As the wheel rotates, its motion is communicated to a mechanical device which makes use of it to raise and lower a plunger, and hence as long as the wind turns the windmill, water is raised. The water thus raised empties into a large tank, built either in the windmill tower or in the garret of the house, and from the tank the water flows through pipes to the different parts of the house. On very windy days the wheel rotates rapidly, and the tank fills quickly; in order to guard against an overflow from the tank a mechanical device is installed which stops rotation of the wheel when the tank is nearly full. The supply tank is usually large enough to hold a supply of water sufficient for several days, and hence a continuous calm of a day or two does not 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).