m; but they are shut up in a
small place in the top of a cylinder. The only thing to hold them up
in this small space, however, is a piston (Fig. 180), and the suddenly
expanding gases shove this piston down and escape. The piston is
attached to the drive wheel of the automobile, and when the piston is
pushed down it gives the automobile a push forward. If it were not for
the expansion of a gas in the cylinder, this gas being confined to a
small space, the piston would not be pushed down.

An explosion is simply the sudden pushing of a confined gas expanding
on its way to freedom. The gasoline vapor and air were the confined
gas. Their chemical combining made them expand; by pushing the piston
out of its way the newly formed gas suddenly freed itself. This was
an explosion, and it gave the automobile one forward push. But the
automobile engine is so arranged that the piston goes up into the
cylinder again, and is pulled down again, drawing a spray of gasoline
and air into the cylinder after it. Then it goes up a second time,
an electric spark explodes the gasoline, the piston is forced down
violently once more, and so it goes on. There are several cylinders,
of course, and the explosions take place within them one after the
other so as to keep the automobile going steadily.

HOW A GUN SHOOTS. Pulling a trigger makes a gun shoot by causing an
explosion. There is a spring on the hammer of a gun. This drives
the hammer down suddenly when you release the spring by pulling the
trigger. The hammer jars the chemicals in the cap and causes them to
explode. The heat and flame then cause the oxygen in the gunpowder to
combine with some of the other elements in the powder to make a gas.
The gas requires more room than the powder and is further expanded
by the heat released by the chemical change. The expanding gas frees
itself by pushing the bullet out of its way. The bullet gets such a
push through the exploding of the gunpowder that it may fly to a mark
and pierce it.

[Illustration: FIG. 181. The most powerful explosions on earth occur
in connection with volcanic activity. The photograph shows Mt. Lassen,
California, the only active volcano in the United States.]

There is a slight explosion even when you shoot an air gun. First you
compress some air in the upper part of the barrel of the air gun; then
you suddenly release it. The only thing in the way of the expanding
air is the bullet; so the air pushes this out in front