WEISE S HYPSOMETER. 23 



number of feet (or yards as the case may be) from the hori- 

 zontal 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 the observer and the foot of the tree, which 

 is obtained in the same way, by directing the tube towards the 

 foot of the tree, reading the height on the prolongation of the 

 scale towards 0. 



The theory of the instrument rests upon the similarity of 



Fig. 15. 



the triangles with the sides R H D and r h d; that is to 

 say, the following equation holds good : 



d : h = D : H 

 and 



If therefore the units of the scales, which give h and d, are 

 of the same size, and d is so fixed that its units are the same 

 number as the units of the measured distance D, it follows 

 that the above formula gives the height. 



Christens Instrument. [t consists of a piece of metal (see 

 Fig. 16) with protruding upper and lower edges (see a and I). 

 The instrument is based upon the theory explained on page 

 17, which avoids the measurement of a base line. A staff 

 of known length = I, say 4 yards, is placed alongside the foot 

 of the tree. The instrument is then held in a vertical position 



