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 yi 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 

 tin- trijxxl. 



Levelling is employed by woodsmen in constructing 

 dams and ascertaining the area of flowngo, in laying out 

 roads and railroads, and for the basis of topographic work. 



