MAW'S FORMULA 447 



Generally speaking, it will be of more practical value to 

 know the percentage increase which is taking place upon 

 that which is timber only, instead of the percentage on the 

 total contents. 



Now, practically all the increase can be reckoned as 

 timber, for in trees of timber size there is always, in the same 

 species of tree, about the same quantity of material which is 

 under timber size ; and, although much of the increment takes 

 place on that which is under timber size, yet it is about 

 counter-balanced by that which was formerly just under 

 timber size, being measured as timber, since the increment 

 upon it has brought it up to timber size. 



Therefore, in the case of trees with small diameters, this 

 percentage will be very much greater than the percentage 

 upon the total contents, for the proportion of that which is 

 timber only to the total contents is very small ; and thus, the 

 mean diameter (or quarter girth) is less. So also, the mean 

 diameter (or quarter girth) is less in the case of trees with 

 large diameters, though not in the same ratio as in the former 

 case. 



It should be noted that this mean diameter is the diameter 

 of a cylinder the full height of the tree, having a total con- 

 tents equal to the cubic contents of that which is timber 

 only. Obviously, this diameter must be less than the 

 diameter of a cylinder whose contents are equal to the cubic 

 contents of the whole tree, including that which is under 

 timber size. 



Now, the previous formula may be used for ascertaining 

 the percentage increase (on standing timber) upon that which 

 is timber only, if the form factor for timber only be substituted 

 for the form factor for the total contents. 



Hence, if 



F// = form factor for timber 



p = percentage increment on timber only 

 Then 



400 



Having regard to the foregoing details, as to gauging the 



