ANGULAR SURVEYING 49 



on its transverse axis slowly and notice whether the point 

 sighted remains on the cross-hair throughout the motion. 

 Should any deviation be discernible, loosen the capstan-headed 

 screws that control the cross-hairs, and by the pressure of the 

 hand, or by tapping lightly against the heads of the screws 

 outside the telescope tube, rotate the cross-hairs very care- 

 fully in the direction opposite that in which they should 

 apparently be rotated, until the point sighted remains on the 

 cross-hair throughout the motion of the telescope. Then 

 tighten the screws sufficiently to bring them to a firm bearing 

 without straining them, and repeat the test. 



This test should be applied before performing the third 

 adjustment for the line of collimation. If the plate levels are 

 in perfect adjustment, it can also be made by sighting at a 

 plumb-line suspended at a suitable height and distance, with 

 the plate levels centered perfectly, and observing whether 

 the vertical cross-hair coincides exactly with the plumb-line. 



TRANSIT FIELD WORK 



To Prolong a Straight Line. Let AB, Fig. 7, be a straight 

 line whose position on the ground is fixed by stakes set at A 

 and B, and let it be required to prolong the line to C. This 



4 B c 



-o 



FIG. 7 



can be done in two ways; namely, by foresight only, or by 

 backsight and foresight, the latter method being commonly 

 called backsight. 



By Foresight. The transit is set over the point at A, and 

 the line of sight directed to a flag held at B; if the point C 

 is to be set at a given distance from B, the chainmen measure 

 the required distance, the head chainman being kept in line 

 by the transitman. When the required distance has been 

 measured, the point C. which evidently lies in the prolongation 

 of AB, is marked by a stake or otherwise. 



By Backsight. Set the transit over the point at B and 

 sight on a flag held at A. Plunge the telescope, which will 

 then be directed along the prolongation of AB. Any required 



