involved in the Sailing Flight of the Albatros. 133 



that HE equals 64. So that if an Albatros starts with a velocity 

 of 115 feet a second, it could maintain a constant height above 

 the sea until its velocity was reduced to 64 feet a second by 

 merely increasing the angle to the horizon at which it was flying 

 from 0° to 7°. 



The velocity of the air in a "fresh sailing-breez©" is about 

 30 feet a second, in a " moderate gale " 60 feet a second, in a 

 "strong gale" 90 feet a second, and in a "great storm" 120 

 feet a second. Now an Albatros can often be seen sailing, 

 though slowly, directly against a strong gale ; his velocity must 

 therefore often be more than 90 feet a second ; he is, however, 

 most at home in a strong breeze or moderate gale, when the ve- 

 locity of the wind is 50 or 60 feet a second, and consequently 

 when his velocity would have to be 70 or 80 feet a second to 

 enable him to fly easily against it. In a calm or light air, when 

 the wind has a velocity of only 10 feet a second, the Albatros 

 rarely sails for so long as a minute at a time — the reason for 

 this being that as, in order to sustain himself in the air, he must 

 move through it with a velocity not less than 64 feet a second, 

 he would, even when flying against the wind, have to travel over 

 the sea at the rate of not less than 54 feet per second, or 36 

 miles an hour, and so could not reach it properly for good, nor 

 stop himself quick enough when he saw anything; so that the 

 velocity and manner of flight observed in the Albatros correspond 

 closely enough with those calculated as necessary from theore- 

 tical considerations. 



We will now proceed to see what the resistance of the air to 

 his forward progress ought to be to enable him to start with a 

 velocity of 115 feet a second and sail for half an hour without 

 flapping his wings, and at the end of that time to have reduced 

 his velocity to 64 feet per second. 



If a body starts with a velocity V, and after moving for t 

 seconds the resistance of the air reduces its velocity to v, it can 

 be shown that 1 _ 1 _ Agk£ 



where W represents the weight of the bird in pounds, A the area 

 of its front surface in square feet, g the force of gravity, and k a 

 constant quantity depending on the form of the surface exposed 

 to the air, and probably on the velocity at which the body moves ; 

 so that, in order to find this constant for the Albatros, we have 



,/i_n w 



\v Y/gtA 



_ W.{Y-v) _ 

 ~~V.v.g.t.A 



