The Adjustments of the Engineers' Transit for 



and for Measuring Horizontal and Vertical Angles. 



These adjustments differ according as the telescope is provided with a fixed 

 level, which is the more usual form in the engineers' and surveyors' transits, or is 

 provided with a striding-level parallel to the telescope, as shown in the illustration 

 of the transit-theodolite No. lla. 



1. A Fixed-Level Parallel to the Telescope. 



This adjustment consists of two distinct operations. The first is to place the 

 horizontal wire of the telescope in the combined geometrical and optical axes of 

 the telescope, or the line of coilimation (see " Some Facts Worth Knowing, " 1',-irt 

 I.); the second is to place the axis of the fixed -level parallel to the line of coili- 

 mation. 



The precision of the adjustments will depend in a measure upon the perfection 

 with which the geometrical axis of the telescope has been made to coincide with 

 the principal optical axis for all focussing positions of the object-slide. The more 

 perfect the continual coincidence of these two axes, the more precise and accurate 

 will be the measurements when the sights are of unequal lengths ; but in practical 

 leveling, where sights of equal length are used, the effect of a non-coincidence of 

 these two axes is mainly confined to " intermediate points," and is of minor conse- 

 quence unless the non-coincidence is very marked. 



The Adjustment of the Horizontal Wire. This may be done with sufficient accuracy 

 In our transits having an erecting telescope, where the eye-piece is non-adjustable 

 laterally, in other words, permanently centered with respect to the optical axis, by 

 merely placing the horizontal wire in the center of the field of view of the eye-piece. 

 The cross-wires of inverting telescopes, however, being stretched on a diaphragm 

 which itself limits the field of view, they will appear in the center of the field whether 

 the instrument is adjusted or not, and therefore another method of adjustment is 

 needed. This can be done roughly in an emergency by merely placing the hori- 

 zontal wire by the eye as nearly as possible in the center of the main tube of the 

 telescope, the eye-piece having been first removed. Another method is that 

 described by Gravatt, see pages 48 and 49. But the method which is the easiest in 

 our shop practice, and sufficiently accurate for all practical purposes, is that followed 

 by us, and as it might be followed with profit by the engineer, should circumstances 

 favor, we give it here also. After the wires have been placed in a position 

 perpendicular to the horizontal axis of revolution, the telescope is removed from 

 the standards and placed in wyes where it can be rotated. Such wyes may bo 

 Improvised by the engineer, by cutting the proper shapes out of thin wood and 

 fastening a pair of them to a board in an upright position. The distance between 

 them should be such, that the telescope may rest upon the outside of the mounting 

 of the object-glass where the cap is placed, taking care to rest it against its shoulder 

 on the inside of the wye, and upon the tube near the cross-wires, when practicable. 

 The improvised wyes being placed on a firm support and fastened so that they -v'll 

 not move, the telescope may be revolved in them and the wires may be placed in the 

 line of coilimation as in a wye-level, using a distant point. 



In some makers' transits the telescope is so arranged that it can be revolved 

 in the center part of the transverse axis of revolution, after the level tube and the 

 screw fastening it in the center of the hub which keeps the telescope and axis 

 together have been removed. In this case the horizontal and vertical wires can be 

 adjusted approximately for coilimation by turning the telescope in its hub. This 

 method, however, does not permit of close work, and besides being clumsy, gen- 

 erally leaves the telescope in a more insecure state of stability, than when the tele- 

 scope and transverse axis are soldered together. On this account, whenever appli- 

 cable, all our instruments p,re made in this latter manner. 



There is another method which may be followed if the principal optical axia 

 passes through the geometric horizontal axis of revolution of the telescope. Select 

 a place for the transit alongside of a house or near a tree, and with an unobstructed 

 viow of at least 300 feet. Set the instrument and level up approximately. Clamp 

 the telescope, and by means of its tangent-screw bisect a distant point with the hor- 



