BOARD OF EDUCATION 
 Ca CATALOGUE 
 OF THE COLLECTIONS IN 
 THE SCIENCE MUSEUM 
 SOUTH KENSINGTON 
 
 | matte DESCRIPTIVE AND HISTORICAL NOTES 
 AND ILLUSTRATIONS 
 
 COMPILED BY 
 
 | GEODESY and SURVEYING 
 
 E. LANCASTER-JONES, B.A. 
 
 LONDON: 
 
 PRINTED AND PUBLISHED BY HIS MAJESTY’S STATIONERY OFFICE 
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 i PREFACE, 
 
 The Science Museum, with its Collections and Library, aims at 
 affording illustration and exposition in the fields of mathematical, 
 physical, and chemical science, as well as their applications to 
 astronomy, geophysics, engineering, and to the arts and industries 
 generally. To that end the Museum includes objects which are of 
 historical interest as marking important stages in development, and 
 others which are typical of the applications of science to current 
 practice. 
 
 A Museum of Science was contemplated as an integral part of the 
 Science and Art Department from its beginning in 1853, and in 1857 
 collections illustrating foods, animal products, examples of structures 
 and building materials, and educational apparatus, were brought 
 together and placed on exhibition. 
 
 The first of the Engineering Collections, that of Marine Construction, 
 was formed in 1864, when the Royal School of Naval Architecture was 
 established at South Kensington, and the ship models belonging to the 
 Admiralty were transferred to the Museum from Somerset House, 
 where they had previously been. This collection of ships of war was 
 of great historical interest, and with the assistance of private donors 
 and by purchase it was rapidly increased by the addition of many 
 models of mercantile ships as well as of later ships of war, with the 
 result that when the Admiralty removed their models to the Royal 
 Naval College, Greenwich, in 1873, an important collection still 
 remained at South Kensington. Engineering and Manufactures were 
 first included in 1867, from which time the development of this portion 
 of the Museum advanced steadily; but the transfer of the Museum 
 of the Patent Office to the Department of Science and Art in 1883 
 added to the collection many machines of the highest interest in the 
 history of invention and greatly increased its scope and value. 
 
 The collections of scientific instruments and apparatus were first 
 formed in 1874, but it was only after 1876 that they became of 
 importance. The Special Loan Collection of Scientific Apparatus 
 which was held in that year in London brought together examples of 
 all kinds from various countries, and a large number of these were 
 acquired for the Museum. 
 
 In 1893, many Mining and Metallurgical objects were transferred 
 to South Kensington from the Museum of Practical Geology in Jermyn 
 Street, and these have subsequently been largely added to. 
 
 Mention should be made, too, of certain special Collections: The 
 Watt Collection was presented to the Patent Museum in 1876 and 
 contains original models made by James Watt; the Maudslay Col- 
 lection, consisting of models of marine engines and machine tools, 
 was purchased in 1900; and in 1903 a valuable collection of engine 
 models, portraits, etc., was bequeathed by Bennet Woodcroft. 
 
 The Museum Collections are being continually increased by gifts 
 and loans, and also by the purchase of such examples as are required 
 to illustrate the application of science and the development of various 
 types of instruments, machinery, etc. 
 
 (15814) Wt. P2183/4455B/1541 3,000 3/25 Harrow G.31. 
 
 F 61700 
 
4 
 
 Notes.—A large number of objects in the Collections have been 
 photographed. Selected prints from the negatives may be seen in 
 guard books at the entrance stiles. Particulars of available prints 
 and lantern slides may be obtained by personal application at the 
 entrances or by letter addressed ‘‘ The Director, The Science Museum, 
 South Kensington, $.W.7.”’ 
 
 A compressed air service furnishes the power for driving such of the 
 machines as are shown in motion, and the service is available daily 
 from Ir a.m. (Sundays 2.30 p.m.) till closing time. Where practicable, 
 these objects are fitted with self-closing air valves, by means of 
 which Visitors may start them at will. Other objects are arranged 
 so that Visitors may work them by other means, and there are a few 
 that can be shown in motion only by an Attendant. 
 
GEODESY AND SURVEYING. 
 
 CONTENTS. 
 
 (The objects in the various Sections are arranged chronologically.) 
 
 Marking Points :— 
 Triangulation and Traverse Marks 
 Bench Marks 
 Beacons and Lamps 
 
 Direct Measurement of Distance :— 
 Route Traversing 
 Field Measurement 
 Base Measurement .. 
 
 Indirect Measurement of Distance :— 
 Rangefinding 
 Tacheometry 
 
 Measurement of Altitude :— 
 Plummet Levels and Clinometers 
 Spirit Levels and Clinometers 
 Accessories ae se 
 Barometers and Hypsometers 
 
 Measurement of Angles :— 
 Fixed Angle Instruments .. 
 Plane Tabling 
 Compasses 
 Early Theodolites 
 Altazimuth Theodolites AN oie 
 Miscellaneous Instruments and Accessories 
 Geodetic Theodolites 
 Field Astronomy 
 Mine Surveying .. 
 Marine Surveying 
 Photographic Surveying 
 
 List of Donors and Contributors 
 Index 
 
 List of Museum Inventory Numbers of Objects with corresponding 
 
 Catalogue Serial Numbers ee te 
 
 Page 
 
List Ol Luu S ERATION»: 
 
 Plate I. 
 Page 
 Fig. 1. Double-Mirror Heliotrope : 
 Fig. 2. Colby’s Compensation Bars 5 
 Plaie If, 
 
 Fig. 1. Early Balance Level .. 8 
 Fig. 2.° “Spirit Level; by Cary .. 3 
 Platesvi{, 
 
 Pig OT. Ac Ola Pee 6 
 Fig. 2. Early Theodolite 5 
 Plaie IV. 
 
 Fig. 1. Circumferentor, by Thos. Wright .. ‘4 6 
 Fig. 2. Early Altazimuth Theodolite, by J. Sisson 7 
 Plate V. 
 
 Fig. 1. 3-ft. Theodolite, by Ramsden : me 8 
 Fig. 2. 2-ft. Theodolite, by Troughton and Simms 3 
 Plaie VI. 
 
 Fig. 1. 12-in. Theodolite, by Watts .. 36 
 
 Fig. 2. Transit, by Brauer ; 
 
 Plate VII. 
 Fig. 1. Equal Altitude Instrument, by Jobin 90 
 Fig. 2. Lean’s Dial, by W. & S. Jones 
 
Ge ee UP of 
 
 the Collections in 
 the Science Museum, 
 South Kensington. 
 
 GEODESY AND SURVEYING. 
 
 Numerical references in the text refer not to the page but to the serial 
 
 numbers placed at the beginning of each catalogue title. When an object 
 
 is twlustrated the reference to the plates of illustrations 1s given immediately 
 
 after the title. The number at the termination of each description 1s that 
 
 under which the object ts registered in the Museum Inventory. If the objec 
 
 has been photographed, the Inventory number is followed by the negative 
 number. 
 
 GEODESY AND SURVEYING. 
 
 The term ‘ geodesy ”’ is usually applied to the task of determining 
 the size and shape of the earth as a whole, whilst the name “ sur- 
 veying ”’ is restricted to the processes used in fixing the position and 
 dimensions of small portions of the earth’s surface, and of the natural 
 or artificial features located thereon. For many centuries geodesy 
 has provided the foundation upon which more detailed surveys could 
 be erected, and the methods and appliances employed in the two 
 have differed only in precision. Consequently, in the collection repre- 
 senting geodesy and surveying, the arrangement and classification of 
 the exhibits are designed to illustrate their essential inter-dependence, 
 
 rather than to emphasize any differences which distinguish one from 
 the other. 
 
 Historically and naturally, geodesy has been intimately associated 
 with astronomy, and the instruments employed in the latter science 
 have been applied with little modification to the former, and even, 
 to a more limited extent, to surveying generally. The evolution of 
 surveying instruments and processes can therefore only be properly 
 appreciated by reference to the corresponding developments in 
 astronomy, and particular attention may be called to the series of 
 sundials, astrolabes, quadrants, sextants, reflecting circles, transits, etc., 
 exhibited in the Astronomy collection. Many exhibits of geodetic 
 interest will also be found in the collections representing Gravity and 
 Terrestrial Magnetism, whilst in the Aeronautical collection a series of 
 cameras of the types recently used for aerial surveying is exhibited. 
 
8 
 
 MARKING POINTS. 
 
 Natural or artificial objects of a fairly permanent and prominent 
 character, such as monuments or boundary stones, were the earliest 
 station marks. The lack of precision in the surveying methods and 
 instruments of the period would make any exact definition of the 
 actual station point upon such objects unnecessary. In the early 
 days of geodesy, church spires were favourite objects for observing 
 upon ; the fact that they could rarely be occupied so as to observe 
 from them did not seriously impair their value in this connection, 
 for it was always possible to occupy some adjacent observing station 
 and allow for the difference in position between the two. In com- 
 paratively uncivilised and sparsely populated countries, it was, and 
 still is, usually necessary to erect an artificial signal which could be 
 observed over a long distance. In the 17th century geodetic obser- 
 vations in Lapland and Peru, such signals were commonly made of 
 timber built up into a conical or pyramidal shape and covered with 
 brushwood or cloth ; space was left beneath the pyramid to permit of 
 the erection of the observing instrument, so as to avoid the employ- 
 ment of satellite stations. Where possible, triangulation stations were 
 permanently “marked ”’ by means of stones buried in the ground, 
 so that they could be re-occupied at any future date. Particular care 
 was usually taken in marking the terminal points of bases. For 
 example, the end points of the Hounslow Heath base in England were 
 marked by means of iron pipes driven into the hubs of old coach 
 wheels, buried in the ground ; these were replaced at a later date by 
 iron cannon, buried mouth upwards, the centre of the bore indicating 
 the exact terminal point. In the latter half of the 18th century, when 
 the errors of observation due to refraction were noted, it became 
 increasingly customary to observe at night time, and to use some form 
 of luminous signal for such observations. In 1818 Gauss noted the 
 powerful illumination obtained by reflecting the‘sun’s rays by means 
 of a mirror, and in 1820 devised a “ heliotrope ’’ (see No. 35) to utilize 
 this fact in daylight observations. By using such luminous signals, 
 sights could be taken over very long distances. The gradual improve- 
 ment in the construction of lamps, of the Argand, acetylene and 
 electric types, has caused these to be employed in the most precise 
 work within recent years. Whenever opaque signals are now employed 
 for daytime observations it is usual to allow space for the theodolite 
 to be set up below the signal. In cases where observations cannot be 
 taken at ground level, a stage is built up which serves both as a signal 
 and an observing platform ; in such cases, it is customary to erect a 
 separate stage for the observer to stand upon, so as to reduce the 
 risks of disturbance of the observing instrument owing to vibration 
 of the platform. 
 
 The actual points of a geodetic station are nowadays carefully 
 marked in a permanent manner, usually by embedding a metal bolt 
 in masonry or concrete, where no naturally fixed rock is available. 
 Lines are engraved upon the metal bolt to define the exact station 
 point. Such control points of a survey are generally buried beneath 
 the ground, and reference marks placed to facilitate recovery of the 
 control mark. 
 
 In surveys of minor precision, the signals are frequently ranging 
 
 poles, fitted with parti-coloured flags or discs to facilitate observation. 
 The marks at such stations are of a less elaborate and permanent 
 
9 
 
 character than in geodetic work; iron pipes, wooden stakes with 
 embedded nails, or occasionally, metal bolts set in concrete blocks at 
 the surface of the ground form satisfactory marks in cadastral or 
 engineering surveys. 
 
 TRIANGULATION AND TRAVERSE MARKS. 
 
 1. DIAGRAM OF TRIANGULATION MARKS OF THE 
 ORDNANCE SURVEY. Made in the Museum. 
 
 The primary (first order) triangulation stations are now marked by means of 
 a gun-metal bolt let into a block of concrete 2-ft. cube. The exact point is indicated 
 by a small hole drilled in the top of the bolt, which is protected by an iron cover. 
 The top of the block is at least two feet below the surface of the ground, except 
 when set in solid rock. A cairn is built over the point when circumstances allow. 
 
 The secondary triangulation stations are marked in the same way, except 
 that the dimensions of the block, bolt and cover are all less than in the case of 
 the primary stations. 
 
 The tertiary stations are marked in the same way as permanent traverse 
 stations. A brass bolt, having a cylindrical hole 7/16ths of an inch diameter 
 drilled in its top, is cemented into a 30-in. iron socket, which is rammed into the 
 ground until it is from 9 to 12 in. beneath the ground surface and then filled with 
 concrete. Inv. 1920-388. 
 
 2. DIAGRAM OF TRAVERSE MARK OF THE ORDNANCE 
 SURVEY. Made in the Museum. 
 
 To provide additional points for the control of the detailed survey, traverse 
 lines are run between third or fourth order triangulation points with a theodolite 
 by which, at each change of direction of the traverse line (traverse point), the 
 azimuth of the forward line is measured. 
 
 All such traverse points in towns are now marked by means of a brass bolt, 
 into the top of which can be fitted the 15-in. observing rod or the foot of the 
 traverse pole. For temporary marks the bolts are cemented into the curb of 
 the pavement. For permanent marks a 30-in. iron socket, similar to the base of 
 an iron telegraph pole (Figs. 1 and 2) is rammed into the ground till the top is 
 9 to 12 in. below the surface. The socket is filled with cement or concrete, and 
 the bolt cemented in at the top. 
 
 The number of points so marked varies according to the nature of the survey. 
 In some continental cadastral surveys the average number is 20 to 50 per square 
 kilometre in the country, and roo to 150 in towns. In the highly cultivated 
 land of Egypt where holdings are small but the land is level and unobstructed 
 by hedges, about 16 to 20 per square kilometre are used. Inv. 1920-386. 
 
 8. TRAVERSE MARK BOLT AND COVER, ORDNANCE 
 SURVEY. Presented by the Ordnance Survey. 
 
 A cylindrical hole 7/16ths in. diam. is drilled in the head of the bolt into which 
 the 15-in. observing rod or the foot of the traverse pole can be fitted. The bolt 
 is protected by an iron cap or hydrant cover according to the nature of the 
 surface. 
 
 An observing pin and small circular spirit level for setting it vertical are 
 also shown. Inv. 1914-154. 
 
 4. BRONZE TRIANGULATION MARK. Presented by the 
 French Government. 
 
 This type of mark set either in rock or masonry is used for marking the first 
 order triangulation stations in France. It consists of a small cylindrical bronze 
 bolt with grooved sides and a cross engraved on its upper face, the intersection 
 of the cross indicating the station point. Inv. 1914-107. 
 
 5. NAME-PLATE FOR TRIANGULATION MARK. Presented 
 by the French Government. 
 
 This elliptical metal plate is fixed in masonry at a first order triangulation 
 station in France, and carries a number corresponding to the particular station 
 point. Inv. 1914-108. 
 
Io 
 
 6. TRIANGULATION STATION MARK. Presented by the 
 U.S. Coast and Geodetic Survey. 
 
 This mark consists of a brass disc and shank cast in one piece. The shank 
 is ribbed so as to grip securely the material—generally rock or concrete—in which 
 the mark is set. One such mark is set at the surface and a similar one about 
 3 ft. directly underneath it at every triangulation station. 
 
 The station point is defined by a small dot within a 20 mm. equilateral triangle 
 engraved on the disc. 
 
 (See U.S. Coast and Geodetic Survey Special Publication No. 26, p. 42.) 
 
 Inv. 1920-81. 
 
 7. MAGNETIC STATION MARK. Presented by the U.S. Coast 
 and Geodetic Survey. 
 
 This mark is the older form of station mark used by the U.S. Coast and 
 Geodetic Survey. Its shank is split so that a wedge can be inserted. When the 
 mark is driven into the drill-hole, the wedge causes the shank to bulge and grip 
 firmly the surrounding material. 
 
 The station point is defined by a dot enclosed in two intersecting 20-mm. 
 equilateral triangles engraved on the disc. 
 
 (See U.S. Coast and Geodetic Survey Special Publication No. 26, p. 42.) 
 
 Inv. 1920-82. 
 
 8. HYDROGRAPHIC STATION MARK. Presented by the 
 U.S. Coast and Geodetic Survey. 
 
 This mark is the same as that used for triangulation station marks, except 
 that the central hole or dot is here surrounded by a 2-5-cm. circle. The mark 
 is set in a post or block of concrete, or wedged into a drill-hole in rock, after which 
 a triangle or circle is chiselled round it. 
 
 For underground station marks a similar disc is set 3 ft. below ground ina 
 block of concrete, in rock or other suitable material. 
 
 (See U.S. Coast and Geodetic Survey Special Publication No. 26, pp. 42-43.) 
 
 Inv. 1920-78. 
 
 9. REFERENCE MARK. Presented by the U.S. Coast and 
 Geodetic Survey. 
 
 This mark also consists of a metal disc and shank and is used to facilitate 
 the recovery of station marks, especially where these are liable to damage or 
 disturbance. The mark is placed so that the arrow engraved thereon points to 
 the station mark, and the horizontal distance between the station mark and the 
 groove cut across the shaft of the arrow is recorded. 
 
 Reference marks are set in concrete posts or blocks, or in rock, at distances 
 up to 20 yd. from the station marks. 
 
 (See U.S. Coast and Geodetic Survey Special Publication No. 26, pp. 42-43.) 
 
 Inv. 1920-77. 
 
 10. TRIANGULATION STATION MARK. Presented by the 
 Geodetic Survey of Canada. 
 
 Stations of the Primary Triangulation of Canada are marked by means of 
 circular metal bolts of the type exhibited, the bolts being embedded in concrete 
 or leaded into rock. Both underground and surface marks are employed, and 
 reference monuments provided to facilitate recovery of the staticn. 
 
 The bolt consists of a 3-in. circular bronze disc, attached to a 3-in. cylindrical 
 shank, which is corrugated and fox-wedged so that the shank expands tightly 
 against the sides of the hole drilled to receive it when the bolt is driven in. The 
 centre of the disc has a triangle engraved upon it, with a central hole punched 
 to define the exact station point. Inv. 1922-726. 
 
 11. REFERENCE MARK. Presented by the Geodetic Survey 
 of Canada. 
 
 Where the station marks of the Primary Triangulation of Canada are in 
 danger of disturbance, or are difficult to find, reference marks of the type shown 
 are set in outstanding rocks or in concrete monuments. Where possible, three 
 such marks are set approximately 120° apart around the station. 
 
 The mark is a similar bronze bolt to the station: mark, but has an arrow 
 engraved upon it to indicate the direction in which the station point lies. 
 
 Inv. 1922-724. 
 
II 
 
 12. REFERENCE MONUMENT PLATE. Presented by the 
 Geodetic Survey of Canada. 
 
 Where no natural rock is available, reference monuments are built by the 
 Geodetic Survey of Canada to mark the station points. Such monuments are 
 erected in convenient places, preferably near a road, and a plate of the type 
 shown is embedded into them. 
 
 The plate is rectangular and has two projecting shanks for fixing it into the 
 monument. Inv. 1922-725. 
 
 13. GEODETIC TRIANGULATION STATION MARK. Pre- 
 sented by the Survey of Egypt. 
 
 This brass bolt forms the station mark at a geodetic station of the Survey of 
 Egypt. It is provided with two saw-cuts intersecting at right angles, and is 
 cemented into a concrete block in the ground. Accurately centred over this and 
 built into the concrete block is an iron cylinder filled with concrete, having gaps 
 in the bottom so that the mark can be seen. A brass and cast iron tribrach 
 (see Cat. No. 14) is accurately centred over the mark and cemented in the top 
 of the pillar. Upon this tribrach the theodolite rests when sighting from the 
 station, whilst to its centre is screwed a heliograph or lamp when sighting to the 
 station. Inv. 1920-630. 
 
 14. SELF-CENTRING GEODETIC STATION TRIBRACH. 
 Presented by the Survey of Egypt. 
 
 This brass tribrach is built into a masonry pier and accurately centred over 
 the concealed mark at geodetic stations in Egypt. It has three radial V-shaped 
 grooves at angles of 120° with one another, and forms a self-centring stand for 
 the three feet of a theodolite or signal lamp. Such a device is especially 
 convenient in countries where the centring of signal lamps over the station 
 point cannot always be entrusted to skilled operators. Inv. 1920-627. 
 
 15. DIAGRAM OF TRAVERSE LINES AND MARKS. Survey 
 of Egypt, Qaliub District. (Scale I : 20,000.) Made in the Museum. 
 
 In the Cadastral Survey of Egypt, the method of traversing with theodolite 
 and chain is used to control the detail chain survey, and this traverse system 
 is connected up to the points of the minor triangulation system, the actual 
 traverse lines being chosen to conform so far as possible to the village boundaries. 
 The azimuths of these lines are measured with a 5-in. vernier theodolite, and 
 their lengths with 20-m. steel chains. There are about 15 traverse stations per 
 square kilometre. 
 
 In the diagram, which shows a portion of the Qaliub district on a scale of 
 I: 20,000, the second and third order triangulation system is shown in red, 
 whilst the traverse lines and points are in black. 
 
 (See “‘ The Cadastral Survey of Egypt ’’—Capt. H. G. Lyons, F.R.S. 
 
 Inv. 1920-384. 
 
 16. “ DRAY ” TRAVERSE MARK. Presented by H. Dray, Esq. 
 
 The liability of ordinary metal bolts and bars used as traverse marks in 
 Egypt to removal, led H. Dray to devise this land-anchor type of mark, which 
 is now successfully employed in the soft soil of the Nile Valley and elsewhere. 
 
 The mark consists of a conical iron cap securely attached to the head of a 
 screw pile which is buried some 6 ft. below the ground and is connected to the 
 cap by a $-in. steel cable. The cap has a central socket to receive the ranging 
 rod and rests on a circular concrete block interposed between it and the ground. 
 
 In order to drive the pile into the ground, its flexible cable is enclosed in an 
 iron pipe of suitable length. A nut threaded on the pile-head keeps this pipe 
 pressed against the pile at its lower end, where the pipe and pile are notched so 
 as to interlock. The two then form a rigid pile which is driven into the ground 
 by rotating the pipe with the pipe-grips shown. When only the head of the pile 
 remains above ground, the nut is unscrewed and the pipe withdrawn, the space 
 round the cable being filled with rammed earth. The concrete block, about 
 1 ft. diameter by 4in. thickness, has a central hole and is next fitted over the 
 projecting pile-head, forming a firm bed for the cap, which is afterwards screwed 
 on to the head by means of the keys shown. The adjacent diagram illustrates 
 the process of fixing the mark in position. Inv. 1921-360. 
 
IZ 
 
 BENCH MARKS. 
 
 17. DIAGRAM OF LEVELLING MARKS OF ANCIENT EGYPT. 
 (Circa 2800 B.C.) Made in the Museum. 
 
 When excavating near the pyramid of Seneferu ({1I Dynasty) at Meidum 
 in 1891, Sir W. Flinders Petrie uncovered four corner walls of crude brick built 
 round the angles of a rectangular tomb structure known as a “‘ Mastaba.”” On the 
 inner faces, which were plastered and whitewashed, level lines were drawn in 
 red at intervals of one cubit, and numbered to show the distance of each below 
 the “‘ ground level’’ (nfrw). The sloping black lines show the inclination of the 
 outer face of the masonry ‘‘ Mastaba.”’ 
 
 The hieroglyphic inscriptions in red record that the line to which they refer 
 is 3, 4 or 5 cubits below ground level. 
 
 (See ‘‘ Medum,”’ by W. M. Flinders Petrie, London, 1892, p. 12 and Plate VIII.) 
 
 Inv. 1920-385. 
 
 18. DIAGRAM OF FUNDAMENTAL BENCH MARK. Made in 
 the Museum. 
 
 A number of fundamental bench marks have been fixed in the principal 
 (first order) levelling lines in the British Isles for the purpose of verification from 
 time to time. These bench marks are fixed in rock in order to ensure permanence 
 and immunity from accidental disturbance. One visible mark and two concealed 
 marks, the altitudes of which are accurately known, are provided. 
 
 A recess having been excavated in sound and unweathered rock, the two 
 reference points A and B are set in a bed of concrete. Mark A is a dome-headed 
 bolt of bronze ; mark B is a polished flint selected as being unlikely to deteriorate 
 in any way. A third mark, C, is a bronze dome-headed bolt bearing the words 
 “height above datum,” and the altitude in feet. This is set in a granite pillar 
 which rises 12 in. above the ground, and on which the name plate D is fixed. 
 
 Examples of the bronze bolts A’and C and the name plate D, and also of the 
 coverplate protecting the two lower marks A, B, are shown near by. The upper 
 mark C is used for all ordinary purposes of levelling, whilst the concealed marks 
 serve to determine whether secular changes of level are in progress. 
 
 The distance between fundamental bench marks varies from 25 to 35 miles. 
 
 (See “‘ The Second Geodetic Levelling of England and Wales, 1912-1921,” 
 p. 14, Plate X.—Ordnance Survey, 1921.) Inv. 1920-387. 
 
 19. BENCH MARK. First Order. Presented by the Ordnance 
 Survey. | 
 This bolt forms the upper reference point at a fundamental bench mark on 
 the first order levelling lines of the Ordnance Survey. It is fixed in the top ofa 
 granite block forming a pillar 12 in. high, and is the mark for use in all ordinary 
 circumstances, its level being recorded on the topographical maps. 
 Inv. 1914-152. 
 
 20. BENCH MARK. First Order. Presented by the Ordnance 
 Survey. 
 
 This bolt forms one of the two concealed reference points at a fundamental 
 bench mark of the Ordnance Survey. It is set in rock concrete 3 ft. below ground 
 in a concrete recess 3 ft. square by 1 ft. 9 in. deep, and will be used when necessary 
 to determine whether any secular changes of level are in progress. 
 
 Inv. 1914-151 
 
 21. BENCH MARK NAME PLATE. Presented by the 
 Ordnance Survey. 
 
 This plate is affixed to the granite pillar of a fundamental bench mark ol 
 the Ordnance Survey. Inv. 1914-153. 
 
 4 22. COVER OF REFERENCE BENCH MARKS. Presented by 
 the Ordnance Survey. 
 
 The cover is placed below ground in a fundamental bench mark chamber to 
 protect the reference marks of the Ordnance Survey from interference or damage. 
 Inv. 1914-155. 
 
13 
 
 23. SECONDARY BENCH MARK. Presented by the Ordnance 
 Survey. 
 
 At intervals of about a mile between the fundamental bench marks of the 
 Ordnance Survey, secondary marks are fixed on lines of first order levelling. 
 These consist of a gun-metal plate set flush with the surface of a wall, and having 
 , an identification number cast on it. The reference point, to which the published 
 altitude corresponds, is the top of the “ broad arrow.’’ In levelling, the foot of 
 the levelling staff is brought to the same altitude as this point with the aid of a 
 detachable bracket and a small spirit-level. 
 
 (See ‘‘ The Second Geodetic Levelling of England and Wales, 1912-1921,” 
 
 p. 14, Plate XI.—Ordnance Survey, 1921.’’) Inv. 1914-149. 
 24. BENCH MARK BRACKET. Presented by the Ordnance 
 Survey. 
 
 This adjustable bracket is used for supporting the levelling staff at a secondary 
 bench mark on the lines of the first order levelling of the Ordnance Survey. 
 
 The bracket is hooked on to the bench mark, and the upper surface, upon 
 which the levelling staff is placed, is made level and in agreement with the 
 reference point by means of the adjusting screw and the spirit level. 
 
 Inv. 1914-150. 
 
 25. BENCH MARK BRACKET. Lent by the Ordnance Survey. 
 
 This bracket is an old type of angle iron used to support the levelling staff 
 at an Ordnance Survey broad arrow bench mark. It has a short arm which is 
 fitted into the horizontal groove engraved above the broad arrow, and a longer 
 inclined supporting arm which abuts against the wall in which the mark was 
 notched. A series of circular depressions are sunk in the upper face to receive 
 the stud on the base of the staff, the lower level of the depressions being a horizontal 
 plane passing through the centre line of the engraved groove. This type of 
 supporting bracket was superseded by the modern one represented in the next 
 example. Inv. 1914-423. 
 
 26. BENCH MARK BRACKET. Lent by the Ordnance Survey. 
 
 This modern type of bracket is used to support the levelling staff at a broad 
 arrow bench mark of the Ordnance Survey. It is broader than the adjacent 
 earlier type, and has an adjusting screw threaded through its inclined arm and 
 a box level mounted on its horizontal arm ; the end of the screw butts against 
 the vertical wall in which the mark is engraved, and the inclination of the upper 
 arm can be adjusted level by turning the screw. The flat base of the metal shoe 
 forming the end of the staff rests on a stud projecting from the top of the bracket, 
 the top of the stud being level with the centre line of the mark when the bracket 
 is horizontal. Inv. 1914-424. 
 
 27. BENCH MARK. Presented by the U.S. Coast and Geodetic 
 Survey. 
 
 This brass disc is fixed with its face flush with the building in which it is set 
 or with its stem vertical in rock or cement. The short line in the centre is placed 
 horizontal and serves as the reference point. 
 
 (See U.S. Coast and Geodetic Survey Special Publication, No. 26, p. 117.) 
 
 Inv. 1920-76. 
 
 28. BENCH MARK. First Order. Presented by the French 
 Government. 
 
 This mark is used at first-order stations of the Nivellement Général de la 
 France. It consists of a metal bolt having a grooved shank and a rectangular 
 plate head with a recess to receive a plate, on which the height of the station 
 point is printed. The reference point is the top of a spherical boss on a horizontal 
 platform which projects from the plate. Inv. 1914-109. 
 
 29. BENCH MARK. Second Order. Presented by the French 
 Government. 
 
 This mark is fixed at second-order stations of the Nivellement Général de la 
 France. It is a metal bolt with a grooved shank and cylindrical head ; on the top 
 of the curved surface of the head is a small spherical boss, the top of which is 
 the reference point. The height of this point is printed on a plate embedded in 
 the cylindrical head. Inv. 1914-110. 
 
14 
 
 30. BENCH MARK. First Order. Presented by the Survey of 
 Egypt. 
 
 This mark consists of an iron bolt 21 cm. long with a hexagonal head 5 cm. 
 thick and 11 cm. wide. It is cemented into a vertical wall or solid rock. The 
 point of reference is the top of a bronze hemispherical stud 1 cm. high, on the 
 top of the upper face of the head. Inv. 1920-628. 
 
 31. BENCH MARK. Second Order. Presented by the Survey 
 of Egypt. 
 
 This mark is similar to the adjacent first order bench mark, except that it 
 has a cylindrical head, of which the highest point is the reference point. 
 Inv. 1920-629. 
 
 32. BENCH MARK. Presented by the Geodetic Survey of Canada. 
 
 Bronze tablets of this type are used by the Geodetic Survey of Canada to 
 mark the station points of the Precise Levelling Network. 
 
 The tablet is a circular disc of diameter 3 in., provided with a corrugated, 
 fox-wedged shank 3 in. long, for fixing it into rock or concrete. The centre of 
 the disc is engraved with a cross, and a number can be stamped on it with a die 
 for reference to the record book, which contains the particulars of the corresponding 
 station. Inv. 1922-723. 
 
 33. DIAGRAM OF BENCH MARKS. Survey of India. Made 
 in the Museum. 
 
 The type of mark now employed in India varies according to the character 
 of the neighbourhood. In towns or cities the standard bench mark is used, as 
 being less liable to interference than the metal bolts employed in Europe. In 
 places where there is an outcrop of hard rock, the mark is made on this and suitably 
 protected, as in the centre diagram. The embedded type of mark is more suitable 
 in country districts, where no ground-rock or permanent stone building is available. 
 
 The “‘ standard ”’ type of bench mark was first employed in 1904. It consists 
 of a stone monolith of 2 ft. square section, 3 ft. high, set upon a foundation of 
 concrete 64 ft. square by 2 ft. deep. The top of the monolith is cut in the form 
 of a frustrum of a pyramid, and terminates in a square of 3 in. side, the central 
 point of which constitutes the mark. The date of construction of the monolith 
 is engraved on it, together with the inscription ‘‘ G.T.S. Standard Bench Mark,” 
 whiist the height of its reference point above sea-level at Karachi is engraved 
 upon a stone slab embedded in a brickwork setting round the base of the monolith. 
 
 The bench-mark inscribed upon ground-rock is protected by a masonry 
 column, closed at the top by means of a stone slab on which is engraved the 
 inscription ‘‘ G.T. Survey, Upper Mark.’’ The “ embedded ”’ bench mark consists 
 of a 4-in. iron pipe embedded in well-rammed earth about 2 ft. below the ground 
 level. A brickwork arch on concrete abutments is built over this mark in order 
 to distribute the weight of traffic passing near. Above this arch is more rammed 
 earth surrounding a central brickwork well, closed by a stone cover with the 
 usual inscription. This type of mark has only recently been introduced, the 
 previous type of “embedded ”’ mark being a cubical stone block buried with its 
 upper face slightly below the ground level. 
 
 (See ‘‘ Great Trigonometrical Survey of India,” Vol. XIX, Chap. IV.) 
 
 Inv. 1920-389. 
 
 BEACONS AND LAMPS. 
 
 34. PHOTOGRAPHS OF TRIANGULATION BEACONS. Pre- 
 sented by Major G. G. Waterhouse, R.E. 
 
 These two photographs illustrate the construction of quadripod triangulation 
 beacons at Ado Rock and Awba Hill stations during the survey of Southern 
 Nigeria, 1910-12. 
 
 The beacons are formed of tree-trunks and branches built up into a four-sided 
 pyramid, the upper part of which is roofed with brushwood to form a prominent 
 land-mark, and at the same time to protect the surveying instruments when 
 taking bearings from the station, A pole carrying at its upper end a white flag 
 is fixed to the top of the beacon to render it more conspicuous. The right hand 
 photograph shows the framework of the beacon built up, but the roof not yet 
 added. The left hand photograph shows a completed beacon, with a heliograph, 
 theodolite and plane table standing beneath it. The ground in the vicinity has 
 been carefully cleared of obstacles to view. Inv. 1921-306, 
 
ia 
 is 
 eP? be 
 
 to 
 
PLATERT, 
 
 Fig. 1. Double Mirror Heliotrope, p. 15. 
 
 Fig. 2. Colby’s Compensation Bars, p. 24. 
 
T5 
 
 35. SEXTANT-HELIOTROPE OR VIZEHELIOTROP. Made 
 by Rumpf, Gédttingen, 1827. Lent by the Admiralty. 
 
 In October, 1818, C. F. Gauss was observing at Luneburg, a station of the 
 Altona-Hanover meridian arc, and noted in his observation book that the 
 triangulation mark in Hamburg was difficult to see, but that the windows, lit 
 up by the rays of the setting sun, facilitated the directing of his theodolite. This 
 suggested to him the construction of a heliotrope, of which the sextant-heliotrope, 
 made in 1920, was the first type. 
 
 The telescope is mounted on a circle capable of rotation about a horizontal 
 axis, and can itself revolve about the centre of the circle. This circle has no 
 divisions and is only for giving a slow motion to the telescope by means of a 
 screw. Through a sleeve on the telescope passes a brass rod carrying a mirror 
 perpendicular to it, the distances of the mirror and the sleeve from the axis of 
 rotation of the telescope being equal. 
 
 In order to send a ray of sunlight in a specified direction, the instrument is 
 set up so that the mark is in the centre of the field when the axis of the telescope 
 is normal to the plane of the mirror. If, then, the telescope is directed to the 
 sun and follows it continuously, the mirror will reflect its light on to the mark. 
 
 On account of its somewhat complicated form, this type of heliotrope was 
 very shortly superseded by the adjacent double-mirror type. 
 
 (See Gauss, Werke, Bd. IX, pp. 461-484, and Poggendorff’s Annalen der 
 Physik, 1829, Bd. XVII, p. 83.) 
 
 Inv. 1914-305, S.M. 1935. 
 
 36. DOUBLE-MIRROR HELIOTROPE, OR SPIEGELKREUZ- 
 HELIOTROP. Made by Rumpf, Géttingen, 1822. Lent by the 
 Admiralty. Plate I, Fig. 1, p. 15. 
 
 C. F. Gauss, in writing to Schumacher in March, 1820, described this heliotrope 
 as being more convenient, simpler, and having a larger reflecting surface than 
 his sextant-heliotrope which it superseded. In its turn it was replaced by 
 Bertram’s heliotrope, which Bessel used on his East Prussian arc. 
 
 For use the telescope was directed on the triangulation mark, the small central 
 mirror being turned parallel to the axis of the telescope. Then the double-mirror 
 was turned until its axis was at right angles to a plane containing the sun, the 
 instrument and the distant mark. This is facilitated by the disc beside the mirror, 
 since, when its shadow is a line only, the mirror is in position. Lastly, the double 
 mirror is rotated until the sun’s image as reflected by the central mirror is seen 
 in the telescope, when the outer mirror reflects the sun’s light on to the distant 
 mark. 
 
 (See Gauss, Werke, Bd. IX, pp. 461-483, and W. Jordan, Handbuch der 
 Vermessungskunde, Bd. III, p. 33.) Inv. 1914-376, S.M. 1936. 
 
 37. HELIOTROPE. Lent by the Ordnance Survey. 
 
 This instrument, sometimes also called a heliostat, was used for reflecting the 
 sun’s rays to an observing station. Three sizes were in use by the Ordnance Survey 
 in the principal triangulation ; the largest were rectangular and measured 20 in. 
 by 16 in., the other two were circular and were 12 in. and 5 in. diam. respectively. 
 
 In each case the mirror, having been centred over the triangulation mark, 
 was moved by hand so as to follow the sun and to reflect its light through a ring 
 placed in the line between the heliostat, or the reflecting station, and the observing 
 station. The ring was placed in position by means of a small theodolite. 
 
 Inv. 1914-425. 
 
 38. HELIOGRAPH. Presented by the Admiralty. 
 
 This instrument, primarily devised for signalling, can readily be used as a 
 heliotrope at small distances ; the sighting arrangement is not suitable for long 
 distances. 
 
 The instrument comprises a circular reflecting mirror of 4 in. diam., pivoted 
 on horizontal bearings in a vertical frame, which is mounted rotatably in a 
 vertical sheath projecting from a tripod head and can be clamped in the sheath 
 by opposing screws. The mirror can thus be set in any desired plane and the 
 reflected beam from the sun caused to fall upon a small sliding plate, which 
 forms part of the foresight. This foresight is mounted on a sliding vertical rod 
 at the end of a supporting arm, which is about 2 ft. long, and is pivoted 
 horizontally to a vertical collar surrounding the sheath supporting the mirror. 
 The arm can thus be pointed in the direction of the distant station and the 
 
16 
 
 V-foresight accurately aligned by viewing through a central aperture left 
 unsilvered in the back of the reflecting mirror. When the supporting arm has 
 thus been directed, the sliding plate is moved over to the V to receive the reflected 
 beam, and has an engraved dot accurately corresponding to the tip of the 
 V-foresight. When the mirror is turned until the image of the sun falls on this 
 plate, with the shadow caused by the unsilvered central hole of the mirror falling 
 on the dot, the reflected beam is then directed towards the distant station. 
 
 A smaller mirror, of diam. 2$in., can be substituted for the foresight rod if 
 necessary owing to the sun’s position. It also has a central unsilvered aperture 
 for sighting. 
 
 The large mirror can be used apart from the stand, its socket having a bayonet 
 joint to connect it to a small knife which is stuck into the ground. 
 
 Inv. I91I-155. 
 
 39. LESUERRE HELIOGRAPH. Made by Molteni, Paris. 
 Lent by the School of Military Engineering, Chatham. 
 
 This instrument was primarily devised for long-distance signalling, but can 
 be used for marking a distant station in triangulation. It is of the double- 
 mirror type, and is provided with a sighting telescope and a smaller collimating 
 telescope for adjusting the reflected beam of sunlight in the direction of the 
 observing station, 
 
 The instrument consists of two elliptical plane mirrors, mounted upon a long 
 arm which can rotate about a horizontal axis in the standard, the latter being 
 furnished with the usual three levelling screws. The arm is provided with a 
 spirit level clinometer and declination compass. Each mirror is so mounted on 
 the arm that it can be set normal to any required direction, and has slow-motion 
 tangent screws for accurate adjustment. There are two telescopes arranged so 
 that their axes are always parallel to each other ata fixed distance apart but 
 point opposite ways. ‘The smaller telescope is directed towards the reflecting 
 mirror, and, by means of it, the beam of sunlight reflected irom this mirror 
 projects upon a small screen, mounted at a suitable distance from its eye-piece 
 end, an image of the sun and the cross-wires of the telescope. When the centre 
 of the sun’s image coincides with the intersection of the image of the cross wires, 
 the axis of the telescope is parallel to the direction of the reflected beam. Hence 
 the beam is directed parallel to the axis of the larger telescope, which is focussed 
 on to the observing station, and the observer will see the flash of the heliograph. 
 The necessary adjustments to effect this are made with the mirrors. 
 
 (See ‘‘ Journal of Telegraph Engineers,” 1878, pp. 351-355.) Inv. 1914-829. 
 
 40. ACETYLENE SIGNAL LAMP. Made in 1907 by E. R. Watts 
 & Son. Lent by the Colonial Office. 
 
 This lamp is an acetylene projector designed by Mr. G. T. McCaw for use in 
 trigonometrical surveys. It was first used in the measurement of an arc of 
 meridian in Uganda, and later in the Royal Siamese Survey Department. 
 
 Its hight can normally be seen at a distance of 30 miles. It is provided with 
 a telescope for directing the beam, and with means for attachment to a rough 
 table so as to be in the direct line between the observer and the mark at its own 
 station. 
 
 The lamp is of ordinary motor design, with a separate carbide container, the 
 gas being conveyed to the burner by flexible tubing. In the barrel is a single lens 
 of about 5-in. aperture, movable slightly along the barrel for correct focussing. 
 
 The projector has independent traversing, azimuthal and vertical motions. 
 When in use its lower plate is firmly screwed to a rough table in an approximate 
 line between the station mark and the observer. Its orientation having been 
 approximately corrected by the azimuthal and vertical motions, the projector 
 is then accurately aligned by the traversing motion, operated by the handle on 
 the left-hand side of the lower plate, and is clamped in this alignment. The axis 
 of the barrel is then moved up or down, according to the indication of the telescope, 
 by the vertical motion screw under the back of the barrel. This screw passes 
 through the upper and middle plates, which are kept down to the lower plate 
 by a spring and stop. Finally, the upper plate to which the barrel is rigidly 
 attached, is swung in azimuth over the middle plate. When the telescope, 
 attached to the upper plate, is directed on the observer’s station, the upper plate 
 is clamped by the wing nut under the front of the barrel. The axis of the projector 
 is then correctly aligned on the distant station and the position of the burner is 
 in the direct line between the local and distant triangulation points. . 
 
 In the original form, a sighting tube affixed to the top of the barrel replaced 
 the telescope. 
 
17 
 
 For work by day a heliograph was attached to the front of the instrument, 
 and projected its beam of sunlight through a hole cut in a board nailed to a post 
 fixed from ro to 20 ft. in front of the instrument. This was found less confusing 
 to the native assistants in Rhodesia and Uganda than the mirrors of a self- 
 contained heliograph such as is ordinarily employed in Army signalling. 
 
 Inv. 1914-238. 
 
 41. ELECTRIC SIGNAL LAMP. Presented by the Geodetic 
 Survey of Canada. 
 
 This electric lamp is used at stations of the Primary Triangulation of Canada 
 to mark the station point during night observations. It is visible at distances 
 up to 130 miles. 
 
 The lamp is of the type used for motor-car headlights, and has a parabolic 
 reflector behind the bulb. It is mounted within a wooden frame which can be 
 supported upon the box within which it is packed for travelling, and can be 
 adjusted upon this box by means of an elevating screw. The top of the frame 
 has metal V-s attached to support the telescope used in aligning the lamp upon 
 the observing station. 
 
 Two bulbs of different sizes are supplied with the lamp for use at smaller and 
 larger distances respectively. They are illuminated by current supplied by a 
 battery of dry cells. An electrically controlled time-switch can be connected in 
 the circuit to light automatically the lamps at regular intervals. Inv. 1923-10. 
 
 DIRECT MEASUREMENT OF DISTANCE. 
 
 The direct measurement of length for the purposes of land and 
 engineering surveys has undergone little modification since the earliest 
 times of which we have records. In order to measure the distance 
 between two points on the earth’s surface, the ancient Egyptians 
 marked out the straight line joining these points by means of a stretched 
 cord (see No. 52), or by a set of intermediate pegs aligned by sighting. 
 Graduated wooden rods or flexible cords were used as measuring units. 
 Similar methods and appliances are still used in minor surveys and 
 give results sufficiently accurate for ordinary purposes. The approxi- 
 mate estimation of distances by pacing, timing, or counting the 
 revolutions of a wheel of known circumference was practised at a very 
 early date, and such methods are still employed in reconnaissance and 
 exploratory surveys. On the other hand, the precise measurement 
 of length which is now a feature of trigonometrical or geodetic surveying 
 had no counterpart in ancient or medieval times, but arose with the 
 development of the general science of measurement in the 17th century. 
 
 Geodetic surveying entered upon its modern epoch with the arc 
 measurements of Snell, about 1615, and Picard, about 1669. Snell, 
 by introducing the method of triangulation from a fixed base, reduced 
 length measurement to the minimum necessary to determine the base. 
 As, however, any error in this base measurement inevitably affected 
 the whole triangulation, the need for precision in its determination 
 was evident ; and the increasing accuracy of angle-measurement due 
 to the application of the telescope to surveying instruments rendered 
 a corresponding increase of precision in base-measurement necessary 
 if the whole work were not to be vitiated. Consequently every effort 
 was made to investigate the sources of error in base-measurement, 
 and to minimize their effects. Amongst these sources, two were 
 particularly prominent, namely, the variation of the effective length 
 of the measuring units due to changes of temperature and the dis- 
 turbance of one unit from its proper position when the next unit was 
 placed in contact with it. As the measuring units employed were 
 
 (15814) B 
 
18 
 
 made practically rigid, so as to avoid bending under gravity or the 
 influence of wind, or stretching more or less premanently under tension, 
 it was very difficult to measure the exact temperature of the units at 
 the time of the observation, or to avoid some disturbance of one unit 
 when its immediate successor was laid in position next to it. To 
 reduce these “temperature ”’ and “contact ”’ errors, various devices 
 were employed, and the measuring rods grew more and more compli- 
 cated in consequence. Picard and his immediate successors had used 
 wooden rods, tipped with brass. These rods were made about four 
 fathoms in length, and were constantly compared during the measure- 
 ment with copies of the standard “ toise’’ or French fathom. In 1750, 
 Boscovitch, when measuring the Rimini base in Italy, adopted the 
 method of placing successive rods nearly but not quite in contact 
 with one another, and measuring the intervals between them with 
 beam compasses. This considerably reduced “contact errors,’ but 
 the “‘ temperature errors ’’ remained relatively large, in spite of the 
 small temperature coefficient of wood, until Borda, in 1792, utilized 
 the unequal expansion of two metallic rods, similar in dimensions but 
 made of different metals, to measure more exactly the mean tempera- 
 ture of either rod during the determination of the base. 
 
 This principle was utilized in a different manner in Colby’s Com- 
 pensation Bars (see No. 64), the object in this latter case being to 
 provide a length which should be absolutely invariable whatever the 
 temperature of the observation. The “ metallic-thermometers ” of 
 Gauss and Bessel, used on the Hanoverian and Prussian arcs in the 
 years 1820 and 1830-35, were similar to those used by Borda, but 
 were placed so as to be separated by small intervals, which were 
 measured by means of graduated glass wedges. Colby used double 
 compensating microscopes, through which the terminal points of 
 adjacent bars were viewed, to render the interval between each pair 
 of bars exactly six inches. The Briinner apparatus, used until recently 
 in France, Spain and other countries, is also similar to the Borda rod 
 in employing the relative expansion of two metals to measure tem- 
 perature changes, but in this apparatus, two independent measures 
 of the base are made, one with each rod. Such an apparatus is termed 
 a ‘‘ bi-metallic ’’ combination, and the Eimbeck Duplex base apparatus 
 (see No. 67) is of this type, and was used on many geodetic bases in 
 the United States previous to 1900. Contact disturbances are avoided 
 in the Briinner apparatus by measuring the distances between micro- 
 scopes, which are fixed at approximately equal distances along the 
 base line. The measuring rod is provided with scales at its ends, 
 and these are viewed in turn through each adjacent pair of microscopes. 
 The Eimbeck apparatus is provided with a spring contact-slide to 
 minimize disturbance, so that the end-pressure between adjacent bars 
 is never big enough to disturb either. Experiments have been made 
 with glass rods and ice-packed bars with a view to reducing temperature 
 errors, but the invention of “ invar”’ near the end of the last century 
 has provided a metallic alloy whose temperature coefficient is so small 
 as to render the exact determination of the temperature during a base 
 measurement unnecessary. 
 
 The temperature errors are also considerably reduced by substi- 
 tuting a very thin tape or wire for the rigid rod of the older type of 
 measuring unit, and thereby ensuring a more even distribution of 
 temperature in the unit itself. The use of flexible as opposed to rigid 
 measuring units originated with Jaderin about the year 1885, although 
 
1g 
 
 a flexible steel chain, made by Ramsden (see No. 61) had been used 
 on the Hounslow Heath base in 1784 by General Roy, but in this 
 case the chain was supported uniformly along its length. Jaderin, 
 on the other hand, suspended his wires, of which two were used to 
 measure each base independently, freely between fixed tripods, and 
 measured the distance between each pair of tripods by means of scales 
 engraved on the tapes (see No. 65). The two wires were of different 
 metals, and thus formed a bi-metallic combination for determining 
 the mean temperature during measurement, and they were stretched 
 by a constant tension so that the sag could be calculated and allowed 
 for. Invar tapes (see No. 69) or wires, either suspended freely between 
 two tripods as in Jaderin’s method, or supported at a few intermediate 
 points on tripods carefully aligned with the two end ones, have super- 
 seded the rigid types of base apparatus in the most recent geodetic 
 and topographical work. By means of these tapes, bases can be 
 measured rapidly and accurately, and it is now a comparatively simple 
 matter to measure “‘check’”’ bases at frequent intervals in a long 
 triangulation. 
 
 The various steel chains, bands, tapes, etc., used in land engineering 
 and cadastral surveys require no general description. Examples of 
 the usual types used in such work are exhibited. With regard to 
 pedometers, etc., it is interesting to note that the invention of a 
 mechanical counter for land and marine surveying is attributed to 
 Hero of Alexandria, who is believed to have lived in the latter half 
 of the 1st century A.D. 
 
 ROUTE TRAVERSING. 
 
 42. PEDOMETER. Early 18th Century. Presented by Evan 
 Roberts, Esq. 
 
 This instrument, for registering the number of paces made by the person 
 carrying it, resembles a watch in size, appearance and general details of construc- 
 tion. The ‘“ escapement”’ is, however, not actuated by a pendulum or balance 
 wheel, but by a rocking lever and spring pawl which engages in a ratchet wheel. 
 
 The lever is depressed by the forward movement of the wearer’s leg, which 
 stretches a connecting cord attached to the split ring at the top of the instrument. 
 This disengages the spring pawl momentarily and permits one tooth of the ratchet 
 wheel to slip past its point. As the motion of the observer’s leg slackens the 
 tension in the cord, a counter spring forces the lever into its former position 
 ready for the next forward movement. The ratchet wheel thus moves one tooth 
 at every alternate stride of the wearer, and is connected with the pinion wheel 
 train, dials and pointers which record the steps taken. There are four such dials 
 and pointers, for the units, hundreds, thousands and ten thousands respectively, 
 the units dial having 50 graduations and its pointer moving over one division 
 every alternate pace of the observer. Inv. 1916-77. 
 
 43. PEDOMETER. Made by J. Alt, Berlin. Presented by 
 Evan Roberts, Esq. 
 
 This is also an 18th century example, and has a similar construction and 
 ‘“escapement’’ to the preceding one. The cord which is attached to the 
 observer’s leg and actuates the rocking lever, is also exhibited ; it passes over a 
 smali pulley wheel mounted in the head of the pedometer. 
 
