[August, 
and when motion at length occurs, it is sufficiently rapid to make 
up for lost time. A non-elastic fluid would not present this rapid 
motion on the compressing surfaces. If we increase the pendant 
weight and take the plane to the lantern of a lighthouse, or 
very high building, we shall find that its lateral motion will be 
greater, and every addition of weight will heighten the lateral 
velocity in a greater ratio than the fall. The reason is that the 
plane meets with small resistance to its edgeways motion, so that 
a slight increase of force in this direction moves it rapidly. But 
however great this motion may be, however closely it may ap- 
proach level translation, by no means could it become horizontal 
as long as the plane itself was level. The entire force producing 
lateral motion comes from motion in the direction of gravity 
down the vertical, and horizontal translation would stop the fall, 
thus cutting off the motive power. But if we slant the plane the 
vertical goes over with it. Its own activity determines the direc- 
tien of its vertical. It is fluid pressures we are dealing with, and 
what would be level translation on our horizontal is a constant 
fall from the bird’s horizontal, and a soaring bird certainly needs 
: no further explanation, neither does a wild duck. Itis not heavy 
= enough relatively to the size of its wings to reach a uniform 
velocity of fall which would produce the thrust needed in the lat- 
eral motion, and resort to flapping is compelled to aid the normal 
_ component of gravity. The wing now comes down with velocity 
= sufficient to give the thrust required for horizontal flight. But 
_ both soaring and flapping would be impotent without the resolu- 
_ tion of the direction of gravity. 
_ The moment the plane is inclined it becomes necessary to 
_ counteract the component of resolved gravity acting in it, op the 
706 The Wings of Birds. 
ous man could move his limbs at a higher speed than mint 
feet per second for a short time at his utmost exertion. a 
planes greatly exceed this velocity in the lateral motio®, © 
