2a 



INSTRT'MENTS T'SED IN IMENSUKATIOK. 



In using the iiistrnment, a position is chosen, from which 

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

 horizontal 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 tlie 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 



i. 





:>?l^'^:- 



Fig. 15. 



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 oft". This number 

 gives the number of feet (or yards, as the case may be) from 

 the horizontal of the eye of the observer to the top of the tree. 

 To this must be added (or deducted) the difference in height 

 between the eye of tlie observer and the foot of the tree, 

 whicl) is obtained in the same way, by directing the tube 

 towards the foot of the tree, and reading the lieight on the 

 prolongation of the scale to^^ards O. An improvement of 

 the instrument would be the addition of a guide for tlie 

 plumb line. 



The theory of the instrument rests upon the similarity of 



