The hand fire-engine depends upon the action of compressed air, which is so compressed by pumping water into the air chamber _a_. The tube is closed at _g_, and the pumps _e e_ drive water into the air chamber. At length the tap is opened, and the air drives the water out as it is continually supplied (fig. 54). Compressed air was also used for driving the boring machines in the Mount Cenis tunnel. In this case also the air was compressed by water, and then let loose, like steam, to drive a machine furnished with boring instruments. A pretty little toy may be made, and at the same time exemplify an interesting fact in Physics. It is called the _ludion_, and it “lies in a nut shell” in every sense. When the kernel has been extracted from the shell, fasten the portions together with sealing wax, so that no water can enter. At one end O, as in the illustration, leave a small hole about as large as a pin’s head; fasten two threads to the sealing wax, and to the threads a wooden doll. Let a weight be attached to his waist. When the figure is in equilibrium, and will float, put it into a jar of water, and tie a piece of bladder over the top. If this covering be pressed with the finger, the doll will descend and remount when the finger is removed. By quick successive pressure the figure may be made to execute a _pas seul_. The reason of the movement is because the slight cushion of air in the upper part of the vase is compressed, and the little water thus caused to enter the nut shell makes it heavier, and it descends with the figure (fig. 56). [Illustration: Fig. 55.—The Pump.] We have now seen that air is a gas, that it exercises pressure, that it possesses weight. We know it can be applied to many useful purposes, and that the air machines and inventions—such as the air-pump and the “Pneumatic Despatch”—are in daily use in our laboratories, our steam engines, our condensed milk manufactories, and in many other industries, and for our social benefit. Compressed air is a powerful motor for boring machinery in tunnels where steam cannot be used, even if water could be supplied, for smoke or fire would suffocate the workers. To air we owe our life and our happiness on earth.