March i, 1889.] 



SCIENCR 



169 



" Hannah Calline's Jim," and an essay on " Simplicity," by Charles 

 Dudley Warner, close the prose articles ; and the poetry includes 

 Mr. Whittier's " The Christmas of 1888," and verses by E. Wilson. 



LETTERS TO THE EDITOR. 



*,' Correspondents are requested to be as brie/ as possible. The writer's 7!ame is 

 in allcases required as proof of ^ood faith. 



Twenty copies of the number containing his communication will be furnished 

 free to any correspondent on request. 



The editor will be glad to publish any queries consonant with the character of 

 the journal. 



The Soaring of Birds. 



Since my paper was reported in Science (xii. p. 267), Messrs. 

 Oliver, Pickering, and MacGregor have favored the journal with 

 letters on the subject. 



Professor Oliver (xiii. p. 15), while admitting that the action I 

 suggest is to some extent efficient in sustaining the bird, questions 

 its sufficiency. I had asked the same question myself, and found 

 no answer; but I am glad to know, through private correspondence, 

 that Professor Oliver and at least one other physicist are disposed 

 to put the question to nature through experimentation. 



Professor Oliver also suggests minute vibratory motions of the 

 wings, or perhaps of the individual wing-feathers, and cites an ob- 

 servation by another. The same hypothesis was advanced in ex- 

 planation of the allied phenomenon of hovering, but was rejected 

 on the strength of what seemed sufficient observation (Nature, viii. 

 p. 324; ix. p. 5). The hovering bird remains in one place for so 

 long a period that he can be deliberately and carefully watched. 



Mr. Pickering and Professor MacGregor each proposes an expla- 

 nation different from mine, and not involving differential air move- 

 ments, but appealing instead to a homogeneous and uniform wind. 

 Their conception of relative velocities is so different from mine, and 

 I am so confident in the correctness of my own, that I am led to 

 suspect I have not made my meaning clear, and I therefore ask the 

 privilege of restatement. 



As I conceive the matter, the horizontal velocity of the bird 

 with reference to the earth has no importance, and should be ig- 

 nored. The earth enters the problem only by means of its attrac- 

 tion. Except for the gravitational pull, we need not consider the 

 bird in relation to any thing except the air. If a cloud intervened 

 between the bird and the earth, so that he could not see the 

 ground, he would not know in which direction the air surrounding 

 him was passing over the earth ; but the possibility of his soaring 

 would not be affected by that ignorance. Provided the air in which 

 the bird floats is not disturbed thereby, his motions would not be 

 affected by the sudden reversal of the direction of rotation of the 

 earth, although such a reversal would enormously change the rela- 

 tive velocity of bird and earth. 



In my analysis of the subject I spoke of winds, that is, currents 

 of air moving horizontally with reference to the earth, because such 

 language afforded me a simple means of expression, and for that 

 reason only. In so doing I took a special case as illustrative of the 

 general case. As this seems to have been misunderstood by some 

 of your correspondents, I will repeat the analysis in more general 

 language. 



Let the line AB represent in section a horizontal plane within the 

 atmosphere. Conceive the body of air above this plane and the 

 body of air below it to have different motions, such that their dif- 

 ferential motion has the direction indicated by the arrows ; that is 



to say, the upper body referred to the lower moves from B to A, or 

 the lower body referred to the upper moves from A to B. The 

 movement of the two bodies collectively may be in any horizontal 

 direction. They may both move toward A, the upper moving the 

 swifter. They may both move toward B, the upper moving the 

 slower. Either may be still and the other move past it, or they 

 may move in directions approximately normal to the paper. My 

 only postulates are, that their motions have no vertical component, 

 and that their differential motion, z, has the direction expressed by 

 the arrows. 



The oval curve represents the assumed orbit of the bird as pre- 

 sented to an eye nearly in its plane. The bird ascending on one 

 side of this orbit through the point C has, just before reaching that 

 point, a velocity V as referred to the lower body of air, in which it 

 is then moving. Immediately after passing C, his velocity referred 

 to the upper body of air, in which he is then immersed, is F + i. 

