Fig. 100.
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.
Fig. 101.
157. The Sucker.—The boy's sucker illustrates the pressure of the air. It is simply a circular piece of leather with a string fastened to its centre, as seen in Fig. 101. When the leather is moistened and pressed upon a smooth stone, on pulling the string a vacuum is made between the middle of the leather and the stone, and the leather adheres by its edges to the stone, just as the receiver adheres to the plate of the air-pump when the air is pumped out. There are many animals that have contrivances of a similar character. The gecko and the cuttle-fish furnish interesting examples, as noticed in my Natural History, pages 198 and 320. Snails, limpets, etc., adhere to rocks by a like arrangement. Some fishes do the same. There is one fish called the remora, that attaches itself by suckers to the side of some large fish or a ship, and thus enjoys a fine ride through the water, without any exertion on his part. In all such cases it is water instead of air that makes the pressure, but the principle is the same. Flies and some other insects can walk up a smooth pane of glass, or along the ceiling over-head, because their feet have contrivances akin to the boy's sucker. The hind-feet of the walrus are constructed somewhat like the feet of the fly, enabling this huge animal to go up smooth walls of ice.
Fig. 102
Fig. 103.
