870 KEPORT— 1893 



It, seems to me that for every pound of liis weight pressing downwards there must 

 be an equivalent force pressing upwards. This can he produced only by his giving 

 downward motion to the air previously at rest, or by his arresting previous motion 

 of air in an upward direction. The latter alternative involves the supposition that 

 the air-currents which soaring birds face are not, as Mr. Lancaster believes, always 

 horizontal, but must have, to some extent, an upward direction. If a parachute 

 were falling in a current of air, which was moving upwards at the same rate as 

 the parachute fell, it would obviously retain its level, yet gravity would be acting. 

 So, if a bird with extended wings were sliding down a stream of air which was 

 tending upwards at the same angle and same velocity, the phenomenon of soaring 

 would be produced. 



Weight of Birds in Belation to their Bulk. — It is generally believed that birds 

 are lighter, bulk for bulk, than other animals, and that to this lightness they owe, 

 in some degree, their power of flight and of floating on water. To account for 

 this it is said that their bone-cavities are filled with air, and that some, though 

 not even all, flj'ing birds have small air-sacs under the skin. It is clear, however, 

 that displacement of external air by air-filled cavities can only assist aerial 

 flotation to an infinitesimal extent, 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 swimmmg, they can keep 

 only a small portion of their heads above the surface, and that by continued exer- 

 tion. 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 similar 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 lb. 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 H inch, or, when 

 closely pressed, say, 1\ inch. The immersed surface I estimated at \^ square 

 foot. The weight oif water displaced by this feather and down jacket, and the 

 consequent extra buoyancy produced thereby, was no less than 9*78 lb. This 

 would account for twivthirds of the bird's body being out of water when swimming, 

 even if the body were of the same specific gravity as water. 



I next procured a freshly-shot wild duck, which weighed 23- lb., 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 

 f inch. The water displaced by this envelope would weigh 1'5 lb., 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 conclusively 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 gi-avityof 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 birds can exert continu- 

 ously 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 



' See pp. 868 and 871. 



