502 



NA TURE 



[September 21, 1893 



is clear, however, that displacement of external air by air-filled 

 cavities can only assist aerial floatation to an infinitesimal ex- 

 tent, unless highly heated. Such cavities would, however, help 

 aquatic birds to swim, if situated under the immersed portion of 

 their bodies, which is not always the case. 



Some aquatic birds, such as .swans, swim with head, neck, 

 wings, tail, and half their bodies out of the water. The specific 

 gravity of fishes and land animals is clearly about the same as 

 water. For, when swimming, they can keep only a small portion 

 of their heads above the surface, and that by continued exertion. 

 Are, then, birds, in the substance of their bodies, less dense 

 than other animals, although also composed of flesh, blood, and 

 bone, and these components in similar proportions and of simi- 

 lar character and texture ? If they are, then land animals might 

 have been made lighter in proportion to their bulk or smaller in 

 proportion to their weight than they have been. If they are 

 not, how is it that some of them can swim and float high out of 

 the water ? 



Having an opportunity recently of inspecting a large wild, 

 or whooper, swan, I ascertained its weight to be 14 lbs. I 

 noticed that the whole of the under-part of the body, which 

 would be immersed when swimming, was covered with feathers 

 and underlined with down to an average depth of not less than 

 \\ inches, or, when closely pressed, say \\ inches. The im- 

 mersed surface I estimated at 15 square feet. The weight of 

 water displaced by this feather and down jacket, and the conse- 

 quent extra buoyancy produced thereby was no less than 

 978 lbs. This would account for two-thirds of the bird's body 

 being out of water when swimming, even if the body were of 

 I he same specific gravity as water. 



I next procured a freshly-shot wild duck, which weighed 

 2| lbs., and placed it in a tank of sea-water. It floated. I 

 found the area of its immersed surface to be 54 square inches, 

 and the average depth of its under-feathers and down to be 

 \ inch. The water displaced by this envelope would weigh 

 I '5 lbs., and would support three- fifths of its entire weight. 

 I then had it denuded of all its feathers and down, and 

 again placed in the tank. It then slowly sank to the bottom. 



These experiments, so far as they go, seem to prove con- 

 clusively that birds are not lighter, bulk for bulk, than other 

 animals, but, on the other hand, about the same specific gravity, 

 and that their floating power lies entirely in the thick jacket 

 or life-belt with which nature has furnished those, and those only 

 which are intended to swim. 



Inasmuch, therefore, as the specific gravity of the actual 

 bodies of all animals appears to be about the same, there is no 

 reason to believe that any could have been constructed of lighter 

 material or to lighter design. 



Weight in Relation to their Energy. — But notwithstanding 

 this uniformity of specific gravity, there remains the curious fact 

 that flying V>irds can exert continuously about three times the 

 horse-power per lb. of weight that man can — and, indeed, 

 about three times what is possible for the horse. This 

 marvellous flow of energy in proportion to weight is 

 probably due to rapidity of limb-action rather than to increase 

 of muscular stress. I have timed sea-gulls and found them to 

 flap their wings two hundred times per minute when flying at 

 about 24 knots per hour, and have estimated eider-ducks, 

 making about 36 knots per hour, to be flapping their wings 

 five hundred times in a minute. I say "estimated," for their 

 movements are too rapid for precise counting. This outpouring 

 of energy, which seems to me to be unequalled in terrestrial 

 animals, is nevertheless maintained by birds for indei'initely 

 long periods of time. 



A proportionately increased rate of combustion and renova- 

 tion of tissue as well as of food-consumption are necessary con- 

 sequences. The higher temperature of the bodies of birds, as 

 compared with other animals.' and the well-known voracity of 

 those which, like sea-birds, are almost continuously on the wing, 

 are circumstances which seem to point to the same conclusion. 

 It is confirmed by what we know of steam and other motors. 

 For instance, if a steamship were so built and proportioned that 

 a ton of coal per hour consumed in the boilers would maintain 

 the pressure at 100 lbs. per square inch and produce locx) horse- 

 power at the propeller ; and then if, without other alteration, 

 firing was slackened until the steam fell to 50 lbs. per square 

 inch and there maintained, it is clear that the horse-power pro- 

 duced would be greatly lessened, and so would the temperature 



^ Chambers Eitcyclopcpdia, " J3ird and Animal Heat"; Lehrbttch der 

 Zoologie, by Prof. Hertwig, p. 538. 



of the steam in the boilers, steam-pipes, and cylinders. Thus, 

 other things being equal, the temperature of the steam would 

 rise and fall with the energy given forth by the mechanism. 



