THEODOLITE. 



be nearly vertical, hang the level on its rod, and reftify the 

 bubble by the finger-fcrcw of the clamp. Set the horizon- 

 tal wire on the fteel Aide, to interfedl the centre of the 

 oblique wires, and place the dart or index at zero on the 

 micrometer head. Then obfcrve fome dittant diftinft ob- 

 ieft covered by the horizontal wire. Invert tin- fcmi- 

 circle, that is, turn the azimuth circle i8o°, and the tcle- 

 fcope upfide down, fo as to bring the wire upon, or nearly 

 upon, the fame objeft. Now, if the level be not right, 

 reftify it by the finger-fcrew at the tail of the clamp. If 

 the telefcope does not now accurately cover the fame objeft, 

 as in tlie former pofition, bifeft the difference by the finger- 

 fcrew of the clamp, and then reftify the bubble by the 

 capftan-nuts under one end of the rod. Repeat this opera- 

 tion until the level is right, when the telefcope fees the fame 

 objcft in both pofitions, and thereby tlie rod will be 

 brought parallel in altitude to the line of coUimation, or 

 «xis of vifion. 



We have defcribed other and more recent conftruftions 

 of large circular inftruments, imder the word Circle, that 

 have all the properties of the theodolite which we have here 

 defcribed, and fome of which have the advantage of a large 

 vertical circle, that renders their ufe in aftronomy co-exten- 

 five with their application to geodetical operations, and 

 which tlierefore we recommend in preference to the bulky 

 inftrument with which the EngUfh trigonometrical furveys 

 were performed. 



A theodoUte of a portable fize, and of Troughton's bcft 

 conftruftion, is exhibited in Plate Y!^. fig. I. of Surveying, 

 fuch as is adapted for land-furveying, or for tlie furveying 

 of harbours. A, B, C, are the three mahogany legs of a 

 tripod, furmounted with brafs joints which allow the legs 

 to form one entire cylinder, about four feet long. The 

 brafs-work above the three joints has a male fcrew, upon 

 which a focket, under the brafs circular plate D E, fcrews, 

 and bears the inftrument, whicli is alraoft entirely of brafs. 

 This plate D E has four fockets made faft into it, projcft- 

 ing both above and below, as feen in the figure, in which 

 are exhibited three out of the four, with as many fcrews 

 with milled heads, that afcend and defcend as they are 

 turned round, forwards or backwards, by means of their 

 connexion with their refpeftive fockets, that have each a 

 female fcrew : the heads of thofe fcrews prefs againft the 

 inferior face of the upper circular brafs plate F G, to 

 vrhich a ball, ending with a vertical axis, is attached, and is 

 embraced by the upper portion of the focket of plate D E, 

 within the four fcrews. The intention of the ball and 

 focket, and of the two plates with the four intervening 

 fcrews, is to place the axis of the ball in a vertical pofi- 

 tion, and to keep it in that pofition while the parts above 

 are employed in making an obfervation ; which office this 

 mechanifm will perform on Hoping as well on level ground. 

 The axis of the ball, however, is made hollow, to admit of a 

 fmaller folid axis within it, and has alfo two fockets or 

 tubes furrounding it ; all which have feparate motions, when 

 the ball is made faft by the prelTure of the four fcrews. 

 The inner tube is attached to the graduated circle L M, of 

 eight inches diameter, the chamfered edge of which circle 

 is folid filver, that receives the dividing ftrokes read by 

 the microfcope K : and the clamping piece, feen feparately 

 mjig. 2, will fix the faid tube and graduated plate in any given 

 fituation by means of the fcrew F, after which the fcrew G 

 with the milled head, attached to the plate F G, will pro- 

 duce the flow motion when neceflary. The lower telefcope 

 H I turns on two pivots not feen, one of which pivots has 

 Its. hole in a cock, borne by a fmaller circular plate under 

 the graduated plate, into which the two fcrews H and I 

 Vol. XXXV. 



