MECHANICAL PARADOXES. 



the air beneath to let it fall further ; whereas 

 when the bird is gliding rapidly forward, the 

 air above it is previously undisturbed by itself, 

 and there is only its own weight to press upon 

 the supporting air beneath. 



With such a gliding movement, therefore, 

 the wings may be quite motionless, and the 

 falling of the bird will be extremely small. 

 Cases have been observed, however, where 



no falling could 

 be noticed at 

 all, and no gliding 

 apparently took 

 place. 



These observa- 

 tions are of a na- 

 ture that makes it 

 difficult to verify 

 them exactly. At a 

 great height small 

 differences of 

 height and con- 

 siderable changes of position can easily escape 

 notice. But for cases where there really is no 

 motion of the bird, the following explanation is 

 quite adequate. 



The behaviour of clouds, and facts observed 

 by means of balloons and kites, show that 

 there are many movements of the upper air 

 not represented by corresponding movements 

 of the air beneath. A moderate breeze in the 



72 



FlQ. 18.-SOARING OF LARGE BIRDS 

 IDENTICAL WITH GLIDING. 



