They all went out of doors then, papa, Mr. Carter, Philip and Kitty, across a narrow court-yard. There was a huge, round box, or drum, with sides as high as those of the carriage-house at home, but with no opening anywhere, “like a great giant’s bandbox,” thought Kitty. Four stout posts, much taller still than the “bandbox” itself, were set at equal distances around it, and their extremities were joined by stout beams which passed across over the top of the gasometer.
As the children went up nearer to it, they saw it was made of great plates of iron firmly riveted together, and that it did not rest on the ground, as they had supposed, but in the middle of a circular tank of water.
“After the gas has been made and purified and measured,” said Mr. Carter, “it is brought by underground pipes into this gasometer, and from here drawn off by other pipes into the houses. The weight of this iron shell bearing down upon the gas, gives pressure or force enough to drive the gas anywhere we wish.”
THE GASOMETRE.
“But why do you put the—the iron thing in water, instead of on the ground?” asked Kitty.
“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.”