needs as that
the steam engine shall be fired with coal possessing a reasonable amount
of heat-producing particles.

The human body requires this supply of raw material for several
different purposes. In the first place, the very fact of living uses up
each minute a number of cells of various kinds in various organs. Each
breath taken, each heart beat, each muscular motion, all tend to the
destruction of tissue and involve its reconstruction. Violent exercise
uses up cell tissue very rapidly, so much so that a football player will
commonly lose from five to ten pounds in weight during a well-contested
game. It is a fundamental principle of training for any athletic event
involving hard exercise, that suitable food in large quantities must be
provided, and a young man training for football or rowing will eat
beefsteak, eggs, and other hearty food to an astonishing amount, all of
it going chiefly to repairing worn-out and used-up tissue.

In the second place, food is needed to supply material for growth, and
so it is that a growing boy eats out of all proportion to his size, and
the fact that he seems to be, as it is said, hollow clear to his feet,
is only his rational endeavor to supply the material needed for his
growing body.

In the third place, food must be supplied for the work to be done by the
body, as distinguished from the loss of tissue due to the performance of
the work, and finally, food must be provided in order to maintain the
bodily temperature, a larger amount being naturally required where the
difference between the temperature of the body and the outside air is
very great, as in the Arctic regions.

The human body being a special kind of machine, the raw material
supplied must be adapted to the needs of the machine, and while a lump
of coal admirably supplies energy for a steam boiler, no one would think
of feeding a lump of coal to a human being, simply because, by
experience, we know that suitable energy is not thereby developed. In
the matter of suitability of foods, much depends upon the local supply.
It is not to be supposed, for example, that the Eskimos eat meat and fat
altogether because it is the best article of food for them, but rather
because it is the only food available. It would be foolish to prescribe
fresh fruit or even white bread for the Eskimos because it is out of the
question for them to get such food. But, in general, it is possible for
the average individual to choose his sup