ELISHA MITCHELL SCIENTIFIC SOCIETY. 63 



Then the action of all the compressed air which passes the 

 ulnar edge, and of all which passes forward upon the under sur- 

 face of the wing is to cause forward motion of the bird. 



Thus we see there are two forces which combine to give a 

 bird, with outstretched wings, forward motion : 



1st. Gravity ; 2d. The resistance offered by the wings to the 

 forward movement of the compressed air. The first acts per- 

 pendicularly to the earth; the second is subject to the will of the 

 bird, and may act horizontally, or obliquely toward the earth, or 

 obliquely from it. The resultant, however, when the air is 

 quiet, and the bird has no momentum, except that initiated by 

 gravity, is always toward the earth, though in some cases it may 

 be on a plane diverging only slightly below the horizontal. The 

 resultant from the two forces has somewhat the same effect upon 

 the bird that a string, in the hands of a running boy, has upon 

 a kite in quiet air. The bird does not move in the line of the 

 resultant of the two forces, but on a plane somewhere between 

 the resultant and the horizontal. This being true, the additional 

 force, or forces, necessary to carry the bird on a horizontal plane, 

 or a slightly ascending plane, would be far less than many would 

 think. 



Many times have I watched the Turkey Vultures in soaring 

 flight, when without flapping their wings they would rise several 

 hundred feet. A case came under my observation in which the 

 bird could not possibly have depended upon successively increas- 

 ing velocities in the currents of air to supply the force necessary 

 to permit it to rise in an ascending plane. I stood on a hill, and 

 watched a Vulture which was soaring in the valley. The wind 

 was blowing a brisk breeze, but the configuration of the land 

 was such that it is not probable there was an upward current. 

 The remarkable thing is that the bird did not move off with the 

 wind as it rose, but the spiral course was perpendicular. In 

 three circles it rose two hundred feet above my head and then 

 passed off at a right angle to the direction of the wind. I 

 noticed that the bird slowly rocked first on one side and then on 

 the other, especially when it faced the wind. At the time I 



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