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 three 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 



S^-r 



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 S, and the lines 

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

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

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



by the formula H = — -— — . 

 SJj 



Now, considering the tree as a paraboloid, its basal 



area times one-half the height will give approximately 



