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 through it into D and forces up the piston.  This up-and-down motion of the piston causes continuous rotation of the wheel L.  If the fire is hot, steam is formed quickly, and the piston moves rapidly; if the fire is low, steam is formed slowly, and the piston moves less rapidly.

The steam engine as seen on our railroad trains is very complex, and cannot be discussed here; in principle, however, it is identical with that just described.  Figure 129 shows a steam harvester at work on a modern farm.

[Illustration:  FIG. 129.—­Steam harvester at work.]

In both engine and turbine the real source of power is not the steam but the fuel, such as coal or oil, which converts the water into steam.

180.  Gas Engines.  Automobiles have been largely responsible for the gas engine.  To carry coal for fuel and water for steam would be impracticable for most motor cars.  Electricity is used in some cars, but the batteries are heavy, expensive, and short-lived, and are not always easily replaceable.  For this reason gasoline is extensively used, and in the average automobile the source of power is the force generated by exploding gases.