"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 see 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.