THE "LUDION. 



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^-, 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 illustra- 

 tion, 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). 



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 con- 

 densed 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. 



Pneumatics, then, deals with the mechanical properties of elastic fluids 



Fig. 55. The Pump. 



