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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 dif- 

 ferent, 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 



boat, of the wind, and of the 

 bird. In the circle the strong 

 line represents the higher por- 

 tion of the circular sweeps and 

 the light line the lower portion, 

 almost in contact with the wa- 

 ter'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 spi- 

 ral, 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 b^en visible, I have no doubt the explana- 

 tion 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 aeroplane, why can not we by arti- 

 ficial means i. e., by a machine do the same ? The article before 

 referred to was written to show why we can not. 



Fig. 4. Arrows show Direction of Wind, 

 OF THE Boat, and of the Circling Bird, 



