192 PRINCIPLES OF AMERICAN FORESTRY. 



one of them, a person may become sufficiently expert in 

 estimating the heights of trees to get on without the 

 instrument. 



The Rate of Growth of a Standing Tree may be deter- 

 mined by removing from the trunk a small cylinder of 

 wood with a hollow auger called an accretion-borer.- On 

 this section of wood the annual rings are counted and 



Fig. 65. — The mirror hypsometer in use. 



their width measured with a pocket rule graduated in 

 inches and tenths, or in millimeters. Where the growth 

 has been slow, and the rings are close, a pocket lens may 

 be necessary to enable one to count them. When a fuller 

 determination of the rate of growth is desirable, trees 

 are felled with an axe, or with a saw, and cut into logs. A 

 small saw is easier to carry around, but a longer, heavier 

 saw does much faster work. The common logging saw 