 The main dial in this instrument has its divisions numbered in two sets, and a 
 pointer corresponding to each set. The outer set records the unit paces, and the 
 inner set the tens. A second dial and pointer registers the thousands up to twelve. 
 The units pointer moves over two divisions at a time, as it is only moved at every 
 alternate step of the observer. Inv. 1916-82. 
 
 (15814) B2 
 
20 
 
 44. PEDOMETER. Made by Spencer & Perkins. Presented by 
 T. MacNamara, Esq. 
 
 This is a late 18th century pedometer, of a construction less sensitive than the 
 adjacent one by the same makers. 
 
 The rocking lever is here attached to a pin fitting across the top of the case, 
 and the long hook is mounted upon the same pin, so as to embrace it tightly. 
 As the hook is swung by the motion of the person carrying the instrument, the 
 pin rotates and causes the lever to rock. A two-pronged spring pawl is fixed to the 
 lever at its upper end and engages the teeth of a ratchet wheel at its lower end. 
 As the lever rocks, the pawl causes the ratchet to slip a tooth, and thus turns a 
 pinion wheel mounted on the same axis. The pinion gears in the train of wheels 
 which record the paces taken. As only alternate paces are recorded, the dials 
 and pointers are arranged accordingly. Miles and decimal parts of a mile are 
 registered on three dials, the instrument recording up to 12 miles at intervals of one 
 thousandth of a mile. 
 
 The long hook serves to suspend the pedometer from the fob so that it rests 
 against the thigh, and thus gets the desired swing as the wearer walks. 
 
 Inv. 1911-182. 
 
 45. PEDOMETER. Made by Spencer & Perkins, London. Lent 
 by F. G. Ogilvie, Esq., C.B. 
 
 This is probably a late 18th or early 19th century pedometer and differs 
 from earlier examples in having a more delicate escapement, actuated by thesudden 
 stoppage in the motion of the wearer’s body at each step. The long hook is used 
 to suspend the instrument from the fob of the person carrying it. 
 
 As usual the escapement is effected by the withdrawal of a spring pawl, 
 attached to one end of a rocking lever, from the teeth of a small ratchet wheel. 
 In this instrument the teeth are very fine, so that the pawl slips over four teeth 
 at each oscillation. The lever is normally kept in the ‘‘down’”’ position by a 
 fairly strong spring, but the pull of this spring is considerably reduced by slightly 
 raising the long hook projecting from the top. The lower end of this hook is 
 fixed to a chain wound round a drum mounted on the same spindle as the rocking 
 lever. The hook is raised when the instrument is fitted in its case by inserting 
 a pin in a hole in the lower end of the hook, the ends of the pin resting in brackets 
 on the top of the case. In this position, the rocking lever is just balanced, and is 
 very sensitive to small jerks. 
 
 The instrument has three dials and pointers registering miles, tenths, and 
 thousandths of a mile. The pointer of the thousandths dial is connected to the 
 rachet wheel and so geared that it moves 176 times in one revolution, 7.e. in one 
 tenth of a mile. 
 
 The pedometer is mounted separately from its case for exhibiting the movement 
 mechanism, Inv. 1912-200. 
 
 46. COMBINED PEDOMETER AND WATCH. Made by Ralph 
 Gout, London.: Presented by Evan Roberts, Esq. 
 
 This instrument was made about 1800 and consists of a pedometer and a 
 watch mounted in the same case and having a common face, with separate dials 
 and pointers. The pedometer movement is effected by the tension of a string, 
 which is attached at one end to the wearer’s leg, and at the other to a delicately 
 poised rocking lever controlling by its oscillations the engagement of a spring 
 pawl in a ratchet wheel. 
 
 The large pedometer dial is graduated to record every hundred double paces ; 
 there is a smaller one to register every ten double paces, and a third to record each 
 double pace. Inv. 1916-83. 
 
 47. PEDOMETER MOVEMENT. Presented by Evan Roberts, 
 Esq. 
 
 The movement here exhibited is of the type invented by W. Payne in 1831 
 and still used in modern pedometers. The escapement of the recording ratchet 
 wheel is effected by the oscillations of a horizontal anchor-shaped pendulum, 
 whose moment about its pivot is just more than balanced by the pressure of a 
 control spring. The sudden jerk caused by the stoppage of the downward motion 
 of the wearer’s body at each step is sufficient to make the pendulum oscillate. 
 
 A spring pawl, fixed at one end to the pendulum arm, engages at its other end 
 the teeth of the ratchet wheel. A counter spring pawl, pivoted in the bed of the 
 instrument, opposes the thrust of the former with a force sufficient to damp the 
 
21 
 
 rotary movement of the ratchet. (This opposing spring is missing in the instru- 
 ment shown.) The teeth of the ratchet are very fine, so that as the pendulum 
 moves from the upper stop to the lower, the pawl passes over several teeth, 
 the number depending upon the adjustment of the lower stop, which is threaded in 
 a fixed block. By this adjustment, the indication of the recording pinion can be 
 varied to correspond roughly to the length of the observer’s stride. The ratchet 
 wheel is geared to a train of pinion wheels which record the distance travelled 
 in miles and furlongs. There is only one dial and pointer, the former graduated in 
 miles up to twelve, with eight sub-divisions to each mile. The numbering of the 
 main graduations is like that used in watches. 
 
 (See British Patent Specification, 1831. No. 6078.) Inv. 1916-81. 
 
 48. PEDOMETER. Made by Pastorelli & Co. 
 
 This instrument has the movement invented by W. Payne in 1831 and shown 
 in the preceding exhibit. The movement of the horizontal pendulum is restricted 
 by stops, the lower one being adjustable to allow approximately for the variation 
 in the length of stride. 
 
 The instrument is carried in the waistcoat pocket and kept in a vertical position 
 by means of the spring clip attached to its ring. It has a single dial, graduated in 
 miles and quarters up to 12 miles. Fractions of quarter-miles can be estimated 
 by the position of the pointer between the graduations. Inv. 1875-25. 
 
 49. WAYWISER. Made by G. Adams. Lent by T. H. Court, 
 Esq. 
 
 The Odometer, Waywiser or Perambulator has been employed since the days 
 of the Romans for measuring distances by counting and recording the revolutions 
 of a wheel of known circumference. The example shown was made in the late 
 18th century. It has a brass wheel of 1-5 {t. circumference geared to a registering 
 mechanism furnished with dials and pointers which record the number of links, 
 poles, chains, furlongs, and miles traversed. 
 
 The bearings for the wheel axle are fixed to metal brackets screwed to the case 
 containing the registering mechanism, which itself is fixed at the lower extremity 
 of a long wooden handle. At one end of the axle is a small pinion wheel the teeth 
 of which engage in those of a perpendicular pinion mounted on the lower end ofa 
 brass rod, which is pivoted at its lower end in the axle bearing block and kept 
 parallel to the supporting bracket by guides. The upper end of this rod is geared 
 within the case to the registering mechanism. This has two pointers and dials, 
 the outer one divided to read links and having divisions both every ten links and 
 every pole, whilst the inner dial is graduated in miles and half furlongs up to 
 four miles. Inv. 1914-895. S.M. 918. LS. 
 
 50. PERAMBULATOR. Made by W. & S. Jones. 
 
 This instrument for measuring distances by recording the revolutions of a 
 rolling wheel was made at the beginning of the 19th century, and is an improved 
 form of the adjacent Waywiser, made by Adams. The wheel is much larger, its 
 circumference being half a pole, and it is carefully mounted in its bearings so as 
 to reduce friction and shake. 
 
 The mechanism differs slightly from that in the Waywiser. The motion of the 
 rotating axle of the wheel is transmitted to a perpendicular shaft running along 
 one bearing fork by a pair of bevel wheels, and from the shaft to the recording 
 mechanism by a worm and worm-wheel. One of the bearing forks is hinged at 
 the top to the case containing the recording mechanism. The outer dial has a 
 diam. of 7 in. and is graduated in yards and poles, its pointer making one revolu- 
 tion every furlong. An inner dial, of 5 in. diam. is graduated in furlongs and miles 
 up to 10 miles and is read by a second index. 
 
 The wheel and mechanism case are made of wood, with metal parts where 
 there is much wear, e.g. the axle and bearings, wheel rim, etc. 
 
 (See Adam’s ‘“‘ Geometrical and Graphical Essays,’’ 1813.) 
 
 Inv. 1906-40. S.M. 939. L.S. 
 
 51. SLEDGE METER. Made by S. Smith & Son. Lent by the 
 British Antarctic (Terra Nova) Expedition, Ig10-13. 
 
 This form of measuring wheel was constructed specially for use on frozen snow 
 surface and ice, and was employed on all journeys made by the above expedition 
 when much reliance had to be placed on dead reckoning. 
 
 (15814) B3 
 
22 
 
 The instrument has a trailing wheel of diam. 18 in. made of steel, with an 
 aluminium band on its rim to prevent snow from clinging to the surface and with 
 steel pegs fitted into the rim so as to enable the wheel to roll without slipping on 
 ice surfaces. The wheel is mounted in ball bearings upon a metal fork to which the 
 recording mechanism is attached. The gearing is completely enclosed and the dial 
 gives reading in geographical miles, subdivided to 25 yd. The fork is fixed to a 
 wooden bar, to which at the other end is attached a cross head connected to the 
 bar by a universal joint. This permits the instrument to follow the track of a 
 sledge without partaking of its lateral movements. The instrument gave satis- 
 factory readings over soft snow surfaces but registered too low on a very hard 
 surface owing to the steel spikes giving a variable periphery. 
 
 Inv. I914-III. 
 
 FIELD MEASUREMENT. 
 
 §2.° PHOTOGRAPH? + OF] 1 -FARIOV (Un GYPTIAN ss SND 
 MEASURERS. Presented by Captain H. G. Lyons, F.R:S. 
 
 Although no map of landed property in ancient Egypt has come down to us, 
 representations of land measurers at work have been made upon tomb walls. 
 This photograph shows the scene depicted upon the walls of the tomb of one Menna 
 at Sheikh Abd el Qurna, in Thebes, a land overseer and inspector of the boundary 
 stones of Amon (Circa, 1400 B.C.). 
 
 Two “ chain ’’ men, or “‘ rope stretchers,’’ are shown measuring a field of corn 
 with a long cord on which are knots or marks at intervals apparently of 4 or 5 
 cubits ; each also carries a spare cord coiled up on his arm. Beside them walk 
 three officials carrying writing materials and accompanied by a boy who also 
 carries writing materials and a bag probably containing documents and plans 
 relating. to the property. An old man and two boys also accompany the surveyor 
 and a peasant brings a loaf of bread and a bunch of green corn. In a similar 
 scene pictured on the wails of a tomb belonging to a certain Amenhotep, also at 
 Sheikh Abd el Qurna; it is possible to see that the measuring cord terminated 
 in a ram’s head. 
 
 (See “‘ Cadastral Survey of Egypt,” p. 50, Lyons, 1908.) Inv. 1913-572. 
 
 53. GUNTER’S CHAIN. 
 
 The chain most generally used in England for land measurement is named 
 after its inventor, Edmund Gunter, and was first employed about 1620. It has a 
 length of 66 ft. made up of 100 links, so that 10 square chains equal one acre. 
 
 The example shown is made of iron, with brass handles, as was formerly the 
 custom. This type has been replaced by the chain made of steel wire, of No. 8 
 or No. 12 B.W.G., which stretches much less than an iron chain after use. 
 
 In all Gunter’s chains, brass tellers are inserted at every 10 links, for con- 
 venience in estimating fractional lengths. Inv. 1872-78. 
 
 54. LAND SURVEYOR’S STEEL CHAIN. Made and lent by 
 J. Chesterman & Co. 
 
 This chain, which is 66 ft. long and has roo links, is representative of the 
 modern land surveyor’s chain. It is made of No. 12 B.W.G. steel wire, with brass 
 handles, and brass tellers every 1o links. A central swivel is fitted to obviate 
 kinks when the chain is twisted in use. 
 
 When not in use, the chain is folded up in the manner shown, and bound 
 together with a leather strap. Inv. 1914-845. 
 
 55. STEEL MEASURING BAND. Made and lent by J. Chester- 
 man & Co. 
 
 A steel band of this type is preferred to a chain by many land surveyors, 
 partly because, length for length, it is lighter and also because it preserves 
 its length much better after the strain of constant usage. On the other hand, 
 it is not so durable as a chain, nor so easily read. 
 
 The example shown is 50 ft. long, and is divided every foot, the two terminal 
 feet being subdivided into inches. The divisions are marked by brass studs of 
 various sizes, an oval plate being substituted for the stud at every 10 ft. and 
 marked by perforation to show the length from the nearer end. The handles are 
 made of brass, with swivel joints. The whole is wound up in a metal case which 
 has a narrow slit for inserting the handle at one end. A strap is attached to the 
 case for carrying it. 
 
 The band is in. wide and of No. 26 B.W.G. thickness. Inv. 1914-846. 
 
23 
 
 56. STEEL MEASURING TAPE. Made and lent by J. Chester- 
 man & Co. 
 
 An accurately graduated steel tape of this type is extensively used for short 
 measurements. This example is 10m. long and is graduated throughout in 
 millimetres. It is }in. wide, and of No. 34 B.W.G. thickness. When not in 
 use, it is wound up in the leather case as shown. Inv. 1914-847. 
 
 57. ‘““ OASSABA ” MEASURING ROD. Presented by Captain 
 Hi-G. Lyons) Ri: 
 
 The “‘ qassaba ”’ (gassaba, a rod, Arabic) is a Syrian unit of length which has 
 been employed in land surveying in Egypt for the past ten or twelve centuries. 
 Previous to a decree dated 1861, its actual length varied considerably at different 
 times and in different parts of the country, but it was then defined as being equal 
 to 3°55 m., the unit of area (feddan) then becoming 3334 square qassabas instead 
 of 400. A “ qassaba’”’ rod, with its ends tipped with metal, is here exhibited, 
 with photographs showing the rod in use. 
 
 The land-measurer (messah) holds the rod by the middle and places it 
 successively along the line to be measured by turning it over on the forward end. 
 Skilled ‘“‘messahin’”’ obtain remarkably good results but they can also easily 
 vary the measurement towards excess or defect when turning over the rod. At 
 the present time all official land-measurements in Egypt are made with the 
 steel chain, but the qassaba-rod is still used in the villages for sub-dividing 
 holdings or measuring up crops. 
 
 (See “‘ The Cadastral Survey of Egypt,’ by Capt. H. G. Lyons, F.R.S.) 
 
 Inv. 1912-244. 
 
 58. WOODEN MEASURING RODS. Made by O. Fennel Sohne, 
 Cassel. 
 
 Wooden rods (Messlatten, Messtangen) similar to the pair here shown are 
 commonly used in Germany in land-measurement, and have been in use in 
 Wirtemburg for several centuries. 
 
 Each rod has alternate metres painted red and white, and each metre is sub- 
 divided by small studs into decimetres. They are made of well-seasoned fir, 
 painted and varnished to prevent variations in length due to atmospheric 
 moisture. The rods are usually round or oval in section and vary in thickness, 
 tapering towards the ends, which are flat and tipped with metal. They are used 
 of various lengths, 3, 4 or 5 metres being the most usual, and 2 metres in towns 
 where narrow alleys occur. In measuring a length, the rods are used in pairs, 
 being placed successively in contact and aligned on the distant point which is 
 marked by a ranging rod. 
 
 (See Jordan’s “‘ Handbuch der Vermessungskunde,”’ Bd. II, Section 18.) 
 
 Inv. 1914-257. 
 
 59. WOODEN MEASURING RODS. Made by O. Fennel Séhne, 
 Cassel. 
 
 These are similar in general characteristics to the adjacent pair, but are of 
 square section, and have wedge-shaped metal ends, which abut at right angles 
 when the rods are laid in contact. In one rod the metres are painted alternately 
 red and white; in the other, black and white. Each rod is 5m. long, graduated 
 in decimetres. Their length is verified before use by laying each rod on a test-bed 
 provided with chisel-shaped stops at a known distance apart. By measuring the 
 interval between the rod and the test-bed with a measuring wedge the length 
 of the rod is obtained. Inv. 1914-258. 
 
 60. STEEL MEASURING WEDGE. Made by O. Fennel, Cassel. 
 
 In comparing a wooden measuring rod of the type used in cadastral surveys 
 with the corresponding standard, it is usually necessary to measure very 
 accurately the small distance between one end of the rod and the end of the 
 standard. For this purpose, a steel measuring wedge of uniformly varying 
 thickness is inserted between the two terminals. The one exhibited has a uniform 
 variation in thickness of I mm. per I cm. of its length. Its length is graduated 
 in millimetres, reading by estimation to tenth-millimetres, and thus giving the 
 corresponding thickness to 0-o1 mm., within a range of error of 0-OoI mm. 
 
 (See ‘‘ Handbuch der Vermessungskunde,” Jordan, Bd. II, Section 20.) 
 
 Inv. 1914-259. 
 
 (15814) B4 
 
24 
 
 BASE MEASUREMENT. 
 
 61. STEEL CHAIN. Made by Ramsden. Lent by the Royal 
 Society. 
 
 This chain was first used by General Roy in measuring the Hounslow Heath 
 base in 1784. Although it was only used as a preliminary to more precise 
 measurements with both fir and glass rods, General Roy was so impressed by 
 its accuracy that, in measuring the base of verification on Romney Marsh in 
 1787, he used the chain only, and discarded the wooden and glass rods. 
 
 The chain has 100 links, each 1 ft. long. The joints at every 1o ft. are at right 
 angles to the remainder, which permits the chain to be folded so as to fit into a 
 box about 14in. by 8in. by 8in. for transport. Brass plates are attached at 
 the joints to indicate 10 ft. lengths along the chain. 
 
 In the rough measurement of the Hounslow Heath base, the chain appears 
 to have been simply stretched along the ground, but in the measurement at 
 Romney Marsh it was supported on deal coffers, 20 ft. long, resting on carefully 
 aligned posts driven into the ground every 20 ft. along the base. Thus supported, 
 the chain was stretched by a weight of 28 lb., the temperature being measured 
 by thermometers laid in the coffers near the chain. Inv. 1900-156. 
 
 62. BONING TELESCOPE. Lent by the Royal Society. 
 
 In measuring the Hounslow Heath base in 1784, General Roy divided the 
 base into sections of 200 yd., and erected tripods of fixed height for supporting 
 the measuring rods at each division point; these sections were subdivided into 
 segments of 20 ft., corresponding to the lengths of the deal measuring rods used, 
 and intermediate tripods set up at the subdivision points with their platforms 
 adjusted so as to be in the line connecting the platforms of the tripods at each 
 end of the section. The telescope here exhibited was used to sight between the 
 two terminal tripods upon boning rods set up at the subdivision points. The 
 movable vanes of these rods being adjusted until their index lines lay on the axis 
 of sight of the telescope, the height of the intermediate tripods was thus 
 ascertained. 
 
 The telescope has a rack-focussing objective lens of 1-2 in. aperture and 13 in. 
 focal length, and a fixed eyepiece of the Huygens type, with a fixed cross-wire 
 diaphragm between the two lenses. The magnifying power of the eyepiece is 
 small, and the telescope can be focussed down to a few feet. It has square brass 
 collars shrunk on to its tube near each end to support it upon the platform of the 
 tripod. 
 
 (See Phil. Trans. Roy. Soc., 1785.) Inv. 1900-157. 
 
 63. PHOTOGRAPH OF STRUVE’S BASE APPARATUS. Pre- 
 sented by the Pulkowa Observatory. 
 
 The apparatus here illustrated was designed by W. Struve and used by him 
 in measuring seven bases of the principal triangulation of Russia, 1817-1855. 
 It consists of four wrought iron bars enclosed in wooden cases, supported near the 
 ends on trestles. The chief peculiarity of the bars is the use of spring contact 
 levers. The temperature was measured directly by thermometers whose bulbs 
 rested in cavities in the bar. 
 
 Struve estimated the mean probable error of his bases as I in 1,250,000. 
 
 Each iron bar is 2 toises (nearly 4 m.) long, and has a square section of 3°4 cm. 
 side. It terminates at one end in a small steel cylinder, having a slightly convex 
 outer face. To the other end is pivoted a lever with unequal arms. On the 
 lower arm is a polished hemisphere, pushed outwards from the bar by a weak 
 spring. The steel cylinder of the adjacent bar makes contact with this hemisphere, 
 and forces the longer arm of the lever to traverse a vertical graduated scale. 
 Any slight variation in length of the bar is correspondingly magnified by the 
 lever on the scale, and can be accurately measured. 
 
 Two mercury thermometers are placed with their bulbs in recesses in each 
 bar, and serve to measure the temperature directly. There is also a spirit level 
 attached to determine the inclination of the bar. 
 
 (See ‘‘ Encyclopedia Britannica,’ XI Edition, Article ‘‘ Geodesy.”’) 
 
 Inv. 1876-1211. 
 
 64. COLBY’S COMPENSATION BARS. Presented by the Ord- 
 nance Survey: Plate [Bizg.2, p15: 
 
 This apparatus was designed by Major-General Colby, R.E., in order to 
 eliminate the errors in measuring bases caused by the impossibility of estimating 
 
25 
 
 accurately the temperature of the measuring bars at the time of observation. 
 The device of Colby consists in so connecting two bars, one of brass and the 
 other of iron, that two reference marks on metal tongues attached to them remain 
 at an invariable distance apart whatever the temperature. 
 
 Two bases have been measured with this apparatus, one in the North of Ireland 
 in 1827-28, and the other on Salisbury Plain in 1849. Ten bases have been 
 measured with similar apparatus in India. 
 
 In this apparatus two bars, one of brass and the other of iron, each 1o ft. 
 1:5 in. long, 0:5 in. broad and 1-5 in. deep at a certain temperature, are placed 
 1-125 in. apart and firmly connected at their centres by two small transverse 
 steel cylinders, nearly in contact. Near each extremity of the double-bar is a 
 metal tongue, 6-2 in. long, which is connected to each bar by pivots so as to 
 permit expansion, but no other movement of the bars. On a silver plug at the 
 extremity of each tongue is engraved a dot—the compensation point. Since the 
 lengths of the tongue between the dot and either bar is proportional to the 
 coefficient of expansion of that bar, no joint expansion of the bars due to tem- 
 perature changes will affect the position of the dot. Consequently the dots are 
 to ft. apart at all temperatures. 
 
 The compound bar is placed in a deal box, and is kept from moving lengthwise 
 by a brass stay fixed firmly to the bottom of the box at the centre and projecting 
 upwards between the two small steel cylinders connecting the bars. <A long 
 level is fixed to the top of the brass bar and read through a glass covered opening 
 in the top of the box. The tongues carrying the compensation points project 
 beyond the box, but are carefully protected. 
 
 In measuring, the boxes are supported at their quarter and three-quarter 
 points on stands which permit lateral and longitudinal motion. The dots on 
 two adjacent bars are brought to the precise distance of 6 in. apart by means of 
 a compensation microscope. This latter is formed by uniting two microscopes 
 by parallel bars of iron and brass in such a manner that the distance between 
 their outer foci remains exactly 6in., whatever the temperature. The whole 
 apparatus consists of six bars and seven microscopes, of which two bars and three 
 microscopes are exhibited. The rate of measurement with this apparatus was 
 six bars per hour on the Lough Foyle base. 
 
 (See Ordnance Survey ‘‘ Report on Measurement of Lough Foyle Base,’’ 
 pp. 163, 164.) Inv. 1876-1201, S.M. 261. 
 
 65. JADERIN BASE MEASURING APPARATUS. Presented 
 by the Survey of Egypt. 
 
 This apparatus for measuring geodetic bases is of the type first employed by 
 Jaderin in Sweden in 1885, and was used in Egypt for determining second order 
 bases from t1go01 to 1907. In Jaderin’s system, the massive rigid base-bars 
 hitherto employed were replaced by light metallic wires or tapes freely suspended 
 under a constant tension. The saving in labour and time thus effected easily 
 outweighs the slight loss in accuracy sustained. Independent measurements of 
 each section of the base are made with two wires of different metallic structure, 
 and the effect of temperature variations is thus calculated. 
 
 The apparatus exhibited comprises two tripods and two pairs of wires, 
 25 and 50 metres long respectively, each pair consisting of one brass and one steel 
 wire. The tripods are provided with steel observing pins mounted upon double 
 ball and socket joints, which permit slight displacements of the pins when the 
 tripods are fixed. The wires are nickel-plated, and are provided at each end with 
 decimetre scales, graduated in millimetres. The centimetre divisions of these 
 scales are numbered from 5 to 15, and the length of the wire between the “ Io” 
 division at each end is the characteristic length. The wires terminate beyond 
 the scales in loops, to which are attached the flexible cords carrying the straining 
 weights. The tension may be applied by hand, and measured by a spring balance, 
 or by attaching weights to the cords. The cord at each end passes over a pulley 
 mounted on a straining rod or trestle, which serves to take the end strain and to 
 adjust the scale of the wire during the actual measurement into close proximity 
 to the side of the observing pin. Observations of the positions of each pin with 
 respect to the corresponding scale are simultaneously recorded. Several dual 
 observations are made with each wire of a pair, the wire being slightly displaced 
 for each observation. 
 
 Previous to the actual measurement of any bay, the pins of the tripods marking 
 the ends of that bay are carefully aligned and their heights determined by a transit 
 theodolite and level. 
 
 With this apparatus, various bases in Egypt were measured with probable 
 errors between I in 200,000 and I in 1,000,000 at a speed of about I,ooo metres 
 
26 
 
 per day. The higher degree of accuracy was consistently attained when the 
 “constants ’’ of the wires were known properly, and the observing parties had 
 gained experience in correctly applying the tension to the wires. Recently the 
 invention of ‘‘invar’’ has obviated the employment of two wires in the Jaderin 
 system, and with improved straining mechanism, it is now possible to measure 
 bases with Jaderin “‘ invar’’ wires or tapes with an accuracy high enough for 
 first order triangulation purposes. 
 
 (See ‘‘ Geodatische Langenmessung mit Stahlbanden und Metaldiapten,’’ 
 E. Jaderin, Stockholm, 1885 ;/ also US; C & G.-S. Report, 1893, Part’ 11, 
 Appendix 5.) Inv. 1913-560. 
 
 66. PHOTOGRAPHS OF SECONDARY (STEEL) BASE-APPA- 
 RATUS. Presented by the U.S. Coast and Geodetic Survey. 
 
 These three photographs illustrate a base-measuring apparatus constructed 
 in July, 1891, and employed by the U.S. Coast and Geodetic Survey Department 
 in measuring the Holton (Indiana) Base in 1891, the Salina (Kansas) Base in 1896, 
 and the Versailles (Missouri) Base in 1897 in connection with the Transcontinental 
 (Primary) Triangulation of the 39th Parallel of Latitude. 
 
 The apparatus consists of two precisely similar base-bars, known as the 
 U.S. C. & G. S. Nos. 13 and 14 measuring bars. Each bar is of the contact-slide 
 metallic-thermometer type, and is 5 m. long at a certain temperature. The 
 actual measuring unit is a steel rod 8 mm. in diam., terminating at one end in an 
 agate plane, and at the other in a contact-slide which itself ends in an agate 
 knife-edge. The slide is connected to the rod by a spring, and has a mark engraved 
 upon it corresponding to a mark engraved upon the rod, so that when these two 
 marks coincide, the knife-edge and plane terminal points are the correct distance 
 apart. Two zinc tubes, 9°5 mm. in diam., are soldered one to each end of the 
 steel rod, their other ends being free to expand or contract relatively to this rod. 
 The relative expansion is measured by means of a vernier on each zinc tube 
 moving over a scale fixed to the steel rod, and the compound rod thus formed 
 enables the mean temperature to be determined. 
 
 The compound measuring rod rests upon rollers in a rectangular wooden case, 
 which itself rests upon tripods during the measurement and is provided at its 
 rear end with a telescope for aligning the bar. When the forward bar has been 
 aligned and adjusted as a whole on its trestles to within a smail distance of the 
 preceding bar, its measuring rod is slowly drawn backwards upon the rollers by 
 a milled screw at its rear end, until the contact-slide touches the agate plane of 
 the preceding bar and is forced gently over the steel rod until the two engraved 
 lines coincide. During this motion, the wooden case remains stationary upon 
 its trestles, and the only pressure upon the rear bar is that due to the spring of 
 the contact-slide, which is quite powerless to disturb the bar. 
 
 Each bar is provided with a mercurial thermometer for use when comparing 
 it with the standard bar. 
 
 The average rate of measurement with this apparatus on the Holton Base 
 was 155m. per hr., with a probable error of about I in 1,500,000. 
 
 (See Special Publication No. 4, U.S. C. & G. S. Department). 
 
 Inv. 1914-388. 
 67: “PHOTOGRAPHS” "OF -EIMBECK Ss DUPLEX cise 
 APPARATUS. Presented by the U.S. Coast and Geodetic Survey. 
 
 These three photographs illustrate a bimetallic contact-slide apparatus 
 designed by W. Eimbeck, and used by the U.S. Coast and Geodetic 
 Survey Department in measuring the Salt Lake Base (Utah) in 1896 in connection 
 with the Transcontinental Triangulation of the 39th Parallel of Latitude. It 
 consists of two similar bars, each 5 m. long, known as U.S. C. & G.S. Nos. 15 and 16 
 duplex bars. The rate of measuring with this apparatus in Utah was about 250 m. 
 per hr., with a probable error of I in 5,000,000. 
 
 Each bar consists of two measuring units of precisely similar construction, 
 one of steel and one of brass, each 5 m. long, and weighs complete 118 lb. The 
 measuring units are made of tubing of in. diam., each having a thickness of wall 
 corresponding to the conductivity and specific heat of the material of which it is 
 made, so that under changing conditions each tube shall keep the same tempera- 
 ture as the other one. The two measuring tubes are carried on rollers in a brass 
 protecting casing, which turns on its longitudinal axis in an outer brass protecting 
 casing which remains stationary. The inner casing is rotated 180° from time to 
 time to equalize temperature distribution. The two measuring units are entirely 
 disconnected, and contact is always made brass to brass and steel to steel; 
 hence two independent measurements of the base are obtained, their difference 
 
27 
 
 determining the average temperature during the measuring, so that each can be 
 corrected accordingly. To overcome the difficulty due to one rod gradually 
 gaining on the other, owing to the different coefficients of expansion of brass and 
 steel, each measuring unit is provided with vernier scales, and the brass bar is 
 occasionally readjusted by an amount thus measured and allowed for in the 
 final calculations. 
 
 Telescopes are affixed to each base-bar for aligning, whilst the tripods are 
 provided with special heads for quick and slow adjustment in the horizontal and 
 vertical planes. The contact slide is similar to the one described in connection 
 with the Secondary Base-Apparatus. 
 
 (See Appendix II, Report for 1897, and Special Publication, No. 4. U.S.. 
 CC. & G45. Department.) Inv. 1914-389. 
 
 68. PHOTOGRAPHS OF TAPE-STRETCHING APPARATUS. 
 Presented by the U.S. Coast and Geodetic Survey. 
 
 These three photographs illustrate a form of tape-stretching apparatus which 
 is an improvement upon the one used by the U.S. Coast and Geodetic Survey in 
 measuring the Stanton Base, Texas, in 1908-09. 
 
 It consists of an iron tube 1-35 m. long and 32 mm. diam., along which is 
 movable a short collar fitted with a frame to carry the spring balance, and a 
 weight to counterpoise it. The position of the sliding collar is easily adjusted 
 by means of a friction lever device. The bottom of the tube ends in a round conical 
 point, which permits it to rotate about its axis, thus allowing the tension to put 
 the balance into alignment with the tape. A short iron projection near the bottom 
 of the tube is used for forcing the latter into the ground by means of the foot. 
 
 The total weight of the stretcher is 114 Ib. 
 
 The rear end of the tape is attached by a strap to a simple wooden staff 
 fitted with a metal shoe, as shown in the left-hand photograph. 
 
 The tension normally applied by this means to an “‘invar’’ tape for primary 
 base-measurem=2nt is, in the United States, 15 kg. 
 
 (seers: Ce kG) So Report) 1910, Appendix *4.) 
 
 Inv. 1914-390. 
 
 69. “INVAR” MEASURING TAPE. Made by J. H. Agar Bough. 
 
 The name “‘invar’’ has been given to a nickel-steel alloy containing from 
 35 per cent. to 36 per cent. nickel, discovered by Dr. C. E. Guillaume in 1896, 
 owing to its low coefficient of expansion for temperature, which is not greater than 
 I-ox10—®per®C. The application of this alloy to geodesy arises from the difficulty 
 of ascertaining the exact mean temperature of a metal measuring unit, and 
 consequently the precise length of the unit at the time of measurement. As the 
 effect of am error of this type is directly proportional to the expansibility of the 
 metal, the importance of a low coefficient of expansion is obvious. 
 
 The use of “‘invar’”’ wires or tapes, suspended under a constant tension of 
 about 10 kg. according to Jaderin’s method, is now almost universal in measuring 
 bases for triangulation of the first or second order. 
 
 The tape exhibited is just over 100 ft. long, and has scales divided to read 
 two-hundredths of a foot at each end. According to the National Physical 
 Laboratory certificate of examination, the length between its zero marks at each 
 end is 100-001 ft. at 58-5° F., under a tension of 10 kg., with an accuracy of 1 in 
 100,000. The width of the tape is 5 mm. and its thickness 0-5 mm. Itis wound 
 upon an aluminium drum of 16 in. diam., which can rotate upon its pivot as the 
 reel is unwound. 
 
 (See “‘ Les nouveaux appareils pour la mesure rapide des bases géodésiques.”’ 
 Guillaume et Bénoit. Paris, 1916.) Inv. 1912-245. 
 
 70. PHOTOGRAPHS OF BASE MEASUREMENT WITH 
 “INVAR” WIRES. Presented by Major G. G. Waterhouse, R.E. 
 
 These four photographs of the measurement of the Narguta base in ig12, 
 during the survey of Northern Nigeria, illustrate the general process of using 
 flexible wires, suspended according to Jaderin’s method, and the superiority of 
 such apparatus over rigid rods in difficult country. In this case a river, the 
 Delimi, having fairly steep banks, was successfully negotiated without any 
 special preparation of the ground. Two of the photographs illustrate the approach 
 to the Delimi and its crossing, a third shows the crossing of a deep irrigation chan- 
 nel, and the fourth shows the completion of a section of the base, the terminal 
 point being marked by a ground pin. Inv. 1920-655. 
 
28 
 
 INDIRECT MEASUREMENT OF DISTANCE. 
 
 RANGEFINDING. 
 
 Problems involving the determination of the distance of an inaccessi- 
 ble object have aroused great interest from very early days, particularly 
 among military surveyors. Thales of Miletus, in the 6th century B.c., 
 already applied his knowledge of geometry to determine the distances 
 of ships at sea from the shore, and Hero of Alexandria, in his treatize 
 on “ the dioptra,” written in the Ist century A4.D., shows how to apply 
 this instrument to solve problems of rangefinding. The surveyors of 
 the Middle Ages and Renaissance periods constantly discuss similar 
 problems, and show how to solve them by triangulating from a short, 
 measured base. The subtense method of rangefinding, in which the 
 base is situated on the distant object, has long been employed for 
 astronomical purposes, and is often still used in marine surveying, 
 where the object ranged upon provides a known length, such as the 
 ship’s mast, which can serve as base (see Nos. 71 and 72). Generally, 
 however, the tendency in land surveying is to employ the ordinary 
 system of triangulating from a “ home” base, with the restrictions 
 that this base shall be very short compared with the distance to the 
 object ranged upon, and very nearly perpendicular to the direction 
 of that object. Under these conditions, a very simple formula connects 
 the required range and the angle of convergence of the rays from 
 either end of the base to the distant object ; this angle can be micro- 
 metrically varied to suit the range, and the latter read off directly on 
 a special scale connected with the mechanism controlling the angular 
 variation. By this means, the necessary rapidity of ranging upon 
 moving objects is attained. Many modern rangefinders, such as the 
 Labbez and Gautier Telemeters and the G.S. Mekometer (see Nos. 73, 
 75 and 76), require either two observers or separate measurements 
 from each end of the base, which is usually fairly long—about 50-100 ft. 
 —but the more precise types have a base fixed in the instrument itself, 
 and are operated by a single observer, who sees simultaneously two 
 views of the distant object obtained by rays from each end of the 
 base. One of the earliest rangefinders of this latter type was invented 
 in 1775 by an optician named Magellan, and had an adjustable base 
 in the instrument itself. In a rangefinder patented in 1858 by General 
 Clark, two views of the object were reflected from mirrors situated 
 one at each end of the fixed base tube, into a central telescope, so that 
 the observer saw two images of the object, separated by a short lateral 
 distance; the range of the object could be rapidly calculated by 
 measuring this distance micrometrically. 
 
 In 1860 Adie’ patented a short, constant-base rangefinder (see 
 No. 77) in which the two images of the distant object were made to 
 “coincide ”’ in the central telescope field, by means of a rotation of 
 the inner tube carrying one end reflector system within the outer 
 tube containing the other end system. Soon after this date, Steinheil, 
 in Germany, and Tavernier, in France, brought out rangefinders in 
 which the “ coincidence ” of the images was effected by rotating one 
 of the end reflectors. The employment of pentagonal prisms, now 
 generally used, as end reflectors instead of plane mirrors or triangular 
 prisms appears to have been first made by Colonel Goulier about 1864. 
 
 In 1888 Messrs. Barr and Stroud invented their well-known short- 
 base coincidence rangefinder (see No. 78), in which the coincidence 
 
29 
 
 was effected by the lateral movement of a deflecting prism interposed 
 between the central telescope and one of the fixed end reflectors. The 
 same firm have since introduced many improvements in their original 
 type of rangefinder, whilst preserving the deflecting prism as an 
 essential feature. 
 
 A different principle is involved in the stereoscopic type of range- 
 finder, in which the capacity of the human eyes to “ converge ’’ upon 
 a distant object is utilized to measure its distance. In such an instru- 
 ment the muscular effort of the eyes during convergence is a delicate 
 measure of the required range, and the optical systems employed 
 merely enhance the range within which this effort can be accurately 
 distinguished. The foundations of stereoscopic measurement were 
 laid by Wheatstone in 1832 and by Helmholz in 1855-58, but the first 
 rangefinder utilizing this principle was conceived by De Grousilliers 
 in 1893 and developed by Messrs. Zeiss in the subsequent years. 
 Stereoscopic rangefinders, whilst very accurate and rapidly manipu- 
 lated in the hands of a skilled observer, necessitate a power of stereo- 
 scopic perception not possessed by everybody. A rangefinder of this 
 type, devised by G. Forbes in 1gor for infantry use, is exhibited in 
 the Collection (No. 80), 
 
 M. STADIOMETER. Made by W. Heath. Presented by the 
 Admiralty. 
 
 This instrument was designed by Capt. G. H. Blakey, R.N. for determining the 
 distance of a ship on the assumption that the height of its mast is known, and that 
 the mast is perpendicular to the direction of the ship from the observer. It is 
 merely a telescopic sighting tube having a pin-hole aperture and pairs of parallel 
 wires set at known intervals apart near the object end of the tube. The distance 
 of these wires from the aperture is given by the graduated scales on the telescopic 
 tube. 
 
 In using the instrument, the mast of the distant ship is viewed and the sighting 
 tube telescoped until the mast lies exactly between a pair of wires. On reading the 
 length of the tube, z.e. from aperture to wires, the range of the mast will be in the 
 same ratio to this length as the mast is to the interval between the wires. As 
 these last two lengths are known, the range is readily calculated, or can be obtained 
 directly from the tables supplied with the instrument. Two mutually perpendicu- 
 lar pairs of parallel wires are fitted in the instrument exhibited, at intervals of 
 one tenth and one twentieth of a foot respectively. A table corresponding to the 
 former interval for lengths of masts between 100 and 200 ft., and all possible 
 lengths of sighting tube is printed on the tube and gives the corresponding ranges 
 in yards. For the smaller interval, these ranges are of course to be doubled. 
 
 Measurements of distance by this apparatus can only be very approximate 
 owing to the absence of magnification, uncertainty as regards the length of the mast 
 and its inclination to the line of sight, etc. Inv. 1876-1209. 
 
 72. STUART’S DISTANCE METER. Presented by T. H. Court, 
 Esq. 
 
 ’ Assuming that the length of a distant ship’s mast is known, its range can be 
 found by this instrument, which was designed in 1895 for use at sea. Two images 
 of the mast are seen in the field of the telescope, one formed by light rays traversing 
 a fixed prism of constant angle, and the other by rays traversing a movable prism 
 of varying angle. These images are made to approach each other until the lower 
 limit of one is level with the upper limit of the other by sliding the movable prism, 
 when the position of an index attached to this permits the range to be read. 
 
 The movable prism consists of a thin piece of glass, ground concave on both 
 faces, and is attached to a slide worked by a tangent screw. It thus forms a 
 prism of varying angle in front of one half of the telescope, the other half being 
 occupied by the fixed constant angle prism. 
 
 Ranges up to 3 miles can be read to the nearest tenth of a mile. 
 
 (See British Patent Specification, No. 23834, 1895.) Inv. 1915--399. 
 
30 
 73. LABBEZ TELEMETER. 
 
 This is a pocket rangefinder suitable for military reconnaissance. To determine ~ 
 ranges up to 3,000 m. two observations are made with it, one from each end of a 
 30 m. base, which is set off approximately at right angles to the direction of the 
 object. 
 
 The instrument consists of a short metal tube having a pin-hole sight at one 
 end and a rectangular window in one side. Opposite this window within the tube 
 are two small plane mirrors inclined at an angle of 45° to each other. Both mirrors 
 can be rotated slightly. The observer views the object A whose range is required 
 directly through the pin-hole sight, and rotates°one mirror until he sees, after 
 double reflection, the image of an object B whose direction is nearly at right angles 
 to A’s. He measures off the base in the direction of B, and again views A directly. 
 By rotating the second mirror he once more gets a doubly ~ reflected image of B. 
 This latter rotation is effected by turning a milled drum in the eye end of the instru- 
 ment, and the position of this drum when B is again sighted is a measure of the 
 range of A. The drum is calibrated to give this range immediately. 
 
 A thin cord of the base length is supplied with the instrument. 
 
 (See “‘ Fieldwork and Instruments,’’ 1900, pp. 236-239, by A. T. Walmisley.) 
 
 Inv. 1920-66. 
 
 74. THREE-PRISM TELEMETER. Presented by T. H. Court, 
 Esq. 
 
 This instrument is used to measure ranges in the ordinary manner, by solving 
 a very acute right-angled triangle. The angle subtended at the distant point is 
 constant, whilst the base is variable, and is measured by pacing. 
 
 Three reflecting right-angled prisms are mounted by hinged joints near the 
 objective of a small telescope, so that any one of the three can be set to reflect rays 
 axially along the telescope from directions at 90°, 88° 51’ 15” and 74° 53’ 15” 
 respectively. The first prism is used to set off the base at right angles to the 
 direction of the distant object. The observer paces in the line of the base until, 
 using one of the other two prisms, he again sees the object by reflection. For 
 both observations the telescope is directed to view another distant point at right 
 angles to the target, 7.e. in the direction of the base. The number of paces taken 
 between the two observations, multiplied by a constant depending upon the length 
 of pace and the reflecting angle of the second prism used, gives the required range. 
 The constant is determined experimentally. Inv. IQII-I51. 
 
 75. GAUTIER’S POCKET TELEMETER. Made by Tavernier- 
 Gravet, Paris. 
 
 This rangefinder was designed by Capt. Gautier, a French artillery officer, 
 about 1865, and has been extensively used in the French army. Its main feature 
 is a thin prism mounted in a ring at the object end of the sighting tube. By 
 rotating the ring the deviation of rays in the plane of the distant object and the 
 horizontal through the instrument can be varied continuously, and the ring 
 calibrated correspondingly. 
 
 The sighting tube is about 5 in. long by r-5 in. diam. and has a protecting tube 
 which can be fitted underneath to serve as a hand-grip when observations are being 
 made. The sighting tube, in addition to the movable ring containing the prism, 
 has a pair of plane mirrors having their faces inclined to each other at an angle of 
 about 45°, and so inclined to the optical axis that rays incident through a window 
 in the side of the tube in a direction nearly perpendicular to the axis, are doubly 
 reflected at the mirrors into the line of the axis, and pass through a small telescope 
 at the eye end of the tube. In the instrument shown, two such telescopes of 
 different powers are fixed side by side. One of the mirrors can be slightly rotated 
 so as to vary the angle of deviation of the reflected rays. 
 
 In using the instrument, the observer sets the range scale on the movable ring 
 at infinity and the index of the movable mirror at the centre. He views the 
 distant object by double reflection at the mirrors and looks directly through the 
 tube and prism for a convenient aiming mark. If such a mark is not quite co- 
 incident with the image of the distant object, he rotates the mirror until co- 
 incidence occurs. Then the object and the aiming mark are in directions nearly 
 perpendicular to each other. Next he paces out a convenient distance in the direc- 
 tion of the aiming mark and takes a second observation similar to the first, but this 
 time effects the coincidence by means of the movable prism. He reads off the 
 scale calibrated on this ring when coincidence is effected and multiplies the distance 
 so read by the length of his base. The result is the range of the distant object. 
 The movable ring is really calibrated to record the reciprocal of the angle of devia- 
 tion caused by the prism, which is the same as the reciprocal of the angle subtended 
 
i 
 
 by the base at the object. As this base has been made nearly perpendicular to the 
 direction of the object, the product of its length and the reciprocal of the angle 
 gives the range. Thus any convenient base length can be used subject to the 
 condition that it subtends an angle of not more than 3° at the distant object. 
 (See ‘‘ Notice sur le Télémétre de Poche,”’ by A. Gautier, Paris.) 
 Inv. 1910-126. 
 
 76. MEKOMETER, GENERAL SERVICE PATTERN. Made 
 and lent by T. Cooke and Sons, Ltd., York. 
 
 This standard rangefinding set is used by infantry units. It comprises two 
 optical sighting instruments, known as the “ Right Angle’? and “ Reading ”’ 
 instruments, and two reels of cord, each 25 yd. long, for setting off the base. 
 The observer at one end of the base uses the “‘ Right Angle ” instrument to make 
 the base line perpendicular to the line joining his position to the distant object, 
 after which the other observer uses his ‘“‘ Reading ’”’ instrument to measure the 
 angle between the base line and the direction of the object from his position. 
 
 The Right Angle instrument has two fixed vertical mirrors inclined to each 
 other at an angle of 45°. The observer using this sees the target directly and the 
 vane on the Reading instrument after double reflection at the two mirrors. He 
 adjusts his position, keeping the base line fully extended, until the two images 
 coincide. 
 
 The Reading instrument differs only in that one of its mirrors can be rotated, 
 and the rotation recorded upon a graduated drum, so that the images of the target 
 (viewed directly) and the vane of the Right Angle instrument (viewed by double 
 reflection on the two mirrors) can be made to coincide. The graduated drum is 
 calibrated to record the range directly. 
 
 (See ‘‘ Handbook of the Mekometer.’’) Inv. 1914-293. 
 
 77. ADIE TELEMETER. Presented by the Admiralty. 
 
 This type of rangefinder was designed by P. Adie in 1860, and was extensively 
 used by the Admiralty until superseded by the Barr and Stroud pattern. The 
 example shown was used on H.M.S. “ Triton’ from 1885 to 1904 during the © 
 survey of the East Coast of England. 
 
 The rangefinder consists of two systems of lenses and right-angled reflecting 
 prisms mounted in a 3-ft. metal tube of square section. Each system forms a 
 “broken ’’ telescope, the objectives being at each end of the tube, along which 
 the rays, originally perpendicular to it, are deflected by right-angled prisms to 
 the centre, where they are again reflected at right angles into the common central 
 eye-piece. in this manner two images of a distant object whose direction is nearly 
 perpendicular to the axis of the tube are obtained, one through each system. 
 These images will be separated laterally by an interval which diminishes con- 
 tinuously as the range increases, but does not become zero until the object is at 
 an infinite distance. If the objective lens and two prisms of one system are 
 separately mounted and can be rotated with respect to the other system, the 
 two images may be made to coincide by such a rotation, and the angle of rotation 
 necessary will vary inversely as the range of the object. This angle can be 
 measured and the range obtained by calculation, or a table of corresponding 
 values. In the instrument exhibited such an arrangement is effected by mounting 
 one object lens and its pair of reflecting prisms in a separate tube within the main 
 tube containing the other set, and pivoted to this so that the rotation can be 
 effected by a screw threaded in the outer tube with its end pressing against the 
 inner. The angle of rotation is measured by a vernier on one tube and a scale 
 on the other to the nearest second of arc, and the range read off on a table fixed 
 to the outer leather protecting cover. 
 
 (See British Patent, 1860, No. 357; and 1863, No. 608.) Inv. 1909-139. 
 
 78. BARR AND STROUD INFANTRY RANGEFINDER. 
 (Early Type.) Lent by Messrs. Barr & Stroud, Lid. 
 
 This instrument was made in 1889, and is one of the first rangefinders con- 
 structed by Messrs. Barr & Stroud for use by infantry. It is a single-observation, 
 coincidence instrument, with a fixed base of 80 cm., and measures ranges between 
 200 and 2,000 yd. with a percentage error directly proportional to the distance. 
 
 The instrument is optically equivalent to a pair of telescopes having a common 
 central eye-piece and their axes in opposite directions along the base-line, at the 
 ends of which are the two objective lenses. Two pencils of rays diverging from 
 the distant object to each end of the instrument are reflected at right angles by 
 mirrors so as to traverse the objectives and form two images at the centre, where 
 
32 
 
 they are again reflected at right angles by a pair of mirrors, one of which is / 
 parallel to each end mirror. The two images are thus viewed in the focal plane 
 of the right eyepiece of the instrument, and are separated laterally by a distance / 
 inversely proportional to the required range. Between the right objective lens/ 
 and the central mirrors there is a small angled deflecting prism, which can move 
 along the axis, and in so doing increases or decreases the amount of separation 
 of the two images. In one position of the prism, corresponding to the distance 
 of the object, the two images coincide. A scale attached to the prism movés 
 with it and is graduated so as to give the range directly. This scale and its fixed 
 index-line are viewed by the observer’s left eye in a separate eyepiece, which also 
 serves in conjunction with a field lens as a small telescope of large field for picking 
 up the target. 
 
 In using the instrument the observer picks up the target with his left /eye, 
 turns the pinion actuating the deflecting prism with his right hand until the two 
 images of the object in the right eyepiece coincide, and reads off the desired range 
 with his left eye. 
 
 (See “‘ The Modern Rangefinder,’ by Prof. F. J. Cheshire. ‘‘ Thepry of 
 Modern Optical Instruments,’’ Appendix on Rangefinders, by Gleichen. British 
 Patent No. 9520-1888.) Inv. 1914-180. 
 
 79. BARR AND STROUD RANGEFINDER ON NAVAL 
 MOUNTING. Lent by Messrs. Barr & Stroud, Ltd. 
 
 This instrument was used by Dr. Bruce in the Weddell Sea Antarctic expedition 
 on the “ Scotia,’’ 1902-04, and afterwards by Capt. R. F. Scott, R.N., on the 
 “Terra Nova ’”’ in the British Antarctic expedition I9I10~-13. 
 
 The rangefinder is of the F.A.2 type, base 4 ft. 6in., mounted on a type N.M. 
 naval mounting which permits the tube to be rotated in azimuth and altitude. 
 Its optical system is functionally similar to that of the adjacent early infantry 
 type, but special reflecting prisms replace the mirrors there employed. The two 
 images formed in the central eyepiece are erect, and are separated by a fine 
 horizontal line bisecting the field. Coincidence of these images is effected by 
 moving a deflecting prism to which is attached a scale whose lateral motion is 
 measured and indicates the required range as in the earlier instrument. The 
 scale can be illuminated for night reading, when a cylindrical lens astigmatiser 
 is put into action and serves to draw out the images of a luminous point into fine 
 vertical streaks suitable for effecting coincidence. The instrument can be adjusted 
 at a known range by “ halving’’ and “coincidence ’’ fittings, and will then 
 read all ranges correctly. 
 
