12 MENSURATION AND INCREMENT [CH. 



no objection to making use of a long measured pole which can 

 be held up against the trunk, to assist in estimating its height. 

 It only remains now to make a further deduction from the 

 quarter-girth under bark to allow for the taper of the stem. 

 What is wanted is the quarter-girth under bark at mid-timber- 

 height. Thus, suppose the timber-height is estimated at 50 feet, 

 and we have taken the quarter-girth at 5 feet from the ground; 

 we now have to estimate the deduction for taper to be made over 

 a length of 20 feet, which is the distance through which we have 

 to raise our point of girthing to arrive at the middle of the 

 timber-length. The deduction to correspond with 20 feet of 

 height would generally be about 3 inches, or something between 

 2 and 4 inches. Here again there may be an opportunity of 

 measuring some felled trees lying on the ground to ascertain 

 what degree of taper actually exists. It may vary very con- 

 siderably according to the local conditions of growth, and 

 especially with the density of the crop, and an error in estimating 

 this deduction will make a considerable difference to the cubic 

 contents. 



Another point that requires to be fixed is up to what size is 

 to be considered measurable timber? Anything above 6 inches 

 in diameter is usually considered as possible timber, or in the 

 case of coniferous trees in regions where pitwood is saleable, 

 the limit may be put at 3 inches diameter. 



The quarter-girth under bark at mid-timber-height, and the 

 timber-height, being now arrived at by this process of estimation, 

 it is only necessary to turn up Hoppus's tables to ascertain the 

 corresponding volume in cubic feet. 



1 1 . True measure of standing timber. 



For purposes of forest management, and for scientific in- 

 vestigations, some more accurate means of ascertaining the true 

 cubic contents, of standing timber than the foregoing method, 

 which relies too much on ocular estimates, must be employed. 



In continental countries, where systematic forestry has been 

 practised for a long time, and where careful data and statistics 

 of all kinds have been recorded and accumulated for a long time, 

 form-factors and volume-tables have been arrived at, which are 



