274 A NATURALIST IN HIMALAYA 



speed, rests horizontally on Its pinions and descends 

 slightly to the earth. Thus the windward motions 

 are the reverse of the leeward motions. The bird's 

 objective is increase in altitude. To windward it 

 gains, to leeward it loses its object, but in a moderate 

 breeze the gain up wind is greater than the loss down 

 wind, and the difference between the two is the measure 

 of the bird's success. 



But why this extraordinary contrast ? Why free 

 gliding to leeward ? why restraint against the wind ? 

 Why is there a lessened speed, an inclined body, and 

 an ascent on one side of the circle and the reverse on 

 the other side ? I believe that the contrast is due to 

 the fact that to leeward the object of the bird is to 

 develop speed, and to windward to convert the energy 

 of that speed into a fresh gain in height. Everything 

 to leeward favours the bird in its accumulation of 

 speed. Its longer and wider sweep, its horizontal 

 attitude, its gradual descent and the fair breeze 

 streaming through its feathers all combine to give it 

 fresh momentum and a new store of energy for the 

 windward rise. 



Nor does it appear difficult to undert^nd the 

 mechanism by which an opposing wind produces an 

 increase in the altitude of a bird already possessing 

 momentum, or, in other words, the method by which 

 the kinetic energy of the leeward speed is converted 

 into the potential energy of the windward height. 

 For each time the kite comes round to the wind it 

 faces a greater obstruction to its forward flight ; but as 

 it raises the fore end of its body and depresses the 

 hind end so as to offer an inclined surface to the wind, 

 then the effect of the wind actin^- against this inclined 



