"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 partitions 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 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.