Fig. 54.—The Hand Fire-Engine
The Pump.
We have seen in the case of the Water Barometer that the pressure of the air will sustain a column of water about thirty feet high. So the distance between the lower valve and the reservoir or cistern must not be more than thirty-two feet, practically the distance is about twenty-five feet in pumps.
We can see by the illustration that the working is much the same as in the air-pump. The suction pipe B is closed by the valve C, the cylinder D and spout E are above, the piston rod F lifts the air-tight piston in which is a valve H. When the piston is raised the valve C opens and admits the water into the cylinder. When the piston is depressed the valve C is closed, the water already in forces H open, and passing through the piston, reaches the cylinder and the spout (fig. 55).
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.
Fig. 55..—The Pump.
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).