
 « But on this Side t, that is, between t and C, many 
 “ of thefe Rays will be tranfmitted through the Bafe: 
 $< And all the moft refrangible Rays which fall upon 
 ** the Bafe beyond p, that is,, between p and B and can 
 “ by Reflection come from thence to the Eye, will be 
 “ reflected thither,, but every where between /and e% 
 « many of thefe Rays will get thro* the Bafe and be 
 “ refraCted j and the fame is to be underftood of the 
 44 meanly refrangible Rays on either Side of the Point 
 “ r. Whence it follows, that the Bafe of the Prifm 
 * muft every where between t and B by a total Re* 
 a. fleCtion: 
 
( 617 ) 
 
 “ flexion of all Sorts of Rays to the Eye, look 
 « white and bright. And every where between p 
 “ and C, by realbn of the Tranlmilfion of many 
 - “ Rays of either Sort, look more pale, obfeure and 
 “ dark. But at r, and in other Places between p 
 a and t, where all the more refrangible Rays are re- 
 “ fleded to the Eye, and many of the lefs refrangi- 
 “ gible are tranfmitted, the Excefs of the molt re- 
 “ frangible in the reflected Light will tinge that Light 
 “ with their Colour, which is violet and blue, this 
 “ happens by taking the Line C p r t B any where 
 “ between the Ends of the Prifin H G and E I. 
 
 If this needs any farther Explanation, let us lup. 
 pole CAB the Sedion of the Prifin in Fig.ro. 
 transferred to Fig . 2,1. at ACB. If R 0 be a red 
 Ray inclined to a Perpendicular to A B in an Angle 
 of more than 41 or 41 Degrees, it will at its Emer- 
 fion under the Surface AB be turned into the Curve 
 onmi , and lo go up again to the Eye at E ; but a- 
 nother red Ray coming in the Direction rn making 
 an Angle with the Perpendicular fufficiently lefs, will 
 after its Emerfion at n, be only bent fo much as 
 to be turned out of the Way, and refraded to q , in 
 an Angle of Refradion agreeable to the Refrangibi- 
 lity of red Light. But V m a violet Ray with the 
 fame Inclination as the laft red one r n lhall not be 
 refraded, but turned up in the Curve m i P, and lo 
 go to the Eye at E. Another violet Ray \m making 
 an Angle fomething lefs with the Perpendicular, will 
 pals through the Glafs, and be refraded in the Line 
 m S. Upon this Account all that Part of the Bale 
 of the Prifin (of which A B is the Sedion ) betweeu 
 A and p will be dark or faint, all that Part between p 
 
 O o o o 2, and 
 
( 6i 8 ) 
 
 and n be tinged with a bluilh Colour, and all be¬ 
 tween o and B of a bright White. 
 
 5 POSTSCRIPT. 
 
 T HE Bending of Rays of Light juft as they 
 come to be reflected or retraced, may be 
 eafily underftood by fuch as are well acquainted with 
 thole Properties of Light, which Sir Ifizac Newton 
 calls their Fits of eajy Reflection , and Fits of eajy 
 Tranfmijflon ; without any Hypothefis, but by Con- 
 fequences fairly drawn from Experiments and Ob- 
 fervations. But as Signior Rizzetti does not feem 
 (in his Book) to have the leaft Notion of thofe 
 Properties of Light, and the nice Obfervations on 
 which they are founded ; and feveral other Perlons 
 have not Time to read thofe Parts of the Opticks 
 with fufficient Application ; to Ihew how the fame 
 Power of the Surface of a denfe Medium may both 
 attradk and repel under different Circumftances ——. 
 I content myfelf here with giving the Hypothefis, 
 which Sir lfaac does before he comes to that Part 
 of his Book where he demonftrates the Fits above- 
 mentioned. 
 
 If GG be the Surface of a denfe Medium 
 GDDG, on which a Tremor is cauled by the 
 Warmth communicated to it by the Rays of Light, 
 lb as to give a Wave-like Motion to the Medium 
 immediately next to the Surface GG; as that vi¬ 
 bratory Motion is performed, the Medium alter¬ 
 nately pulhes from the Surface, and returns towards 
 it (as is reprefented by the Pofition of the Darts in 
 
 the 
 
( 6^9 ) 
 
 the Darts in Figure) and pufhes back the Light fo as 
 ro refled; it when the Vibration is contrary to its 
 Direction, but brings it down to be refra&ed when 
 the Vibration confpires with the laid Motion. See 
 a further Account of this in Sir Ifaac NewtonV 
 Of ticks. Book II. Part 3. Proportion ix. 
 
 The Perfons prelent at the Experiments above- 
 mentioned, tried them as well as myfelf, and be¬ 
 ing fatisfied with the Succels of them, allowed me 
 to mention it, and make ufe of their Names in this. 
 Account. 
 
 Of the Royal Society. 
 
 Sir Hans Sloane , Prefident. 
 
 Dr. Scheutzer. 
 
 Mr. Grey. 
 
 Mr. Georges. 
 
 Mr. Thigood. 
 
 Other Gent l e m e n. 
 
 Colonel Sfotfwocd. 
 
 Mr. Haily. 
 
 Mr. Graham. 
 
 Mr. He wet. 
 
 Foreigners, 
 
 The Abbot Lercari, a Nobleman of Genoa , Cou- 
 fin to Cardinal Lercari. 
 
 The Abbot Cuzzoni. 
 
 The Abbot Rolli, and his Brother, 
 
 Ilf. The 
 
( 6 ]o ) 
 
 III. The Method of making Tin-Plates, extracted 
 from the Memoirs of the Academy of Sciences , 
 for the fear 1725, by William Rutty, M. T>. 
 (f. S. Seer . 
 
 T H E making of Tin-plates, or Lattin, as it is 
 called, being not commonly praCtifed in Eng¬ 
 land, though there is fo great a Confumption of it, 
 either becaufe the Method is not fufficiently known, 
 or becaufe that in Ufe to make fmall Quantities for 
 particular Purpofes is much too dear to aufwcr the 
 Artificer’s Expectation in making larger, whereby 
 vve are obliged to export our own Tin to Germany , 
 to receive it back again manufactured, I thought it 
 not improper to lay before the Society the Method 
 the Germans themfelves make uie of as I have ex¬ 
 tracted it from a Differtation of Mr. He Reaumur, prin¬ 
 ted in the laft Volume of the Memoirs of the Aca¬ 
 demy of Sciences of Rar'ts, in which alfo he lays 
 down fome Improvements, as he thinks, of his 
 own. 
 
 He takes notice then that the making of Tin-plates, 
 (which is called in France, white Iron) does not pro¬ 
 perly begin, fill they go about to prepare the Leaves 
 or Plates of Iron that are to be tinned, which are fup- 
 pofed to be fufficiently thin and flat, and cut into 
 Squares : But there are only certain Sorts of Iron 
 which can be reduced into thefe Leaves, of which 
 thofe arc the molt proper, that when heated are eafi- 
 eft extendible, and yet can be forged with a Hammer 
 when cold ; the more foft and extremely flexible, as 
 
 well 
 
( ) 
 
 well as the more brittle being to be rejected. Thefe 
 Leaves are drawn from Bars of Iron, about an Inch 
 fquare; which being made a little flat, they cut into 
 thin Pieces or Soles ( femelles) which they fold toge¬ 
 ther, and having made them intp Parcels containing 
 forty Leaves each, beat them all at once with a Ham¬ 
 mer that weighs from 600 to 700 ft* Weight. After this, 
 the principal Part of the whole Art is to prepare thefe 
 Leaves ; for the lighted Duft, or the leaft Rud upon 
 their Surface will prevent the Tin from uniting with 
 them. This may indeed be taken ofF by filing, but 
 that being much too expenfive, the fame may be 
 brought about by deeping the Plates in acid Waters, 
 for a certain Time, to what Number they pleafe, and 
 when they are taken out, fcouring them with Sand, 
 in order to fetch off any Thing that may remain upon 
 the Surface: And by this Method a Woman may clean 
 more Plates in an Hour, than the mod expeditious 
 Workman can file in many Days. Of thefe Waters 
 the Author mentions feveral ; but what the Germans 
 themfelves ufed, and which they make a mighty Se¬ 
 cret of, he found to be only common Water made ea¬ 
 ger with Rye, which requires very little Pains. For 
 after they have ground the Grain grofly, and pounded 
 it,they leave it to ferment in common Water for a certain 
 Time, and with a little Patience they are fure to have 
 an eager Mendruum. With this Mendruum they fill 
 Troughs or Tuns, into which they put Piles of Iron 
 Plates ; and to make it grow eager the better, and to 
 have more A&ivity, they keep thefe Vedels in Vaults 
 or Stoves which have little Air, and in which they 
 keep lighted Charcoal. The Workmen go into thefe 
 Vaults once or twice in a. Day, either to turn the 
 
 Plates 
 
( * 3 * ) 
 
