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strumui':, but in common hands its want of solidity and 

 firmness xould probably have been felt. It would be 

 impossible as well as useless to give an account oi' the 

 various constructions of different artists at home and 

 abroad. The general properties of a theodolite, that it 

 should be firm, well balanced, &c., will be easily recog- 

 nised by a -person who knows how to make good use oi' 

 the instrument, and we shall advert in the course of this 

 article to some of the qualities which are, and to others 

 which are not, essential. 



\\~r have given here a sketch of the theodolite in its 

 Miupii'^ liirm. such as would be proper for the secondary 



.^illation of a national survey, or for the nu 

 private .-uney. The tripod which cariies the instrument 

 .siththree foot -screws in brass notches let into the 

 top of a v :d. The legs ct tin- stand are not fully 



d, but the tuo parts of which each is com 

 end below in a strung and sharply-pointed metal 

 The circle is iixcd, and the upper works, telescope, vei- 

 -. turn on a centre, which may I 

 M of the telescope. The adjustments arc 

 \eiy simple. The wooden stand is first set down wl 



. ami the top nearly horizontal. 

 The i - are placed in their notches, the ] : 



ok, below the centre of the circle, and 

 the Ich-M-opc turned round till one le\el is parallel to the 

 line joining two foot-screws, while the other level is in a 

 the third toot-screw to the centre. Briny; the 

 bubble d! the fust-mentioned level into the midd 

 raising one of the two foot-screw.-, and depressing the other, 

 :iml then adjust the cross-level by raising or depressing the 

 alone. Now turn the telescope round 

 180, and if the bubbles are not in the middle, bring tlvm 

 half way there by touching the foot I the other 



hall' by screws which adjust the levels themselves. \Vhen 

 this has been nicely done, the bubbles will remain in the 

 middle in cu iv position of the telescope. If the object.; 

 to be observed lay all in the horizon, or in a plane parallel 

 to it, the above adjustment would be sufficient ; but when 

 the i out of the horizontal plane they in 



perpendicular, that is, the plane de- 



: -ope must be a great, circle, and must, 



also pi>.-s through ihe zenith. There are generally two 



ni the focus ol the telescope, one horizontal 



and the other vertical. Place the eye-piece to give sharp 



visio ,; es, and turn the milled screw, seen towards 



. until the objects you are going to ob- 



ai-e distinct. Place the vertical wire on any well- 

 defined object, making the bisection near the crossing 

 of the wire ; raise or depress the telescope until the 

 object is nearly at the bottom or top of the field ; if it is 

 still bisected, 'the wire is rightly placed, but if not, twist, 

 the tube carrying the eye-piece so as to effect a bisection. 

 To make the telescope describe a great circle, select some 

 well-defined object near the horizon, and bisect it: now 

 take the telescope very e.trefulty out of its Y's, reverse it, 

 and look a&p.in at the object. If it is still bisected, there is 

 no error ; but if not, the bisection is to be effected half by 

 the tangent-screw of the instrument and half by the screw* 

 which carry the wire-plate, screwing up one and releasing 

 the other. Restoring the telescope to its first position, it 

 will be seen whether the adjustment is correct, and if not, 

 the process must be repeated until the bisection is the. 



in both positions of the telescope, the clamp and 



it-screw remaining fixed. For the adjustment of the 

 axis of the telescope a level would be convenient, but in 

 this instrument the axis is supposed to have been correctly 

 placed by the maker, and the only mode of correcting any 

 is by filing tire Y's. It may be ascertained whether 

 the axis is tolerably correct as follows : Bisect an object 

 as far above or below the horizon as the motion of the 



ope will allow. Reverse the telescope, and if the 

 object is still bisected, the pivots of the telescope are the 

 same size : if not, the observer must deduce the difference 

 of the pivots from the altitude and the error observed, 



is not difficult. When this has been satisfactorily 

 . as in the last instance, an object as far as 



'e from the horizon, and read off the verniers. Turn 

 the instrument round 180, return the telescope end for 

 i.isect the object again, and read off the verniers. If 

 the mean readings differ exactly 180, the axis is hori- 

 zontal ; but, if they do not, the observer will have sufficient 

 data from this, and the altitude or depression, for deter- 

 mining the quantity and direction of the error, which he 

 may correct, by the file or by calculation, according to his 

 pleasure. There is a much easier method of examining 

 the position of the axis by observing an object directly 

 and as seen by reflexion from a fluid, as mercury, oil, or 

 water. The axis is truly horizontal when the vertical wire 

 bisects the object and its reflected image without moving 

 the tangent-screw. It must be recollected that the ad- 

 justments of the horizontal circle already described must be 

 previously and very scrupulously performed before attempt- 

 ing the examination or adjustment of the cross-axis. 



the objects in a survey are at very different dis- 

 tances, an adjustment is required for forming the image 

 exactly on the wires. The use of the milled screw, seen 



i- the object-end of the telescope for this purpose, 



'ready been mentioned. 



In use, this theodolite should be placed on a repeating 

 table or tripod, such ns is to be found figured and described 

 in REPEATING CHICLE, and the repeating-tripod upon the 

 stand. This was not done in the present, plan for the sake of 

 clearness. To adjust the repeating tripod, place the lev 

 described in the first adjustment, and clamp the theodolite. 

 Bring the foot-screws of the theodolite over the foot- 

 of the repeating-tripod by the motion of the tripod, and 

 then by touching the foot-screws of the tripod or theodo- 

 lite set the level-bubbles in the middle. Turn the d] 

 plate i;f the tripod half-round, and again bring the bub- 

 bles into the middle, half by the tripod foot-sri 

 by those of the instrument, and repeat the operation uniil 

 ulution of the repeating-table does not alter the 



.n of the level-bubbles. The rcpcaling-stund is 

 now clamped, and the instrument itself is to be adjusted 



we have described above. 



The course of observation after the instrument is ad- 

 justed i, very simple. The problem is i 

 horizontal angle between two objects. Turn the 

 two or three times round in the direction in \vhie!i 

 intend to observe, then bisect one of the objects, read <.li' 

 the verniers, and take a mean ; bisect the second object, 

 read the verniers, and take a mean. The diffcrem 

 tween the two means is the angle required. ThU is :ill 

 that can be done by the instrument as usually a 

 but. with :i rtpeating-tftble the operation is continued 

 thus. Bring the telescope back on the first object, by 

 the motion of the repeating-table, using its clamp and 

 tangent-screw, and by the motion of the instrument, bring 



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