h supply is being
sucked in below.

[Illustration: FIG. 174.]

THE FORCE-PUMP.

[Illustration: FIG. 175. Force-pump; suction stroke.]

[Illustration: FIG. 176. Force-pump; delivery stroke.]

For driving water to levels above that of the pump a somewhat different
arrangement is required. One type of force-pump is shown in Figs. 175,
176. The piston now is solid, and the upper valve is situated in the
delivery pipe. During an upstroke this closes, and the other opens; the
reverse happening during a downstroke. An air-chamber is generally
fitted to the delivery pipe when water is to be lifted to great heights
or under high pressure. At each delivery stroke the air in the chamber
is compressed, absorbing some of the shock given to the water in the
pipe by the water coming from the pump; and its expansion during the
next suction stroke forces the water gradually up the pipe. The
air-chamber is a very prominent feature of the fire-engine.

A _double-action_ force-pump is seen in Fig. 177, making an upward
stroke. Both sides of the piston are here utilized, and the piston rod
works through a water-tight stuffing-box. The action of the pump will be
easily understood from the diagram.

[Illustration: FIG. 177.]

THE MOST MARVELLOUS PUMP

known is the _heart_. We give in Fig. 178 a diagrammatic sketch of the
system of blood circulation in the human body, showing the heart, the
arteries, and the veins, big and little. The body is supposed to be
facing the reader, so that the left lung, etc., is to his right.

[Illustration: FIG. 178.--A diagrammatic representation of the
circulatory system of the blood.]

The heart, which forces the blood through the body, is a large muscle
(of about the size of the clenched fist) with four cavities. These are
respectively known as the right and left _auricles_, and the right and
left _ventricles_. They are arranged in two pairs, the auricle
uppermost, separated by a fleshy partition. Between each auricle and its
ventricle is a valve, which consists of strong membranous flaps, with
loose edges turned downwards. The left-side valve is the _mitral_ valve,
that between the right auricle and ventricle the _tricuspid_ valve. The
edges of the valves fall together when the heart contracts, and prevent
the passage of blood. Each ventricle has a second valve through which it
ejects the blood. (That of the right ventricle has been shown double for
the sake of convenience.)

The action of