244 



THE MEASUREMENT OF STANDING TREES 



JJach instrument is constructed for use with a specified length of pole. The 



-. instrument described above is one designed by the author for 



convenience with the use of English units. It was constructed 



in the following way: The distance be on the instrument was 



chosen arbitrarily as 15 inches and the length of the pole as 10 



feet. It would, of course, be possible to construct an instrument 



for a pole 12 feet or any other length and on a basis of any 



desired length of instrument. The theory of the construction of 



Christen's instrument may be shown by Fig. 52. When used as 



above described, two pairs of similar triangles are formed: ABC, 



bcXDC bcXDC 



and Abe: ADC, and Ade, in which BC = and de= . 



dc BC 



With a known value of DC and be, dc may be determined for all 



different heights which are likely to be required. Thus it may be 



assumed that it would not be necessary to measure trees less than 



20 feet high, so that the lowest graduation on the instrument is 



made for that height. To find the proper point for the 20-foot 



graduation on the scale, the following formula was used : 



BC be 20 15 150 



DC = ^ °^ To^Jc °^ ^^ = ^ = 



= 5.7 inches. 



md'^ 



''^ , I ■■V tiiiw.w<s^> 



Fig. 51.— The 

 Christen hyp- 

 someter. Fig. 52. — Method of application of the Christen hypsometer. 