 Plates that they may be equally expofed to the Addon 
 of the acid Liquor, or to take out thofe that are fuffi- 
 ciently cleanfed, or to put others in their room: And 
 as the Liquor is more acid, or the Heat of the Vault or 
 Stove is more intenfe, the Plates are fooner cleanfed } 
 but it requires at leaft two Days, and fometimes a great 
 deal more. This is the Method which the Germans 
 employed in the Tin*works in France, conftantly made 
 ufe of to prepare the Iron-plates to receive the Coat of 
 Tin: But as the Author obferved, that the conftant 
 Attendance upon them in the Stoves was very labori¬ 
 ous, the Heat therein being almoft infupportable to 
 thofe who are not ufed to it, he propofes fome other 
 Methods which are attended with very little Trouble, 
 and as fmall, if not a lefs Expence j and which upon 
 Trial fucceeded full as well. Having therefore ob¬ 
 ferved that the Iron-leaves or Plates are covered with a 
 Scale or Layer, half vitrified by the Fire, on which 
 Acids have none or very little Effeft, he imagined that 
 inftead of diffolving the Iron in thefe acid Waters, it 
 would be better to make it ruft , and thereby put it in 
 a Condition to be eafier cleanfed from thefe Scales^ as 
 Ruft is accompanied with a fort of Fermentation and 
 Rarefaftion, and the Matter which rufts takes up a great¬ 
 er Space, and raifes up whatever oppofes it. To this 
 Purpofe he fteeped Iron Plates in different eager Men- 
 fttuums, as in Water in which Alum, common Salt and 
 Sal-armoniack were feparately diffolved ; and others of 
 the fame Iron he only dipped' into the fame Waters, 
 and inftantly taking them out expofed them to the Air. 
 Thefe latter were rufted by all of them, but fooner 
 by that in which the Sal-armoniack was diffolved. Af¬ 
 ter two Days, during which every Plate had been dip¬ 
 ped 
 
( ) 
 
 ped into the Menftruum but twice or thrice, he fcour- 
 ed them, and likewife thofe he had left to fteep for 
 that Time; and comparing them together, found that 
 thofe, which had been only wetted at different Times, 
 cleanfed better than thofe which were fteeped j the 
 Ruft covering all the Surface of the latter without raif- 
 ing the Scale; whereas in the former, as foon as one 
 Part of the Metal is detached, it is attracted by the 
 Menftruum, and the Surface is raifed into Blifters of 
 Ruft. Thefe Diifolvents,the Author takes notice, tho’ 
 weak in themfelves, yet produce the Effeft as well 
 as the ftronger, which are much dearer: But amongft 
 the latter he prefers Vinegar, which being very plen¬ 
 tiful in France , may be ufed with little Coft. For 
 you need only dip each Leaf into it, and take it out 
 again immediately, leaving it afterwards in fome moift 
 Place, and it will be fcaled in eight and forty Hours, 
 if you take care to repeat this 3 or 4 Times in a Day. 
 The fcaling will ftill be more expeditious, if you dif- 
 folve a little Sal-armoniack in the Vinegar, a Pound 
 or two to a Puncheon j for as the Vinegar dilfolves I- 
 ron well, fo Sal-armoniack, as juft obferved, rufts it 
 fooner than any other Salt: But this muft be ufed very 
 moderately, and the Leaf muft be left to fteep in clean 
 Water to diifolve any Particles of it that may flick to 
 its Surface, which may otherwife make it ruft after it 
 is tinned. If you fcale with Vinegar, and want to do 
 it at a lefs Expence, you need only plunge the Leaves 
 once or twice at fartheft, and when the Vinegar is dri¬ 
 ed upon the Surface, fprinkle it with Water; or dip 
 them into it, and take them out immediately. There 
 are feveral other Ways of making Iron ruft, as keeping 
 it in a moift Cellar, expofing it to the Dew, fprinkling 
 
 P p p p it 
 
( <J4 ) 
 
 it with Ample Water, feveral Times in a Day, which 
 will ftill ad quicker by diffolving Sal-armoniack in it. 
 In thofe Countries where the Pyrites is common, the 
 Vitriolick Waters will fcale them foon enough, which 
 are almoft as cheap as common Water : You need only 
 heap the Pyrites together, and leaving them to moulder 
 in the Air, make afterwards a Lixivium with them and 
 common Water, which Lie will have the defired Effed : 
 But as the Leaves of Iron are fenfibly much eafier cleanf- 
 ed on one Side than the other, the bad Side rarely 
 taking the brilliant Polifti in the tinning, but having 
 always forne Spots, which proceeds in that in the beat¬ 
 ing one Side is more expofed to the Adion of the Ham¬ 
 mer,. and is therefore better plained, the Author again 
 advifes not to deep them, but only to moiften them, in 
 order to make them ruft, whereby you need moi¬ 
 ften that Side only that wants it moft: Whereas 
 if you fteep them, as the bad Side will take double 
 or triple the Time of the other, the acid Menftruum 
 will diflblve the Surface, and occafion a Lofs of Iron. 
 He next gives two Cautions neeeffary to be followed : 
 the firft is in the Management of the Plates before they 
 come to be prepared ; which is in the beating of them,, 
 to change the Place of each in its Turn, that every 
 one may receive the immediate Adion of the Hammer, 
 other wife they will not extend equally: the fecond is 
 to fteep them in Clay or Fuller’s earth tempered with 
 Water before you heat them, to prevent their foldering 
 with one another. He then clofes this Part of the Ope¬ 
 ration with remarking that whatfoever of thefe Me¬ 
 thods are pitched upon, whether the old one, of which 
 he has learnt the Secret, or any of the new,, which he 
 has here (hewn, it is abfolutely neceflary after the Plates 
 
 are 
 
( *15 ) 
 
 are fufficiently (baled, to fcour them with Sand, and 
 when there remains no more black Spots upon their 
 Surface, to throw them into Water to prevent their 
 rufting agairf, and leave them in it till the Inftant you 
 would tin them, or in the Term of Art, blanch them. 
 This he obferves is the very Objebt of the whole Art, 
 and is kept as much a Secret by the Blancher , as the a- 
 cid eroding Menftruum is by the Scaler : But the Man¬ 
 ner of doing it is thus. They flux the Tin in a large 
 iron Crucible, which has the Figure of a broken Pyra¬ 
 mid with four Faces, of which the two oppofite ones 
 arelefs than the two others. This Crucible they heat 
 only from below, its upper Border being luted in the 
 Furnace quite round. The Crucible is always deeper 
 than the Plates which are to be tinned are long, which 
 they always put in downright, and the Tin ought to 
 fwim over them. For this Purpofe Artificers of differ¬ 
 ent Trades prepare the Plates in different Manners, 
 which are all exceptionable : But the Germans he per¬ 
 ceived made ufe of no Preparation whatfoever, except 
 putting the fcoured Plates into clean Water, as juft 
 remarked i, but when the Tin is melted in the Crucible, 
 they cover it with a Layer of a Sort of Suet, an Inch 
 or two thick, through which the Plate mud pafs before 
 it comes to the Tin : The firfl Ufe of which is to 
 keep the Tin from burning, and if any Part (hould take 
 Fire, as the Suet will foon moiftenit, to reduce it to its 
 natural State again. This Suet is compounded, as the 
 Blanchers fay, and is of a black Colour, which the Au¬ 
 thor thought might be given it with Soot or the Smoak 
 of a Chimney,only to fpread a Myftery over theirWork $ 
 but he found* it true fo far, that common unprepared Suet 
 was not fufficient: For after feveral Attempts, there 
 
 P p p p z v/as 
 
{6^6) 
 
 was always fomething wanting to render the Succefs of 
 the Operation certain. The whole Secret then of 
 Blanching lies entirely in the Preparation of this Suet: 
 And this he at laft difcovered to confift only in firft 
 frying and burning it} which not only gives it the Co¬ 
 lour, but puts it into a Condition to give the Iron a 
 Difpofition to be tinned, which it does furprifingly. 
 The Tin itfelf ought to have a certain Degree of Heat} 
 for if it is not hot enough it will not flick to the Iron } 
 if it is too hot, it will cover it with too thin a Coat, and 
 the Plates will have feveral Colours, as a Mixture of 
 red, blue, and yellow, and the whole appear of a villain¬ 
 ous yellow Caft. To prevent this, by knowing when 
 the Tin has a proper Degree of Heat, they might firft 
 make an Effay with fmall Pieces of the fcaled Plates, and 
 they would learn from them when the Tin is in proper 
 Order: But generally fpeaking, they dip the Plates in¬ 
 to Tin that is more or lefs hot, according to the Thick- 
 nefs they would have the Coat to be of. Some Plates 
 they only give one Layer to, and thefe they plunge in¬ 
 to Tin, that has a lelfer Degree of Heat than that in¬ 
 to which they plunge thofe Plates which they would 
 have take two Layers } as alfo when they give thefe 
 thefecond Layer, they put them into Tin that has not 
 fo great a Degree of Heat, as that into which they were 
 put the firft Time : Befides which, it is to be obferved, 
 that the Tin, which is to give the fecond Coat, ought 
 to be frefti covered with Suet, but only with the common 
 Sort without Preparation } for melted Tin is fuffici- 
 ently difpofed to attach it felf to folid Tin ; and in 
 this Cafe it is to tin itfelf, to which the new Tin is 
 to be joined. As to the Choice of the Tin, the Man¬ 
 ner of making it is as bright as poflible, with a Num¬ 
 ber 
 
( <*37 ) 
 
 ber of little Articles neceflary to thePradice,the Author 
 refers them to another Time, as more properly belong¬ 
 ing to the Defcription of the whole Art, than to a 
 Memoir in which he only gives the Principles of it. 
 
 IV. A Letter from the <3$everend Mr. James Brad¬ 
 ley SaVtlian Trofeffor of Aftronomy at Oxford, 
 andF.^S. to £>r.Edmond Halley Aftronom. 
 Reg. &c. giving an Account of a new dif- 
 coVered Motion of the Fix’d Stars. 
 
 SIR , 
 
 Y O U having been pleafed to exprefs your Satis¬ 
 faction with what I had an Opportunity fome- 
 time ago, of telling you in Conversation, concerning 
 fome Obfervations, that were making by our late wor¬ 
 thy and ingenious Friend, the honourable Samuel 
 Molyneux Efquire, and which have fince been conti¬ 
 nued and repeated by my felf, in order to determine 
 the ‘Parallax of the fixt Stars ; I (hall now beg leave 
 to lay before you a more particular Account of them. 
 
