50 ANGULAR SURVEYING 



distance BC may then be measured from B in the direction 

 indicated by the line of sight. 



Measurement of Horizontal Angles. The horizontal circle 

 of the transit, like that of the compass, measures only horizontal 

 angles; that is, angles between the horizontal projections of the 

 lines of sight. Let AB and AC, Fig. 8, be two lines on the 

 ground the angle between which it is desired to measure with 

 the transit. Set up the instrument precisely over the vertex 

 A , level it carefully, loosen the upper clamp, and turn the upper 

 plate until the zero of the vernier to be read (say vernier A) 

 nearly coincides with the zero of the graduated circle. Clamp 

 the plates, and by turning the upper tangent screw bring the 

 zero of the vernier exactly in line with that of the limb. This 



operation is called setting the vernier at zero. Loosen the lower 

 clamp (if it is not already loosened), and direct the telescope 

 to a flag held at B. Next, loosen the upper clamp, and direct 

 the telescope to a flag held at C. The arc of the graduated 

 circle traversed by the zero point of the vernier will measure 

 the angle BAG, whose value can be determined by reading the 

 instrument; that is, by adding the reading of the vernier to 

 that of the limb. 



It is not always necessary nor convenient to set the vernier 

 at zero before measuring an angle. The upper clamp being 

 set, whatever the position of the vernier may be, the tele- 

 scope is directed to B, as explained, and the reading of the 

 instrument taken. The upper clamp is then loosened, the 

 telescope directed to C, and the instrument read again. The 

 difference between the two readings is the value of the angle. 



