OH. V] MECHANICAL RECREATIONS 107 



incessant occurrence of gusts of wind separated by lulls, and 

 Dr Bryan's of vortices in the atmosphere. 



It now seems reasonably certain that the second and third of 

 these sources of energy account for at least a portion of the 

 observed phenomena. The effect of the third cause may be 

 partially explained by noting that the centre of gravity of the 

 bird with extended wings is slightly below the aeroplane or wing 

 surface, so that the animal forms a sort of parachute. The effect 

 of a sudden gust of wind upon such a body is that the aeroplane 

 is set in motion more rapidly than the suspended mass, causing 

 the structure to heel over so as to receive the wind on the under 

 surface of the aeroplane, and this lifts the suspended mass 

 giving it an upward velocity. When the wind falls the greater 

 inertia of the mass carries it on upwards causing the aeroplane 

 to again present its under side to the air; and if while the 

 parachute is in this position the wind is still blowing from the 

 side, the suspended mass is again lifted. Thus the more the 

 bird is blown about, the more it rises in the air ; actually birds 

 in flight are carried up by a sudden side gust of wind as we 

 should expect from this theory. 



The fact that the bird is in motion tends also to keep it up, 

 for it has been recently shown that a horizontal plane under 

 the action of gravity falls to the ground more slowly if it is 

 travelling through the air with horizontal velocity than it would 

 do if allowed to fall vertically, hence the bird's forward motion 

 causes it to fall through a smaller height between successive 

 gusts of wind than it would do if it were at rest. Moreover it 

 has been proved experimentally that the horse-power required 

 to support a body in horizontal flight by means of an aeroplane 

 is less for high than for low speeds: hence when a side-wind 

 (that is, a wind at right angles to the bird's course) strikes the 

 bird, the lift is increased in consequence of the bird's forward 

 velocity. 



CURIOSA PHYSICA. When I was writing the first edition 

 of these Recreations, I put together a chapter, following this 

 one, on " Some Physical Questions," dealing with problems such 

 as, in the Theory of Sound, the explanation of the fact that in 



