CUERENT RATE OF INTEREST. 159 



(d.) From these amounts must be deducted all returns 

 realised between the formation of the wood and the 

 end of the year in = 



T„ X 1-0//'-" + T„ X VQir-' + ...+ '/', X l-Qp'"-' + T,,,. 



The current forest per cent. ""'''' Pf, with which the invested 

 capital has worked during the year v/i+l, is, therefore, 

 expressed by the formula — 



(F,„+i - F,, - e) 100 ^ 



0S<..+£ + c)xi-Oiy'^-(r«xi-o/"-" + nxi-OK^'+ . . . + ^».+ £)' 



The denominator in the above formula can also be written 

 thus : — 



0S'.+£) (l%"'-l) + cX l%"'-('i'„X l-Oi/"~" + . . . + 7'J+^',. 



On reference to page 138, it will be seen, that this expression 

 represents the cost value of the growing stock immediately 

 after the thinning in the year m has been made, plus the cost 

 value of the soil ; hence the formula for the forest per cent, 

 reduces to— 



curr. ^ (Y^^i-Y^-e)XlOO* ^ (Y^^i-Y^-e)XlOO 

 Sc + ^G, -Fe 



By substituting the utilisation value of the growing stock 

 for its cost value, the formula becomes — 



curr. ^ ^ (Y „^i - Y „ - e) X 100 



Se"+Y„ 



The latter formula is used, when m is anywhere near the 

 end of the rotation. 



* This formula differs from tliat usually giveu in coutinental works, which is 

 as follows : — 



(F„. + i- F,„)i00 



■Pf 



It is easy to show, that this formula is correct only for the year, in which 

 ""■'■• j^;|- = general per cent. ^;. 



