4 
A MANUAL FOR NORTHERN WOODSMEN 
be carried in the pack or slung from the shoulder. A 
folding-sight compass with too small a box and needle of 
less than full length should not be employed on work of 
importance, as it is impossible with such an instrument to 
read bearings and set marks with accuracy. 
Compasses are either mounted on a tripod or fitted for 
attachment to a single staff called a Jacob-staff, which 
the surveyor may make for himself, when needed, from a 
straight sapling. The former is the firmer mounting and 
better adapted to accurate work, but the latter is much 
more portable, except on bare rocks is more quickly set up, 
and is generally employed for the ordinary work of the 
forest surveyor. 
2 . Adjustments of the Compass 
. 
A compass in first-class order will meet the following 
tests: 
a. The plate must be perpendicular to the axis of the 
socket. 
b. The plane of the level bubbles must be perpendicular 
to the same axis. 
c. The point of the pivot must be in the center of the 
graduated circle. 
d. The needle must be straight. 
e. The sights must be perpendicular to the plane of the 
bubbles. 
In these tests it is presupposed that the circle is accurately 
graduated and that the plane of the sights passes through 
the zero marks. These are matters that belong to the 
maker of instruments, and in all modern compasses accu¬ 
rate adjustment of them may be assumed. 
The general principle of almost all instrumental adjust¬ 
ments is the Principle of Reversion, whereby the error 
is doubled and at the same time made more apparent. 
Thorough mastery of this principle will generally enable 
one to think out the proper method of adjusting all parts 
of any surveying instrument. In the case of the compass 
the above-named tests may be applied and the instrument 
adjusted as follows. The order of the adjustments is 
essential. 
