EXPLANATION OF THE PLATES. 331 



PLATE IV. 



Fig. 1 . is a section of the parts of a high pressure engine with a four-passaged 

 cock. The engine is supposed to be partly within the boiler, of which D B is the 

 top plate. P is the steam piston, and R the piston rod, A is the four-passaged 

 cock ; the steam enters from the boiler at S, and passes through t to the top of the 

 piston, and the steam below escapes through the passage b, and pipe a and E, to 

 the atmosphere ; the pipe E is surrounded by water, which the escaping steam 

 warms ready for the boiler. By turning the cock the motions are reversed, but it 

 is obvious we cannot in this engine employ the expanding force of the steam. The 

 motion is regulated by a throttle valve V. See art. 356 361. 



Fig. 2. and 3. show a section and plan of a similar engine, with a D-slide 

 instead of a cock ; the steam enters from the boiler at S, and by the passages being 

 shut and opened close to the extremities of the cylinder, there is no loss by the 

 communicating pipes being filled with strong steam. See art. 364. This engine 

 will not work expansively unless the construction of the slide be altered. See 

 art. 371. Contrary to the usual practice, the packing of the slide is on the sliding 

 part ; the advantage of this plan is obvious, but the practical difficulty of boring 

 a semi-cylinder is incurred. 



Fig. 4. is a simple arrangement of the high pressure engine by which the 

 expanding power of the steam may be used ; it is the invention of Murdoch. The 

 passages are opened and closed by pistons sliding in a pipe : the steam enters this 

 pipe at S, and the steam is supposed to be just shut off by the upper piston, so that 

 by the expansion of that in the cylinder the rest of the stroke is completed, the 

 passage E to the atmosphere being still open. See art. 371 380. The slide 

 would be improved by making it of the form of a D-slide. 



The construction of the pistons of the slide is a suggestion which may perhaps 

 answer better than the common ones, (art. 450. and note.) 



Fig. 5. is an arrangement to illustrate the action of a high pressure engine to 

 work expansively by means of combined cylinders. See art. 381 383. 



