94 



THE MANAGEMENT OF WOODLANDS. 



Section oF Perpendicular 



rod 



in a small metal disc at one end, and by a hair-line stretched across a 

 metal frame at the other end. In Fig. 22 it is set at 26 yards, and the 

 reading on the mirror gives the result as 8 yards or 24 ft., to which, if 

 the observer be on a level with the tree, must be added the 5 or 54 ft. 

 from the ground to the eye, or a supplementary observation has to be 

 made from the horizontal to the foot of the tree, and added to or sub- 

 tracted from the original reading. 



Weise's Telescope-Hypsometer (Fig. 23) also rests on the same prin- 

 ciple, but is made of brass, with a heavier weight to steady the free 

 perpendicular, and a toothed base to help to bring the perpendicular 



soon to rest and 



Fig. 23. simplify the read- 



ing of the height. 

 The distance from 

 the tree having 

 been measured and 

 the movable up- 

 right arm set ac- 

 cordingly, the ob- 

 servation to the 

 top of the tree (or 

 to the "timber- 

 height ") is made 

 through the tele- 

 scopic tube, which 

 is turned gently 

 round to the side 

 from time to time 



The end near the eye-hole takes off, so that the upright and to assist the loaded 

 the weighted rod (which is triangular, to catch more easily perpendicular to 

 on the toothed scale) can be placed inside the tube when , ^ M A V n A 



not in use. teaay ai 



let the perpendic- 

 ular fall into the toothed scale along the base. The perpendicular rod 

 being finally brought to rest, the number it registers gives the height 

 of the tree above the observer, to which the usual addition or subtraction 

 must be made if the horizontal base-line be above or below the tree-base. 



In place of measuring the girth as is usual in Britain, Continental 

 foresters measure the diameter at breast-height by means of callipers 

 graded so as to give either the girth or the diameter {or both). In using 

 callipers it is best to measure the stems alternately N. and S., or 

 E. and W., so as to eliminate errors arising from eccentricity in growth ; 

 but the most exact way is to measure two diameters at right angles to 

 each other and take the mean of the two. The best form of calliper 



EYE 



CD 



The Telescope-Hypsometer. 