 The approximate uncertainty of measurement varies from 2 yd. in 1,000 to 
 200 in 10,000, the percentage error increasing directly with the range. The 
 magnification of the main telescope system is 26 diam., its angular field of view 
 1° 20’ horizontally and 0° 40’ vertically. The finder telescope magnifies 2:5 diam. 
 and has a 7° field of view. 
 
 (See “‘ The Modern Rangefinder,’’ by Prof. F. J. Cheshire. ‘“‘ Theory of Modern 
 Optical Instruments,” Appendix on Rangefinders, by Gleichen.) Inv. 1914-179. 
 
 80. FORBES’ STEREOSCOPIC RANGEFINDER. Lent by 
 Prof. G. Forbes, F.R.S. 
 
 Prof. Forbes designed this type of rangefinder in 1901, for use by infantry 
 in measuring ranges up to 3,000 yd. The power of the eyes to “ converge ”’ to 
 a varying degree upon objects at different distances, and to measure distances 
 by the muscular effort so required, is limited normally to small distances of a 
 few yards. Consequently, in this, as in all stereoscopic rangefinders, optical 
 means are employed to bring the image of the object ranged upon within the 
 limit of normal eye convergence. The use of an optical base, whose function is to 
 increase the inter-ocular distance from 2-5 in. to 72 in., and a pair of binoculars 
 of magnification 8 diam. enables the radius of stereoscopic vision to be increased 
 240 times. 
 
 In this instrument, the optical base consists of two pairs of doubly reflecting 
 pentagonal prisms, mounted one at each end and two at inter-ocular distance 
 apart in the centre of a metal tube, which is hinged in the centre so as to fold 
 into two portions each 38 in. long. When in use these two portions are clamped 
 to form a single tube with the end prisms 72 in. apart. The binoculars are fitted 
 with their objectives opposite the centre prisms, there being windows in the tube 
 opposite all the prisms. After double reflection at the end and centre prisms, an 
 image of the distant object is formed in the focal plane of each objective of the 
 binoculars, in which plane there is a glass plate carrying a photograph of a 
 
33 
 
 balloon. The distance between the two balloon images is varied by a micrometer 
 screw. If the observer combines these balloon images stereoscopically into one, 
 and does the same with the images of the distant object, he can judge the 
 apparent distances of the two images, and turn the screw until these distances 
 coincide. The rotation of the screw thus serves as a measure of the required 
 range, and this can be read off directly on a circular scale, attached to the 
 screw and suitably graduated. 
 
 With practice, an observer having good stereoscopic vision can determine 
 ranges with a rapidity and accuracy comparable with that of a coincidence 
 rangefinder, and with the additional advantage for military purposes that the 
 object need not be seen very distinctly. Inv. 1920-442. 
 
 TACHEOMETRY. 
 
 The method of tacheometry consists in obtaining, with only one 
 “ setting ”’ of a theodolite specially adapted for rangefinding, the three 
 polar co-ordinates—distance, azimuth and altitude—of every distant 
 point which is visible and accessible from the observing station. Thus 
 the positions of all such points with reference to the observing station 
 as origin are fully determined, without any direct measurement of 
 length, both rapidly and accurately enough for engineering, recon- 
 naissance and topographical surveys. 
 
 By setting up a staff at the distant point, and taking the altitudes 
 and azimuths of two points on it at a known distance apart, it is 
 possible to use any ordinary theodolite for finding the required distance 
 of the staff from the observing point. The calculation of this distance 
 from the data is, however, somewhat laborious, so that a tacheometer 
 is a theodolite fitted with special devices both for accurately estimating 
 the small angles involved and for reducing the necessary calculations. 
 Generally, the staff is set up in the vertical plane through the line 
 joining the observing station to the distant point, and is set either 
 vertically or perpendicularly to this line. The two points on the staff 
 may be at a constant distance apart, or the angle subtended by these 
 points at the observing station may be constant. In the former case 
 the angle subtended varies, whilst in the latter case the length cut off 
 upon the staff varies with the distance, and consequently in this case 
 the staff is graduated. The angle-measuring device special to the 
 tacheometer may take the form of a microscope mounted so as to 
 make a fixed angle with the telescope, and rotate with it about a 
 common transverse axis (see No. 83); or may consist of a special 
 diaphragm fitted at the focal plane of the object glass of the telescope. 
 This diaphragm contains two parallel lines—defined by metallic fila- 
 ments, hairs, spider’s web, or some similar device—and the angle 
 subtended by the interval thus defined at the optical centre of the 
 object glass is equal to the angle subtended by the known length on 
 the staff at the same point. The interval at the diaphragm is accur- 
 ately measured by a micrometer-screw—or is made constant—and the 
 focal length of the object-glass being known, the distance of the staff 
 from the object-glass can be easily expressed in terms of the three 
 known quantities—diaphragm-interval, focal length, and staff-interval. 
 
 The tacheometric-diaphragm was first introduced into the Keplerian 
 telescope by a young English astronomer named Gascoigne, in 1639. 
 Gascoigne fitted into his telescope a movable micrometer diaphragm, 
 made of metal threads whose distance apart could be calculated to 
 1/400th inch and varied by a micrometer screw, and used it for 
 measuring astronomical and terrestrial distances. His device was not, 
 
34 
 
 however, made known generally until 1667, and meanwhile a similar 
 appliance had been invented and used on the continent for astronomical 
 purposes. Hooke suggested the substitution of hairs for the metallic 
 filaments in the diaphragm, and this was carried out in 1674 by 
 Montanari, who used his instrument for terrestrial work. He used 
 12 to 15 hairs and a vertical staff of constant length. About a century 
 elapsed, however, before the method became at all familiar to surveyors 
 by the publication of a book on surveying by William Green, in 1778. 
 Green described an instrument constructed by him and used for 
 measuring distances on the same principle as the tacheometer, but 
 called by Green a stadiometer. A similar contrivance had been 
 employed by James Watt in surveys for the Tarbert and Crinan Canals, 
 in 1771. About the same period Brander employed a diaphragm 
 constructed of lines engraved on glass by means of a diamond, and 
 applied it specially to the sight rule of his plane-table. These methods 
 were rarely used, however, until Porro, in 1823, designed his “ anallatic”’ 
 telescope, which reduced the calculations hitherto necessary for 
 tacheometry. Many instruments were constructed which embodied 
 Porro’s principle of the anallatic lens (see No. 81), the most famous 
 being the “‘Cleps’’ tacheometer, made in 1867. Since that time, 
 several mechanical appliances have been designed with the object of 
 reducing calculation to a minimum, but many surveyors still prefer 
 to use an ordinary theodolite whose telescope is fitted with “ stadia ”’ 
 lines, in conjunction with a graduated staff for tacheometric purposes, 
 and to perform the necessary calculations with the aid of tables. 
 
 81. TACHEOMETER. Made by Troughton & Simms. 
 
 A Porro “ anallatic’’’ lens is inserted between the objective and eye lenses 
 of this instrument and its horizontal and vertical circles are divided centesimally. 
 It is used in conjunction with a graduated tacheometric staff, upon which the 
 interval subtended between the fixed cross wire varies directly as the distance 
 of the staff from the transverse axis of the instrument, which is itself vertically 
 above the observing station point. 
 
 In the case of an ordinary telescope, the interval subtended by the fixed 
 diaphragm wires upon the staff varies directly with the distance of the staff from 
 the first principal focus of the object lens. Thus, to get the distance from the 
 station point, a constant equal to the distance between this focus and the station 
 point, or the transverse axis of the instrument, must beaddedinevery case. Thisis 
 unnecessary when an anallatic lens is used, for this lens is so placed that the trans- 
 verse axis is the conjugate point of its first principal focus with respect to the ob- 
 jective lens, and is thus the first principal focus of the combination of the two 
 lenses, 
 
 In the diaphragm of this instrument there are five horizontal wires and one 
 vertical wire, so that a pair of cross wires may be chosen to suit the magnitude 
 of the distance measured. 
 
 The focal length of the objective lens is 9-5 in., that of the anallatic lens being 
 6in. Theeye-piece is of the Ramsden type, and can be traversed vertically through 
 a small distance relative to the telescope. The anallatic lens is fitted in a separate 
 tube inside the telescope tube carrying the objective lens, and can be adjusted by 
 a rack and pinion arrangement actuated by a special key. The divided circles are 
 4 in. diameter and are read by verniers to centesimal minutes. Inv. 1890-55. 
 
 82. GENTILLI’S TACHEOMETER. Made by Starke & Kam- 
 merer, Vienna. 
 
 To convert a theodolite into a tacheometer for measuring distances, many 
 devices have been proposed which permit the telescope, when directed upon a 
 graduated staff, to be displaced through a constant small angle, whose limits 
 are determined by two mechanical stops. The distance so measured on the staff 
 will vary directly as the horizontal distance of the staff from the observing station 
 
35 
 
 and inversely as the square of the cosine of the altitude angle. As the latter is 
 known, the former can be calculated by using special tables. In the tacheo- 
 meter invented about 1868 by A. Gentilli, the Austrian engineer, a rigid vertical 
 arm can be clamped to the transverse axis of the telescope in any position. The 
 lower end of this arm can only move a short distance, being limited by two mechan- 
 ical stops. Consequently the vertical angle through which the telescope can move 
 when clamped to the arm is likewise limited. 
 
 The instrument in other respects is like an ordinary theodolite of the type made 
 by Starke about this period. It has a 5-in. horizontal circle and a 3° 5-in. vertical 
 quadrant, both reading by vernier and reading microscope to 30” and having 
 the usual clamps and tangent screws. The telescope has an objective lens of 12 in. 
 focal length and 1-5 in. aperture. The instrument has four levelling screws, two 
 small spirit levels mutually perpendicular on the horizontal circle, and a more 
 sensitive one on the telescope. Inv. 1876-1205. 
 
 83. ECKHOLD’S OMNIMETER. 
 
 An early example of a theodolite modified to enable distances to be measured 
 with it simultaneously with azimuth and altitude angles is here represented in the 
 ‘*Omnimeter,’’ invented by C. A. C. Eckhold in 1868. This instrument is used 
 in conjunction with a special tacheometric staff having a fixed interval, usually 
 10 ft., marked upon it. A microscope, rigidly attached with its axis perpendicular 
 to that of the observing telescope, rotating with this telescope about a common 
 transverse axis, Measures upon a scale fixed horizontally upon the upper plate 
 of the theodolite the distance traversed when the observing telescope is rotated from 
 the upper mark to the lower mark of the staff. As this staff is always set up 
 vertically, its horizontal distance from the centre of the instrument is inversely 
 proportional to the distance traversed on the scale. 
 
 The microscope scale is mounted parallel to the telescope at a distance of 
 exactly 6 in. below the transverse axis. It is 4 in. long and is graduated to read 
 0:02 in. directly, whilst it can be moved on the plate, parallel to its own length, 
 through a small interval by means of a micrometer screw with a graduated drum, 
 which measures 0-002 of each division of the scale. In this manner the ultimate 
 graduation of the scale corresponds to 4 x Io-° in. so that, if 7 such divisions are 
 traversed when the theodolite rotates from the upper to the lower mark of the 
 staff, the distance of this staff from the instrument will be 15 x 10° —mn ft. In 
 other respects the instrument is similar to an ordinary transit repeating theodolite. 
 It has 5-in. vertical and horizontal circles reading by vernier to single minutes. 
 The focal length of the telescope is II in., and its aperture 1-25 in. Itis fitted with 
 a Ramsden eyepiece. Slow-motion tangent screws are provided for each circle, 
 and the usual spirit-levels on the plate and telescope. 
 
 A diagonal eyepiece is fitted to the microscope for convenience in taking steep 
 sights. Inv. 1900-90. 
 
 84. PHOTOGRAPHS OF JEFFCOTT DIRECT-READING 
 TACHEOMETER. Made and lent by Messrs. Cooke, Troughton & 
 Simms, Ltd. 
 
 Ordinary tacheometers are designed for horizontal sights, and the readings 
 have to be corrected by computation if the sight is inclined, whilst a further 
 calculation is necessary to determine the difference in height between the observed 
 and observing stations. Many devices have been introduced in the design of 
 tacheometers with the object of rendering such calculations unnecessary, and 
 enabling the distance and altitude to be read off directly, whether the line of sight 
 is horizontal or inclined. The tacheometer illustrated embodies Prof. H. H. 
 Jeffcott’s arrangement for rendering the readings independent of the inclination, 
 by varying the diaphragm-interval proportionally with the inclination. <A 
 similar arrangement of varying diaphragm-interval enables the height also to be 
 obtained by reading the staff-interval intercepted. 
 
 The diaphragm has one fixed and two movable horizontal platinum-iridium 
 pointers, the former for collimation and the latter to limit the intervals for height 
 and distance readings respectively ; the distance pointer is on the same side as 
 the collimation pointer, and the height pointer on the opposite side. Each movable 
 pointer is fixed on the end of a pivoted lever, of which the other end is pressed 
 against a suitably shaped cam, which is rotated by the movement in inclination 
 ot the telescope, so that the distance of either movable pointer from the fixed one 
 is suitably varied to make the interval intercepted upon the distant staff exactly 
 one-hundredth of the range and one-tenth of the height respectively. The cams 
 are made to suit elevations and depressions up to 30°, and are mounted inside 
 a protecting case at one end of the transit axis of the telescope. 
 
36 
 
 The instrument in other respects resembles the ordinary tacheometric theodo- 
 lite. Ithasa5-in. horizontal circle, graduated on silver to 20 minutes and read by two 
 verniers; one coarse vernier for ordinary use reads to I minute whilst the other, 
 reading to 20 seconds, is only used in adjusting or testing theinstrument. No vertical 
 circle is supplied, unless specially desired. The telescope is provided with an 
 anallatic lens, an objective of 1-65 in. aperture, and a screw-focussing, inverting 
 eyepiece of power 18. The eyepiece is mounted in a movable slide for reading at 
 extreme angles. Three bubble-levels are provided, one on the cover-plate, another 
 above the telescope, and the third on the control arm of the cams; this last is 
 particularly sensitive, having a bubble shift of 2 mm. per 15-20 seconds of 
 elevation. 
 
 (See Brit. Patent Spec. No. 18721 of 1912. Transactions of the Institution of 
 Civil Engineers of Ireland, Jan. 6th 1915.) Inv. 1925-7 
 
 85. TACHEOMETRIC STAFF. Made by Troughton & Simms. 
 
 This is a 4-metre wooden staff, graduated in centimetres and used in 
 conjunction with the tacheometer, Cat. No. 81. It is inextensible, and does not 
 materially differ from a levelling rod. 
 
 The graduation marks are black bands, I cm. wide, at intervals of 1 cm. 
 Those corresponding to metres and decimetres extend right across the face, the 
 rest only half across. The metres are numbered by large red numbers and the 
 letter M, the decimetres by smaller black numbers. The half-decimetre points 
 are marked by small black arrows. 
 
 The rod has a rectangular section, 3 in. wide by 1}in. deep, and the face is 
 slightly sunk below the edges to prevent abrasion of the graduations. 
 
 Inv. 1890-55. 
 
 MEASUREMENT OF ALTITUDE. 
 
 The task of determining the relative altitudes of two places was 
 one of fundamental importance in the earliest days of civilization. 
 It arose primarily in connection with the vast irrigation projects of 
 which traces still remain, but was also important in architectural and 
 military work. For the solution of this problem we find both direct 
 and indirect methods employed in the earliest times, as in our own 
 day. Where both the places were readily accessible, the direct 
 method of levelling was customary; where only one of the places 
 could easily be occupied recourse was had to indirect or trigonometrical 
 methods, which involved the measurement of the distance of the second 
 place from the first, and the determination of the angle of elevation 
 of one above the other. Thus Vitruvius, in his treatise on architecture, 
 refers to the employment of the “ chorobates ’’—a combined plummet 
 and water-level—for setting out aqueducts, whilst Hero of Alexandria, 
 in his work on the “ dioptra,”’ describes how this instrument could be 
 used as a water-level for directly measuring altitudes and also as a 
 sighting instrument for their indirect determination by angular measure- 
 ments. As early as the 6th century B.c., Thales of Miletus is reported 
 to have calculated the heights of mountains by measuring the lengths 
 of shadows cast by them and comparing these with the shadow cast 
 at the same time by a vertical rod of known height. 
 
 For some 2,000 years little change took place in the classical 
 methods of altitude measurement, except that instruments for angular 
 measurement were provided with graduated scales which facilitated 
 the necessary calculations. In the 16th and 17th centuries, the normal 
 surveying instrument was adapted to the measurement of angles in 
 any plane, and could be as readily employed for measuring altitude 
 angles as for determining horizontal ones. Occasionally at this period 
 we find the sight-rule or diopter provided with a movable vane on the 
 
SY, 
 
 foresight to facilitate altitude measurements, whilst in other instru- 
 ments the altitude quadrant could be attached to or detached from 
 the remainder of the instrument, which was specially arranged for 
 measurements in horizontal planes only. The period is characterized 
 by the use of indirect methods of measurement rather than the more 
 precise, but more laborious, direct methods, such as levelling. In the 
 latter part of the 17th century, however, when large aqueducts and 
 canals were constructed in France, the necessity for more precise 
 determinations of relative altitudes led to the design of elaborate levels 
 provided with telescopes for sighting over large distances. The line 
 of sight in these instruments was set horizontal by means of a plummet, 
 of which the heavy telescope itself formed a portion of the weight 
 (see No. 88). Levels of this ‘ pendulum ” type were constructed by 
 Picard, Huygens and others. De la Hire, on the other hand, devised 
 an elaborate water-level for use in such work. 
 
 In 1661 M. Thévenot described an appliance, the air or bubble- 
 level, destined to revolutionize the construction of levelling instru- 
 ments. This invention remained in obscurity for many years, owing 
 principally to the difficulty of manufacture of accurate bubble-tubes. 
 In 1702, however, A. M. Mallet, in his ““ Géometrie pratique,”’ describes 
 several air-levels, or spirit-levels as they are now called, and his book 
 shows that in France, at any rate, the new appliance was gaining 
 ground over the cumbersome plummet and water-levels. In the 18th 
 century, it rapidly superseded these latter, and the bubble-tube 
 became an essential adjunct to the telescope of the theodolite, which 
 rendered that instrument capable of acting at will as a pure level, or 
 of measuring altitude angles for indirect levelling. It was not until 
 the beginning of the 19th century that surveying instruments designed 
 purely to perform the functions of a level, but not those of a theodolite, 
 came into general use, principally in canal and railway engineering 
 surveys. Such instruments had rarely a divided circle, but were 
 usually provided with a compass for measuring azimuth angles. They 
 had very powerful telescopes, specially designed for taking short sights, 
 and particularly sensitive bubble-tubes. Many improvements in the 
 design of such levels have been made during the roth and zoth centuries, 
 with the view of ensuring greater accuracy and power without increasing 
 their size, and of facilitating the necessary instrumental adjustments. 
 Most modern levels are provided with tacheometric or stadia dia- 
 phragms in their telescopes which permit them to measure distances 
 at the same time as the altitudes are read off. 
 
 The earlier staves used with direct levelling instruments were 
 provided with movable sight-vanes, the position of which on the 
 graduated rod could only be read by the stafi-bearer (see No. 122). 
 With the increasing power of telescopes, it became possible for the 
 observer to read the graduated rod distinctly, and the movable vane 
 became superfiuous. | 
 
 For trigonometrical levelling, which is more rapid but less accurate 
 than direct levelling, the modern theodolite with its divided vertical 
 circle and bubble-tubes is well adapted. Where still less accuracy is 
 sufficient, and where other methods of altitude determination are 
 impracticable, heights can be approximately obtained by means of 
 barometers and hypsometers, the action of which depends upon the 
 diminution of atmospheric pressure with increasing altitude. 
 
 In 1648, five years after the invention of the barometer by Torricelli, 
 this instrument was employed by Pascal in the Puy de Dome area to 
 
38 
 
 investigate the diminution of atmospheric pressure as the altitude 
 increased. The exact law of variation was not properly established, 
 however, for another I50 years. Gay-Lussac and Fortin devised 
 important modifications of the mercury-barometer to adapt it specially 
 to the determination of altitudes, but the invention of the aneroid 
 type of barometer by Vidie in 1847 provided a much more portable 
 instrument for the approximate measurement of heights. 
 
 The hypsometer, or boiling-point thermometer, which depends for 
 its action on the lowering of the boiling-point temperature with 
 diminishing atmospheric pressure and therefore with increasing altitude, 
 was first devised by Wollaston in 1817 (see No. 131). Modified forms 
 are often employed to supplement aneroid readings in modern 
 exploratory surveys of mountainous regions. 
 
 PLUMMET LEVELS AND CLINOMETERS. 
 
 86. PHOTOGRAPH OF TWO ANCIENT EGYPTIAN LEVELS. 
 Presented by Captain H. G. Lyons, F.R.S. 
 
 The original levels, which are of the A-shaped plummet type, are exhibited 
 in the Cairo Museum and.are of considerable antiquity. Such levels were used by 
 builders and engineers of ali the early civilizations and have survived in use to the 
 present date without fundamental modification. 
 
 Each level consists of a wooden frame in the shape of an isosceles triangle, 
 having its sides equally prolonged beyond the base. The ends of the legs are cut 
 parallel to the cross piece, so that the frame may rest upon a flat surface. A 
 plummet is suspended from the apex of the frame, and its cord passes over the 
 front of the cross piece, in the middle of which is engraved a vertical mark. If the 
 cord lies along this mark when the plummet hangs freely, the surface on which the 
 level rests is horizontal. 
 
 Hieroglyphics are engraved in the face of the lower level. Inv. 1914-219. 
 
 87. INSTRUMENT , FOR ASCERTAINING HEIGHTS. Pre- 
 sented by T. H. Court, Esq. 
 
 This consists of an isosceles, right-angled metal triangle which can be freely 
 suspended by means of two brass pins so that its two equal sides are vertical and 
 horizontal respectively. All objects in the prolongation of its third side will then 
 be at a horizontal distance from the instrument equal to their height above it. 
 The former distance can be measured, and the height thus ascertained. The 
 hypotenuse of the metal triangle has a pair of sights which enable objects in its 
 line to be viewed. 
 
 When using the instrument, the observer takes up a position from which he can 
 sight the object whose height is required, the instrument being freely suspended. 
 He then measures his horizontal distance from a point vertically beneath the 
 object. This distance is equal to the height of the object above the eye-level of 
 the observer. 
 
 When not in use, the instrument fits in the wooden case exhibited with it. 
 
 Inv. 1917-39. 
 
 88. EARLY BALANCE LEVEL. Lenteby T. H:' Court; Esa: 
 Plate Il, Fig. 1, p. 38. 
 
 This is a level of a type much used for rough levelling during the 17th century, 
 and probably dates from the latter half of that period. It consists of a heavy 
 tube freely suspended from the top of a wooden case by a thread attached to 
 its centre. The tube has a horizontal slit at one end, and a corresponding cross 
 wire at.the other; glass windows in the ends of the wooden case permit a free 
 view along the sights. The instrument as a whole can be rotated in azimuth 
 upon its stand. ; 
 
 When not in use, the sighting tube can be fixed by means of two screws 
 passing through the bottom of the wooden case. 
 
 In this instrument, the centre of gravity of the suspended tube is very little,. 
 if any, below the line of sight. Hence the tube is very sensitive to dis- 
 turbances, which is a source of trouble. Moreover, there is no easy method 
 
PLATE IT. 
 
 Fig. 1. Early Balance Level, p. 38. 
 
 Fig. 2. Spirit Level, by Cary, p. 40. 
 
39 
 
 of adjusting the line of sight to be truly horizontal. Owing to these defects, 
 both of which were remedied in the more precise instruments on the “‘ balance ”’ 
 principle designed by Huygens, this type of level is not very accurate. 
 
 The body of the tube is made of lead, the ends containing the sights being of 
 brass. The wooden case is screwed into the top of a small column, provided 
 with a brass socket at its other end. The socket serves to attach the level to 
 its stand, which may be a tripod or simply a “ stick ’”’ stand. 
 
 Inv. 1913-258, S.M. 951, L.S. 
 
 89. PLUMMET CLINOMETER. 
 
 This instrument was made in the late 17th or early 18th century, and consists 
 of a brass alidade attached to a semi-circle, pivoted upon a heavy stand, which is 
 set with its base horizontal by means of a small brass plummet. The semi- 
 circle and alidade can be rotated in the vertical plane, and the inclination of the 
 line of sight read by a fixed index on the stand. 
 
 The semi-circle is graduated in degrees and has a diameter of 4:2in. The 
 sight rule is a hollow tube fitted with dual sights at each end, one set being 
 within the tube and the other on top of it in the opposite sense. 
 
 Inv. 1883-118. 
 
 90. PLUMMET LEVEL AND CLINOMETER. Made by Jacob 
 Kranewetter. 
 
 This is a German instrument made in 1755, and consists of a hollow wooden 
 stand with a flat base and brass facings. A brass plummet is suspended down 
 one face, and by its position on a scale of degrees engraved on the lower part 
 of the face measures the inclination of the base of the instrument. No sights 
 are provided. A brass knob, screwed into the top of the stand can be detached 
 and used as a plummet; a cord wound round a drum inside the stand is used 
 to suspend the knob. 
 
 The plummet has a length of 4-8 in., which is the radius of the graduated arc. 
 This extends:15° on either side of the vertical. Inv. 1880-201. 
 
 91. BALANCE LEVEL. Made by Susse Bros., Paris. 
 
 This instrument can be used for testing whether a surface is horizontal, _ 
 vertical or inclined at 45°. 
 
 It consists of a loaded disc, pivoted between the vertical faces of a cylindrical 
 brass case fixed to a brass ruler. Marks engraved on the edge of the revolving 
 disc show by their coincidence with marks on the fixed case whether the ruler, 
 which is placed on the surface to be tested, is horizontal, vertical, or inclined at 
 45°. The ruler has a length of about 6 in., and one of its bevelled edges is graduated 
 in millimetres. The disc has a diameter of 2 in. Inv. 1857-25. 
 
 2K EE LECTING “PLUMMET LEVEL. Presented byIeeH. 
 Court, Esq. 
 
 This type of level was invented by Col. Burel in 1829, and operates upon the 
 principle that in a vertically suspended mirror the eye sees a reflected image of 
 itself at its own level. 
 
 The instrument consists of a rectangular frame, suspended in gimbal bearings 
 so as to hang vertical owing to its own weight. The frame contains a piece of 
 plane glass, of which one vertical half is silvered and the other half left free for 
 direct vision of a distant object. A horizontal cross wire bisects the two parts. 
 The observer holds the instrument in front of his eye and moves it until he sees 
 the reflected image of his eye bisected by the cross wire. Any distant object 
 then intersected by the wire will be at the level of his eye. 
 
 (See ‘‘ Memorial de l‘Officier du Génie,”” No. 10, 1829.) Inv. 1911-183. 
 
 93. REFLECTING PLUMMET LEVEL. Presented by T. H. 
 Court, Esq. 
 
 This is very similar to the adjacent Burel type, but has a heavy screw threaded 
 in the base of the frame. By working the screw the inclination of the plane of 
 the mirror can be varied, and adjusted to be truly vertical by means of a test 
 sight. The mirror also occupies much more than half of the frame, leaving a 
 small part only for direct vision. Inv. 1919-331. 
 
40 
 
 94. DE LISLE’S REFLECTING CLINOMETER. Made and 
 lent by T. Cooke & Sons, Ltd., York. 
 
 Of the many arrangements for converting a Burel reflecting level into a 
 clinometer, the one here represented, devised by General A. De Lisle, R.E., is 
 the most satisfactory. 
 
 A heavy arc is attached to the lower part of the mirror frame and can be turned 
 into the same plane as the mirror for convenience of carriage, or into a perpen- 
 dicular frame for measuring angles of elevation or depression. In the latter 
 position the weight of the arc causes the mirror plane to take up an inclination 
 to the vertical, which can be varied by means of a movable radial arm, concentric 
 with the arc and carrying a sliding counterpoise at its extremity. The arc is so 
 graduated on its edge that the position of one edge of the movable arm gives the 
 inclination of the mirror, and, therefore, of the line of sight between the observer’s 
 eye and its reflection in the mirror. The arc is graduated to read gradients from 
 zero to I in 5. Inv. 1921-519. 
 
 SPIRIT LEVELS AND CLINOMETERS. 
 
 95. SPIRIT LEVEL. Made by Troughton & Simms. Lent by 
 the Royal Society. 
 
 This level was constructed in the early part of the 19th century and used in 
 1831 by J. A. Lloyd, F.R.S., in ascertaining the differences of level between the 
 Thames at London Bridge, Sheerness and various intermediate points. The 
 telescope has a focal length of 28 in. and is finally levelled by means of a I2-in. 
 spirit level mounted at one side parallel to its axis. 
 
 The objective lens of the telescope has a 2-in. aperture, and is fixed in the 
 tube, the eyepiece tube being focussed by a rack and pinion arrangement and 
 having a fixed diaphragm and a slide focussing Ramsden inverting eyepiece. 
 The telescope is attached to a transverse horizontal axis, which rests in Y-s on 
 standards fixed to the horizontal plate. The telescope is supported near both 
 ends by vertical screws threaded in the ends of a rigid bar fixed to the horizontal 
 plate parallel to the axis of the telescope. These screws are used in the final 
 “adjustment of the telescope axis, which is tested by the main spirit level. This 
 is suspended from the ends of a metal rod fixed on one side of the telescope, 
 parallel to its axis, and has ivory scales graduated to thirtieths of an inch mounted 
 above its bubble-tube. A small level is also fixed transversely at one end of the 
 main level. The bubble of the latter moves over one division of the scale when 
 the telescope is elevated about 2 seconds of arc. The horizontal plate can rotate 
 about a vertical axis, and has a clamp and tangent screw for slow motion. It is 
 attached to a pair of parallel plates, connected by four levelling screws of the usual 
 type. Asmall circular spirit-level is fixed to the plate above its centre for setting * 
 it horizontal. 
 
 The telescope can be reversed in its Y-s end for end, or rotated through 
 180° about its own axis, as in all Y-levels. 
 
 (See Phil. Trans. Roy. Soc., 1831.) Inv. 1900-155. 
 
 96. SPIRIT LEVEL. Made by Cary. Lent by the Royal Society. 
 Plater Fister 73s: 
 
 This level was made about the year 1830 and was used in 1831 by J. A. Lloyd, 
 F.R.S., in measuring the differences of level between the Thames at London 
 Bridge, Sheerness and various intermediate points. It has a 30-in. telescope 
 mounted in Y-s in a manner similar to the ordinary Y-level, but is fitted with 
 an extra spirit level suspended above the telescope, and also has a circular sup- 
 porting base resembling that of a large theodolite and resting on three footscrews. 
 
 The objective lens of the telescope has a 2-in. aperture and is fixed in the tube, 
 the eyepiece tube having a rack and pinion focussing movement. The eyepiece 
 is of the Ramsden inverting type and is also focussed by a rack and pinion 
 movement. Gun-metal collars are shrunk on to the telescope near its extremities 
 and rest in Y-s fixed to the horizontal circle. One of these Y-s can be adjusted 
 vertically by means of a screw. This adjustment serves to level the telescope 
 more precisely than is possible with the footscrews. The 14-in. horizontal circle 
 can rotate about a vertical axis, and has a clamp and tangentscrew. Italso carries 
 a central 53-in. compass, graduated in degrees, and a small thermometer. The 
 
4I 
 
 supporting base of the instrument consists of two horizontal circles of 14 in. 
 diameter connected by three vertical pillars to which the three footscrews are 
 fixed. Beneath the telescope a 17-in. spirit level is mounted in adjustable 
 bearings, and a 22-in. level of similar construction can be suspended above the 
 telescope from pivots carried by brackets fixed to the end of the tube. Both 
 pivots are adjustable. Each level has ivory scales, graduated to twentieths of 
 an inch mounted above its tube. Small transverse levels are also fixed to the 
 telescope and at one end of the suspended level. 
 
 (See Phil. Trans. Roy. Soc., 1831.) Inv. 1900-154, S.M. 1938. 
 
 97. SPIRIT LEVEL (Troughton type). Presented by A. 
 Stannard, Esq., A.M.I.C.E. 
 
 This level was formerly the property of George Stephenson, and was used by 
 him at Chat Moss in the survey for the Manchester and Liverpool Railway, 1829. 
 
 It is of the type designed and constructed by Troughton, the spirit-level 
 being rigidly fixed in the telescope, which is not reversible, as in the Ramsden 
 type. Such an instrument, whilst incapable of preliminary adjustment, is very 
 simple and rigid in construction, and is well suited for engineering surveys. A 
 small compass, supported on four pillars, is provided for traversing. 
 
 The telescope has an aperture of 1}in. and a focal length of 15 in., and is 
 provided with stops. The eyepiece tube is focussed by a rack and pinion 
 arrangement actuated by.a screw. The diaphragm can easily be disclosed for 
 examination by removing the eyepiece. There are two eyepieces provided, 
 one erect and the other inverting. The bubble-tube is roin. long, and has a 
 radius of curvature of about 50 ft., giving a motion of the bubble of ¢ in. for every 
 minute of arc. 
 
 The compass ring is 2} in. in diameter, and is graduated to read single 
 degrees. Inv. 1892-21. 
 
 98. SPIRIT LEVEL. Made by Troughton & Simms. Lent by 
 the Ordnance Survey. 
 
 This instrument was made about the middle of the r9th century, and was 
 used until 1907 for levelling minor lines. The spirit level is attached by adjusting 
 screws to the telescope, which fits in Y-shaped bearings, and can be rotated or 
 reversed in these for adjustment of the line of collimation. 
 
 The parallel plate arrangement for levelling the instrument is of the usual 
 type, with four levelling-screws, axis clamp and tangent-screw for slow-motion. 
 
 The telescope has a focal length of 12 in., and an aperture of 1-25 in., and is 
 provided with stops. The focussing is carried out at the objective end by means 
 of a rack and pinion arrangement, actuated by an external screw. 
 
 The diaphragm containing a horizontal and vertical wire is easily accessible 
 after unscrewing the eyepiece, which is arranged to give an erect image of the 
 staff. The bubble-tube is 7}in. long, and is graduated in tenths of an inch. 
 The internal radius of curvature of the tube is about 80 ft., giving a movement 
 of the bubble of 0-1 in., corresponding to a change of elevation of 20 seconds. 
 
 Inv. 1912-197. 
 
 99. SPIRIT LEVEL. Made by Troughton & Simms. Lent by 
 the Ordnance Survey. 
 
 This level was made between 1840 and 1850 and was used for levelling of 
 second-order lines. Thespirit levelis graduated to read 5seconds of elevation, and 
 is attached to the telescope, which rests in Y-shaped bearings, and can be rotated 
 or reversed for adjustment of the line of collimation. A compass is provided for 
 rough traversing, and an elevating screw permits the telescope to be finally 
 levelled with great accuracy. In the diaphragm are two vertical and one 
 horizontal wires. 
 
 The telescope has a focal length of 18 in. and an erecting eyepiece. The 
 aperture is 1-5in. The eyepiece is adjusted for parallax by a sliding tube 
 arrangement, whilst the telescope is focussed on the staff by a rack and pinion 
 which moves the objective. 
 
 The compass-ring has a diameter of 3}in. and is graduated to read single degrees. 
 
 The spirit level tube is toin. long, and is graduated at intervals of {in.. 
 each interval corresponding to 5 seconds of arc. The parallel plate arrangement 
 with four levelling screws is of the usual type, and a slow-motion tangent screw 
 is provided for azimuth setting. Inv. 1912-1908. 
 
42 
 
 100. SPIRIT LEVEL. Made by Troughton & Simms. Lent by 
 the Ordnance Survey. 
 
 This level was made in 1841 and was used in first-order levelling for the 
 Ordnance Survey. It has a telescope of 2:2-in. aperture and 23-in. focal length, 
 with a 12-in. spirit level attached, reading to single seconds of arc. The telescope 
 rests in Y bearings, in which it can be rotated and also reversed end for end, for 
 adjustments. A small transverse level is also provided on the telescope for rough 
 levelling, whilst the final accurate setting is performed by means of an elevating 
 screw, which moves one bearing. 
 
 The instrument has a 54-in. compass fitted to its alidade for rough traversing, 
 the compass being graduated in single degrees. There is also the usual parallel- 
 plate levelling arrangement, with four screws, for setting the alidade approxi- 
 mately horizontal. A slow motion tangent screw is provided for setting the instru- 
 ment accurately in azimuth. 
 
 The telescope has a micrometer eyepiece, and is focussed on the staff by moving 
 the eyepiece tube. 
 
 The spirit-level has an ivory scale above each end, graduated in 1/20 in., each 
 division corresponding to about 2 seconds of arc of elevation. Inv. 1912-199. 
 
 101. SPIRIT SLEVEL2 Mades* by 9G. Sickler Surecemrecmmy 
 T. H. Court, Esq. 
 
 A level of this type, serving also for measuring inclines and distances by means 
 of the elevating screw, is particularly adapted by reason of its compactness and 
 simplicity of construction for preliminary surveys where no great accuracy is 
 required. 
 
 It consists of a small telescope and spirit-level separately mounted upon a 
 platform connected with a tripod by means of a ball-and-socket joint of a special 
 type, often employed in small surveying instruments of German construction. 
 This joint permits the orientation of the platform to be varied, whilst leaving it 
 perfectly free to rotate in azimuth about the socket, which is fixed to the tripod. 
 The platform terminates underneath in a metal ball, at the lower end of which is 
 a cylindrical pin. The socket consists of two parts screwed together, an upper 
 spherical dome enclosing the ball, and a lower tubular sheath surrounding the pin, 
 Two levelling screws, set perpendicularly to each other, work in threads in this 
 sheath, and impinge upon a metal collar which encircles the pin sufficiently 
 tightly to prevent shake, but not to hinder the free azimuthal rotation of the pin, 
 and consequently the platform. Opposite the two screws, and likewise impinging 
 on the collar is a spring which takes the pressure of the screws. By means of these 
 screws the platform can be set level in all directions. 
 
 The spirit level can be adjusted upon the platform by the small screws at one 
 end of its case, whilst the telescope can be elevated or depressed about an axis at 
 one end fixed in the platform by means of a fine-threaded screw at the other end. 
 The pitch of the screw-threads is half a millimetre, and the movement of this end 
 of the telescope is read to a tenth of a millimetre with the aid of a graduated disc 
 and vertical millimetre scale. For every tenth-millimetre, the line of sight of the 
 telescope moves through an angle whose circular measure is one-thousandth. 
 By this means, as in tacheometry, distances can be measured with fair accuracy. 
 
 Inv. 1913-256. 
 
 102. SPIRIT LEVEL. Made by Zimmer Bros., Stuttgart. 
 
 This instrument, primarily designed for levelling, can also be employed for 
 measuring angles in any plane, and is likewise provided with stadia lines in the 
 diaphragm of the telescope for measuring distances. It is thus a type of cheap, 
 portable, universal instrument, specially adapted for surveys of minor precision. 
 
 The special features which distinguish it from an ordinary small level are the 
 graduated semicircle and the peculiar method of attaching the spirit level to the 
 telescope and the whole instrument to the tripod. The lower parallel plate is not 
 attached directly to the tripod head, but to an intermediate knuckle-joint piece, 
 which permits the instrument as a whole to be set with the semicircle in any plane. 
 The spirit level can also be attached to the telescope in two places, one serving 
 when the semicircle is horizontal and the other when it is vertical. 
 
 The telescope can rotate on the semicircle, which is graduated to 30 minutes and 
 reads by vernier to 2 minutes, and the semicircle can likewise rotate irrespective 
 of the telescope, thus permitting angles to be ‘‘ repeated.”’ 
 
 The diameter of the graduated semicircle is 18 cm. The parallel plate arrange- 
 ment is of the usual type, with four levelling screws. Inv. 1876-1196. 
 
43 
 
 103. BREITHAUPT’S COMPENSATION LEVEL. Made by 
 F. W. Breithaupt and Son, Cassel. 
 
 This is a later modification of a level designed and constructed by F. W. 
 Breithaupt in 1845, with the idea of minimizing the instrumental adjustments 
 usually necessary as a preliminary to taking levels. The method of weighting the 
 main spirit level and suspending it freely in pin bearings attached to the telescope 
 enables the surveyor to compensate by double readings the errors arising from 
 non-parallelism of the spirit-level axis, telescope axis, and line of sight. 
 
 In this level the telescope can be rotated in its collars through a complete half- 
 turn, the extreme positions being determined by mechanical stops. The spirit 
 level, whilst revolving with the telescope, does not rotate, but is kept with its 
 engraved scale uppermost. Consequently the bubble is always visible against 
 the scale, and its variation during the rotation measures the extent to which the 
 axis of the spirit level is inclined to the axis of rotation of the telescope. Such an 
 inclination, if small, will be automatically rendered ineffective provided two 
 readings of the levelling staff are made with the spirit level “‘ centred,”’ one before 
 and the other after ihe telescope is rotated in its collars through 180°. This double 
 reading also corrects for lack of coincidence between the mechanical axis of rotation 
 of the telescope and the line of sight. The mean of the two readings will be the 
 reading which would be obtained if the three axes of spirit level, telescope rings 
 and vision were mutually parallel. Thus it is not necessary to adjust the instru- 
 ment beforehand for this parallelism. It is, however, essential that the spirit level 
 itself should be so adjusted that when its bubble is central, its own axis of rotation 
 in its bearings should be horizontal. This can be tested by centring the bubble, 
 reversing the spirit level in its bearings without moving the other parts of the 
 instrument, and reading the bubble again. Half the difference is removed by 
 means of the two capstan-headed screws which fix one end of the bubble tube 
 to its case. To enable the spirit level to be so reversed, one bearing pin can be 
 withdrawn irom its supporting bracket. 
 
 The lower part of the instrument is provided with a circular box level, which 
 serves to set the main axis nearly vertical by means of the footscrews. No 
 attempt is made to render this axis exactly vertical, as the telescope is always finally 
 levelled by means of the elevating screw. The head of this latter is divided, and 
 serves to measure very accurately small gradients. The diaphragm of the telescope 
 has two horizontal subtense wires in addition to the central levelling wires, so 
 that the instrument can be used to measure distances. 
 
 The “suspension ’’ spirit level of the type here shown was soon superseded 
 in compensating levels by Amsler’s two-sided “reversion ”’ level, which is much 
 less liable to be injured by shocks than Breithaupt’s pattern. 
 
 See “‘ Dingler’s Polytechnisches Journal,’’ Bd. 153, p. 401, 1859. 
 
 Inv. 1876-1198. 
 
 104. COOKE’S REVERSIBLE LEVEL (14-in.). Made and lent 
 by T. Cooke & Sons, Ltd., York. 
 
 The Y-s of an ordinary reversible level are replaced in this instrument by a 
 strong and rigid socket, from which the telescope can be bodily withdrawn and 
 replaced after reversion end for end in the “ collimation ’’ adjustment. Accur- 
 ately turned flanges on the telescope fit in collars fixed at the ends of the socket. 
 A stop flange at the eyepiece end of the telescope abuts against the end of the 
 socket when the telescope is in the correct position, and can be fixed to the socket 
 by a screw. The socket is fixed to the horizontal rotating arm by end fittings 
 which permit it to be adjusted on this arm by elevating or depressing one end. 
 The main spirit level is similarly attached in adjustable bearings above the socket, 
 parallel to its axis. A small cross level is fitted at right angles to the axis above 
 the socket at one end. Each division of the main level is 0-1 in. long, corresponding 
 to 20 seconds of elevation. 
 
 The rotating arm carries acentral circular compass, of 3-5in.diameter, graduated 
 in half-degrees, and read by a prismatic reader. The needle can be lifted from its 
 pivot and clamped by turning a screw at one end of the arm. The conical socket 
 carrying the axis of rotation is fixed to the tribrach, in the arms of which engage 
 three footscrews. The feet of these are firmly attached to a trivet stage which can 
 be screwed to the tripod head. This stage has three pointed feet which serve to fix 
 the instrument to a platform when the tripod is inconvenient. 
 
 The telescope has an objective of 14 in. focal length and 1-55 in. clear aperture, 
 and an inverting screw-focussing eyepiece magnifying 28 times. The objective 
 tube is focussed by a rack and pinion arrangement actuated by a screw at one side 
 of the telescope near the eye end. The glass diaphragm has two vertical and one 
 horizontal lines engraved upon it, whilst short horizontal lines etched parallel to 
 the sight line can be used for stadia observations. Inv. 1921-560. 
 
44 
 
 105. DUMPY LEVEL (g-in.). Made and lent by C. F. Casella & 
 Can ete 
 
 The compactness and rigidity which are the chief features of the dumpy level 
 have been increased in instruments of recent construction by the employment of 
 an internal focussing lens, which moves between the fixed objective and diaphragm. 
 The telescope tube can thus be rendered more compact, dust-proof and water- 
 tight, whilst the act of focussing is less likely to disturb the line of collimation. 
 
 In the instrument exhibited, the telescope has a focal length of 9 in. and a 
 clear aperture of 1-4 in. and is provided with a tribrach mounting, with three 
 levelling screws. A 2-in. ring, divided every 2°, is fitted round the upper rim of the 
 tribrach and enables the azimuth angle to be read by means of an index on the 
 telescope socket. A tangent clamp and fine motion screw are also fitted to the 
 tribrach. The feet of the levelling screws pass through a locking plate having a 
 threaded central aperture to receive the clamping piece which attaches the 
 instrument to its tripod. 
 
 A sensitive tube level, the casing of which also supports a small circular box 
 level, is mounted in adjustable bearings at one side of the telescope. 
 
 The eyepiece of the telescope is inverting and screw focussing, and has a 
 magnification of 24 diameters. Inv. 1921-733. 
 
 106. DUMPY LEVEL. (10-in.). Made and lent by E. R. Watts 
 and Son, Lid. 
 
 This is a modern dumpy level having a fixed objective of 1-5 in. aperture and a 
 fixed glass diaphragm sealing the other end of the 1o-in. telescope tube, which is 
 thereby rendered dustproof ; a movable internal concave lens is actuated by an 
 external milled head for focussing, and a screw-focussing inverting eyepiece is 
 provided. 
 
 A 5-in. bubble tube is adjustably mounted on the left of the telescope and has a 
 sensitivity of 20 seconds of elevation per 0-1 in. division ofits scale; by means ofa 
 hinged reflecting mirror mounted above its scale, it is read by the operator without 
 moving his other eye from the telescope. 
 
 The telescope tube is permanently fixed to the inner vertical axis of the 
 instrument and rotates within the conical socket of the tribrach levelling base, 
 which has the usual three footscrews connecting it to the trivet stage; dust- 
 proof caps are fitted above the heads of the footscrews and their feet are firmly 
 gripped in the grooves of the trivet stage; this latter has a central screwed aper- 
 ture for attachment to the tripod head, but has also three pointed feet for mounting 
 on a platform without the tripod. The axis clamp and tangent screw of the 
 azimuth motion are of the usual type. - Inv. 1922-443. 
 
 107. ZEISS PATTERN LEVEL. Made and lent by E. R. Watts 
 and Son, Ltd. 
 
 The optical arrangements for adjusting the collimation axis of the telescope and 
 viewing the bubble are the same in this instrument as in the well-known Zeiss- 
 Wild level, but the elevating screw mechanism and tribrach are of the makers’ 
 own design. 
 
 In order to obviate the necessity for reversing the telescope end for end in the 
 collimation adjustment, it is provided with two similar objectives, one fixed at 
 each end of the tube, and an internal focussing lens. The eyepiece can be fitted at 
 either end, and the nearer objective lens then becomes the diaphragm ; each 
 lens is graticuled for this purpose. A screw-focussing eyepiece tube is fitted at the 
 normal eyepiece end, and the objective cap has a central sheath aperture in which 
 the eyepiece can slide at the other end. The normal diaphragm lens has horizontal 
 stadia lines as well as the usual cross lines ; the other lens has only a simple cross. 
 
 The telescope is supported in a central horizontal socket, in which it can rotate 
 through 180° about its own axis. The bubble tube is adjustably fixed at one side 
 of the telescope and rotates with it in the socket; it has apertures at the top and 
 bottom for reading its bubble before and after rotation. A system of prisms 
 mounted above the bubble tube permits the observer to view the two ends of the 
 bubble in his normal position for observing the staff, the bubble being viewed 
 with the left eye and the staff with the right. 
 
 The socket supporting the telescope is itself mounted upon an elevating plat- 
 form which can turn in azimuth about the vertical axis of the tribrach. The 
 telescope socket is pivoted centrally to this platform about a horizontal axis 
 intersecting the main vertical axis of rotation, and can be rotated about this pivot 
 by means of a micrometer elevating screw threaded in the platform at one end ; 
 
45 
 
 the screw does not impinge on the socket, but on an intermediate lever arm 
 pivoted horizontally at the other end of the platform and in contact with the 
 socket near this pivot. A spring connecting the socket and platform keeps the 
 former pressed against the lever arm. By this arrangement the weight of the 
 telescope is taken off the elevating screw and its rotation about the central 
 pivot is only a small part of what it would be without the lever arm. 
 
 The micrometer drum of the elevating screw is divided to read single minutes 
 of elevation of the telescope and can be used for measuring distances ; the stadia 
 lines of the telescope diaphragm can also be used for this purpose. 
 
 Inv. 1922-440. 
 
 108. Y-LEVEL (14-in.). Made and lent by E. R. Watts and 
 Son, Ltd. 
 
 The telescope of this level has a rack-focussing objective of 1-3-in. aperture 
 and 14-in. focal length. A 6-in. bubble-tube is mounted adjustably beneath it 
 and has a sensitivity of 22 seconds per o-1 in. division of its graduations. 
 
 The telescope is supported in Y-s by means of gun-metal collars, 8 in. apart, 
 and can be rotated in its Y-s or reversed end for end in the usual manner. The 
 Y-s are permanently fixed to a supporting platform of very rigid construction, 
 but two screws threaded in the arms of one Y permit the telescope to be raised 
 or lowered in that bearing to take up wear. The hinged retaining clips of the Y-s 
 have metal springs attached which keep the collars pressed down on their bearings ; 
 one of these springs has a pointed toe which engages in a groove notched in the 
 collar when the telescope is in the normal observing position, 7.e., the bubble 
 vertically below its axis. 
 
 The supporting platform has a conical] axis and can rotate in azimuth within 
 the tribrach levelling base; an axis clamp and tangent screw are provided. 
 Three footscrews are threaded in the tribrach and have their feet firmly clamped 
 in a trivet stage, which can be screwed to a tripod head or can stand on a 
 platform by means of its three pointed feet. The footscrews have dustproof 
 caps threaded in the tribrach to protect their heads and to take up wear. 
 
 Inv. 1922-442. 
 
 109. PRECISE LEVEL (16-in.). Made and lent by T. Cooke & 
 Sons, Ltd. 
 
 A“ precise ’’ level, designed for work of the highest possible degree of accuracy, 
 in addition to its greater sensitivity and power, possesses several features not 
 found in ordinary levels. The chief characteristics are the additional facilities 
 for finally levelling the telescope by means of a micrometer screw and a very 
 sensitive bubble, which is read by a supplementary optical arrangement at the 
 same time as the staff, and the protection of the bubble and all movable parts 
 affecting the axis of collimation from temperature changes. In the level exhibited, 
 which was introduced by the U.S. Coast and Geodetic Survey about the year 
 1900, the main bubble tube is sunk well inside the outer telescope tube, which 
 is also covered by felt cloth. A system of mirrors and a supplementary telescope 
 with its eyepiece at binocular distance from the main telescope, enable the 
 observer to read at the same time the staff with his right eye and the ends of 
 the bubble with his left. 
 
