d covered thickly with a paste made of lime."

"Lime like what the masons used when they plastered the new kitchen?"
asked Philip.

"About the same thing. The boxes are made air-tight, and the gas enters
the first box at one of the lower corners. Then before it can get through
the connecting-pipe into the next box, it has to wind its way around
among these plates coated with lime. This lime takes up the sulphur and
other things that we do not want in the gas, and so by the time it gets
through all the boxes it is quite pure and fit to use."

Then the party all went into the room where the gas was measured. It was
a little office with a queer piece of furniture in it; something that
looked like a very large drum-shaped clock, with several different dials
or faces. This, Mr. Carter said, was the metre or measurer, and by
looking at the dials it could be told exactly how much gas was being made
every day.

[Illustration: KITTY IN THE GAS-WORKS.]

"As soon as the gas gets through the purifiers," said he, "it comes, by
an iron pipe, in here, and is made to pass through and give an account of
itself before any of it is used. And now I suppose you would like to know
how it does report its own amount, wouldn't you?"

[Illustration: THE METRE.]

Philip and Kitty both were sure they did want to know, so he sketched a
little plan of the metre on a piece of paper, and then went on to explain
it:

"This shows how the metre would look if you could cut it through in the
middle. The large drum-shaped box A. A. is hollow, and filled a little
more than half way up with water. Inside it is a smaller hollow drum, B.
B. so arranged as to turn easily from right to left, on the horizontal
axis C. This axis is a hollow pipe by which the gas comes from the
purifiers to enter the several chambers of the metre in turn, through
small openings called valves. The partitions P. P. P. P. divide the drum
B. B. into--let us say--four chambers, 1, 2, 3, 4, all of the same size,
and capable of holding a certain known amount of air or gas. The chamber
1 is now filled with gas, 3 with water, and 2 and 4 partly with gas and
partly with water. The valves in the pipe C are so arranged that the gas
will next pour into the chamber 2. This it does with such force as to
completely fill it, lifting it quite out of the water and into the place
that 1 had occupied before. Then as 1 is driven over to the place which 4
had occupied, the gas with which it was f