126 
KIKTLEY F. MATHER 
By means of the vernier it may be read in minutes. The vernier 
is an auxiliary scale qn which there are 30 graduations occupying 
a space equal to that of 29 graduations on the main scale. That 
is, each division on the vernier is just one-thirtieth smaller than 
a division on the main scale. If, therefore, the zero line of the 
vernier is directly opposite a line on the main scale, no other 
line on the vernier scale will coincide with a division of the main 
scale except the thirtieth. If, then, the arc be moved one minute 
( = one-thirtieth of one division) to the right, the first line on the 
left of the vernier zero will coincide with a line on the main scale; 
if the arc be moved 15 minutes to the right, the fifteenth line 
on the left of the vernier zero will coincide with a line on the 
main scale, etc. On this principle the arc graduated only to 
half degrees may be read in minutes. On an arc graduated 
from right to left, read the highest division to the right of the 
vernier zero line; this will be either an even degree or a degree 
plus 30 minutes. Observe which line on the vernier coincides 
with a line on the main scale; add its value in minutes to the 
reading of the main scale. For example: the vernier zero is 
between the 24° 30' and 25° graduations of the main scale; line 
16 on the vernier coincides with a division of the main scale; the 
arc reading is therefore 24° 46'. 
A vertical angle of 1 minute subtends a chord of 0.3 feet at 
a distance of 1000 feet; hence it is imperative that no mistake be 
piade in selecting the vernier division which conicides most 
closely with a line of the main scale. Most surveyors make it a 
practice always to use a pocket magnifier in reading the vernier. 
It is also easier to detect offsets of the main scale and vernier 
division lines if one looks obliquely along the lines at an angle of 
30 or 40 degrees with the face of the scale than it is when observ- 
ing the vernier face from a direction perpendicular to it. The 
most common of the serious errors which may involve the vernier 
reading is to overlook the | degree division of the main scale and 
count it as an even degree; guard against that blunder by com- 
puting the position of the vernier zero twice for each angle. 
Most alidades are equipped with adjustable vernier and with 
main scale so graduated that the vertical arc may be set to read 
