JOURNAL OF FORESTRY 



SUBTRACTION OF VARIABLES 



Tlie method of handling subtraction as represented by an equation of 

 the form c = .v — y is merely a modification of that for addition. There 

 are two variations. One method is to apply an addition chart in a re- 

 verse direction. In figure t, for example, AB and CD may be con- 

 sidered the initial and EF the final axis. If CD is the .r axis. AB the y, 

 and EF the .:::, then obviously .r = 3' -\- r, and therefore .cr :^ .r — y. 



-S 



-/O 



Fig. 5.— Elementary chart for multiplication. 



The second method involves retaining the final axis in its inter- 

 mediate position, as in the case first described, but graduating one of the 

 initial axes negatively — that is, in a reverse direction. The principle 

 here involved is the obvious one that .cr = .r — y may be written z = x 

 -f- ( — y). The geometrical principle is indicated in figure 3, in which 

 the letters are used in identically the same way as in figure i, except 