 The main telescope has an objective of 1:7-in. aperture and 16-in. focal 
 length, mounted in a tube containing the rack focussing eye-piece tube. The 
 objective tube is pivoted near the object end to the cloth-covered outer tube and 
 near the eyepiece end rests on a micrometer screw. By turning this the objective 
 tube can be slightly inclined in the vertical plane through the staff and station 
 point until the bubble, which is fixed to it, is centred. The outer tube itself 
 can rotate about the vertical axis of the instrument and has a tangent clamp 
 and screw for slow-motion in azimuth. A tribrach with three footscrews supports 
 the whole instrument and serves to level it approximately, with the aid of a small 
 box level on the outer telescope tube. The telescope can be lifted from contact 
 with the micrometer screw and clamped inside the outer tube by a lever bar near 
 the micrometer. This arrangement preserves the micrometer from unnecessary 
 use and friction. The micrometer drum is graduated in 100 divisions, each 
 corresponding to the 2-mm. divisions on the main bubble, 7.e., to 2 seconds of eleva- 
 tion. The eyepiece tube of the main telescope has a cross-wire diaphragm, with two 
 horizontal stadia lines; two inverting eyepieces, of magnifications 32 and 44, 
 are supplied. The supplementary telescope has a plain aperture eyepiece and 
 two diagonal reflecting prisms which, in conjunction with a plane mirror fixed 
 to the outer telescope tube, reflect the image of each end of the bubble and scale 
 down the axis of the telescope. All the parts of the main telescope which affect 
 _ the adjustment of its line of collimation are made of “invar.” Inv. 1922-206. 
 
46 
 
 110. PRECISE LEVEL (13-in.). Made and lent by E. R. Watts 
 and Son, Ltd. 
 
 The special features of this level are the bubble-tube and an optical arrange- 
 ment mounted in front of the objective lens to enable the staff to be read 
 accurately to o-oor ft. and by estimation to o-ooo1 ft.; both these fittings are 
 modified examples of the ones originally mounted on the Zeiss Precise Levels 
 about 1913. 
 
 The telescope has a screw-focussing inverting eyepiece and a fixed glass 
 diaphragm, engraved with cross lines and horizontal stadia lines. The horizontal 
 cross line is in the form of a very narrow Y, having its stem horizontal and its 
 arms intersecting at a small angle on the vertical line; by this arrangement any 
 division of the staff is bisected by one half and enclosed by the other half of 
 the Y. If the telescope axis, when levelled, does not bisect a staff division, the 
 optical axis is displaced slightly along the staff by means of the ante-objective 
 fitting until a division is accurately bisected. The fitting consists of a plane 
 glass slab of cylindrical shape, pivoted horizontally within a cap-sheath fitted 
 over the objective end of the telescope; the slab can be rotated about its pivot 
 axis by means of a radial lever fixed on the axis and having a pinion wheel at 
 its outer end, which engages in the toothed face of a fixed arc mounted on the 
 cap sheath. The opposite face of this arc is visible to the observer, and is 
 graduated into 20 parts, each division representing o-oo1 ft. on the staff, 7.e., 
 the optical axis is displaced vertically by this amount when the slab is pivoted 
 through the corresponding arc; an index mark on the rotating lever serves to 
 measure the arc to a tenth division, 7.¢., to o-ooo1ft. The telescope objective 
 has an aperture of 1-8 in. and is fixed in the tube; an internal lens operated 
 by an external milled head is provided for focussing the staff image on the 
 diaphragm. 
 
 The micrometer elevating mechanism of the telescope, the Zeiss. pattern 
 bubble-tube and the tribrach mountings, with clamp and tangent screw for slow 
 azimuth rotation, are of the same type in this instrument as in the small Zeiss 
 pattern level by the same makers, and are described in connection with the 
 adjacent exhibit (Cat. No. 107). The micrometer drum of the elevating screw is 
 graduated into 20 parts, each corresponding to an elevation of 1: 10,000, whereby 
 distances can be accurately estimated. 
 
 This type of level is used in conjunction with an Invar Precise Levelling 
 Staff of the type represented in an adjacent exhibit (Cat. No.125). Inv. 1922-441. 
 
 111. CLINOMETER. Made by W. Bandermann, Berlin. Pre- 
 sented by T. H. Court, Esq. 
 
 This clinometer is not provided with sights, but has two plane edges at right 
 angles and a movable spirit-level with a vernier at one end. The instrument is 
 set on the face of the rod and the level rotated until the bubble is centred, when 
 the zero mark of the vernier gives the inclination to a sixteenth of a degree. 
 The arc over which the vernier moves is graduated in half degrees, each of the 
 eight intervals between the vernier graduations being seven-eights of a half degree. 
 
 An instrument of this type is principally used for setting levelling-rods 
 vertical, or for measuring the gradient of a rod used for determining distances 
 in cadastral surveys. It can be set upon some kind of sighting rod and used for 
 measuring gradients or for levelling. The spirit level is very sensitive, and the 
 bubble moves about 0-5 cm. for every variation of a sixteenth of a degree in 
 the inclination. Inv. 1917-38. 
 
 112. CLINOMETER. Made by Thos. Jones, London. Presented 
 by the Admiralty. 
 
 This instrument was made in the first half of the 19th century, and is used 
 for measuring the elevation to the horizontal of any inclined plane on which it 
 can be set. It is not provided with sights. 
 
 It consists of two brass rulers, hinged at one end. The upper ruler carries a 
 sensitive 6-in. spirit level and an index, which moves over a graduated brass arc 
 fixed to the lower ruler. This latter is placed upon the surface to be measured 
 and the other ruler revolved until the bubble of the level is centred, when it can 
 be clamped by a screw on the index plate and the reading taken. 
 
 The arc is graduated in degrees from 0° to 50°. Inv. I9QII-174A. 
 
47 
 113. CLINOMETER. Presented by the Admiralty. 
 
 In this instrument two brass rulers are hinged together at one end. The 
 upper one has perpendicular sights at each end, each of which has both an aperture 
 backsight and a cross-wire foresight, and carries an index plate which moves 
 over a graduated arc fixed to the lower ruler and can be clamped to this are by 
 a screw. A 5-in. spirit level is supported parallel to the upper ruler in bearings 
 on the sights. The lower ruler has a socket for fixing the instrument to a stand. 
 
 In using the instrument, the upper ruler is lowered until it rests on the lower 
 and the instrument is levelled by the spirit level, after which the upper ruler is 
 raised until the object whose angular elevation is required is sighted, and the 
 reading of the arc taken. 
 
 The arc has a radius of 4 in. and is graduated from 0° to 30° in degrees, read 
 by estimation to tenths. Inv. I91I—-174B. 
 
 114. ABNEY LEVEL AND CLINOMETER. Made by Cary, 
 London. Presented by Captain H. G. Lyons, F.R.S. 
 
 In a hand-level it is desirable that the object sighted and the levelling 
 mechanism should be simultaneously visible. The combined level and clinometer 
 designed by Capt. Abney, R.E., about the year 1870, utilizes the idea of reflecting 
 the bubble of a spirit level into the line of sight of the sighting tube by means 
 of a diagonal mirror fixed in one half of the latter, the other half being left free 
 for sighting the object. This device had been previously used on a sextant. 
 
 In the instrument exhibited, a graduated semicircle is rigidly fixed to the 
 rectangular sighting tube, which has a pin-hole aperture and a diagonal mirror 
 fixed beneath a slit in its upper face, vertically beneath the centre of the arc. 
 Through this centre passes an axis, carrying a small transversely mounted spirit 
 level at one extremity and a vernier arm near the other, which terminates in a 
 large milled wheel. On rotating this wheel the spirit level is set at any angle to 
 the line of sight, the angle being measured by the position of the vernier on the 
 graduated arc. The cover of the spirit level has apertures above and below, the 
 latter permitting an image of the bubble to be seen after reflection on the mirror, 
 whilst the former is used when the clinometer is set on the surface whose gradient 
 is required. The arc is graduated in degrees up to 60° on both sides of the vertical, 
 and reads by vernier on the movable arm to Io minutes. It also has a scale of 
 gradients to I in 1, read by the edge of the vernier. 
 
 The sighting tube is 44 in. long and the graduated arc has an effective radius 
 OL. i. Inv. 1914-220. 
 
 115. ABNEY LEVEL AND CLINOMETER. Made by Elliott 
 Bros., London. Presented by T. H. Court, Esq. 
 
 This example differs from the preceding one in minor details. The graduated 
 arc is fixed to the spirit level and revolves with it, being read for angles up to 
 45° either way by means of a fiducial mark on the supporting bracket, which is 
 fixed to the sighting tube. 
 
 Scales of gradients up to I in 1 are also marked on the arc beyond the degree 
 graduations and are read by the bevelled edges of the supporting bracket. The 
 edge of the arc is milled, and gears with a small pinion wheel centred in the fixed 
 bracket and rotated by a milled screw. This permits the spirit level and arc to 
 be rotated more slowly and evenly than in the other form. The sighting tube 
 has a circular section, which is not so convenient when the instrument is to be 
 placed on the slope as the rectangular type. Inv. 1913-254. 
 
 116. ABNEY LEVEL AND CLINOMETER. 
 
 This is similar to the preceding examples, but has also a I}-in. compass fixed 
 on the upper surface of the sighting tube. The compass ring is graduated in 
 degrees, numbered every 10° by quadrants from the N—S line. Inv. 1887-62. 
 
 117. FIELD CLINOMETER (Mark III). Made by J. Casartelli & 
 Son. 
 
 This is a modern Service clinometer for measuring to the nearest minute the 
 angle of elevation of a plane surface. The angle is measured to the nearest degree 
 by a rotating arm which traverses a fixed graduated sector attached to the base. 
 The arm is slightly curved on its upper and lower surfaces, and carries a sliding 
 spirit level whose position, when the bubble is central with respect to the scale 
 
 -graduated on the arm, gives the number of minutes in the angle. 
 
48 
 
 There are two flat mutually perpendicular base plates, one painted black 
 and the other red, and the sector of degrees is graduated on both sides, one set 
 numbered in black from o to 44 and the other in red from 50 to 89, for use with 
 the corresponding base. The black base is used for angles below 45° and the 
 red for those above. The radial arm has a radius of 5 in., and is pivoted at the 
 centre of the graduated sector. The other end of the arm has a spring clutch, 
 with a toothed face engaging in the teeth on the inner face of the sector and 
 locking the arm in any desired position. The scale of minutes on the arm extends 
 through a complete degree and occupies a length of 2 in. The arm is thus curved 
 in a circle of radius about 9:5 ft. There are such scales of minutes on the front 
 face and the rear face of the arm, the former numbered with the zero nearest 
 the pivot and the latter in the reverse direction. The front scale is used with 
 the black base and the rear scale with the red one. Inv. 1922-42. 
 
 118. WATKIN CLINOMETER. Made and lent by Messrs. Cooke, 
 Troughton & Simms, Ltd. 
 
 Instead of a graduated arc, this type of clinometer is provided with a rotatable 
 drum, upon the face of which is a graduated spiral groove, upon which is recorded 
 the amount of elevation of the surface tested. 
 
 The bubble level is mounted within a gunmetal frame which has two surfaces 
 accurately machined at right angles to each other. One end of the level is pivoted 
 to the frame, and the other end is kept pressed against the base of the drum which 
 rotates upon a threaded screw carried by the frame. As the drum rotates, an index 
 fixed to the frame records on the scale the resulting elevation of the free end of the 
 level above the pivoted end, and hence, when the bubble is centred, the elevation 
 of the plane upon which the frame is set. 
 
 No sights are provided with this instrument, which is designed for recording 
 the slope of a plane surface with respect to the vertical or horizontal, according 
 to which of the two plane surfaces of the instrument is in contact with the surface 
 tested. 
 
 The range of the drum scale is from 40 minutes below to 45° above zero, the 
 first 5° being subdivided by dots to 1 minute; thereafter lines indicate every 
 5 minutes. Inv. 1925-4. 
 
 119. INDIAN CLINOMETER. Made and lent by Messrs. Cooke, 
 Troughton & Simms, Ltd. 
 
 In this clinometer, one sight is fixed and the other is movable over a graduated 
 scale engraved upon the vane; the position of the movable sight registers the 
 inclination of the line of sight. 
 
 The instrument consists of a hollow bubble arm mounted upon a gunmetal 
 base, which has three studs on its lower face. The bubble arm is pivoted at one 
 ~ end to the base, and carries an adjusting screw at its other end for setting the 
 bubble central. A folding sight vane is fitted to each end of the bubble arm, the 
 shorter vane having a fixed aperture backsight and the longer one carrying the 
 sliding foresight. The latter vane has two scales engraved upon it, one of degrees, 
 subdivided to 20 minutes, and the other of tangents. The range of the scale is 
 0°—23° of elevation or depression. Inv. 1925-5. - 
 
 ACCESSORIES. 
 120. AXIS LEVEL. Lent by the Royal Society. 
 
 In large astronomical and surveying instruments, specially 1ong and sensitive 
 levels are generally provided for testing the horizontality of the telescope axis. The 
 levels are usually fitted above the axis by bearing forks, but are sometimes sus- 
 pended from it by curved brackets. The one exhibited is of the latter type, and 
 was probably made in the early part of the 19th century. It has a bubble tube 
 about 15 in. long by 1fin. diameter, and an ivory scale mounted on the casing 
 and divided in tenth-inches. 
 
 The brass casing of the bubble tube has end plates which fit loosely upon 
 screwed vertical pins and can be clamped in any desired position by upper and 
 lower capstan nuts threaded on the pins. The latter project from a horizontal 
 brass tube, somewhat longer than the bubble tube and having the suspension 
 brackets fastened to its ends. The upper portions of these brackets are curved 
 
49 
 
 and their ends turned over and provided with inverted V-plates which rest on the 
 telescope axis. The scale divisions are numbered symmetrically from the centre. 
 A small transverse spirit level is mounted on one end of the casing of the main 
 level. The vertical pin at one end of the base tube is slightly adjustable laterally 
 by means of capstan nuts. Inv. 1900-167. 
 
 121. SET OF BUBBLE TUBES. Made and lent by T. Cooke & 
 Sons, York. 
 
 This exhibit comprises twenty-two bubble tubes of various shapes, sizes and 
 sensitivities, all except two being unmounted. 
 
 Six are of the circular or “ pill-box ”’ type, the upper surface of the container 
 being internally ground spherical, so that the bubble, when centred, occupies the 
 space inside a circle engraved on the glass. Two of this type, which are not very 
 sensitive and are only used for rough levelling, are shown mounted in brass cases, 
 attached to the base-board. 
 
 The remaining sixteen are tube levels, six of them having their glass con- 
 tainers bent as a whole into a circular shape, and the other ten being straight tubes, 
 internally ground to various curvatures according to the desired sensitivity. The 
 first six are not very sensitive, and are of the type used by carpenters, masons, etc. 
 The other ten are used on surveying instruments, etc., and are usually mounted 
 in brass cases, attached to the instrument by adjustable screws. 
 
 On the larger levels of this type, division marks are graduated at o-1I in. 
 interval for reading the positions of the ends of the bubble, and the sensitivity is 
 indicated by the angle of elevation of the level necessary to cause the bubble to 
 move over one division. The most sensitive one exhibited has a sensitivity of 
 1 division per 5:5 seconds of elevation. This particular bubble tube has also a 
 separate chamber at one end. When the bubble expands owing to increasing 
 temperature, a portion of the air can be detached and transferred to the 
 supplementary chamber through a small aperture in the separating wall, and 
 the bubble thus reduced to its normal size. Inv. 1921-563. 
 
 12242 LEVEELING VRODI Lent by Po Hs Court; Esq: 
 
 This staff was made in the 18th century and consists of a graduated wooden 
 rod 6ft. long, 14in. wide, and 8in. deep, over which can slide a sighting vane, 
 which has a. sharply defined diamond of dark wood inlaid upon a face of light wood, 
 and fits closely to the face of the rod. The vane is moved by the staff holder until 
 the observer signals that it is in line with the telescope, when the former takes the 
 reading of an index mark on the bevelled edge of a small opening in the vane. 
 This method of levelling was superseded about 1835 by the modern one, in which 
 the observer himself reads the staff through the telescope, an improvement ren- 
 dered possible by the improved telescopes used. 
 
 The face of the staff is graduated both in inches, divided to tenths, and also 
 in hundredths of a foot along its whole length save the top 3in. Another rod of 
 equal length but slightly smaller section can slide along the back of the first, being 
 retained in position by a metal band which serves as a friction clamp. This 
 second rod is not divided and merely serves as a stand for extending the graduated 
 rod. Both are tipped with brass at their ends. 
 
 A second vane, similar to the first, is supplied. Each has a circular shape, 
 of diameter 5in. The inlaid diamond has diagonals 5 in. and 2} in., the longer 
 bisecting the vane horizontally. Pins and a metal spring fixed to the back of the 
 vane keep it in position on the rod. Inv. 1917-107. 
 
 123. LEVELLING STAFF. Lent by the Ordnance Survey. 
 
 This is an inextensible wooden staff 10-5 ft. long, having a rectangular section 
 2 in. wide by 1 in. deep, and is graduated in hundredths of a foot so as to be read 
 through the telescope by the observer himself. The face of the staff is sunk 
 slightly below the edges to preserve the graduations from abrasion. 
 
 The graduation marks consist of black horizontal bands o-o1 ft. wide at 
 uniform intervals of o:o1 ft. apart. The o-1-ft. bands stretch right across the 
 staff, the remainder occupying the left portion only, save at the 0-05-ft. points, 
 where there is an additional black mark to the right of the corresponding white 
 band. Large black numbers, horizontally set, mark the feet, the tenths being 
 marked by smaller black numbers set vertically. A diamond is used instead of 
 the number “ five ”’ on the latter graduations, and a dot is placed near the “ nine ”’ 
 to distinguish it from the ‘‘ six ’’ when the numbers are seen inverted in the field 
 of the telescope. Inv. 1914-422. 
 
 (15814) c 
 
50 
 
 124. GRADUATED RULES FOR LEVELLING RODS. Made 
 and lent by E. R. Watts & Son, Ltd. 
 
 This exhibit illustrates the method of engraving wooden and invar rules for 
 their levelling rods now adopted by Messrs. Watts. The portions to be painted 
 black on the white surface of the rule are first gouged out to a sufficient depth 
 and the black filled in nearly flush with the white surface; the black bands are 
 thus rendered very durable. 
 
 Two of the rules are portions of those used in the ordinary Sopwith type of 
 staff, one showing the graduations cut and the strips gouged out, the other the 
 finished rule, with the strips filled in and figured. The third rule is made of invar, 
 for use in a precise levelling rod, and the bands of black at 0-02-ft. intervals are 
 not so wide as in the other type of rod, where they are exactly o-o1 ft. wide. 
 In the latter case the lower edges of the bands define the odd hundredths of a 
 foot and the upper edges the even hundredths ; in the invar rod, only the upper 
 edges are used, the cross wire of the telescope being made to coincide with this 
 edge in taking a reading. The graduations are not figured on the invar strip, but 
 on the surrounding wood of the rod, as in the adjacent exhibit of a portion of the 
 complete rod. Inv. 1922-446. 
 
 125. PORTION OF INVAR PRECISE LEVELLING ROD. 
 Made and lent by E. R. Watts & Son, Ltd. 
 
 The advantages of a metal scale for levelling staves have until recently been 
 counterbalanced by the tendency of such scales to expand when heated, and the 
 difficulty of ascertaining their temperature in order to apply the necessary cor- 
 rection. The invention of ‘‘ invar ’’—a nickel-steel alloy of negligible temperature 
 coefficient—has now led to its employment in levelling rods used in precise surveys. 
 The example shown is the lower portion of such a rod. 
 
 The invar rule is painted white and engraved with black bands at intervals of 
 0-02 ft., every fifth band extending over the complete width (about 2 cm.) of the 
 rule, and the others over half its width. The rule is then screwed into a central 
 recess in the face of the wooden rod, which has an I section for strength and 
 lightness, and is shod with a metal shoe terminating in a flat base plate; the 
 bottom of the invar rule is flush with this plate. The face of the wooden rod is 
 painted white and numbered opposite the marks corresponding to intervals of 
 o:1 ft. on the invar rule. The numbers are in black, and are inverted so as to be 
 seen erect in the telescope of the level ; a V is used instead of 5 and an N instead of 
 g. At the divisions of the rule corresponding to an integral number of feet from 
 the base plate, large red numbers are painted on the wood, and are twice repeated 
 in smaller sized numbers on the opposite side of the face so that the observer has 
 always one of the red numbers in the telescope field. 
 
 In estimating the position of the cross wire with respect to the graduated 
 scale of the rod, the nearest o-o02-ft. mark is taken and the remaining interval 
 found either by elevating or depressing the telescope by a fine-motion screw, or 
 by moving an optical device which causes the image of the rod to be raised or 
 lowered in the telescope field. The position of the cross wire is thus readily found 
 to o-oo! ft. Inv. 1922-467. 
 
 BAROMETERS AND HYPSOMETERS. 
 
 126. MOUNTAIN BAROMETER. Made by West. Lent by 
 Gade Oo VIONS) ch ons: 
 
 This instrument is a closed cistern mercury barometer of the type common 
 in the late 18th and early 19th centuries ; it is specially adapted for portability 
 and thus capable of employment in measuring altitudes. When it is being trans- 
 ported, the mercury can be forced up so as to fill the tube completely by turning 
 an adjusting screw, which depresses the flexible leather base of the cistern. The 
 aperture in the wooden case, used for reading the height of the mercury and the 
 temperature, can also be closed by a rotating brass sheath. 
 
 The barometer is graduated in inches, and reads by a vernier index, operated 
 by a rack and pinion mechanism, to o-002in. A thermometer, provided with 
 Centigrade and Fahrenheit scales, is placed near the top of the instrument. A 
 suspension ring is fixed at the top, and is covered by a ferrule when the instrument 
 is being carried. A screw cap at the bottom likewise serves to enclose the adjusting 
 screw which operates on the cistern. The wooden case which encloses the lower 
 part of the tube is graduated in inches, so that the instrument can serve as a 
 rough measuring rod. Inv. 1893-134. 
 
ay 
 
 127. MOUNTAIN BAROMETER. Made by Dollond. Lent by 
 G. J. Symons, F.R.S. 
 
 This resembles the adjacent instrument, made by West, in its general character- 
 istics of compactness and portability, but differs in the arrangement of the cistern. 
 This is of the usual flexible base type, but has a supplementary syphon tube, 
 connected to the cistern by a narrow neck, which can be closed by means of a cock. 
 When the instrument is being used, the cock is opened, and the mercury allowed 
 to attain a fixed height in the syphon tube. This is the zero of the barometer 
 tube scale. For transporting the barometer, the mercury is withdrawn from the 
 syphon tube and the cock closed, after which the flexible base of the cistern is de- 
 pressed by means of the adjusting screw until the mercury completely fills the 
 barometer tube. 
 
 The barometer tube is graduated in inches, and read to 0:002 in. by a vernier 
 attached to the movable index. A thermometer with Centigrade and Fahrenheit 
 scales is fixed near the top. The window for reading this and the barometer can be 
 closed by means of a rotating brass shutter, and a similar shutter at the bottom 
 of the instrument serves to protect the cistern and syphon tube when required. 
 
 Inv. 1893-133. 
 
 128. MOUNTAIN BAROMETER. Made by Troughton & Simms. 
 
 The Gay-Lussac syphon-tube mercury barometer, in which both the long 
 and short tubes are closed completely at the end, but a small aperture in the 
 side of the short tube permits the air to enter whilst preventing the mercury 
 escaping, was essentially a portable instrument, and was, therefore, often used 
 for measuring altitudes prior to the invention of the aneroid. The instrument 
 exhibited is of this type, and was constructed probably about 1840. 
 
 The barometer tube is enclosed in a brass case, at the ends of which are cork 
 cushions. Scales graduated directly to 0-02in. and read by movable verniers 
 to 0-001 in. are engraved upon the case opposite the ends of the mercury. Each 
 vernier has an index which can be set level with the top of the mercury column. 
 The difference between the two readings then gives the height of the barometer. 
 A thermometer, with Centigrade and Fahrenheit graduations, is fixed near the 
 centre of the tube. This instrument is attached by brackets to a wooden stand, 
 and can be set accurately vertical by means of three screws threaded in the lower 
 bracket with their ends pressing against the case of the barometer. For mountain 
 barometers a tripod stand was usually provided. Inv. 1907-65. 
 
 129. ANEROID BAROMETER. Made and lent by W. F. Stanley. 
 
 This instrument was made about 1890, for the use of surveyors in measuring 
 altitudes up to 4,000 ft. 
 
 The zero mark of the altitude scale corresponds to a height of 32 in. in the 
 mercury barometer. The altitude scale has a diam. of 3:5 in., and is graduated 
 every 10 ft., single feet being estimated. The movable glass cover carries an 
 index mark, which is set to the last height read, and a small lens for facilitating 
 reading. The altitude scale is slightly raised on a step so that its inside graduated 
 edge is flush with an aluminium plate fixed to the tip of the hand and provided 
 with a mark. By this arrangement errors due to parallax are avoided. The 
 instrument is compensated for temperature variations. Inv. 1893-119. 
 
 130. WATKIN ANEROID BAROMETER. Made by J. Hicks. 
 
 In aneroid barometers many attempts have been made to provide scales 
 suitable for a very extended range of altitudes. This instrument shows the three 
 concentric circular scale arrangement devised by Major H. S. S. Watkin, by 
 which altitudes from zero to 10,860 ft. can be measured. The zero graduation 
 corresponds to a height of 31 in. in the mercury barometer. 
 
 Altitudes from zero to 3,440 ft. are marked at 20 ft. intervals on the outermost 
 scale, which has a diameter of 3-6in. The middle scale, of diam. 2-9 in., 
 records altitudes from 3,440 ft. to 6,860 ft., whilst the innermost scale, of diam. 
 2°2in., completes the range to 10,860 ft., every 20 ft. being marked on each 
 scale. The corresponding mercury barometer scales are marked at 0:02 in. 
 intervals from 31 in. to 20:8 in. To show which scale should be used, an indicator 
 exhibits the number 1, 2 or 3, corresponding to the outer, middle and inner 
 scales respectively. The number indicated is marked upon a plate worked by 
 
 (15814) Ce 
 
52 
 
 the mechanism, and is automatically altered when the barometer hand reaches 
 the end of the scale concerned. The breaks in the circles are indicated by 
 black bars. 
 
 The revolving glass cover carries an index hand, which can be set to the last 
 reading taken. 
 
 The instrument is compensated for temperature variations. 
 
 (See British Patent Specification, No. 14730, of 1886.) Inv. 1907-67. 
 
 131. EARLY THERMOMETRICAL BAROMETER. Made by 
 Francis J. Hyde Wollaston. Presented by the Rev. T. P. W. Worswick. 
 
 A new “ thermometrical barometer ”’ of the type here exhibited was invented 
 by Wollaston and described by him before the Royal Society in 1817. The one 
 exhibited was made before 1821. Itis essentially a very sensitive ‘‘ boiling-point ”’ 
 thermometer, whereby the temperature at which water boils may be found to 
 within a thousandth of a degree Fahrenheit, and the altitude of the place 
 determined accordingly. 
 
 The bulb of the thermometer is spherical and has a diameter of about an inch. 
 The tube has a length of 30 in. and a very fine bore, so that one degree Fahrenheit 
 corresponds to a movement of the mercury in the tube of 4in. This is measured 
 by means of a brass scale fixed on the face of the rod which supports the tube. 
 The scale is graduated to read 0-02° F. directly, and 0-002° F. by means of a 
 movable vernier. The rod carries a double walled tin cylinder surrounding the 
 bulb of the thermometer. The cylinder can be detached and partly filled with 
 wate1, which is then boiled by means of a spirit lamp so that the steam surrounds 
 the bulb of the thermometer. 
 
 Along the side of the rod is attached a strip of paper on which is marked a 
 scale of corrections to be applied to the temperature scale, owing probably to 
 inequalities in the diameter of the bore of the tube. 
 
 (See Phil. Trans., 1817, No. XIII and 1820.) Inv. 1914-735. 
 
 132. HYPSOMETER. Made and lent by L. Casella. 
 
 Although not so accurate as a standard barometer, a hypsometer for 
 determining altitudes by means of the boiling point of water is more convenient 
 for rapid surveys in mountainous districts. The example shown is of the 
 “pocket ’’ type much used by Alpine travellers. Its thermometer is graduated 
 from 180° to 215° F. by 0:02°. 
 
 The apparatus consists of a brass water container and steam jacket, in which 
 the thermometer is suspended and which is fixed above a small spirit lamp for 
 boiling the water. A brass case surrounds the lamp, which is further screened 
 from wind by means of a black japanned tin box, provided with air holes covered 
 by wire gauze. The thermometer is suspended with its bulb inside an inner tube 
 connected to the boiler. An outer telescopic tube surrounds this and serves to 
 support the thermometer in addition to forming a steam jacket enclosing the 
 mercury as much as possible. The steam escapes by a hole in the top of this 
 outer tube. In improved hypsometers it is made to retrace its path downwards 
 and escape at the bottom of the steam jacket. 
 
 With this apparatus altitudes are determined by means of specially prepared 
 tables provided with the instrument, giving the heights above sea level corres- 
 ponding to a particular temperature at which water boils, and also corrections 
 to be made for latitude, mean temperature of the air, and decrease of gravity 
 with increase of elevation. Inv. 1893-149. 
 
 MEASUREMENT OF ANGLES. 
 
 Primitive notions of direction were probably derived from celestial 
 observations, and the practice, universal among early civilizations, of 
 orienting temples in a definite direction may be considered to mark 
 the commencement of angular measurement. An early Egyptian 
 instrument (merkhet), used by the astronomer priests to determine a 
 meridian line is represented in the Astronomy Collection, and illustrates 
 the antiquity of the plummet as a sighting appliance for determining 
 direction and time. The “ gnomon,” or primitive sun-dial, has much 
 in common with the Egyptian “ merkhet,” and was possibly derived 
 
53 
 
 from it. The evolution of the later Greek and Roman instruments 
 for measuring celestial and terrestrial angles can readily be traced 
 from these two primitive appliances for meridian, direction and time 
 determination. For the execution of ordinary surveys upon a hori- 
 zontal plane, the ancients relied principally upon the direct measure- 
 ment of the sides of triangles or quadrilaterals, supplemented partly 
 by an empirical theory of similar triangles, and partly by a frequent 
 use of right angles. For these purposes they employed either a species 
 of ‘‘surveyor’s cross,’ such as the Roman “ Groma”’ (see No. 135), 
 by which two rectangular directions could be simultaneously sighted, 
 or else a “ dioptra,’’ consisting of a rotatable sight-rule, which could 
 be successively directed in two mutually perpendicular directions 
 defined by stops or index lines. Hero of Alexandria (civea 50 A.D.), 
 described such a “ dioptra ’’ in which the sight-rule could rotate over 
 a circular plate which could be set in any desired plane. For astro- 
 nomical purposes, such as latitude determinations, the circular plate 
 was graduated in degrees, and the instrument thus arranged bore a 
 striking resemblance in appearance and function to the modern 
 altazimuth. 
 
 The development of portable astronomical instruments, such as 
 the astrolabe and quadrant, by the Arabs, and the study of trigono- 
 metry, led to a modification of surveying methods, consisting in the 
 increasing substitution of angular for linear measurement, and the 
 corresponding evolution of portable surveying instruments for measur- 
 ing angles in any plane. Hence arose the surveying theodolites, 
 graphometers and quadrants of the Renaissance period, the former 
 being principally used for measuring angles in a horizontal plane and 
 the latter for measuring altitude angles. Both are essentially simple 
 modifications of the astronomical and nautical astrolabes of the Arabs, 
 the divided circle, semicircle or quadrant being mounted upon a 
 stand, usually by means of a knuckle or universal joint, and provided 
 with a pivoted compass needle for orientation. At this period many 
 surveyors preferred instruments based on linear rather than circular 
 graduated scales. The Geometrical Square of Purbach was of this 
 type, and was an obvious modification of the quadrant. Other linear 
 instruments, however, such as the cross-staff and jacob-staff, were 
 copies of Ptolemy’s Triquetrum, in which the chord of an angle was 
 measured on a linear scale instead of its arc on a circular one. 
 
 In 1571, Thomas Digges, in his Pantometria, described the various 
 surveying instruments in use at that period, including a special type 
 of theodolite, designed by his father, Leonard Digges, in which the 
 sight-rule was mounted upon a graduated vertical semicircle which 
 could rotate upon a vertical rod; the latter was in turn supported 
 upon a horizontal ruler which could revolve over a graduated circle. 
 This instrument thus foreshadowed the altazimuth theodolite, but it 
 does not seem to have found much favour. About the same time, 
 in 1590, Praetorius modified the ordinary plane-table by providing 
 it with a clinometric sight-rule enabling altitude angles to be measured 
 simultaneously with azimuths. 
 
 During the 17th century several auxiliary appliances were invented 
 and incorporated in the design of angle-measuring instruments. 
 Gallileo’s telescope was not suited to surveying, but the invention of 
 the Keplerian eyepiece transformed it into a sighting appliance far 
 superior to the ordinary sight-rule. Gascoigne’s invention about 1640 
 of the movable micrometer wire for the telescope eyepiece also added 
 
 (15814) C3 
 
54 
 
 to its convenience and precision. Picard appears to have been the 
 first to apply, in 1666, the telescope to a surveying instrument. In 
 1631, Pierre Vernier devised the scale bearing his name, and thereby 
 greatly facilitated the accurate determination of the position of the 
 sight-rule with respect to the graduated scale. Verniers soon became 
 necessary adjuncts of all accurate angle-measuring instruments. 
 M. Thévenot invented the spirit level about 1661, but this useful 
 appliance does not seem to have been utilized on surveying instru- 
 ments until the commencement of the next century, when it rapidly 
 superseded the plummet as a means for verticalizing the axes of 
 theodolites. 
 
 When, at the beginning of the 18th century, telescopes began to 
 supersede sight-rules on theodolites, the consequent restriction of the 
 field of view necessitated a vertical as well as a horizontal motion of 
 the telescope, and therefore the provision of a vertical graduated arc 
 to measure this rotation. This century therefore witnessed the 
 evolution of the altazimuth theodolite, in which a vertical graduated 
 arc was fixed to the telescope and revolved with it between standards 
 mounted upon the rotating horizontal vernier plate ; a compass was 
 usually mounted upon this latter plate, and a bubble level was attached 
 first to the telescope and later to the plate also. With such an instru- 
 ment, azimuth angles and altitude angles were measured separately 
 but simultaneously. For many purposes, however, where it was 
 desired to measure an angle formed by rays from the station to two 
 distant points, an instrument in which the plane of the graduated 
 circle could be set so as to pass through the two distant points was 
 obviously convenient. The reflecting sextant, developed by Newton 
 and Hadley in the late 17th and early 18th centuries, fulfilled this 
 purpose and presented the additional advantages that both distant 
 points could be sighted simultaneously, and that the instrument could 
 be held in the observer’s hand. The reflecting circles of Tobias Mayer 
 (1767) and Borda (1775) combined the reflecting principle of the 
 sextant with the principle of the repetition of angles that had been 
 propounded by the former. Such reflecting instruments found more 
 favour on the continent than in England, where the altazimuth theo- 
 dolite, as improved by Jonathan Sisson and Jesse Ramsden, became 
 the standard instrument for the precise measurement of surveying 
 angles. In minor surveys, the combination of the plain theodolite 
 and compass known as the “ circumferentor,” without telescope, was 
 commonly employed. With this instrument magnetic bearings could 
 be quickly read off directly by means of the compass, whilst relative 
 angular azimuths could be more precisely measured by the verniers on 
 the divided limb of the theodolite. 
 
 During the latter half of the 18th century, the interest aroused by 
 Newton’s theory of gravitation combined with military exigencies to 
 bring about a rapid succession of geodetic arc measurements and 
 triangulation which called for terrestrial angular measurements of a 
 precision not hitherto deemed necessary. In England, Ramsden 
 constructed two large altazimuth theodolites (see No. 217) and many 
 smaller ones for the projected trigonometrical survey of the country ; 
 these instruments embodied all the recent improvements due to other 
 inventors, and many due to Ramsden himself. He constructed a very 
 accurate dividing machine for graduating the circles, devised special 
 micrometer microscopes for increasing the precision with which the 
 circles could be read, applied tangent clamps and slow motion screws 
 
oe 
 
 to enable the telescope to be correctly adjusted upon the distant point, 
 and mounted all the movable parts upon well-designed and machined 
 axes to ensure easy manipulation and accurate rotation. 
 
 In the 19th century, the tendency has been towards reducing the 
 size of theodolites without decreasing their precision. The unceasing 
 progress in the construction of optical and mechanical parts has 
 enabled the desired accuracy of measurement to be attained with 
 instruments having from 3 in. to 12 in. graduated circles, according 
 to the precision required. The principal modifications in design have 
 consisted in the adoption of the transit principle whereby the telescope 
 can be completely revolved on its horizontal axis of rotation, even 
 upon the smallest instruments, and the tendency to employ micro- 
 meter microscopes in preference to verniers whenever special accuracy 
 of measurement is desired. The elaborate circular compass has been 
 gradually replaced by a simple trough compass for occasionally orienting 
 the theodolite, although, for precise work, no reliance is placed upon 
 the magnetic needle owing to local attraction. 
 
 The telescopes of modern theodolites are usually provided with 
 tacheometric diaphragms for the indirect measurement of distances, 
 and, in recent years, with movable internal lenses for focussing. The 
 circles are accurately graduated on silver by dividing machines, and 
 the practice of enclosing these circles in dustproof cases is increasing. 
 
 FIXED ANGLE INSTRUMENTS. 
 
 133. PHOTOGRAPHS OF A CARVING OF A GROMA. Taken 
 by the Municipal Museum, Ivrea, N. Italy. 
 
 Upon the tombstone of a Roman ‘ Mensor,”’ L. Aebutius Faustus, who lived 
 about 100 A.D., is carved a crude representation of the ‘“‘ groma,”’ or surveyor’s 
 cross, used by Roman surveyors to set out right angles. The photograph 
 exhibited was taken from this tombstone, which is preserved in the Municipal 
 Museum at Ivrea, North Italy. 
 
 The carving is a somewhat conventionalized perspective sketch, showing the 
 two crossed arms of the “ stella,’”’ with only two ‘“ pondera’’ (weights) instead 
 of the four usually suspended, one from each end of the “stella.”” The 
 *‘ ferramentum,” or staff, to support the “‘ stella ’’ is also depicted, passing through 
 the intersection of the two arms of the latter. 
 
 In using such a groma, it would be set up so that one plummet hung over the 
 station point, and one of the adjacent plummets lay in the direction from which 
 the right angle was to be set off; a sight through the other adjacent plummet 
 would then be taken from the station plummet. 
 
 (See Schdéne, ‘‘ Das Visirinstrument der r6mischen Feldmesser,’’ Jahrb. d. 
 Deutsch. Archaolog. Instituts, XVI, I901, pp. 127-132.) Inv. 1923-492. 
 
 134. DRAWING OF A RESTORATION OF A SMALL GROMA. 
 
 Another example of the Roman groma, found near Eichstatt, Bavaria, has 
 served as basis for Professor Schdéne’s reconstruction illustrated in this drawing. 
 The groma discovered consisted of an iron ‘stella’’ mounted upon a “‘ ferra- 
 mentum,” also of iron. The ends of the arms of the stella are bent and transfixed 
 by iron bolts, which served to attach a square wooden frame—since perished—to 
 the “‘stella,”” and to carry the plumb lines. 
 
 The length of each side of the wooden framework was 35 cm., and the height 
 of the “‘ ferramentum,”’ exclusive of the upper and lower axis pins, is 35:5 cm. 
 This instrument was, therefore, a small type of groma, and was probably mounted 
 upon a wooden tripod, as shown in the drawing, so that the “stella’’ was at 
 approximately the height of a man from the ground. 
 
 (See Schone, ‘‘ Das Visirinstrument der rémischen Feldmesser,’’ Jahrb. d. 
 Deutsch, Archaolog. Instituts, XVI, I9g01, pp. 127-132.) Inv. 1923-493. 
 
 (15814) C4 
 
56 
 
 135. REPRODUCTION OF A ROMAN GROMA. Made in the 
 Museum. Plate III, Fig. 1, p. 56. 
 
 During excavations in the Via Abbonanza at Pompeii, in 1912, under the 
 direction of Cav. M. Della Corte, fragments of a Roman groma were discovered. 
 These fragments were sufficiently preserved and complete to enable Dr. Della 
 Corte to reconstruct the original instrument with a certainty not hitherto attained 
 owing to the meagre data available. The instrument exhibited has been con- 
 structed in accordance with the measurements and drawings supplied by 
 Dr. Della Corte. 
 
 As previously reconstructed, the groma consisted of a cross, composed of two 
 equal perpendicular arms intersecting at their centres, mounted upon a supporting 
 staff (ferramentum), upon which the cross could revolve in azimuth; from the 
 ends of the cross were suspended four plummets, through which sights were 
 taken by the surveyor. This reconstruction comprises all the foregoing portions, 
 with the additional modification that the cross is not supported directly upon 
 the staff, but is pivoted upon the outer end of a curved bracket, which is in turn 
 mounted rotatably upon the head of the staff. This arrangement enables the 
 centre of the cross to be set above the mark on a boundary stone, whilst the 
 staff is driven into the ground beside the stone; also, sights could be taken 
 through opposite, not adjacent, plumb lines without interference from the staff. 
 
 The wooden staff, which is nearly 2 metres long, terminates at its lower end 
 in a fluted iron shoe, and is encircled at its upper extremity by a brass collar, 
 which forms a pivot for the curved bracket. This bracket is of wood, reinforced 
 along its upper and lower faces by curved brass strips, and terminating at the 
 staff end in a brass collar and at the cross end in a brass pivot upon which the 
 cross is mounted. The horizontal distance between the cross and bracket pivot 
 centres is 23:4cm. The cross has a brass centre, and four tapered wooden arms, 
 completely enclosed in thin iron sheeting and reinforced at their intersection 
 by brass angle brackets. The bronze plummets are suspended through holes in 
 the arms at distances of 45 cm. from the centre of the cross. Each plummet 
 resembles its opposite in size and shape but differs from the neighbouring ones. 
 
 The actual portions discovered included all the metal parts, with a few traces 
 of wood fibres embedded therein. The total height of the instrument is alone 
 considered problematical. 
 
 (See M. Della Corte’s article ‘‘ Groma,’’ in Monumenti Antichi, Accademia 
 dei Lincei,,,.Vol. XXVIII, 1922:) Inv. 1923-395, S.M. 1828. 
 
 136. CROSS STAFF. Made by Baldantoni, Ancona. 
 
 This instrument consists of a hollow brass sphere of 24 in. diam., with vertical 
 great circle sighting slits cut every 45° in its surface. It is used like the ordinary 
 sight-rule cross staff for setting off angles of any multiple of 45°, and was probably 
 made towards the end of the 17th century. 
 
 A socket fixed to the sphere serves to connect it to a stand. Inv. 1889-22. 
 
 137. CROSS STAFF. Made by Dollond. 
 
 This instrument, which was made in the late 18th or early 19th century, has 
 two 6-in. brass sight rules fixed accurately at right angles to each other and a 
 central screw thread for attaching it toastand. Backsights and foresights can be 
 taken through either rule, every vertical arm having both a slit backsight and 
 a wire foresight. This type of instrument is mostly used in detail chain and 
 offset surveying, for setting the offset rod at right angles to the chain. 
 
 Inv. 1921-81. 
 
 138. OPTICAL SQUARE. Presented by T. H. Court, Esq. 
 
 This instrument, made probably near the end of the 18th century, has two 
 mirrors fixed to the base of a cylindrical brass box, with their planes at an angle 
 of 45° to each other. One mirror is half unsilvered for direct vision, the other 
 half and the whole of the second mirror being silvered, to reflect rays from objects 
 at right angles to the line of vision. A circular aperture and rectangular windows 
 are pierced in the side of the box opposite the mirrors. Inv. 1917-22. 
 
 139. OPTICAL OPPOSITE. Made by Adie, London. Presented 
 by T. H. Court, Esq. 
 In this instrument, made in the middle of the 19th century, two right-angled 
 
 reflecting prisms are mounted one upon the other, with their reflecting faces 
 perpendicular, 
 
PLATE Iii. 
 
 Sg ‘d ‘ayjopoeyy Apreq 
 
 ‘2 “314 
 
 9S 
 
 d ‘euro3y 
 
 "I '3Iy 
 
Jf 
 
 In using the instrument the observer takes off the cap and screws it beneath 
 the base to serve as a handgrip. Looking at the common face of the two prisms, 
 he views objects on his left hand through the lower prism, and those on his 
 right through the upper. Two objects which appear in the same vertical line 
 will be at opposite ends of a line through the instrument, perpendicular to the 
 direction of sight. Inv. 1911-152. 
 
 140. OPTICAL OPPOSITE AND SQUARE. Made by Beaulieu, 
 Brussels. Presented by T. H. Court, Esq. 
 
 In this instrument, made probably towards the end of the 18th century, 
 three pairs of mirrors are mounted successively in a rectangular brass box, 
 whose sides are provided with the necessary windows. The three pairs of 
 mirrors make angles of 224°, go° and 45° respectively with each other, and one 
 mirror of each pair is left partly unsilvered for direct vision. The direct and 
 reflected rays for each pair are thus inclined at 45°, 180° and go° respectively. 
 
 Inv. 1911-154. 
 
 141. OPTICAL OPPOSITE. Presented by the Admiralty. 
 
 By means of this instrument an observer can place himself in the direct 
 line between two objects. It consists of two mirrors set in small parallel sighting 
 tubes connected by a longer perpendicular tube. The planes of the mirrors 
 are perpendicular and one mirror is partly unsilvered. The observer views the 
 front object directly through the unsilvered portion and the rear object by 
 double reflection at the two silvered surfaces. When the images coincide, he 
 is in the direct line between the two objects. The sighting tubes are closed by 
 glass windows where necessary. Inv. 1876-1208. 
 
 PLANE TABLING. 
 
 142. TELESCOPIC” PLANE TABLE ' ALIDADE.” Made ” by 
 A. Baldantoni, Ancona. 
 
 The application of the telescope and vertical arc to the alidade of a plane 
 table took place about the same time as the corresponding addition to the 
 theodolite. 
 
 In this exhibit, made in the late 17th century, a vertical pillar fixed perpen- 
 dicularly to the alidade carries the vertical semicircle and attached telescope, 
 which can rotate about a horizontal axis in the top of the pillar. A vernier 
 fixed to the pillar records the altitude angle of the line of sight. The alidade 
 is 2 ft. 4 in. long by 1# in. wide and has one edge bevelled. Over this edge, 
 at the two extremities of the rule, are attached two plain sights, a slit backsight 
 and a vertical wire foresight. A few inches in front of the backsight is the 
 vertical pillar carrying the telescope axis. By means of its milled edge and a 
 wormwheel, the socket of this pillar can be slightly rotated in azimuth, and 
 carries the telescope with it. This serves to adjust the line of sight parallel 
 to the bevelled edge of the rule. The semicircle has a diameter of 5 in. and 
 is graduated in degrees up to 50° of elevation or depression. Its edge is milled 
 and gears with a wormwheel attached to the pillar. 
 
 The telescope rests in semicircular collars fixed to the diameter of the semi- 
 circle, and is kept in place by hinged straps and screws. It has a rack-focussing 
 objective lens of # in. aperture and a fixed eyepiece tube containing a compound 
 field lens and eyepiece with a fixed cross wire diaphragm between the two 
 lenses of the eyepiece. A spirit level is mounted upon the telescope parallel 
 to its axis. The vernier attached to the pillar is slightly adjustable by means 
 of a slide and lever, and reads the vertical arc to 3 minutes. Inv. 1889-21. 
 
 143. PLANE TABLE AND ALIDADE. Made by W. & 5S. Jones. 
 Lent by T. H. Court, Esq. 
 
 This is an early 19th century outfit comprising plane table, tripod and brass 
 alidade. The table, which is 17 in. by 15 in. can be rotated and levelled inde- 
 pendently of the tripod. The alidade is nearly 2 ft. long and has a detachable 
 combined back and foresight at each extremity, the slits and wires being vertically 
 above the bevelled edge of the ruler. 
 
58 
 
 Various useful scales of length are engraved upon the alidade, and the 
 rectangular boxwood frame forming the edge of the table is graduated on both 
 faces in half degrees and tenth-inches. This frame is detachable and serves 
 to clamp the paper to the table. Three centres corresponding to the half-degree 
 graduations are marked on the table proper, by which means it can be used 
 also as a theodolite. There is a dovetailed recess beneath the table for supporting 
 a compass box (missing in the instrument exhibited) to orient the instrument, 
 and a small spirit level is used to level the table. The brass piece connecting 
 the table to the tripod has a conical axis and socket, and a spherical ball and 
 socket joint for orienting and levelling the table independently of the tripod. 
 The latter has a brass tripod head and circular tapering legs, 4 ft. long, jointed 
 in the middle, and fitted with brass shoes at the extremities. Inv. 1917-106. 
 
 144. PLANE TABLE OUTFIT. Made by T. Cooke & Sons. 
 
 This is a small outfit of modern construction suitable for rough reconnaissance 
 and engineering surveys. The table is 17 in. by 14 in. and has its under edges 
 removable for inserting and clamping the drawing paper. The boxwood ruler 
 is 12 in. long and has folding metal sights at each extremity. 
 
 The back of the table has a brass plate, recessed to receive the knob of the 
 bolt clamping it to the tripod. A small circular spirit level is used to level the 
 table, the slope of the tripod legs being changed to effect this. A small trough 
 compass with a 3-in. needle is provided for orienting the table. The bevelled 
 edges of the alidade are divided in scales of 6 and 8 inches to the mile, to read 
 10 yd. on each scale. The sights consist of a slit backsight and strip foresight, 
 each vertically above the centre line of the alidade. Inv. 1899-52. 
 
 145. JOHNSON PLANE TABLE OUTFIT. Made by W. & L. E. 
 Gurley. 
 
 A type of modern plane table outfit with telescopic alidade, suitable for 
 topographical surveys of a comparatively high degree of precision, is here 
 exhibited. Similar outfits have been extensively employed by the topographers 
 of the U.S. Geological Survey. 
 
 The table is 30 in. by 24 in., and is carefully made in sections to preserve its 
 shape. Brass screw pins fitting into sockets at the corners and middle of the 
 sides are used for fastening the drawing paper to the table, which is slightly 
 recessed at these points to allow the heads of the pins to be flush with the upper 
 surface of the board and thus permit the alidade to slide freely over the pins. 
 
 The board is connected to the tripod by means of a Johnson quick-levelling 
 socket, which permits it to be easily levelled and rotated in azimuth inde- 
 pendently of the tripod. Two butterfly nuts control these movements of levelling 
 and rotation, and, when tightened, clamp the table rigidly to the tripod. This 
 latter has three split legs, 4 ft. long, attached by bolts to the metal tripod head 
 and terminating in metal shoes. 
 
 Any point on the sketch can be set vertically above the corresponding station 
 point, after the instrument is levelled, by means of the wooden plumbing arm 
 which consists of two pieces so jointed and marked at the joint that, when the 
 upper piece is set flat on the table, with its point over the point on the sketch, 
 and the joint clamped with its two marks coincident, the metal hook at the 
 end of the second arm lies vertically beneath the point. The plumb-bob is sus- 
 pended from this hook and the table moved until it lies over the station point. 
 A flat 44-in. square metal plate, upon which are mounted two mutually perpen- 
 dicular spirit levels and a central compass box with a 3-in. needle and ring 
 divided in degrees, is used for levelling and orienting the table. 
 
