WEISE'S METHOD 



431 



Hence by this rule the average tree is one of 12 inches 

 diameter. 



Having ascertained this, the height of several trees having 

 this diameter is accurately measured, and the average height 

 arrived at ; then the average form factor, for the species of 

 tree having this height, is ascertained from tables ; and then 

 the contents of this average stem are found, e.g. t basal area x 

 height x form factor ; and the contents of the whole area are 

 equal to the contents of the average stem x number of stems 

 on the area. 



If the crop be mixed, the species of trees must be kept 

 separate ; and their contents separately computed. 



Whenever there is no objection to the contrary, a few of 

 the average stems should be felled, and their contents accu- 

 rately measured ; for by this means, still greater accuracy 

 will be obtained, and the use of an average form factor 

 dispensed with. 



A method giving still greater accuracy is to find the basal 

 area and height for each class ; and then the contents of all 

 the trees in each diameter class ; and finally the contents, on the 

 whole area, by adding the contents of each class together thus : 



