The last degree of perfection has been conferred on this principle by connecting the governor with the mechanism by which the slide is moved, so that the governor instead of acting on the throttle-valve, is made to act upon the slide. By this means when, by reason of any diminution of the resistance, the motion of the engine is accelerated, the balls of the governor diverging shift the cam or lever which governs the slide, so that the steam is cut off after a shorter portion of the stroke, the expansive principle is brought into greater play, and the quantity of steam admitted to the cylinder at each stroke is diminished. If, on the other hand, the resistance to the machine be increased, so as to diminish the velocity of the engine, then the balls collapsing the levers of the governor shift the cam which moves the slides, so as to increase the portion of the stroke made by the piston before the steam is cut off, and thereby to increase the amount of mechanical power developed in the cylinder at each stroke. The extent to which the expansive principle is capable of being applied, more especially in marine engines, has been hitherto limited by the necessity of using steam of very high pressure, whenever the steam is cut off after the piston has performed only a small part of the stroke. A method, however, is now (March, 1840) under experimental trial, by [Pg235] Messrs. Maudsley and Field, by which the expansive principle may be applied to any required extent without raising the steam in the boiler above the usual pressure of from three to five pounds per square inch. This method consists in the use of a piston of great magnitude. The force urging the piston is thus obtained not by an excessive pressure on a limited surface, but by a moderate pressure diffused over a large surface. The entire moving force acting on the piston before the steam is cut off, is considerably greater than the resistance; but during the remainder of the stroke this force is gradually enfeebled until the piston is brought to the extremity of its play. [Illustration: _Fig._ 55.]