Page:Transactions and Proceedings of the New Zealand Institute - Volume 1 (2nd ed.).djvu/502

466 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 postition; 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 albatros 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 albatros 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 albatros receives motion by means of the momentum it has previously acquired by strokes of its wings in the air, or of its 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 116 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 its velocity was reduced to 58 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 albatros to sail for half an hour without employing his wings, and only reducing his velocity from 115 to 58 feet per second. He allowed 0.16 square feet as the effective area of resistance to the forward progress of the bird; and, by ably arranged and accurately defined formulæ, 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 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, 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 10 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 albatros, 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