The suggestion is that the higher temperature of birds, a( 

 compared with other animals, is similarly connected with their 

 superior power of producing and maintaining energetic effort. 



Aerial Navigation. 



Let us now consider what man has done, and may be able to 

 do, in aerial navigation by aid of contrivances which, as in the 

 case of railway locomotives and ocean steamers, are propelled 

 by a power other than that of his own body. 



The scientific world is greatly indebted to Mr. Hiram S. 

 Maxim, of London, for recording, in a clear and readable form, 

 the present position of aeronautic mechanisms.' So far, 

 the only contrivances which have been fairly successful are 

 balloons, which, unlike birds, depend on atmospheric displace- 

 ment for their power of sustaining weight or rising or falling. 



In balloon experiments our French neighbours have led the 

 way, from the first attempt of the Montgolfier brothers in 

 1783. During the last twenty years they have made numerous 

 experiments and sul>stantial improvements. Captain kenard 

 and other officers of the French army have con.structed a fish- 

 shaped apparatus, and inflated it with hydrogen. It is driveD 

 by an electric motor of 84 horse-power, and has sulficient 

 buoyancy to carry two aeronauts and all necessary accessories. 

 In fair weather Captain Renard has succeeded in travelling at 

 the rate of 12\ miles per hour, in steering in any direction, and 

 even in returning to his point of departure. The balloon, it 

 is said, always keeps level, and so far there have not been any 

 accidents ; but no expedition has been attempted in wet or 

 windy weather. 



Except that a more powerful motor, going at a higher speed, 

 might be fitted to such an apparatus, Mr. Maxim thinks that 

 it is as near perfection as is ever likely to be reached by a 

 machine depending on aerial flotation. He proceeds to give 

 an account of some experiments made by Prof. S. P. Langley, 

 of the Smithsonian Institution, Washington, and of others by 

 himself, to ascertain how much power is required to produce 

 artificial flight by means of aero-planes, after the manner of 

 birds, and whether such power can be obtained without ex- 

 ceeding the weight which it would itself sustain. 



He says that heavy birds, with relatively small wings, carry about 

 150 lbs. per horse-power exerted, and birds such as the albatross 

 and vulture probably about 250 lbs. Prof. Langley, with small 

 slanting planes, was able to carry 250 lbs. per horse-jjower 

 exerted ; and Mr. Maxim, using heavier weights in propartion 

 to plane-area, 133 lbs. per horse-power, and using lighter ones, 

 nearly the same as Prof. Langley. 



Mr. Maxim has lately developed his energies to constructing 

 a motor which should meet the requirements of the case, and has 

 succeeded, he says, in producing one : a steam-engine burning 

 naphtha and with atmospheric condenser, with a total weight 

 of 8 lbs. per horse-power. He thinks, however {Engineer, 

 January 13, 1893, p. 28), that by using light naphtha and its 

 vapour in the boiler instead of water, as well as m the furnace 

 as fuel, a weight as low as 5 lbs. per horse-power may be 

 reached. 



Meanwhile Prof. Langley's ideas have been embodied in an ex- 

 perimental flying-machine, a drawing and description of which 

 will be found in the iJaj/y Cra//4(V for July I, 1893. The body, 

 which resembles that of a bird and is 15 feet long, contains the 

 propelling machinery in duplicate. The wings, which are 40 feet 

 across, are of China silk spread on a tubular framework, 

 stiffened with wire trusses. The boilers use liquid fuel and con- 

 tain a highly volatile fluid. The capabilities of the machine 

 have not yet been practically tested. 



Promising as are the results hitherto obtained, they are as yet 

 far from placing us on a level with birds in power to utilise the 

 atmosphere as a navigating medium. The absolutely necessary 

 power of delicate guiding, in rising, falling, and turning, what- 

 ever the direction or force of the wind, has yet to he considered 

 and worked out. What would happen in case of a temporary 

 breakdown of the aero-plane machinery we shudder to think of. 

 An important step has been effected by the discovery that 

 parachutes with tubular orifices at the top ate comparatively safe 

 appliances for descending to the earth from indefinitely high 

 altitudes. Perhaps it may be arranged that each aeronaut should 



1 "Progress in Aerial Navigation," by Hiram S, Maxim, /='t'r/«4'A^^.»' 

 ez'iew, October, 1802. 



NO. T247. VOL. 48] 