 Before I proceed to give you the Hiftory of the Ob¬ 
 fervations themfelves, it may be proper to let you know, 
 that they were atfirft begun in hopes of verifying and 
 confirming thofe, that Dr. Hook formerly communicat¬ 
 ed to the publick, which feemed to be attended with 
 Circumftances that promifed greater ExaCtnefs in them, 
 than could be expected in any other, that had been 
 made andpublilhed on the fame Account. And as his 
 Attempt was what principally gave Rife to this, fo his 
 Method in making the Obfervations was in fome 
 
( <38 ) 
 
 Meafure that which Mr. Molyneux followed : For 
 he made Choice of the fame Star ? and his Inftrument 
 was conftru&ed upon almoft the fame Principles. But 
 if it had not greatly exceeded the Doctor’s in Ex- 
 achiefs, we might yet have remained in great Uncer¬ 
 tainty as to the Tarallax of the fixt Stars ^ as you will 
 perceive upon the Comparifon of the two Experiments* 
 
 This indeed was chiefly owing to our curious Mem¬ 
 ber, Mr. George Graham , to whom the Lovers of 
 Aftronomy are alfo not a little indebted for feveral o~ 
 ther exaft and well-contrived Inftruments. The Ne- 
 ceflity of fuch will fearce be difputed by thofe that 
 have had any Experience in making Aftronomical Ob- 
 fervations^ and the Inconfiftency, which is to be met 
 with among different Authors in their Attempts to de¬ 
 termine fmall Angles, particularly the annual Paral¬ 
 lax of the fixt Stars , may be a fufficient Proof of it 
 to others. Their Difagreement indeed in this Article, 
 is not now fo much to be wondered at, fince I doubt 
 not, but it will appear very probable, that the In¬ 
 ftruments commonly made ufe of by them, were 
 liable to greater Errors than many times that Pa¬ 
 rallax will amount to. 
 
 The Succefs then of this Experiment evidently 
 depending very much on the Accuratenefs of the In¬ 
 ftrument that was principally to be taken Care of: 
 In what Manner this was done, is not my prefent 
 Purpofe to tell you ; but if from the Refult of the 
 Obfervations which I now fend you, it fhall be 
 judged neceftary to communicate to the Curious the 
 Manner of making them, I may hereafter perhaps 
 give them a particular Defcription, not only of 
 Mr. Molyneux'$ Inftrument, but alfo of my own, 
 
 which 
 
( 6w ) 
 
 which hath fince been eredted for the fame Purpofe 
 and upon the like Principles, though it is fomewhat 
 different in its ConftrudHon, for a Reafon you will 
 meet with prefently. 
 
 Mr. Mclyneux's Apparatus was compleated and 
 fitted for obferving about the End of November 17x5, 
 and on the third Day of December following, the 
 bright Star in the Head of Draco (marked y by 
 Bayer) was for the firft Time obferved, as it parted 
 near the Zenith, and its Situation carefully taken 
 with the Inftrument. The like Obfervations were 
 made on the fth, nth, and nth Days of the fame 
 Month, and there appearing no material Difference 
 in the Place of the Star, a farther Repetition of them 
 at this Seafon feemed needlefs, it being a Part of the 
 Year, wherein no fenfible Alteration of Parallax in 
 this Star could foon be expected. It was chiefly 
 therefore Curiofity that tempted me (being then at 
 KeWy where the Inftrument was fixed) to prepare 
 for obferving the Star on ‘December 17th, when 
 having adjufted the Inftrument as ufual, I perceived 
 that it parted a little more Southerly this Day than 
 when it was obferved before. Not fufpecfting any 
 other Caufe of this Appearance, we firft concluded, 
 that it was owing to the Uncertainty of the Obfer¬ 
 vations, and that either this or the foregoing were 
 not fo exacft as we had before fuppofed ; for which 
 Reafon we purported to repeat the Obfervation again 3 
 in order to determine from whence this Difference 
 proceeded 5 and upon doing it on December %oih T 
 I found that the Star parted (till more Southerly than 
 in the former Obfervations. This /enftble Alteration 
 the more furprized us ? in lt was f he contrary 
 
 way 
 
( < 4 ° ) 
 
 way from what it would have been, had it pro¬ 
 ceeded from an annual Yarallax of the Star : But 
 being now pretty well fatisfied, that it could not be 
 entirely owing to the want of Exaitnefs in the Ob- 
 fervations ; and having no Notion of any thing elft, 
 that could caufe fuch an apparent Motion as this in 
 the Star} we began to think that fome Change in 
 the Materials, &c. of the Inftrument itfelf, might 
 have occafioned it. Under theft Apprehenfions we 
 remained fome time, but being at length fully con¬ 
 vinced, by ftveral Trials, of the great ExaCtnefs of 
 the Inftrument, and finding by the gradual Increafe 
 of the Stars Diftance from the Pole, that there mud 
 be fome regular Caufe that produced it ; we took 
 care to examine nicely, at the Time cf each Obfer- 
 vation, how much it was: and about the Beginning 
 of March 1726, the Star was found to be 20" more 
 Southerly than at the Time of the firft Obftrvation. 
 It now indeed ftemed to have arrived at its utmoft 
 Limit Southward, becaufe in feveral Trials made a- 
 bout this Time, no fenfible Difference was obferved 
 in its Situation. By the Middle of April it appear¬ 
 ed to be returning back again towards the North ; and 
 about the Beginning of June , it paffed at the fame 
 Diftance from the Zenith as it had done in ‘Decern - 
 her , when it was firft obferved. 
 
 From the quick Alteration of this Star’s Declina- 
 nation about this Time (it increafing a Second in 
 three Days ) it was concluded, that it would now 
 proceed Northward, as it before had gone Southward 
 of its prefent Situation } and it happened as was con¬ 
 jectured : for the Star continued to move Northward 
 till September following, when it again became fta- 
 
 tionary 5 
 
( <>4 l ) 
 
 tionary, being then near 2.0" more Northerly than in 
 June , and no lefs than 39" more Northerly than it 
 was in March . From September the Star returned 
 towards the South, till it arrived in 'December to 
 the fame Situation it was in at that time twelve 
 Months, allowing for the Difference of Declination 
 on account of the Preceflion of the Equinox. 
 
 This was a fufficient Proof, that the Inflrument 
 had not been the Caufe of this apparent Motion of 
 the Star, and to find one adequate to fuch an Effect 
 teemed a Difficulty, A Nutation of the Earth’s 
 Axis was one of the firfl; things that offered itfelf 
 upon this Occafion, but it was toon found to be 
 infufficient ; for though it might have accounted for 
 the change of Declination in y Draconis yet it would 
 not at the fame time agree with the Phenomena in 
 other Stars ; particularly in a fmall onealmoft oppofite 
 in right Afcenfion to y Draconis , at about the fame 
 Diftance from the North Pole of the Equator: For, 
 though this Star feemed to move the fame way, as a 
 Nutation of the Earth’s Axis would have made it, 
 yet it changing its Declination but about half as 
 much as y Draconis in the fame time ( as appeared 
 upon comparing the Obfervations of both made upon 
 the fame Days, at different Seafons of the Year) thi$ 
 plainly proved, that the apparent Motion of the 
 Stars was not occafioned by a real Nutation, fince if 
 that had been the Caufe, the Alteration in both Stars 
 would have been near equal. 
 
 The great Regularity of the Obfervations left no 
 room to doubt, but that there was fome regu/ar 
 Caufe that produced this unexpected Motion , which 
 did not depend on the Uncertainty or Variety of the 
 
 CLqqq Seafons 
 
( «4* ) 
 
 Seafons of the Year. Upon comparing the Obfer- 
 vations with each other, it was difcovered, that in 
 both the fore-mentioned Stars, the apparent Dif¬ 
 ference of Declination from the Maxima ,, was al¬ 
 ways nearly proportional to the verfed Sine of the 
 Sun’s Diftance from the Equinoctial Points. This 
 was an Inducement to think, that the Caufe, what¬ 
 ever it was, had fome Relation to the Sun’s Situa¬ 
 tion with relpedt to thofe Points. But not being 
 able to frame any Hypothefis at that Time, fuffi- 
 cient to fclve all the Phenomena, and being very 
 defirous to fearch a little farther into this Matter; 
 
 I began to think of erecting an Inftrument for my 
 felf at IVanjled , that having it always at Hand, I 
 might with the more Eafe and Certainty, enquire 
 into the Laws of this new Motion. The Confide- 
 ration likewife of being able by another Inftrument, 
 to confirm the Truth of the Obfervations hitherto 
 made with Mr. Molyneux’s , was no fmall Induce¬ 
 ment to me; but the Chief of all was, the Oppor¬ 
 tunity I fliould thereby have of trying, in what 
 Manner other Stars were affecfted by the lame Caufe, 
 whatever it was. For Mr. Molyneux's Inftrument 
 being originally defigned for obferving y c Draconis (in 
 order, as I faid before, to try whether it had any 
 fenfible Parallax ) was fo contrived, as to be capable 
 of but little Alteration in its Direction, not above 
 feven or eight Minutes of a Degree : and there being 
 few Stars within half that Diftance from the Zenith 
 of Kew , bright enough to be well obferved, he 
 could not, with his Inftrument, throughly examine 
 how this Caufe affe&ed Stars differently fituated with 
 
 refpedt 
 
( «4? > 
 
 refpetft to the equino&ial and folftitial Points of the 
 EcJiptick. 
 
