Usually we think of explosions as harmful, and they often are, of course. Yet without them we could no longer run automobiles; gasoline launches would stop at once; motorcycles would no longer run; gasoline engines for pumping water or running machinery would not be of any use; and all aviation would immediately cease. Tunneling through mountains, building roads in rocky places, taking up tree stumps, and preparing hard ground for crops would all be made very much more difficult. War would have to be carried on much as it was during the Middle Ages; soldiers would use spears and bows and arrows; battleships would be almost useless in attacking; modern forts would be of little value; cannon, guns, rifles, howitzers, mortars, and revolvers would all be so much junk. [Illustration: FIG. 179. The explosion of 75 pounds of dynamite. A "still" from a motion-picture film.] [Illustration: FIG. 180. Diagram of the cylinder of an engine. The piston is driven forward by the explosion of the gasoline in the cylinder.] WHAT MAKES AN AUTOMOBILE GO. In all the above cases the explosions are caused by chemical action. When gasoline mixed with air is sprayed into the cylinder of an automobile, an electric spark makes the gasoline combine with the oxygen of the air; the gasoline suddenly burns and changes to steam and carbon dioxid. As you already know, when a liquid like gasoline turns to gases such as steam and carbon dioxid, the gases take much more room. But that is not all that happens. Much heat is released by the burning of the gasoline spray, and heat causes expansion. So the gases formed by the burning gasoline are still further expanded by the heat released by the burning. Therefore they need a great deal more room; 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.