 The alidade consists of a flat brass ruler 20 in. long by 4 in. wide, having 
 one edge bevelled and a central pillar projecting vertically from its upper face 
 and supporting the telescope and attached vertical circle, and the fixed vernier 
 for reading this latter. The vertical circle has a diameter of 4:5 in., and is 
 divided on silver to half degrees, read by the vernier to single minutes. It is 
 fixed with the telescope to the horizontal axis, which can rotate within a socket 
 fixed to the vertical pillar. A tangent clamp and slow motion screw is fitted 
 to this axis and connected to the vernier. 
 
 The telescope is of the pattern usually fixed to 5-in. transit theodolites. It 
 has a rack-focussing objective lens of 1-2 in. aperture and 8 in. focal length, 
 and a rack-focussing eyepiece magnifying 20 diameters. The fixed diaphragm 
 has two horizontal stadia wires as well as the usual cross wires. Inv. 1914-742. 
 
59 
 
 146. JAHNS’S POLYMETER. Made by Schmidt & Hansch, 
 Berlin. 
 
 The ‘‘ Vielmesser ’’ or Polymeter, invented by R. Jahns in 1873, is an early 
 example of a plane table alidade adapted to measure the distance, altitude 
 and azimuth of a point simultaneously. It is mounted upon a horizontal plane 
 table, and used in conjunction with a tacheometric staff of constant interval, 
 set vertically at the distant station. 
 
 The alidade consists of a T-shaped base piece, mounted on three levelling 
 screws and provided with two spirit levels. A vertical arm projecting from 
 the base piece carries two metal rulers, which can be rotated independently 
 about a common horizontal axis in the vertical arm by means of quick and 
 slow motion screws. The axis is perpendicular to the longer arm of the base 
 piece, and there is a marking pin attached to the latter vertically beneath the 
 axis, so that the alidade can be centred above the observing station point on 
 the plan and rotated as a whole about this point. After this has been done, 
 the inner edges of the two rulers are directed upon the upper and lower reference 
 marks on the distant staff by means of a telescope fixed to the upper ruler. 
 A vertical slide, which carries a distance piece and corresponding wooden scales, 
 is then moved along the alidade until the distance piece—which can move up 
 and down on the slide—fits accurately between the two rulers. There is a 
 lever multiplying device to enable this adjustment to be made with extreme 
 precision, and the final motion of the slide is performed with a slow motion 
 screw. The marking pin attached to the slide vertically beneath the distance 
 piece is now depressed, and the point so defined will correspond to the horizontal 
 projection of the distant staff, both as regards azimuth and distance. The 
 position of the distance piece on the wooden scale on the vertical slide gives 
 the altitude of the staff. The scale of representation in each case is the ratio 
 of the width of the distance piece to the staff interval. Five separate distance 
 pieces are provided, with the corresponding wooden scales, so that a suitable 
 
 scale of representation can be chosen. Inv. 1876-1200. 
 147. PRISMATIC ALIDADE. Made and lent by C. F. Casella & 
 TOA: 
 
 In this alidade devised by the Rev. A. J. Potter for use with a plane table, 
 the usual folding sights are replaced by a single right-angled prism which is 
 mounted at one end of the ruler, with its reflecting base vertical and parallel 
 to the edge of the latter. Any distant object can be viewed directly above 
 the prism, whilst an image of the object can be seen through the prism after 
 refraction on each face and reflection on the base. When the two images thus 
 seen are made to coincide, the edge of the ruler must point at the distant object, 
 and the direction line of the latter can be plotted. 
 
 The reflected image is inverted left for right compared with the direct image 
 and is displaced laterally if the ruler is not sighted correctly so that coincidence 
 is readily effected by adjusting the ruler. 
 
 The advantage of this type of alidade lies in the rigidity of the sights, absence 
 of parallax with clearness of illumination, and the increased accuracy of align- 
 ment rendered possible by the fact that, for a given angular displacement of the 
 ruler, the displacement of the reflected image is double that of the ruler. 
 
 The ruler is 15 in. long, and has its bevelled edge graduated in inches and 
 tenths. A circular bubble level is mounted upon it near the prism. 
 
 Inv. 1924—466- 
 
 COMPASSES. 
 148. EARLY COMPASS. 
 
 This consists of a square brass plate with an engraved circle graduated in 
 degrees over which moves an index arm carrying a small compass box. It 
 dates from the 17th century and was probably used for plotting the bearings of 
 objects taken with a circumferentor. 
 
 The side of the square plate is 3 in. long, as is also the diameter of the graduated 
 circle. The compass box has a diameter of nearly 1 in. The N.-S. and E.-W. 
 lines are engraved upon the base of the compass. When the needle is set over 
 the N.-S. line, and the index arm which is always parallel to this line is set 
 upon the graduation corresponding to the bearings, two sides of the square 
 plate will be in the proper direction for plotting this bearing. The circle is 
 divided into four quadrants and each quadrant is graduated from 0° to 90°, 
 all the graduations beginning on the same line parallel to two sides of the plate. 
 
 . Inv. 1880-198. 
 
60 
 
 149. AZIMUTH COMPASS. Made by Dollond. Presented by 
 TeblaGourtslisa: 
 
 This instrument, made by Dollond about 1770, is typical of the surveyor’s 
 compass previous to the invention of Kater’s mirror compass or Schmalcalder’s 
 prismatic compass. Here the position of the needle upon the graduated compass 
 ring, which is fixed, cannot be read at the same time as the object is viewed through 
 the sights. Consequently it is necessary to use a stand to support the compass, 
 and a socket is attached to the base of the wooden case for this purpose. The 
 fixed compass ring is graduated in degrees continuously from 0° to 360° in a 
 counter-clockwise direction. The slit backsight and wire foresight are fixed to the 
 compass box at the o° and 180° respectively. When the object is sighted, the 
 reading of the S. end of the needle upon the graduated ring gives the correct 
 magnetic bearing. 
 
 The compass ring is 44 in. diameter, and thesights are 64. in. apart. These latter 
 can be folded down upon the face of the compass for transport, and a recess is 
 provided in the lid of the wooden case for their reception. This lid, on being closed, 
 depresses a pin which lifts the needle from its pivot and clamps it against the 
 glass cover. Inv. 1912-243. 
 
 150. AZIMUTH COMPASS. Made by Adams. 
 
 This compass, probably made about the end of the 18th century, differs only 
 in minor details from the adjacent one made by Dollond. It has a 3}in. compass 
 ring graduated in degrees, mounted in a brass case to which sights can be fitted, 
 and which has a threaded socket for attaching the instrument to a stand when 
 taking bearings. 
 
 The graduations are numbered every 10° in quadrants, with the o° at the N. 
 and S. points. Opposite these points are sliding sockets in the outer case. The two 
 sights, each having a slit and a wire for use as backsight and foresight respectively, 
 are mounted in these sockets when in use, or fitted within the lid of the compass 
 case for transport. The sights are 3in. long. The compass needle is provided 
 with a lifting lever actuated by a pin which is depressed when the lid is fitted on 
 the case, but has no clamping arrangement. Inv. 1909-140. 
 
 151. MIRROR AZIMUTH COMPASS. Made by Thos. Jones. 
 Lent by W. Shackleton, Esq., F.R.A.S. 
 
 The mirror attachment to the eye lens here shown was invented by Capt. 
 Kater at the beginning of the 19th century, and enables the observer to view the 
 graduations of the horizontal compass ring at the same time as he takes a sight. 
 The compass ring is fixed to the rotating needle and is so graduated in half-degrees 
 consecutively from 0° to 360° that correct bearings are read off immediately. 
 The diameter of the compass card is 44 in. 
 
 The usual slit backsight and wire foresight are fixed to the compass box at a 
 distance apart of 5in. A small pin fixed in the case vertically beneath the back- 
 sight serves as fiducial mark, and its position on the graduated scale is seen through 
 the mirror very clearly when the instrument is held about one foot away from 
 the eye. 
 
 The foresight can be folded down upon the top of the case, and the compass 
 card and needle are thereby automatically lifted from the pivot and clamped. 
 
 Inv. 1016-6.) CS. Di soo74, 1L.S, 
 
 152. PRISMATIC COMPASS. Made by Schmalcalder. 
 
 The reflecting mirror used by Captain Kater for viewing the graduated 
 compass ring was superseded in 1812 by the application of Schmalcalder’s 
 prismatic eyepiece to the compass. By this means the observer can read the 
 bearing and sight the object simultaneously, and can thus dispense with a stand 
 for the instrument. For this reason the prismatic compass has come into general 
 use for rapid reconnaissance surveys. The example exhibited was constructed 
 by Schmalcalder in the early part of the 19th century. 
 
 The compass card is 2} in. in diameter, and is fixed to the magnetic needle. It 
 is graduated in consecutive half-degrees from 0° to 260° in a clockwise direction, 
 the N. end of the needle being under the 180° mark. The diagonal prism is attached 
 to the plate which has a slit serving as backsight, and is ground convex on one 
 perpendicular face so as to magnify the image of the graduated card seen by 
 reflection at its inclined face, which is silvered. This backsight plate is jointed 
 
61 
 
 to tne compass box, and can be folded down when the instrument is not in use. 
 The foresight consists of a vertical wire fixed in a frame diametrically opposite the 
 backsight, and similarly jointed to the base so as to fold down when not in use. 
 
 Mirrors and coloured glasses are attached to the sights for taking bearing upon 
 the sun and other celestial objects. A lever is provided for lifting the needle from 
 its pivot and locking the compass card when desired, and there is a steadying pin 
 underneath the foresight for damping the oscillations of the card. 
 
 Inv. 1905-155. 
 
 153. TELESCOPIC PRISMATIC COMPASS. Made by Schmal- 
 
 calder. 
 
 This instrument, constructed in the early part of the rgth century, is essen- 
 tially a combined theodolite and prismatic compass. The telescopic alidade, 
 which carries the prismatic eyepiece and a vernier underneath this, can be rotated 
 about the vertical axis independently of the compass ring—the instrument thus 
 acting as a theodolite—or can be rotated together with the compass ring, in which 
 case it fulfils the functions of a prismatic compass. The compass ring is graduated 
 directly to 20’ in a clockwise direction and is read by vernier to single minutes. 
 
 The instrument is attached to a stand by means of a ball and socket arrange- 
 ment, and can be levelled independently of the stand by four screws which engage 
 in threads in the socket and press against the vertical axis. There is also a tangent 
 clamp and slow motion screw for rotating the whole instrument in azimuth, 
 whilst a circular rack and pinion inside the compass box serves to rotate the alidade 
 independently. This is actuated by the screw projecting beneath the compass. 
 The telescope carries a small spirit level, and is screwed at the eyepiece end to a 
 brass plate hinged to a sliding piece, whilst it rests at the objective end in a Y-piece 
 fixed to another sliding piece. These two sliding pieces terminate at their lower ends 
 in screws, which work in threaded plates screwed to the compass box. Thus the 
 telescope can be elevated at either end with respect to the compass box and can 
 be adjusted so as to be horizontal. It is focussed by sliding the objective lens. 
 
 . Inv. 1911-180. 
 
 154. PRISMATIC COMPASS AND CLINOMETER. Made by 
 Cary. 
 
 The quadrant attached to this instrument, which is in other respects similar 
 to the Schmalcalder prismatic compass, enables it to be used for measuring alti- 
 tudes as well as taking magnetic bearings. The quadrant has separate sights 
 and a small spirit level fixed to a movable radial arm. A mark engraved on the 
 outer end of this arm registers the altitude upon the graduated arc of the quadrant 
 when the arm is horizontal and the sights are directed upon the distant object. 
 
 The compass ring is 24in. in diam. and is fixed to the rotating magnetic 
 needle. Itis graduated continuously in half-degrees from 0° to 360°. As the N. 
 point of the needle is attached at the 180° mark of the ring, the correct bearing 
 of the object sighted is read off directly through the prismatic eyepiece attached 
 to the slit backsight. A small spirit level is fixed to the bottom of the case inside 
 and serves to set the compass horizontal. 
 
 The altitude quadrant has a radius of 2 in. and is attached to the wire foresight 
 ofthe compass. It is graduated in degrees from 0° to 90°, the radius corresponding 
 to the former being parallel to the compass plane, and carrying the supplementary 
 sights, a pin-hole backsight and a cross-wire foresight, for determining altitudes. 
 
 Mirrors and coloured glasses are attached to the two compass sights for taking 
 bearings on high objects such as the sun. Inv. 1917-78. 
 
 155. PRISMATIC COMPASS. Made by Chamberlain. 
 
 In this instrument, constructed about the middle of the 19th century, the 
 method of lifting and locking the compass card is similar to that employed by 
 Schmalcalder. The card is graduated in degrees, in two sets numbered consecu- 
 tively in the clockwise sense from 0° to 180°. There are no mirrors or coloured 
 glasses for celestial observations. 
 
 The diameter of the compass ring is 2$in. and it can be read by estimation 
 to the nearest 10’. Inv. 1872-79. 
 
 156. PRISMATIC COMPASS. Lent by F. Newton & Co. 
 
 This instrument, which was made about the middle of the 1gth century, 
 differs very little from those made by Schmalcalder about 50 years previously. 
 The compass card is not graduated continuously from 0° to 360°, but in two sets 
 of 180°, both in the clockwise sense. It reads directly to half-degrees, and by 
 estimation to 10’. The compass card is lifted and clamped by folding down the 
 foresight. 
 
62 
 
 The diameter of the compass ringis 34in. Theusual mirrorand coloured glasses 
 are provided for observing celestial objects. The eye prism is adjustable for 
 focussing, and there is a steadying pin under the foresight for damping the 
 oscillations of the compass needle. Inv. 1865-7. 
 
 157. PRISMATIC COMPASS. Made by Cary, Porter, Ltd. 
 Presented by the Admiralty. 
 
 Except for the arrangement to lift and lock the compass card, this instrument 
 differs very little from those made by Schmalcalder nearly a century previously. 
 The compass card, prismatic eyepiece, steadying pin, etc., remain the same, 
 but the eyepiece is here arranged so that the eye must be nearer the slit for 
 focussing than in Schmalcalder’s instruments, whilst the folding down of the fore- 
 sight upon the compass automatically lifts and clamps the card. 
 
 The compass card is 5 in. in diameter, and a duplicate card is provided. The 
 cardis graduated in half-degrees in the usual clockwise sense. Both the sights can 
 be folded down, and the prismatic eyepiece can be adjusted for focussing the gradua- 
 tions. There are the usual mirrors and coloured glasses for taking bearings of 
 celestial objects. A socket is rigidly fixed to the base of the compass box for 
 attaching it to a stand. When the compass is so mounted, bearings can be taken 
 to within 15’, and estimated to the nearest 5’. Inv. I9I1I-157. 
 
 158. PRISMATIC COMPASS (Mark VIII). 
 
 This is a modern type of compass specially designed for rough surveying 
 and for night marching upon a given bearing, previously set off on the edge of the 
 compass. It has a hair foresight upon the hinged lid, and a slit backsight upon 
 the hinged mounting carrying the prismatic reader. Various marks in luminous 
 paint upon the lid, cover plate and dial of the compass are used in night marching. 
 The compass has a spring damping pin for checking the oscillations of the dial 
 as well as a lifting pin for locking it when not in use. 
 
 The luminous marks on the lid are at the extremities of the hair foresight. 
 The glass cover plate of the compass box has also a luminous mark and can be 
 independently rotated so as to set this mark at any angle to the sight line. The 
 angle so set is measured by means of an index mark on the cover in conjunction 
 with the graduated edge of the compass box. This is set to the bearing upon which 
 it is desired to march, and the cover plate locked by means of a spring catch which 
 engages in the milled edge of the plate. The dial of the compass, which is fixed to 
 the magnetic needle and pivoted so as to rotate freely, has a luminous arrow 
 marked above the north pole of the magnet. When the compass is turned until 
 the luminous mark on the cover plate les above the luminous arrow on the dial, 
 the luminous marks at the extremity of the hair line on the lid point in the direction 
 it is desired to march. 
 
 For surveying purposes the compass dial is graduated in degrees round its 
 edge, the numbers being inverted so as to appear upright when seen through the 
 prism, and so arranged that the correct bearing of the line of sight is read directly 
 through the prism. There is a supplementary set of numbered graduations every 
 5° for direct reading without using the prism. A ring is hinged to the box to steady 
 it when taking bearings, and a lever on the lid automatically actuates the lifting 
 pin when the lid is closed. The compass has a diameter of 2in. Inv. 1922-41. 
 
 159. LIQUID PRISMATIC COMPASS. Made by Kelvin, 
 Bottomley & Baird, Ltd. 
 
 This instrument combines the advantages of prismatic reading with the 
 rapid and accurate setting afforded by nearly floating the compass card in a 
 liquid, which reduces the friction of the pivot and rapidly damps the motion of 
 the card. The dial can be illuminated for night reading by means of an electric 
 bulb fixed beneath the transparent base of the compass box. A socket attached 
 to the base enables the compass to be mounted upon the tripod exhibited with it. 
 
 The compass card has a diameter of 2-5 in. and is graduated in degrees, read 
 by an internal index point mounted beneath the centre of the prismatic reader. 
 The card carries two long cylindrical magnetic needles fixed below it with their 
 axes parallel, and has a central buoyant chamber resting upon the pivot. The 
 card, needles and pivot are enclosed in a watertight chamber filled with a mixture 
 of water and alcohol, which has a low freezing point. The card just rests upon 
 the pivot, without pressing much on it, so that there is little frictional resistance 
 to its rotation, and its motion is nearly “ critically damped ”’ by the liquid so 
 that it takes up its new setting after rotation within a few seconds. The prismatic 
 
63 
 
 reader has a V-shaped backsight used with the long foresight bracket upon which 
 is a travelling foresight. Owing to the freedom of the needle from vibration, 
 the reading can be taken to the nearest fifth degree. 
 
 The electric bulb serving to illuminate the dial for night reading is operated 
 by a plug switch and a battery of two small cells, carried in the leather case used 
 for transporting the compass. Inv. 1922-44. 
 
 160. BURNIER’S COMBINED COMPASS AND CLINOMETER. 
 Made by Bulbreck, Paris. Lent by T. H. Court, Esq. 
 
 In this type of military compass, invented by Capt. Burnier about 1850, 
 the graduated edge of the compass ring, which is fixed to the needle and rotates 
 with it, is perpendicular to the plane of the needle. The bearing of the sighting 
 thread can therefore be viewed at the same time as a sight is taken. 
 
 A weighted disc is mounted parallel to the compass ring, and has a similar 
 graduated edge. On clamping the compass ring and bringing the planes of both 
 rings vertical, the disc is unclamped and serves as a clinometer. Either ring is 
 viewed through a cylindrical eye lens, which closes the brass case and is about 
 2cm. distant from the graduated edges. 
 
 A double-jointed socket attached to the base of the case permits the instrument 
 to be rotated in a vertical or a horizontal plane upon a stick stand, and serves 
 as a support for holding the instrument when no stand is used. 
 
 The compass ring is graduated in degrees continuously from 0° to 360°, and 
 the magnetic bearing of the object sighted is read off directly. The clinometer 
 ring is likewise graduated in degrees, and is weighted at the 90° portion. Each 
 ring is 4 cm. diameter and has an edge 2 mm. deep. 
 
 The usual folding slit and wire sights are attached to the top of the case, 
 and are easily viewed simultaneously with the graduated rings when the instrument 
 is held about a foot from the eye. A wire mounted within the case near the 
 graduated edges in the plane of these two sights serves as fiducial mark. 
 
 The whole instrument is compact and serviceable, and is in general use on the 
 Continent for military and reconnaissance surveys. 
 
 (See Laussedat’s ‘“‘ Recherches sur les Instruments Topographiques,’”’ Vol. I, 
 Pp. 306.) Inv. 1916-64. 
 
 161. EXPERIMENTAL “ POCKET ALTAZIMUTHS.”  Pre- 
 sented by C. F. Casella & Co., Ltd. 
 
 These two examples of Casella’s ‘‘ Pocket Altazimuth,”’ or combined compass 
 and clinometer, illustrate the experimental stages of development of the instru- 
 ment, which is represented in its modern form by an adjacent exhibit. 
 
 Each instrument consists of two graduated aluminium rings separately 
 mounted upon pivots along the axis of a single cylindrical metal box, which is 
 closed by glass faces. A magnetic needle is fixed to one ring, which acts as a 
 compass to determine azimuths, whilst the other ring is loaded at one point and 
 serves as a plummet clinometer for measuring altitude angles. When not in use, 
 each ring can be lifted from its pivots and clamped by means of a lever actuated 
 by a sliding knob. 
 
 In one instrument the graduated edges of the rings are read by means of two 
 small telescopes, one for each ring, mounted on the metal box; a thin wire is 
 stretched across the box between the telescopes and the rings and defines the 
 index line, or line of sight. In the other instrument, a single telescope can be 
 fitted so as to read either ring alternately, and also to sight upon a distant object. 
 This telescope has a single objective lens and a dual eyepiece, comprising a lens 
 for sighting on the distant object and a prism for patel the graduations upon 
 the upper face of the compass or clinometer ring. 
 
 In both instruments the rings are graduated in Bee res and a flat footing is 
 attached to the box so that it can be placed upon any surface of which the 
 inclination is to be measured. Inv. 1923-414. 
 
 162. POCKET ALTAZIMUTH. Made by Casella. 
 
 This is a compact form of combined telescopic compass and clinometer of a 
 design embodying improvements due to Sir Francis Galton, F.R.S. Either 
 magnetic bearings or altitude angles can be measured independently, but not 
 simultaneously, with it. It is particularly suitable for very approximate 
 reconnaissance surveying. 
 
64 
 
 It consists of two graduated rings, separately pivoted within a cylindrical 
 case, having glass-covered ends parallel to the planes of the two rings. One of 
 the rings is graduated at the 32 points of the compass, and has a diametrical 
 magnetic needle whose centre rests on the pivot. The other ring is loaded at one part 
 of its circumference and is pivoted to the case on each side. It is graduated in 
 degrees, numbered in quadrants, with the zero point opposite the centre of gravity 
 of the load. Both rings have a diameter of 1? in., the external diameter of the 
 cylindrical case being 2}in. The compass ring only moves when its plane is 
 horizontal, the clinometer ring when its plane is vertical. Each ring is clamped 
 when the other is in use, and is read by the position of an index mark on the 
 cylindrical case. The latter carries a telescope, whose axis is parallel to the ends 
 of the cylinder and passes through the line of pivots of the ring. The telescope 
 has its objective tube and eyepiece tube separately fixed to the case, the latter 
 being partly without and partly within the case, and having a slide-focussing 
 eyepiece. A hair line is stretched across the case just behind the objective lens. 
 A base piece for supporting the instrument is attached to the case, with its plane 
 base parallel to the axis of the telescope. There is also a central threaded hole 
 in the case for fixing it to a tripod. A damping spring is provided for use with 
 the compass ring. The complete instrument weighs only 8 oz. 
 
 163. TELESCOPIC COMPASS. 
 
 This is a modern example of a French type of surveying compass, with an 
 eccentrically mounted telescope, used on the Continent on minor work. The 
 compass is fixed in a flat, rectangular wooden box, to one side of which is attached 
 the telescope, rotating about an axis fixed perpendicularly to this side. The 
 whole instrument can rotate in the plane of the compass ring, which can itself 
 be varied by means of a ball-and-socket joint. 
 
 The instrument is attached to a “‘ peg’’ stand by means of a brass socket 
 connected to the compass by a ball-and-socket joint. By this means the compass 
 can be set horizontal without adjusting the stand. The compass ring has a diameter 
 of 5 in., and is graduated in half-degrees, numbered every 10° in the clockwise 
 sense. The plane of rotation of the telescope is parallel to the 0° to 180° line 
 of the ring. The telescope has a single fixed objective lens with a focal length 
 of 6in. The cross wires are also fixed, the single eye lens being slide focussing. 
 A set-screw serves to clamp the socket to the tripod peg, and a clamping screw 
 to prevent the horizontal rotation of the compass upon its platform. The ball- 
 and-socket joint is fixed by means of a nut and bolt. 
 
 A small tube spirit level is mounted on one side of the compass box, and the 
 compass needle can be elevated from its pivot by a locking lever. 
 
 Inv. 1919-4106. 
 
 Inv. 1893-148. 
 
 EARLY THEODOLITES. 
 164. EARLY CIRCUMFERENTOR. Lent by T. H. Court, Esq. 
 
 The date of this instrument is indicated by the allowance for magnetic 
 declination on the compass ring, on which the meridian is engraved at the point 
 10° KE. of N. This corresponds to the year 1590, in England. 
 
 It consists of a brass circle, 4} in. diam., having a small suspension ring, 
 and a pair of fixed sights which define a horizontal line when the instrument is 
 suspended from the ring. This circle acts as the base plate for a compass box, 
 34 in. diam., which has a separate graduated circle of 3 in. diam. The circular 
 side of the compass box is movable about its centre, and carries a pair of sights, 
 with pointers attached, the latter moving over the graduated circle on the 
 base plate. 
 
 The instrument can be screwed on toa stick stand, for taking magnetic bearings 
 in a horizontal plane, or suspended by the ring for taking altitude angles. In 
 each case the movable sights are set along the line whose direction is required. 
 
 A lid is provided for protecting the instrument when it is not in use. 
 
 Inv. I918-124. 
 
 165. EARLY ‘THEODOLITE:~— Lentiby 12 He Gourr Esq. 
 
 This instrument probably dates from the early 17th century. It consists of 
 a brass circle, 8 in. diam., graduated in degrees, having two pairs of fixed sights 
 along perpendicular diameters, and a movable alidade fitted with pointers at 
 each end. It can be set upon a stand for measuring angles in any plane, or 
 suspended from a ring attached to the circumference for taking altitudes. 
 
 On the face of the circle is engraved a dial indicating the points of the compass, 
 and a geometrical square with ‘“‘ Umbra Recta’’ and ‘‘ Umbra Versa ”’ scales, 
 
 The circle is graduated continuously from 0° to 360°. Inv. 1918-123. 
 
65 
 
 166. EARLY THEODOLITE, Lent’ by T. H.' Court, * Esq. 
 Plate III, Fig. 2, p. 56. 
 
 In this instrument, which was probably made in the early 17th century, only 
 one pair of fixed sights is provided, defining a horizontal line when the instrument 
 is suspended from the ring attached to its circumference. 
 
 The diameter of the graduated circle is 10 in., and the movable alidade carries 
 the usual slit sights and terminates in pointers. There is a circular ring on one 
 arm of the instrument which probably supported a magnetic compass. 
 
 The circle is graduated in degrees in four sets from 0° to go®. On the back 
 of the instrument is attached a socket with a fixing screw for receiving the head 
 of a stand. The suspension ring is specially jointed to permit the plane of the 
 circle to hang freely vertical. Inv. 1918-126, S.M. 1856. 
 
 167. COMBINED GRAPHOMETER, SECTOR AND COM- 
 PASSES. 
 
 This instrument of Italian make was probably constructed in the late 16th 
 or early 17th century, and consists of a graphometer whose alidades are fitted 
 with points to serve as compasses, and have various scales engraved on them 
 corresponding to those usually found on sectors, and used in measuring lines, 
 setting off angles, taking weights of spherical shot, etc. A small magnetic compass 
 is attached to the junction of the two alidades. 
 
 There is no evidence that this instrument was fitted to a stand, and it was 
 presumably employed in conjunction with a plane table, on which it rested when 
 angles were observed. 
 
 The semicircle is graduated in degrees, and is 6-5 in. in diam. The sights on 
 the alidades are 8 in. apart. Inv. 1880-202. 
 
 168. [TPODIMETER. 
 
 This is a 17th-century theodolite of Continental design, the alidade being 
 mounted near the edge of the circle and connected to a centrally mounted parallel 
 index arm used to read the circle. The circle has a diameter of 64in., and a 
 24-in. compass box is centrally fixed above the movable index arm. 
 
 The circle is divided on its outer edge into 360 degrees, but has a concentric 
 circle, of 5 in. diam., divided into 320 parts, also read by the diametral index 
 arm. Each circle is divided into octants and the divisions of each octant numbered 
 in the clockwise sense from o to 45 on the outer circle and from o to 4o on the 
 inner. The outer octants are successively designated by the letters T, G, L, S, 
 O, A, P, M, which are the initial letters of the old Italian names for the compass 
 points. The inner octants each commence at the central points of the outer ones, 
 and are named Septentrio, Aquilo, Solanus, Eurus, Auster, Africus, Favonius, 
 and Chaurus respectively, which are the old Latin titles for the winds corres- 
 ponding to the cardinal points. The central index arm has bevelled edges at each 
 extremity for reading the divisions. The alidade is about g in. long and has folding 
 combined back and foresights at each end. It rotates with the index arm about 
 the axis of the circle. The compass box, fixed to the circle above the index arm, 
 has its base divided into octants, but no subdivisions. Inv. 1908-122. 
 
 169. GRAPHOMETER. Made by Christoff Kohler, Dresden, 1663. 
 
 This instrument is a combined graphometer, or surveying semicircle, and 
 clinometer. The graduated semicircle can be detached from the frame and 
 used as a protractor for plotting the angles in the field. A compass is attached 
 to the frame for orienting the instrument, and the whole is attached to a stand 
 by means of a double-knuckled joint, permitting it to be set in any plane. It 
 can also rotate as a whole in that plane. 
 
 The semicircle has one fixed and one movable alidade, and is 5 in. in diam. 
 It is graduated in degrees, and has a diagonal scale enabling it to read to 5’. 
 This is particularly interesting as showing that the much simpler vernier system 
 was not yet in general use, although invented over 30 years previously. 
 
 The compass ring has a diameter of 14 in. and is graduated in degrees. On 
 the back of the plate carrying it is a simple quadrant pendulum clinometer, 
 also graduated in degrees, and provided with ‘‘ Umbra Versa’”’ and ‘‘ Umbra Recta’”’ 
 scales for measuring heights. 
 
 The clinometer is attached to an alidade, which is connected to the semi- 
 circle by a spring clip and can be disconnected for taking altitudes. 
 
 Inv. 1894-106. 
 
66 
 
 170. EARLY SURVEYING QUADRANT. Made by Lusuerg, 
 Rome, 1674. 
 
 This instrument has a radius of 6 in. and is graduated in half-degrees. By 
 means of a vernier arc covering 31° and divided into 60 parts, angles can be 
 measured to the nearest half-minute. The index arm carrying this vernier is 
 pivoted at the centre of the quadrant and carries three folding sights, one at its 
 centre and the other two at the ends of its arc. Two similar sights are fixed 
 at the ends of the quadrant arc. The quadrant is attached to a socket for 
 fixing it upon a stand by a ball and socket joint permitting it to be set in any 
 plane, including the vertical. 
 
 On the upper surface of the quadrant are engraved a tangent scale (“‘ Scala 
 altimetra ’’), concentric with the divided arc, and also linear scales and a rect- 
 angular network for use in plotting. On the lower surface is engraved a clino- 
 meter scale for artillery purposes and the arms and coronet of the Duke of 
 Modena. The inscription ‘‘ Jacobus Lusuerg Mutinensis Faciebat Romae 
 A° 1674”’, engraved on the upper surface, gives the date and maker of the 
 instrument. Inv. 1880-200. 
 
 171. EARLY THEODOLITE. Made by Joannes Macarius, 
 Mirandola (Modena). 
 
 The date of construction, 1676, and maker’s name are inscribed on this very 
 fine example of an early theodolite, which has a circle of 12 in. diam., graduated 
 every 10’ and read by a vernier on the alidade to 1’. A 2$-in. compass is fixed 
 at the centre of the circle and has its ring divided in degrees. 
 
 The circle carries four fixed combined sights at the ends of mutually perpen- 
 dicular diameters. Each sight has a vertical slit, a small aperture, and a pair 
 of thin cross-pieces intersecting at the aperture and forming cross-wire foresights. 
 The alidade, pivoted to rotate about the centre of the circle, has a similar pair 
 of sights at the extremities of its index arm, and these have bevelled edges for 
 reading the circle directly. The double vernier plate is screwed to the alidade 
 at right angles to its direction of sight. Each vernier extends over 61° of the 
 circle, and is divided into 60 parts. There are two vernier plates provided, 
 the one fixed to the alidade having engraved upon it the various numbers of 
 foreign feet equal to 1,000 Roman feet. The other vernier plate can be sub- 
 stituted for this one if required. The circle graduations have three sets of 
 numbers, one consecutively from o° to 360° in the clockwise sense, the other 
 two by quadrants having the zero points beneath each of the fixed sights. Scales 
 of sines, tangents and sides of inscribed polygons are also engraved on the circle, 
 as well as various scales of length on its cross-bars. On the back of the circle 
 are engraved other scales used in artillery and fortification work with the Latin 
 inscription ‘“ Bonifacius March® Rangonus Faciedum jussit Mutinae Anno 
 Domini, 1676”’. . 
 
 A suspension shackle is fixed to the top of the circle for setting it in a vertical 
 plane, and a socket is provided for mounting it horizontally upon a stand. 
 
 Inv. 1880-195. 
 
 172. GRAPHOMETER. Made by Deens, 1682. 
 
 This instrument consists of one fixed and one movable alidade, each provided 
 with sights, intersecting each other at the centre of a semicircle graduated to 
 read half-degrees. The whole is mounted upon a ball and socket joint, and can 
 be used to measure angles in any plane. 
 
 In this instrument, no compass is attached for orientation, and the alidades 
 are not provided with verniers. The graduated semicircle has a diameter of 
 II cm., with two sets of supplementary figures. 
 
 An inscription on the movable alidade reads ‘‘ Iodocus Deens Louaniensis 
 Belga Austriorum Viennes fecit, 1682”’’. Inv. 1892-9. 
 
 173. EARLY CIRCUMFERENTOR. Made by Lusuerg, Rome. 
 
 This example was constructed in 1686, and exhibits many characteristics 
 of that period. The alidade plate is square, and can be detached from its 
 support and employed as a protractor. On it are engraved three concentric 
 graduated circles, of diameters 8 in., 7 in. and 34 in. The outer circle is divided 
 in degrees, with two sets of numberings. The central circle is a “‘ Scala alti- 
 metra,’’ or tangent scale, for measuring heights, and the inner circle is divided 
 into 32 parts corresponding to the points of the compass. The movable alidade 
 
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67 
 
 carries a 2}-in. compass box, and is pivoted at the centre of the plate. At one 
 end is a vernier arc which moves over the outer graduated circle, and reads 
 this to single minutes, the 60 divisions of the vernier extending over 61°. 
 
 The fixed and movable sights have circular apertures, with vertical slits 
 above their centres, and are carved and ornamentally engraved. Two edges 
 of the square plate are parallel to the two fixed sights, which can be folded down 
 upon the plate when notin use. The conical socket for attaching the instrument 
 to its stand has a ball and socket joint permitting the alidade plate to be set 
 in any plane, including the vertical. It is kept in position by the friction of the 
 joint, there being no clamping screw. 
 
 The compass plate is graduated in degrees, numbered left and right from 
 the N. and S. points from o° to 90°. A compass rose indicating the 32 main 
 points is engraved inside the circle. On the back of the alidade plate are 
 engraved a clinometer scale for use of artillery, and the inscription “‘ Jacobus 
 Lusuerg Mutinensis Faciebat Romae A®° 1686’’, with the arms and coronet of 
 the Duke of Modena. Inv. 1880-199. 
 
 174. CIRCUMFERENTOR. Made by Thos. Wright. Plate IV, 
 Pigrep.\07: 
 
 This early 18th-century example is typical of the circumferentor of that 
 period. It is a combined theodolite—having a graduated limb of 11 in. diam., 
 reading by vernier to 5’—and compass, with a 6-in. needle reading to half- 
 degrees. 
 
 The theodolite limb is graduated in single degrees in the clockwise sense 
 from o° to 360°, and carries a pair of fixed sights, each with a slit and window 
 for sighting both ways, at its o° and 180° marks. The movable alidade, carrying 
 a similar pair of sights, has a vernier at one end reading to 5’ and a central 
 fiducial mark at the other end. The compass box is fixed to this alidade with 
 its N. point above the zero of the vernier. The compass ring is graduated 
 consecutively in half-degrees from 0° to 360° in a counter-clockwise sense, so that 
 the position of the N. end of the needle gives the magnetic bearing of the alidade 
 sights. 
 
 Both pairs of sights are employed when the instrument is used as a theodolite, 
 but only the alidade sights when taking magnetic bearings. The instrument is 
 mounted upon a tripod when in use. Inv. 1914-561, S.M. 937, L.S. 
 
 175. CIRCUMFERENTOR. Made by J. Howard, London. 
 
 This early 18th-century instrument has a t1o%-in. graduated circle, reading 
 by vernier to 5’. The vernier is attached at one end of the movable alidade, 
 to which is also fixed the compass ring, which has a 6-in. diam. and is graduated 
 in degrees. 
 
 The sights on the movable alidade, and those fixed on the large circle are 
 each fitted with both slits and vertical wires, and can be used either as back or 
 fore sights. They are sufficiently high to permit the measurement of angles 
 in the horizontal plane, and the whole instrument is designed for such measure- 
 ments, and not for determining angles in any plane. 
 
 The outer circle is graduated in degrees, and reads from o° to 360° in a 
 clockwise sense. The fixed sights are attached at the 0° and 180° points. The 
 compass ring has two sets of graduations, one set reading degrees from o° to 
 360° in a counter-clockwise sense, the other set giving every 10° only from 0° to 
 go° in each quadrant, the N. and S. points being the zero points. The eight 
 main points of the compass are likewise indicated. 
 
 The whole instrument fits upon a stand by means of a socket fixed on the 
 back of the large circle. Inv. 1913-257. 
 
 176. GRAPHOMETER. Made by Macquart, Paris. 
 
 This is an early 18th-century example of the graphometer, or surveying 
 semicircle, much used in France in preference to the English theodolite. It has 
 a diameter of 11 in. and is graduated in degrees, read by vernier on the rotating 
 alidade to 5’. A 3-in. compass, with its ring divided in degrees, is mounted 
 eccentrically on the semicircle. 
 
 Combined slit and wire sights are fixed at opposite ends of the diameter of 
 the semicircle, and their orientation determined by means of the compass. 
 A similar pair of sights is attached to the alidade, which rotates about the centre 
 of the semicircle, and has bevelled verniers at each end. The axis of the semi- 
 circle terminates below in a spherical ball and socket arrangement for mounting 
 the instrument on a tripod and setting its semicircle in any plane. 
 
68 
 
 The graduations of the semicircle are numbered both ways from 0° to 180°, 
 those of the compass ring in four quadrants, with the zeros at the N. and S. points. 
 The compass needle is supplied with a lifting needle for clamping it to the top 
 of its box when transporting the instrument. Inv. 1908-118. 
 
 177. CIRCUMFERENTOR. Made by Baradelle, Paris. 
 
 In this instrument, which was constructed towards the end of the 18th 
 century, the compass has a diameter of 1? in. and is graduated only at the 
 eight cardinal points. It is arranged for a magnetic variation of 16° W. 
 
 The main circle has a diameter of 6 in. and is graduated in degrees, numbered 
 both ways from 0° to 180° from the diameter underneath the pair of fixed sights. 
 The alidade carrying the movable sights is fixed to the compass box and rotates 
 with it about the vertical axis. The compass needle is pivoted upon a plate 
 fixed to the horizontal circle. There are no verniers, but only bevelled reading 
 edges, on the alidade, and the sights are not suited for sighting both ways, but 
 consist of a simple slit backsight on one end of the alidade and a wire foresight 
 at the other end. The graduated circle can be set in any desired plane by means 
 of a ball and socket joint connecting it to the stand. Inv. 1908-119. 
 
 178. CIRCUMFERENTOR. Made by W. & S. Jones. Presented 
 by Evan Roberts, Esq. 
 
 This instrument, made about the end of the 18th century, is a good example 
 of the circumferentor of that period. It has two small spirit levels fixed in the 
 base of the compass box for setting the instrument horizontal. 
 
 The g-in. circle is graduated in degrees in the clockwise sense from 0° to 360°, 
 and read by the vernier at one end of the alidade to 3’. The compass ring, 
 with a 4-in. needle, is graduated in degrees in the counter-clockwise sense from 
 0° to 360°. Both fixed and movable sights are provided with slits and windows 
 for sighting in either direction. The small spirit levels upon the base of the 
 compass box were not considered necessary by the makers, but were provided 
 owing to the wishes of many surveyors. The socket attached to the base does 
 not appear to have the customary screw for clamping the instrument to its 
 
 tripod. 
 (See Adams’s ‘‘ Geometrical and Graphical Essays,” p. 213, and Plate XIV, 
 Fig. 5.) Inv. 1912-308. 
 
 ALTAZIMUTH THEODOLITES. 
 
 179. EARLY ALTAZIMUTH THEODOLITE. Made by Thomas 
 Wright. Presented by P. A. Newton, Esq. 
 
 This instrument, dating from the early part of the 18th century, illustrates 
 a fundamental stage in the development of the theodolite. Upon the alidade of a 
 “common theodolite’’ of the type very generally used in the 17th century, is 
 mounted a fixed graduated vertical arc. A telescope upon which is fixed a 
 longitudinal spirit level, carries a vernier arm which traverses the graduated 
 arc as the telescope rotates, and thus measures the altitude of the object sighted 
 at the same time as the alidade measures its azimuth. 
 
 The horizontal circle in this instrument has a diameter of Io in. and is 
 graduated in degrees, numbered from 0° to 260° in the clockwise sense. The alidade 
 has a vernier at one extremity reading the horizontal circle to 5’. A 5-in. 
 compass box is mounted centrally upon this alidade and gives magnetic bearings 
 in single degrees. The vertical arc is graduated in degrees, with the zero at the 
 point vertically beneath the telescope axis, and reads by vernier to 10’. Altitude 
 angles up to 40° of elevation or depression can be measured. 
 
 The telescope has a focal length of 8 in., an aperture of I in., and an erecting 
 eyepiece with a simple cross-wire diaphragm. The spirit level is rigidly attached 
 to the telescope tube. The instrument is attached to its tripod by means of a 
 socket fixed beneath the centre of the horizontal circle. 
 
 Inv. 1918-240. S.M. 1098. LS. 
 
 180. EARLY ALTAZIMUTH THEODOLITE. Made by » 
 JeSisson? Lentipy a HeCourt, Hsq.syPlate LVeie 2! p. 67. 
 
 Among the early 18th-century makers of surveying instruments, Sisson is 
 generally regarded as the most expert, and his design of altazimuth theodolite, 
 of which the instrument exhibited is typical, remained the standard until Rams- 
 den’s time. This example has a 4}-in. horizontal circle, graduated in degrees and 
 
69 
 
 read by three symmetrically placed verniers to 6’. The vertical arc, reading 
 angles up to 70° of elevation or depression, has a radius of 2 in. and is similarly 
 graduated and read by a single vernier centrally placed on the horizontal vernier 
 late. 
 ‘ Both circles have toothed edges, in which engage pinion wheels, the motion 
 of rotation being effected by turning these pinions. The telescope is attached 
 above the vertical arc by screw bearings which permit it to be adjusted to sight 
 horizontally when the vertical arc reads zero. A small spirit level fixed above the | 
 telescope, and a smaller one rigidly joined to the vertical arc enable this adjust- 
 ment to be effected. The horizontal axis of rotation of the vertical arc and 
 telescope rests on standards projecting from the central plate carrying the three 
 horizontal vernier arms. To one of these is attached a clamp and screw which 
 regulates the friction between the movable vernier plate and the fixed horizontal 
 circle. In another vernier arm is fixed the axis of rotation of the pinion wheel 
 which engages in the teeth of the horizontal circle. Beneath the horizontal circle 
 is screwed a compass box of 4 in. diameter. The compass ring is not graduated, 
 but has index marks beneath the 0° and the 180° graduations of the horizontal 
 circle. This enables the instrument to be oriented. The conical socket for attach- 
 ing the theodolite to a staff, or to a levelling device consisting of two parallel 
 plates and four levelling screws, is fixed beneath the compass box. 
 The telescope has a fixed objective of 4-in. aperture, and a sliding Huygens 
 
 ocular, with a simple fixed cross-wire diaphragm. Inv. 1921-588. S.M. 1858. 
 
 181. EARLY ALTAZIMUTH THEODOLITE. Made by Cole. 
 
 This instrument was probably constructed about 1760 and resembles the 
 adjacent theodolite made by Wright. The vertical arc is fixed to the alidade, and 
 the vernier to the telescope. 
 
 The 8-in. horizontal circle is graduated in degrees and read to 5’ by means 
 of a vernier scale on the alidade. The vertical arc has a radius of 3 in. and is 
 graduated in degrees up to 50° on each side of the zero. It also is read to 5’ 
 by its vernier. The 4-in.compass mounted on the alidade is graduated in degrees 
 in the counter clockwise sense. 
 
 The telescope of 1o-in. focal length and o-5-in. aperture, has a single lens, 
 draw-focussing eyepiece and a simple cross-wire diaphragm. Spirit levels are 
 fixed on the telescope tube and the base of the compass box. The two parallel 
 plates for fixing the instrument to its tripod are connected by four levelling screws 
 and a spherical joint. Inv. 1910-125. 
 
 182. ALTAZIMUTH THEODOLITE. Made by G. Adams. Lent 
 by T. H. Court, Esq. 
 
 In this theodolite, made probably about 1770, the alidade is replaced by a 
 circular plate carrying two widely separated standards upon which is mounted the 
 horizontal axis of rotation. The vertical semicircle is fixed to this axis, whilst the 
 vernier is fixed on the circular plate. The telescope rests in jointed rings attached 
 above the semicircle. 
 
 The 7-in. horizontal circle is graduated in degrees, and read by a vernier scale 
 on the alidade plate to 5’. The vertical semicircle is of the same diam. and is 
 graduated in degrees on one side, with scales of feet and links on the other. It is 
 also read by vernier to 5’, and can measure angles up to 60° on either side of the 
 vertical. The compass fixed on the alidade plate has a diam. of 3 in. and is 
 graduated in degrees. 
 
 The telescope, of 7-in. focal length and 0-5-in. aperture, has a single-lens, draw- 
 focussing eyepiece. A spirit level is fixed by adjustable screws beneath the tele- 
 scope tube, parallel to its axis. 
 
 The horizontal circle is attached by a clamping screw to the upper of two 
 parallel plates, which are connected to each other by four levelling screws and a 
 spherical joint. Inv. 1917-104. S.M. 950. LS. 
 
 183. SMALL ALTAZIMUTH THEODOLITE. Made by 'G. Adams. 
 Lent by the Royal Society. . 
 
 In its essential features this theodolite is a smaller example of the adjacent 
 type by the same maker. A supplementary telescope is fixed to the horizontal 
 circle for keeping the back station in view, whilst both the vertical arc and hori- 
 zontal circle have milled edges and are rotated by small pinions gearing with the 
 edges. The base of the horizontal circle forms the upper parallel plate for levelling. 
 
70 
 
 The 5-in. horizontal circle and 3:+5-in. vertical arc are graduated in degrees, and 
 read by verniers to 5’. Angles up to 65° on either side of the vertical can be 
 taken. The telescope has a 4°5-in. objective, with an aperture of 0-3 in. and a 
 simple cross-wire diaphragm. It has a small magnification—about 2 diam.— 
 and no focussing arrangement. The supplementary telescope is of similar con- 
 struction and dimensions. A small spirit level is fixed in adjustable bearings 
 beneath the main telescope parallel to its optical axis. The telescope rests in Y-s, 
 fixed to the ends of the vertical semicircle, and is kept in position by retaining 
 straps. The bracket supporting the axis of rotation of the vertical are carries the 
 vernier and is supported on the alidade above the compass box. This latter has 
 a compass ring of 3-in. diam., graduated in degrees. The needle is missing. 
 
 The lower parallel plate is attached to the base of the horizontal circle by a 
 ball and socket joint, and carries a conical socket with a clamping spring and screw 
 for mounting the instrument on the peg of a tripod. 
 
 On the back of the vertical circle are graduated scales of feet and links, the 
 former being a tangent series and the latter one of hypotenusal increments. 
 
 Inv. 1900-153. S.M. 938. LS. 
 
 184. ALTAZIMUTH THEODOLITE. Made by Ramsden. Lent 
 by T. H. Court, Esq. 
 
 This instrument, made towards the end of the 18th century, illustrates the 
 many improvements in design due to Ramsden. The alidade and horizontal plates 
 have their graduated edges bevelled to facilitate reading, and are supported by 
 accurately made conical axes. A tangent clamp and slow-motion screw control 
 the horizontal motion, and the ball and socket joint connecting the two parallel 
 plates of the levelling attachment is very solidly and accurately constructed. 
 
 The telescope has an achromatic, rack-focussing objective lens of 8-5-in. focal 
 length and 1-in. aperture, and an erecting eyepiece. It rests in Y-s attached to 
 the ends of the vertical semicircle, which has a radius of 3 in. and is graduated on 
 one face in half-degrees. This arc is read by a fixed vernier to 1’, and can 
 measure angles of elevation or depression up to 60°. 
 
 The 7-in. horizontal circle is divided in half-degrees and read by two opposing 
 verniers to 1’... The compass box, has a 3-in. ring graduated in degrees and a 
 sensitive needle which can be lifted from its pivot by means of a lever when 
 required. The box is mounted between the telescope standards upon an adjustable 
 stage, which is connected to the horizontal vernier plate by capstan-headed 
 Screws. 
 
 Spirit levels are mounted on the telescope and the vernier plate. 
 
 Inv. 1913-571. 
 
 185. ALTAZIMUTH THEODOLITE. Made by Troughton. 
 
 This instrument was made in the early part of the 19th century, and in details 
 resembles the adjacent example made by Ramsden. There is, however, a 
 supplementary telescope used for keeping the back-set station in view. 
 
 The horizontal circle and vertical semicircle have diameters of 7in., are 
 graduated in half-degrees, and read by vernier to single minutes. Both are rotated 
 by rack and pinion movements. There is a tangent clamp and slow-motion screw 
 to the horizontal circle. The telescope rests in Y-s attached to the vertical arc, 
 and moves with this. The standards supporting the horizontal axis of rotation are 
 fixed upon an adjustable stage, attached to the alidade piate by three capstan- 
 headed screws. This stage carries the compass box, which has a 3}-in. ring, gradu- 
 ated in degrees. The two telescopes are of precisely similar optical construction. 
 Each has a 7-in. achromatic objective, with a 1-2-in. aperture, and a rack and 
 pinion-focussing mechanism. Their eyepieces are erecting and consist of four 
 lenses, with a draw-focussing mechanism. The supplementary telescope has a 
 full azimuthal motion round the vertical axis of the instrument, and a small 
 altitude movement. The construction of the parallel plate mounting is of the 
 usual Ramsden type, with four levelling screws. There is a longitudinal spirit 
 level fixed in adjustable bearings beneath the telescope, and two levels with their 
 axes at right angles on the alidade plate. Inv. 1909-138. 
 
 186. ALTAZIMUTH THEODOLITE. Made by W. Harris. 
 
 This instrument, although made in the middle of the 19th century, retains 
 most of the features characterising the Ramsden type of theodolite. Tangent 
 clamps and slow-motion screws replace the Ramsden rack and pinion movements 
 
7h 
 
 for both horizontal and vertical circles, and small reading microscopes are pro- 
 vided near the verniers. The supporting standards for the horizontal axis of 
 rotation are not mounted upon an adjustable stage, as in Ramsden’s instruments, 
 but are fixed directly to the alidade plate. 
 
 The horizontal circle has a diameter of 6 in. and the vertical semicircle one of 
 54in. Both are graduated in half-degrees and read by vernier to 1’. The 
 telescope is mounted in Y-s attached to the vertical arc. It has a 9-in. achromatic 
 objective, with a 1-in. aperture and a rack-focussing arrangement. The eyepiece 
 is erecting, with four lenses, and a draw-focussing mechanism. The diaphragm has 
 one vertical and one horizontal wire. A sensitive longitudinal spirit level is fixed 
 in adjustable bearings beneath the telescope. On the alidade there are two 
 smaller levels at right angles to each other, and a 3-in. compass graduated in 
 degrees. The parallel plates are of the usual Ramsden type, with four levelling 
 screws. Inv. 1872-80. 
 
