L>29 



combining these two equations we get 



W (C - P) 



= gP ' 



now when the velocity of the bird equalled that of the wind, P. would be o, in 

 this case 



c w 



.". t = = oc 



o 



so that the bird could never actually acquire a velocity equal to that of the 

 wind, and there would always be a force of C — v. acting on it, and as the bird, 

 its wings and its feathers, would be inclined at an angle, which I will call Q, 

 to the horizon, and therefore to the direction of the wind, this force would be 

 resolved into two — one, equal to (C — v) sin 2 Q, tending to drive it backwards 

 — and the other, equal to (C — v) sin Q cos Q, tending to delay its fall or even 

 to raise it, supposing it to be sufficiently great to overcome the force of gravity. 

 But even in this case C — v. is constantly decreasing as it approaches its limit 

 v=C, so that there must always come a time when (C — v) sin Q cos Q is not 

 sufficient to support the bird, and it must commence to fall ; so that in all 

 cases it would reach the water in a curved line at a certain distance behind 

 the first position of the bird, the form of the curve depending on C, "W., and 

 Q. I have dwelt thus minutely on these simple facts, because it has been 

 supposed that in a gale of wind, a certain position merely of its wings or 

 feathers, might enable an albatros to sail against the wind, without any 

 momentum of its own, which is quite impossible. 



Another explanation that has been given is that the albatros can fly 

 almost against the wind, in the same way that a ship beats to windward. This, 

 however, is manifestly incorrect. A ship is placed in two different media, one 

 of which — the water — is practically stationary, and it is enabled to sail at an 

 acute angle with the wind, because the pressure of the wind, being met by the 

 resistance of the water, is resolved into forces having other directions, and 

 advantage being taken of this by trimming the sails, it ultimately results that 

 the ship is moved in the direction of least resistance, viz., forwards. But the 

 case of the albatros presents no analogy to this ; it is placed in one medium 

 only — the air, the whole of which is moving in the same direction, and with 

 the same velocity, and it has no means, unless by using its wings, of offering 

 any resistance, except its inertia which we have already seen is not sufficient, to 

 the wind and so resolving its direction into others more advantageous to itself, 

 in fact it is analagous to a balloon, which, except by the aid of machinery, can 

 only drift with the wind. 



Having, therefore, seen that while the wings are stationary, no forward 

 movement can be commenced by the bird, we are forced to the conclusion that 

 the albatros sails along by means of the momentum that he had previously 

 acquired by strokes of his wings on the air, or of his feet on the water when 

 rising from it, or from both combined, and that so soon as the resistance of the 

 air has reduced his velocity so much that it no longer prevents him falling, 

 fresh impulses of the wing have to be given. It will now be observed that 

 the difficulty has been shifted from the means of obtaining motion through 

 the air, to that of keeping up a velocity but slightly diminished, for so long a 

 time as the albatros is known to sail without using his wings, or in other 

 words, to the very small resistance that the air must offer to his progress ; and 

 if it could be shown that this resistance is not too great to allow for the longest 

 observed time of sailing, all difficulties with respect to this part of the flight of 

 the albatros would disappear. I do not profess to have done this, but I think 

 that I can show that there is no insuperable difficulty in the way. 



