416 THE EMPIRE OF THE AIR. 



Near the " Abbassich" gate at Cairo, amid inoimtain.s of potsherds, 

 dead animals are deposited by the scavengers. They are not buried; 

 there is no need, for between vagrant dogs and rapacious birds, they 

 are eaten up in a few hours. As soon as the carcass has been laid 

 down and the knackers have finished skinning it, carniverous birds 

 appear. They pass in the zenitli of the observer and arrive at their 

 destination, all the time visible through the telescope. The distance is 

 known, and only the time need be noted which is consumed in the 

 journey. 



This trip is performed with the same velocity by the three birds 

 named, but the force expended is evidently much less when the mass 

 is great. The actual speed, of course, varies with the velocity and 

 direction of the Avind. 



As a final result, deduced from such daily observations, I think that 



I closely approximate the fact when I state that a kite, soaring to 

 survey the hunting ground beneath him, has a mean ])roper speed of 



II miles an hour when the wind blows also 11 miles an hour ; this is the 

 sailing bird which seems able to obtain support with the feeblest cur- 

 rent of air. The tawny vultures, in order to rise on such velocity of 

 breeze, need to unfurl their entire i)ossible wing surface. For them it 

 seems that the wind velocity should be at least 17 mihvs an hour to be 

 in full accord with their faculties. 



Effect of speed. — Theorists frequently set themselves this problem: 

 What is the power required to obtain support in the air! 



The lifting force, (ascensional power if you please so to call it), is 

 under many circumstances so slight that it may be neglected, and 

 is reduced to the force necessary to sustain the aj)paral:us. 



In soaring flight this ascensional power is only indispensable when 

 there is no wind. The problem would be better stated thus: What 

 velocity must be imparted to an aeroplane, bird or nuichine, in order 

 that it may be sustained on the air and may rise ? Xow for this, as 

 for all aspects of this ])roblem, we find a solution provided by nature. 



Birds whose pectoral nniscles have not power to raise them bodily 

 are not rare. Sailing birds can, unaided, compass but little rise; 

 especially the very large birds. A tawnj^ vulture can not rise 20 yards 

 on a start of 45°; lie can not rise 10 yards vertically. So this king of 

 soarers may be kept a prisoner in a roofless cage, ])rovided the sides or 

 walls are 20 yards high and 20 yards apart. Among birds with nar- 

 row wings this peculiarity is still more marked. 



The Swift, this wild, darting, rustic inhabitant of the air, can not 

 rise vertically G feet. It is perfectly caged in a large box without a 

 cover, and yet if any creature is thoroughly eciuipped for flight it is he. 

 The same is true of the large sea birds. A frigate bird is impotent 

 with less than a certain spaee to perform its evolutions, whereas as soon 

 as the two birds I have named have acquired speed, or are launched in 

 a current of air (which amounts to the same thing), they become forth- 