 187. ALTAZIMUTH THEODOLITE. Made by W. & T. Gilbert, 
 London. Lent by T. H. Court, Esq. 
 
 This instrument, probably constructed about 1840, differs in many respects 
 from the more usual Y-theodolite of the period. Its telescope is rigidly attached 
 to the horizontal axis of rotation, which carries the vertical arc and rests in 
 bearings upon two standards fixed to the rim of a case whose bottom forms the 
 alidade. This case is closed at the top by a glass cover, so that the horizontal 
 ‘circle and compass are enclosed. Reading microscopes with prism reflectors are 
 attached to the case for reading the verniers of the horizontal circle. Tangent 
 clamps and slow-motion screws are fitted to both circles. The upper part of the 
 instrument is suspended in a gimbal mounting within a hollow cylinder screwed 
 upon the tripod plate, and can be levelled independently of the tripod by two 
 screws threaded into the cylinder, with diametrically opposite springs. These 
 screws press against a square nut fixed to the lower end of the axial cone which 
 carries the horizontal circle. 
 
 The 6-in. horizontal circle is graduated to 20’, and read by vernier to 30’. 
 The 5-in. vertical arc is similarly graduated and read by vernier, and has a range 
 of 50° on either side of the zero mark. The telescope is achromatic, with a I-in. 
 aperture and 8-in. focal length. The objective is focussed by a rack and pinion 
 mechanism. The eyepiece is a simple Ramsden inverting draw-focussing one, 
 and the diaphragm has one vertical and one horizontal wire. A diagonal reflecting 
 mirror can be attached in front of the objective for taking steep sights. There is 
 one longitudinal adjustable spirit level attached beneath the telescope, and a 
 parallel one fixed to the standards but no transverse level. The alidade carries 
 a 3-in. compass, graduated in degrees. Inv. 1914-894. 
 
 188. ALTAZIMUTH THEODOLITE (Everest Type). Made by 
 T. Cooke & Sons. Presented by the Ordnance Survey. 
 
 The Everest pattern theodolite, although not possessing the advantage of a 
 transit instrument, has been much used by the British and Indian Survey depart- 
 ments since its inception, due to Sir George (then Captain) Everest, about the 
 year 1835. The one exhibited is a late 19th-century 5-in. model, the horizontal 
 circle and vertical arcs being graduated to 20’ and read by two opposing verniers 
 to 30”. 
 
 In the Everest type of theodolite two opposing vertical arcs are used instead 
 of a complete vertical circle, and the horizontal axis to which the telescope and 
 arcs are fixed, is mounted upon two short standards projecting from a central 
 pillar, supported on the horizontal vernier plate. The vernier bracket and attached 
 spirit level are mounted on the axis, and are clamped to one standard by opposing 
 screws. Two perpendicular spirit levels are likewise mounted on the horizontal 
 vernier plate. The horizontal circle has a bevelled graduated edge and a separate 
 mounting on the vertical axis cone. Thus the vernier plate and circle have 
 independent motions, controlled by clamps and tangent screws, for repeating 
 azimuth angles. All verniers are provided with reading microscopes. The 
 instrument has a tribrach mounting, the feet of the three levelling screws resting 
 in grooves in a trivet plate and being locked therein by a spring-controlled locking 
 plate—also a feature of the original Everest design. 
 
 The telescope has a rack-focussing objective of 1-2-in. aperture and 9-in. 
 focal length, and a slide-focussing Ramsden eyepiece. 
 
 Owing to the low standards, angles of elevation beyond 30° and of depression 
 beyond 45° cannot be measured, as the telescope then fouls the horizontal plates. 
 The vertical arcs consequently only extend about 50° on each side of the horizontal 
 Zeros. Inv. 1895-49. 
 
72 
 189. TRANSIT THEODOLITE. Made by Troughton & Simms. 
 
 The introduction of the transit principle into medium and small-sized theodolites 
 in England took place in the early part of the 19th century. This exhibit is an 
 early example of such a theodolite. Below the horizontal circle it resembles 
 Ramsden’s Y-theodolite, but the upper part has standards sufficiently high to 
 permit the telescope to transit at either end. Both horizontal and vertical circles 
 are 6 in. diam., divided on silver to 10’ and read by opposing verniers to 10”. 
 
 The vernier plate and horizontal circle have independent axes of rotation for 
 repeating angles, and are bevelled at the graduations to facilitate reading. 
 Reading microscopes are provided for both circles, and also tangent clamps and 
 slow-motion screws, as well as a similar clamp and screw connecting the vernier 
 plate to the horizontal circle. The axis of this circle rests in a conical socket 
 attached to the upper of two parallel levelling plates, connected by four screws 
 and a ball and socket joint. The vernier plate carries a single spirit level, parallel 
 to the transit axis, and a central circular compass box, with a 34-in. stepped ring 
 graduated in degrees. The needle, of rectangular section with its longer side 
 vertical, is provided with a lifting lever for clamping it. The vertical circle is 
 attached to the telescope and transit axis, and is read by horizontal verniers on a 
 bracket fixed to one standard. This bracket carries the clamp and tangent 
 screw. The other standard carries a spirit level perpendicular to the transit 
 axis. The telescope, fixed in the centre of the transit axis, has a rack-focussing 
 objective of 1-5-in. aperture and 1o-in. focal length, a fixed diaphragm having 
 one vertical and three equidistant horizontal wires, and a slide-focussing Ramsden 
 eyepiece. A sensitive spirit level is mounted parallel to the axis of the telescope. 
 
 Inv. 1909-137. 
 
 190. TRANSIT THEODOLITE. Made by Thos. Jones. Presented 
 by the Admiralty. 
 
 In this instrument, made in the early part of the 19th century, the 8$-in. 
 horizontal and the 7$-in. vertical circles are graduated every 10’ and read by 
 vernier to 10”. The vertical circle and telescope are rigidly joined to the transit 
 axis, and rotate with it, the verniers being fixed to one standard. The horizontal 
 circle and vernier plate can rotate independently about the vertical axis, for 
 repeating azimuth angles. 
 
 The three footscrews for levelling the instrument are fixed to the base circle 
 of a framed stand having a central conical socket, into which fits the hollow 
 conical axis attached beneath the horizontal circle. The axis of the vernier 
 plate fits within this and is kept in position by a screw and washer fitted in its 
 lower end and abutting against the socket in the stand. The vernier plate carries 
 three symmetrically placed vernier arms, with reading microscopes suspended 
 above their verniers, and is provided with a sensitive spirit level, with its axis 
 perpendicular to the transit axis. The standards supporting the transit axis 
 project from the vernier plate, and have Y-s at their upper ends, one of which 
 is adjustable. The axis is kept in position in these Y-s by metal straps; it is 
 hollow, and permits the telescope to be illuminated by means of a lamp supported 
 by a bracket fixed to one standard. The vernier bracket for reading the vertical 
 circle carries a spirit level parallel to its two vernier arms, for setting their zeros 
 horizontal. It has a projecting piece which fits between screws fixed to the 
 standard, and keeps the verniers stationary when the telescope and circle rotate. 
 The telescope has a fixed objective of 11-in. focal length and 1-8-in. clear aperture, 
 and a central mirror for illuminating the axis at night. The eyepiece tube can 
 be focussed by a rack and pinion mechanism, and carries the diaphragm, having 
 five vertical and one horizontal threads, and the inverting, draw-focussing eye- 
 piece, which fits in a slide permitting a slight lateral motion of the eyepiece. 
 
 Clamps and tangent screws for fine motion are fitted to both circles and the 
 horizontal vernier plate. Inv. 1876-1206. 
 
 191. TRANSIT THEODOLITE. Made by Cary. Presented by 
 the Admiralty. 
 
 This is an early 1gth-century example of a medium-sized transit theodolite. 
 The t0-in. vertical and horizontal circles are graduated on silver every 10’, and 
 read by two opposing verniers to 10” of arc. The telescope transits at the 
 eyepiece end, and its axis can be illuminated for night reading through the hollow 
 horizontal axis of rotation. The instrument is mounted upon a tribrach having 
 three footscrews for levelling; these rest upon a plate screwed to the tripod 
 head and are kept in position by a locking plate of the Everest pattern. 
 
figs: 
 
 The horizontal circle can rotate freely about the vertical axis, or can be 
 clamped to it and rotated slowly by means of a tangent screw. The vernier 
 plate can be similarly clamped to the horizontal circle, or rotated freely. Reading 
 microscopes are fixed on brackets above each vernier. An adjustable spirit level 
 is fixed to the vernier plate, perpendicular to the line joining the two standards 
 which support the vertical circle and telescope. The vertical circle is mounted 
 upon the axis of rotation of the telescope, and is read by verniers at the ends of 
 a bracket fixed to one standard. Reading microscopes are attached to this bracket 
 for use with each vernier, and a spirit level is mounted in adjustable bearings 
 on the bracket with its axis parallel to the line joining the vernier zeros. A 
 tangent clamp and slow-motion screw connects the vertical circle to one standard. 
 Upon the top of the telescope, parallel to its axis, is set a 7-in. trough compass. 
 The lamp for illuminating the telescope axis rests upon a bracket fixed to one 
 standard, with its lens opposite the end of the horizontal axis of rotation. A 
 mirror in the centre of the telescope reflects the light down its axis. 
 
 The telescope has an achromatic objective of 1-5-in. aperture and 15-in. 
 focal length. Three eyepieces are provided, one inverting, one erecting, and one 
 diagonal for taking steep sights. There is a sensitive striding level for testing 
 the horizontal axis, one of the trunnions being adjustable on its standard. A 
 plumb-bob for centring the instrument over a station point is also provided. 
 
 Inv. Ig1I—160. 
 
 192. TRANSIT THEODOLITE. Made by Robinson, London. 
 Presented by the Admiralty. 
 
 This example of a 5-in. transit theodolite was probably constructed about 
 1850. Both horizontal and vertical circles are graduated in half-degrees and 
 read by vernier to single minutes. A trough compass is fixed above the telescope 
 instead of the spirit-level more usually found in the Ramsden type of altazimuth. 
 The footscrews of the tribrach are designed to be inserted in a locking plate 
 attached to the tripod, from which the vertical axis of the instrument is 
 completely separated. 
 
 The vernier plate for the horizontal circle carries two spirit levels with their 
 axes mutually perpendicular, and has three symmetrically disposed verniers, 
 with small reading microscopes suspended above them. A tangent clamp and 
 slow-motion screw mechanism is fixed to this plate to clamp it to the horizontal 
 circle. This latter can rotate independently about the vertical axis, or can be 
 clamped to it by means of a tangent screw attached to thetribrach. The standards 
 supporting the upper part are fixed rigidly to the vernier plate. The horizontal 
 axis rests in bearings on these standards, and is retained in position by metal 
 straps. This axis carries the telescope and two vertical circles, one on each side of 
 thetelescope. Only one of these is graduated, and is read by means of two verniers 
 on a bracket mounted upon the axis, but fixed to one standard. Two reading 
 lenses are mounted on a movable arm for use with these verniers. To the other 
 standard is fixed a tangent clamp and slow-motion screw arrangement for fixing 
 the vertical circles and telescope. The trough compass box is 5 in. long, and 
 rests upon the distance pieces which connect the two circles, parallel to the 
 telescope. The needle can be raised from its pivot and clamped when required. 
 
 A striding level is provided for testing the horizontality of the axis of rotation. 
 
 The telescope has an objective of I-in. aperture and 8-in. focal length. The 
 diaphragm is fixed in the eyepiece tube, which is focussed by a rack and pinion 
 mechanism. There are three vertical and one horizontal wires. <A diagonal 
 erecting eyepiece is provided in addition to the normal inverting one. The 
 telescope transits at the eyepiece end. Inv. I9gII-159. 
 
 193. TRANSIT THEODOLITE. Made by Henry Barrow & Co. 
 Presented by the Admiralty. 
 
 This instrument, made in the middle of the 19th century, has 6-in. vertical 
 and horizontal circles, very finely graduated on silver to 10’ of arc, and reading 
 by vernier and microscopes to 10”. In general construction, it resembles rather 
 a Borda repeating circle than an English theodolite, the whole upper part being 
 supported by a single central pillar, so that the telescope is slightly eccentric 
 and needs a counterpoise. Both the vertical and horizontal circles can rotate 
 independently of their verniers, so that angles can be “‘ repeated ”’ on both circles. 
 
 Three symmetrically disposed verniers for reading the horizontal circle are 
 attached to a light frame to which the central pillar is fixed. Reading microscopes 
 are suspended above each vernier, and a tangent clamp and screw-fitting permits 
 the vernier bracket to be clamped to the horizontal circle, which can 
 
74 
 
 itself be clamped to the vertical axis by a tangent screw. There are two 
 verniers, with reading microscopes, rigidly attached to the _ telescope. 
 Upon the same axis, but capable of independent rotation, are the vertical circle 
 and a counterpoise circle; the axis also carries a spirit level fixed horizontally 
 on adjustable bearings. The vertical circle is clamped by a screw attached to 
 the counterpoise, a slow-motion screw being provided. A similar clamp and 
 screw connect the vertical circle and the vernier bracket. 
 
 The telescope has a rack-focussing objective of aperture 1-1 in. and focal length 
 9:5in. The diaphragm is fixed in the telescope tube and has two wires forming 
 an acute angled cross instead of the usual vertical wire, together with a single 
 horizontal wire. Three eyepieces are provided, all inverting; one is diagonal 
 for taking steep sights, whilst the other two have different magnifications. The 
 instrument has a tribrach mounting, with three footscrews set in a locking plate 
 of the Everest pattern. The telescope transits at the objective end. A supple- 
 mentary telescope is provided and can be fixed near the edge of the horizontal 
 circle. It is of equal dimensions with the main telescope, and is used for keeping 
 the back-set station in view, and also as a test of the fixity of the horizontal circle 
 when the upper part is rotated. Inv. I91I-158. 
 
 194. TRAVELLER’S THEODOLITE. Made by Robinson, 
 London. Lent by T. H. Court, Esq. 
 
 This is a highly compact transit theodolite, with vertical and horizontal circles 
 of diameter 3:5 in., graduated in half-degrees and read by vernier to single 
 minutes. It rests on three levelling screws, which can be fitted within three 
 V-shaped notches in the top of the case. The instrument was probably 
 constructed in the middle of the roth century. 
 
 The horizontal circle is fixed to the tribrach, which carries a supplementary 
 telescope, tangential to the circle, for keeping the back station in view. A single 
 central standard projects vertically from the horizontal circle, and serves as 
 axis for the hollow cylinder which carries the vernier bracket for the horizontal 
 circle at its lower extremity, and a bracket to which is fixed the vertical circle 
 at its upper end. The telescope is fixed to an arm carrying opposing verniers for 
 the vertical circle, and can rotate with this arm about a horizontal axis through 
 the top of the standard. A spirit level is attached to the back of the vertical 
 circle, at right angles to the standard. Tangent clamps and slow-motion screws 
 are fixed to the vernier brackets of both circles, and there are magnifying lenses 
 for facilitating the reading of the vernier. There are three verniers to the 
 horizontal circle, symmetrically disposed round its circumference. The vertical 
 circle has two verniers at opposite ends of a diameter. The main and supple- 
 mentary telescopes are of equal size and power. Their objectives have a focal 
 length of 3:5in., and a clear aperture of o-6in. They slide in the telescope 
 tubes for focussing. The eyepieces are of the Ramsden type. Diaphragms 
 fitted with cross wires are fixed inside the telescope tubes. The one on the main 
 telescope has two vertical wires and one horizontal wire. A diagonal eyepiece 
 is provided for taking steep sights. As the telescope is slightly eccentric to 
 the vertical standard, a counterpoise to balance its weight is fitted on the 
 standard. Inv. 1917-105. 
 
 195. TRANSIT THEODOLITE. Made by O. Fennel, Cassel. 
 
 The horizontal circle in this instrument, which was made about 1875, has a 
 separate axis with clamps and tangent screw for repeating angles, and is covered 
 by a metal plate attached to the vernier plate. It has a diameter of 14 cm. and 
 is graduated every 20’ and read by opposing verniers and reading microscopes 
 to 20”. The reversible level fixed to the telescope has its upper and lower faces 
 graduated, and can be used before and after this is transited. The circular metal 
 footplates attached to the extremities of the footscrews by ball and socket 
 joints are peculiar to German instruments. They allow the theodolite to be 
 moved over the surface of the tripod head for centring, which movement is 
 permitted by the special arrangement for connecting the instrument and tripod. 
 
 The vertical circle has a diameter of 12 cm. and is graduated in half-degrees 
 and read by a single vernier to minutes. It is rigidly fixed to the transit axis 
 and telescope. This has a fixed objective of 22-cm. focal length and 3-cm. 
 aperture, and a rack-focussing eyepiece tube, carrying the fixed diaphragm and 
 a slide-focussing inverting eyepiece. The reversible levelis attached by adjustable 
 screws parallel to the axis of the telescope. Any error in adjustment between the 
 level and telescope is automatically corrected by transiting the telescope and 
 again levelling it after rotation horizontally through 180°. An axis clamp and 
 
75 
 
 tangent screw for fine vertical motion is attached to one standard, the Y-bearing 
 of this being adjustable vertically. The axis is kept in the bearings by metal 
 straps. Between the standards is a small circular spirit level, for roughly setting 
 the plate horizontal, the final levelling being tested by the spirit level attached 
 to the telescope. The vernier plate has a clamp and tangent screw in addition 
 to that on the horizontal circle. The instrument has a tribrach mounting, with 
 three levelling screws, and is connected to the tripod head by a central screw and 
 spring. On loosening this the theodolite can be moved laterally for centring 
 above the station point. Inv. 1876-1193. 
 
 196. TRANSIT THEODOLITE. Made by A. & R. Hahn, Cassel. 
 
 This is a repeating theodolite, made about 1875, and resembles the adjacent 
 one by Fennel in general construction. It differs, however, in having an ordinary, 
 one-sided spirit level attached to the telescope, which has also a Huygens, not a 
 Ramsden, eyepiece. The horizontal circle has a diameter of 14 cm., is graduated 
 every 20’ and read by two opposing verniers and reading microscopes to 30”. 
 The vertical circle, of 10-5 cm. diam., is divided to 30’ and read by opposing 
 verniers to single minutes. 
 
 The telescope has a fixed objective, of 2-5-cm. clear aperture and, with the 
 field lens of the eyepiece, a focal length of 23 cm. The eyepiece is focussed 
 by a rack and pinion arrangement, the cross-wire diaphragm being fixed between 
 the two lenses. The telescope, rigidly attached to the horizontal axis and 
 vertical circle, transits at the eyepiece end, and has a sensitive longitudinal 
 spirit level mounted on it in adjustable bearings. The vertical vernier bracket 
 is screwed to one standard, whilst the other carries the tangent screw and 
 clamping spring for the transit axis. The horizontal vernier plate carries a 
 metal cover which protects the horizontal circle and supports the standards, 
 a central circular spirit level, and the reading microscopes. There are windows 
 in this plate above the verniers, and reflectors are attached to the microscopes 
 for illuminating the field. The horizontal circle has a separate axis, with clamp 
 and tangent screw for repetition. This fits in the socket fixed to the tribrach, 
 which has three levelling screws furnished at their feet with small metal plates, 
 to which they are connected by ball and socket joints. The instrument is 
 connected to its tripod by the usual German central screw, with a flange and 
 Opposing spring which fix the instrument tightly, but on loosening the screw 
 permit the theodolite to be displaced laterally for accurate centring over the 
 
 station point. Inv. 1876-1192. 
 197. TRANSIT THEODOLITE. Made by Dennert & Pape, 
 Altona. 
 
 The horizontal circle and vernier plate in this instrument, which was con- 
 structed about 1875, have independent motions about the vertical axis, for 
 repeating azimuth angles. Clamps and tangent screws are provided for both 
 circles, and for the vertical circle. The horizontal and vertical circles have 
 diameters 21 and 13 cm. respectively, are both covered, divided to 10’ and 
 read by opposing verniers and reading microscopes to 10”. 
 
 The telescope is mounted centrally on the horizontal axis, and can transit 
 at either end. It has a fixed objective of aperture 3-5 cm. and focal length 
 25 cm., and a rack-focussing eyepiece tube, carrying a fixed cross-wire diaphragm 
 and a screw-focussing inverting eyepiece. A sensitive spirit level is mounted 
 in adjustable bearings longitudinally above the telescope, and another is fixed 
 at right angles to this on the vernier plate, between the standards. The vertical 
 circle is fixed to the telescope and transit axis, the vernier plate being fixed to 
 one standard, and an axis clamp and tangent screw to the other. The trunnions 
 on both standards are adjustable vertically. The instrument has a tribrach 
 mounting, with three footscrews for levelling. These screws have flat ball 
 and socket jointed footplates which rest on the tripod head. The attachment 
 for fixing the theodolite to the tripod is the central screw and spring usual in 
 German instruments, which permits a lateral adjustment of the theodolite over 
 the station point without moving the tripod. Inv. 1876-1194. 
 
 198. TRANSIT THEODOLITE. Made by J. Wanschaff, Berlin. 
 
 This instrument, constructed about 1875, has the telescope and vertical 
 circle fixed at opposite extremities of the horizontal axis, outside the bearings. 
 This arrangement reduces the height of the instrument and permits the telescope 
 to transit completely, but has the disadvantage that the telescope is not 
 centred above the station point. 
 
76 
 
 The horizontal circle has a diameter of 16 cm., is divided to 10’ and read 
 by opposing verniers and reading microscopes to 10”. It is covered by a metal 
 plate attached above the vernier plate, and fits fairly tightly on the vertical 
 axis, but can be rotated independently for taking readings of an azimuth angle 
 at various parts of the limb. The vernier plate has an axis clamp and tangent 
 screw, and carries a central socket in which the vertical axis rotates. A horizontal 
 bracket fixed across the top of this socket carries the bearings for the transit 
 axis. Both bearings can be adjusted on the bracket, and tested for horizontality 
 by a sensitive striding level. The transit axis has a clamp and slow-motion 
 screw to fix it to the vertical socket. The vertical circle has a diameter of 
 10*5 cm., is divided to 20’ and reads by opposing verniers fixed to the axis 
 bracket and magnifying lenses to 1’. The telescope has a fixed objective of 
 2*5-cm. aperture and 22-cm. focal length, and a rack-focussing eyepiece tube 
 carrying a fixed cross-wire diaphragm and a slide-focussing inverting eyepiece. 
 The instrument is mounted on a tribrach, with three footscrews. 
 
 Inv. 1876-1195. 
 
 199. TRANSIT THEODOLITE. Made by Elliott Bros. Presented 
 by the Admiralty. 
 
 This instrument was made at the end of the roth century and used in the 
 British Antarctic Expedition, t901. It has 7-in. vertical and horizontal circles, 
 graduated every 10’ and reading by two opposing verniers and reading micro- 
 scopes to 10”. All adjustable screws and parts are covered with wash 
 leather to protect the operator’s fingers from frost. 
 
 The horizontal and alidade circles are enclosed in a metal case, the verniers 
 being covered by hinged plates. Clamps and tangent screws are fitted to both 
 circles, and to the vertical circle, which is not covered, and is fixed to the transit 
 axis. The vertical vernier bracket is clamped to a lug on one standard by 
 Opposing screws, and carries a sensitive spirit level perpendicular to the transit 
 axis. The transit axis rests in Y-bearings on the standards, one trunnion being 
 adjustable vertically, and is kept in position by metal retaining straps; it is 
 hollow and has a lens at one end for illuminating the telescope axis. The telescope 
 is rigidly fixed to this axis and transits at the eyepiece end only. It has an 
 objective of r1-in. focal length and 1:5-in. aperture, focussed by a rack and 
 pinion mechanism controlled from the eyepiece end. The diaphragm has one 
 vertical and one horizontal thread, both adjustable laterally. Three slide- 
 focussing é€yepieces are provided, one inverting, one diagonal, and one erecting. 
 There is a central reflector in the telescope for illuminating its axis by light 
 coming through the transit axis from a lamp fixed to one standard. A striding 
 level is provided for testing the transit axis. The vernier cover plate carries a 
 spirit level, with its axis parallel to the transit axis, whilst a perpendicular level 
 is fixed to one standard. A central compass with a 4-in. ring graduated in degrees 
 is attached to the vernier plate. 
 
 The instrument has a tribrach mounting with three levelling footscrews, 
 whose points rest in a trivet plate; this is provided with a locking plate of the 
 Everest type attached to the tripod head. Inv. IOLI—10T. 
 
 200. TRAVELLER’S TRANSIT THEODOLITE. Made by 
 Taselia. 
 
 This is a compact theodolite specially designed for Alpine and military sur- 
 veying. It has 3-in. vertical and horizontal circles, divided to half-degrees and 
 read by verniers to single minutes. A diagonal eyepiece is provided for astro- 
 nomical observations of stars near the zenith. 
 
 The construction of the instrument is similar to that of a modern vernier 
 theodolite of twice the size. The transit axis rests on standards projecting 
 from the vernier plate, and carries a central telescope and the vertical circle 
 fixed to it. The vernier arm projects from one standard, and a tangent clamp 
 and slow-motion screw is fitted to the other, which also carries a spirit level 
 perpendicular to the transit axis. A similar level parallel to the transit axis 
 is fixed on the vernier plate, which also carries a central compass box, having 
 a 14-in. ring divided in degrees. The compass needle is provided with a lifting 
 lever for clamping it when desired. The vernier plate rotates independently 
 about the vertical axis and has a clamp and tangent screw for connecting it 
 to the horizontal circle, which has a similar clamp and tangent screw connecting 
 it to the tribrach. This latter has three footscrews, with their feet clamped in 
 a trivet plate by a locking plate of the Everest type. The trivet plate can be 
 screwed to the tripod head. The telescope has a rack-focussing objective of 
 
77 
 
 4°5-in. focal length and 0-65-in. aperture, a fixed cross-wire diaphragm and a 
 set of slide-focussing eyepieces, one erecting, one inverting, and one diagonal. 
 A reflector for illuminating the cross wires at night is fitted in front of the objective. 
 
 Inv. 1893-147. 
 
 201. ENGINEER’S TRANSIT THEODOLITE. Made by W. & 
 L. E. Gurley, Troy, New York. 
 
 This instrument illustrates many of the characteristics of modern American 
 transit theodolites of the type used in reconnaissance and engineering surveying. 
 The solar attachment to the telescope for determining meridians and latitudes 
 by observations on the sun is a special feature, whilst the levelling base, with its 
 “shifting centre ’’ and four footscrews, is typical of American construction. 
 
 The horizontal circle has a diameter of 6-7 1n., and is graduated on silver 
 to 30’, reading by two opposite verniers to 1’. It is covered by a metal plate 
 attached to the vernier plate and having windows above the verniers. The 
 graduations on the limb are in two sets, one completely round the circle from 
 0° to 360°, the other in four quadrants from 0° to go°. The figures are slightly 
 inclined in the direction of increasing numbers. The vernier cover plate carries 
 two spirit levels with their axes respectively parallel and at right angles to the 
 transit axis. A large compass box, with a 5-in. ring graduated in half-degrees, 
 is so mounted on the centre of the plate that its ring can be rotated in its own 
 plane by a rack and pinion arrangement. The pivot of the needle is attached 
 to a large plate, which can be lifted as a whole for clamping the needle. The 
 standards for the transit axis terminate in Y-bearings, one of which is adjustable. 
 The transit axis carries at one end a small spirit level, which can rotate about it, 
 and be clamped in any position to rotate with the axis. At the opposite end 
 is similarly mounted the vertical arc, its vernier being adjustably fixed to the 
 corresponding standard. The arc is graduated every 20’ and reads by vernier 
 to 30”; angles of elevation or depression up to 80° can be read. The telescope 
 rigidly fixed to the axis, transits at either end. It has an 11 in. objective, of 
 13/16ths in. aperture, with a rack and pinion focussing movement, and an erecting 
 eyepiece of magnification 26 diam., also rack focussing. Three parallel hori- 
 zontal and one vertical platinum wires are mounted in the diaphragm, the two 
 outer horizontal ones being for stadia work. A sensitive level, 7§-in. long, with 
 a metal scale for reading the bubble is mounted beneath the telescope and can 
 be adjusted by capstan screws at both ends. 
 
 The solar attachment is of the type invented by Burt, and is fixed to the 
 top of the telescope, with its polar axis perpendicular to that of the telescope 
 and the transit axis. This axis, and the hour circle attached to it, can be 
 adjusted on the telescope to ensure perpendicularity. The hour circle is gradu- 
 ated every 5’, and has a diameter of 1-5 in. The declination arc, movable 
 about the polar axis has a radius of 5 in., with a silver limb graduated every 
 15’ from o° to 33°, and read by a vernier on the sighting arm to 30”. This 
 sighting arm is mounted on the centre of the arc, and can be rotated with respect 
 to it by a fine threaded screw. At each end of the arm is fixed a lens with a 
 pin-hole stop and a silver reflecting plate on which are engraved parallel lines 
 in two perpendicular directions, at a distance apart equal to the diameter of 
 the sun’s image. In using this attachment to find the true meridian, the co- 
 latitude of the place is set off on the vertical arc, the declination of the sun 
 corresponding to the date and time on the arc, and the hour on its circle, and 
 the telescope is then rotated in azimuth until the sun’s image, caused by refraction 
 through the lens of the sighting arm, falls exactly between the lines on the 
 mirror. The telescope and polar axis are then in the meridian. Inv. 1914-741. 
 
 202. TRANSIT THEODOLITE. Made and lent by C. F. Casella 
 &Con Ltd: 
 
 This is a 5-in. vernier theodolite of modern construction, the telescope having 
 a fixed objective and internal focussing lens. A gradienter scale is fitted to 
 the clamp of the vertical circle and is used for measuring distances. 
 
 The horizontal circle has a diameter of 5 in., and is divided to 20’ and read 
 by two opposing verniers and reading microscopes to 20”. A metal cover plate 
 is fixed to the vernier plate and protects the horizontal circle from dust. The 
 standards and two mutually perpendicular spirit levels are mounted on this 
 plate. Tangent clamps and slow-motion screws are fitted to both the vernier 
 plate and the horizontal circle, so that azimuth angles can be repeated. A 4-in. 
 trough compass can be attached beneath the horizontal circle, and_ rigidly 
 connected to it, for orienting the instrument. The vertical circle has a diameter 
 
78 
 
 of 4 in., is divided to 20’ and read by opposing verniers and microscopes to 30”. 
 A sensitive spirit level is fixed to the vernier bracket, perpendicular to the 
 transit axis. The telescope has a fixed objective of 1-25-in. aperture, and an 
 internal lens, movable for focussing by means of a central screw operating the 
 rack and pinion. This screw is conveniently placed at one end of the transit 
 axis. The eyepiece is inverting, screw focussing and magnifies 20 diam. It 
 can readily be removed to permit access to the diaphragm, which is fixed in 
 the main telescope tube and is of the star pattern, with one horizontal and two 
 equally inclined wires intersecting on the former. The gradienter scale is fixed 
 to the clamping bracket of the vertical circle, the head of the tangent screw 
 having a graduated drum reading hundredths of the scale divisions, each of 
 which is one-hundredth of the distance between the transit axis and the axis 
 of the tangent screw. In using the scale for measuring distances, the altitude 
 angle of the telescope introduces a factor depending on the square of its cosine, 
 the staff being vertical. The instrument has a tribrach mounting with three 
 levelling screws, the feet of which rest in grooves in a trivet stage and are kept 
 fixed by a locking plate. A knurled clamping screw serves to fix this stage to 
 the tripod head, permitting a small centring movement without disturbing the 
 tripod. Inv. 1921-732. 
 
 203. VERNIER TRANSIT THEODOLITE. Made and lent by 
 E. R. Watts & Son, Ltd. 
 
 The 4:6-in. horizontal and 4-in. vertical circles in this instrument are divided 
 on silver to 20’ and read by two opposing verniers and reading microscopes to 20”, 
 The telescope has an internal focussing lens and transits at both ends. The hori- 
 zontal circle is enclosed in a dustproof and waterproof box, fixed to the vernier plate. 
 The theodolite is clamped to the levelling tribrach by a centring device which 
 permits the vertical axis to be set over the station point without moving the 
 tribrach and tripod. 
 
 The telescope hasa fixed objective of 1 - 7-in. aperture and a fixed glass diaphragm 
 on which are engraved the usual cross lines and stadia lines. Owing to the 
 anallatic properties of the internal focussing lens, no “‘ constant ’”’ is necessary 
 when using the theodolite as a tacheometer. The eyepiece is inverting and 
 screw focussing. 
 
 The transit axis, on which are fixed the telescope and vertical circle, is per- 
 manently fixed on the standards, but the bearing at one end can be adjusted by - 
 means of a capstan screw. The horizontal circle is mounted on a repeating stage 
 and has a separate axis clamp and tangent screw, carefully distinguished in shape 
 from those operating the plate motion. The vertical vernier bracket is fixed 
 adjustably to the standard in the ordinary manner, and carries the usual spirit 
 level with its axis parallel to the vernier zeros. A similar bubble tube is mounted 
 on the horizontal plate, parallel to the transit axis, and a very small bubble is 
 fixed across one standard, perpendicular to the transit axis. A trough compass 
 with the usual edge-bar needle and lifting arrangement is screwed across the other 
 standard, and can be detached if required. The feet of the three levelling foot- 
 screws are tightly clamped in a trivet stage which screws on the tripod head, but 
 has also pointed feet for setting it on a platform when the tripod is not necessary. 
 The heads of the footscrews have dustproof caps, threaded in the tribrach ; these 
 also serve to take up any wear on the footscrews. The graduation of the circles 
 and the numbering of the divisions at every 5° are very distinct. 
 
 Inv. 1922-444. 
 
 204. MICROMETER MICROSCOPE TRANSIT THEODOLITE. 
 Made by A. Lingke & Co., Freiberg. Lent by the Admiralty. 
 
 The precision of smail theodolites has been greatly enhanced in recent years 
 by the provision of micrometer microscopes for reading the circles to within a 
 few seconds. The instrument exhibited, salved from a German warship sunk at 
 Scapa, illustrates a type of theodolite of Continental design, with micrometer 
 reading to both circles, the telescope and vertical circle being mounted eccentri- 
 cally at one end of the transit axis. The horizontal and vertical circles, of dia- 
 meters 14 cm. and 13 cm. respectively, are graduated on silver to 10’, and read 
 by two opposing micrometer microscopes to 2”, the micrometer drums being 
 graduated every 20”, and the tenths estimated. 
 
 The eccentric arrangement of the telescope has obvious disadvantages, but 
 permits the telescope to transit independently of the height of the standards above 
 the horizontal circle, and also enables longer microscopes to be employed with the 
 vertical circle than is possible when the telescope is above the vertical axis. 
 
79 
 
 These microscopes are mounted in a stage which is set on the transit axis and is 
 permitted a slight rotation about this by means of a tangent screw attached to 
 one standard. The stage carries a spirit level mounted in adjustable bearings, 
 whereby the microscope can be set accurately horizontal. A second adjacent 
 spirit level is mounted in a bracket on the transit axis, to which it can be clamped, 
 and is prevented from fouling the microscope by stops, which abut against the 
 vertical axial cone, but allow a small motion of the level. A separate axis clamp 
 is connected with the tangent screw fitted to the second standard, and when 
 tightened, permits a slow motion only of the transit axis. The telescope is screwed 
 upon one end of this axis, the vertical circle being attached to it by a friction 
 clutch permitting independent setting of the circle, but ensuring that the telescope 
 and circle move together with the axis. An auxiliary vertical circle, 1o-cm. 
 diam., graduated in degrees and read by index, is mounted on the other end of the 
 transit axis, and serves both to balance the instrument and to check the larger 
 units of the vertical reading. The standards are fixed to a central hollow conical 
 socket, and also carry the brackets supporting the two horizontal reading micro- 
 meter microscopes. This conical socket revolves about a central cone carried 
 by the horizontal circle, which is fixed to the tribrach, and is provided with an axis 
 clamp and tangent screw for slow motion. 
 
 The telescope has a sliding eyepiece, actuated by a rack and pinion arrange- 
 ment and a fixed objective, of 22-cm. focal length and 2-5-cm. aperture. The 
 eyepiece tube carries the diaphragm and a screw-focussing inverting eyepiece. 
 
 Inv. 1920-88. 
 
 205. MICROMETER TRANSIT THEODOLITE. Made and lent 
 by T. Cooke & Sons, Ltd. 
 
 This design of a small transit theodolite exhibits many special features which 
 render it capable of very accurate work. The horizontal circle, of 5-in. diam. and 
 graduated directly to 10’, is completely enclosed in a metal sheath and is read by 
 two micrometer microscopes to 10”, or even less by estimation. The vertical 
 circle is likewise graduated and read, but is not enclosed. The telescope is focussed 
 by means of an internal lens, the position of which can be varied by rotating a 
 milled screw at one end of the transit axis. 
 
 In other respects the theodolite resembles an ordinary modern vernier reading 
 type, with the usual tribrach mounting, the feet of the three levelling screws being 
 clamped in a trivet plate which screws on to the tripod head. Tangent clamps 
 and slow-motion screws are fitted to the independently rotating horizontal plate 
 and limb, as also to the vertical circle, which is fixed to the transit axis. The 
 microscopes for the horizontal circle are held rigidly by brackets projecting from 
 the cover plate, and are “‘ broken ”’ near their eyepiece ends, so that they can be 
 read more comfortably. At the other end their tubes are attached to the cover 
 plate by translucent glass tubes which exclude dust but permit illumination of the 
 limb. The microscopes for reading the vertical circle are rigidly fixed to a bracket 
 to which is attached a long spirit level, perpendicular to the transit axis, and which 
 is clamped to one standard by means of a projecting tongue. A perpendicular 
 spirit level is also mounted on the cover plate. Both levels can be adjusted. 
 
 The telescope is mounted centrally on the transit axis and transits at either end. 
 It has a 1-5-in. aperture objective lens, internal focussing lens and a screw-focussing 
 inverting eyepiece of magnification 25 diam. The diaphragm is of glass, and has 
 horizontal stadia lines of I : 100 ratio on each side of the main line, and is so fixed 
 that, by a special key, it can be withdrawn and cleaned without upsetting the 
 collimation setting. 
 
 A special type of trough compass, with a flat-nosed needle, read from one end, 
 can be screwed to one of the standards. The needle is clamped or released by 
 turning the milled screw at the non-reading end. Inv. 1922-205. 
 
 206. MICROMETER TRANSIT THEODOLITE. Made and lent 
 by E. R. Watts and Son, Ltd. 
 
 In this example of a modern theodolite of small size, both the vertical and 
 horizontal circles are read by two opposing micrometer microscopes, rigidly fixed 
 to the standards; the objective tubes of these microscopes are vertical, but 
 prismatic reflectors are fitted to deflect the light into the horizontal eyepieces 
 and also, in the case of the vertical circle microscopes, into the objectives. 
 
 Both circles have a diameter of 5 in. and are divided on silver to 10’, every 
 degree being numbered. The micrometer drums are graduated into sixty parts, 
 each division thus corresponding to 10” on the circle ; it is, however, quite easy to 
 
80 
 
 set the micrometer wire to within a single second of arc, and to read the drum 
 to this amount. The circles are both enclosed completely in dustproof metal 
 boxes, fixed to the standards. 
 
 The telescope is centrally mounted on the transit axis, and has a fixed objective 
 of 1-5-in. aperture and 8-in. focal length ; its tube is dustproof, being terminated 
 at the other end by a fixed glass diaphragm upon which the image is focussed by 
 means of a movable internal concave lens, operated by a milled screw outside the 
 tube. Slide-focussing eyepieces of different powers are provided. The vertical 
 circle is fixed to one side of the telescope and revolves with it on the transit axis. 
 
 The horizontal circle can revolve independently of the upper plate for repeating 
 azimuth angles, and has a separate axis clamp and tangent screw for slow motion. 
 The instrument has a tribrach mounting, with three levelling footscrews. A self- 
 centring arrangement is fitted whereby the vertical axis can be displaced laterally 
 through a range of #in. in any direction without disturbing the tribrach or the 
 tripod to which it is screwed. 
 
 Two small bubble tubes are fixed to the horizontal cover plate with their axes 
 respectively perpendicular and parallel to the transit axis ; a more sensitive level 
 is mounted on the vertical microscope brackets, and a specially sensitive striding 
 level is provided for testing the horizontality of the transit axis. Inv. 1922-445. 
 
 MISCELLANEOUS INSTRUMENTS AND ACCESSORIES. 
 207. SMALL UNIVERSAL INSTRUMENT. 
 
 This is a 17th-century instrument for measuring angles in either a vertical or 
 a horizontal plane. 
 
 It consists of a wooden rectangular frame to which can be screwed a brass 
 socket in either of two positions for setting the plane of the rectangle horizontal or 
 vertical. Two adjacent sides of the rectangle have folding brass sights at their 
 corners for setting off right angles. A movable brass index arm, provided with 
 folding sights, is centred on one of the sighted sides of the rectangle and rotates 
 over a brass scale mounted on the other side. This scale is graduated to read 
 tangents, to the nearest tenth. A clamp and screw fixed to the index arm serve to 
 clamp it in any desired position to a brass arc fixed to the rectangle. The outer 
 sight on the arm carries a plumb-line when the rectangle is horizontal. When 
 vertical angles are being measured, the plumb-line is detached and fixed to a screw 
 so that it hangs parallel to the brass scale in the plane of the rectangle. 
 
 Inv. 1917-77. 
 
 208. TROUGHTON DIP SECTOR. Made by Thos. Jones. 
 Presented by the Admiralty. 
 
 The Dip-Sector was first designed and made by Troughton for measuring the 
 “ dip of the horizon ’’ from an elevated observing point. The example shown was 
 made in the early part of the 19th century. 
 
 The graduated sector, index arm, index mirror and horizon mirror are the same 
 as in the sextant, but the telescope has its eyepiece at right angles to its objective, 
 a reflecting mirror being interposed between the two. This enables the observer 
 to measure the zenith angle between the apparent horizon lines to his left and right. 
 By rotating the handle through a half turn, he can measure the corresponding 
 nadir angle. The quarter of the difference between these two readings is the re- 
 quired dip angle between his apparent and true horizons. The index error is 
 eliminated in this method. The sector has a radius of 7 in. and is divided on silver 
 from 5° to 40° every 10’ and read by vernier to 10”. The actual angles are only 
 half the graduation angle, as in the sextant, but the angle read is the correct one 
 between the two sight lines. The horizon mirror, fixed to the sector, is partly 
 silvered and parly unsilvered. The index glass, which rotates with the index arm, 
 is wholly silvered, and has a central index. The arm carries the vernier, and can 
 be clamped to the sector at any point, further rotation being effected by a tangent 
 screw. The telescope can be adjusted to vary its lateral distance from the sector 
 by a screw, and the handle can be rotated and clamped in any position parallel 
 to the face of the sector. Inv. 1876-1207. 
 
 209. BOX SEXTANT. Made and lent by Newton & Co. 
 
 This example is typical of the box sextant invented by William Jones at the 
 beginning of the 19th century. In principle, it measures angles like the sextant, 
 but its fixed and movable mirrors are enclosed in a circular brass box, which has 
 
SI 
 
 an aperture sighting hole and opposing window cut in its circular side. The index 
 mirror is rotated from without the box by means of a milled head and pinion gears, 
 and the external vernier arm is fixed on the same pivot axis as this mirror and 
 measures its rotation on a graduated arc engraved on a silver strip embedded 
 in the upper external surface of the box. 
 
 The arc is divided to quarter-degrees, numbered as if they were half-degrees, 
 since the angle sighted is double the angle through which the index mirror and 
 arm rotate, and is read by vernier on the index arm to single minutes ; a pivoted 
 magnifying lens is fitted for assisting the reading of the vernier. The brass cover 
 which protects the external parts when the instrument is not in use, can be screwed 
 to the under part of the box when sights are taken, and serves as a hand grip. 
 The radius of the divided arc is only 2 in., so that the instrument is less accurate 
 than an ordinary sextant, but it is preferred for rough reconnaissance surveying 
 on account of its portability and the protection of its parts afforded by the box. 
 
 The horizon mirror can be adjusted from outside the box by means of a 
 key, and coloured glasses are provided for taking sights on the sun. 
 
 Inv. 1865-5. 
 
 210. PAGET ANGLE SEXTANT. Made by Hughes & Son. 
 Presented by the Admiralty. 
 
 This instrument is very similar to the ordinary box sextant with open sides 
 and is used for measuring angles in any plane. One object is viewed directly 
 through a lateral aperture and the unsilvered half of a fixed mirror, whilst the 
 other is viewed after double reflection at the surface of a rotating mirror and the 
 silvered half of the fixed one. 
 
 An index geared to the rotating mirror measures the angle between the two 
 objects on the graduated limb of the top surface of the instrument. A hand grip 
 is screwed to the bottom surface. The index arm is so geared to the revolving 
 mirror that it rotates through four times the angle through which this mirror 
 turns ; this is twice the angle through which the doubly reflected ray rotates. 
 Hence the limb is graduated in degrees, numbered as if they were half-degrees. 
 Both mirrors have their faces perpendicular to the plane of the angle measured. 
 A metal index in front of the axis of the rotating mirror marks the central line 
 of sight. Inv. I911I—220. 
 
 211. AERIAL POSITION FINDER. Presented by the Admiralty. 
 
 This instrument is a crude form of altazimuth theodolite, designed for 
 approximately locating the position of aerial objects. The altitude and azimuth 
 angles of such an object can be measured at any moment to the nearest tenth 
 of a degree. : 
 
 It consists of a massive, horizontal, graduated circle of 16 in. diam., divided 
 in degrees and read by estimation to the nearest tenth-degree by means of the 
 pointed end of a rotating alidade, pivoted at the centre of the circle. Three 
 footscrews and a pair of perpendicular spirit levels are fixed to the circle for 
 levelling it. The feet of the footscrews rest on three footplates, of which the 
 upper surfaces have a plane, a straight groove and a conical hole respectively. 
 The footplates are fixed to a stand and serve to keep the circle fixed without 
 straining it. The alidade has a vertical bracket fixed at the end opposite to its 
 pointer for supporting the vertical arc and sighting tube, which are pivoted about 
 a horizontal axis at its upper extremity. The vertical arc is a quadrant of 8 in. 
 radius, graduated in degrees and read by an index fixed to the bracket. The 
 sighting tube is fixed parallel to one radius of the quadrant and moves with it. 
 The tube is 30 in. long and carries a blade foresight and aperture backsight, the 
 latter having a rubber protecting eyepiece. There are no clamps to the motions, 
 the friction at the pivots being great enough to prevent the instrument moving 
 after it has been set. Two small electric bulbs set in conical reflectors on the 
 alidade can illuminate the circles for night reading, a battery and operating 
 switch for this purpose being provided and exhibited with the instrument. 
 
 Inv. 1920-583. 
 
 212. COMBINED THEODOLITE AND CROSS HEAD. 
 
 This instrument, probably of fairly modern French construction, has a hollow 
 brass cylinder of 13 cm. diam., divided into two parts independently movable 
 about the common vertical axis, instead of the horizontal and vernier plates of 
 an ordinary theodolite. 
 
 The upper half of the cylinder carries the standards for supporting the 
 horizontal axis of rotation of the telescope. A spirit level is mounted in adjustable 
 
 (15814) D 
 
82 
 
 bearings between these standards and is used for setting the axis of the cylinder 
 vertical. The curved face of this half is pierced by two pairs of vertical sighting 
 slits at opposite ends of perpendicular diameters, and the lower circular edge, 
 which is in contact with the lower half cylinder, has two opposing vernier scales 
 engraved on it. The corresponding edge of the lower half is graduated in degrees, 
 and read by the verniers to 2’. This half, which has a single pair of vertical 
 sighting slits pierced in its surfaces, is mounted upon a tribrach provided with 
 the usual three footscrews, and is fitted with a clamp and tangent screw for slow 
 motion. The upper half cylinder can be rotated with respect to the lower half 
 by a toothed wheel and pinion mechanism, actuated by a milled screw beneath 
 the cylinder. 
 
 The telescope is mounted so as to be always in the same vertical plane as the 
 vernier zeros of the upper half cylinder. It has a fixed objective of 1-in. aperture 
 and 7-in. focal length, and a rack-focussing eyepiece tube with a slide-focussing 
 inverting eyepiece. 
 
 The head of the tripod used with this instrument is also exhibited and 
 illustrates the type usual in French construction. It is of wood, with brass slots 
 for the feet of the tribrach, and is connected to the theodolite by a central plunger 
 screw with opposing spring, which permits the small displacement of the instrument 
 necessary for setting it horizontal. Inv. 1919-415. 
 
 213. PERISCOPE THEODOLITE. Made by Troughton & Simms. 
 Lent by the War Office. 
 
 This instrument was designed during the war, 1914-18, for surveying from 
 the front line trenches. The periscope attachment to the objective end of the 
 telescope enables the observer to take azimuth angles without exposing himself 
 or the theodolite. The latter is similar to an ordinary 3-in. vernier theodolite, 
 except that the telescope is ‘‘ broken’’ by a diagonal mirror in the transit axis, 
 one half of which is hollow and forms the eyepiece tube, perpendicular to the 
 objective tube. When in use the objective tube is veftical, pointing to the zenith, 
 and a counterpoise fixed to its lower end below the transit axis balances the 
 weight of the periscope. The axis of rotation of the periscope mirror is perpen- 
 dicular to the transit axis, and the mirror is adjusted about this axis by means 
 of a long brass chain. This adjustment varies the vertical angle of the line of 
 sight, but does not change its azimuth. 
 
 The 3-in. horizontal circle is graduated in half-degrees and read by opposing 
 verniers and readers to 1’. A similar vertical circle is attached to the transit 
 axis and read by opposing verniers fixed to the standards, but is not used with 
 the periscope attachment. The horizontal vernier plate covers the divided circle 
 except at the verniers and carries a spirit-level parallel to the transit axis. A 
 perpendicular spirit-level is attached to the vertical vernier bracket. A trough 
 compass is fitted beneath the horizontal circle and rotates with it about the vertical 
 axis. Clamps and tangent screws are fitted to the vertical and horizontal circles, 
 and the horizontal vernier plate. The theodolite has a tribrach mounting, with 
 three levelling screws, which rest in a trivet stage and are locked in position by 
 a locking plate of the Everest type. A shifting centre arrangement serves to 
 attach the trivet stage to the tripod head, but permits a relative horizontal 
 displacement of the two when the clamping screw is loosened. 
 
 Inv. 1919-495. 
 
 214. THREE READING MICROSCOPES. Made by O. Fennel 
 
 Séhne, Cassel. 
 
 These represent three types of microscope for reading the circles of small 
 theodolites. In each case, the circles are completely enclosed in metal, dust- 
 proof cases, and the microscopes are consequently provided with mirror reflecting 
 tubes for illuminating the graduations. 
 
 The first microscope is a “‘ Schatz ’”’ or ‘‘ Estimation ”’ Microscope, and has a 
 single fixed line marked in its focal plane, parallel to the graduations of the 
 circle. It is used in theodolites having ro-cm. circles, graduated every 10’, The 
 position of the fixed microscope line between these graduations is estimated to 
 a single minute. 
 
 (See “ Zeitschrift fir Vermessungswesen,”’ 1902, p. 214-216.) 
 
 The second microscope, known as a “‘ Nonien-Mikroskop,”’ has a fixed vernier 
 scale marked in its focal plane, and is used for theodolites having 13-cm. circles 
 graduated to 5’. Ten divisions of the vernier scale correspond to nine of the 
 circle, whence the position of the zero vernier line can be read to 30”, or even 
 to 15” by estimation. Alternate graduations of the vernier scale are numbered 
 consecutively from o to 5, and represent minutes. 
 
 (See ‘‘ Zeitschrift fir Instrumentenkunde,” 1912, Part V.) 
 
