THEODOLITE. 



for diftinft vifion, and for power. The fcrew of tlie ver- 

 tical micrometers has feventy-two threads in tJie inch, and 

 the notches that indicate the fifteen minutes on tlie micro- 

 metrical fcale of fteel, are formed by this fcrew, which we 

 mention particularly, becaufe this mode of reading was pro- 

 bably an original mode, though now become commou- 

 Thefe vertical microfcopes, G and H, have each a ftage, 

 reprefentcd by Plate IX. /^. 4 ; and a dot made on a thin 

 flip of gold, C3\\tA 2i gold tongue, lies under the objeft-lens in 

 fuch a fituation, that the capflan fcrews can adjull it to a 

 given place in the field of view of each microfcope, fo as to 

 become a mark for making the adjuftments by, and for 

 bifeding the circle at reverfed oppoiite readings. The 

 pofition of the three glaffes of each microfcope is feen 

 in Jig. 3. of the fame plate, together with the magnified 

 appearance of the notched fcale and divifions of the 

 circle. In Jig. 5. the general plan of the micrometer is 

 exhibited, and in Jig. 6. is the plan of the ftage where the 

 pillars enter, that fupport the microfcope. Fig. 7. (hews 

 the lower or fteel Aide, and fg. 8. the upper or 

 brafs Aide, that feparate the wires of the micrometer ; 

 while^^. 9. is a reprefentation of the long horizontal micro- 

 fcope that reads the divifions of the femi-circle above 

 noticed, at the letter I in the large figure of Plate VIII. 

 As it was not eafy to defcribe the conftruAion of the three 

 feet, two of which, we have faid, only are vifible, oppofite 

 to D and E, we have added ^^j. 3. and 5. in Plate VIII. 

 to illuftrate their pofition. In Jg. 3. the piece F F, as 

 before, is a portion of the ftand, and D E a portion of the 

 oftagonal board, to which the fcrew F is attached, that 

 prefles againft the angulai- corner of the ftand, into which 

 a piece of brafs is let for the fcrew to prefs againft, an end 

 fettion of which is fhewn in_^^. j, with the fame letters of 

 reference to the fame parts : M N is a feclion of the bafe 

 of the inftrument, and O one of the thi-ee brafs branches 

 that bears the foot-fcrew P, and its two fide fcrews or tight- 

 ening fcrews Q, R, as feen in Jig. 5. At S is a curved piece 

 of box, faft to M N, which bears the principal part of the 

 weight laid on this fcrew, and does not gall the parts 

 on which it Aides, when a circular motion is given ; but, to 

 take off a part of the weight, a cylindrical roller near S is 

 put to a horizontal fpring bearing the central pin of the rol- 

 ler, which fpring prefTes the roller even with the face of the 

 curved block of box, and may be made to take more or lefs 

 of the weight by a fcrew preffing upon it from above, and 

 giving it more or lefs tenfion. Hence all the parts of this large 

 inftrument are ftrong, and yet the moving parts are made to 

 go freely and fmoothly ; and the only alteration that can appa- 

 rently be made for the better, is the addition of a third vertical 

 microfcope, for which the conftm Aion is peculiarly adapted ; 

 for each branch of the triple bar that carries the three feet, 

 being braced firmly, is made as though each was intended 

 to have a microfcope over it, which is the cafe with only 

 one ; and an additional foundation for the fecond ftage is 

 made to receive the fecond microfcope, as we think, unne- 

 ceffarily ; for if three equidiftant microfcopes had been 

 ufed inftead of two oppofite ones, not only would the 

 errors of divlfion and of eccentricity have been leflened 

 thereby, but in the reverfed pofition new parts of the circle 

 would have been pointed to, equidiftant from the former 

 three, fo that fix portions of the circle would thus have 

 been employed in meafuring a reverfed obfervation, which 

 the prefent aftronomer royal firft pointed out to be an ad- 

 VMtage pecuhar to three readings on a horizontal circle. 

