150 



ELEMENTARY FORESTRY. 



person may become sufficiently expert at estimating the heights 

 of trees to get along without an instrument. 



The Rate of Growth of a Standing Tree is determined 

 by removing from the trunk a small cylinder of wood with a hol- 

 low auger, called an accretion borer. On this section of wood 

 the annual rings are counted, and their width measured with a 

 pocket rule graduated in inches and tenths, or in millimeters. 



Figure 41. The mirror hypsometer 



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 ax, 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 of the Min- 

 nesota woods is six feet in length. In making an examination 

 of the end of a log the rough graining of the saw must often be 

 smoothed away before the rings can be counted readily, and this 



