e the gasometer, or gas reservoir."

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."

[Illustration: 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