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velocity of 40 feet a second ; it is clear that the bird would remain stationary 

 with regard to the sea over which he was flying, nevertheless he would have a 

 velocity of 40 feet a second through the air, just as much as if the day was 

 quite calm, and he was flying both through the air and over the water equally at 

 the rate of 40 feet a second. This being understood, I will first suppose an 

 albatros, on a perfectly calm day, to be placed in the air at some distance above 

 the sea-level, with its wings and neck stretched out in the attitude of flight, 

 but without any forward movement. It is clear that the moment the support 

 was withdrawn, it would commence falling in a nearly vertical direction, unless, 

 indeed, it had power to buoy itself up by inflating its air-cells with hot air. 

 That it has not this power I have elsewhere shown (See " Ibis," July, 1865), 

 but for the sake of completeness I may perhaps be allowed to repeat it here. 

 " The temperature of the albatros, as taken by Sir G. G-rey, by placing a 

 thermometer under the tongue, is 98° F, and if we add 10° F. to this, in order 

 to allow for the difference between the head and the body, we shall have the 

 temperature of the air-cells at 108° F. The temperature of the surrounding 

 air cannot be taken lower than 48° F. ; the bird, therefore, could not raise the 

 temperature of the air taken into these cells more than 60° F. This would 

 increase its volume not quite one eighth ; and taking 100 cubic inches of air 

 to weigh 31 grains, and the average weight of an albatros to be 17 lbs., it 

 would be necessary, in order that the specific gravity of the bird might be 

 brought to that of the atmosphere, that these cells should contain 1820 cubic 

 feet of air, or in other words, they must be more than 1000 times the size of 

 the body of the bird. In fact it would require a sphere of more than 15 feet 

 in diameter to contain the necessary quantity." This objection being disposed 

 of, it follows that the bird must fall. 



If now we take the area of the under surface of the body, neck, and 

 expanded wings and tail of the albatros to be 8 square feet, and its weight 

 17 lbs., we see that it would take an upward pressure of 2 -12 lbs. per square 

 foot, to suppoi't it. This pressure would be given by a current of air moving 

 upwards with a velocity of 20 feet a second, so that on a perfectly calm day 

 the bird would fall downwards at a constantly increasing rate, until it had 

 attained a velocity of 20 feet a second, which velocity it would keep until it 

 fell into the sea. This is called its " terminal velocity." 



It is necessary to notice that as the position of the body, wings, and wing- 

 feathers of the bird would be inclined to the horizon, the direction of descent 

 would not be quite vertical, but would be inclined in the same direction as the 

 body and feathers of the bird, namely, backwards. 



I will next suppose that instead of being calm, a breeze is blowing with a 

 velocity of C feet a second . The bird would now, of course, be forced back in 

 the direction of the wind at the same time that it was falling. But it is well- 

 known that when a body at rest is set in motion, by a force acting upon it, 

 the body commences to move gradually, and acquires a certain velocity in a 

 certain time which is represented by the formula 



P.g.t. n , 



v. — ° (1) 



where v. is the velocity acquired in the time t., when a force P. acts upon a 

 body weighing W. lbs., g being the force of gravity. This is called the 

 " inertia " of the body. If then we suppose the bird to be facing the wind, 

 the backward velocity, comnnmicated by the wind would increase, while the 

 force of the wind upon it would decrease, but of course P + v would always 

 be equal to C. 



.-. v = C-P (2) 



