Mar. 19, 1 8 74 J 



NATURE 



381 



and the others as successively less recent in the order in 

 which I have named them, and comparing similar parts 

 of any two zones, the height of the anticlinals is greater, 

 the dip less, and the difference between their axes greater 

 in the more recent." The pamphlet is ably written and 

 very deserving of study. Henry H. Howorth 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. A^o notice is taken of anonymous 

 commutticaiions.l 



Animal Locomotion 



Tn Nature, vol. ix. p. 301, there is a letter from Mr. 

 Wallace on .a very important point connected with the Theory 

 of Flight. The question he discusses is " whether a bird's wing 

 during onward flight moves do-iinzvards and baihairds or dir.vit- 

 7vards and fot-ikirds ;" and Mr. Wallace supports Mr. Pettigrew 

 in affirming that the movement is dincniwaids and/ot-cards. 



As tliis is a subject to which I have paid long and close 

 attention, I desire to express my conviction that neither of the 

 two motions thus described by Mr. Wallace is the true motion 

 of a bird's wing in forward fliglit. 



The true motion is one strictly vertical to the axis of the bird's 

 body ; and as that axis is ordinarily horizontal in flight, the wing- 

 stroke is a vertical stroke, that is simply downwards, and neither 

 " downwards and forwards " nor " downwards and backwards." 



This is rot a question of theory, but a question of fact, to be 

 determined liy observation. The wing-stroke of most birds is 

 indeed so rapid that the eye cannot distinctly follow the opera- 

 tion. But there are birds whose wing is so large and whose 

 flight is so slow, that the wing-stroke can be followed with the 

 greatest distinctness. Such is the common heron— common, 

 alas, no longer in most parts of England, but numerous on the 

 west coast of Scotland. When at home I am in the daily habit 

 of watching their flight ; and the truly vertical character of the 

 wing-stroke is a fact which I have verified by the eye under 

 every possible condition which could supply the evidence. 



There are indeed two slight modifications of the perfect per- 

 pendicularity of the stroke which result (i) from the attachment 

 'of the wing to the body of the bird, and (2) from the structure of 

 the wing-feathers. The first of these two modifications consists 

 in this — th.at as the wing moves upon a hinge, its extremity must 

 move downwards, not absolutely vertically, but describing an 

 arc. The segment of a circle, however, through which the 

 wing thus moves, is generally a very short one : and in so 

 far as the movement of the exti'emity departs from the 

 vertical, it departs therefrom neither "backwards" nor "for- 

 wards," but (.as it were) "inwards," — that is, in the direction of 

 a circle encon;pa.ssing the axis of the bird's body as with a hoop. 

 Pigeons, as an amusement and in play, often complete this 

 circle — making their primary quills clash against each other over 

 their backs, and downwards again under their breasts. But in 

 ordinary forward flight, when birds are intent only on progres- 

 sion, the wings move through a very small arc indeed of the 

 complete circle referred to. 



The second modification of the perpendicularity of the stroke 

 arises from the "set " of the wing-feaihers — which curve back- 

 wards and downwards from the wing-bones. In some birds, and 

 notably in the heron, and all the storks, the concavity thus 

 formetl is very deep, and of course a surface which is thus not a 

 plane surface, but a concave one, however truly it may he struck 

 downwards, cannot have a purely vertical reaction on the air. 



When we observe, however, that in the case of many birds, 

 and some of these the most powerful fliers in the world, this con- 

 cavity of the wing-feathers is very slight indeed, and that the 

 whole vane is very narrow, flat, and " taut," it is obvious that a 

 purely vertical stroke, or one as near it as possible, is the really 

 essential stroke for flight. 



The great secret of flight is the exquisite and complicated 

 adaptation of structure in the' feathers of a bird's wing which 

 derives from this one simple action the resultant of a force which 

 is both sustaining and propelling. It is an adajilation which, 

 when thoroughly grasped and understood, at once dispenses 

 with as needless, and condemns as mechanically erroneous, all 

 the explanations which assume either a "downward and for- 

 ward " or a " downward and backward " movement, 



I venture to think that Mr. Wallace is certainly in error when 



he ascribes to Mr. Pettigrew the merit of having been the first to 

 .show that " horizontal forward motion is a general resultant of 

 the ujjward and downward action of the wings under the in- 

 fluence of gravitation." 



