The charge of mixture is drawn into the cylinder just as a pump sucks in water. At a time when the piston is at the closed end of the cylinder, a valve is opened connecting the space above the piston with the device that forms the mixture; then by moving the piston outward, mixture is sucked into the space above it. When the piston reaches the end of its stroke the cylinder has been filled with mixture, and the valve then closes. It would be useless to set fire to the mixture at that time, for the piston is as far down the cylinder as it can be, and pressure could not move it any farther. To get the piston into such a position that the expanding mixture can move it, it is forced back to the closed end of the cylinder. This squeezes or _compresses_, the cylinderful of mixture into the small space, called the _combustion chamber_, between the piston and the cylinder head. If the mixture is now burned, the piston can move the length of the cylinder, and in so doing it develops power. The cylinder is cleared of the burned and useless gases by opening a valve and pushing them out by moving the piston back to the inner end of the cylinder. When this has been done, the valve is closed, and, by opening the inlet valve and moving the piston outward, a fresh charge is sucked in, and the several steps of the _gas engine cycle_ are repeated. The name _cycle_ is given to any series of steps or events that must be gone through in order that a thing may happen. Thus the empty shell must be taken out of a gun and a fresh cartridge put in before the gun can be fired again, and that series of steps might be called the gun cycle. The gas engine cycle requires the piston to make four strokes. An outward stroke sucks in a charge of mixture, and an inward stroke returns the piston to the firing position and compresses the charge. Then comes the outward stroke when the piston moves under power, followed by the inward stroke that clears the cylinder of the burned gases. For every stroke of the piston the crank shaft makes a half-revolution; the crank shaft therefore makes two revolutions to four strokes of the piston and to each repetition of the gas engine cycle. Of these four strokes of the piston only one produces power. The other three strokes, called the _dead strokes_, are required to prepare for another power stroke. A gas engine cylinder thus produces power for only one quarter of the time that it runs. This is one of the striking differences between the gas engine and the steam engine, for the piston of a steam engine moves under power all of the time that the engine runs.