92 
A MANUAL FOR NORTHERN WOODSMEN 
rate results may nevertheless be had by taking backward 
and forward sights of equal length, and this check it is easy 
to secure by pacing. It is important that the rod should 
be held plumb during the levelling operation. This position 
is secured by careful attention on the part of the rodman 
and by waving the rod slightly. The length of sight varies 
with the instrument, the condition of the air, and the ac¬ 
curacy desired. About 300 feet is stated to be in general 
the best length on the score of accuracy, but speed will 
often require that much longer shots be taken. In accu¬ 
rate work, it should be remembered that error may be 
introduced by the slightest causes, such as disturbance of 
the tripod. 
Levelling is employed by woodsmen in constructing 
dams and ascertaining the area of flowage, in laying out 
roads and railroads, and for the basis of topographic work. 
