THE STEAM-ENGINE. 



479 



mitted 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. 



The last degree of perfection has been conferred on this principle by con- 

 necting 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 in- 

 creased, 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 in- 

 crease 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 cyl- 

 inder 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 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 dif- 

 fused 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. 



Mr. Samuel Seaward, of the firm of Messrs. Seawards, engineers, has con- 

 trived an improved system of slides, for which he has obtained a patent. A 

 section of Seaward's slides is represented in fig. 36. The steam-pipe pro- 

 Fig. 36. 



ceeding from the boiler to the cylinder is represented ai A A, and it commu- 

 nicates with passages S and S' leading to the top and bottom of the cylinder. 