 Thefe Confiderations determined me ; and by the 
 Contrivance and Direction of the fame ingenious 
 Perion, Mr. Graham , my Inftrument was fixed up 
 Auguft 19, 172,7. As I had no convenient Place 
 where I could make ufe of fo long a Telefcope as 
 Mr. Molyneux's , I contented my felf with one of 
 but little more than half the Length of his ( viz. of 
 about 12- Feet, his being 241) judging from the 
 Experience which I had already had, that this Ra¬ 
 dius would be long enough to adjuft the Inftrument 
 to a fufficient Degree of Exadtnefs, and I have had 
 no Reafbn fince to change my Opinion: for from all 
 the Trials I have yet made, l am very well fatisfied, 
 that when it is carefully re<ftified, its Situation may 
 be fecurely depended upon to half a Second. As the 
 Place where my Inftrument was to be hung, infome. 
 Meafure determined its Radius, fo did it alfo the 
 Length of the Arch, or Limb, on which the Divifions 
 were made to adjuft it: For the Arch could not con¬ 
 veniently be extended farther, than to reach to about 
 6 i° on each Side my Zenith. This indeed was fuffi¬ 
 cient, fince it gave me an Opportunity of making 
 Choice of feveral Stars, very different both in Mag-, 
 nitude and Situation; there being more than two, 
 hundred inferted in the Britijh Catalogue,that may be 
 obferved with it. I needed not to have extended the. 
 Limb fo far, but that I was willing to take in Capella r 
 the only Star of the firft Magnitude that comes fo 
 near my Zenith. 
 
 My Inftrument being fixed, I immediately began, 
 to obferve fuch Stars as I judged moft proper to 
 
 Q-qqq *-■ give 
 
( ^44 ) 
 
 give me light into the Caufe of the Motion already 
 mentioned. There was Variety enough of fmall 
 ones; and not left than twelve, that I could obferve 
 through all the Seafons of the Year} they being 
 bright enough to be feen in the Day-time, when 
 neared the Sun. I had not been long obferving, be¬ 
 fore I perceived, that the Notion we had before en¬ 
 tertained of the Stars being farthed North and South* 
 when the Sun was about the Equinoxes, was only 
 true of thofe that were near the folditial Colure: And 
 after I had continued my Obfervations a few Months, 
 I difcovered, what I then apprehended to be a gene¬ 
 ral Law, obferved by all the Stars, viz. That each 
 of them became dationary, or was farthed North or 
 South, when they pafled over my Zenith at fix of 
 the Clock, either in the Morning or Evening. I per- 
 ' ceived likewife, that whatever Situation the Stars 
 were in with relpedt to the cardinal Points of the 
 Ecliptick, the apparent Motion of every one tend¬ 
 ed the fame Way, when they pafled my Indrument 
 about the fame Hour of the Day or Night ; for they 
 all moved Southward, while they pafled in the Day, 
 and Northward in the Night ; fo that each was far- 
 thed North, when it came about Six of the Clock in 
 the Evening, and farthed South, when it came a- 
 bout Six in the Morning. 
 
 Though I have fince difcovered, that the Maxima 
 in mod of thefe Stars do not happen exadtly when 
 they come to my Indrument at thofe Hours, yet not 
 being able at that time to prove the contrary, and 
 fuppofmg that they did, I endeavoured to find out 
 what Proportion the greated Alterations of Decli¬ 
 nation in different Stars bore to each other} it being 
 
C <*45 ) 
 
 very evident, that they did not all change their De¬ 
 clination equally. I have before taken notice, that 
 it appeared from Mr. Molyneaux's Obfervations, 
 that y ‘Draconis altered its Declination about twice 
 as much as the fore-mentioned fmall Star almoft op- 
 polite to it; but examining the matter more particu¬ 
 larly, I found that the greateft Alteration of Declina¬ 
 tion in thefe Stars, was as the Sine of the Latitude 
 of each refpedtively. This made me fufpedt that 
 there might be the like Proportion between the 
 Maxima of other Stars ; but finding, that the Ob- 
 jervations of fome of them would not perfedUy cor- 
 refpond with fiich an Hypothefis, and not knowing, 
 whether the fmall Difference I met with, might not 
 be owing to the Uncertainty and Error of the Ob- 
 fervations, I deferred the farther Examination into 
 the Truth of this Hypothefis, till I fhould be fur- 
 nifhed with a Series of Obfervatious made in all 
 Parts of the Year ; which might enable me, 
 not only to determine what Errors the Obferva- 
 tions are liable to, or how far they may fafely be 
 depended upon ; but alfo to judge, whether there had 
 been any fenfible Change in the Parts of the Inftru- 
 ment itl'elf. 
 
 Upon thefe Confiderations, I laid afide all Thoughts 
 at that Time about the Caufe of the fore-mentioned 
 Phaenomena, hoping that I fhould the eafier difcover 
 it, when I was better provided with proper Means to 
 determine more precisely what they were. 
 
 When the Year was compleated, I began to exa¬ 
 mine and compare my Obfervations, and having pret¬ 
 ty well fatisfied my felf as to the general Laws of the 
 Thanomena, I then endeavoured to find out the 
 
 Caufe 
 
 i 
 
( 64^6 ) 
 
 Caufe of them, I was already convinced, that the 
 apparent Motion of the Stars was not owing to a 
 Nutation of the Earth’s Axis. The next Thing that 
 offered itfelf, was an Alteration in the Direction of 
 the Plumb-line, with which the Inftrument was con- 
 ftantly rectified ; but this upon Trial proved infuffL 
 cienr. Then I confidered what Refraction might do, 
 but here alfo nothing fatisfactory occurred. At laft 
 I conjectured, that all the c Phanome 7 ta hitherto men¬ 
 tioned, proceeded from the progreflive Motion of 
 Light and the Earth’s annual Motion in its Orbit; 
 For I perceived, that, if Light was propagated in 
 Time, the apparent Place of afixt Object would not 
 be the fame when the Eye is at Reft, as when it is 
 moving in any other Direction, than that of the Line 
 paffing through the Eye and Object; and that, when 
 the Eye is moving in different Directions, the appa¬ 
 rent Place of the Object would be different* 
 
 I confidered this Matter in the fol¬ 
 lowing Manner. I imagined CA to 
 be a Ray of Light, falling perpendi¬ 
 cularly upon the Line B D ; then if 
 the Eye is at reft at A, the Object 
 muft appear in the Direction A C, 
 whether Light be propagated in Time 
 or in an Inftant. But if the Eye is 
 moving from B towards A, and Light 
 is propagated in Time, with a Velo¬ 
 city that is to the Velocity of the 
 Eye, as C A to B A ; then Light mov¬ 
 ing from C to A, whilft the Eye 
 moves from B to A, that Particle of 
 
( 6 47 ) 
 
 it, by which the ObjecSt will be difcerned, when the 
 Eye in its Motion comes to A, is at C when the 
 Eye is atB. Joining the Points B,C, I fuppofed the 
 Line CB, to be a Tube (inclined to the Line B D in 
 the Angle D B C ) of fuch a Diameter, as to admit 
 of but one Particle of Light; then it was eafy 
 to conceive, that the Particle of Light at C (by 
 which the Object mult be feen when the Eye, as it 
 moves along, arrives at A) would pals through the 
 Tube B C, if it is inclined to B D in the Angle D B C, 
 and accompanies the Eye in its Motion from B to A, 
 and that it could not come to the Eye, placed behind 
 fuch a Tube, if it had any other Inclination to the 
 Line BD. If inftead of fuppofing CB lo fmall a 
 Tube, we imagine it to be the Axis of a larger; then 
 for the lame Reafon, the Particle of Light at C, could 
 not pals through that Axis, unlefs it is inclined toBD, 
 in the Angle CBD. In like manner, if the Eye 
 moved the contrary way, from D towards A, with 
 the fame Velocity ; then the Tube mull be inclined 
 in the Angle BDC. Although therefore the true or 
 real Place of an Objebt is perpendicular to the Line 
 in which the Eye is moving, yet the vifible Place 
 will not be fo, fince that, no doubt, mull be in the 
 Direction of the Tube ; but the Difference between 
 the true and apparent Place will be ( cateris paribus ) 
 greater or lefs, according to the different Proportion 
 between the Velocity of Light and that of the Eye. 
 So that if we could fuppofe that Light was propagat¬ 
 ed in an Inltant, then there would be no Difference be¬ 
 tween the real and vifible Place of an Object, altho’ 
 the Eye were in Motion, for in that cafe, AC be¬ 
 ing infinite with Relpedt to A B, the Angle A C B (the 
 
i 648 ) 
 
 Difference between the true and vifible Place) vaniflies. 
 But if Light be propagated in Time (which I prefume 
 will readily be allowed by mod of the Philolbphers 
 of this Age) then it is evident from the foregoing 
 Confiderations, that there will be always a Difference 
 between the teal and vifible Place of an Objed, un- 
 lefs the Eye is moving either diredly towards or from 
 the Objed. And in all Cafes, the Sine of the Dif¬ 
 ference between the real and vifible Place of the Ob¬ 
 jed, will be to the Sine of the vifible Inclination of 
 the Objed to the Line in which the Eye is moving, 
 as the Velocity of the Eye to the Velocity of 
 Light. 
 
 If Light moved but 1000 times fafler than the Eye, 
 and an Objed (fuppofed to be at an infinite Diftance) 
 was really placed perpendicularly over the Plain in 
 which the Eye is moving, it follows from what hath 
 been already laid, that the apparent Place of fuch an 
 Objed will be always inclined to that Plain, in an 
 Angle of 89° ; lo that it will conftantly appear 
 
 3' i from its true Place, and feem fo much lels inclin¬ 
 ed to the Plain, that way towards which the Eye tends. 
 That is, if AC is to AB(or AD) as 1000 to one, 
 the Angle AB C will be 89° 56' I, and ACB = 3 'i, and 
 BCD =iACB=7'. So that according to this Sup- 
 pofitio n > the vifible or apparent Place of the Objed 
 will be altered 7', if the Diredion of the Eye’s Mo¬ 
 tion is at one time contrary to what it is at ano¬ 
 ther. 
 
