5
lbs. to the square inch. Suppose that to a very long tube having a
sectional area of one square inch we fit an air-tight piston (Fig. 172),
and place the lower end of the tube in a vessel of water. On raising the
piston a vacuum would be created in the tube, did not the pressure of
the atmosphere force water up into the tube behind the piston. The water
would continue to rise until it reached a point 34 feet perpendicularly
above the level of the water in the vessel. The column would then weigh
15 lbs., and exactly counterbalance the atmospheric pressure; so that a
further raising of the piston would not raise the water any farther. At
sea-level, therefore, the _lifting_ power of a pump by suction is
limited to 34 feet. On the top of a lofty mountain, where the
air-pressure is less, the height of the column would be diminished--in
fact, be proportional to the pressure.

[Illustration: FIG. 172.]

[Illustration: FIG. 173.]

THE SIPHON

is an interesting application of the principle of suction. By its own
weight water may be made to lift water through a height not exceeding 34
feet. This is explained by Fig. 173. The siphon pipe, A B C D, is in the
first instance filled by suction. The weight of the water between A and
B counter-balances that between B and C. But the column C D hangs, as it
were, to the heels of B C, and draws it down. Or, to put it otherwise,
the column B D, being heavier than the column B A, draws it over the
topmost point of the siphon. Any parting between the columns, provided
that B A does not exceed 34 feet, is impossible, as the pressure of the
atmosphere on the mouth of B A is sufficient to prevent the formation of
a vacuum.

THE BUCKET PUMP.

We may now pass to the commonest form of pump used in houses, stables,
gardens, etc. (Fig. 174). The piston has a large hole through it, over
the top of which a valve is hinged. At the bottom of the barrel is a
second valve, also opening upwards, seated on the top of the supply
pipe. In sketch (_a_) the first upstroke is in progress. A vacuum forms
under the piston, or plunger, and water rises up the barrel to fill it.
The next diagram (_b_) shows the first downstroke. The plunger valve
now opens and allows water to rise above the piston, while the lower
closes under the pressure of the water above and the pull of that below.
During the second upstroke (_c_) the water above the piston is raised
until it overflows through the spout, while a fres