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ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 4 



though how they do so no one has yet managed to explain. Physical 

 research may one day show us that upcurrents of air arise near the 

 leading surfaces of traveling waves, and that the surface gliders are 

 able to use these currents. Or it may be that these birds have solved 

 the problem of gust flying — gaining height when they meet a gust, 

 and losing height again as the gust dies away, rather as a tennis ball 

 can be kept in the air by a succession of small upward hits from a 

 racquet. 





x-v//^ 



X"".:-> 



Figure 8. — The flight of the albatross. Note how the bird makes semicircular turns, climb- 

 ing into the wind and descending. 



All observers agree that an albatross cannot maintain its gliding 

 flight if there is no wind. These great birds seem to rise in a slanting 

 direction against the wind to a height of about 20 feet, depending on 

 the strength and direction of the wind; this climb is followed by a 

 wide semicircular turn as the albatross rapidly descends downwind; 

 and then the cycle is repeated (fig. 8) . As the bird turns into the wind 

 again, its speed relative to the air has been increased and the kinetic 

 energy (energy due to movement) thus gained can be used for gaining 

 height ; this results in a gain in potential energy (energy due to height) 

 which is then available for maintaining the bird's flight downwind; 

 and so it goes on. Notice that this interpretation of the flight of the 

 albatross depends on the existence of an adequate difference between 

 the wind speed at the surface of the sea, and that at a level perhaps 

 20 feet up. 



Gulls behind a steamer give us an example of gliding that is by 

 no means as simple as it may appear. No doubt there is an upward 

 deflection of air by the hull of a ship, and the heating effect of the 

 vessel's smoke stacks may cause certain upward thermal currents, 

 but there is also a mass of air, stationary in relation to the ship, and 

 therefore traveling over the sea at the same speed as the ship. How 

 far the gulls rely on the upcurrents, and how far they glide down 

 in the stationary air, would form an interesting subject of study. 



Stability in flight is of the greatest importance in an airplane; it 

 must be designed so that any slight disturbance in balance between 

 lift, drag, and weight is automatically and immediately corrected. 