 If the Earth revolve round the Sun annually, and 
 the Velocity of Light were to the Velocity of the 
 Earth’s Motion in its Orbit (which I will at prelent 
 fuppofe to be a Circle) as 1000 to one; then tis eafy 
 
 * to 
 
( ^49 ) 
 
 to conceive, that a Star really placed in the very Pole 
 of the Ecliptick, would, to an Eye carried along with 
 the Earth, feem to change its Place continually, and 
 (negle&ing the finall Difference on the Account of 
 the Earth’s diurnal Revolution on its Axis) would 
 feem to defcribe a Circle round that Pole, every Way 
 dirtant therefrom 3'f. So that its Longitude would 
 be varied through all the Points of the Ecliptick every 
 Year; but its Latitude would always remain the fame. 
 Its right Afcenfion would alfo change, and its Decli¬ 
 nation, according to the different Situation of th& 
 Sun in relpedi to the equinoctial Points ; and its ap¬ 
 parent Diflance from the North Pole of the Equator 
 would be 7 / lefs at the Autumnal, than at the vernal 
 Equinox. 
 
 The greateft Alteration of the Place of a Star in 
 the Pole of the Ecliptick (or which in Effect amounts 
 to the fame, the Proportion between the Velocity of 
 Light and the Earth’s Motion in itsOrbit) being known; 
 it will not be difficult to find what would be the Dif¬ 
 ference upon this Account, between the true and ap¬ 
 parent Place of any other Star at any time ; and on 
 the’contrary, the Difference between the true and appa¬ 
 rent Place being given ; the Proportion between the 
 Velocity of Light and the Earth’s Motion in its Or¬ 
 bit may be found. 
 
 As I only obferved the apparent Difference of De¬ 
 clination of the Stars, I fhall not now take any far¬ 
 ther Notice in what manner fuch a Caufe as I have 
 here fuppofed would occafion an Alteration in their 
 apparent Places in other RefpecSts ; hut, fuppofing the 
 Earth to move equally in a Circle, it may be gather¬ 
 ed from what hath been already faid, that a Star which 
 
 R r r r is 
 
( 6 5 ° ) 
 
 is neither in the Pole nor Plain of the Ecliptick, will 
 feemto defcribe about its true Place a Figure, infenfi- 
 bly different from an Ellipfe, whofe Tranfverfe Axis 
 is at Right-angle to the Circle of Longitude palling 
 through the Stars true Place, and equal to the Diame¬ 
 ter of the little Circle defcribed by a Star (as was 
 before fuppofed) in the Pole of the Ecliptick ; and 
 whofe Conjugate Axis is to its Tranfverfe Axis, as the 
 Sine of the Stars Latitude to the Radius. And al¬ 
 lowing that a Star by its apparent Motion does ex- 
 ajftly defcribe fuch an Ellipfe, it will be found, that 
 if A be the Angle of Pofition (or the Angle at the 
 Star made by two great Circles drawn from it, thro’" 
 the Poles of the Ecliptick and Equator) and B be 
 another Angle, whofe Tangent is to the Tangent of 
 A as Radius to the Sine of the Latitude of the Star; 
 then B will be equal to the Difference of Longitude 
 between the Sun and the Star, when the true and ap¬ 
 parent Declination of the Star are the fame. And if 
 the Sun’s Longitude in the Ecliptick be reckoned 
 from that Point, wherein it is when this happens; 
 then the Difference between the true and apparent 
 Declination of the Star (on Account of the Caufe I 
 am now conftdering) will be always, as the Sine of 
 the Son’s Longitude from thence. It will likewife be 
 found, that the greatefl Difference of Declination 
 that can be between the true and apparent Place of 
 the Star, will be to the Semi-Tranfverfe Axis of the 
 Ellipfe (or to the Semi-diameter of the little Circle de¬ 
 fended by a Star in the Pole of the Ecliptick) as the 
 Sine of A to the Sine of B. 
 
 If the Star hath North Latitude, the Time, when 
 its true and apparent Declination are the fame, is be¬ 
 fore: 
 
{* 5 * ) 
 
 fore the Sun comes in Conjun&ion with or Oppofition 
 to it, if its Longitude be in the .firft or laft Quadrant 
 (viz. in the afcending Semi-circle) of the Ecliptick ; and 
 after them, if in the defending Semi-circle ; and it wilt 
 appear neareft to the North Pole of the Equator, at the 
 Time of that Maximum (or when the greateft Differ¬ 
 ence between the true and apparent Declination happens) 
 which precedes the Sun’s Conjun&ion with the 
 Star. 
 
 Thefe Particulars being fufficient for my prefent 
 Purpofe, I fhall not detain you with the Recital of 
 any more, or with any farther Explication of thefe. It 
 may be time enough to enlarge more upon this Head, 
 when I give a Defcription of the Inftruments &c. if 
 that be judged neceflary to be done; and when I (hall 
 find, what I now advance, to be allowed of (as I flat¬ 
 ter my felf it will) as fomething more than a bare Hy¬ 
 pothecs. I have purpofely omitted fome matters of no 
 great Moment, and confidered the Earth as moving in a 
 Circle, and not an Ellipfe, to avoid too perplexed a 
 Calculus , which after all the Trouble of it would not 
 fenfibly differ from that which I make ufe of, efpecial- 
 ly in thofe Confequences which I (hall at prefent draw 
 from the foregoing Hypothefis. 
 
 This being premifed, I fhall now proceed to deter¬ 
 mine from the Obfervations, what the real Proportion is 
 between the Velocity of Light and the Velocity of the 
 Earth’s annual Motion in its Orbit; upon Suppofition 
 that the ‘Phenomena before mentioned do depend upon 
 the Caufes I have here afligned. But I i«uft firft let 
 you know, that in all the Obfervations hereafter men¬ 
 tioned, I have made an Allowance for the Change of 
 the Star’s Declination on Account of the Preceflion of 
 
 R r r r 2 the 
 
(6 5 \> 
 
 the Equinox, upon Suppofition that the Alteration 
 from this Caufe is proportional to the Time, and regu* 
 lar through all the Parts of the Year. I have deduced 
 the real annual Alteration of Declination of each Star 
 from the Obfervations themfelves ; and I the rather 
 choofe to depend upon them, in this Article, becaufe alt. 
 which I have yet made, concur to prove, that the Stars 
 near the Equino&ial Colure, change their Declination at 
 this time i" I or 2" in a Year more than they would do 
 if the Preceflion was only 50", as is now generally fup- 
 pofed. I have likewife met with fome fmall Varieties 
 in the Declination of other Stars in different Years, 
 which do not feem to proceed from the fame Caufe, par* 
 ticularly in thofe that are near the folftitial Colure, 
 which on the contrary have altered their Declination 
 iefs than they ought, if the Preceflion was $o n \ But 
 whether thefe fmall Alterations proceed froth a regular 
 Caufe, or are occaftoned by any Change in the Mate* 
 rials &c. of my Inftrument, I am not yet able fully 
 to determine. However, I thought it might not be a- 
 mifs juft to mention to you how I have endeavoured ta 
 allow for them, though die Refult would have been 
 nearly the fame, if I had not confidered them at all. 
 What that is, I will fhew, firft from the Obfervations 
 of y Draconic , which was found to be 39" more South* 
 erly in the Beginning of March , than in September. 
 
 From what hath been premifed, it will appear that 
 the greateft Alteration of the apparent Declination of 
 y'Draconic, on Account of the fucceffive Propagation 
 of Light, would be to the Diameter of the little Circle 
 which a Star (as was before remarked) would feem to 
 defcribe about the Pole of the Ecliptick, as 39^ to 
 40", 4. The half of tills is the Angle A C B*(as repre- 
 
 fented 
 
f <S ?3 ) 
 
 fented in the i^.)This therefore being i, A G will 
 be to AB, that is, the Velocity of Light to the Velo¬ 
 city of the Eye (which in this Cafe may be fuppofed 
 the fame as the Velocity of the Earth’s annual Motion 
 in its Orbit) as ioxio to One, from whence it would 
 follow, that Light moves, or is propagated as far as 
 from the Sun to the Eartlvin 8' 1 i! 1 . 
 
 It is well kno wn,that Mr, Romer, who firfl attempted 
 to account for an apparent Inequality in the Times of the 
 Eclipfes of Jupiter ’s Satellites, by the Hypothefis of 
 the progreflive Motion of Light, fuppofed that it fpent 
 about ii' Minutes of Time in its PalTage from the Sun 
 to us: but it hath fince been concluded by others from 
 the like Eclipfes, that it is propagated as far in about 
 7 Minutes. The Velocity of Light therefore deduced 
 from the foregoing Hypothefis, is as it were a Mean 
 betwixt what had at different times been determined 
 from the Eclipfes of Jupiter 's Satellites. 
 
