204 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



mention an easy and interesting application of the laws of reflection. 

 Its object is to measure the height of objects such as trees, houses, 

 edifices, etc. A small plane mirror (Fig. 143) is placed on the ground 

 in a horizontal position between the eye and the object to be measured. 

 Then, on drawing further away in the direction of the line which 

 connects the base of the object and the mirror, the image A' of the 



FIG. 141. Window mirror, or espion. 



summit A is seen. It is easy to understand that at this moment the 

 ratio of the height of the eye above the plane I to the height of the 

 top A of the tree is exactly that of the horizontal distance 10 to the 

 horizontal distance of the foot of the tree at 0. An easy calculation 

 then gives the required height. 



In the Forces of Nature we have described various ingenious appli- 



