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 hut 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| lbs. 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 lbs. to a height of 
3| feet; or one-eighth that amount, namely 47,700 lbs., 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. JSTow 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 lbs. The quotient is 
318, which is the number of seconds , during which the ex¬ 
penditure of a force that would raise 47,700 lbs. 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 
