illed passes out by another pipe
and off to the large reservoir you will see by and by, its place being
filled with water. At the same time 4 is driven around to the place of 3,
and 3 to that of 2. The water always keeps the same level, and simply
waits for the chambers to come round and down to be filled.

"Next, 3, being in the place of 2, receives its charge of gas from the
entrance pipe, is in turn lifted up into the central position, and sends
all the other chambers around one step further. And when the drum gets
completely around once, so that the chambers stand in the same places as
at first, you know each chamber must have been once filled with gas and
then emptied of it. If then we know that each chamber will hold, say two
and a half cubic feet of gas, we are sure that every time the drum has
turned fully around it has received and sent off four times two and a
half feet, or ten feet in all. Now we connect the axis C with a train of
wheel-work, something like that in a clock, and this wheel-work moves the
pointers on the dials in front, so that as the gas in passing in and out
of the chambers turns the drum on the axis, it turns the dial pointers
also.

"The right hand dial marks up to one hundred. While its pointer is
passing completely around once, the pointer on the next dial (which marks
up to one thousand) is moving a short space and preserving the record of
that one hundred; and then the first pointer begins over again. The two
pointers act together just like the minute and hour hands on a clock.
Then the next dial marks up to ten thousand, and acts in turn like an
hour-hand to the thousands' dial as a minute-hand, and so on. You see
each dial has its denominations, 'thousands,' 'hundred thousands,' or
whatever it may be, printed plainly below it. And now, when we want to
read off the dials, we begin at the left, taking in each case the last
number a pointer _has passed_, and read towards the right, just as you
have learned to do with any numbers in your 'Eaton's Arithmetic.' There
is one thing more to remember, however; the number you read means not
simply so many cubic feet of gas but so many hundred cubic feet."

Philip and Kitty immediately set to work to read the dials on the office
metre, and found that they were not now so very mysterious.

"But how do you know how much people use?" asked Philip. "There is
something like this metre, only smaller, down cellar at home, and a man
came and looked at