104 FOKESTKY IN GREAT BRITAIN AND IRELAND. 



where Yr represents the value of the final yield m the year r ; 

 Ta, T6 . . . Tg-the values of thinnings, &c., in the years a, 6, . . . 

 q, c the cost of formation ; e the annual expenses for administra- 

 tion, &c. ; and p the rate of interest, under which Sc is equal to 

 Se. The object is to determine the value of p corresponding to a 

 certain cost value of the soil which is the average rate of interest 

 yielded by the invested capital. It changes with every change 

 in the value Sc of the soil. 



Unfortunately, the above equation is mathematically not 

 soluble, so that a graphical solution must be resorted to. 

 Different values of p are introduced into the right side of the 

 equation and the corresponding values of Se ascertained. These 

 values are plotted and a curve drawn through them, from 

 which the per cent, corresponding to different cost values 

 of the soil can be read off. It is, of course, necessary to con- 

 struct a curve for each species or even for each quality of soil, 

 though that is, to a large extent, expressed by the cost value 

 of the soil. The Sub-Committee were satisfied to make the 

 calculation for a soil of average quality for each species. It 

 selected the five species which are Hkely to be planted on a 

 large scale, and determined the following matters for each of 

 these species : — 



(1) Length of rotation for Douglas fir (green variety), 60 years ; 



larch, 70 years ; Sitka spruce, 70 years ; Scots pine, 

 80 years ; and Common spruce, 80 years. 



(2) The quantity of timber Hkely to be produced by each 



species in the course of one rotation on fully stocked areas 

 managed according to efficient silvicultural rules, that 

 is to say, under a 100 per cent, efficiency. The data were 

 obtained in the following way : For Scots pine and spruce 

 from statistics collected and tabulated in continental 

 yield tables, modified by measurements made in this 

 country and by local experience ; for larch and 

 Douglas fir from measurements made in this country ; 

 for Sitka spruce from a comparison with common 

 spruce and the somewhat meagre records available in 

 this country. 



(3) The average prices obtained for the different classes of 



timber immediately before the war. 



