"So as to make it air-tight, and give it a chance to move freely up and down. Of course if the iron shell were empty its own weight would make it sink directly to the bottom of the water-tank and stay there. But gas, you know, is so much lighter than common air that it always makes a very strong effort to rise higher and higher, carrying along whatever encloses it. You saw that illustrated in the balloon that went up last Fourth of July. Now, as the gas from the works pours into the reservoir from beneath, it is strong enough to lift the iron box up a little in the water. Of course that gives a little more room. Then as more gas comes in to take up this room, the gasometer keeps on rising slowly. We make sure of its not rising above the water and letting the gas leak out, by means of the beams you see stretched across above it. They are all ready to hold it down in a safe position if the need should come. "On the other hand, as the people in town draw off the gas to burn, the gasometer would, of course, tend to sink down gradually. So we have the water-tanks made deep enough to allow for every possible necessity in that direction. In very cold weather we keep the water from freezing by passing a current of hot steam into it. If it should ever freeze, the gasometer might as well be on the ground, for it could not move up and down, or be trusted to keep the gas from leaking out around the edges. With these precautions, however, we know it is perfectly trustworthy." "I saw it one morning early, when I was out coasting on the hill," said Philip, "and it wasn't more than half as high as it is now." "A great deal had been drawn off during the night and we had not been making any more during the time to take its place." "Does it ever get burned out too much?" "No, there's no danger of it. We make enough to allow a good large margin above what we expect will be used." The children looked about a little longer, and then, with good-byes and many thanks to Mr. Carter, walked home again with papa, over the crisp, hard snow. Next week Philip had a composition to write at school. He took "Gas" for his subject, and wrote: "Gas that you burn is made out of soft coal. They put it in Ovens and cook it until it is not coal any longer. The Ovens are so hot you cant go anywhare near them but the men do With poles and big lether aprons. I would not like to shovle in the coal. I would rather have a Balloon. They use two or three tons every day. it makes coke and Tar and the gas that goes up the pipes. They make the gas clean and mesure it in a big box of water, and tell how much there is by looking at the clock faces in front. Then it goes into a big round box made of iron and then we burn it. but I do not like to smell of it. you must not blow it out for if you do you will get choked. This is all I Remember about gas. "PHILIP RAYMOND LAWRENCE." RACING A THUNDER-STORM.