LOO 



of course, the projection of the line of sight will not deviate from a straight line for 

 intermediate points. Sec on tln^ point "Sotm? Facts Worth Knowing." It i> 

 assumed Unit the point of suspension of the plumb bob is in the prolongation of the 

 vertical axis of revolution ol the in^tiununt, and that the instrument is corm-tly 

 set over the point chosen for its position. 



method of adjusting the vertical wire for collimation is sufficiently de- 

 scribed on page 49. The result of this process is, however, affected by any variation 

 of the transverse axis from a true right angle to the vertical axis of revolution, 

 when the two objects sighted at are not precisely in tho same horizontal plane, a 

 condition somewhat inconvenient to meet at all times in the field. Therefore, 

 in instruments not provided with a transverse striding-level, it will be best to 

 t two points in opposite directions that are as near as possible in the same 

 horizontal plane, then make the vertical adjustment as described on page 51, which, 

 however, is a crude substitute for the more perfect methods of the striding-ieve], 

 or two collimators arranged in a vertical plane. When this adjustment is com- 

 pleted it will be necessary in these instruments to again verify the adjustment of 

 tlu vertical wire for collimation and also to repeat the adjustment of the vertical 

 plane, as neither of them can be made at once correctly independently of the 

 other. 



The adjustment of the vertical wire of telescopes whose transverse axis is pro- 

 vided with cylindrical bearirgs resting in wyes, as in Transit-Theodolites Nos. 11 and 

 12, etc., may be made by the same process of reversing through the standards, or 

 by the following process, which is the only one applicable to instruments having low 

 standards, such as Theodolite No. 14. 



The process is as follows. Level up the instrument approximately; clamp the 

 plates; set the vertical wire in a vertical position by the method described on page 

 48. Then select a well-defined distant point and bisect it with the vertical wire. 

 Next, lift the telescope out of its bearings, and reverse the transverse axis end for 

 end, replacing the axis in the bearings with the telescope pointing in the same 

 direction as before. If the distant point is again bisected by the vertical wire, its 

 adjustment is correct. If not, move it in the direction stated in the footnote on 

 page 49, by moans of the capstan-headed screws, one-half of the distance between 

 its present position and the point previously bisected, and repeat the process until 

 the adjustment is correct. 



Tests of the assumed conditions. 



In the foregoing explanation of the adjustment of the transverse axis by means 

 of the striding-level resting on special collars, it has been assumed that the special 

 collars are of equal diameters and that they are concentric with the telescope's axis 

 of revolution. Now to see whether the line of collimation moves in a truly vertical 

 plane, after the horizontal axis of revolution has been adjusted to lie at right angles 

 to the vertical axis of revolution of the Engineer's Transit, proceed thus : 



Having previously verified the adjustment of the striding-level and by it that 

 of the wye adjustment of the standard as well as that of the vertical wire for colli- 

 mation, set up the instrument as near to a church with a spire, or a building with a 

 well defined object elevated sufficiently to require the telescope to move through an 

 altitude of about90 when it is elevated to the object and depressed to the base of the 

 building. Now level up the instrument, bring the bubble of the striding-level to the 

 center of its graduation, and with the plates clamped bisect the object at the top, 

 then depress the telescope and find or make a mark at the base which is also 

 bisected hy the vertical wire. Now reverse the telescope, turn the instrument in 

 azimuth and with plates clamped again bisect the elevated object and upon depress- 

 ing the telescope see whether the mark at the base is also bisected. If so, the adjust- 

 ment of the line of collimation for motion in the vertical plane by means of the 

 striding-level is correct but, if not it will show that the collars are worn and that 

 correction of the difference must be made by means of the wye adjustment of the 

 standard, over a little less than one quarter of the error, taking care to make the 

 final adjustment by an upward movement of the capstan-headed screws. Then 

 repeat this adjustment, and find or make a new mark at the base upon each succes- 

 sive trial when the instrument is depressed, until correct. 



Now note to what extent and towards which collar the bubble of the stridiim- 

 ievel has moved and make a memorandum of it so that it may be applied as a cor- 



