168 



PRINCIPLES OF AMERICAN FORESTRY. 



basal area of a tree is taken at breast-height, to avoid 

 the excessive swelling near the ground. Breast-height 

 is usually considered as four feet six inches above the 

 ground, at which point the diameter is measured by a 

 pair of calipers in inches, and the area in square feet of 

 the corresponding circle is found in a prepared table of 

 such areas. The height of the tree may be determined 

 by triangulation, in which various instruments are used, 

 as the transit, the altimeter, or a mirror hypsometer. A 



A 



FIG. 60. Diagram showing method of measuring the height of a 

 tree by a simple geometrical method. 



simple geometrical method is illustrated in the figure. 

 A measuring-rod is set up at a convenient distance from 

 the tree AB, the eye of the observer is at $, and the lines 

 of sight to the top and bottom of the tree intersect the 

 rod at a and 6. Then, by measuring the distances from 

 the observer to the rod and to the tree the height is given 



abXSC 



by the formula H = . 

 o/>> 



Now, considering the tree as a paraboloid, its basal 

 area times one-half the height will give approximately 



