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. [Illustration: Fig. 102] [Illustration: Fig. 103.] [Illustration: Fig. 104.] 158. =Density of the Air Dependent upon Pressure.=--The fact that the degree of the density of the air is dependent on pressure has been already shown in § 152. The same thing can be shown in various ways with the air-pump. If a small bladder, partly filled with air, Fig. 102, and loaded with a weight so as to sink in water, be placed in a jar of water, and the whole be set under the receiver of the air-pump, on exhausting the air the bladder will swell out with the expanded air in it, and will rise as seen in the figure. The reason is, that the pressure being taken off the surface of the water, the bladder bears only the pressure of the water, and not that of the air with the water, and so the air in it expands and becomes less dense. If an India-rubber bag be partly filled with air, Fig. 103 (p. 119), and put under the receiver, on exhausting the air, the surrounding pressure being thus taken off from the bag, the air in it becomes expanded, that is, rarefied. For the same reason, if a vessel with soap-bubbles in it be placed under the receiver, on pumping out the air the bubbles will become much enlarged. A very pretty experiment illustrating the same thing may be tried in this way. Let an egg with a hole made in its small end be suspended in a receiver, as represented in Fig. 104, a wine-glass being beneath it. On exhausting the air the egg will all run out of the shell into the wine-glass, and then, on admitting the air, it will run back again into the shell. The explanation is this: There is air in the large end of the egg. As soon as the