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. 



6. 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. 



