The operation of the air-pump can be made clear by the plan in Fig. 95. But one pump-barrel, _a_, is represented, with a piston, _c_, working in it. In the piston there is a valve, _i_, opening upward, and also one at _b_, in the beginning of the passage leading to the centre of the plate where is the receiver, _d_. The working of the instrument is thus: If the piston be forced down, the air under it, being compressed, will close the valve at _b_, and will rush upward through the valve _i_ in the piston. Let the piston now be raised; the resistance of the air above it will close the valve _i_, while the valve _b_ will be opened by the air rushing from the receiver, _d_, through the passage, _e_, to fill the space between the piston and _b_. You see, then, that every time that the piston is drawn up air passes out of the receiver through the valve _b_ into the space between this valve and the piston. None of this air which has passed out can go back again, for the moment that you press upon it by forcing downward the piston the valve _b_ is shut down, and the air escapes from the pressure by passing out through the valve _i_. Each time, therefore, that you work the piston up and down you pump out some of the air from the receiver; and if you pump for some time there will be exceedingly little air left in it, and that will of course be diffused throughout the receiver. It will be thin, like that in the upper regions of the atmosphere. [Illustration: Fig. 96. Fig. 97.] [Illustration: Fig. 98.]