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 The third microscope, designated a ‘‘ Skalen-Mikroskop,” is of the type 
 devised by Hensoldt in 1879, and is used on theodolites having 13°5-cm. circles, 
 graduated in 20’. A fixed scale in the focal plane of the microscope has ten 
 divisions, numbered right to left, exactly covering the interval between consecutive 
 graduations of the circle. The position of one circle graduation on this scale 
 can be read to one-tenth of a scale division, 7.e., to 12”. In this and the second 
 microscope, the reading edges of the two scales slightly overlap each other, 
 which ensures a sharper reading. 
 
 (See ‘‘ Zeitschrift fiir Vermessungswesen,” 1879, pp. 497-507.) 
 
 Inv. I9Q14—-553, 554, 555- 
 
 215. QUICK-LEVELLING TRIPOD HEAD. Made and lent by 
 T. Cooke & Sons, Ltd., York. 
 
 For setting up a surveying instrument on uneven ground, the tripod head is 
 often provided with an arrangement permitting the instrument to be quickly 
 levelled,. without moving the legs of the tripod or the foot-screws, which last 
 are only used in the final accurate levelling. 
 
 Such a device is here exhibited. The trivet stage of the instrument is screwed 
 to a central spherical section of a hollow ball, which fits sufficiently closely into 
 a spherical socket fixed to the tripod legs to prevent any motion other than a 
 rotation of the ball in the socket, permitting the top of the ball to be levelled 
 irrespective of the top of the tripod socket. A circumferential clamp, provided 
 with three thumb pieces, is threaded upon the outside of the socket, and serves 
 to clamp the ball tightly within this when the instrument is approximately level, 
 as shown by a small circular spirit level fixed in the top of the ball. A shank 
 attached beneath the top plate of the ball marks its centre, and has a loop of 
 cord attached for centring the instrument over the station point. 
 
 Inv. 1921-561. 
 
 216. SHIFTING CENTRE TRIPOD HEAD. Made and lent by 
 T. Cooke & Sons, Ltd., York. 
 
 This tripod head is fitted with a ‘‘ shifting centre ’’ arrangement, permitting 
 the centre of the attached theodolite to be displaced any distance up to about. 
 1 in. in any direction without removing the theodolite from the tripod or 
 altering the position of this latter. This property is very useful in setting the 
 theodolite accurately over the station point. 
 
 The tripod head consists of five metal parts, namely a threaded plate to which 
 the theodolite is screwed, a shoulder plate to which the tripod legs are attached, 
 and three other parts which together form a collar, between the flanges of which 
 the other two plates can be clamped. The outer diameter of the collar is 1}in., 
 whilst the theodolite base plate and tripod shoulder plate have central apertures 
 2in. and 24in. diam. respectively, so that either of them can be moved between 
 the flanges independently, and clamped in the required relative position. 
 
 A small circular spirit level is embedded in the top of the tripod plate and is 
 used to set this approximately level. Inv. 1921-559. 
 
 GEODETIC THEODOLITES. 
 
 217. RAMSDEN’S 3-ft. THEODOLITE. Presented by the Ord- 
 nance Survey. Plate V, Fig. 1, p. 83. 
 
 The main angles of the Principal Triangulation of Great Britain and Ireland 
 were mostly measured with this altazimuth theodolite, or with a very similar one 
 now in the possession of the Royal Society. The latter was made by Ramsden 
 in 1787 and the one here exhibited about 1790 ; both were in constant use between 
 1792 and 1862, and several sights were taken with them during the Principal 
 Triangulation at distances exceeding 100 miles. 
 
 The theodolite here exhibited has a 3-ft. horizontal circle, divided to 10’ and 
 read to 1” by three micrometer microscopes. The circle is mounted upon ten 
 conical radii, which project from the outer conical vertical axis ; this revolves 
 upon an inner fixed axis to which the reading microscopes and levelling foot 
 screws are fixed. A flat horizontal bar rigidly mounted across the top of the outer 
 axis supports the telescope axis. Two 10:5-in. vertical circles, divided to 30’, 
 are fixed to the telescope and read to 3” by single microscopes mounted on the 
 supporting bar. The telescope axis and supporting bar are made accurately 
 horizontal by means of two spirit levels, one 24 in. and the other 21 in, long. 
 
 (15814) D2 
 
84 
 
 The telescope objective has a clear aperture of 2:5 in. and a focal length of 36 in. 
 The erecting eyepiece generally employed magnifies 54 diam., but other eye- 
 pieces were used in special cases. The telescope diaphragm has fixed cross wires 
 and a horizontal travelling wire controlled by a micrometer screw, and could be 
 illuminated through the horizontal axis of rotation for night readings. Clamps 
 and tangent screws for fine adjustment are fitted to the horizontal and vertical 
 circles. 
 
 The probable error of a single observation with this instrument was about 
 2” for distances up to 70 miles. 
 
 The mechanism of the microscope is illustrated in the accompanying diagram. 
 (See No. 218.) 
 
 (See ‘‘ Phil. Trans. Roy. Soc.,’’ Vol. LXXX, 1790, p. 111. ‘“‘ Account of the 
 Principal Triangulation,” ‘‘Ordnance Survey, 1858.’’) 
 
 Inv. 1876-1203, S.M. 259. 
 
 218. DIAGRAM OF MICROMETER MICROSCOPE. (Ramsden’s 
 3-ft. Theodolite.) Constructed in the Museum. 
 
 The method of reading a graduated scale by means of a micrometer microscope 
 having at its focal plane a movable cross wire, was first applied to theodolites 
 by Ramsden in the construction of his 3-ft. instruments. The diagram shows 
 the details of his later microscopes, which were added to the original two of his 
 theodolites, and which differ very slightly from those. 
 
 The microscope has a single objective lens fixed in a tube, which is mounted 
 in the supporting bracket and can be slightly adjusted towards or from the 
 graduated circle of the theodolite. The base of the micrometer box is screwed 
 to the top of this tube and has a smaller one screwed into its top; the latter 
 carries a slide-focussing eyepiece of the usual Ramsden type. 
 
 The micrometer box contains two adjustable stages having their inner surfaces 
 contiguous and their outer surfaces touching the walls of the box. The lower 
 edge of the aperture of the upper stage has divisions notched in it at intervals 
 of a hundredth of an inch, every fifth notch larger than the rest. The interval 
 
 - between the larger scale divisions is the same as that between adjacent divisions 
 of the graduated circle when the latter interval is magnified by the microscope 
 objective. Thus the interval between consecutive notches is 1’ of arc on the 
 circle. The lower stage has a cross hair attached to its inner face, and can be 
 moved as a whole across the notched scale by rotating the micrometer head ; 
 this head is threaded on to a screwed pin fixed to one end of the stage, and is 
 pressed against the micrometer box by a counter helical spring inside the box. 
 The pin has 100 threads per inch, so that the cross hair moves over one division 
 of the notched scale at each revolution of the micrometer head. The periphery 
 of the drum on this head is divided into 60 parts, and its rotation read by the 
 position of a knife edge fixed to the box. Thus a rotation of the drum through 
 one division of its periphery causes the cross hair to travel through 1” of arc 
 on the circle. 
 
 The stage carrying the notched scale can be adjusted in the box by means of 
 a screw and counter V-spring acting on its ends until the cross hair lies over a 
 notch when the micrometer drum reads zero. Inv. 1921-556. 
 
 219. 2-ft. TRANSIT THEODOLITE. Made by Troughton and 
 Simms. Presented by the Ordnance Survey. Plate V, Fig. 2, p. 83. 
 
 This instrument was made at the commencement of the Irish survey and 
 was first used in 1828 for checking the measurement of the Lough Foyle base 
 by triangulation. It was further employed between 1829 and 1862 in measuring 
 the main angles of the Principal Triangulation of Great Britain and Ireland. 
 
 This theodolite differs from the adjacent 3-ft. one made by Ramsden in that 
 its telescope can transit completely between the supporting pillars, and the 
 horizontal circle is mounted upon a repeating stage so that both circle and 
 microscopes can be independently rotated. The horizontal circle is 2 ft. in diam., 
 and is divided on silver to 5’ and read by five microscopes to 1”. Two vertical 
 circles of 12-in. diam. are fixed to the telescope; they are divided to 5’ and 
 read by single microscopes to 2”. The telescope has a fixed objective of 27-in. 
 focal length and 2:-1-in. aperture, and a rack-focussing eyepiece tube carrying 
 the diaphragm and erecting eyepiece. The diaphragm has one fixed horizontal 
 and two fixed diagonal wires, with five equidistant vertical wires moved laterally 
 by a micrometer screw. 
 
85 
 
 The repeating stage rests by means of three footscrews upon an iron triangle 
 fixed upon the head of the wooden tripod stand. It has an upper rotating 
 tribrach supporting the horizontal circle and vertical axis of the theodolite. 
 The five microscopes reading this circle rotate with the telescope and upper 
 part about this axis. Clamps and tangent screws are fitted to both circles and 
 the repeating stage, and four spirit levels are provided for adjusting the telescope 
 and horizontal axis of rotation. 
 
 (See “‘ Account of the Measurement of the Lough Foyle Base,’’ Ordnance 
 Survey, 1847, p. 108.) Inv. 1876-1204, S.M. 260. 
 
 220. 40o-cm. TRANSIT THEODOLITE. Made by Brunner Freres, 
 Paris. Lent by the Physical Department, Ministry of Public Works, 
 
 Egypt. 
 
 This instrument was ordered in 1858 for use in a projected general survey of 
 Egypt. It has horizontal and vertical circles, each 40-cm. diam., graduated 
 direct on silver to 5’, and each read by four micrometer microscopes direct to 2”, 
 by estimation to 0:2”, 
 
 The vertical circle, together with a circle of equal dimensions as a counter- 
 poise, is fixed directly to the horizontal axis of rotation of the telescope. This 
 axis rests in bearings upon two standards which carry, opposite the two ends 
 of the axis, fixed brackets for supporting the circle carrying the microscopes 
 at one end and a counterpoise at the other. The axis of the telescope is pierced 
 by a cylindrical hole for illuminating the interior, and a lamp can be attached 
 to one of the brackets for this purpose. The standards project from a horizontal 
 stage fixed to the alidade platform by adjustable screws, operated by a worm- 
 wheel and pinion mechanism. These serve to level the stage more precisely 
 after the alidade has been approximately levelled by the three footscrews attached 
 to the horizontal circle. <A striding level fitted parallel to the axis of rotation 
 of the telescope, together with a perpendicular level fixed to the microscope 
 circle, is used for levelling. The alidade platform carries the four microscopes 
 for reading the graduated horizontal scale, and can be rotated in azimuth, 
 carrying the whole upper portion with it. The horizontal circle itself is fixed 
 to the three footscrews, but its graduated circle can be independently rotated 
 for ‘‘ repeating ”’ angles. In addition to the microscopes, there is an index arm 
 with a reading lens for checking the major readings of each circle. Tangent | 
 clamps and slow-motion screws are fitted to both the vertical circle and the 
 alidade. 
 
 The telescope has an objective of 2:2-in. aperture and 30-in. focal length 
 and an inverting draw-focussing Ramsden eyepiece, to which is fitted a micro- 
 meter stage having one horizontal and five vertical fixed threads, and one 
 vertical thread movable by means of the micrometer screw. 
 
 The reading microscopes are fitted with metallic mirrors, and the vertical 
 circle ones have prism reflectors in addition, for illuminating the field. 
 
 Inv. 1921-479. 
 
 221. 12-in. EVEREST THEODOLITE. Made by T. Cooke & 
 Sons, Ltd. Lent by the Ordnance Survey. 
 
 Theodolites of this type were used by the Ordnance Survey in the Secondary 
 Triangulation of the United Kingdom during the latter half of the 19th century. 
 The example shown was made in 1884, and the probable error of a single obser- 
 vation made with it was about one second of arc. 
 
 The telescope is mounted on very low standards, so that it cannot be transited, 
 whilst the vertical arcs only extend for some 40° on each side of the horizontal. 
 The horizontal circle is mounted on a separate vertical axis for repeating azimuth 
 angles. When this axis is clamped the verniers revolve with the standards ; 
 when unclamped, the circle, verniers and standards rotate together. The circle 
 is of 12-in. diam., divided on silver to 5’ and read by four symmetrical verniers 
 to 5”; four small microscopes are mounted on a separate rotating bracket for 
 reading these verniers. The circle is mounted in a tribrach stiffened by a circular 
 ting, used for lifting the theodolite. The three levelling screws threaded in the 
 tribrach arms rest in radial V-shaped grooves in a base plate. The latter rests 
 in turn on a trivet stage having a massive curb and three footscrews ; three 
 horizontal radial screws threaded in this curb press on the central boss of the 
 base plate and serve to adjust the theodolite axis over the station point. 
 
 The four horizontal verniers are rigidly fixed to the standards which are 
 about 6 in. high and support the central telescope, vertical arcs, vertical verniers 
 and counterpoise. The two vertical arcs are portions of a circle of 12-in. diam., 
 
 (15814) D3 
 
86 
 
 divided on silver to 10’ and read by opposing verniers and reading microscopes 
 to 10”. The arcs are fixed to the axis of rotation carrying the telescope and 
 counterpoise, whilst the verniers are clamped to one standard. The telescope 
 has a rack-focussing objective of 2-5-in. aperture and 20-in. focal length, a fixed 
 star diaphragm, and a slide-focussing Ramsden eyepiece. 
 
 The usual clamping and tangent screws are provided for all motions in azimuth 
 and altitude, and two sensitive levels are supplied, one on the vertical vernier 
 bracket, the other at right angles on the base of the standards. Inv. 1895-48. 
 
 222. 12-in. TRANSIT THEODOLITE. Made by E. R. Watts & 
 Son. Lent by the Ordnance Survey. Plate VI, Fig. 1, p. 86. 
 
 This theodolite was constructed in 1910 for use on the Primary (First Order) 
 Triangulation of the United Kingdom and has already been used to observe 
 the angles of a piece of test triangulation in North-East Scotland in 1911-12, 
 when 29 angles were observed with a mean error of 0-517” per angle. 
 
 It is a transit theodolite with a repeating horizontal circle of 12-in. diam., 
 read by three micrometer microscopes to 1” and by estimation to o-1”. 
 A separate pointer microscope serves to check the integral degrees of the reading. 
 The circle is protected from dust by a cover plate provided with apertures for 
 the tubular microscope reflectors which form dustproof connections to the 
 microscopes. The outer vertical axis is rigidly connected to a strong tribrach 
 fitted at the ends of its arms with levelling footscrews and strengthened by a 
 strong metal ring, covered with leather, which serves to lift the instrument. 
 The footscrews rest in radial V-shaped notches on a lower tribrach plate which 
 rests upon a strong circular baseplate having a massive curb. The lower 
 tribrach has a central boss projecting through an aperture in the base plate; 
 three radial screws threaded in the curb impinge on this boss and serve to move 
 the whole instrument accurately over the station point. The vertical axis has 
 a central socket in which can be inserted a nadir telescope for this centring 
 operation. 
 
 The massive U-shaped standards for supporting the telescope axis of rotation 
 are cast in one piece and attached to the cover plate of the horizontal circle, 
 which also carries two mutually perpendicular spirit levels for setting the plate 
 horizontal. The telescope axis is hollow and thus permits the diaphragm to 
 be illuminated by an electric lamp supported on one standard, the light being 
 ‘reflected down the axis of sight by a central diagonal reflector inside the telescope. 
 The latter has a fixed objective of 3-in. aperture and 24-in. focal length and a 
 rack-focussing micrometer eyepiece tube in which various inverting eyepieces 
 of powers 72, 50 and 30 diam. can be fitted. A 6-in. vertical circle, reading by 
 two fixed verniers to 1’ is attached to the telescope and rotates with it; the 
 vernier bracket, which carries a sensitive spirit level parallel to the telescope 
 axis, is clamped to one standard. A very sensitive striding level is mounted 
 on the telescope axis pivots; its bubble moves about o-1 in. per 1” difference 
 of elevation. 
 
 For testing the cylindricity of the telescope axis, an arrangement designed 
 on the principle advocated by Sir David Gill is provided (see ‘‘ Report of Geodetic 
 Survey of South Africa,” 1896, p. 123). Its micrometer drum reads to 0-ooooT1 in, 
 
 All the circles are fitted with clamps and tangent screws for slow motion. 
 
 Inv. 1914-103, 5.M. 1366, 1367 and 1368. 
 
 , FIELD ASTRONOMY. 
 
 Survey stations have to be referred to the standard system of lines 
 of latitude and longitude by means of occasional astronomical observa- 
 tions. These observations also serve as checks upon the ordinary 
 survey measurements, and can even replace such measurements in 
 special cases. The determination of the latitudes of important towns 
 and the directions of the meridians through them was an important 
 task of ancient astronomy, and the Alexandrian geographer Ptolemy 
 showed his appreciation of the necessary astronomical basis of map- 
 making by giving the latitude and longitude of every important place 
 
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 known in his day. Unfortunately, Ptolemy’s figures are mostly 
 illusory, since the actual astronomical measurements made up to his 
 time were too crude and scanty to provide an accurate basis for his 
 maps. In a few cases, latitudes had been observed by means of 
 stationary gnomons; in many cases, however, he relied upon the 
 observations of travellers who professed to have reckoned the lengths 
 of the longest days at the various places visited by them; whilst for 
 longitudes few data besides the crude dead reckoning of sailors and 
 travellers were available. The Arabs developed both stationary and 
 portable astronomical instruments for determining latitudes and 
 meridians, and did much to equip the early European navigators 
 with the astrolabes, quadrants, compasses, dials and hour-glasses, 
 which enabled them to determine their positions and construct the 
 celebrated sea-charts of the Middle Ages. During the great arc 
 measurements of the 17th and 18th centuries, the determination of 
 latitudes and meridians was of fundamental importance, and instru- 
 ments of large proportions were devised to obtain the necessary pre- 
 cision in these measurements, particularly in the latitude ones. For 
 the latter purpose, sectors of circles of several feet radius were con- 
 structed to measure the altitudes or zenith distances of stars. The 
 sectors made by Graham for the Lapland Arc Measurement in 1736, 
 by Ramsden about 1800 for the Trigonometrical Survey of Great 
 Britain, and by Troughton about the same date for use in America 
 (see No. 224), illustrate the type of instrument used at that period. 
 Airy’s Zenith Sector (see No. 223), which replaced Ramsden’s on the 
 Trigonometrical Survey of Britain, shows the tendency to replace the 
 plummet of the earlier sectors by the spirit level now generally em- 
 ployed. During the roth century the increasing accuracy of graduation 
 induced geodetic surveyors to use transits or altazimuths provided with 
 small but complete divided circles instead of the earlier sectors, and 
 Struve, in his arc measurements in Northern Europe, relied upon 
 instruments of the type exhibited (No. 227). An important feature 
 of such instruments is the micrometer eyepiece, with which in many 
 cases the actual observations were made, the circles only being em- 
 ployed for approximately setting the telescope. In the zenith tele- 
 scope, now mostly used in Europe and America for latitude deter- 
 minations by Talcott’s method of measuring the small difference in 
 the zenith distances of two stars, this reliance upon micrometer 
 measurement with the telescope immovable is also illustrated. The 
 prismatic astrolabe (see No. 229), devised about 1900 by Claude and 
 Driencourt for determining latitudes by the method of equal altitudes, 
 represents another method of avoiding measurements upon vertica! 
 arcs and has found increasing favour where the highest degree of 
 precision is not required. 
 
 The determination of longitude was for long a matter of extreme 
 difficulty. Lunar observations were troublesome and required elaborate 
 calculations, whilst chronometers were disturbed during long voyages 
 and journeys. With the rapid development of electrical signalling 
 during the 1gth and 2oth centuries, however, the difficulties of com- 
 paring the local time found by celestial observations with the standard 
 time have been largely overcome, and longitudes can now be obtained 
 with a high degree of accuracy. In determining local time by means 
 of timing the transits of stars in the meridian, the electric chronograph, 
 especially when controlled by a transit micrometer fitted to the transit 
 telescope, has much reduced the personal errors of observation. 
 
 (15814) D4 
 
88 
 
 223. AIRY’S ZENITH SECTOR. Presented by the Ordnance 
 
 Survey. 
 
 On the destruction by fire at the Tower of London of Ramsden’s zenith. 
 sector, which had previously been used to determine latitudes in the Principal 
 Triangulation of Great Britain and Ireland, this sector was designed by Sir George 
 Airy and constructed under his direction. It was used between 1842 and 1850 
 for determining the latitudes of twenty-seven stations of the Principal Triangu- 
 lation. 
 
 The telescope is pivoted horizontally to a revolving gunmetal frame which 
 is itself pivoted vertically in a fixed cast-iron framework. The latter is supported 
 by three levelling screws upon a rectangular plate which in turn rests upon a 
 cast-iron trough; this trough was usually screwed upon heavy wooden pickets 
 driven into the ground. 
 
 Four micrometer microscopes attached to the telescope serve to read its 
 inclination to the zenith upon two graduated arcs, mounted upon the revolving 
 frame. Each arc extends over 45° of a circle of radius 20°5 in., and is divided 
 to 5’ and read by the microscopes to one-quarter of a second. 
 
 The telescope has an objective of aperture 3-7 in. and 46 in. focal length, 
 a micrometer diaphragm with five fixed and one movable wires, and an inverting 
 eyepiece of magnification 70. 
 
 Three sensitive spirit levels are fixed in adjustable bearings across the back 
 of the revolving frame. 
 
 In taking observations of a star’s transit, the frame is quickly rotated to 
 permit face east and face west readings of the telescope. 
 
 The probable error of a single observation of latitude with this instrument 
 was about 1”. 
 
 (See “‘ Astronomical Observations made with Airy’s Zenith Sector,’’ Ordnance 
 Survey, 1852.) Inv. 1876-1202. 
 
 224. ZENITH SECTOR. Made by Troughton. 
 
 This portable sector was made at the beginning of the 19th century for use 
 in America and was afterwards transferred to Copenhagen. 
 
 The main telescope for observing stars near the zenith is pivoted horizontally 
 near its objective end to the top of a vertical hollow brass cylinder, which carries 
 at its lower end the fixed graduated sector over which the eyepiece of the 
 telescope rotates. Two micrometer microscopes fixed to the telescope serve to 
 read its inclination to the vertical upon separate scales engraved on the sector. 
 The scales, each of which covers about 15° and is divided to 3’, start at opposite 
 ends of the sector and overlap about 5° in the centre. <A third scale, continuous 
 across the whole sector, is read by an index and serves as a check. 
 
 The telescope has an objective of 2:5-in. aperture and 45-in. focal length, 
 and a diagonal eyepiece. Its motion over the sector is controlled by a clamp 
 and tangent screw. 
 
 The base of the vertical cylinder is connected to a horizontal circle by six 
 vertical pillars and can rotate with this circle about a vertical axis coincident 
 with the cylinder axis. A long plummet suspended within the cylinder along 
 its axis rests in a water vessel, inside the pillars, and serves to set the axis vertical. 
 Its position is observed by two perpendicular microscopes fixed in the cylinder 
 below the graduated sector. 
 
 A small telescope pivoted to the cylinder is used to observe circumpolar stars 
 for setting the sector in the meridian. 
 
 The horizontal circle has a diameter of 9 in. and is divided to 10’. The 
 vertical axis revolves inside a conical socket fixed to the tribrach, which carries 
 three verniers for reading the circle to 10”. The three levelling foot screws of 
 the tribrach rest on the top plate of a strong mahogany tripod. A small spirit 
 level is mounted on the cylinder for rough levelling, and an adjustable counter- 
 poise balances the telescope. 
 
 (See Pearson’s Astronomy, 1829, Vol. II, p. 546.) Inv. 1891-24, S.M. 1943. 
 
 225. PHOTOGRAPHS OF ZENITH SECTOR. Lent by Prof. J. 
 Norman Lockyer, C.B., F.R.S. 
 
 The zenith sector represented in these two photographs was designed by 
 Lt.-Col. A. Strange and constructed by Troughton and Simms in 1869 for use 
 in the Great Trigonometrical Survey of India. By means of this and a somewhat 
 improved instrument of similar design and make, many latitudes were determined 
 in India during the period 1884-1900 ; from 1890, however, a more convenient 
 zenith telescope has been mostly employed in India for determining latitudes 
 by Talcott’s method. 
 
89 
 
 The instrument illustrated consists of a telescope of 4-in. aperture and 48-in. 
 focal length, mounted rigidly upon a double sector which forms part of a graduated 
 vertical circle of 3 ft. diam. The telescope can rotate upon a horizontal axis 
 supported by a vertical metal pillar, and the vertical rotation is measured by 
 means of four microscopes fixed to the frame carrying the axis. The instrument 
 can be accurately adjusted upon its base frame so that the vertical sector revolves 
 in the meridian plane, and its zero line is adjusted to be horizontal by means of 
 a sensitive spirit level; the zero line is perpendicular to the telescope axis of 
 sight, which therefore points to the zenith when the former is horizontal. 
 
 The sector is graduated on silver every 5’; each limb extends over 55° and 
 is read by two microscopes, separated 20° from each other, to single seconds ; 
 a separate smaller microscope serves to read degrees and minutes on the sector 
 graduations. 
 
 The telescope has a micrometer eyepiece fitted with a diagonal reflecting 
 prism. A setting circle, of diameter 8 in., divided to 10’ and read by vernier 
 to 10”, is mounted on the telescope near its eyepiece, and is provided with a 
 small level and also a very sensitive one, which latter permits the instrument 
 to be used as a zenith telescope for observations by Talcott’s method. The 
 probable error of latitude determination with this instrument was about 0-05”. 
 
 (See ‘‘ Great Trigonometrical Survey of India,”’ Vol. XI, 1890.) 
 
 Inv. 1883-14, 
 
 226. PHOTOGRAPH OF ZENITH TELESCOPE (3-in.). Lent 
 by T. Cooke & Sons, Ltd. 
 
 This photograph illustrates a 3-in. portable zenith telescope for determining 
 latitudes by the Talcott method. The original instrument was supplied to the 
 Survey of India in 1909, and is a typical example of the modern zenith instrument 
 for geodetic work. 
 
 The telescope, of 3-in. aperture and about 42-in. focal length, is mounted at 
 one end of the horizontal axis by means of a centrepiece, which also carries the 
 Talcott level frame. The latter has two chambered levels, running o-1 in. to 
 2” of arc, enclosed in special sealed cases lined with thermal insulating material, 
 and is provided with a long sensitive clamp and slow motion screw permitting 
 an accurate adjustment in altitude. The horizontal axis has a weight relieving 
 spring and ball race at the telescope end and a counterpoise at the other end ; 
 it is hollow to permit illumination of the telescope field by means of an oil or 
 electric lamp. The telescope has a steel micrometer eyepiece, giving a value 
 of about 50” to one revolution of the micrometer screw. A 7-in. setting circle, 
 divided on silver to 30’ and reading by vernier to 1’, is mounted on the telescope 
 near the eyepiece end. 
 
 A to in. diam. horizontal circle, divided on silver to 30’ and read by vernier 
 to 1’, is mounted on the tribrach base, which is provided with three levelling 
 footscrews. A clamp and slow motion screw controls the vertical axis motion, 
 and two adjustable stops are provided to enable the telescope to be rapidly 
 rotated in azimuth through 180°. 
 
 A striding level is mounted above the horizontal axis of the instrument. 
 
 Inv. 1921-513. 
 
 227. PORTABLE TRANSIT. Made by Brauer, Pulkowa Obser- 
 vatory. Lent by the Royal Observatory, Greenwich. Plate VI, 
 Fig. 2, p. 86. 
 
 A transit of this type was used about 1850 by F. G. Struve in his measurement 
 of an arc of the 52nd parallel from Haverfordwest to Eastern Russia. 
 
 The telescope has a 2-7 in. objective and is “‘ broken ’”’ where it meets the 
 hollow transit axis, a diagonal prism being here interposed to deflect the light into 
 the eyepiece tube at one end of this axis. The diaphragm of the Ramsden eye- 
 piece, which has a pair of movable wires actuated by a micrometer screw in addi- 
 tion to an adjustable reticule, is illuminated by a lamp mounted at the other end 
 of the transit axis. A vertical circle of 11 in. diam., divided on silver to 10’, is 
 fixed to the transit axis near the eyepiece, and is read by an index on the adjacent 
 standard. The steel pivots of the transit axis rest on high standards projecting 
 from the levelling platform, which is supported on the wooden tripod stand by 
 three levelling screws, having a point, line and plane contact respectively with the 
 bearing plates. Two small bubble-levels are used to set the base horizontal by 
 means of the footscrews, and the transit axis is finally adjusted horizontal by a 
 sensitive hanging level. 
 
go 
 
 The telescope can be lifted from its bearings on the standards and reversed end 
 for end by means of a rotating framework mounted inside the levelling platform 
 and carrying two extending forks which support the transit axis during the 
 rotation. Inv. 1914-304. S.M. 1941 & 1942. 
 
 228. PHOTOGRAPH OF TRANSIT INSTRUMENT (3-in.). 
 Lent by T. Cooke & Sons, Ltd. 
 
 Transit instruments of the type here illustrated are sufficiently portable tobe 
 used for time and longitude observations in geodetic surveys. Transits very simi- 
 lar to the original of this photograph have been supplied to the Colonies in recent 
 
 ears. 
 
 ¢ The telescope, of 3-in. aperture and about 36-in. focal length, has two setting 
 circles of 6in. diam., reading by verniers to 20”, mounted near its eyepiece end, 
 and is mounted upon a rigid horizontal axis fitted with a long clamp and slow- 
 motion screw. The axis rests in bearings upon a massive U-shaped frame which 
 permits the telescope to transit, and also carries a mechanism for lifting the 
 telescope axis from its pivots and reversing it through 180° in azimuth. The sup- 
 porting frame is adjustably mounted upon a base plate which can be fixed on a 
 masonry pier. ; 
 
 The diaphragm of the telescope is of glass and has seven lines ; it is illuminated 
 through the transit axis by means of an electric lamp at one end of the latter. 
 A sensitive striding level is mounted above the axis. Levels, tangent and slow 
 motion screws are fitted to both setting circles. Inv. 1921-514. 
 
 229. EQUAL ALTITUDE INSTRUMENT. Made by A. Jobin, 
 ParisPlate VIP Pir? a p60" | 
 
 This instrument, also known as a “‘ Prismatic Astrolabe,’’ was designed by 
 MM. Claude and Driencourt about the year 1goo, and is used for the astronomical 
 determination of latitude, local time and azimuth. It acts on the principle of 
 determining the exact moment at which a known star crosses a horizontal circle of 
 constant angle, namely 60°. 
 
 The instrument exhibited is the ‘‘ geodetic ’’ model of the firm, and has a 
 telescope of aperture 40mm. and magnification 75 diam. Latitudes can be 
 0+04 
 sin A 
 
 determined with it to 0:4” and time to sec., where / is the co-latitude of 
 
 the observing station. 
 
 The telescope can rotate in azimuth upon one end of a horizontal plate, which 
 carries a mercury artificial horizon at its other end and can itself rotate above the 
 graduated horizontal circle of the instrument. This circle is divided in degrees, 
 and enables the telescope to be set approximately in the desired azimuth for ob- 
 serving any particular star. 
 
 An equilateral glass prism of sides and height 45 mm. is mounted in front of 
 the objective lens of the telescope, with its base vertical and apex edge horizontal. 
 When the star is nearly at an altitude of 60°, two images are seen of it in the 
 telescope, one by light refracted through the prism and telescope, and the other 
 by light reflected from the mercury horizon before refraction in the prism. At the 
 precise moment that the star crosses the 60° horizontal circle, the two images 
 meet on the same horizontal line of the telescope graticule. 
 
 By a series of similar observations of the times of crossing of known stars, 
 both latitude and local time can be ascertained from tables. 
 
 (See ‘*‘ Handbook of the Prismatic Astrolabe,”’ by J. Ball and H. Knox-Shaw, 
 19IQ.) Inv. 1920-131. S.M. 1939 & I940. 
 
 MINE SURVEYING. 
 
 Underground surveys, made in the confined space of mine shafts 
 and galleries, are usually effected by the method of traversing rarely 
 employed above ground for precise work. The magnetic needle has 
 always been a favourite instrument for use in traversing, and we find 
 it adapted to mine-surveying in the earliest known treatize on mining, 
 Agricola’s ““ De Re Metallica,” published in 1556. This book shows 
 
PLATE VII. 
 
 Fig. 1. Equal Altitude Instrument, by Jobin, p. 9o. 
 
 Fig. 2. Lean’s Dial, by W. & S. Jones, p.gz. 
 
OI 
 
 a species of plane-table compass, the early German “ setz-compass,” 
 in which the compass box is mounted within a rotatable tablet provided 
 with several concentric rings of differently coloured wax and a rotating 
 ruler. The compass has no graduations beyond an index mark, above 
 which the needle was made to set by rotating the tablet, after which 
 the line joining the observing station to the next one was marked 
 upon one of the wax rings by means of the ruler. No sights were 
 employed, but the stations were connected by a stretched, knotted 
 cord, which served both to measure the intervening distance and to 
 indicate the direction in which the ruler must point. A mine compass 
 of this type, dated 1541, is preserved in the Clausthal School of Mines. 
 At this period the underground survey results were plotted upon the 
 surface, full scale, principally to see how near the tunnels had been 
 driven to the boundaries of the concession. 
 
 The magnetic compass continued in use for mine surveying, both 
 on the Continent and in England, until the middle of the rgth century, 
 and is still employed in minor work. In Germany, “ hanging ” 
 compasses and clinometers were used to measure the bearing and 
 inclination of the cord or chain stretched between successive stations 
 of the traverse. A compass of this type, suspended in a gimbal 
 mounting, was invented by Roessler and first described in Voigtel’s 
 “Geometria Subterranea,” 1686. An improved pattern, known as 
 the “ Freiburg hanging compass,’ was introduced by the Freiburg 
 instrument-maker, Studer, about 1810. In England the compass 
 was provided with a graduated circle and a vernier sight-rule during 
 the 18th century, at the end of which the telescopic type, fitted with 
 a vertical graduated arc, known as Lean’s Dial, was introduced (see 
 No. 230). Many surveyors, however, still used the plain Miner’s Dial 
 (see No. 231), with sight-rule, the compass being mounted upon a 
 swivel joint which allowed its plane to be set vertical, when inclinations 
 could be measured upon the compass lid, which was fitted as a plummet 
 clinometer. In 1850 John Hedley brought out the improved dial 
 known by his name ; this had the sight-rule fixed upon a rocker frame 
 pivoted to the compass box, so that steep sights could be taken without 
 disturbing the compass (see No. 232). A vertical arc could be attached 
 to the rocking frame and the position of an index fixed to the compass 
 could be read off on this arc to measure altitude angles. The Hedley 
 dial, provided with vernier arc and supplementary telescope to replace 
 the sight-rule, has always been popular in England. 
 
 Modern investigations of terrestrial magnetism have, however, 
 established the uncertainty of traverses depending upon the magnetic 
 needle, and the custom has arisen of employing a dial fitted with 
 outside verniers attached to the telescope, or using a special kind of 
 theodolite, so that the angles could be measured as in surface surveys. 
 For this type of traverse, the instrument has to be accurately centred 
 over the observing station point, so that modern mining theodolites 
 are usually provided with some type of shifting centre to permit this 
 adjustment to be made. In the best class of work, it is also customary 
 to have special targets, which can be mounted upon the same tribrach 
 stage as the theodolite so that target and theodolite can be inter- 
 changed as the survey progresses. Provision is made for illuminating 
 the target and theodolite cross wires. 
 
 In order to connect up the underground survey with the surface 
 network, it is necessary to fix either a single point or a single line of 
 one survey with respect to the other. For this purpose, special plumb- 
 
g2 
 
 lines, made of wire, may be used. A heavy plummet is attached to 
 this line and allowed to swing freely ; its oscillations may be damped 
 by vanes moving in water. The position of the line is measured above 
 ground by the theodolite in the normal manner and below ground by 
 observing the oscillations and deducing the mean position, for which 
 purpose the eyepiece of the telescope may be fitted with a special 
 scale, or such a scale may be fixed beside the swinging plumb-line and 
 observed through the telescope. Where it is necessary to fix a line, 
 not merely a point, it is usual to employ a transit theodolite to sweep 
 out this line. The ordinary mining theodolite can be adapted for this 
 purpose by providing it with a supplementary telescope mounted 
 above or at the side of the ordinary one, so that vertical sights can 
 be taken. Another method often used is to set out the underground 
 and surface lines by means of a delicate magnetic needle (see No. 237), 
 but great care is necessary if the readings of the needle above and 
 below ground are to be reliable. 
 
 230. .LEANS .DIAL.. Made by. W sci.) Oles on late Le 
 Fig. 2, p. 90. 
 
 In this instrument, made in the early part of the 19th century, the vertical 
 semicircle is fixed to the alidade with its curved edge upwards, and the telescope 
 rotates tangentially upon it. This arrangement permits nearly vertical sights to 
 be taken and renders the instrument suitable for mine surveying. The vertical 
 arc and telescope can easily be detached from the alidade for packing the instru- 
 ment. 
 
 The horizontal circle has a diameter of 4 in. and is graduated in degrees. The 
 vertical arc has a diameter of 34 in. and is graduated in degrees on one side. On 
 the other side is a scale of corresponding hypotenuse increments. Both circles 
 read by vernier to 5’, but have no slow-motion fittings. The alidade also carries a 
 compass box with a 2 in. ring, graduated in degrees. 
 
 The telescope has a focal length of 5 in., a clear aperture of 0-5 in., anda simple 
 inverting eyepiece with a cross-thread diaphragm. There is no focussing arrange- 
 ment. A spirit level is mounted in adjustable bearings beneath the telescope. 
 
 (See ‘““ Geometrical and Graphical Essays,’’ by G. Adams, 1813, p. 231.) 
 
 Inv. 1908--120. S.M. 2197. 
 
 231. MINER’S DIAL. Made by Henry Barrow & Co. 
 
 The plain mine surveyor’s dial is very similar to the old circumferentor and 
 has been much employed in England owing to its adaptability to surveying in 
 low galleries and suitability for traversing. The instrument exhibited is a mid- 
 1gth-century dial, with a 5-in. compass ring graduated in degrees and a vernier 
 fixed to the movable alidade reading to 3’. 
 
 The compass ring and base are mounted upon an axial socket perpendicular 
 to their planes and can be rotated about the central axis which fits in this socket 
 and terminates in a spherical ball. This latter is gripped by another socket and 
 can be clamped in any desired position so that the plane of the compass faces any 
 direction. A slit in the socket allows the compass to take up a vertical position 
 for measuring altitude angles. When measuring azimuth angles, the plane of the 
 compass is levelled by means of a pair of perpendicular tube spirit levels fixed 
 to the alidade. This is attached to the compass case, and carries combined back- 
 sights and foresights at each end. The alidade and compass case, with the internal 
 vernier scale can be rotated independently of the compass ring and angles measured 
 as in a theodolite, or the case can be clamped to the compass ring and the two rotated 
 together about the axis of the latter for taking magnetic bearings, as in the ordinary 
 compass. In the former case, the alidade is fitted with a tangent screw for slow- 
 motion adjustment. 
 
 The sights are 6 in. high and fixed at a distance of 1 ft. apart on the alidade. 
 Each sight has both a slit and a pin-hole backsight and a long single-wire and 
 a cross-wire foresight. The aperture.and cross-wire sights are for use in measuring 
 altitude angles. For this purpose the compass needle is locked by operating the 
 lifting lever, the plane of the compass is set vertical, and the lid is placed in position 
 on the box, and a plumb-line suspended from a stud in this lid. The opposite 
 edge of the lid is graduated in degrees up to 45° each way and has a scale of hypo- 
 tenusal increments engraved on it. Inv. 1907-66. 
 
93 
 232. HEDLEY DIAL. Made by B. J. Hall & Co., Ltd. 
 
 The chief feature of this mining dial, which is a modern example of the type 
 invented by John Hedley in 1850, is the use of a rocking circle, pivoted externally 
 on horizontal bearings to the compass box, to carry either plain sights or a tele- 
 scope. This arrangement permits a fairly steep vertical angle to be measured, 
 the angle being read by an index revolving with the rocking circle over a graduated 
 semicircle fixed to the compass box. The compass ring can rotate inside the box 
 about the vertical axis, for measuring azimuth angles independently of the needle, 
 or can be clamped to the box for taking magnetic bearings. 
 
 The compass ring has an outside diameter of 5 in. graduated in degrees for 
 reading with the vernier, which is fixed to the box above one pivot of the rocking 
 circle. The graduations extend to the inner edge of the ring, which has a diameter 
 of 44in. and is read by the needle. The base of the compass is also graduated 
 every 10° and the cardinal points are marked on it. It also carries two spirit 
 levels with their axes mutually perpendicular. 
 
 The vertical semicircle has a diameter of 5 in. and is graduated in degrees, 
 with an inner scale of hypotenusal increments. The rocking circle has projecting 
 lugs at right angles to its axis of rotation. The plain sights or standards for the 
 telescope can be fixed to these lugs. The plain sights are 6 in. high and have both 
 slits and wires for taking azimuth angles either way. In addition, one sight has 
 a pin-hole aperture and the other a pair of cross wires for measuring vertical angles. 
 The telescope has a rack-focussing objective, of focal length 9-5 in. and aperture 
 Iin., a simple cross-wire diaphragm, and a slide-focussing inverting eyepiece. 
 It carries a sensitive spirit level fixed to it by adjustable bearings. The compass 
 ting, which has a tangent clamp and slow-motion screw, reads by vernier to 3’. 
 The compass box and upper part can rotate about a vertical axis and can be fixed 
 by a tangent clamp. The socket is threaded to fit a pair of parallel plates for 
 levelling. Inv. 1922-43. 
 
 233. MINING THEODOLITE. Made by F. W. Breithaupt & 
 Son, Cassel. 
 
 Theodolites with eccentrically mounted telescopes and vertical circles, which 
 can be transited without requiring high standards projecting from the horizontal 
 vernier plate, have been very generally employed by Continental mine surveyors 
 owing to their suitability for work in low galleries. The example exhibited is of 
 this type, the transit axis being mounted just above the vernier plate and carry- 
 ing a telescope and vertical circle at one end, just clear of the horizontal circle. 
 Both circles are of 3-in. diam., graduated every 30’ and read to 1’ by opposing 
 verniers. A 3-in. compass, mounted upon a large circular spirit level, is fixed to 
 the upper part above the transit axis. 
 
 The horizontal circle and vernier plate have independent vertical axes of 
 rotation for repeating azimuth angles, with separate clamps and tangent screws. 
 The axial socket of the circle carries a tribrach fitted with levelling footscrews. 
 The feet of these rest in small footplates, as is commonly the case in German 
 instruments of this period—the latter half of the 19th century. The foot- 
 plates rested on the tripod head, which was connected to the instrument by a 
 central plunger screw. On loosening this, the theodolite could be accurately 
 centred without moving the tripod. 
 
 A stout tubular supporting piece projects centrally above the vernier plate 
 and carries the socket for the transit axis, and the ring for the compass and 
 spirit level. The vertical circle is fixed to the transit axis socket and the telescope 
 to an inner rotating vernier plate, which also carries a tangent clamp and slow- 
 motion screw fitting. A spirit level is mounted on the telescope, parallel to the 
 line of sight. The telescope has a fixed objective of #-in. aperture and a rack- 
 focussing eyepiece tube carrying a fixed Huygens ocular, with a pair of cross 
 wires fixed between the two lenses. A diagonal reflecting prism is screwed to the 
 eyepiece for convenience in sighting. The compass ring is graduated in half- 
 degrees, and a lifting lever is provided for clamping the needle against the cover, 
 the glass cover of the circular spirit level forms the base of the compass box. 
 
 Inv. 1876-1197. 
 
 234. RAND MODEL MINING THEODOLITE. Made and lent 
 by T. Cooke & Sons, Ltd., York. 
 
 The special features of a modern mining theodolite are well illustrated in this 
 example, of which the chief characteristics are the protection of all delicate 
 
uf 
 
 parts from the dust and dirt inseparable from mine surveying, and the provision 
 of means for taking sights at very short ranges and at extreme vertical 
 angles. 
 
 othe 4*5-in. horizontal and 4-in. vertical circles are graduated directly to 
 20’ and read by two opposing verniers and reading microscopes to 30”. Both 
 circles are encased in metal, the verniers being protected by hinged glass covers, 
 which can be raised for cleaning. The circles and the horizontal vernier plate 
 have independent motions, and are provided with clamping and slow-motion 
 screws, the two screws to each circle being distinguished by differently shaped 
 heads. All the verniers have ground glass reflectors. 
 
 The main telescope, of 6:5-in. focal length and 1-1-in. aperture, transits at 
 either end, and has an internal focussing lens actuated by a screw at one end of 
 the transit axis. This arrangement permits the telescope tube to be rendered 
 dustproof, and to be focussed down to 5 ft. An objective focussing auxiliary 
 telescope, of 5-in. focal length and o-9-in. aperture, can be fitted either on top or 
 at the side of the main telescope, for use in taking steep sights, and is balanced 
 by a counterpoise. Both telescopes and reading microscopes have screw- 
 focussing inverting eyepieces. An additional diagonal erecting eyepiece is 
 provided for use with either telescope. The diaphragm of the main telescope 
 is of the web type, with 1 : 100 horizontal stadia lines, and can be readily withdrawn 
 for repairs. An interchangeable glass diaphragm is also supplied. 
 
 The standards supporting the vertical circle and telescope axis are fixed to 
 the vernier plate, which carries a small circular spirit level and a long adjustable 
 tubular one, with its axis parallel to the transit axis. A similar tubular level is 
 mounted upon the telescope, and can be adjusted parallel to its optical axis. 
 The horizontal circle is attached to a tribrach, in the arms of which three foot- 
 screws engage. The screws are provided with dustproof covers, and their feet 
 are firmly fixed in a trivet stage, which can be screwed to the tripod head but has 
 also three pointed feet for fixing it upon a platform independently of the tripod. 
 The tripod head has a centring arrangement of 2}-in. range, permitting the 
 instrument to be centred over the station point without moving the tripod legs. 
 
 A delicate compass of special design is attached to one standard and serves 
 to give a magnetic zero line. Inv. 1921-558. 
 
 235. BRUNTON MINING TRANSIT. Made and lent by C. F. 
 Casella & Co., Ltd. 
 
 This so-called transit is essentially a combined compass and clinometer designed 
 for filling in the minor details of a survey, more particularly in mine surveying, 
 as the sights can be adjusted to steep slopes without displacing the compass box 
 from the horizontal. The position of the compass needle and the bubble of the 
 clinometer can be read at the same time as the sight is taken by means of the 
 mirror mounted inside the hinged lid of the compass box. 
 
 The instrument consists of a circular aluminium compass box, to which are 
 attached on opposite sides a hinged lid and a hinged slit and aperture backsight. 
 The inner face of the lid is a mirror and has a diametral line engraved upon it, 
 which bisects an aperture pierced in the lid near the hinge; this aperture serves 
 as foresight for viewing any object, though the reflection of the slit of the backsight 
 in the mirror, together with the line engraved thereon, serve to determine the 
 exact bisection of the object viewed. In taking sights, the lid is inclined at an 
 angle of 45° to the face of the compass, so that a reflected image of the compass 
 ring and clinometer is seen. 
 
 The compass has a stepped ring, about 24-in. diam., with its edge graduated 
 in degrees. An edge-bar needle is pivoted at the centre and can be lifted from 
 the pivot by means of a lever, actuated by pressing a button. The act of closing 
 the lid of the compass depresses this button and clamps the needle. The 
 clinometer arrangement is pivoted to the base of the compass below the needle 
 and consists of a rotatable vernier arm upon which is mounted a small bubble 
 level. The arm traverses a graduated arc engraved on the base of the box, and 
 the vernier reads this arc to 10’. The movement of the arc is operated from 
 outside the box by means of a small rotating arm having a milled button at its 
 extremity. Two flats are filed upon the side of the compass box, which is rested 
 on either of these when vertical angles are being measured by means of the 
 clinometer. 
 
 The aperture plate of the backsight bracket is separately hinged to this, so 
 that, when the bracket is adjusted to the angle convenient for taking a sight, 
 the aperture plate can remain vertical. Inv. 1924-465. 
 
95 
 
 236. MINER’S DIAL. Made and lent by Messrs. Cooke, Trough- 
 ton & Simms, Ltd. 
 
 The requirements of minor and routine mine surveying, particularly in 
 collieries, can often be met by a dial of this type, which has recently been designed 
 by the makers to replace their Rand Model Mining Theodolite (see No. 234), 
 where particular accuracy is not a necessity. A special feature of the dial 
 exhibited is the sighting arrangement which is claimed to afford the advantages 
 of open sights without the focussing difficulties of the latter. 
 
 The compass box, mounted upon the tripod head by means of a ball and 
 socket levelling arrangement, is designed for loose or fixed needle reading. It 
 has a 5-in. horizontal circle, graduated in degrees, numbered o—360° clockwise 
 and read by opposing verniers to 1’, and a needle circle also graduated in degrees, 
 numbered o-360° in the anti-clockwise sense. The readings of the two circles 
 thus correspond and may be checked by comparison. The horizontal circle is 
 provided with a clamp and slow-motion screw, whilst the compass box as a whole 
 can be clamped to the base. The needle is 4:25 in. long and is mounted freely 
 upon a steel pivot; it is provided with a lifting and clamping arrangement, 
 designed to prevent shocks. 
 
 Upon the rim of the compass box is pivoted a solid arm, carrying at its outer 
 end the vertical circle with the sighting tube fixed thereto. The circle can rotate 
 upon a horizontal axis in the arm, to which a vernier bracket is attached for 
 reading the circle to 6’. The circle is of 3-in. diam., and is protected by a metal 
 guard. 
 
 The sighting tube is provided with a simple optical system of unit magnifi- 
 cation, whereby the image of the target is brought into the same plane as the 
 reference mark, which is a point or cross wires engraved on a glass diaphragm. 
 The line of sight is above the centre of the compass box, and the supporting 
 arm can be adjusted to enable vertical sights to be taken. 
 
 (See ‘‘ The Colliery Guardian,” July 27th, 1923.) Inv. 1925-3. 
 
 237. MINE MAGNETOMETER. Made and lent by Messrs. Cooke, 
 Troughton & Simms, Ltd. 
 
 Many mining surveys, being based upon magnetic bearings, require an accurate 
 determination of the magnetic meridian to serve as a periodical check and to 
 connect them with the surface survey. The ordinary magnetic needle is not 
 sufficiently sensitive to respond to the daily changes in the magnetic meridian, 
 and the instrument exhibited was designed by Mr. T. Lindsay Galloway to 
 enable the meridian to be fixed with the required accuracy. 
 
 The instrument consists of a tribrach levelling stage, supporting a long 
 cylindrical case within which is suspended an arrangement consisting of a plane 
 mirror and a pendant connected by a fibre of unspun silk. Both mirror and 
 pendant are mounted upon stems to which the ends of the fibre are fixed, and a 
 small bar magnet is attached across the back of the mirror. Either the pendant 
 stem or the mirror stem, or both together, can be clamped to the case by means 
 of adjustable stirrup clamps, and the case can be inverted upon the tribrach 
 levelling stage so that either the pendant or the mirror is uppermost. 
 
 In using the instrument the case is first set up with the mirror above and the 
 pendant below, and the latter is then unclamped and allowed to turn freely until 
 it comes to rest, whenit is clampedupagain. By this means any twistis taken out 
 of the fibre. The case is next inverted so that the mirror is below the pendant, 
 and the mirror is unclamped, whereupon it oscillates between two stops. If 
 necessary, the case is rotated in azimuth to enable the mirror to come to rest 
 with its plane in the magnetic meridian. If now a theodolite is directed on the 
 mirror and adjusted until it centres the reflected image of a plummet suspended 
 below its own axis, the line of sight will be perpendicular to the meridian. 
 
 This type of magnetometer is now obsolete. 
 
 (See ‘‘ A Method of Determining the Magnetic Meridian,” by T. Lindsay 
 Galloway, Trans. Institute of Mining Surveyors, Vol. LVI, Part 4.) 
 
