FLIGHT 79 



is some internal work in keeping the wings outstretched 

 and taut. 



(4) In going down the wind the bird sinks a little ; in 

 going up into the wind it rises again, and it may rise higher 

 than it was before. Moreover, the albatross not only circles 

 round the ship but it keeps pace with it. In all probabiHty 

 the energy of locomotion acquired by increased velocity 

 when sailing with the wind is changed into energy of 

 position as it rises with reduced velocity against the wind. 



(5) The theory that the bird takes advantage of strong 

 upward currents may probably be ruled out, since it does not 

 readily account for horizontal progress and since sailing 

 has been clearly demonstrated while light objects like feathers 

 sank slowly in the air. But our knowledge of the currents 

 in the air is still very young. 



When a gull flying against the wind from the fields 

 shorewards comes just above the edge of the cliffs, it often 

 meets a strong upward current, caused by the cliff diverting 

 the breeze from the sea. In such a case the gull often 

 rests on its oars ; it not only rises, but continues its previous 

 course without further strokes of its wings. But this 

 gliding continues for a very limited time and it is not com- 

 parable to true sailing ; the gull is taking advantage of a 

 considerable velocity attained by previous strokes of the 

 wings and of the strong upward current off the cliffs. 



(6) A widely accepted view, supported by Lord Rayleigh 

 and others, is that the sailing bird takes advantage of the 

 unequal velocity of the air at different heights. In its 

 sinking as it sails with the wind the bird will be meeting 

 currents which are more and more retarded by proximity 

 to the surface of the earth. If at the lowest level of its 

 trajectory the bird tilts its body and rises facing the wind, 

 it will meet successive currents of air which are less and less 

 retarded by proximity to the earth ; the air-resistance, 

 which is the lifting force, will be greater than if the air were 

 in uniform movement at all levels. An argument in favour 

 of this theory is that the sailing birds, though moving in 

 great circles or ellipses on the whole horizontal, arc seen to 



