PRESSLE1TS FORMULA 443 



Now, in order to shorten and simplify the calculations, 

 various formulae have from time to time been suggested for 

 readily obtaining the percentage increment on standing 

 timber. 



But in most cases it is necessary to assume that the 

 height is the same for both periods, and so also the same form 

 factor. Now, the assumption of the same height will not 

 materially affect the results obtained. But, the assumption 

 of the same form factor as well, will, in most cases, give 

 results which are materially defective. 



However, if the height and form factor are the same at 

 both periods, the cubic contents at both periods are respectively 

 proportionate to the basal areas at both periods ; and so also 

 to the square of the diameters at both periods. 



Hence, the rate of compound interest can be found by 

 reference to the square of the diameters only, at the respective 

 periods. Hence, if 



D = future diameter (under bark) 



d = present diameter (under bark) 



n = number of years in period 



p = the percentage 



T , 200 D 2 -^ 2 



Then p x D ,-^ 



and this is Pressler's formula. 



Now, this formula has been still more simplified, thus : 



This latter simplification must, however, be used with 

 very great care. It gives practically the same result, provided 

 D and d are very nearly equal ; but if there be a big 

 difference, then the rate per cent, that is indicated will be far 

 too great. 



Hence, with this method, n must never represent a large 

 number of years ; and, the smaller the diameter of the tree, 

 the greater will be the error. 



If, however, n be taken for only one year, the percentage 

 will be very slightly too much, if the percentage for the 

 coming year is under consideration. If the formula be 