 We have been informed, that it is yet intended to have thi-ee 

 microfcopes attached to the horizontal circle of this large 

 inftrument, to be ufed as we have here fuggefted. For the 



advantage of a triple reading, fee our table for tlu-ee veniiers 

 in the article ClRCj-E. 



After having given a defcription of Ramfden's great 

 theodolite, we proceed, in the next place, to explain how 

 its different adjuftments are made, for putting it into a ftate 

 proper for ufe, which we cannot do better, tlian by adopt- 

 ing the dircftions laid down by general Roy himfelf. 



The AdjuJlmenl oj the Axis Level. — The axis of the upper 

 or tranfit telefcope, being brought over any one of the feet, 

 and the circle being clamped, hang the axis level on the 

 pivots, or anfx of the telefcope, and bring the bubble to 

 the two indices ; then reverfe the level, that is, turn it end 

 for end, and note the difference. BifeA this difference, 

 one half by the level's adjufting fcrew, and the other half 

 by that foot-fcrew only which is in a line with the axis. 

 This operation being repeated until the difference wholly 

 vanifties, the level will be truly adjufted, that is to fay, 

 the bubble will reft between the fame points in both 

 pofitions. 



Adjujlment oJ the Eleniation Level. — This level being fuf- 

 pended on the rod attached to the outfide of the tranfit 

 telefcope, fcrew the ereft eye-tube on, to make that end 

 preponderate. Adjuft the bubble to the indices by the 

 fteel finger-fcrew at the tail of the femi-circle's clamp, 

 reverfe the level, and note the difference. Then bifect 

 that difference, and correft one half by the finger- 

 fcrew, and the other half by the proper adjufting fcrew 

 under the level, and fo on, repeatedly, until the difference 

 wholly vanifties. The level may then be hung on the two 

 pins that projeft from the horizontal bar which carries the 

 telefcope, where, being parallel to the axis level, it will 

 ftiew, when that is removed (as is commonly the cafe when 

 terreftrial objefts only are obferved) whether the plane of 

 the inftrument fuffers any alteration. If this ftiould have 

 happened, the level on the horizontal bar is at all times 

 fufficient to correft it. 



To fit the vertical Axis perpendicular. — This may be done 

 by either level, but beft with the axis level, which, being 

 fufpended on its pivots, muft be brought parallel with two 

 of the feet of the inftrument ; and by the fcrews of thefe 

 two feet, the bubble is to be brought between its indices. 

 The circle being then turned round 180°, if the bubble 

 changes its place, half the difference is to be correfted by 

 one of the feet -fcrews, and the other half by two capftan- 

 headed fcrews, that aft againft each other, under and be- 

 longing to one of the Y's, or fupports, in which the pivots 

 reft. When the bubble is found to be juft in thefe two 

 pofitions, turn the circle 90°, which will neceffarily bring 

 the axis over the third foot of this inftrument : then correft: 

 any error there may be by that foot-fcrew. In this manner 

 the circle will be made to revolve again and again, without 

 any alteration whatever of the bubble, which ftiews that 

 the vertical axis is then truly perpendicular to the horizon. 



To make the Line oJ Collimation in the Telefcope at right angles 

 nuith the tranfverje Axis. — The pivots refting in their Y's, 

 direft the telefcope to fome diftant well-defined objeft, and 

 let the circle be clamped. Then reverfe the axis, that is, 

 turn the telefcope upfide down. If the intcrfeftion of the 

 wires does not coincide with the objeft in both pofitions, 

 half the difference muft be correfted by the motion of the 

 circle with the Hooke's joint, and the other half by the 

 motion of the brafs Aide in the eye-end of the telefcope, by 

 applying the milled-head key in the fmall focket feen in 

 the figure ; and fo repeatedly until the difference whoUy 

 difappears. 



Tofet the Rod on tuhieh the Elevation Level hangs parallel to 

 the Line of Collimation. — The vertical axis being fuppofed to 



be 



