currents must be greater than this small, amount. As the bird is immersed in the current, and if he uses the whole available differential 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 differential 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 advantage 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 aëroplane, 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 phenomenon 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 observations. 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 wind, 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 evolution. 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. Doubtless, also, with head wind, advantage is taken of eddies in the wake