156. =Experiments.=--When the receiver is full of air it can be moved about on the plate easily, and can be lifted from it. But work the pumps a few strokes and you will find that the receiver is firmly fastened to the plate, for the air within, being made thin, presses with little force compared with the air outside. If the pumps be worked for some time no force could release the receiver from the pressure without breaking it. But loosen the screw, _g_, and thus let the air in, and the equality of the pressure on the outside and inside is at once restored. Take off now this large receiver, and place a small glass jar, open at both ends, on the plate, with the hand covering the upper opening, as represented in Fig. 96. On exhausting the air the hand is so firmly pressed into the glass that it requires considerable force to disengage it from the pressure. If we tie a piece of bladder or India rubber over this jar, as in Fig. 97, and then pump out the air, the bladder at first is pressed in as represented, and if we pump on it at length bursts with a loud report. It would make no difference in the result of the experiment if the jar were shaped as in Fig. 98, for the pressure is the same in all directions. The resemblance between air and liquids in this respect may be illustrated thus: Suppose that a flat fish covers with one of its sides the end of the tube of a pump. He feels no uncomfortable pressure, because the water in the pump and that below it press equally upon him. If, now, the pressure of the water in the pump could be suddenly taken off by the piston, the fish would be pressed upward into the tube, as the bladder is pressed upward in Fig. 98, or downward in Fig. 97, or as the hand is pressed downward in Fig. 96. The Magdeburg Hemispheres, Fig. 99, illustrate very impressively the pressure of the atmosphere. They consist of two hemispheres whose edges at A fit very accurately upon each other. The air is exhausted through the stem where you see the stop-cock, and then the handle B is screwed on. The force required to pull these hemispheres apart depends upon the extent of their surface. In the famous experiment at Magdeburg, in 1654, by Otto von Guericke, the inventor of the air-pump, two strong hemispheres of brass of a foot in diameter were employed, and it required the force of thirty horses to separate them. In Fig. 100 you see a receiver with an opening at the top. Cemented in this opening is a wooden cup, _a_, terminating in a cylindrical piece, _b_. If mercury be poured into the cup, on exhausting the air from the receiver the mercury will be forced through the pores of the wood by the external air, and will fall in a silver shower. A tall jar, _c_, is placed there to receive it, to prevent any of it from going down into the opening in the metallic plate. [Illustration: Fig. 99.] [Illustration: Fig. 100.] [Illustration: Fig. 101.]