44 MOMENTUM IN FLYING. 



means of resisting the reaction backwards, it might 

 move its wings both long and vigorously without 

 advancing a single inch. That the bird derives the 

 greater part of that hold on the air, which enables it 

 to take as effective a leap from that element as it 

 could from a solid substance, and even more so, 

 directly by the action of the wings, is true ; but 

 there is no doubt that it receives considerable assist- 

 ance from the general muscular action. When 

 birds are on long and smooth flight, they also acquire 

 a momentum in proportion to their velocity, and the 

 difference between their specific gravity and that of 

 the air. In consequence of this momentum, they con- 

 tinue their progressive motion with much less effort ; 

 and the superiority of this momentum, in a rarer 

 atmosphere, may be an additional reason why they fly 

 high upon their long journeys. But there are many 

 birds which proceed by a succession of jerks or 

 leaps, in the pauses of which they are almost or 

 altogether at rest, and these birds can acquire but 

 little momentum, but must renew their whole impulse 

 at every jerk. These are, for the most part, birds 

 of low flight; and it is probable that the greater 

 resistance of the denser atmosphere is as advanta- 

 geous to them as the rarity of the upper strata is to 

 those species which fly with a momentum. Birds of 

 smooth flight also, however, often shoot onward with 

 great rapidity, after having hovered so long over the 

 same spot, as that all the momentum which they ac- 

 quired in arriving at that spot must be exhausted ; 

 and they must take the whole of their new velocities 

 from the resistance of the air. 



The flight of those birds which proceed by jerks 

 affords a good illustration of the fact which has been 

 stated, of there being a buoyant or upward tendency 



