II] THE PRINCIPLE OF SIMILITUDE 27 



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

 dimensions, would be compelled to fly (if it flew at all) with 

 a minimum velocity of 5 x 14, or 70 miles an hour. 



The same principle of necessary S'peed, or the indispensable 

 relation between the dimensions of a flying object and the minimum 

 velocity at which it is stable, accounts for a great number of 

 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 effectively "caged" in a small enclosure 

 open to the sky. 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 sHght and 

 scarcely perceptible velocity relatively to the air being sufficient for 

 their support and stability. And again, since it is in all cases 

 velocity relative to the air that 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. 



It is not improbable that the ostrich has already reached 

 a magnitude, and we may take it for certain that the moa did 

 so, at which flight by muscular action, according to the normal 

 anatomy of a bird, has become physiologically impossible. The 

 same reasoning applies to the case of man. It would be very 

 difficult, and probably absolutely impossible, for a bird to fly 

 were it the bigness of a man. But Borelli, in discussing this 

 question, laid even greater stress on the obvious fact that a man's 

 pectoral muscles are so immensely less in proportion than those 

 of a bird, that however we may fit ourselves with wings we can 

 never expect to move them by any power of our own relatively 

 weaker muscles ; so it is that artificial flight only became possible 

 when an engine was devised whose efficiency was extraordinarily 

 great in comparison with its weight and size. 



Had Leonardo da Vinci known what GaHleo knew, he would 

 not have spent a great part of his life on vain efforts to make to 

 himself wings. Borelli had learned the lesson thoroughly, and 

 * Cf. U empire de Vair ; oniitkologie appliquee a Vaviation. 1881. 



