240 THE MEASUREMENT OF STANDING TREES 



The Faustmann Hypsometer. Instead of graduating a circular arc 

 in per cents, which requires a decreasing scale with increasing per cent 

 (since the tangents of the angles increase faster than the angle), the 

 height scale corresponding with this arc may be placed on a straight 

 arm as in other hypsometers (§ 195) and graduated evenly. 



The Faustmann hypsometer employs this principle of the pendulum, 

 using a plumb-bob to determine the angles BAD and CAD, and indicat- 

 ing the height of the tree above and below the point D by the intersec- 

 tion of this plumb-bob string with the " height "scale on the base of the 

 hypsometer. This instrument is illustrated in Fig. 47. Its method 

 of use is shown in Fig. 48. 



Fig. 47. — The Faustmann hypsometer. 



The slide is first moved upwards until the number of units on the 

 vertical scale, from zero, thus set off, equals the distance to the tree 

 in feet (or in yards). When sighted at the upper point on the tree, 

 the plumb-bob falls to the near side towards the eye, and the number of 

 units or height is read in the mirror. The second reading is shown in 

 Fig. 48, the plumb-bob falling to the far side. The horizontal scale thus 

 extends in both directions from, zero. On fairly level ground, this 

 second reading is sometimes omitted, providing the height of the eye 

 above the base of tree is regarded as a constant and added for total 

 height. For accurate measurements both readings must be taken. 



Practice has demonstrated that the use of a plumb-bob and weight 

 reduces the serviceable character of the instrument, since the seweights 

 are easily lost and the strings broken. The mirrors also are easily 

 damaged. 



Weise Hypsometer. The Weise hypsometer (Fig. 49) is the same 

 in principle as the Faustmann but substitutes a metal pendulum for 



