July 4, 1884.] 



• KNO\A/'LEDGE ♦ 



The fact appears to be that wlien a horizontal plane 

 traverses the air in a horizontal direction, the supporting 

 power of the air is increased in proportion as the plane 

 moves more quickly, or in proportion to the actual quan- 

 tity of air it glides over, so to speak. Indeed, we have 

 clear evidence to this effect in the beliaviour of the 

 comniou toy-kite, the supporting power of which is in- 

 creased in proportion to the force of the wind. For a kite 

 held by a string in a strong horizontal current of air, cor- 

 responds exactly to an inclined plane surface drawn swiftly 

 in a horizontal direction during a calm. The same sup- 

 porting power which results from the rapid passage of the 

 air under the kite will be obtained during the rapid passage 

 of the kite over still air. 



When we study the flight of birds we are confirmed in 

 the opinion that velocity of horizontal motion is a point of 

 extreme importance as respects the power of flying. For 

 though there are some birds which seem to rise almost 

 straight from the ground, yet nearly all, and especially the 

 larger and heavier birds, have to acquire a considerable 

 horizontal velocity before they can take long flights. 

 Even many of those birds which seem, when taking flight, 

 to trust rather to the upward and downward motion of 

 their wings than to swift horizontal motion, will be found, 

 when carefully observed, to move their wings up and 

 down in such sort as to secure a rapid forward motion. 

 I have been much struck by the singularly rapid for- 

 ward motion which pigeons acquire by what appears like 

 a simple beating of their wings. A pigeon which is about 

 to fly from level ground may be seen to beat its wings 

 quickly and with great power ; and yet instead of rising 

 with each downward stroke, the bird is seen to move quite 

 horizontally, — as though the wings acted like screw- 

 propellers. I believe, in fact, that the wings during this 

 action do really act, both in the upward and downward 

 motion, in a manner resembling either screw-propulsion or 

 the action by which seamen urge a boat forward by means 

 of a single oar over the stern.* The action of a fish's tail 

 is not dissimilar ; and as the fish, by what seems like a 

 simple beating of its tail from side to side, is able to dart 

 swiftly forwards, so the bird, by what seems like a beating 

 of its wings up and down, is able — when occasion requires 

 — to acquire a swift forward motion. At the same time it 

 must be understood that I am not questioning the un- 

 doubted fact that the downward beat of a bird's wing 

 is also capable of giving an upward motion to the bird's 

 body. The point to be specially noticed is that when a 

 bird is taking flight from level ground, the wings are so 

 used that the downward stroke gives no perceptible up- 

 ward motion. 



But since a horizontal velocity is thus effective, we 

 might be led to infer that the larger flying creatures, 

 which, co'teris paribus, travel more swiftly through the 

 air than the smaller, would require a smaller relative 

 extent of supporting surface. We are thus led to the 

 consideration of that point which has always been regarded 

 as the great, or rather the insuperable difiiculty, in the 

 way of man's attempts at flight, — his capacity or incapacity 

 to carry the requisite extent of supporting surface. We 

 are led to inquire whether a smaller extent of supporting 

 surface than has hitherto been deemed necessary may not 

 suffice in the case of a man, and a fortiori in the case of a 

 large and powerful flying-machine. 



The inference to which we have thus been led, is found 

 to accord perfectly with the observations which have been 



* Sailors call this sculling, a term more commonly applied to 

 the propulsion of a boat by a single oarsman using a pair of oars, 

 or sculls. 



made upon flying creatures of different dimensions. It 

 has been found that the supporting surface of these crea- 

 tures, — whether insects, birds, or bats, — by no means 

 varies in proportion to their weight. This is one of the 

 most important results to which the recent inquiries into 

 the problem of flight have led ; and I believe that my 

 readers cannot fail to be interested by an account of the 

 relations which have been observed to hold between the 

 weight and the supporting surface of different winged 

 creatures. 



We owe to M. de Lucy, of Paris, the results of the 

 first actual experiments carried out in this direction. 

 The following account of his observations (made in the 

 years 18G8, 18G9) is taken from a paper by Mr. Brearey, 

 the Honorary Secretary to the Aeronautical Society. 

 " M. de Lucy asserts," says Mr. Brearey, " that there is 

 an unchangeable law, to which he has never found any 

 exception, amougst the considerable number of birds and 

 insects whose weights and measurements he has taken, 

 viz., that the smaller and lighter the winged animal is, the 

 greater is the conijiarative extent of supporting surface. 

 Thus in comparing insects with one another — the gnat, 

 which weighs 460 times less than the stag-beetle, has four- 

 teen times greater relative surface. The ladybird, which 

 weighs 150 times less than the stag-beetle, possesses five 

 times more relative surface, &c. It is the same with birds. 

 The sparrow, which weighs about ten times less than the 

 pigeon, has twice as much relative surface. The pigeon, 

 which weighs about eight times less than the stork, has 

 twice as much relative surface. The sparrow, which weighs 

 .339 times less than the Australian crane, possesses seven 

 times more relative surface, &c. If we now compare the 

 insects and the birds, the gradation will become even more 

 striking. The gnat, for exam])Ie, which weighs 97,000 

 times less than the pigeon, has forty times more relative 

 surface ; it weighs .3,000,000 times less than the crane 

 of Australia, and possesses relatively 140 times more sur- 

 face than this latter, which is the heaviest bird M. de Lucy 

 had weighed, and was that also which had the smallest 

 amount of surface, the weight being nearly 21 lb. ; and 

 the supporting surface 139 inches per kilogramme 

 (2 lb. 3J oz ). Yet of all travelling birds the Australian 

 cranes undertake the longest and most remote journeys, 

 and, with the exception of the eagles, elevate themselves 

 the highest, and maintain flight the longest." 



M. de Lucy does not seem to have noticed the law to 

 which these numbers point. It is exceedingly simple, and 

 amounts in fact merely to this, that instead of the wing- 

 surface of a flying creature being proportioned to the weight, 

 it should be proportioned to the surface of the body (or 

 technically, that instead of being proportioned to the cube, 

 it should be proportioned to the square of the linear dimen- 

 sions). Thus, suppose that of two flying creatures one is 7 

 times as tall as the other, the proportions of their bodies 

 being similar, then the body surface of the larger will be 49 

 times (or 7 times 7) that of the other, and the weight 343 

 times (or 7 times 7 times 7) that of the other. But instead 

 of the extent of wing-surface being 343 times as great, it is 

 but 49 times as great. In other words, relatively to its 

 weight the smaller will have a wing-surface 7 times greater 

 than that of the larger. How closely this agrees with what 

 is observed in nature, will be seen, by the case of the 

 sparrow as compared with the Australian crane; for M. de 

 Lucy's experiments show that the sparrow weighs 339 times 

 less than the Australian crane, but has a relative wing- 

 surface 7 times greater. 



It follows, in fact, from M. de Lucy's experiments, that, 



as we see in nature, birds of similar shape should have wings 



, similarly jiroportioned, and not wings corresponding to the 



