io8 ANIMALS AT WORK AND PL A Y 



be accounted for by the action of the same laws as 

 the flying of a kite. The wind, striking the in- 

 clined face of the kite, divides into two forces, one 

 parallel, the other at right angles to the face. The 

 last is again resolved into two forces, one of which 

 acts vertically, the other horizontally. The horizontal 

 force, would cause it to drift, but it is checked by 

 the action of the string. The other is an upward 

 force and drives the kite aloft. So, when the bird 

 sets its wings at an angle to the wind, the horizontal 

 force of the wind current should make it drift to 

 windward. But the bird resists this drift by its 

 inertia, and only yields to it gradually, and at a 

 uniform rate of increase. Moreover, by the form 

 of its body and the angle at which it presents its 

 wings to the blast, it offers very little surface for 

 this horizontal force to act upon ; at the same time, 

 it presents all the surface possible to the second, or 

 upward, force of the wind. Its tendency then is to 

 rise much, and to drift backward little and gradually. 

 The bird thus gains greatly in height, and loses a 

 little in progress, until the wind drops somewhat- 

 variations in the force of the wind are a postulate of 

 the theory which most people who have flown a kite 

 will be disposed to grant. The bird then changes 

 the angle of its wings, and glides forward and down- 

 wards with all the added force gained by its ascent, 