enter, and the other pivot enters the butermoft tube; fo 

 that an elevation or depreffion bf about 20° can be ef- 

 fefted by this telefcope, before its motion is limited by 

 the fupcrior and inferiof plates. This telefcope has 1 

 proper motion in azimuth, independently of the graduated 

 plate, which motion is produced by the thumbjfci-ew H, 

 the axis of which has a pinion afting with a concealed 

 wlieel made faft to the graduated plate ; but when the 

 clamping-fcrcw I is maHo faft, then the telefcope and gra- 

 duated plate have but one common motion, Tvhich is com- 

 manded by the tangcnt-fcrew G of the clamping-piece F G. 

 The ufe of the feparate motion of this telefcope will be 

 explained prcfently. Above the graduated plate L M, and 

 in clofe contaft with it, the veniier-platc revolves with the 

 folid or innermoft axis, that is faft to it, and its clamp and 

 fcrew of flow motion are hid from tlie fight by the fuper- 

 incumbent frame. This plate, which has four oppofite ver- 

 niers, each reading to the accuracy of 15", will move fepa- 

 rately, or may be clamped to the graduated plate, as occa- 

 fion may require. Upon the plane of this vernier-plate, two 

 fpirit-levels arc placed with their proper adjufting-fcrews, 

 one of which is feen at N ; but the other, which ftands at 

 right angles to this, is concealed in the drawing by the 

 frame-work. A compafs and magnetic needle are alfo con- 

 cealed within the frame, but may be conceived to be con- 

 centric with the vernier-plate witiiin the faid frame. The 

 tail-piece of the revolving microfcope K fits into a circular 

 groove under the graduated plate L M, and, without hav- 

 ing a centre of motion, wiU Aide along the groove into 

 any of the four pofitions, where the verniers require to be 

 read, without interfering with any of the other motions. 

 The inftrument now before us has been fome years in ufe, 

 and is drawn on an enlarged fcale, that all the parts may be 

 the better defcribed ; but the moft recent theodolites of this 

 conftruftion have only three verniers, in preference to four, 

 by reafon of the property, which this number has, of mea- 

 furing ztjix different and equidiftant points on the gra- 

 duated limb, when the meafurement of an angle is repeated 

 in a reverfcd pofition of the fuperior telefcope ; fij that 

 whatever errors of excentricity or of graduation may exift 

 in the horizontal circle, they will be made to vanifti, in a 

 great meafure, by their counteraftion in the reverfed pofition 

 of the verniers. Upon the plane of the vernier-plate is 

 fcrewed, by three fcrews, forming an ifofceles triangle, the 

 frame which fupports the pivots of the horizontal axis 

 of the femi-circle P O, on which the upper telefcope T U 

 is placed. The arm, which beai-s the microfcope O for 

 reading the altitude or depreflion meafured by the femi- 

 circle, has a tube that Aides upon the projefting horizontal 

 axis, that allows of fome degrees of motion between the 

 end-bars of the frame ; and another arm, that clamps the 

 oppofite end of the faid axis, has a tangent-fcrew of flow 

 motion at R, which finifties the final contaft of the inter- 

 fefting point of the fpider's lines, within the eye-end of the 

 telefcope, with the objeft viewed. The vernier for the 

 femi-circle is fcrewed to the frame, after fpanning over the 

 compafs-box ; and its exaft place may be adjufted by the 

 fcrew of the frame above M, which ftands at the apex of 

 the ifofceles triangle formed by the three fcrews. The 

 level, that is feen under and parallel to the upper telefcope, 

 is attached to it by two pairs of fcrews, one pair of wliich 

 adjufts for the elevation or deprcffion of one end of the tube 

 that holds the bubble, and the other pair adjufts laterally 

 for true parallelifm in this refpeft : when the pins T and U 

 are removed, the upper femi-circle of each ring, V and W, 

 will turn back each on a hinge, and allow the telefcope to be 

 taken out of its Y's, for the purpofe of being reverfed in 

 3 O po-ltion; 



