22 INSTRUMENTS USED IN MENSURATION. 



piece (E) at the other. (2) A scale fastened longitudinally to 

 the tube (called the height scale, H, Fig. 14) ; it is toothed 

 on one side, and has the zero point some distance from its 

 end. (8) A second scale, D, moving at the zero point of 

 the height scale and at right angles to it (called the distance 

 scale). From the upper or zero point of this scale depends a 

 plumb line P. 



OP 

 Fig. 14. Weise's Hypsometer. 



When not used, the distance scale and plumb line are kept 

 in the tube. * 



In using the instrument, a position is chosen from which 

 both the top and foot of the tree can be seen ; then the hori- 

 zontal distance from the point of observation to the tree is 

 measured, and the distance scale drawn out until it indicates 

 at the zero point of the height scale the number of units in the 

 distance ; then the tube is raised and directed towards the 

 top of the tree, taking care that the up and down line of the 

 objective keeps a vertical position. As soon as the horizontal 

 line of the cross covers the top of the tree, the tube is gently 

 turned from left to right, thereby causing the plumb line, which 

 hitherto swung free, to be caught by the toothed edge of the 

 height scale. The instrument is then taken down and the 

 number of units, from the zero point to the point where the 

 plumb line was arrested, read off. This number gives the 



