92 



THE MANAGEMENT OF WOODLANDS. 



to ascertain the height, the girth or diameter at breast-height 

 (4J ft.), and the form-factor or proportion which the true cubic 

 contents of the tree bears to a cylinder of similar height and 

 basal area [form - factor = actual contents of tree -j- (height x 

 basal area)]. This method gives the true cubic contents 

 and makes no allowance for wastage or bark ; but it is only 

 applicable to tree- crops growing in much closer canopy than 

 is customary in Britain. 



The height of the tree is measured with instruments called 

 Hypsometers, of various kinds, which are all based upon the 



Fig. 21. 



The Measuring-Board. 



geometrical fact that in similar right-angled triangles the 

 respective sides are proportionate. The best and handiest of 

 these instruments are the Measuring - Board, the Mirror- 

 Hypsometer, and the Telescope - Hypsometer ; this last being 

 perhaps the most convenient and reliable. 



The Measuring - Board (Fig. 21), about 8 inches square and an inch 

 thick, is divided into squares numbered off in units, fives, or tens, and 

 has a deep notch along the line of sight to the top of the tree. It may 

 either be held in the hand or fixed sideways, by a screw in the centre, 

 to an upright staff shod with iron, which can be inserted into the'ground 



