IMPOSSIBILITY OF HUMAN FLIGHT. 511 
= 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 distanee 
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 
at a man of ordinary strength can raise 134 Ibs. to a height 
z feet per second ; and can continue this exertion for 
sight hours in the day. He will then exert a force capable of 
raising (13i x 60 X 60 X 8) 381,600 Ibs. to a height of 
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 thaf a Man could by any means concen- 
wate 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 
eight, which we may take to be 150 lbs. The quotient is 
918, which is the number of seconds, during which the ex- 
penditure of a force that would raise ‘47,700 Ibs. to a height 
if 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 (Mecuanics, § 285), 
hat by no combination of mechanical powers can force be 
reated ; as these only enable force to be more advantageously 
ipplied. Hence, the problem of human flight will never be 
