12 THE POPULAR SCIENCE MONTHLY. 



We see that the gnat, one of the lightest of insects, has an expanse 

 of wing uf no less than 48-9 square feet for each pound of weight, 

 while the heavy cockchafer has only 5*1 square feet for each pound. 

 With birds, the sparrow has 2*7 square feet of wing-surface for each 

 pound of weight, while the great Australian crane has only 0"41 of a 

 square foot, and yet this bird undertakes remote journeys, and, the 

 eagle excepted, flies higher, and keeps on the wing longest, of all the 

 travelers. 



It would appear, then, that our flying-machine, while heavy, need 

 not necessarily have a very broad expanse of flying surface. Indeed, 

 l)aradoxical as it may seem, weight is really an essential feature. Set 

 in motion by muscular effort, the weight of a bird acts somewhat like 

 the fly-wheel of an engine : the power is stored up during the down- 

 ward stroke of the wing, to be given out again on its upward stroke, 

 and ])robably it is weight also that enables the bird to successfully 

 combat and take advantage of the force of the wind. It is noteworthy 

 that all sailing-birds, like the hawk or vulture, have comparatively 

 heavy bodies. Tlie magnificent albatross, in rising from the water, is 

 said to beat the air with great energy, but, when fairly launched, in a 

 brisk gale, will sweep around in broad circles for hours together, hardly 

 ever deigning to flap a wing. Darwin, in his " Voyage of the Beagle," 

 speaks of watching the condor sailing in a similar way at a great height, 

 without, so far as he could notice, any flapping action whatever. 



At the same time, it is hard to understand how such a condition of 

 affairs could exist. The condor's wings, inclined to the wind, have 

 been compared to a kite, and if there were a string stretching from 

 the bird to some fixed point, the whole thing would be clear ; but every 

 boy knows to his cost that, if the string slips or breaks, the kite 

 quickly seeks some other point of support — probably a telegraph-wire. 

 But Professor Pettigrew has suggested that the string is the invisible 

 one representing the attraction of gravitation, and that ''the string 

 and the hand are to the kite what the weight of the flying creature is 

 to the inclined planes formed by its wings." This, however, does not 

 make the matter much clearer, for the force of gravity acts in vertical 

 lines, and a vertical kite-string, with the kite flying directly overhead, 

 is a thing, it is safe to say, no boy ever saw. Why should not our 

 bird drift with the wind unless he uses some muscular effort to over- 

 come its force or to keep himself from falling ? 



Once elevated, he can utilize his weight in a number of ways. 

 A body will naturally fall along a line of least resistance, and if the 

 front edge of the wings be tipped slightly downward the bird will 

 glide forward while falling, gaining velocity and momentum ; and 

 then, by reversing the inclination of the wings, he can again glide up 

 an aerial incline until this stored-up energy has been expended. But 

 the resistance of the air must be overcome, and there must be con- 

 tinual loss from the imperfect sustaining power of the wings. 



