January 23, 1891.] 



SCIENCE. 



45 



LETTERS TO THE EDITOR. 



*** Correspondents are requested to be as brief as possible. The vtriter's name 

 is in all cases required as proof of good faith. 



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

 of the journal. 



On reqziest, twenty copies of the number containing his communication vnll 

 be furnished free to ani/ correspondent. 



The Flight of Birds. 



One would suppose that there could be little difference of 

 opinion in regard to such fundamental principles of avian flight 

 as the direction in which the down-stroke of the wings is deliv- 

 ered, and the relative positions to a horizontal plane of the anterior 

 and posterior margins of the wings during this and the up-stroke. 

 Nevertheless the other day I was completely astounded at some 

 ideas expressed in "Aniaial Locomotion; or. Walking, Swimming, 

 and Flying," by G. Bel! Pettigrew, M.D,, F.R S., F.R S.E., 

 F.R.C.P.E., and connected with several other scientific and edu- 

 cational institutions (International Scientitic Series, 1888). 



Never having happened to see any review or remarks upon this 

 remarkable work, I am in ignorance of how it has been received 

 by the scientific world. To me it appears so completely illogical 

 in parts, that I cannot refrain from presenting these remarks ; so 

 that, if I be as completely mistaken as to me appears to be this 

 author, some one may kindly put me aright, that my ignorance 

 of some fundamental points of aerostatics and animal mechanism 

 may not vitiate my further observations in this line. It is with 

 considerable tiifHdeuce that I venture to advance my opinion 

 against that of one who has spent some twenty years upon the 

 subject, and who, judging by the position that he occupies, cer- 

 tainly should be capable of coming to satisfactory conclusions on 

 the subject; but my utter inability, after considerable study of 

 the matter, to admit the possibility of what is given as the main 

 principle of avian flight, induces me to bring the matter for- 

 ward. 



I will put the case in the author's own words, here as elsewhere, 

 with his Italics (p. 197): "Reasons why the effective stroke should 

 be delivered downwards and forirards. — The wings of all birds, 

 whatever their form, act by alternately presenting oblique and 

 comparatively non-oblique surfaces to the air, — the mere exten- 

 sion of the pinion, as has been shown, causing the primary, sec- 

 ondary, and tertiary feathers to roll down till they make an angle 

 of 30° or so with the horizon, in order to prejjare it for giving the 

 effective stroke, which is delivered with great rapidity and energy, 

 in a downward and forward direction."' My first impression was 

 that such a movement would drive the bird upwards and back- 

 wards, and subsequent study of the subject only makes me the 

 more positive of this. Theoretically I believe that any body sus- 

 pended in a fluid medium will tend to move in a direction opposite 

 to that in which the medium is forced by the members of that 

 body. Take a wing of a bird and vibrate it rapidly, as its move- 

 ments are described by Dr. Pettigrew, before the flame of a candle, 

 and we shall find that the flame is driven downward and for- 

 ward. 



On p. 95 we are told, " In the water the wing, when most effec- 

 tive, strikes downwards and backivards, and acts as an auxiliary 

 of the foot ; whereas in the air it strikes downwards and forwards.'^ 

 I fail to see why a movement that produces locomotion in one 

 direction in water should be reversed in the air to produce locomo- 

 tion in the same direction; and my mystification is increased when 

 I read on p. 108, " Plight may also be produced by a very oblique 

 and almost horizontal stroke of the wing, as in some insects, e.g., 

 the wasp, blue-bottle, and other flies," for here I am left in 

 doubt whether opposite directions of applying the wing produce 

 the same direction of locomotion, or whether I am to believe that 

 an "almost horizontal stroke of the wing" forwards produces a 

 forward movement of the body. For the present I am inclined to 

 believe neither the one nor the other. Again, on p. 204, in the 

 explanation of Fig. 107, we read, " The Red-headed Pochard 

 (Fuligida ferina, Linn.) in the act of dropping upon the water; 

 the head and body being inclined upwards and forwards, the feet 

 expanded, and the wings delivering vigorous short strokes in a 

 downward and forward dh-ection. — Original." The questions 

 presented to my mind by this are these : " Does the duck really 

 wish to increase its speed just before ahghtiug upon the water, or 



does the fact of the strokes being ' vigorous short strokes' diamet- 

 rically change their effect on the body from what would be pro- 

 duced by leisurely short strokes or vigorous loug strokes?" I 

 imagine that if the bu-d were in its right mind it would wish to 

 check its course, — in other words, to give an upward and back- 

 ward impulse to its body before coming in contact with the water, — 

 and I should approve of its giving downward and forward strokes 

 to its wings in order to accomplish this end. 



Many other of Dr. Pettigrew's illustrations, both pictorial and 

 verbal, also do violence to my ideas without convincing me : in 

 fact, I seem to see exactly the opposite in tliem to what he haa 

 found. For instance : in Figs. 53 and 54, illustrating the action 

 of the wing, the hinder edge of the wing must be below the 

 anterior on the up-stroke and above it on the down-stroke, which 

 is exactly the reverse of what he tells us occurs in flight. On pp. 

 156 and 157 we read, " It is a condition of natural wings; and of 



FIG. 1 (FIG. 81 IN ORIGINAL). 



artificial wings consti-ucted on the principle of living wings, that 

 when forcibly elevated and depressed, even in a strictly vertical 

 direction, they inevitably dart forward. This is well shown in 

 Fig. 81. If, for example, the wing is suddenly depressed in a 

 vertical direction, as represented at a b, it at once darts down- 

 wards and forwards in a curve to c, thus converting the vertical 

 down-stroke into a down oblique forward stroke. If, again, the 

 wing be suddenly elevated in a strictly vertical direction, as at 

 c d, the wing as certainly darts upwards and forwards in a curve 

 to e, thus converting the vertical up-stroke into an upward oblique 

 fonvard stroke. The same thing happens when the wing is de- 

 pressed from eto/, and elevated from gtoh." Admitted. But 

 the posterior margin of the wing must be elevated during this 

 movement, or one of two things must take place. If this margin 

 be depressed, the wing will move in a contrary direction; i.e., 

 backwards and downwards. If this does not take place, then 

 force must be used which will cause an appreciable upward and 



PIG. 2 CFIG. 116 IN ORIGINAL). 



backward recoil to the hand moving the wing. In the same way 

 the posterior margin of the wing will be lower than the anterior 

 instead of above it, as the author states, during the upward stroke 

 of the wing. Also I had imagined that the buoyancy and pro- 

 gression of a bird depended on the resistance that the wing en- 

 countered. If it be allowed to move in the plane of least re- 

 sistance, it will move forward while the body remains stationary; 

 whereas if not allowed to move forward, or forced slightly back- 

 ward, then, and only then, can a forward impulse be given to the 

 body. I might cite my personal observations of the movements 

 of the wings of flying birds against the observations of Dr. Petti- 

 grew; but in that case he would have in his favor the longer 

 length of time during which his observations have taken place. 



To draw the discussion to a close, which, if I am in the wrong, 

 has sufiBciently exposed my ignorance, I will call attention to Fig. 

 116. On p. 331 we read, " Instead of the two wings forming one 



