The sucking-fish and the shark.
Flies and other insects, that walk along so well on the ceiling and on smooth glass, have suckers on their feet, that work very much in the same way that the boy’s sucker does upon the stone. Some fishes have suckers by which they can stick to rocks or any thing else. In this case, it is water that makes the pressure instead of air. Here is the drawing of a fish that has a sucker, or, rather, a set of suckers, on the upper part of its head. With this it can adhere to any thing very firmly. A singular story is told by a traveler about one of these sucking-fishes. He saw a shark attempt to seize it, but the fish dodged him, and then fastened itself to the shark’s back by its suckers. It so happened that one of the sailors had tied to the fish a stick of wood by a short line. The shark dashed off with this fish thus fastened to him towing the stick of wood astern. He soon stopped, and, getting hold of the cord, jerked the fish off, and then dove at it as before. The fish dodged him again, and got hold with its suckers a second time, and when last seen, the shark was struggling in vain to get rid of the troublesome fellow.
Questions.—Describe the air-pump, and tell how it works. Tell about the experiment with the India-rubber ball, with the soap-bubbles, and with the shriveled apple. How much is the pressure of the air on every square inch of your body? How much is it on your whole hand? Why do you not feel this pressure? What experiment with the air-pump makes this plain? Give the other experiment that shows the same thing in another way. How is the boy’s sucker made? Explain how it holds on to the stone. How do flies and other insects walk on ceilings and on glass? Tell about the sucking-fish.
CHAPTER VIII.
GASES.
How the gas that we burn differs from air.
I have told you about the air which we breathe, and which is all around us; but there are other kinds of air. When we light the gas, what is it that we set on fire? It is an air, or gas, as we call it, that comes through the pipe to the burner. It is like the air which we breathe in some respects. It is transparent; that is, you can see through it as you can through common air. It moves about as easily as air does. But it is different from the air in some things. It is lighter. The air has no smell; but this gas has a very bad smell, as you may know when it leaks out of the pipes. Air does not burn, but this gas does; and it is curious that when it burns the bad smell is all gone.
Sometimes, when the gas leaks out of a pipe, it is very dangerous. If a close room should get very full of it, and you should go into it with a light, the gas in the room would all take fire and explode. Persons have been killed in this way. It is well that the gas does smell badly, for this lets us know when it leaks, so that we may guard against the danger. We should let the gas out by opening doors and windows before we bring a light in.
Persons have sometimes been killed by the gas in another way. You know that there is in every gas-pipe something that you can turn so as to shut the pipe, and thus keep the gas from coming out. Now persons that do not know how the gas is managed have blown it out instead of shutting it off. When this is done, the gas continues to come out from the open pipe just as it did when it was burning, and gradually fills the room; and if the person in the room goes to sleep, he will be injured, and perhaps even killed by breathing the gas.
Gas burning in a common fire.