112 
HINTS TO TRAVELLERS. 
to measure half of it with each micrometer, the sum of which measures 
will, of course, be the whole angle subtended. The distance from the 
staff is computed in the following manner :—Multiply the total number 
of divisions used in each micrometer by the value of a single division of 
that micrometer, add the results together, and this will be the value of 
angle in seconds. Divide the length of the staff, in feet, by the angle in 
seconds and multiply the result by the cosecant of 1" = 206265. This 
will give the distance between the instrument and the staff, in feet. 
Example :—Length of staff, 12 feet; divisions used. Left Micrometer, 
581*9, value of each division, 2"*31; Right Micrometer, 575*2, value of 
each division, 2" *04. 
Left Micrometer. 
581*9 
2*31 
5819 
17457 
11638 
i 544* l8 9 
ft. 
Log.12 = 1-079181 
Log. 2517-6 = 3-400986 
3*678195 
Cosec. 1 "=. 206265 Log. = 5-314425 
Right Micrometer. 
575-2 
2-04 
23008 
11504 
1173-408 
1344*189 
The angle in seconds = 2517-597 
Log. distance in feet, 983-2 = 2-992620 
The rod, though convenient, is not, however, absolutely necessary, as 
distances can be measured by this class of tacheometer without it, by 
making an assistant set up two staves at a carefully-measured distance 
from one another, and at right angles to the line of sight. The angle sub¬ 
tended by these staves is measured with the micrometers, and the distance 
computed in the manner already shown. 
A theodolite with fixed hairs, such as described (p. 39), may often 
be used for measuring distances approximately when it is impossible to 
read the markings on a graduated staff. This is done in the following 
manner:—An assistant should be sent to the object, the distance of which 
is required, and directed to place a staff in the ground. The surveyor 
must then cover the staff with one of the fixed hairs in the instrument, 
after which the assistant must move, very slowly, in a line at right 
