THft TACHEOMETEft. 
§7 
In Fig. 1, suppose the instrument to be at A, and a staff of known 
length to be represented by BO; then if the angle BAC is measured, and 
A 
B 
D 
the length of the staff BC is known, the distance AD can be easily 
computed. In order, however, to measure the angle BAC, the value of 
the micrometer divisions must be determined in the following manner:— 
Set the telescope to solar focus , and carefully measure the distance AD 
from the instrument to a staff of known length; measure the angle BAC 
subtended by the staff with each micrometer, carefully noting the number 
of divisions and decimals of a division used with each. Divide the length 
of the rod by the distance AD between the instrument and the rod, and 
multiply this by the cosecant of 1" = 206265, and the result will be the 
value of the angle BAC in seconds as measured by that micrometer. Now 
divide BAC in seconds by the number of micrometer divisions used in 
taking it, and the result will be the value of each division of the micro¬ 
meter in seconds and decimals of a second. As the value of the divisions 
will not be exactly the same in both micrometers their values must be 
separately determined. It should he home in mind that the values of the 
micrometer divisions must he determined at solar focus and the instrument 
used subsequently at solar focus, otherwise wrong values will he given for 
the micrometer divisions. 
Example :—Number of divisions used (Right Micrometer), 1157*1; 
length of rod, 12 feet; distance between rod and instrument, 983*2 feet. 
Log 12 = I *079181 
Log distance 983*2 = 2*992642 
2*086539 
Cosecant of t" — 206265 Log = 5‘Ji44 2 5 
The Whole = 2517 *"46 =s Log 3*4°°9 6 4 
20326 
11571 
87550 
