The petrol engine resembles a steam engine in these respects: it has a cylinder in which the driving force is compressed, a piston-rod this driving power pushes down, and a fly-wheel the piston actuates, and which carries round the piston-rod by its momentum, pushing it towards the top of the cylinder again after one down stroke, so that it may obtain another thrust. What the fly-wheel does, in a word, is to store up energy between each thrust upon the piston, and thus keep the motor in regular motion. In starting an engine, the petrol tap is turned on, and some of the spirit allowed to run into the carburettor. Then, usually with a handle, the engine is made to revolve so that the piston-rod moves down inside the cylinder and sucks in the explosive mixture. As the piston sinks in the cylinder, it draws in a charge of the gas; then rising again, it compresses this charge between the head of the piston and the top of the cylinder. Now comes the moment when, if the most forcible thrust is to obtained from the air and gas, it should be ignited, and this is done by causing an electric spark to jump between two metal points on what is called the sparking-plug. This plug is screwed into the head of the cylinder; the sparking end is inside the cylinder where the gas charge is compressed, and to the outside are fixed wires which run to the magneto--a small electrical machine which, driven by the motor, makes sparks in the plug each time the gas is to be fired. Just at the right instant, therefore, the spark flashes in the cylinder, and the gas is exploded. Being compressed within the walls and top of the cylinder, the explosion can only exert its force in one way--upon the head of the piston, to which it gives a sudden downward thrust. The power is transmitted to the fly-wheel, which is set in motion, and thus the engine runs, driven by the series of explosions which takes place in the cylinder. The majority of petrol engines operate upon what is known as the four-stroke principle. This action is as follows: First the pistons are driven down by a charge of gas, then they ascend in the cylinders so that the spent and useless gas fumes may be forced out through valve ports; then the pistons descend again so that a fresh charge may be drawn into the cylinders, and then, for a fourth time, the pistons move so that the gas may be compressed and fired. During the period the engine is not driven by the explosions, the fly-wheel has to do its work, carrying the piston up and down while it expels waste gas and obtains a new supply. In Fig. 32 is a diagram to illustrate the principle of the petrol engine and it amplifies this explanation.