HI] MENSURATION AND INCREMENT 15 



enumeration would be made over the whole area. If, on the other 

 hand, the forest is too large for a complete enumeration to be 

 practicable, the estimate of its volume may be made either by 

 linear surveys or by sample plots. In either case at least 5 per 

 cent, of the area should be enumerated in order to obtain reliable 

 results; less in young and regular crops, and more in old and 

 irregular ones. Linear surveys are preferable to sample plots if the 

 crop is irregular, or the ground hilly. In such a case a gridiron of 

 lines of one or two chains in width should be taken right across 

 the map in parallel lines at right angles to the contour lines as far 

 as possible, or in both directions at right angles to one another. 

 If sample plots are chosen, small areas of not less than half an 

 acre in extent should be selected at various points in the forest 

 so' as to give a correct representation of aU varieties of soil, 

 situation, age, and condition of crops over the whole area. 



12. Weise's method of finding the average tree. 



Another method of finding the average tree in a fairly regular 

 crop with a close canopy is Weise's. An enumeration of all stems 

 on the area is made by i-inch diameter or 3-inch girth-classes, 

 and the number of stems is then totalled. A count-back of 

 40 per cent, of this total number of the trees is made beginning 

 from the largest size. The size-class into which this count-back 

 leads will contain the average stem of the whole crop. A few 

 sample trees of this size should then be selected, felled, cut up, 

 and measured. 



13. Increment. 



In order to ascertain what financial return is being obtained 

 from the capital invested in a forest, and in order to frame a 

 plan on business principles, it is necessary to know the increment 

 in volume, quality, and price that is taking place at any given 

 time. 



For this purpose we have to find out the volume increment 

 per acre per annum that is accruing in a standing crop of trees. 



There is the rare case in which we might happen to have 

 accurate measurements of the crop previously taken, with 

 which, after a known period of years, we could easily ascertain 



