FORMULA FOR SD 



35 



Ta the stumpage value of the thinnings from one acre a 

 years old, etc. ; 



c the cost of planting, or otherwise reproducing, one acre; 

 e the regular yearly expenses of tax and care for one acre 

 of land ; 



r the rotation; 

 .p the interest rate employed or assumed; 



Se the expectation or income value of one acre of land or 

 soil.* 



(x.opri) 



In the above formula the term e may also be writ- 



(i.op i) 



e e 



ten - - (i.op r i)=E(i.op r i) where E = - =e 



(i.op i) .op 



capitalized so that E is a sum of money which will bear a yearly in- 

 terest e at p per cent and therefore, if put out at interest will "take 

 care" of the regular yearly expenses e. 



The formula then may also be written: 



_ Yr + Ta(i.op r - a )) 4- etc. c(i.op r ) E(i.op r i) 



(i.op'-i) 



which may be and usually is simplified into : 



Yr + Ta(i.op r - il )+ etc. c(iop r ) _ E ^ 



(i.op r i) 



* It is of great 'help to the student to use these terms or symbols con- 

 sistently and use no more, terms than are actually necessary. To use Y and 

 Yr interchangeably or use Yr for one acre and for a given area of many acres. 

 etc.. soon confuses even able students. Where new cases come up, especially 

 cases which are not really standard, but exceptional, it is much better to write 

 out fully what each part means and use the full analysis, as in the above 

 sample case, rather than try to force it into a formula. 



** This formula is written by Emdres as follows : 



which means exactly the same but changes the order for c so that it confuses 

 the beginner and the general student. As stated before, the average student 

 will find it helpful to stay closely with the simple forms and write them out 

 in full so that the formula shows clearly just how it was derived. 