 Inv. 1925-8. 
 
 MARINE SURVEYING. 
 
 Information regarding the relative positions of various coastal 
 features, depth of water, and similar data of importance to navigators, 
 must have been sedulously collected and compared by early mariners, 
 and it is significant that the Alexandrian geographer, Ptolemy, relied 
 
96 
 
 largely upon the statements of his predecessor, Marinus of Tyre, for 
 the data which enabled him to construct his maps. Marinus had 
 himself constructed a map based upon his voyages. Four centuries 
 previously, Pytheas of Marseilles, a contemporary of Alexander the 
 Great, had made important observations of the shores of Gaul and 
 Britain. These early navigators relied largely for their estimates of 
 relative positions upon observations of the comparative lengths of 
 the day between sunrise and sunset and records of the number of days 
 required to sail between various points, but occasionally checked their 
 estimates by erecting sundials at the places visited by them and 
 determining the latitude by this means. It is significant of the early 
 development of marine surveying that in all early maps the Mediter- 
 ranean coast districts are comparatively accurately delineated. 
 
 Similarly, during the late Middle Ages and Renaissance periods, 
 the portolanos or nautical charts exhibited a marked superiority over 
 the existing land maps; this was probably due to the fact that 
 navigators were equipped with portable astrolabes, quadrants or cross- 
 staffs for estimating latitudes, and with magnetic compasses for 
 determining directions. The first arc measurement made in England, 
 by Richard Norwood in 1633, was more related to hydrographical 
 than to land surveying, for Norwood was the author of a work on 
 navigation, and his object was to ascertain the length of a degree for 
 the purpose of correcting the nautical charts then extant. 
 
 With the invention of the back-staff by John Davis about 1594 
 and Newton’s reflecting octant a century later, the marine surveyor 
 became better equipped with instruments for measuring celestial and 
 terrestrial angles. Finally, in 1731, Hadley published the account of 
 his new reflecting octant, which was a great improvement upon any 
 existing nautical observing instrument, and required few modifications 
 to transform it into the modern sextant. This instrument, provided 
 with tangent screw, telescope and vernier scale, enabled the navigator 
 to determine his latitude very accurately, and was equally well adapted 
 for coast surveying by triangulation. In 1733 Hadley added a spirit 
 level to his octant to define the horizontal when the natural horizon 
 was obscured by fog or by darkness, but the attempt to provide a 
 nautical sextant with an artificial horizon for use at sea has never 
 attained much success. 
 
 The main difficulty of determining the ship’s position astronomically 
 has always lain in the comparative inaccuracy with which longitudes 
 could be obtained. For long, mariners had to rely upon dead reckoning 
 for this purpose. Attempts were made during the 17th and 18th 
 centuries to utilize the variation of the magnetic declination and 
 observations of lunar phenomena or stellar occultations as means of 
 ascertaining the longitude, but the problem was not solved successfully 
 until John Harrison, in 1735 and the subsequent years, devised a 
 method of compensating the temperature variations of watches and 
 thus originated the chronometer, which enabled standard time to be 
 maintained throughout a voyage. Local time could always be 
 determined by means of the sextant. MHarrison’s invention con- 
 sisted in the application of a bi-metallic compensating device to 
 watches. 
 
 With the invention of the sextant and chronometer, marine 
 surveying was put upon the same accurate basis as land surveying 
 had been by the development of the theodolite, and the era of modern 
 
97 
 
 hydrography begun. In 1795 the Admiralty Hydrographic Depart- 
 ment was founded for coast and harbour surveying around Great 
 Britain, and similar activities were inaugurated on the Continent at 
 the same period. 
 
 For coast surveying on shore the sextant was often employed, but 
 modern hydrographic surveyors have favoured the use of the theodolite 
 for onshore work. For taking angles from the sea, the sextant remains 
 the most suitable instrument. Attempts have been made to increase 
 its angular range by means of suitably disposed reflectors, and to 
 substitute a complete graduated circle for the sextant, whilst the 
 superior rigidity of reflecting prisms has induced some makers to 
 substitute these for the ordinary mirrors. 
 
 In many respects, hydrographical surveying is merely a repetition 
 of land surveying, but it differs principally in the use of the sextant 
 owing to the usual instability of the observing station, and the more 
 frequent employment of the method of resection ; in this method the 
 position of the observing station is fixed by angular observations upon 
 three fixed points, and plotted by means of the station pointer. Also, 
 to contour the sea bottom, special sounding appliances are necessary, 
 
 and in this connection hydrography owes much to the genius of 
 Lord Kelvin. 
 
 238. SEXTANT. Made and lent by F. Newton & Co. 
 
 This is a sextant of the usual type employed in marine surveying. It has a 
 graduated arc of 8-in. radius, divided on silver to 10’ and read by vernier and 
 magnifier to 10”. The arc extends from 0° to 150°, each numbered degree being 
 really only half a degree. 
 
 The vernier moves with the index arm, which is pivoted at the centre of the 
 arc and carries the index mirror. A long telescope is mounted in the usual up and 
 down piece on one outer radius of the arc and faces the horizon mirror, mounted 
 on the other outer radius. Coloured glasses are provided for astronomical sights, 
 and a handgrip is mounted on the under side of the frame. Inv. 1865-6. 
 
 239. SEXTANT. Made by H. Haecke, Berlin. Presented by 
 the Admiralty. 
 
 This sextant is believed to have belonged to the German surveying ship 
 ‘*Moewe.” It is of the usual construction and has an arc of radius 20cm., 
 divided to 30’ and read by vernier to 1’. 
 
 The index arm, carrying the index mirror at its centre and the vernier at its 
 extremity, is provided with a clamp and tangent screw for slow motion. Coloured 
 glasses are fitted on pivots near both mirrors. A handgrip fixed beneath the 
 frame is used when taking vertical angles, and has a screw socket beneath it for 
 attaching a perpendicular handgrip used for taking horizontal angles. The 
 telescope has two erecting eyepieces, of magnification 3 and 4 diam. respectively. 
 A plain sight tube is also provided. The telescope supporting ring can be elevated 
 or depressed by a fine screw in the usual manner. Inv. 1920-84. 
 
 240. REFLECTING CIRCLE SEXTANT. Made by Pistor & 
 Martins, Berlin. Lent by the Admiralty. 
 
 This instrument, which was salved from a German warship sunk at Scapa, 
 has a complete graduated circle instead of a sextant, and a totally reflecting 
 right-angled prism instead of the usual horizon mirror; it is of the type first 
 made by Pistor and Martins in 1845. The sustitution of the prism for the horizon 
 mirror enables angles up to 180° to be taken and also gives a clearer image at 
 extreme angles, whilst the use of a complete circle and an index arm fitted with 
 two opposing verniers, minimizes errors due to inequalities of graduation or 
 inaccuracy in centring the index mirror and arm. 
 
 The circle has a diameter of 13 cm. and is graduated to 10’, corresponding 
 to 20’ of the angle measured. This angle is read by the vernier to 20”. Only part 
 of the graduations, namely, those under one vernier, are numbered and in these 
 
98 
 
 a gap is left where the telescope and head of the observer foul the line of sight 
 of the index mirror. This latter has its plane inclined at about 20° to the zeros 
 of the index arm to which it is attached. The horizon prism is mounted with 
 its reflecting face parallel to the index mirror when the verniers are at zero. It 
 is opposite the lower half of the telescope, which is mounted in the usual “ up 
 and down ”’ piece, the upper half of the field being left free from direct vision. 
 The index arm has the usual clamp and tangent screw at one extremity, and a 
 wooden handgrip for holding the instrument is attached beneath the circle. A 
 reading microscope is fixed to a rotating bracket for magnifying the scales, and 
 a set of smoked glasses for varying the illumination of the telescope field is 
 attached to a rotating pivot near the supporting ring. Inv. 1920-89. 
 
 241. STATION KEEPERS (Bell Type). Presented by the 
 Admiralty. 
 
 The station keeper is an instrument enabling an observer to keep himself 
 at a fixed distance from another object, e.g., an observer in a boat from his ship, 
 by keeping constant the angle which a known length on the distant object 
 subtends at his position. 
 
 In this type of station keeper, of which three examples are exhibited, a small 
 glass prism of very small apex angle is mounted in a holder. The observer 
 holds this at arm’s length and views the distant fixed length partly through the 
 prism and partly directly. If the direct sight of one extremity of the known 
 length coincides with the image of the other extremity after refraction through 
 the prism, his distance from the object must be invariable. The actual distance 
 will depend upon the fixed length and the apex angle of the prism. 
 
 Inv. IgI1I-172. 
 
 242. STATION KEEPER (Wharton Type). Presented by the 
 Admiralty. 
 
 This type of station keeper is essentially similar in construction to a box 
 sextant, having a graduated arc of 6-in. radius extending over 20°. By measuring 
 on this arc the angle subtended at his position by a fixed length on the distant 
 object, the observer can calculate his distance from that object, or, having 
 calculated the necessary angle, can set it on the instrument and keep his distance 
 constant, 
 
 The instrument consists of a sector-shaped box having its wider end open 
 and its narrower end closed, save for a pin-hole aperture for sighting. At the 
 open end, within the box, is mounted a fixed ‘‘ horizon ’’ mirror, whilst a movable 
 index mirror is pivoted near the closed end and geared to an external rotating index 
 arm, which moves over a graduated arc on the upper face of the box. The gearing 
 is done by rack and pinion gear, which causes the index arm to rotate through 
 twice the angle through which the index mirror moves. The arc is graduated in 
 half-degrees from 0° to 20° and the index arm carries a vernier reading to 5’. 
 
 In using the instrument the observer sights one end of the distant base‘directly 
 through the space below the horizon mirror, and the other end by double reflection 
 at the two mirrors, and moves his position or the index arm until the two ends 
 appear coincident. Inv. 1911-173. 
 
 243. STATION POINTER. Made by H. Haecke. Lent by the 
 Admiralty. 
 
 The arms of a station pointer are often fitted with sights which permit the 
 instrument to act as a theodolite and thus to measure the angles subtended by 
 three known points at the observing station. On detaching the arm piece from 
 the circle and applying it to the chart like an ordinary station pointer, the position 
 of the observing station can be plotted. The instrument exhibited is of this 
 combination type, and was salved from a German warship sunk at Scapa Flow. 
 
 It has a graduated circle of 11-in. diam., with its edge divided on German 
 silver to single degrees. This circular plate can rotate within a hollow central 
 axis which terminates below in a spherical ball; this ball forms part of a ball 
 and socket joint connecting the instrument to a stand. The plate can thus be 
 set in any plane and clamped by means of a threaded collar acting on the joint ; 
 it can then be rotated in this plane and clamped by a radial screw threaded in 
 its outer axis and impinging on its inner one. The arms of the instrument are 
 mounted on a separate circular ring which has a central pin fitting within a hole 
 in the centre of the plate. Each arm is pivoted round this pin and can be turned 
 
99 
 
 about it and clamped to the edge of the circular ring when the arm is directed 
 towards the corresponding fixed point. The central pin acts as backsight for 
 the observation, whilst each arm has a folding slit foresight fixed at its outer 
 end. The subtended angles can then be read off on the graduated circle if 
 required and the arm piece removed and applied to the chart. The bevelled 
 edges of the arms are then made to pass through the corresponding fixed points 
 on the chart, when the central pin marks the position of the observing point. 
 This pin is mounted loosely in the arm piece and its point kept slightly above 
 the plane of the arms by a spring. On depressing the head of the pin, its point 
 makes an indentation in the chart and thus plots the position of the observing 
 station. 
 
 The arms are 14 in. long from central pin to sights, but can be extended 
 to 20 in. by means of the three extension pieces shown. ‘Two of the bevelled 
 edges can be brought to coincide, but the third cannot be set less than 14° from 
 the other two. Inv. 1920-22. 
 
 244. NORDENFELT SEXTANT. Presented by C. F. Casella & 
 Co., Ltd. 
 
 This modified form of sextant was designed by T. Nordenfelt about 1884 
 for rangefinding at sea. Observations are taken with the plane of the sextant 
 vertical from a point on board ship at a known height above sea level upon 
 the water line of a distant object and simultaneously upon the horizon. When 
 the horizon and water line images are made to coincide by revolving the index 
 arm, the reading of the index on the graduated arc gives the range of the distant 
 object directly. 
 
 The index mirror is not mounted upon the pivot of the rotating arm, but 
 upon an eccentric pivot, connected to the arm by a cam and pin mechanism, 
 which causes the mirror to rotate through a much smaller angle than the arm ; 
 this enables the angle swept out by the reflected ray to be varied very gradually 
 between narrow limits, whilst the index traverses an arc of 60° or thereabouts. 
 As the angle of depression of the distant object below the horizon varies inversely 
 as the range, if the base—7.e. the height of the observer above sea level—is 
 constant, the arc of the sextant can be graduated to read the range directly. 
 Different arcs are, however, necessary for different observing heights, and three 
 are provided, two for heights of 96 ft. and the third for a height of 47 ft.; any 
 arc can be fixed in the sextant frame by means of clamping pins. 
 
 The effect of the dip of the horizon is allowed for on the graduated arcs. 
 The rotating arm is provided with one fixed index mark and another index 
 which can be displaced relative to the fixed one by means of a tangent screw ; 
 the arm has a separate clamp and tangent screw. Two telescopes, of different 
 fields and magnifications, are provided. 
 
 (See British Patent Specification No. 16029 of 1884.) Inv. 1923-405. 
 
 PHOTOGRAPHIC SURVEYING. 
 
 Although perspective sketches have been employed for many 
 centuries, and their theory studied by various geometricians such as 
 Lambert, it was not until the end of the 18th century that an attempt 
 was made to reproduce a horizontal plan from a series of such sketches. 
 At this period the French hydrographer Beautemps-Beaupré con- 
 structed a series of coast charts based on oriented perspective drawings, 
 but his method found few disciples. In the middle of the 19th century 
 the recent invention of the Daguerre photographic process led A. Lausse- 
 dat to study the application of photography to surveying purposes. 
 He developed methods and instruments for this purpose, and, under 
 his direction, a few test surveys of small areas were made. Laussedat’s 
 methods only created a temporary interest in France, but were 
 developed by Italian,German and Austrian engineers during the last four 
 decades of the century. Various types of surveying cameras, or photo- 
 theodolites, were devised by Laussedat in France, Paganini in Italy, 
 
I00O 
 
 Meydenbauer, Finsterwalder and Koppe in Germany, Pollack and 
 von Hubl in Austria, and Deville in Canada, and were employed in 
 numerous branches of topographical surveying, but principally for 
 surveys of mountainous districts. Paganini and Deville also designed 
 plotting devices to assist in reproducing the plans from the photographs. 
 At the beginning of the present century successful attempts were made 
 to apply stereoscopic methods to photo-surveying. Deville in Canada 
 made experiments with this aim in view, but the efforts of Seliger and 
 Pulfrich in Germany, which resulted in the production by the firm of 
 Zeiss in 1901 of the first Pulfrich stereo-comparator and a precision- 
 phototheodolite designed for stereo-surveying (see Nos. 247 and 248), 
 created a new impetus in photographic methods of survey. The 
 stereoscopic examination of photographs greatly facilitated the recog- 
 nition of prominent features upon a pair of prints, which had hitherto 
 been one of the chief obstacles in the method, and efforts were next 
 directed towards a mechanical process of cartographic reproduction. 
 F. V. Thompson, working at the School of Military Engineering, 
 Chatham, evolved in 1907 a stereo-plotter (see No. 249), which, in 
 conjunction with the Pulfrich stereo-comparator, reduced the process 
 of measuring the photographs and plotting the detail to one of a 
 purely mechanical nature, but the stereo-autograph invented by an 
 Austrian military engineer, von Orel, in 1908 was produced by Zeiss 
 and is now considered the best instrument for mechanically plotting 
 from stereograms. By means of this apparatus, not only can isolated 
 points be rapidly plotted or measured, but contour lines or feature 
 lines, such as rivers, hedges, etc., can be directly plotted by simply 
 tracing over them in the stereo-photograph. An improved model of the 
 Zeiss-von Orel stereo-autograph was produced in 1911 (see No. 250). 
 
 In ordinary ground photo-surveying, or photogrammetry, it is 
 usual to employ vertical photographic plates only, but much labour 
 has been expended in investigating the laws of inclined perspectives, 
 which in certain situations have obvious advantages. Photography 
 from the air is naturally more suited to the employment of horizontal 
 or inclined plates than vertical ones. The first attempt to take photo- 
 graphs from the air was made by Nadar at Paris in 1858, using a 
 captive balloon, and he suggested that such photographs could usefully 
 be employed for making plans. The collodion plates were, however, 
 very unsuitable for aerial photography, which languished until 1885, 
 when Tissandier and Ducom, using gelatine plates, obtained excellent 
 horizontal photographs of the environs of Paris and constructed a 
 plan from these. The first comprehensive employment of aerial 
 photographic surveying, however, was undertaken by the Japanese 
 Army in the Manchurian campaign of 1904, and the development of 
 the aeroplane and airship in the succeeding years has led to an increasing 
 employment of this method of surveying, especially during war time 
 and over mountainous districts. During the Great War, 1914-1918, 
 much reconnaissance work was accomplished by aeroplane photography, 
 both over country which had been well surveyed trigonometrically, 
 and in which, therefore, many “control’’ points existed, and also 
 over territory, such as Palestine, where few trigonometrical control 
 points existed. In the latter type of survey, steadiness of flying is an 
 essential for aerial mapping, for the data as to orientation and inclina- 
 tion of the photographic plate, provided by instrumental means, are 
 often unreliable. Where a previous trigonometrical network exists 
 for the country surveyed by aerial methods, it is usually possible to 
 
IOI 
 
 deduce the inclination and orientation of the camera axis, and all 
 other necessary data, from the actual negatives by the laws of inclined 
 perspective. It is generally advantageous to “rectify ”’ the plates, 7.e. 
 to produce the perspective which would have been obtained if the axis 
 of the camera had been truly vertical ; to perform this function various 
 auxiliary appliances have been devised. These are mostly in the 
 nature of special enlarging cameras having facilities for twisting the 
 negative into the position relative to a horizontal plane which it had 
 during exposure. The instrument devised by Captain Cole, of the 
 Egyptian Survey Department, is of this type. On the Continent 
 attention has been concentrated on the microscopical measurement 
 of aerial photographs by the so-called “ Bildmess theodolites ”’ of 
 Hugershoff and Pulfrich, which are designed for computing the data 
 for photographs taken in an inclined position. The principles of 
 stereogrammetry and autocartography have also been applied to 
 aerial surveying by these two scientists. 
 
 245. PHOTO-THEODOLITE. 
 
 This instrument was designed for photographic surveying by Prof. E. J. 
 Mills, F.R.S., about the year 1897. It consists of an accurately adjustable 
 camera of nearly constant focus, fixed to the vernier plate of a 6-in. theodolite 
 which gives the azimuth angle of the optical axis of the camera to 30”, and 
 is provided with an ordinary parallel plate four levelling screw mounting for 
 attachment to the tripod. The photographic plates are 5 in. by 4 in. The 
 camera box, which is made of teak, has a pair of perpendicular spirit levels and 
 a 24-in. compass box attached to its top. The front consists of a brass plate 
 mounted in horizontal and vertical slides, and adjusted in both directions by 
 means of fine screws. It carries a centrally mounted Zeiss extreme angle (about 
 130°) anastigmat lens of 141-mm. focal length. The vertical and horizontal 
 position of the lens is measurable by scales and verniers to 0-04 mm. The 
 back of the camera can slide in the box and is moved by a twin rack and pinion 
 arrangement, actuated by thumb wheels at each side. The focal length of the 
 camera for any position of the back is read by a scale and vernier to 0:04 mm. 
 The ground glass is graduated in centimetre squares, the central vertical and 
 horizontal lines having millimetre graduations. Four triangular brass teeth 
 are fixed in the centres of the edges of the back, and press against the front 
 face of the plate during exposure. Their images on the plate after exposure 
 serve to determine the horizon line and principal vertical line of the picture. 
 The slides, twelve in number, are “ single,’’ and must be changed after each 
 exposure. The base of the camera box has guides and screw threads for fixing 
 it to the vernier plate of the theodolite, so that the principal point of the lens 
 is vertically above the centre of the horizontal circle. This latter is graduated 
 in half-degrees and read by opposing verniers to 30”. Clamps and tangent 
 screws for fine motion are fitted to both vernier plate and horizontal circle. 
 The instrument is levelled by means of the spirit level on top of the camera box, 
 and the four footscrews. Inv. 1917-113. 
 
 246. PHOTO-THEODOLITE. Made by R. Lechner, Vienna. 
 Lent by the Survey of Egypt. 
 
 In this photo-theodolite, which is of the type designed by V. Pollack for 
 engineering surveys in mountainous districts of Austria, the camera is centrally 
 mounted above the horizontal circle and carries the telescope and vertical circle 
 at one side, these being counterbalanced by a weight carried on the other side. 
 To reduce the weight, the camera box, telescope tube and support for the latter 
 are made of aluminium. 
 
 The camera objective is a Zeiss anastigmat lens f/18 of focal length 212 cm., 
 mounted in a movable plate which can be elevated or depressed to the extent 
 of 5 cm. by means of a rack and pinion arrangement; the amount of elevation 
 or depression is measured by a vernier and scale at the side of the plate. At the 
 back of the camera box is a rectangular brass frame, 21 cm. by 15 cm., the inner 
 edges of which are notched every centimetre. The photographic plate is pressed 
 against this frame and the graduations are photographed round its edges; these 
 serve to locate the principal and horizon lines on the picture and to discover 
 
102 
 
 any subsequent distortion due to development. The ground glass screen which 
 normally fits in the back of the camera has a telescopic eyepiece mounted at 
 its centre; this eyepiece, which is pivoted on a horizontal axis, forms a central 
 telescope with the camera lens as objective. 
 
 The horizontal circle of the instrument is divided to 30’ and reads by verniers 
 to 1’; it is fitted with a clamp and tangent screw and carries two perpendicular 
 bubble levels. 
 
 The vertical circle and telescope are mounted upon a curved bracket carried 
 by the horizontal circle. The circle is divided to 30’ and read by verniers to 1’. 
 The telescope has a rack-focussing eyepiece tube, fitted with stadia lines. 
 
 Inv. 1924-103. 
 
 247. PHOTOGRAPHS OF A _ ZEISS-PULFRICH PHOTO- 
 THEODOLITE. Lent by Messrs. Carl Zeiss, Ltd. 
 
 Two views are here shown of a photo-theodolite designed by Dr. C. Pulfrich 
 specially for use in stereo-photographic surveys. Above the camera is mounted 
 a telescope which can sweep out a field of view perpendicular to the camera axis, 
 so that, when the other station point of the base is sighted, the optical axis of 
 the camera must be perpendicular to the base. 
 
 The camera box is of light metal and is mounted upon the rotatable limb 
 of a graduated horizontal circle, which can in turn rotate within a levelling 
 tribrach. The footscrews of the latter rest on a trivet plate, which can be 
 clamped upon the tripod head after the vertical axis of the instrument has been 
 centred above the station point. The camera objective is mounted in a sliding 
 front plate and has a constant focal length of 127 mm. The plate rack takes 
 12 cm. by 9 cm. plates, upon which the horizon and principal vertical lines are 
 automatically recorded during exposure. 
 
 The camera box can be detached from the tribrach by unclamping the screw 
 which fixes the vertical axis of rotation. A similar tribrach, trivet plate and 
 tripod is set up at the other base observing point and the camera can readily 
 be transferred from one station to the other. In the tribrach which does not 
 contain the camera, a signal pin can be clamped, centred above the station point, 
 and observed from the camera station through the special telescope mounted 
 above the camera box. This telescope has its optical axis parallel to the camera 
 axis, but has a pentagonal prism mounted in front of its objective for deflecting 
 the axial rays at right angles to the optical axis. The telescope fits within a 
 sleeve rigidly fixed to the camera box, and can rotate within this sleeve about 
 an axis coinciding with its optical axis. During this rotation it carries the 
 deflecting prism with it, and also a vertical circle for measuring the amount of 
 rotation. Any point viewed through the prism and telescope during this rotation 
 must lie in the vertical plane perpendicular to the telescope axis, and therefore 
 perpendicular to the optical axis of the camera. 
 
 To measure the length of the base, the telescope is sighted upon a horizontal 
 rod of fixed length, which is mounted upon the tripod at the other base point, 
 and the instrument rotated in azimuth by means of the tangent screw of the 
 horizontal limb, which has a graduated screw-head for measuring the rotation 
 necessary to traverse from one end of the distant rod to the other. By this 
 means the base is measured tacheometrically. Inv. 1914-456, S.M. gio. 
 
 248. PHOTOGRAPH OF ZEISS-PULFRICH’S STEREO-COM- 
 PARATOR. Lent by Messrs. Carl Zeiss, Ltd. 
 
 The stereo-comparator is a special type of stereoscope, designed by Dr. C. 
 Pulfrich about 1901, for measuring the relative displacement of corresponding 
 points in the two views of an area surveyed from each end of a base, and deducing 
 therefrom the position of the corresponding ground point with respect to the base. 
 The position of each ground point is separately measured in terms of its azimuth, 
 altitude and range (perpendicular to the base) from the left-hand base terminal 
 
 oint. 
 
 i The comparator consists of a binocular telescope, very similar to a stereoscopic 
 rangefinder, mounted above a traversing frame which carries the two negatives. 
 Each eyepiece of the telescope carries in its focal plane a fixed index mark. 
 When the negatives are viewed through the twin eyepieces, the observer sees 
 a combined image of the index mark apparently suspended in space above the 
 country surveyed, which is presented in bold relief. By operating the mechanism 
 controlling the movements of the negative and telescope carriages the mark can 
 be made to “‘ touch ’”’ any point of the landscape, and the readings of the scales 
 then give the position of that point. 
 
103 
 
 ~ 
 
 The two negatives can be traversed together in a lateral direction parallel 
 to their horizon lines by means of a rack operated by the wheel shown on the left 
 of the instrument and known as the “ direction ”’ wheel, or the right-hand plate 
 can be moved alone in the same direction by means of the wheel on the right-hand 
 side of the instrument. The latter is known as the “ parallax ”’ or “‘ distance ”’ 
 wheel. Each movement is registered by scales and verniers, whilst the ‘‘ distance ” 
 wheel has a micrometer drum head for recording the smaller movements. The 
 telescope can also be moved across the negative carriage in a direction parallel to 
 the principal vertical lines of the views by means of the “‘ height ”’ wheel, shown 
 in front of the middle part of the instrument, and the amount of motion measured 
 on the “ height ”’ scale. 
 
 The instrument is originally adjusted so that the mark appears to be at an 
 infinite distance from the observer, in a direction coinciding with that of the 
 principal point of the negatives, 7.e. perpendicular to the base of the survey. 
 When the mark is made to coincide with any point of the landscape, the readings 
 of the direction, distance and height scales give the azimuth, range and altitude 
 of,that point with respect to the left base point. All the scales read too-or mm. 
 
 a@The instrument is designed to take negatives of dimensions 12 cm. by 9cm., 
 such as are used in the photo-theodolite, by Zeiss, illustrated in an adjacent 
 photograph. 
 
 (See “‘ La stéréoautogrammétrie,” by P. Corbin in ‘‘ Revue générale des 
 Sciences,” “Vol! 25, 1914, p. 1231.) Inv. 1914-456. S.M. gog. 
 
 249. PHOTOGRAPHS OF THOMPSON’S STEREO-PLOTTER. 
 Presented by W. Watson & Sons, Ltd. 
 
 Early in 1907, Lieut. F. V. Thompson, R.E., at the School of Military Engineer- 
 ing designed the instrument illustrated in these two photographs for the purpose 
 of automatically plotting the positions of points obtained by a stereo-photographic 
 survey. The apparatus consists of a Pulfrich stereo-comparator, with a pair of 
 plotting boards traversed by rulers connected to its measuring mechanism in such 
 a manner that the co-ordinates of the point viewed in the comparator can be 
 automatically plotted on the boards. 
 
 In the Pulfrich comparator the three co-ordinates of any point surveyed, 
 namely its direction, altitude and range, are measured by the motions of the 
 main slide, the microscope bridge slide and the parallax slide respectively. The 
 two plotting boards of the stereo-plotter, which may be called the plan and ele- 
 vation boards respectively, are provided with one common “ ordinate ’’ ruler and 
 separate ‘“‘ abscissa ’’’ rulers. The two latter are geared respectively to the main 
 slide and microscope siide mechanism of the comparator, and are moved automatic- 
 ally over their boards by the operation of setting the comparator on the view 
 point. The ordinate ruler has to be set by hand at a distance from the plan 
 point of the camera station equal to the “‘ range ”’ of the view point as recorded 
 on the range drum of the comparator, which drum is operated by the mechanism 
 of setting the parallax slide. After this setting of the ordinate ruler, its point of 
 intersection with the abscissa ruler of the plan board gives the plan point of the 
 view point, whilst its intersection with the abscissa ruler of the elevation board 
 gives a reading upon a scale fixed to the ordinate ruler which registers the elevation 
 of the view point above the camera station. 
 
 (See “‘ Geographical Journal,’ Vol. XXXI, 1908, pp. 534-551.) 
 
 Inv. 1914-297. 
 
 250. PHOTOGRAPH OF ZEISS—VON OREL’S STEREO- 
 AUTOGRAPH (1911 Model). Lent by Messrs. Carl Zeiss, Ltd. 
 
 In 1908, Lieut. E. von Orel, of the Military Geographical Institute, designed 
 the first model of his stereo-autograph, by means of which the points of a stereo- 
 photographic survey could be viewed in a Pulfrich stereo-comparator and auto- 
 matically plotted, one by one, by the movements used to set the comparator. In 
 this early model, which resembled Thompson’s stereo-plotter, the altitudes of 
 the various points had to be separately calculated, but in 1909 Messrs. Zeiss, in 
 conjunction with von Orel, constructed a new model in which the contours could 
 also be automatically plotted upon the plan. In 1911 the instrument represented 
 in the photograph exhibited was evolved. It performs all the functions of the 
 1909 model and in addition permits the trace of any line upon the photographs, 
 z.é., upon the ground surveyed, to be plotted directly upon the plan ; thus contours, 
 lines of rivers, roads, fences, etc., can all be automatically plotted by simply 
 causing the index mark of the comparator to traverse such lines. Moreover, 
 
104 
 
 with this model, the optical axis of the camera used in obtaining the two stereo- 
 photographs need not be parallel or at right angles to the base of the survey, 
 but must be horizontal. 
 
 The instrument consists of a stereo-comparator mounted upon the front of a 
 massive base frame, and a plotting board fixed to the rear of the frame near its 
 right-hand corner. A complicated framework composed of various rotating and 
 traversing arms is connected to the comparator mechanism and carries the 
 marking pencil, which automatically plots the plan, contours, etc., as the index 
 mark of the comparator is set upon any point or line of the landscape viewed 
 therein. The movements of the framework are controlled by the usual ‘“‘ direc- 
 tion,’ ‘‘ distance ’’ and “ height ’’ wheels, duplicated in the latter case by a foot 
 pedal, so that all the necessary movements can be performed simultaneously. 
 No scales to register the movements are necessary, as the points and lines are auto- 
 matically plotted. 
 
 The whole instrument is mounted upon a table, and two operators are usually 
 required to manipulate it. One operator controls the mechanism and the other 
 sees that the pencil is recording properly and inserts names, figures, conven tional 
 signs, etc., on the plan. 
 
 The plates used are 18 cm. by 13 cm. The accuracy of surveying and plotting 
 with this apparatus compares very favourably with that of the more usual methods, 
 such as plane-tabling, and the speed is much greater. The instrument is more 
 particularly adapted to large scale surveys, from 1: 1,000 to I : 20,000. 
 
 (See “La stéréoautogrammétrie,”’ by P. Corbin, in “‘ Revue générale des 
 sciences... Vol. 25,1914, p.' 240. 
 
 ‘‘Stereographic Survey,” by A. R. Hinks, F.R.S., in the ‘‘ Geographical 
 Journal,” Vol. LIX, April, 1922, pp. 273-284.) 
 
 Inv. 1914-456. SIM OEL. 
 
105 
 
 LIST OF DONORS AND CONTRIBUTORS. 
 
 Page 
 
 Admiralty, Lords Commissioners 
 ofthe .. 15, 29, 31, 46, 47, 57, 62, 
 72, 13, 76, 78,, 80, 81, 97, 98 
 Barr & Stroud, Messrs., Ltd. AOR SD Fis 
 
 British Antarctic (Terra Nova) 
 Expedition, 1910-13 a 21 
 
 Casella, C. F. & Co., Ltd. 44502; 
 59,/63,777, 94,:99 
 Chesterman, J. & Co. . 22, 23 
 
 Colonial Office .. wa 16 
 Cooke, T. & Sons, Ltd., York . 31, 40, 
 45, 49, 79, 83, 89, 90, 93 
 
 Cooke, Troughton & Simms, Ltd. 35, 43, 
 48, 95 
 
 Court, T. H., Esq. 21, 29, 30, 38, 39, 
 42, 47, 49, 56, 57, 60, 63, 64, 
 
 65, 68-71, 74 
 
 Dray, bie esd? i - oy 11 
 Forbes, Prof. G., F.R.S gs 32 
 French Government 9,13 
 Geodetic Survey of Canada : vi iE 
 140k) 
 
 Lockyer, Prof. Norman, C.B., 
 Teekoeyaiae a 88 
 Lyons, Capt. H. G., me R. S, Pipl is Reta 
 MacNamara, T., Esq. .. Lie 20 
 Newton, F. & Co. . 61, 68, 80, 97 
 
 Ogilvie, F. G., Esq., C.B. “ie 20 
 
 Page 
 Ordnance Survey 9, 12, 13, 15, 24, 41, 
 42, 49, 71, 83-86, 88 
 
 Physical Department, Ministry 
 of Public Works, Egypt ae 85 
 
 Pulkowa Observatory .. hid 24 
 Roberts, Evan, Esq. 19, 20, 68 
 Royal Observatory, Greenwich 89 
 Royal Society . 24, 40, 48, 69 
 School of Military Sa tnbee tt 
 
 Chatham Ay 16 
 Shackleton, W., Esq., F. R. A. S. 60 
 stanley. WE, ESq.yy).'. 51 
 
 Stannard, A., Esq., A.M.I.C. E. 41 
 
 Survey of Egypt . 11, 14, 25,101 
 Survey of India : ue 14 
 Symons, G. J., Esq., ERS. a OU, os 
 Troughton & Simms, Ltd. "3 51 
 U.S. Coast & Geodetic Survey .. 10, 13, 
 26027 
 
 Waterhouse, Major G.G., R.E. 14, 27 
 Watson, W. & Sons, Ltd. 103 
 Watts, E. R. & Sons, Ltd. . 44, 45, 
 46, 50, 78, 79 
 
 War Office a oS 82 
 Worswick, Rev. T. P. Ww. ae 52 
 Zeiss, Carl, Ltd. 102, 103 
 
INDEX 
 
 Page Page 
 Abney Level Al fe 47 | Clinometers . 39, 40, 46-48, 61, 63 
 Acetylene Signal Lamp an 16 | Colby’s Compensation Bars . 18, 24 
 Adams, G. : 21,69 | Compass-Clinometer, Burnier’s 63 
 Adie Rangefinder | ; 28, 31 | Compass-Clinometer, Casella’s. . 63 
 Aerial Position Finder .. as 8i | Compasses, Surveying .. -- 99-64 
 Airy’s Zenith Sector .. vi 88 | Compensation Bars, Colby’s .. 18, 24 
 Alidades, Plane Table .. .. 57-59 , Compensation-Level, Breithaupt’s 43 
 Anallatic Lens .. 34, 36, 78 | Compensation Microscopes, 
 Ancient Egyptian Levels De 38 Colby’s i .. 18, 25 
 Aneroid Barometers 38, 51 | Cooke’s Reversible Level eb 43 
 Astrolabe 53, 87, 96 | Cross Staff Hy ive of 56 
 
 Astrolabe, Prismatic by 90 
 Axis, Tiumination Oraaecure: 76, 84-86 
 . 89 
 
 Axis Level ate aN ee 48 
 
 Balance Level 
 Bands, Measuring Ae a 22 
 
 Barometers, Aneroid SS, Os 
 Barometers, Levelling .. 37, 50-52 
 Barr and Stroud Rangefinder .. 28, 31 
 
 32 
 Base, Hounslow Heath 8, 19, 24 
 Base, Lough Foyle 25, 84, 85 
 Base, Romney Marsh .. as 24 
 Base, Salisbury Plain .. A 25 
 Base Measuring Apparatus, 
 
 Eimbeck Duplex # b8; i126 
 Base Measuring Apparatus, 
 
 Jaderin . 18, 19, 25-27 
 Base Measuring Apparatus, 
 
 Struve’s . ! 24 
 Beacons, Triangu! ation... .. 14 
 Blakey Stadiometer .. ok 29 
 Boning Telescope at as 24 
 Borda ‘ ae 18, 54 
 Boscovitch ne Ay Ais 18 
 Box Sextant ae 80 
 
 Breithaupt’s Compensation Level 43 
 British Antarctic Expedition, 
 
 1901 Oe 76 
 British Antarctic Expeditio: on, 
 
 1910-13 pal Here 
 Broken Telescope 31, 82, 89 
 Briinner .. ae 18 
 Brunton Mining Transit “ 94 
 Bubble Level, Invention of aie 37 
 Bubble Tubes Ay im 49 
 Burel Reflecting Level . 39, 40 
 Burnier’s Compass- Clinometer. ; 63 
 Burt Solar Attachment ee aa, 
 Cary 72 
 Casella’s “ Pocket Altazimathe ss 63 
 Chains, Measuring ve .. 22, 24 
 Chorobates aye 1 ste 36 
 Chronograph .. Bis 87 
 Chronometer, Invention of ae 96 
 Circumferentors : 64-68 
 “ Cleps ’’ Tacheometer . . os 34 
 
 De Lisle’s Reflecting Clinometer 40 
 
 Dialsj;-Miner’s' +)... .. 91-95 
 Diaphragm, Tacheometric .. 33-35 
 37, 55 
 Digges, Thos. .. Vis vi 53 
 Dioptra, Hero’s . 28, 36, 53 
 Dip of the Horizon 80, 99 
 Dip Sector wa oi 80 
 Distance Meter, Stuart’s 0 29 
 Dollond oF 51, 56, 60 
 Dray Traverse Mark .. a 11 
 Dumpy Level .. is 44 
 Duplex Base Apparatus, Eim- 
 beck’s 18, 26 
 
 Early Egyptian Land Measurers 22 
 
 Early Theodolites 64-68 
 Eccentric Telescope as 75, 78, 93 
 Eckbold’s Omnimeter .. 35 
 
 Eimbeck Duplex Base Apparatus 18, 26 
 Electric Signal Lamp .. 17 
 Enclosed Circles on Theodolites 71, 74, 
 
 76-80, 94 
 Equal Altitude Instrument... 90 
 Everest Theodolites Bh ad Lee 
 Field Clinometer at ee 47 
 Forbes’s Stereoscopic Range- 
 finder .. ; * rs 4 
 Gascoigne 33, 53 
 CeatISS Cov ke, SW 8, 19, 24 
 Gautier Telemeter 28, 30 
 Gentilli’s Tacheometer .. As 34 
 Gradienter Scale ti, do 
 Graphometers .. 65-67 
 Cron eet ae 53..0D800 
 Gunter 5.Chain Ne ae 44 
 Hedley Dial o5 ms ee 93 
 Heliographs leulon so 
 | Heliotropes : LOG 
 Hero of Alexandria “19, 25, 30,008 
 Hooke .. = 34 
 Hounslow Heath Base 8, 19, 24 
 Huygens .. é 37, 39, 69, 75, 93 
 Hypsometers he sah Oreo 
 
Page 
 Indian Clinometer 48 
 Internal Focussing Lens 44, 46, 55, 78 
 79, 80, 94 
 Invar 19, 26, 27, 45, 46, 50 
 Ipodimeter oe Ae 65 
 Jaderin Base Apparatus 18, 19, 25-27 
 Jahns’s Polymeter wa 59 
 Jeffcott Direct- REA RERS Tacheo- 
 mecer)o. F 35 
 Johnson Plane Tablet. ee 58 
 Jones, Thos. dy a . 4) 2-2, 80 
 Jones, Wierd, Oe. ri Hie Oo? OZ 
 Kater’s Mirror Compass ws 60 
 Labbez Telemeter . 28, 30 
 Lamps, Signal 1G}? 
 
 Latitude, Determination of 77, 87-90, 
 
 96 
 Lean’s Dial 3f ¥) is 92 
 Level, Axis ve Aye “pr 48 
 Levelling Barometers 37, 50-52 
 Levelling Staves 37, 49, 50 
 Levelsu es ae .. 36-48 
 Levels; Abney... ae re 47 
 Levels, Precise «.. os are 46 
 Lesuerre Heliograph .. an 16 
 Liquid Prismatic Compass a 62 
 Longitude, Determination of .. 87, 90, 
 96 
 Lough Foyle Base 25, 84, 85 
 Magnetic Needle, first pag ia to 
 Mine Surveying ae 90 
 Magnetometer, Mine .. ae 95 
 Marine Sextants.. po pete 
 Marks, Bench .. ce .. 12-14 
 Marks, Traverse . aut Oe LA: 
 Marks, Triangulation Ms Geet | 
 Measuring Bands a oe 22 
 Measuring Chains ; ny) 22, a4 
 Measuring Rods a .. 23-26 
 Measuring Tapes a .. 23-27 
 Measuring Wedge A Mf 23 
 Mekometer, G. S. 4; Ric ae te dete a! 
 Microscopes, Reading .. 82-86 
 Mine Magnetometer .. HY 95 
 Mine Theodolites si a 93 
 Miner’s Dials... : .. 91-95 
 Mining Transit, Brunton A 94 
 Mirror Compass, Kater’s ne 60 
 Nordenfelt Sextant me Se ap 
 Omnimeter, Eckhold’s .. be 35 
 Optical Opposites ee, 7 
 Optical Squares .. . 56, 57 
 Paget Angle Sextant .. es 81 
 Pedometers Ld s By4| 
 
 Perambulator .. i a 21 
 
 Page 
 Periscope Theodolite .. 82 
 Photo-Theodolites 101, 102 
 Picard, J. 17, 18, az 54 
 Plane Table Alidades . 57-59 
 Plane Tables a ont LOS 
 Pocket-Altazimuth, Casella’ Sul ay 63 
 Polymeter, Jahns’s i ng 59 
 Porro é a 34 
 Potter’s Prismatic Alidade oe! 59 
 IPAetorius {i 7h is ce 53 
 Precise Levels ay Us a 46 
 Prismatic Astrolabe .. Ai 90 
 Prismatic Compasses .. 60-62 
 Ptolemy 86, 87, 95 
 Oassaba Measuring Rod ay 23 
 Quadrant, Surveying .. 66 
 Quick- Levelling Socket, Johnson 58 
 Quick-Levelling Tripod Head . 83 
 Ramsden, J. 24, 54, 70, 83, 84, 87, 88 
 Ramsden’s Steel Chain : 24 
 Ramsden’s 3-ft. Theodolite . 83, 84 
 Rangefinders | .. 28-33 
 Reading Microscopes .. 82-86 
 Reflecting Clinometer, De Lisle’s 40 
 Reflecting Level, Burel.. 39, 40 
 Reflecting Sextant ag me 97 
 Reversible Tube-Level .. Le 40, 74 
 Reversible Level, Cooke’s = 43 
 Rods, Measuring ie 23-26 
 Romney Marsh Base .. Ay 24 
 Rope Stretchers ie ey) pao 
 Roy, General 19, 24 
 Salisbury Plain Base .. 25 
 Schmalcalder’s Prismatic Com- 
 pep Shy ae ‘ 60-62 
 Self-Centring Tribrach . By! 1] 
 Sextant, Angle .. a a 81 
 bole) qa We eed ol >. ann tae a we 80 
 Sextants, Marine ath tls. 99 
 Shifting Centre .. 77, 78, 80, 82, 91, 94 
 Shifting Centre Tripod Heady ts. 83 
 Signal Lamps ~. 16-17 
 sisson, J... ae .. 04, 68 
 Sledge Meters ii. a x 21 
 Snell, W. a 17 
 Solar Attachment, Burt Me 77 
 Stadiometer, Blakey .. NK 29 
 Station Keepers se iy 98 
 Station Pointer Aga w 98 
 Staves, Levelling ae 37, 49, 50 
 Staves, Tacheometric 36 
 Stereo-Autograph, Zeiss-Von Orel 103 
 Stereo-Comparator, Zeiss-Pulfrich 102 
 Stereo-Plotter, Thompson 103 
 Stereoscopic Rangefinders 28, ee 
 Struve, W. : bi 24, 87, 89 
 Stuart’s Distance Meter ‘ 29 
 
Page 
 Tacheometers : Me 33-36 
 Tacheometric Staff AE 5% 36 
 Tape Stretching ap eT B, 27 
 Tapes, Measuring ‘ e. 23/27 
 Telemeters i 28, 30, 31 
 Telescope, first applied to Sur- 
 veying Instruments .. 54 
 Telescope, Boning qs oe 24 
 Telescope, Zenith SR dys 89 
 Telescopic Alidade 58, 61 
 Thales of Miletus 28, 36 
 Theodolites, Altazimuth 68-80 
 Theodolites, Early He .. 64-68 
 Theodolites, Everest Jig oO 
 Theodolites, Geodetic .. .. 83-86 
 Theodolites, Micrometer 79, 83 
 Theodolites, Mine 93 
 Theodolites, Photographic 101, 102 
 Theodolites, Transit . 72-80, 84— 86 
 Thévenot, M. Od a 
 Transit Instruments .. 89, 90 
 Transit Theodolites 72-80, 84-86 
 Traverse Marks .. 9-11 
 Triangulation Marks 9-11 
 
 Page 
 Triangulation Beacons .. x 14 
 Tribach, Self-centring .. 11 
 Troughton, E. 70, 87, 88 
 Vernier, P. a 3 .2 54 
 Water-level ae ig 37 
 Watkin Aneroid Barometer .. 51 
 Watkin Clinometer aie am 48 
 Waywiser ins <s 21 
 Wright, Thos. Why 68 
 Y-Levels ats = ee 45 
 Zeiss Level ; : We 44 
 Zeiss Photo- Theodolite | 102 
 Zeiss Rangefinder ae % 28 
 Zeiss Stereo-Autograph 103 
 Zeiss Stereo-Comparator 102 
 Zenith Sectors .. oe ah 88 
 Zenith Telescope te ae 89 
 
109 
 
 LIST OF MUSEUM INVENTORY NUMBERS of objects with 
 Corresponding Catalogue Serial Numbers. 
 
 Inventory | Catalogue} Inventory | Catalogue} Inventory | Catalogue 
 No. No. | No. No. No. No. 
 1857-25 91 1899-52 144 1913-258 88 
 1865-5 209 1900-90 83 1913-560 65 
 1865-6 238 1900-153 183 1913-571 184 
 1865-7 156 1900-154 96 1913-572 52 
 1872-78 53 1900-155 95 1914-103 222 
 1872-79 155 1900-156 61 1914-107 a 
 1872-80 186 1900-157 62 1914-108 5 
 1875-25 48 1900-167 120 1914-109 28 
 1876-1192 196 1905-155 152 1914-110 29 
 1876-1193 195 1906—40 50 1914-111 51 
 1876-1194 197 1907-65 128 1914-149 23 
 1876-1195 198 1907-66 231 1914-150 24 
 1876-1196 102 1907-67 130 1914-151 20 
 1876-1197 233 1908-118 176 1914-152 1S 
 1876-1198 103 1908-119 177 1914-153 21 
 1876—1200 146 1908-120 230 1914-154 3 
 1876-1201 64 1908-122 168 1914-155 22 
 1876—1202 223 1909-137 189 1914-179 79 
 1876-1203 217 1909-138 185 1914-180 78 
 1876-1204 219 1909-139 77 1914-219 86 
 1876-1205 82 1909-140 150 1914-220 114 
 1876—1206 190 1910-125 181 1914—238 40 
 1876—1207 208 1910-126 75 1914—257 58 
 1876-1208 141 1911-151 74 1914-258 59 
 1876-1209 71 1911-152 139 1914—259 60 
 1876-1211 63 1911-154 | 140 1914-293 76 
 1880-195 171 1911-155 38 1914-297 249 
 1880-198 148 1911-157 157 1914-304 227 
 1880-199 173 1911-158 193 1914-305 35 
 1880—200 170 1911-159 192 1914-376 36 
 1880-201 90 1911-160 191 1914-388 66 
 1880-202 167 1911-161 199 1914—389 67 
 1883-14 225 1911-172 241 1914-390 68 
 1883-118 89 1911-173 | 242 1914-422 123 
 1887-62 116 1911-174A 112 1914-423 25 
 1889-21 142 1911-174B 113 1914-424 26 
 1889-22 136 1911-180 153 1914-425 37 
 1890-55 81 1911-182 44 ( 247 
 1890-55 85 1911-183 92 1914-456 , 248 
 1891-24 224 1911-220 | 210 | 250 
 1892-9 172 1912-197 | 98 1914-553 ii 
 1892-21 97 1912-198 | 99 1914-554 r 214 
 1893-119 129 1912-199 100 1914-555 J 
 1893-133 127 1912-200 45 1914-561 174 
 1893-134 126 1912-243 149 1914—735 131 
 1893-147 200 1912-244 57 1914-741 201 
 1893-148 162 1912-245 69 1914-742 | 145 
 1893-149 132 1912-308 | 178 1914-829 | 39 
 1894-106 169 1913-254 115 1914-845 54 
 1895-48 221 1913-256 101 1914-846 55 
 1895-49 188 1913-257 175 1914-847 56 
 
IIo 
 
 LIST OF MUSEUM INVENTORY NUMBERS of objects with 
 Corresponding Catalogue Serial Numbers—continued. 
 
 Inventory 
 
 No. 
 
 1914-894 
 1914-895 
 1915-399 
 1916-6 
 1916-64 
 1916-77 
 1916-81 
 1916-82 
 1916-83 
 1917-22 
 1917-38 
 1917-39 
 1917-77 
 1917-78 
 1917-104 
 1917-105 
 1917-106 
 1917-107 
 1917-113 
 1918-123 
 1918-124 
 1918-126 
 1918-240 
 1919-331 
 1919-415 
 1919-416 
 1919-495 
 1920-22 
 1920-66 
 1920-76 
 1920-77 
 1920-78 
 1920-81 
 
 Catalogue 
 No. 
 
 Printed by H.M.S.O. Press, Harrow. 
 
 Inventory 
 No. 
 
 1920-82 
 
 1920-84 
 
 1920-88 
 
 1920-89 
 
 1920-131 
 1920-384 
 1920-385 
 1920-386 
 1920-387 
 1920-388 
 1920-389 
 1920-442 
 1920-583 
 1920-627 
 1920-628 
 1920-629 
 1920-630 
 1920-655 
 1921-81 
 
 1921-306 
 1921-360 
 1921-479 
 1921-513 
 1921-514 
 1921-519 
 1921-556 
 1921-558 
 1921-559 
 1921-560 
 1921-561 
 1921-563 
 1921-588 
 1921-732 
 
 Catalogue 
 No. 
 
 Inventory 
 No. 
 
 1921-733 
 1922-41 
 1922-42 
 1922-43 
 1922-44 
 1922-205 
 1922-206 
 1922-440 
 1922-441 
 1922-442 
 1922-443 
 1922-444 
 1922-445 
 1922-446 
 1922-467 
 1922-723 
 1922-724 
 1922-725 
 1922-726 
 1923-10 
 1923-395 
 1923-405 
 1923-414 
 1923-492 
 1923-493 
 1924-103 
 1924-465 
 1924-466 
 1925-3 
 1925-4 
 1925-5 
 1925-7 
 1925-8 
 
 Catalogue 
 No. 
 
 105 
 158 
 
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