56 THE FLIGHT OF BIRDS 



is more conspicuous. The big bird, on the other 

 hand, dare not lose altitude in so reckless a way, 

 since when he has once lost it he with difficulty 

 recovers it. He ploughs steadily on, whereas the 

 small bird with a few rapid strokes gains altitude, 

 then, like a bicyclist utilising his free wheel, glides 

 restfully and rapidly onward, not minding the loss 

 of some of the height he had gained. His stroke 

 is more rapid, but he is able to take frequent 

 easies.* 



I have now compared and contrasted big birds 

 and small, but even the biggest birds that fly are 

 not very big ; as compared with the larger mammals 

 they are diminutive. Why, among all the birds 

 that fly, are there none that weigh even half as 

 much as, for example, a Zebra ? 



Helmholtz, dealing with this subject, produced a 

 formula which, backed as it was by the authority of 

 a great man, was too readily accepted, regardless of 

 the fact that it was not founded on data obtained 

 by experiment. There is nothing so misleading 

 as mathematics when the premises are unsound. 

 Helmholtz started with undeniable facts. If a bird's 

 linear dimensions be multiplied by 4, then the area 

 is multiplied by 16 (4 2 ), and the bulk, which must 

 nearly correspond to the weight, by 64 (4 3 ). So far 

 good. He showed that the weight increased more 

 rapidly than the supporting area. But when he 

 went on to maintain that the power required to lift 

 the bird increased at a still more rapid rate — that, in 

 the case I have taken, it would be 128 (4i) — then he 

 was building a theory without a proper foundation 



* On big birds and small birds see Chap, i, pp. 18-22. 