 He moves faster in the upper air, because when he enters it his 

 direction is opposed to the direction of the differential motion of 

 the upper air referred to the lower. His absolute motion both be- 

 fore and after passing the plane of separation is the same ; but his 

 relative velocity, that is, his velocity referred to the air through 

 which he is passing, has been increased by the quantity z. Con- 

 tinuing on his circling orbit, he first ascends and then descends, 

 reaching the plane of separation at the point D. While he ascends, 

 gravity retards his motion ; while he descends, his motion is accel- 

 erated by gravity to the same extent ; so that he returns to the 

 plane at D with the same velocity ( F + z') with which he left it at 

 C. He now passes from the upper body of air to the lower body 

 in such direction that he again increases his relative velocity. As 

 soon as he has passed D, his velocity referred to the lower air is 

 V + ii. Continuing to C, he first descends and then ascends, his 

 velocity being first accelerated by gravitation, and then retarded by 

 the same amount ; so that he reaches C with the velocity V -\- iz, in 

 place of the previous velocity V, having gained the velocity ii by 

 passing in suitable directions to and fro between the differentially 

 moving bodies of air. 



The various qualifications of this theorem, and its relation to the 

 problem of soaring, need not be repeated here. All that is now 

 attempted is to show that the essential parts of the analysis are 

 absolutely independent of the direction and velocity of air move- 

 ment as referred to the ground. 



It appears to me that Mr. Pickering and Professor MacGregor, 

 by referring the motions of the bird partly to the ground and partly 

 to the air, engender confusion, and are led to assume untenable 

 positions. Mr. Pickering (xiii. p. 31) says that a piece of paper 

 floating on the air is carried along horizontally with the velocity 

 of the wind, but that a soaring bird does not drift so fast. Then, 

 to account for the floatation of the bird, he appeals to the " force 

 exerted on him by the wind, owing to the fact that he does not 

 move along as fast as the surrounding air." Thus he assumes a 

 force tending to prevent the bird from drifting horizontally with 

 reference to the earth ; and this assumption reduces the problem 

 to practical identity with the problem of the ascent of a boy's 

 kite. In point of fact, the assumed force does not exist : the only 

 re-action between the bird and the earth is through gravity, and 

 the direction of gravitation is vertical. If it be true that the 

 soaring bird drifts less rapidly than the piece of paper, the ex- 

 planation lies in something that the bird does ; and that thing, 

 whatever it is, costs energy. Appealing to the bird's net move- 

 ment against the wind as a source of energy merely shifts the point 

 of difficulty, for his net movement against the wind must then be 

 explained. 



Professor MacGregor says, " Let us suppose, now, that a bird is 

 at any instant moving horizontally, in the same direction as the 

 wind, and with a small velocity relative to the earth. . . . As his 

 speed increases, the velocity of the wind relative to him diminishes " 

 (xiii. p. 152). Now, if the velocities of bird and wind relative to 

 the earth are so related that increase of the bird's speed dimin- 

 ishes the velocity of the wind relative to him, then it must be that 

 the wind is moving faster than the bird, or is overtaking him. 

 The context shows that Professor MacGregor conceives the bird 

 to face in the direction toward which the wind blows, and it follows 

 that with reference to the air the bird is moving tail first. I am 

 confident that no ornithologist will admit the possibility of such 

 flight ; and its implicit postulation could hardly have occurred had 

 the problem been stated wholly in terms of bird and air instead 

 of being stated partly in terms of bird and ground. A little 

 further on he says, " Let us suppose that in wheeling he maintains 

 his velocity relative to the earth as well as his elevation. Then 

 [after wheeling], starting upwards with a considerable velocity, he 

 will clearly be able to rise through a certain height before his velo- 

 city has been reduced to its initial value." The assumption that 

 the bird in wheeling maintains his velocity relative to the earth 