 Thefe different Methods of finding the Velocity of 
 Light thus agreeing in the Refult, we may reafonably 
 conclude, not only that thefe ‘Phenomena are owing 
 to the Caufes to which they have been afcribed but 
 alfo, that Light is propagated (in the fame Medium ) 
 with the fame Velocity after it hath been refle&ed as 
 before: for this will be the Confequence, if we allow 
 that the Light of the Sun is propagated with the fame 
 Velocity,, before it is reflected,, as the Light of the fixt 
 Stars. And I imagine this will fcarce be queftioned, 
 if it can be made appear that the Velocity of the Light 
 of all the jixt Stars is equal, and that their Light moves 
 or is propagated through equal Spaces in equal Times, 
 at all Diftances from them: both which points (as I ap¬ 
 prehend) are fufficiently proved from the apparent Alte¬ 
 ration 
 
t &S4 ) 
 
 ration of the Declination of Stars of different Luftre $ 
 for that is not fenfibly different in fuch Stars as feem 
 near together* though they appear of very different 
 Magnitudes. And whatever their Situations are (if I 
 proceed according to the foregoing Hypothefis) I find 
 the fame Velocity of Light from my Obfervations of 
 fmall Stars of the fifth or fixth, as from thofe of the 
 fecond and third Magnitude, which in all Probability 
 are placed at very different Diflances from us. The 
 fmall Star,for Example, before fpoken of^ that is altnoft 
 oppofite to y Draconis (being the 3 5th Camelopard. 
 Hevelii in Mr. Flamjleed's Catalogue) was 15^ more 
 Northerly about the Beginning of March than in Sep - 
 tember. Whence I conclude, according to my Hypo¬ 
 thefis, that the Diameter of the little Circle defcribed 
 by a'Star in the Pole of theEcliptick would be 40", 2. 
 
 The laft Star of the great Bear’s-tail of the 2d 
 Magnitude (marked yj by Bayer) was 36'' more South¬ 
 erly about the Middle of January than in July . 
 Hence the Maximum , or greateft Alteration of Decli¬ 
 nation of a Star in the Pole of the Ecliptick would be 
 40", 4, exa&ly the fame as was before found from the 
 Obfervations of y 'Draconts . 
 
 TheStar of the yth magnitude in the Head of Berfeus 
 marked r by Bayer y was 25^ more Northerly about 
 the End of 'December than on the 29th of July fol¬ 
 lowing. Hence the Maximum would be 41". This 
 Star is not bright enough to be feen as it pafles over my 
 Zenith about the End of June , when it thould be ac¬ 
 cording to the Hypothefis fartheft South. But becaufe 
 I can more certainly depend upon the greateft Alterati¬ 
 on of Declination of thofe Stars, which I have frequent¬ 
 ly obferved about the Times when they become ftatio- 
 2. nary. 
 
( *55 ) 
 
 nary, with refpeft to the Motion I am now confider- 
 ing ^ I will fet down a few more Inftances of fucli* 
 from which you may be able to judge how near it may 
 be poffible from thefe Obfervations, to determine with 
 what Velocity Light is propagated. 
 
 ol Terfel Bayero was ^3" more Northerly at the 
 beginning of January than in July . Hence the Maxi¬ 
 mum would be 40", 2. cx, Cajfiopea was 3 4'' more 
 Northerly about the End of ‘December than in June . 
 Hence the Maximum would be 40", 8. /g Draconic 
 was 39" more Northerly in the beginning of Septem¬ 
 ber than in March ^ hence the Maximum would be 
 40", 2. Capella was about 16" more Southerly 
 in Auguft than in February ; hence the Maximum 
 would be about 4o' r . But this Star being farther from 
 my Zenith than thofel have before made ufe of, I can¬ 
 not fo well depend upon my Obfervations of it, as of 
 the others \ becaufe I meet with fome finall Alterations 
 of its Declination that do not feem to proceed from the 
 Caufe I am now confidering, 
 
 I have compared the Obfervations of feveral other 
 Stars, and they all confpire to prove that the Maximum 
 is about 40" or 41". I will therefore fuppofe that it 
 is 40"4- or (which amounts to the fame) that Light 
 moves, or is propagated as far as from the Sun to us in 
 8' 13". The near Agreement which I met with among 
 my Obfervations induces me to think, that the Maxi¬ 
 mum (as I have here fixed it) cannot differ fo much as 
 a Second from the Truth, and therefore it is probable 
 that the Time which Light fpends in paffmg from the 
 Sun to us, may be determined by thefe Obfervations 
 within 5" or 10"; which is fuch a degree of exaftnefs as 
 we can never hope to attain from the Eclipfes of Jw- 
 pter'% Satellites, Having 
 
( 656 ) 
 
 Having thus found the Maximum , or what the great- 
 eft Alteration of Declination would be in a Star pla¬ 
 ced in the Pole of the Ecliptick, I will now deduce 
 from it (according to the foregoing Hypothefis) the 
 Alteration of Declination in one or two Stars, at fuch. 
 times as they were actually obferved, in order to fee 
 how the Hypothefis will correfpond with the Pheno¬ 
 mena through all the Parts of the Year. 
 
 It would be too tedious to fet down the whole Se¬ 
 ries of my Obfervations ^ I will therefore make Choice 
 only of fuch as are moft proper for my prefent Pur- 
 pofe, and will begin with thofe of y < Draconis. 
 
 This Star appeared fartheft North about September 
 7th, 17x7, as it ought to have done according to my 
 Hypothefis. The following Table Ihews how much 
 more Southerly the Star was found to be by Obfervati- 
 on in feveral Parts of the Year, and likewifehow much 
 more Southerly it ought to be according to the Hy¬ 
 pothefis. 
 
 172.7. D. 
 
 The Difference of 
 Declination by 
 Obfervation. ^ 
 
 H 
 
 ~ % 
 zr 0. n 
 
 « CLd 
 
 Iff 
 
 re 0 
 
 n 
 
 1728. D. 
 
 The Difference of 
 Declination by 
 Obfervation. ^ 
 
 The Difference of 
 Declination by 
 the Hypothefis. ^ 
 
 October 20th — 
 
 41 
 
 44 
 
 March * 24 
 
 37 
 
 38 
 
 November - 17 
 
 nf 
 
 1 1 
 
 April - - 6 
 
 3 « 
 
 3 <H 
 
 December - 6 
 
 1 7 i 
 
 iSi 
 
 May - ~ <5 
 
 284 
 
 ipr 
 
 - » - 28 
 
 a r 
 
 16 
 
 1 
 
 1 
 
 <!a 
 
 sa 
 
 i8i 
 
 20 
 
 00 
 
 *-a 
 
 
 
 . - - iy 
 
 ! 7 f 
 
 l 7 
 
 January - 24 
 
 34 
 
 34 
 
 July - - 3 
 
 iif 
 
 nf 
 
 February ' - 10 
 
 38 
 
 37 
 
 Auguft - 2 
 
 4 
 
 4 
 
 March - -* 71 
 
 3 P 
 
 3 P 
 
 September - 6 
 
 0 
 
 0 
 
 Hence 
 
( *57 ) 
 
 Hence it appears, that the Hypothefis correfponds 
 with the Obfervations of this Star through all Parts of 
 the Year} for the fmall Differences between them feem 
 to arife from the Uncertainty of the Obfervations, 
 which is occafioned (as I imagine) chiefly by the tre¬ 
 mulous or undulating Motion of the Air, and of the 
 Vapours in it j which caufes the Stars fometimes to 
 dance to and fro, fo much that it is difficult to judge 
 when they are exactly on the Middle of the Wire that 
 is fixed in the common Focus of the daffies of fhe 
 Telefcope. 
 
 I mult confefs to you, that the Agreement of the 
 Obfervations with each other, as well as with the Hy¬ 
 pothefis, is much greater than I expeded to find, be¬ 
 fore I had compared them; and it may poflibly be 
 thought to be too great, by thofe who have been ufed to 
 Aftronomical Obfervations, and know how difficult it 
 is to make fuch as are in all refpeds exad. But if it 
 would be any Satisfadion to fuch Perfons (till I have 
 an Opportunity of deferibing my Inftrument and the 
 manner of ufing it) I could affiure them, that in above 
 70 Obfervations which I made of this Star in a Year, 
 there is but one (and that is noted as very dubious on 
 account of Clouds) which differs from the foregoing 
 Hypothefis more than 2", and this does not differ 3". 
 
 This therefore being the Fad. I cannot but think it 
 very probable, that the ‘Phenomena proceed from the 
 Caufe I have affigned, fince the foregoing Obfervations 
 make it fufficiently evident, that the Effed me real 
 Caufe, whatever it is, varies in thi® ™ r h e ^ am ® 
 Proportion that it ought ac^ming to the Hypothefis. 
 
 But leaft y ‘Draw* may be thought not fo proper 
 to fhew the Proportion, in which the apparent Altera- 
 
 Sfff tion 
 
( M ) 
 
 tion of Declination is inereafed or diminifhec3, : a* 
 thofe Stars which lie near the Equino&ial Colure: I 
 will give you alfo the Comparifon between the Hypo¬ 
 thefis and the Obfervations of * Vrfa Majoris , that 
 which was fartheft South about the 17th Day of Janu¬ 
 ary 17x8, agreeable to the Hypothefis. The following 
 Table fhews how much more Northerly it was found 
 by Obfervation in feveral Parts of the Year, and alfo 
 what the Difference Ihould have been according to the 
 Hypothefis. 
 
 J7Z7. d. 
 
 The Difference of 
 Declination byOb- 
 fervation. ^ 
 
 The Difference of 
 Declination by the 
 Hypothefis. ^ 
 
 1728. 
 
 ,d. 
 
