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Papers read:
(1.) “On the Mechanical Principles involved in the Flight of the Albatross,” by Captain F. W. Hutton, F.G.S.
(Extracts.)*
The author commenced by remarking that, though most other branches of ornithology had been treated of, that of “flight” had received little attention, though it was a subject of considerable difficulty and importance. His first illustration was that of an albatross, 17 Ib. in weight, poised in mid-air ready for flight; the temperature of its air cells, as scientifically ascertained, being 108 F., and that of the surrounding
[Footnote] * This paper, by Captain Hutton, could not be printed, in extenso, as it was found impossible to procure the necessary type for the algebraic formulæ, contained therein, in Wellington. — Ed.
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air 48 F. In that case, it would require a sphere of more than fifteen feet in diameter, to sustain the quantity of air necessary to render the specific gravity of the bird equal to that of the atmosphere. Taking its under surface to be eight feet in all, it would require a pressure of 2.12 lb. per square foot to support it, and an upward velocity of twenty feet a second to sustain the bird in the air. If the breeze were blowing with a velocity of one hundred feet a second, the bird would be forced backwards and downwards in the direction of the wind. The essayist, having proceeded to show, in algebraic formulæ, the comparative degrees of inertia of a body, proved that in all cases the bird would reach the water in a curved line, at a certain distance behind its first position; and concluded that the common notion, that a certain position of the bird's wings and feathers enabled it to sail against the wind, was erroneous, and opposed to the known laws of physical science. He also combated the theory, that an albatross could fly almost against the wind, in the same manner that a ship beats to windward, pointing out that in the one case, the pressure of the wind was resolved in forces, having other directions, by the resistance it received from the water: whereas, the albatross was placed in only one medium, having a uniform direction, affording no opportunity, as in the case of the ship, of resolving its direction into that most advantageous to itself, viz., forwards.
The author then propounded his own theory, that the albatross receives motion by means of the momentum it has previously acquired by strokes of his wings in the air, or of his feet in the water, or both combined. He then went on to illustrate, that duration of sailing might be supposed to depend upon the relative momentum and resistance. He showed, by algebraic formulæ, that a velocity, at starting, of one hundred and sixteen feet a second, sailing at an angle of five degrees to the horizon, would enable the bird—by gradually increasing the angle at which he was flying, to ten degrees—to maintain a uniform height until his velocity was reduced to fifty-eight feet a second. He then went on to show—by means of comparing the resistance offered to a round shot—the amount of resistance required, to allow an albatross to sail for half an hour without employing his wings, and only reducing his velocity from one hundred and fifteen to fifty-eight feet per second. He allowed 0.16 sq. ft. as the effective area of resistance to the forward progress of the bird; and, by ably arranged and accurately defined formula, arrived at the conclusion, that the resistance would be much less than one-fortieth of that calculated for round shot. He also showed that the greater the
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weight of the bird, and the smaller the velocity at which it was compelled to fly in order to maintain its position in the air, and the less the front area, the greater would be the period during which the bird could sail without using its wings. Thus, it might be said, that the sailing power of a bird depended upon its weight, and resistance to the downward force of gravity being great, while the resistance to its forward movement was small. He then took a Cape pigeon as an illustration: and, calculating its terminal velocity at ten feet a second, and the rate of flying at an angle of five and ten degrees to the horizon, at fifty-eight and twenty-nine respectively, showed that it would be able to sail only about eight minutes, or one-fourth as long as the albatross; the resistance of the air being in a similar ratio in both cases. However, the pigeon could not sail so long as eight minutes without being carried away by the wind, as the bird would have to use its wings some time before it had reached its least possible velocity. Bearing this in mind, it was shown that a diminution in velocity of 11.6 ft. a second, could be compensated for by an increase of one degree in the angle at which the bird happened to be flying; and that, therefore, it was extremely probable, that the albatross used its air-cells to enable it to slightly shift its centre of gravity with respect to the position of its wings, and so, with little muscular exertion, to alter the angle at which it was flying. The essayist concluded his able and instructive paper by stating, that he did not pretend to have solved the problem connected with the flight of the albatross, but merely to have suggested method of solving it. Experiments required to be made, respecting the resistance offered by the front and under surface of the bird, to different velocities of wind, before any satisfactory conclusion could be arrived at.
A vote of thanks was tendered to Captain Hutton, for the care and ability he had shown in the preparation of this paper.