"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 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.