materially affect the house flow. When once built,
a windmill practically takes care of itself, except for oiling, and is
an efficient and cheap domestic possession.

[Illustration: FIG. 125.--The windmill pumps water into the tank.]

178. Steam as a Working Power. If a delicate vane is held at an
opening from which steam issues, the pressure of the steam will cause
rotation of the vane (Fig. 126), and if the vane is connected with a
machine, work can be obtained from the steam.

When water is heated in an open vessel, the pressure of its steam is
too low to be of practical value, but if on the contrary water is
heated in an almost closed vessel, its steam pressure is considerable.
If steam at high pressure is directed by nozzles against the blades of
a wheel, rapid rotation of the wheel ensues just as it did in Figure
121, although in this case steam pressure replaces water pressure.
After the steam has spent itself in turning the turbine, it condenses
into water and makes its escape through openings in an inclosing case.
In Figure 127 the protecting case is removed, in order that the form
of the turbine and the positions of the nozzles may be visible.

[Illustration: FIG. 126.--Steam as a source of power.]

[Illustration: FIG. 127.--Steam turbine with many blades and 4
nozzles.]

A single large turbine wheel may have as many as 800,000 sails or
blades, and steam may pour out upon these from many nozzles.

The steam turbine is very much more efficient than its forerunner, the
steam engine. The installation of turbines on ocean liners has been
accompanied by great increase in speed, and by an almost corresponding
decrease in the cost of maintenance.

179. Steam Engines. A very simple illustration of the working of a
steam engine is given in Figure 128. Steam under pressure enters
through the opening _F_, passes through _N_, and presses upon the
piston _M_. As a result _M_ moves downward, and thereby induces
rotation in the large wheel _L_.

[Illustration: FIG. 128.--The principle of the steam engine.]

As _M_ falls it drives the air in _D_ out through _O_ and _P_ (the
opening _P_ is not visible in the diagram).

As soon as this is accomplished, a mechanical device draws up the rod
_E_, which in turn closes the opening _N_, and thus prevents the steam
from passing into the part of _D_ above _M_.

But when the rod _E_ is in such a position that _N_ is closed, _O_ on
the other hand is open, and steam rushes