94 A MANUAL FOR NORTHERN WOODSMEN 
or, if difference in elevation is required, a pole or level rod 
may be used to measure the amount. 
The instrument may be used to find the difference in 
elevation between any two points without the use of a 
level rod. To do this the observer begins at the lower 
point, and, after levelling the instrument, sights in the 
desired direction and notes the point on the ground ahead 
intersected by the cross-wire. He then advances to that 
point and repeats the operation, and so moves on up the 
grade until the upper point is reached. As between every 
two observations he has advanced to a height equal to the 
distance from the ground to his eye, the height of the hill 
will be the product of that distance by the number of 
sights taken. 
The instrument may also be used as a clinometer to 
measure slope. To do this the observer sights along the 
slope parallel to the ground, and then uses the hand wheel 
to turn the level tube until the bubble shows it is level. 
The measuring arm, turning with the wheel and the level, 
sweeps the scale and indicates the slope in degrees, or in 
per cents, according as the instrument is graduated. 
In the same way, and with the aid of a table of tangents, 
one may use the instrument to obtain the height of a tree 
or a hill. This process is explained and illustrated on 
page 166. 
For an improved form and more complicated use of 
the instrument, see pages 130-131. 
SECTION IV 
COMPASS AND PACING 
The staff compass, with folding sights, cross levels, and 
a needle from 2\ to 4 inches long, is familiar to most 
woodsmen. It is a very compact and practical instrument, 
has long been employed for retracing lines, and of late 
years, as forest lands have come to be handled more 
systematically, has attained a great extent and variety of 
uses. It has also been constructed in a variety of forms, 
combined with other instruments in some cases. The form 
