INCREMENT. 



101 



unscrews for this purpose. The borer is placed against the 

 stem at breast-height and firmly screwed in at right angles to 

 the surface, and as far as desired. Then the toothed pin c is 

 pressed home, the gimlet reversed for a few turns, and the spill 

 d extracted. 



A spill having been taken by means of this borer, the 

 number of annual rings in the last inch of radius of wood 

 formed can be counted, not including bark or outer cambium, 

 and this gives the number of years taken for the stem to 

 increase by the last 2 inches in diameter, or 6f in. in girth ; 

 and from this one gets a very good indication as to whether 

 the tree is making fair increment or not. The most correct 

 results are got with two borings at right angles to each other, 

 and taking the mean of the two (as also in case of measuring 

 the diameter of a tree). And as, except when a thinning takes 

 place, the rate of growth for the next 5 or 10 years will prob- 

 ably be much the same as during the last 5 or 10, one can esti- 

 mate the mean percentage of increment approximately by either 



D 2 -d* 200 D-d 400 



of the formulae - x or - x , where D is the 

 2 



n D + d n 

 present diameter (below bark) and d the diameter n years 

 previously (Pressler's formula, and Schneider's). 



For example, in a stem of 20 in. diameter (free of bark) two spills taken 

 at right angles at breast-height show 11 and 9 annual rings for last inch of 

 radius, then the mean is 10 years for the girth- increase of 6f in. ; and the 

 first formula will indicate a mean percentage of increment = 1520-^724 = 

 2-099, while the second indicates =; 80 + 38 = 2 '105. 



3. Estimate of the Increment in whole crops of timber 

 can be made by the examination of sample plots or by means of 

 Average Yield Tables, although the latter method is only ap- 

 plicable when the woods are fairly regular and in normal 

 density. In general, the height of the crop and its appearance 

 give a fair indication as to whether the wood is still growing 

 well or not. But if one wishes to ascertain whether it is more 



