ii8 FORESTRY 



ment when not in use; and (3) the wedge-needle, which is 

 toothed on one side, to press and hold fast the little cylinder of 

 wood when breaking it from the body of the stem. On the 



FIG. 24 

 PRESSLER'S INCREMENT BORER 



a. Hollow metal handle. 



b. Boring tube. 



c. Wedge-needle with scale. 



d. Wedge-needle showing 



toothed edge. 



reverse side the wedge-needle bears a scale for measurement. 



In using the instrument the auger-like borer is made to 

 enter the stem in as radial a direction as possible, i.e. at right- 

 angles to the axis of the tree. At first the boring should 

 proceed slowly, later at any desired speed, until the instru- 

 ment has entered deeply enough. The wedge-needle is then 

 carefully inserted between the side of the boring tube and the 

 wood within. A turn of the handle backward is given in 

 order to detach the cylinder of wood at its further end, 

 and with the toothed needle the little column of wood is 

 drawn out. The latter shows along its length the annual 

 rings of the stem; these rings are measured, and the rate 

 of increase computed. The instrument is thus of assistance 

 in deciding whether or not the trees have arrived at their 

 financial maturity. 



In height measurement the geometric method is often 

 adopted ; it rests upon the theory of similar triangles. 

 Referring to Fig. 25, a vertical staff is placed in the ground 

 at some distance (6 to 8 feet) from the eye (D) of the 

 observer. The staff forms a line parallel with the stem of 

 the tree, and the rays or lines of vision to the top and foot 



