Feh. 12, 1 8 74 J 



NA TURE 



281 



divided the true area by 12, for so 234^ (if we neglect the half 

 inch) gives just igi, and 312 instead of 311 gives 26. 



But (as a second matter) myiriend's notion of Dr. Pettigrcw's 

 arithmetic receives some colour from the sentence following the 

 one before quoted, viz., "The wings of the gannet, therefore 

 [each wing being supposed 19.^ square inches], furnish a support- 

 ing area of 3 feet 3 inches square." So of the heron. Having 

 told us that the area of each wing is 26 square inches, he says, 

 " Both wings consequently furnis>> an area of 4 fe:t 4 inches 

 square." Here, surely, square inches have been treated as if 

 they were linear, and only 12 of them instead of 144 reckoned to 

 the square foot ! 



Once more (as was observed in your review a week or so ago), 

 Dr. Pettigiew maintains, in opposition to all other experimenters, 

 that in flight the downward stroke of the wing is directed for- 

 wards and not backwards. Now, to say nothing of the "singu- 

 larity " of representing the wings in his ovni case as concavo- 

 convex, and in that of his opponents as flat (much to the detri- 

 ment of the latter), the whole of Dr. Pettigrcw's "mathematical 

 demonstration " of his position is so extrcnely original that I 

 fear for the uninitiated it is only explaining ohscurum per obscu- 

 ritts. Would he condescend to accepted methods a' d prove his 

 c ise by the parallelogram of forces ? As it is, his proof amounts 

 simply to this :— " A.S the under surface of the wing, which is a 

 true kite, looks upwards and forwards, it tends to carry the bird 

 upwards and forwards." No doubt, if the wing remain still, 

 and the bird have already a sufficient velocity. A kite is sus- 

 tained or elevated by an extraneous force, either the wind im- 

 pinging against its under surface, or vice I'ersii when the boy 

 runs. But whence comes the bird's motion, before its wings can 

 act as kites? Dr. Pettigrew nowhere tells us this, but starts 

 with his bird already flying. Thus he says : — " The bird, when 

 flying, is a body in motion. It has already acquired momentum. 

 Ifa grouse is shot on the wing, it does not fall vertically down- 

 wards, as Borelli and his successors assume [Shade of Borelli !], 

 but downwards and forwards. The flat surfaces of the wings 

 are consequi ntly made to strike downwards and forwards, as 

 ihey in this manner act as kites to the falling bodv, which they 

 bear or tend to bear upwards and forwards." Here it is unmis- 

 takeable that the function of the wings in generating velocity is 

 confounded with their function in directing the velocity already 

 generated ; ju-t as if one should confound a steamer's rudder 

 with her screw. The question is, How do the wings generate 

 velocity? In this respect it is immaterial whether the bird is at 

 rest or in motion. But to this there can be but one answer, at 

 least if we are still to believe that " action and reaction are equal 

 and opposite ;" the answer that is, that everybody gives but 

 Dr. Pe'tigrew. The downward motion of the wing is wholly 

 concerned with sustaining or elevating against gravity. A back- 

 ward movement must carry the bird forward ; Dr. Pettigrew's 

 forward movement must make it fly tail first. 



James Ward 



Trinity College, Cambridge, Feb. 2 



Your reviewer resorts to very strong language, without, it 

 appears tome, justifying his procedure. In reviewing my volume 

 he exclaims, " Imagine our disappointment on finding that, 

 instead ol the work being by the hand of a master, its author is 

 deficient in the knouledge of the first principles of physics, 

 and of the undoubttd meaning of smie of llie most simple 

 terms employed in the science ; his argument, if it may 

 be so called, being but little more than a long series of vague 

 and fanciful analogies, incorrectly stated physical facts, and un- 

 tenable theories." . . . "We must say that we expected better 

 things of Dr. Pettigrew, and regret that he has not, before now, 

 learned ihat there are errors in his methods and results that can- 

 not be tolerated by a thinking public, which prefers accurate 

 reasoning rather than dogmatic statement, and well-grounded 

 fact 10 fanciful analogy " (Nature, vol. ix. p. 221). One would 

 naturally have expected after such announcements an exposure 

 of fa'se theories and a criticism of the nomenclature employed, 

 but Mr. Garrod ondescends upon neither. He takes refuge in 

 general statements and implies what he does not attempt to 

 prove. 



He states, e.g. " that it is at present impossible to obtain from 

 any f m of fuel, a sufficient percentage of the potential. ty which it 

 poiseises for doing work, to work an engine sufficiently compact 

 and light for the wings which it has to drive." Now this is utter 

 nonsense. In 1868 Mr. Stringfellow, of Chard, Somersetshire, 



exhibited at the Crystal Palace a flying machine which with its 

 engine, boiler, water, fuel, flying surfaces, and propellers only 

 weighed 12 lbs. The engine of this machine exerted the third of 

 a horse power and obtained the 100/. prize of the Aeronautical 

 Society of Great Britain as being at once the lightest and most 

 powerful steam-engine ever made. 



