II] OF NECESSARY SPEED 45 



That is to say, the larger machine must be capable of a speed of 

 40 X 1-414, or about 56|, miles per hour. 



An arrow is a somewhat rudimentary flying-machine; but it is 

 capable, to a certain extent and at a high velocity, of acquiring 

 "stability," and hence of actual flight after the fashion of an aero- 

 plane; the duration and consequent range of its trajectory are 

 vastly superior to those of a bullet of the same initial velocity. 

 Coming back to our birds, and again comparing the ostrich with 

 the sparrow, we find we know little or nothing about the actual 

 speed of the latter ; but the minimal speed of the swift is estimated 

 at 100 ft. per second, or even more — say 70 miles an hour. We 

 shall be on the safe side, and perhaps not far wrong, to take 20 miles 

 an hour as the sparrow's minimal speed; and it would then follow 

 that the ostrich, of 25 times the sparrow's linear dimensions, would 

 have to fly (if it flew at all) with a minimum velocity of 5 x 20, 

 or 100 miles an hour*. 



The same principle of necessary speed, or the inevitable relation 

 between the dimensions of a flying object and the minimum velocity 

 at which its flight is stable, accounts for a considerable number of 



* Birds have an ordinary and a forced speed. Meinertzhagen puts the ordinary 

 flight of the swift at 68 m.p.h., which talhes with the old estimate of Athanasius 

 Kircher (Physiologia, ed. 1680, p. 65) of 100 ft. per second for the swallow. Abel 

 Chapman {Retrospect, 1928, ch. xiv) puts the gliding or swooping flight of the swift 

 at over 150 m.p.h., and that of the griffon vulture at 180 m.p.h.; but these skilled 

 fliers doubtless far exceed the necessary minimal speeds which we are speaking of. 

 An airman flying at 70 m.p.h. has seen a golden eagle fly past him easily; but 

 even this speed is exceptional. Several observers agree in giving 50 m.p.h. for 

 grouse and woodcock, and 30 m.p.h. for starling, chaffinch, quail and crow. A 

 migrating flock of lapwing travelled at 41 m.p.h., ten or twelve miles more than 

 the usual speed of the single bird. Lanchester, on theoretical considerations, 

 estimates the speed of the herring gull at 26 m.p.h., and of the albatross at about 

 34 miles. A tern, a very skilful flier, was seen to fly as slowly as 15 m.p.h. 

 A hornet or a large dragonfly may reach 14 or 18 m.p.h.; but for most insects 

 2-4 metres per sec, say 4-9 m.p.h., is a common speed (cf, A. Magnan, Vol. 

 des Insectes, 1834, p. 72). The larger diptera are very swift, but their speed is much 

 exaggerated. A deerfly (Cephenomyia) has been said to fly at 400 yards per second, 

 or say 800 m.p.h., an impossible velocity (Irving Langmuir, Science, March 11, 1938). 

 It would mean a pressure on the fly's head of half an atmosphere, probably enough 

 to crush the fly; to maintain it would take half a horsepower; and this would need 

 a food- consumption of 1^ times the fly's weight per second\ 25 m.p.h. is a more 

 reasonable estimate. The naturalist should not forget, though it does not touch 

 our present argument, that the aeroplane is built to the pattern of a beetle rather 

 than of a bird; for the elytra 'are not wings but planes. Cf. int. ah, P. Amans, 

 Geometrie. . .des ailes rigides, C.R. Assoc. Frang. pour Vavancem. des Sc. 1901. 



