194 THE INCREMENT. 



Hence : 



3820x5 + 710x1 Q7 

 Q= "4530- -4-37 pence. 



And: 



-- ', 



4-37^3-44 xl'op, 10 



And 



p, = 2-42 per cent. 



Approximate value : 



4-37-3-44 200 QQ 



X TO =238percent - 



Calculating the quality increment per cent, for timber only 

 with the data given in the table at page 122, the following 

 values are obtained for p q according to the formula : 



Period 30 40 years . . . p q = 2'26 



40 50 . . . = 1-84 



50 60 . . . = 2-92 



60 70 . . . = 2-26 



70 80 . .; . = 1-84 



80 90 .. ; v . . = 1*55 



90100 . v ' . = 1-34 



100110 . . . = 1-18 



110120 . . = 1-06 



What has been said above can also be applied to the inter- 

 mediate returns. Indeed, the quality increment of that part 

 of a wood which yields the thinnings can be very considerable, 

 especially while the wood is still young. Here a few years 

 extra growth may cause a great rise in the quality per unit of 

 measurement. On the other hand, if thinnings are kept over 

 too long, they interfere with the proper development of the 



