MOTIVE POWER 4] 
till we lose almost all that we have gained. But the 
bird has to deal with a material that seems far more 
shifty and undependable than small pebbles or 
volcanic ashes, or than snow atits 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 
