II] OF FLIGHT 43 



fish or the dolphin swims, and how the bird flies, are up to a certain 

 point analogous problems ; and stream-lining plays an essential part 

 in both. But the bird is much heavier than the air, and the fish 

 has much the same density as the water, so that the problem of 

 keeping afloat or aloft is negligible in the one, and all-important in 

 the other. Furthermore, the one fluid is highly compressible, and 

 the other (to all intents and purposes) incompressible ; and it is this 

 very difference which the bird, or the aeroplane, takes special 

 advantage of, and which helps, or even enables, it to fly. 



It remains as true as ever that a bird, in order to counteract 

 gravity, must cause air to move downward and obtains an upward 

 reaction thereby. But the air displaced downward beneath the 

 wing accounts for a small and varying part, perhaps a third perhaps 

 a good deal less, of the whole force derived; and the rest is generated 

 above the wing, in a less simple way. For, as the air streams past 

 the slightly sloping wing, as smoothly as the stream-lined form 

 and polished surface permit, it swirls round the front or "leading" 

 edge*, and then streams swiftly over the upper surface of the wing; 

 while it passes comparatively slowly, checked by the opposing slope 

 of the wing, across the lower side. And this is as much as to say 

 that it tends to be compressed below and rarefied above; in other 

 words, that a partial vacuum is formed above the wing and follows 

 it wherever it goes, so long as the stream-lining of the wing and its 

 angle of incidence are suitable, and so long as the bird travels fast 

 enough through the air. 



The bird's weight is exerting a downward force upon the air, in 

 one way just as in the other; and we can imagine a barometer 

 delicate enough to shew and measure it as the bird flies overhead. 

 But to calculate that force we should have to consider a multitude 

 of component elements; we should have to deal with the stream- 

 lined tubes of flow above and below, and the eddies round the fore- 

 edge of the wing and elsewhere; and the calculation which was too 

 simple before now becomes insuperably difficult. But the principle 

 of necessary speed remains as true as ever. The bigger the bird 



* The arched form, or "dipping front edge" of the wing, and its use in causing 

 a vacuum above, were first recognised by Mr H. F. Phillips, who put the idea into 

 a patent in 1884. The facts were discovered independently, and soon afterwards, 

 both by Lilienthal and Lanchester. 



