The ordinary steam engine labors under the disadvantage of having to start and stop its pistons at the end of each stroke. Every body possesses inertia, whether it be moving or at rest. If it be at rest, it takes much more energy to set it in motion than to keep it moving. In fact it would keep on moving without further expenditure of energy were there no friction and no forces acting against it. In order to stop the body energy must be expended to overcome its inertia. The more rapidly a body is started and the more quickly it is stopped, the more work must be done in overcoming its inertia. In a steam engine not only the piston but other parts connected to it may be required to reciprocate several hundred times per minute. A great deal of energy is uselessly expended in starting and stopping these parts. Many efforts have been made to produce a rotary engine in which the piston rotates instead of reciprocating, thus doing away with the work of overcoming inertia. However, there are serious obstacles to the construction of such an engine, and as yet no truly efficient and practical rotary engine has been built. DE LAVAL’S STEAM TURBINE However, there is another type of engine in which the steam is applied continuously and all the parts revolve. Such an engine was the reaction turbine invented by Hero, to which reference has already been made. Modern turbines, however, are of very different construction. They resemble the Pelton wheels and turbines used in developing water power, differing from them mainly in the fact that use is made of the expansive energy of steam which is lacking in water. In the De Laval steam turbine a wheel is used which has a series of curved buckets all around its periphery that are closed at the outer end by a circular rim. (See Figure 48.) Steam is directed against this bucket, not tangentially as in a Pelton wheel, but from the side. Several steam nozzles are employed and as the steam jets strike the buckets and sweep around their curved surfaces they react against the buckets and drive the wheel around. In order to operate efficiently the velocity of the steam must be very high and the wheel must also turn at high speed. When steam flows through a diverging nozzle its velocity is greatly accelerated by its expansive effort. Such nozzles are used in the De Laval turbines and the steam issues from them with a velocity which may be higher than that of a rifle bullet. The buckets are forged and the hard-scale surface is left on them; otherwise they would wear away quickly under the action of the powerful jets of steam. [Illustration: FIG. 48.--THE DE LAVAL STEAM TURBINE]