Part 4 (1/2)

The next room they entered had a row of great, square chests on each side, as they walked through.

”These are the purifiers,” explained Mr. Carter again. ”They are boxes with a great many fan-like shelves inside, projecting out in all directions, and 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.

[Ill.u.s.tration: 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 pa.s.s 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?”

[Ill.u.s.tration: 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 part.i.tions 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 filled pa.s.ses 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 pa.s.sing 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 pa.s.sing completely around once, the pointer on the next dial (which marks up to one thousand) is moving a short s.p.a.ce 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 pa.s.sed_, 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 pa.s.s through the metre the very first thing, before any of it is used; and each little metre keeps as strict an account of what pa.s.ses through from the main to the burners, as the large one here in the office does of that which pa.s.ses 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 pa.s.ses 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 pa.s.sed 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 see the gasometer, or gas reservoir.”