ORNITHOLOGICAL OBSERVATIONS 275 



surface will be not only to check the onward motion 

 of the bird, but also to raise it in the air. The 

 mechanism seems much the same as that by which a 

 child's kite is enabled to rise when an additional 

 momentum is given to it by running onward with the 

 line. The bird develops its fresh store of momentum 

 by sweeping round to leeward and then coming up to 

 windward ; the child develops its momentum in the 

 kite by pulling rapidly on the line. But as the bird is 

 lifted upward by the wind, so at the same time does 

 its rate of motion rapidly diminish and at length 

 becomes so slow that the bird can no longer face the 

 wind. Its momentum is exhausted and the kite can 

 in this circle gain no more in height, for without 

 momentum it can do nothing. It must turn away to 

 leeward, develop a new store of energy in its wheel 

 and come up again to windward with increasing speed. 

 It has renewed its store of momentum ; it again 

 presents its inclined wings and body to the opposing 

 breeze and repeats the windward rise. 



If the kite can ascend only when circling to wind- 

 ward, it follows that, in the absence of wind, the bird 

 is unable to circle with gain in height. It can certainly 

 sweep round in circles on the calmest day, and may 

 even use its momentum to make a short temporary 

 rise ; but without wind, it is quite unable to make a 

 permanent addition to its altitude in each successive 

 circle, and must of necessity flap its wings if it wishes 

 to rise higher into the sky. A bird circling in the 

 absence of wind is all the time sinking ; at the end of 

 each circle it is a little nearer the ground than at the 

 end of the preceding circle, and as it sinks lower and 

 lower it will have to abandon its passive circling and 



