it the other day, to see how much gas had been burned
in the house he said, when I asked him what he was going to do."

"The metre you have at home works in the same way as this," said Mr.
Carter, "and the dial-plates are read in the same way. But the gas that
your little metre registers is only that which you take from the main
supply-pipe, to light your parlors and bed-rooms.

"When a stream of gas from the main enters the house, it has to pass
through the metre the very first thing, before any of it is used; and
each little metre keeps as strict an account of what passes through from
the main to the burners, as the large one here in the office does of that
which passes from the purifiers to the reservoir. But there is this
difference between the two: the gas keeps pouring through the office
metre as long as we keep making it in the retorts, but it passes through
your metre at home only just as long as you keep drawing it off at the
burners. So if we find by looking at the metre that 5450 feet have passed
through during a given time, we send in our bill to your papa for that
amount, knowing it must have been burned in the house.

"But most likely the metre doesn't say anything directly about 5450. It
says, perhaps, 11025. 'How can that be?' you would think. 'We haven't
burned so much as that,' and you haven't--during this one quarter. But
after the metre had been inspected at the end of the last quarter, the
pointers did not go back to the beginning of the dials and start anew;
they kept right on from the place where they were, so that 11025 is the
amount you paid for last time and the amount you want to pay for this
time, lumped together. Now this is what we do. We turn to our books and
see how much you were charged with last time, and subtracting that record
from the present record leaves the amount you have used since the last
time of payment.

"Then suppose another case. Your metre registers only as far as 100,000.
At the end of the last quarter it marked 97850; now it records but 3175.
How would you explain that, master Philip?"

Philip looked puzzled a moment, and then said,

"I should think it must have finished out the hundred thousand and begun
over again."

"Exactly. And to find the amount for this quarter you would add together
the remainder of the hundred thousand (2150) and the 3175, and get 5325,
the real record. But I guess you've had arithmetic enough for the
present, so we'll go out now and se