In February 1865 I published in Good Words a paper i>n the 

 mechanism of flight, in which this effect of the wing-stroke was 

 fully explained, and elaborately illustrated. This paper sub- 

 sequently appeared as chap. iii. in the "Reign of Law "pub- 

 lished in the end of i8''i6. Mr. Pettigrew's lecture before the 

 Royal Institution (in which I believe his views were first promul- 

 gated) was delivered on March 22, 1S67. I had the pleasure of 

 hearing that lecture, and the amusement of recognising parts of 

 it (including even a poetical quotation) as t.aken directly from 

 my chapter on flight. The pleasure, however, was somewhat 

 abated by the strange mixture of much that was quite correct, 

 with a great deal more which I believed then, and believe now, 

 to be wholly erroneous. Ari;yll 



March II 



Mr. Wallace has well said that the question. How a bird's 

 wing moves in flight, " is a very important question." In these 

 days, when scientific attention is being directed to the problem 

 of aerial navigation, it is especially important. I have the less 

 hesitation, therefore, in troubling you with some further remarks 

 in reply to the strictures of this very accurate observer. 



At the outset I must deny that I assumed either that a bird's 

 wing is inflexible or that it is a plane. Of its flexibility I had no 

 cause for speaking at all ; but so far from regarding it as a plane, 

 I expressly objected to Dr. Pettigrew so representing it in his 

 supposed refutation of the orthodox view. The point in dispute 

 is entirely conceniing the down stroke ; against Mr. Wallace's 

 account of the up stroke I make no objection. 



First, what may we infer a priori concerning the down stroke? 

 (i) Its efficiency is independent of the velocity of the bird : this 

 is simply a consequence of the second law of motion. We have 

 to su])pose a bird fixed in still air, and to ascertain the effect 

 which ensues on a downward blow of the wing. The subsequent 

 forward velocity of the bird, so far as that depends on the down 

 stroke, is but a consequence or an accumulation of these effects. 

 It is thus only needful to analyse the single effect itself To this 

 end the shape and varying flexibility of the wing must be noted. 

 .Mong the exterior margin we have a rigid area, comparable to 

 the blade of an oar, and formed for ihe most part ol bone, in 

 the top side of which the rigid tubes of the primary and secondary 

 feathers are inserted. On the under side of this, which we may 

 term the oar part of the wing, there is thus a considerable con- 

 cavity, the direction of which w'hen the wing is extended is 

 decidedly backward. The area towards the middle line of the 

 wing is flat and I10ri20nt.il, approximately so at all events, when 

 the bird is freely suspended in the air. Of the posterior, the 

 larger, half of the wing it is true, as Dr. Pettigrew says, that the 

 aspect is forward, more especially in heavy birds with broad and 

 rounded wings. The flexible extremities of the feathers readily 

 turn upwards like vanes in the manner so well shown in Fig. So 

 of Dr. Pettigrew's work. We may thus roughly distinguish four 

 areas, beginning from the front : ((/) the oar area ; (l>) the plane 

 or flapping area ; (c) the kite area ; {d) the vane area. (2) Now 

 we may inquire what will be the effect of each when the wing is 

 struck downward. The reaction from the o.ar area will be («) a 

 force directed upwards .and forwards ; that from the plane area 

 (/■) a force directed upwards simply. Against the kite area will 

 impinge the air sent backwards and rebounding from the blow of 

 the oar area ; the effect of this (c) is all that corresponds to what 

 Dr. Pettigrew calls the kite action of the wing. Lastly, the 

 same air in escaping through the feathers, and especially in raising 

 the tips in the vane area, will produce the forward motion (d) to 

 which Mr. Wallace refers, besides contributing something (,•) to 

 support the bird's weight. The horizontal component of (a) to- 

 gether with (i/) will cany the bird forward. The -lighter hori- 

 zonlal component of (c)— slighter because proceeding only from 

 the rebounding air and from a yielding surface— will tend to 

 hinder the forward motion : hence the absence, more or less 

 complete, of this area in quick fliers. The forces (b), greater 

 part of (<-), and (<•) will sustain the bird against gravity. 



Neither Dr. Pettigrew, nor apparently Mr. Wallace-, distinguishes 

 the motion consequent on a surface striking against the air from 

 that of a surface gliding through it. If I incline a sheet of paper 

 to the horizon and let it slip from my hand it will descend with 

 a similar incline towards the ground ; but if, having stiffened it, 

 I strike it against the air at the same inclination it will tend to 

 rise in a direction at right angles to that inclination. The blovi 



