ANGULAR SURVEYING 43 



used when angles are turned to the left, that is, when the zero 

 of the vernier slides in the direction AB, and the degrees are 

 indicated by the upper figures (60, 70, 80, etc.) on the graduated 

 circle. The vernier MN' is used when angles are turned to 

 the right, and the degrees are indicated by the lower figures 

 (90, 100, 110, etc.) on the graduated circle. Nearly all transits 

 have two combinations of verniers similar to NN', the zeros 



FIG. 4 



of which are 180 apart. Each of these combinations, although 

 it really consists of two verniers, is referred to as one vernier, 

 one of them being called vernier A and the other vernier B. 

 For very accurate work, both verniers are read, and if they 

 do not agree, the mean of the two readings is taken as the true 

 reading. The circle is divided into degrees and halves, and the 

 vernier is divided into 30 equal parts covering 29 of the half- 

 degree divisions of the circle; the vernier therefore reads to 

 minutes. 



Suppose that the center of the graduated circle is over the 

 vertex of an angle to be measured; also, assume that its zero 

 is on one of the sides, that the vernier has been slid to the left 

 along the graduated circle until the other side of the angle 

 passes through the zero mark of the vernier, and that the vernier 

 has then the position shown in Fig. 4. Since the vernier has 

 moved to the left, the side MN is to be read. The twenty- 

 third mark of the vernier coincides with a division mark of 

 the scale, and, as the least reading of the vernier is 1', its reading, 

 in this case, is 23'. The reading of the scale, up to the division 

 mark immediately preceding the zero of the vernier, is 74. 

 The reading of the instrument, or the measure of the angle, 



