1,500,000 foot pounds is, then, the work done. He does it in 10 hours,
or 36,000 seconds, which gives an amount of work of only 42 foot
pounds per second performed by his muscles of locomotion.

At the end of the ten hours the man is exhausted, while the bird
delights in further flight. To what is this difference of condition
due? _It is due simply to the difference in the machine;_ but this,
you say, is not explaining the unknown in terms of the known. Let us
see, then, if we cannot do this. In the two accounts of work done as
above cited in the case of the man and the bird, an amount of energy,
i.e., heat of the system, has been expended just proportional to the
work done.

Now while the bird has expended more energy in this particular work of
locomotion than has the man, we find the bird machine has done little
else; he has consumed but little of his available heat force in
exercising his brain or the other functions of his system, or in
preserving the temperature of the body, and but little of his animal
heat, which is his strength, has been radiated into space. In short,
we find the bird machine so devised by nature that a very large
proportion of the available energy of the system can be used in
working those parts contrived for locomotion, and resist the force of
gravity, or, what is the same thing, nature has placed a greater
relative portion of the whole furnace at the disposal of these parts
than she has in man. The breast muscles of the bird are so constructed
as to burn a far greater proportional amount of the fuel from which
all energy is derived than do the muscles of the rest of the body
combined.

Let us see how it is with the man who has climbed the mountain. In