9 



feet, the distance between the poles 2 feet; then the height of the tree 



6x30 

 equals — o — =90 feet. 



Another simple method, where possible, is to measure the shadow of 

 the tree and of a pole or man, when the unknown height [h) of the tree 

 is in the same ratio to the known length of its shadow {s) as the length 

 of the pole (_p) to that of its shadow (ps), both of which are also knowu j 



that is to say, the height is equal to the product of the tree's shadow 

 and the pole's length divided by the length of the pole's shadow 

 sxp 



(h='^ ). 



V ps J 



There are various instruments for measuring the height of a stand- 

 ing tree, based on the same principles as the first-meutioned simple 

 method. The calculations are usually placed on the scale of the instru- 

 ment and the height can be read off at once. The simplest one is a 

 right-angled isosceles triangle, which may easily be made of paste- 

 board or wood. In using this triangle the observer should select a 

 spot on the same level with the base of the tree at a distance approxi- 

 mately equal to the height of the tree (fig. 2). 



