66 



of investigations made at the age o£ exploitability. Tlie number 

 of stemSj the basal area, and the dimensions of the average stem 

 at all previous periods must be known. Hence to trace the" course 

 o£ growth of a given crop it must be surveyed from its earliest 

 youth annually, or at regularly recurriug periods, until it becomes 

 exploitable. This method of repeated survey has been adopted in 

 order to measure the influence of different niodes of treatment on 

 crops that in all other respects are similar. 



■ To follow out this system rigidly on one and the same crop 

 through the whole of its life would be an extremely long and slow^ 

 process. In order to curtail it and obtain the results sought as 



' quickly as possible, several similar crops, under the same treatment 

 but of xJifTerent ages, are experimented upon, and the' various 

 observations made from time to time in the several crops are com- 



■ bined and interpolated ■ together into a connected whole, that is 

 nearly, if not quite, as accurate as if the figures it comprises had 

 been obtained from investigations in one and the same crop from 

 the time of its constitution to its maturity. 



There is yet another and still shorter way of obtaining the re- 

 quisite figures. A series of crops growing under similar conditions 

 of species, soil, locality and treatment,but of various ages differing 

 from each other by as short intervals as possible, is chosen with 

 great discrimination and care, and each crop is measured once for 

 all, the results being tabulated together. Most of the yield-tables 

 hitherto compiled have been obtained by this inethod, and the 

 results have been proved, to be quite correct enough to justify its 

 adoption as often as there is no time to wait for the-outcome of the 

 longer and more elaborate methods. 



Whichever of the two last described methods is adopted, there 

 are always certain ages for which figures are wanting, and the 

 gaps must be filled up by interpolations which are most conveni- 

 ently obtained graphically thus : — The various successive ages are 

 marked on a horizontal line, and at these points perpendiculars are 

 raised, the perpendiculars for the ages for which the volumes or 

 increments are known being made of the lengths corresponding 

 to these volumes or increments, as the case may be. The ends of 

 these perpendiculars being joined by a continuous curve, the 

 lengths of the other perpendicular up to where they are cut by 

 the curve, give respectively the remaining volumes or increments. 



