MOTIVE POWER 41 



til] we lose almost all that we have gained. But the 

 bird has to deal with a material that seems far more 

 shifty and ^independable than small pebbles or 

 volcanic ashes, or than snow at its worst. He over- 

 comes the difficulty by means of levers calculated 

 to give the utmost rapidity of movement. The 

 muscular effort required is great, but his muscles 

 are strong, and it is long before they tire. 



Propulsion. — Phases of the Wing-stroke. 



But the bird has not only to lift himself, or to 

 maintain the altitude he has already gained. He 

 must also have onward momentum. Were this 

 wanting, he could not even lift himself, for air has 

 little or no supporting power when it has just 

 been disturbed. He must, therefore, be perpetually 

 advancing to fresh columns of air that have not yet 

 been shattered by the beating of his wings. I have 

 already pointed out that, when birds are flying in 

 flocks, each takes care to keep clear of the backwash 

 of the bird in front of him — takes care to avoid tracts 

 of air that have already been disturbed ; that a 

 Pigeon, when he has a string tied to his leg, cannot 

 maintain himself in air, however wildly he may ply 

 his wings, when he has reached the end of his tether ; 

 that when a Kestrel hovers without advancing there 

 is always a breeze, so that each wing-beat descends 

 on fresh, unbattered air. For ordinary flight the 

 wings must be so adjusted as to propel as well as 

 lift. This the bird can effect only if the front part 

 of the wing is lower than the hinder part. Thus the 

 parallelogram of forces comes once more to his aid. 



Let f b (fig. 16) represent a section through the 



