478 



THE STEAM-ENGINE. 



Fig. 35. 



and the second just before the termination of the stroke. Let the position of 

 the slide, at the commencement of the stroke, be represented in fig. 33, and 

 let it be required that the steam shall be cut off at one half of the stroke. When 

 the piston has made half the stroke, the rod governing the slide is moved 

 downward, so. as to throw the slide into the position represented in fig. 34. [ 

 The passage between the steam-pipe and the cylinder is now stopped at both ' 

 ends ; but the passage from the bottom of the cylinder to the condenser re- j 

 mains open. During the remainder of the stroke, therefore, the steam in the < 

 cylinder works expansively. As the piston approaches the bottom of the cyl- ] 

 inder, another motion is imparted to the rod governing the slide, by which the < 

 latter is thrown into the position represented in fig. 35. Steam now flows be- j 

 low 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. 



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 sud- 

 . denness of opening would be effected. 



The great advantages in the economy of fuel resulting from the application 

 of the expansive principle have, of late years, forced themselves on the atten- 

 tion of engineers, and considerable improvements have been made in its ap- 

 plication, 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 adjust- 

 ment, 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 ad- 



