ire is very hot, you
cannot see the smoke. The burning of the meat makes heat. Heat in a
steam engine makes the machine do work.

Every fire must have plenty of air. If air is shut off, the fire goes
out. When meat burns, the air unites with the meat and makes smoke, and
ashes, and gives out heat. Air unites with something in every fire.

=106. Burning inside the body.=--In every part of a man's body a very
slow fire is always burning. The blood brings to the cells food from
the intestine, and air from the lungs. The food and air join in a
burning. The smoke goes back to the blood and is carried to the lungs,
and breathed out with the breath. The ashes, also, go back to the
blood, and are carried away by the skin and kidneys. The burning makes
no flame or light for it goes on very slowly. You cannot see the
smoke, but you can feel the warmth of the burning. Some of the heat is
turned to power, and gives the body strength to do work. The body is
like a steam engine. It burns up all its food.

=107. How the body is warmed.=--The body is warmed by the slow burning
in the cells. This burning keeps the body always at the same warmth.
On a hot summer's day you feel warmer than on a cold snowy morning.
But your body is no warmer. Only your skin is warmer.

If the skin is warm, the whole body feels warm, but if the skin is
cold, the whole body feels cold. On a hot summer's day the heat is
kept in the skin, and we feel warm. On a cold winter's day a great
deal of heat passes off from the skin, and we feel cold. Yet our
bodies have the same warmth in winter as in summer.

=108. How the sweat keeps us cool.=--When your hands or feet are wet,
they are cold. On a hot summer's day, your body becomes wet with
sweat. This cools the body as if water were poured over it. So
sweating keeps you from getting too warm, and from being sunstruck.

We are sweating all the time, but the sweat usually dries as fast as
it forms. When we are too warm it comes out faster than it dries. On a
winter's day we sweat only a little, and so we save the heat. But more
heat passes off from the skin into the cold air, and we do not grow
warmer.

=109. Clothes.=--We wear clothes to keep the heat in the body. They do
not make heat, but they keep it from going off. Wool and flannel
clothes keep the heat in better than cotton. We wear woolen in the
winter, and cotton in the summer.

Fur keeps in heat the best of all. In very cold lands only fur is worn.