IMPOSSIBILITY OF HUMAN FLIGHT. 511 



helpless offspring ; and the rapidity of their flight is such, 

 that they can scarcely traverse less than seven or eight hun- 

 dred miles in that time, although they go but a short distance 

 from home. The flight of Insects is even more remarkable 

 for its velocity in proportion to their size ; thus a Swallow, 

 which is one of the swiftest-flying of Birds, has been seen to 

 chase a Dragon-fly for some time without success ; the Insect 

 always keeping about six feet in advance of the Bird, and 

 turning to one side and the other so instantaneously, that the 

 Swallow, with all its powers of flight, and its tact in chasing 

 Insects, was unable to capture it. 



673. If the preceding estimate of the power expended by a 

 Bird in sustaining itself in the air be correct, it may be easily 

 proved that it would be impossible for a Man to sustain him- 

 self in the air by means of his muscular strength alone, in 

 any manner that he is capable of applying it. It is calculated 

 that a man of ordinary strength can raise 13 J Ibs. to a height 

 of 3^ feet per second; and can continue this exertion for 

 eight hours in the day. He will then exert a force capable of 

 raising (13| X 60 X 60 X 8) 381,600 Ibs. to a height of 

 3J feet ; or one-eighth that amount, namely 47,700 Ibs., to the 

 height of twenty-six feet, which, as we have seen, is that to 

 which the Bird would raise itself in one second, by the force it 

 is obliged to exert in order to sustain itself in the air. Now if 

 we suppose it possible that a Man could by any means concen- 

 trate the whole muscular power required for such a day's 

 labour, into as short a period as the accomplishment of this 

 object requires, we might find the time during which it would 

 support him in the air, by simply dividing this amount by his 

 weight, which we may take to be 150 Ibs. The quotient is 

 318, which is the number of seconds, during which the ex- 

 penditure of a force that would raise 47,700 Ibs. to a height 

 of twenty-six feet, will keep his body supported in the air ; 

 and this is but little more than five minutes. There is no 

 possible means, however, by which a Man could thus concen- 

 trate the force of eight hours' labour, into the short interval in 

 which he would have to expend it while supporting himself 

 in the air. And we have elsewhere seen (MECHANICS, 285), 

 that by no combination of mechanical powers can force be 

 created; as these only enable force to be more advantageously 

 applied. Hence, the problem of human flight will never be 



