Most boys understand, at least generally, how an automobile or motor-boat engine works. Scientists call gasoline engines "internal combustion motors," and that means that the force is gathered from the explosion of the gasoline vapour in the cylinder. Enough gasoline to supply fuel to run an aeroplane motor for as much as eight or nine hours can be carried in the tank. From the tank a small pipe carries the gasoline to a device called the carbureter. The carbureter turns the gasoline into gas by spraying it and mixing it with air, for gasoline turns into a very inflammable and explosive gas when mixed with the oxygen in the air. So this gas, if lighted in a closed space, will explode. The explosion takes place in the motor-cylinder by the application of an electric spark, and the force pushes the piston, which turns the crank and drives the aeroplane propeller, automobile wheels, or motor-boat screw. Thus we have the piston driven out and creating the first downward thrust, but the thrusts must be continuous. The piston must be drawn back to the starting place, the vapours of the exploded gas expelled, and the new gas admitted to the cylinder ready for the next explosion. On the ordinary four-cycle motor two complete revolutions of the flywheel are necessary to do all the work. First, we must have the explosion that causes the initial thrust; second, the return of the piston rod in the cylinder by the momentum of the flywheel as it revolves from the initial thrust, thus forcing out the burned gas of the first explosion; third, the next downward motion to suck in a fresh supply of gas; and, fourth, the next upward thrust to compress it for the second explosion. It sounds simple enough, but it isn't, as every one knows who has tried to run a gasoline motor for himself. The carbureter must do its work automatically and convert the air and gasoline into gas in just the right proportions. A slight fault with the feed of gasoline or air would cause trouble. Also the electric-spark system that ignites the gas and causes the explosions must be in perfect running order. The explosions cause great heat, so some system of cooling the cylinders either by air or water must be used. Only one cylinder has been explained here, but most engines have several, each working at a different stage, so that the power is exerted on the shaft continuously. For instance, take a four-cylinder engine; on the instant that the first cylinder is exploding and driving the shaft, the second cylinder is compressing gas for the next explosion, the third is getting a fresh supply of gas, and the fourth is cleaning out the waste gas of the explosion of a second before. Thus it will be seen why the explosions are almost constant. Now think of the aeroplane motor that has fourteen cylinders and develops 140 horsepower! This is probably the most powerful aeroplane engine in the world, although there are many motor boats that have engines developing 1,000 horsepower.