JVBW LIGHTS OJV THE PROBLEM OF FLYING. 749 



currents must be greater than this small, amount. As the bird is 

 immersed in the current, and if he uses the whole available dif- 

 ferential force in rising, none is left over for progress against the 

 wind. He therefore drifts with the wind. 



I have spoken thus far only of differential currents in altitude, 

 for these can always be depended upon, but there may be also dif- 

 ferential currents side by side. These might be utilized in the 

 same way. The same may be said also of differential currents in 

 time i. e., successive puffs or gusts. The bird may take advan- 

 tage of these. If so, he would slope down with the gust and come 

 back and rise in the interval. 



5. Sailing. Many large birds fly with alternate intervals of 

 flapping and sailing. But in such cases the bird always loses 

 either velocity or height during the sail, which it recovers only 

 by flapping. There is nothing remarkable in this. But some sea 

 birds which live almost continuously on the wing and usually in 

 a high wind, acquire an almost incredible expertness in the use of 

 the wings as an aeroplane, and sometimes go for hours and over 

 many miles of space without flapping once. The most wonderful 

 bird in this regard is probably the albatross. On several voyages 

 from. Oregon to San Francisco I have watched these birds with 

 their long, narrow wings rigidly extended, skimming the surface 

 of the sea, then rising and wheeling and swooping, and again 

 skimming, but without moving a feather for hours. I will briefly 

 describe the i^henomenon as I have seen it. The explanation will 

 be brought out as I proceed, 



I will suppose a wind aft, as was the case in most of my obser- 

 vations. The bird follows the boat, skimming the very surface of 

 the sea perhaps for several hours ; then, finding that he is losing 

 ground, wheels about, facing the wiild, shoots up to forty or fifty 

 feet above the sea, then turns again with the wind, swoops down 

 a steep incline, acquiring great speed both by the high velocity of 

 the upper stratum and by the descent, then skims the surface 

 again, and quickly overtakes the boat, to repeat the same evolu- 

 tion. With head wind these evolutions are more frequent. As 

 before, he skims the surface behind the boat, but more quickly 

 begins to lose ground; then rises and then wheels and swoops 

 downward, leaving the boat ; then, having acquired the necessary 

 velocity, he again turns and skims the surface in air of small 

 velocity, and in spite of head wind overtakes the boat, to repeat 

 the same evolution. For hours these evolutions are repeated, the 

 wings remaining motionless, with only varying position toward 

 the wind. 



Here, again, the bird takes advantage of the great difference of 

 velocity between the lower and the upper strata of the air. Doubt- 

 less, also, with head wind, advantage is taken of eddies in the wake 



