DIAMETER INCREMENT. 81 



two measurements. The latter are made with the calliper, 

 care being taken to mark the place of measurement without 

 causing an unusual swelling at that part of the tree. Where 

 immediate results are required, the increment can be ascertained 

 with Pressler's increment borer. The number of years for 

 which it can be ascertained depends on the length of the 

 cylinder which can be extracted, and on the rate of growth. 

 As most trees grow irregularly, it is necessary to ascertain the 

 increment at opposite sides, or at four sides, and to take the 

 mean. These investigations rest on the assumption that the 

 concentric rings are distinguishable, and that each ring repre- 

 sents one year's growth. 



The increment can be ascertained with much greater accuracy 

 by felling a tree and measuring the breadth of the desired 

 number of rings in the section, the latter being laid at right 

 angles to the axis of the stem. The measurements are made 

 with a scale subdivided to a sufficient degree. This is either 

 laid on the section and the breadths read off, or the latter are 

 taken off with a pair of compasses, and the dimensions then 

 taken from the scale. In either case care must be taken to 

 obtain averages by measuring along two, four, or more radii, 

 equally arranged over the section, and then taking the mean of 

 the several readings. 



In the case of standing trees the increment can only be 

 ascertained for a limited number of years. If a tree is felled, 

 the increment can be ascertained for the several periods of its 

 life, say for every five, ten, or more years. The result can be 

 graphically represented, and a mean curve of increment con- 

 structed, from which the increment for any desired intervals 

 can be easily determined. 



By repeating the above operation at successive heights from 

 the ground, the increment can be ascertained in the several 

 parts of the stem. 