 The Difference of j 
 Declination byOb- 
 fervation. 1 
 
 The Difference of 
 Declination by the 
 Hypothefis. ^ 
 
 September - 14 
 
 iP f 
 
 2 . 8 * 
 
 slpnl - 
 
 - 1 6 
 
 I 8 i 
 
 18 
 
 - - " H 
 
 Hi 
 
 
 May - 
 
 - f 
 
 247 
 
 23# 
 
 October — id 
 
 iPf 
 
 ip# 
 
 'June - 
 
 - f 
 
 3 Z 
 
 31 f 
 
 November - 11 
 
 n| 
 
 IO| 
 
 - 
 
 - 2 f 
 
 3 T 
 
 34 * 
 
 December - 14 
 
 1728 
 
 4 
 
 3 
 
 July - 
 
 - l 7 
 
 3 ^ 
 
 36 
 
 February - 17 
 
 2 
 
 3 
 
 Juguft 
 
 - 2 
 
 3f 
 
 3 fl 
 
 March - - 2 J 
 
 ill 
 
 IOI 
 
 2 
 
 September 
 
 - 20 
 
 2(51 
 
 
 I Tind noon Examination, that the Hypothefis a- 
 grees altogetfic* ^ exactly with the Obfervations of 
 this Star, .as. the former, f or j n a b ou t; y 0 that were 
 made of it in a Year, I do not meet with a Difi 
 ference of fo much as 2", except in ox^ which is 
 
 mark’d 
 
( <5?9 ) 
 
 mark'd as doubtful on Account of the Undulation 
 of the Air, &c . And this does not differ 3" from 
 the Hypothefis. 
 
 The Agreement between the Hypothefis and the 
 Obfervaticns of this Star is the more to be reguard¬ 
 ed, fince it proves that the Alteration of Declination, 
 on account of the Proceffion of the Equinox, is (as 
 I before fuppoied) regular thro’ all Parts of the Year; 
 fo far at lead, as not to occafion a Difference great 
 enough to be difcovered with this Inftrument. It like- 
 wife proves the other part of my former Suppofition, 
 'viz. that the annual Alteration of Declination in 
 Stars near the Equinoctial Colure, is at this Time 
 greater than a Preceffion of $o u would occafion : for 
 this Star was zo" more Southerly in September 1718, 
 than in September 1717, that is, about z" more than 
 it would have been, if the Preceffion was but 50". 
 But I may hereafter, perhaps, be better able to deter¬ 
 mine this Point, from my Obfervations of thofe 
 Stars that lie near the Equinoctial Colure, at about 
 the fame Diftance from the North Pole of the E- 
 quator, and nearly oppofite in right Afcenfion. 
 
 I think it needlefs to give you the Comparifon 
 between the Hypothefis and the Obfervations of any 
 more Stars ; fince the Agreement in the foregoing is a 
 kind of Demonftration (whether it be allowed that 
 I have difcovered the real Caufe of the ‘Phenomena 
 or not;) that the Hypothefis gives at lead the true 
 Law of the Variation of Declination in different Stars, 
 with RefpeCt to their different Situations and Af- 
 peCts with the Sun. And if this is the Cafe, it mud 
 be granted, that the Parallax of the fixt Stars is much 
 Fmaller, than hath been hitherto fuppofed by thofe, 
 
 Sfffz who 
 
( 66o ) 
 
 whet have pretended to deduce it from their Obfervati- 
 ons. I believe, that I may venture to fay, that in 
 either of the two Stars laft mentioned, it does not 
 amount to a". I am of Opinion, that if it were i", I 
 fhould have perceived it, in the great number of Ob- 
 fervations that I made,efpecially of y ‘Draconis ; which 
 agreeing with the Hypothefis (without allowing any 
 thing for Parallax) nearly as well when the Sun 
 was in Conjunction with, as in Oppofition to, this 
 Star, it feems very probable that the Parallax of it 
 is not lb great as one Angle Second ; and confequent- 
 ly that it is above 400000 times farther from us than 
 the Sun. 
 
 There appearing therefore after all, no fenfible 
 Parallax in the fixt Stars, the Anti - Copernicans have 
 ftill room on that Account, to objeCt againft the Mo¬ 
 tion of the Earth; and they may have (if they pleale) 
 a much greater Objection againft the Hypothefis, 
 by which I have endeavoured to folve the fore-men¬ 
 tioned Thanomena ; by denying the progreflive Mo- 
 tion of Light, as well as that of the Earth. 
 
 But as I do not apprehend, that either of thefe Po- 
 ftulates will be denied me by the Generality of the 
 Aftronomers and Philofophers of the prefent Age; 
 fo I lhall not doubt of obtaining their Aflent to the 
 Conlequences, which I have deduced from them j if 
 they are luch as have the Approbation of fo great 
 a Judge of them as yourlelf. I am, 
 
 Sir, Tour moji Obedient 
 
 Humble Servant 
 
 J. Bradley,' 
 
( 66 1 ) 
 
 POSTSCRIPT. 
 
 A S to the Obfervations of Dr. Hook y I muft own to 
 you, that before Mr. Molyneux's Inftrument was 
 erected, I had no fmall Opinion of their Corrednefs; 
 the Length of his Telefcope and the Care he pretends 
 to have taken in making them exad, having beenftrong 
 Inducements with me to think them fo. And fince I 
 have been convinced both from Mr. Molyneux's Obfer- 
 rations and my own, that the Doctor’s are really very 
 far from being either exad or agreeable to the ‘Pheno¬ 
 mena, \ I am greatly at a Lofs how to account for it. 
 I cannot well conceive that an Inftrument of the Length 
 of 36 Feet, conftruded in the Manner he defcribes his. 
 could have been liable to an Error of near 30" (which 
 was doubtlefs the Cafe) if rectified with fo much Care 
 as he reprefents. 
 
 The Obfervations of Mr. Flamjleed of the differ¬ 
 ent Diflances of the Pole Star from the Pole at differ¬ 
 ent Times of the Year, which were through Miflake 
 looked upon by fotne as a Proof of the annual Paral¬ 
 lax of it, feem to have been made with much greater 
 Care than thofe of Dr. Hook . For though they do not 
 all exadly correfpond with each other, yet from the 
 whole Mr. Flamjieed concluded that the Star was 35^ 
 4^" or 45" nearer the Pole in ‘December than in May 
 or July: and according to my Hypothefis it ought to 
 appear 40" nearer in December than in June . The 
 Agreement therefore of the Obfervations with the Hy- 
 pothefis is greater than could reafonably be expeded, 
 confidering the Radius of the Inftrument, and the Man¬ 
 ner in which it was conftruded. 
 
 * 
 
 x 
 
V. c '£ I SI O l T £ 0 c 
 
 ■A' "5 
 
 EKKA S V A. 
 
 TWl *’ 399 * Tab. content * P a g* *• Ho* * 4 - leg. Curienfis. N*‘ id. 
 J^i lin. 19. leg. latifolia • ibid. 1. n. leg. pyridncifis. pag. 315. 
 Curienfis . pag. 317. 1 . 11. leg. cuticula . ibid. 1 . 17. leg. Heifiert. p 
 leg. differentes . N*. 400. p. 351. 1 . 7. leg. Cafes, N°*403. p.471. 
 ri/>. ibid. pag. 489. 1. 15. leg. hexaedra . p.490. 1. 27. leg, ^r. £ 
 5B5-1- 14. leg* thirtythree , 
 
 c 
 
 pag. 294* 
 1, 23. le 
 '. 322.1. 
 
 1. 26. leg. 
 l P ’ 4 <M* P* 
 
 w OTQ 
 
An INDEX to the Thirty Fifth Volume of the 
 
 Philosophical Transactions. 
 
 A. 
 
 A BDOMEN, Two extraor dinary Tiimours of it , n. 40$. 
 p. 562. 
 
 /Equations, their impoffible Roots determined, n. 404. p. 
 515 - 
 
 Anatomy, Cerebra Epilep/ia mortuorum obfervata, n. 399. p. 315. 
 Catara&as fubfiantia, ibid p. 317. preternatural ‘Perforation in 
 the Stomach , n. 400. p. 3^1. Of the poifoncus apparatus of a 
 Rattle-Snake, n. 401. p. 377. TJiJfettion of three Subjetts , ib. 
 p. 413. Ancuryfm differed, n. 402. p. 436'. Villi of the Sto¬ 
 mach of Oxen, n. 404. p. 532. , 
 
 Ancuryfm of the Aorta, n. 402. p. 436'. Obfervations on them, ib. 
 
 p. 440. An artificial one , ib. p. 443. 
 
 Aurora Borealis feen at Petworth, n. 399. p. 501* At Liverpool, 
 ib. p. 304. At Paris, n. 402. p. 454. A Thiers, ib. p. end. 
 At Treggiaia, ib. p. 455. At Florence, ib. p. ead % At Bologna, 
 ib. p. 45 6. 
 
 B. 
 
 Barometer, the Heights of feveral Mountains meafured by it, n. 
 
 405. p. 537. n. 406-. p. 5 7 7- 
 
 Births and Burials, an Account of them , n. 400 . p. ^6^. 
 
 Bladder, fixty Stones found in it , n. 401. p. 413. Extreamly 
 finally ib. p. 314. thirty three Stones cut out of it , n. 404. 
 P- 335 - 
 
 Blindnefs, Obfervations of a Perfon brought to Sight by couching , 
 
 n. 402. p. 447. 
 
 Book, a farther Accoimt of one entitled. Vegetable Staticks, ffic. 
 n. 399. p. 323. A Refutation of one entitled , De Luminis af- 
 fe&ionibus, <Ac- n. 40(5. p. 569. 
 
 C. 
 
 Camphire, its Efficacy in maniacal Diforders , n. 400. p. 347. 
 Cataraft, its Sub fiance, n. 399. p. 317. 
 
 Cerebra, Epilepfia mortuorum obfervata , n. 399. p. 315. 
 
 Chirurgical Queftions, n. 399. p. 318. 
 
 Chronology, Sir If. Newton ’5 Syfiem of it defended, n. 399. p. 29^, 
 
 D. 
 
 Damps drawn from Mines , n. 400. p. 353. 
 