What bird weighing 12 lbs. can Mr. Gan-od inform me exert; 

 a third of a hors- power in flying ? 



This one fact proves that in the ordinary steam-engine we have 

 a po>ver more than equal to the production of flight. 



Mr. Garrod takes exception to my statement that " weight 

 when acting upon wings, or, what is the same thing, twisted in- 

 clined planes, must be regarded as an independent moving 

 power." 



This point will be best illustrated by an example. Ifa gannet 

 drops from a cliff with expanded motionless '<i'in^s it can sail for 

 an incredible distance, the weight of the body dragging upon the 

 wings, doing the prinC'pil part of the work. This is a matter 

 of observation, and the principle may be exemplified by the follow- 

 ing simple experim^^nc. If an apparatus composed of two quill 

 feathers stuck in the end of a cork be made to fall from a height 

 it will be found to travel do'i'nioards and fonoai ds in a curve, the 

 forward curve equalling half the space through which the appa- 

 ratus descends. Here we have no muscular movement to direct 

 or influence the motion in any way, and it certainly seems to me 

 to afford an explan.ilion of the manner in which mere -.-height, or 

 gravity acting up n weights, may by the aid of wings be made to 

 propel a body from one point to another. 



Mr. Garrod proceeds — "After such indications of imperfect 

 knowledge, nothing in the way of mechanical theories could 

 cause surprise, and we ate therefore not astonished to find it laid 

 down as the fundamental principle of flight, that tlie np-stroke op 

 the iving aids in pyopiilsion^ and that in the down-stroke the 

 inferior sttrfaee of the wi^ig is directed do^unioards and forn'ards." 

 If Mr. Garrod attempts to elevate a natural wing or an aitificial 

 one properly constructed, even in a strictly vertical direction, he 

 will find that it inevitably darts upwards and forwards in a curve 

 and carries the hand ^\■ilh it. In this manner, as experiment 

 proves, the ascent of the -win:: aids in propulsion. If again Mr. 

 Garrod attempts to depress tlie wing verticdly downwards, he 

 will as certainly find that it darts doxomvards and forwards in a 

 curve, the hand being carried in the direction specified. The 

 upward forward and downward forward curve-;, being united as 

 they are in flight, give a -waved track. If the wings did not dart 

 fonvards both during their ascent and de-cent the body of the bird 

 could rot be transferred from one place to another in a horizontal 

 waveii line which it is. Mr. Garrod is evidently imperfectly in- 

 firmed on the subject of flight, for he inquires " Who can see any 

 close relation between the llijht of birds and that of a kite ? " 

 The merest tyro in mechanics will, I think, perceive this on a 

 moment's reflection. The kite is ^nW^iX forwards on the moving 

 air by the string. The kite formed by the wings of a bird is 

 pushed forwards on the moving air by the weight of the body. 



I do not forget, as Mr. Garrod insinuates, th.at a kite requires 

 a string. The following passage, written in 1867, will show this. 

 "The wing of a bird acts after the manner of a boy's kite, the 

 only difference being that the kit- is pulled forwards upon the 

 wind l>v the string and the hand, whereas in the bird the wing is 

 pushed forwards on the wind by the weight of the bjdy and the 

 life residing in the pinion itstll."* 



Mr. Garrod's words are — " Dr. Pettigrew seems to forget that 

 a kiten^eds a string, and yet, backed by his false analogy, he has 

 the presuniplioJi to quote ihe experimental verifications and 

 opinions of such able and ingenious thinkers as Borelli and 

 Marey, the authors of the true theory of flight, only 'to reject 

 thtm." To one who has experimented on the subject of flight 

 for the last 10 years, the term presumption in this sentence sounds 

 strange. One may, I venture to think, without p esumption, 

 differ from another after such mature deliberation. M.arey's 

 theory of flight, which is nearly, if not identical, with my own, 

 was not promulgated till neatly two years after I had published 

 mine. This point will be fully discussed in \\\& Athenaiivi of 

 Feb. 14. In fsct Marey frankly admitted this in a letter to the 

 French Academy of Sciences in reply to a reclamation lodged by 

 me with that learned body. 



His words are : — "J'ai constate qu'effectivement M. Pettigrew 

 a vu avant moi, et represen'e dans son Memoire, la formt en S 

 du parcours, de I'aile de I'insecle ; que la methode optique a 



* On the various modes of flight in relation to aSronautics; Proc. Roy. 

 Instit. of Great Britaio, March 21, 1867. 



