MEAN RATE OF INTEREST. 161 



the mean annual forest per cent, under the intermittent 

 working is — 



^ X 100. 



If >S'->N,, then""""^V>jj. 



If the expectation vakie of the soil is equal to the cost 

 value, then the mean annual forest per cent, is equal to the 

 general per cent. ]), which proves the correctness of the above 

 formula. 



The highest mean annual forest per cent, is obtained under 

 that rotation, foi' which the expectation value of the soil 

 culminates ; it is then equal to the current annual forest per 

 cent. 



Example.— Given the same conditions as before — 



,S', = 404.S. 

 If the cost value of the soil is also 404.s., then 



404 



4U4 



equal to the general per cent, j), with which the value of Se has 

 been calculated. 



If S, = 300 shillings, then 

 404 



mean ,, ^v^ . , n " .t < j 



Pf — OQQ X 2-0 = 3-4 per cent. 



If S, = 500 shillings, then 



404 



'"-»«, = 5^ X 2-5 = 2-0 per cent. 

 ^ ^ 500 ^ 



In the latter case money is lost ; hence the capital had 

 better be taken out of the forest, and invested elsewhere, if 

 at least 2J per cent, can be obtained with equal security. 



F.M. M 