 Du£his alimentalis of Oxen defcribed, n. 404. p. 532. 
 
 E. 
 
 Earth, an uncommon Sinking of it, n. 405. p. 551. 
 
 Earthquake felt in Kent, n. 399. p. 305. 
 
 Ttt t 
 
 Eclipfes 
 
I N D E X. 
 
 Eclipfes of Jupiter's Satellites, n. 4.01. p. 408. n. 402. p. 415. n e 
 
 404. P-, 534 - n. 405. 5. 55 3 - **>. P* 55 7 - 
 Eclipfe of the Moon objerved at Liston, n. 400. p. 338. At Pekin 
 China, n. 405. p. 554. 
 
 Eclipfe of the Sun obferved ^Lisbon, n. 400. p. 335. At Vera 
 Cruz, n. 401. p. 388/ Near Lisbon, n. 403. p. 471. At Rome, 
 ib. p. 473. At Bologna, ib. p. 477. At Padua, ib. p. 479. At 
 Ingolftadt, n. 405. p. 558. 
 
 Elephants, their Teeth and Bones found under Ground, n. 403. p. 457. 
 n. 404. p. 497- 
 
 Experiments on the Fffeffs of the Poifon of a Rattle-Snake, n. 399. 
 p. 309. n. 401. p. 380. For drawing Damps out of Mines, n. 
 400. p. 353- For meafuring the 'Depths of the Sea, n. 405. p. 
 559. Several in Opticks , n. 4.06. p. 607. 
 
 Eyes, a new Operation on them , n. 402. p. 451. 
 
 F. 
 
 Falling Stars, the Geographical Longitude of places determinable by 
 them , n. 400. p. 351. 
 
 Finlanders, an Account of them , n. 400. p. 357. 
 
 Fire Subterraneous, a Species of it obferved in Kent, n. 399. p. 307 * 
 Fixt Stars, a new Motion difcovered in them , n. 406'. p. 637. 
 
 Forces of moving Bodies considered , n. 400. p. 343. n. 401. p. 381. 
 Foflil Teeth and Bones of Elephants , n. 403. p. 457. n. 404. p.497. 
 
 G. 
 
 Gage to meafure the Depth of the Sea , n. 405. p, 559. 
 
 Gout, its Nature and Caufes , n. 403. p. 491. 
 
 Ground, an uncommon Jinking of it, n. 405. p. 551, 
 
 H. 
 
 Hernia aquofa, a remarkable one, n. 401. p. 414. 
 
 Holt-Waters, their Impregation whence, n. 403. p. 489. 
 Hydraulicks, Remarks upon fome Experiments in them, relating to 
 the Forces of moving Bodies, n. 400. p. 34.3. 
 
 L 
 
 Iris contrasted , its Cure, n. 402. p. 451. 
 
 Iron Oar, different Species of it, n. 403. p. 480. 
 
 Jupiter, his Satellites eclipfed, n. 401. p. 408. n, 402. p c 415. n.404 
 p. 5 34. n. 405. p. 5.5 3. ibid p. 5 5 7-. 
 
 K. 
 
 Kent, an Earthquake felt there, n. 399. p. 305. A Species of fuh- 
 terraneous Fire in that County , n. 399. p. 307. An uncommon 
 Sinking of the Ground in the fame, n. 40 5. p. 551. 
 
 Kidneys, remarkable Stones found in them, n.402. p. 452. 
 
 Lateral 
 
■J 
 
 INDEX. 
 
 L. 
 
 Lateral Operation, Thirty three Stones cm out of the Bladder by 
 it, n.404. p. 535. 
 
 Latitude of Lisbon, n. 401. p. 405?. 
 
 London and Paris, their Magnitude compared , n. 402. p. 432. 
 
 Longitude Geographical of ‘Places determiifable from IE ailing Stars,, 
 n. 400. p. 351. Of Vera Cruz, n. 401. p* 389. 
 
 Long Life, Examples of it, n. 400. p. 364. 
 
 M. 
 
 Maniacal Diforders cured by Camphire, n. 400. p. 347. 
 
 Marriages, an Account of them , n. 400. p. 3 <55. 
 
 Mars, an Occultation of it by the Moon, n. 405. p. 55^. 
 
 Materia purulenta per triennium ex ore defluens, n. 400. p. 374. 
 
 Meteors, ^ new Method for a natural Hiftory of them , n. 401. p. 390. 
 
 Metals, Obfervations towards a natural Hiftory of them, n. 401. p. 
 402. n. 403. p. 480. 
 
 Mines, Experiments for drawing Damps out of them , n. 400. p._ 
 353. Obfervations towards a natural Hiftory of them, n. 401. p. 
 402. n. 403. p. 480. 
 
 Mifleto, its germinating ‘Principle, n. 3 99. p. 5 otf. Of different Sex , 
 n. 405. p. 54.7. 
 
 Moon, an Eclipfe of it obferved at Lisbon, n. 400. p. 338. At Pe¬ 
 kin in China, n. 405. p. 554. 
 
 Mortality, Bills of it, n. 400. p. 365. 
 
 Motion, A new one difcovered in the fixt Stars , n. 4 c6. p. 637. 
 
 Moving Bodies, their Forces conftdered , n. 400. p. 343. n. 401. p. 381. 
 
 Mountains, the Heights of feveral meafur'd by the Barometer, n! 405. 
 p. 537. n. 40 6. p. 577. 
 
 N. 
 
 Needle, its Variation , n. 401. p. 389. 
 
 Newton, Sir Ifaac ’5 Chronology defended , n. 399. p. 29^. Of ticks 
 defended , n. 406V p. 596. 
 
 Norwegian Finns, an Account of them , n. 400. p. 357. 
 
 Occultation of Venus, n. 404. p. 535. Of Mars by the Moon, n a 
 405. p. 5 5 6. 
 
 Omentum, a great Part of it found in the Scrotum , n. 401. p. 414.. 
 
 Opticks, fever al Experiments concerning them , n. 406'. p. 596. 
 
 Oxen, their Du&us alimentalis deferibed, n. 404. p. 532. 
 
 P. 
 
 Paris and London, their Magnitude compared, n. 402. p. 432. 
 
 Plants, a Catalogue of fifty , n. 399. p. 293. Exotick, n. 403. p, 
 485. 
 
 Poifonous Apparatus cf a Rattle-Snake differed, n. 4or. p. 377. 
 
 Preternatural perforation in the Stomach , n. 400. p, 36L 
 
 j 
 
 
 2 
 
 Pumice 
 
INDEX. 
 
 Pumice Stones on the main Ocean, n. 402 p. 444. <■ 
 
 - Q. - v - 
 
 Queftions Chirurgical ft ate d and anfwered, n. 399. p. 318. 
 
 R. 
 
 Rattle-Snake, the EffeHs of its Poifon, n. 399. p. 309. n. 401. p 
 380. Itspoifonous apparatus , n. 401. p.377. 
 
 Rizzetti, his Book of Of ticks refuted, n. 406". p. 59^. 
 
 Roman Pavement in Lincolnfhire, n. 402. p. 428. 
 
 Roots impojftble of adfeEled Aquations, n. 404. p. 515. 
 
 # S. 
 
 Saffron, its Culture, n. 405. p. 566. 
 
 Sea, a Gage-taweafure the Depth of it , n. 405. p. 559. 
 
 Seeds exotick raifed in England, n. 403. p. 485. 
 
 Skeleton of an extraordinary Large?iefs, n. 400. p. 3^3. 
 
 Specifick Gravity of Tin-Cryftals to the Metal , n. 403. p. 484. 
 
 Spleen of an enormous Size , n. 401. p. 414. 
 
 Stars, a new Motion difcovered in them , n 40 6 . p. eT3 7 . 
 
 Stomach, a preternatural perforation in it , n. 400. p. 3^1. 
 
 Stones, Sixty found in the Bladder, n. 401. p. 413. Several in the 
 Veficulae Seminales, ibid. p. 414. Remarkable Ones in both Kid¬ 
 neys, n. 402. p.452. Thirty three cut fuccefsfully out of the 
 Bladder, n. 404. p. 535. 
 
 Sun, an Eclipfe of it obferved at Lisbon, n. 400. p. 338. At Vera 
 Cruz, n. 401. p. 388. Near Lisbon, n. 403. p. 471. At Rome, 
 ibid. p. 473. At Bologna, ibid, p.477. Ad Padua, ibid. .479. At 
 Ingoldffadt, n. 405. p. 558. 
 
 Surgery, two ghieftions in it ftated and anfwered, n. 399. p. 318. 
 
 Swifferland, the Heights of its Mountains meafured, n. 405. p. 537. 
 n. 406". p. 577. 
 
 T. 
 
 Teeth and Bones of Elephants found under Ground, n. 403. p. 459. 
 n. 404. p. 497- 
 
 Tin Oar, its Compofnion, n. 403. p. 482. Specifick Gravity of its 
 Cryfials to the Metal, ibid. p. 484. 
 
 Tin-Slates, the Method of making them, n. 406. p. 637. 
 
 Tumours, two extraordinary ones of the Abdomen , n. 405. p. 562. 
 
 V. 
 
 Variation of the Needle, n. 401. p. 389. 
 
 Venus, an Occultation of it by the Moen,cbferved at Eologna, n. 404. 
 p. 5 5 5 • 
 
 Vera Cruz, Longitude of it, n. 401. p. 389. 
 
 Veficulae Seminales, Stones found in them, n. 401. p. 414. 
 
 Villi of the Stomach of Oxen, n. 404. p. 5 32. 
 
 Virgo eclipfed by the Moon, n. 404. p. 5 34. 
 
 FINIS . 
 

 
 
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