of the boat. In all cases, if the wind slackens, the bird flaps now and then. If it stops, he flaps all the time. It can not be done in still air.
With the wind on the side the evolution must of course be different, but, as I have not watched this case, I will describe it as given by a writer in Nature (Nature, vol. xliii, p. 223, 1891).
Fig. 4 represents the stern of the boat and the circle of the bird's path. The arrows show the directions of the motion of the 
Fig. 4.—Arrows show Direction of Wind, of the Boat, and of the Circling Bird. boat, of the wind, and of the bird. In the circle the strong line represents the higher portion of the circular sweeps and the light line the lower portion, almost in contact with the water's surface. It is seen, then, that the bird swoops down with the wind, skims the surface, and then rises against the wind. The sweep of the bird is here represented as a circle, because the boat is standing still; but really it is an advancing spiral, following the moving boat. The explanation here is exactly the same as in soaring, except that the differential force of the air currents is utilized in progress instead of in rising.
One more example: On the ferry boat going across the bay to San Francisco, with strong wind ahead and a little to the right (i. e., coming through the Golden Gate), I have several times seen a gull place himself behind on the left, just opposite the hind deck, and maintain his position with motionless wings for a half mile or more—I say motionless with confidence, because he was so near that I could see his eyes wink.
In this case I feel sure that the motion of the boat created an eddy in which the air was still or perhaps moving in contrary direction—i. e., with the boat, and perhaps also a little upward. If the air currents had been visible, I have no doubt the explanation would have been obvious. What I could not see, the bird felt and skillfully utilized.
Now, if birds, even the largest and heaviest of them, can thus play and gambol in the fiercest wind with the greatest ease and grace, and without serious expenditure of energy, but only by the skillful use of wings as an aëroplane, why can not we by artificial means—i. e., by a machine—do the same? The article before referred to was written to show why we can not.
