44 ANGULAR SURVEYING 



is, therefore, 74+23' = 74 23'. It can readily be seen that 

 when the angle is measured from B toward A the reading of 

 the instrument is 105 37'. 



The vertical circle, or arc, V, of the engineers' transit, Fig. 1, 

 is often graduated to degrees and halves, and the vernier 

 ', which is double, like the vernier of the horizontal circle, 

 reads either to single minutes or to 5 minutes. If the vernier 

 is attached to the standards, it is stationary; and instead of 

 its sliding along the vertical arc, the vertical arc slides on it. 

 Care should always be taken to read that side of the vernier 

 whose numbers increase in the same direction as those by 

 which degrees are measured on the graduated circle. 



ADJUSTMENTS OF THE TRANSIT 



When a transit is in perfect adjustment, it must, after being 

 leveled, fulfil the following conditions: 



1. The centers must revolve on a truly vertical axis, so 

 that the plate levels will remain centered during a complete 

 revolution. 



2. The line of collimation that is the line of sight must 

 be perpendicular to the transverse axis of the telescope, so 

 that it will be in the same straight line when the telescope is 

 plunged. 



3. The axis of revolution (the transverse axis) of the 

 telescope must be horizontal, and, therefore, perpendicular to 

 the vertical axis of the instrument. 



When a transit has a level and a vertical arc or circle attached 

 to the telescope, it should fulfil the following additional 

 conditions: 



4 . The line of collimation must be parallel to a line tangent 

 to the tube of the telescope level at its middle point, so that the 

 line of collimation will be horizontal when the bubble of the 

 telescope level stands at the middle of its tube. 



5. The vernier of the vertical arc or circle must read zero 

 when the line of collimation is horizontal. 



The adjustments establishing these conditions should be 

 made in the order in which the conditions are stated. The 

 best time for adjusting an instrument is on a cloudy day or 

 in the early morning before the air has become heated and 





