46 ON MAGNITUDE [ch, 



observed phenomena. It tells us why the larger birds have a 

 marked difficulty in rising from the ground, that is to say, in 

 acquiring to begin with the horizontal velocity necessary for their 

 support; and why accordingly, as Mouillard* and others have 

 observed, the heavier birds, even those weighing no more than a 

 pound or two, can be effectually caged in small enclosures open 

 to the sky. It explains why, as Mr Abel Chapman says, "all 

 ponderous birds, wild swans and geese, great bustard and caper- 

 cailzie, even blackcock, fly faster than they appear to do," while 

 "light-built types with a big wing-areaf, such as herons and harriers, 

 possess no turn of speed at all." For the fact is that the heavy 

 birds must fly quickly, or not at all. It tells us why very small 

 birds, especially those as small as humming-birds, and a fortiori the 

 still smaller insects, are capable of "stationary flight," a very slight 

 and scarcely perceptible velocity relatively to the air being sufficient 

 for their support and stabihty. And again, since it is in all 

 these cases velocity relatively to the air which we are speaking 

 of, we comprehend the reason why one may always tell which 

 way the wind blows by watching the direction in which a bird starts 

 to fly. 



The wing of a bird or insect, like the tail of a fish or the blade 

 of an oar, gives rise at each impulsion to a swirl or vortex, which 

 tends (so to speak) to chng to it and travel along with it;' and the 

 resistance which wing or oar encounter comes much more from 

 these vortices than from the viscosity of the fluid. J We learn as a 

 corollary to this, that vortices form only at the edge of oar or wing — 

 it is only the length and not the breadth of these which matters. 

 A long narrow oar outpaces a broad one, and the efficiency of the 

 long, narrow wing of albatross, swift or hawkmoth is so far accounted 

 for. From the length of the wing we can calculate approximately 

 its rate of swing, and more conjecturally the dimensions of each 

 vortex, and finally the resistance or Ufting power of the stroke; 

 and the result shews once again the advantages of the small-scale 



* Mouillard, L' empire de Vair; essai d'ornithologie appliqu4e a Vaviationf 1881; 

 transl. in Annual Report of the Smithsonian Institution^ 1892. 



t On wing-area in relation to weight of bird see Lendenfeld in Naturw. Wochenschr. 

 Nov. 1904, transl. in Smithsonian Inst. Rep. 1904; also E. H. Hankin, Animal 

 Flight, 1913; etc. 



X Cf. V. Bjerknes, Hydrodynamique physique, n, p. 293, 1934. 



