cylinder, another motion is imparted to the rod governing the slide, by which the latter is thrown into the position represented in _fig._ 54. Steam now flows below the piston while the steam above it passes to the condenser. In a similar manner, by two motions successively imparted to the slide during the ascent of the piston, the steam may be cut off at half stroke; and it is evident that by regulating the time at which these motions are given to the slide, the steam may be worked expansively, to any required extent. [Illustration: _Fig._ 52.] [Illustration: _Fig._ 53.] [Illustration: _Fig._ 54.] It is easy to conceive various mechanical means by which, in the same engine, the point at which the steam is cut off may be regulated at pleasure. In cases where the motion of the piston is very rapid, as in locomotive engines, it is desirable that the passages to and from the cylinder should be opened very suddenly. This is difficult to be accomplished with any form of slide consisting of a single aperture; but if, instead of admitting the steam to the cylinder by a single aperture, the same magnitude of opening were divided among several apertures, then a proportionally less extent of motion in the slide would clear the passage for the steam, and consequently greater suddenness of opening would be effected. [Pg234] The great advantages in the economy of fuel resulting from the application of the expansive principle have, of late years forced themselves on the attention of engineers, and considerable improvements have been made in its application, especially in the case of marine engines used for long voyages, in which the economy of fuel has become an object of the last importance. The mechanism by which expansive slides are moved, is made capable of adjustment, so that the part of the stroke at which the steam is cut off, can be altered at pleasure. The working power of the engine, therefore, instead of being controlled by the throttle-valve, is regulated by the greater or less extent to which the expansive principle is applied. Steam of the same pressure is admitted to the cylinder in all cases; but it is cut off at a greater or less portion of the stroke, according to the power which the engine is required to exert.