s meter indicates the number of cubic
feet of gas consumed.]

The instrument itself is somewhat complex. It will suffice to say that
within the meter box are thin disks which are moved by the stream of
gas that passes them. This movement of the disks is recorded by
clockwork devices on a dial face. In this way, the number of cubic
feet of gas which pass through the meter is automatically registered.

89. The Relation between Pressure and Volume. It was long known that
as the pressure of a gas increases, that is, as it becomes compressed,
its volume decreases, but Robert Boyle was the first to determine the
exact relation between the volume and the pressure of a gas. He did
this in a very simple manner.

Pour mercury into a U-shaped tube until the level of the mercury in
the closed end of the tube is the same as the level in the open end.
The air in the long arm is pressing upon the mercury in that arm, and
is tending to force it up the short arm. The air in the short closed
arm is pressing down upon the mercury in that arm and tending to send
it up the long arm. Since the mercury is at the same level in the two
arms, the pressure in the long arm must be equal to the pressure in
the short arm. But the long arm is open, and the pressure in that arm
is the pressure of the atmosphere. Therefore the pressure in the short
arm must be one atmosphere. Measure the distance _bc_ between the top
of the mercury and the closed end of the tube.

[Illustration: FIGS. 54, 55.--As the pressure on the gas increases,
its volume decreases.]

Pour more mercury into the open end of the tube, and as the mercury
rises higher and higher in the long arm, note carefully the decrease
in the volume of the air in the short arm. Pour mercury into the tube
until the difference in level _bd_ is just equal to the barometric
height, approximately 32 inches. The pressure of the air in the closed
end now supports the pressure of one atmosphere, and in addition, a
column of mercury equal to another atmosphere. If now the air column
in the closed end is measured, its volume will be only one half of its
former volume. By doubling the pressure we have reduced the volume one
half. Similarly, if the pressure is increased threefold, the volume
will be reduced to one third of the original volume.

90. Heat due to Compression. We saw in Section 89 that whenever the
pressure exerted upon a gas is increased, the volume of the gas is
decreased; and that wheneve