THE INDICATOR.] 



APPLIED MECHANICS. 



807 



every precaution, there is a diminution of pressure to 

 the extent of several pounds per square inch, especially 

 when the initial pressure is considerable.* Moreover, 

 before the steam enters the cylinder, it has to pass the 

 throttle-valve, which may be partially closed by 

 the governor, and the steam is thus, as it were, 

 vrire-drawn, or made thinner, less dense, and con- 

 sequently capable of pressing with less force on 

 the piston. Were the piston at rest, however 

 small might be the passage for the steam, yet, 

 like water finding its level, its pressure would 

 very quickly become uniform throughout every 

 cavity to which it might have access. But as the 

 piston is in motion, the steam has to flow along 

 the pipe and passages with sufficient rapidity to 

 follow up the piston in its progress ; and if its 

 course be arrested or impeded, that portion of it 

 beyond the impediment must necessarily be of 

 less density than that before it. Again, as the 

 alternate flow of the steam above and below the 

 piston is controlled by means of valves or a slide, 

 the apertures covered by them cannot be opened 

 or closed instantaneously, and there must, there- 

 fore, be at every alternation, moments of transi- 

 tion, during which the passages are throttled and 

 the steam wire-drawn. When the slide is moved 

 by an eccentric, the opening and closing of the 

 port is gradual, and the amount of passage open 

 f'.r e steam is continually changing. If the 

 slidi i.rid eccentric be so adjusted that the upper 

 port is just beginning to open when the piston 

 is at the top, it continues to open for some time 

 during the descent of the piston, until it attains 

 its extreme width. It then begins to close ; and 

 some time before the piston reaches the bottom 

 it must be closed by the slide, which has moved 

 onwards a sufficient distance to be ready for 

 opening the bottom port to admit the steam below 

 the piston. The slide and eccentric are generally 

 so adjusted that each port is closed when the 

 piston has passed through ijrds of its stroke. 

 During the remaining Jrd of the stroke, the 

 pressure on the piston must therefore gradually 

 diminish, for no fresh steam being admitted into 

 the cylinder, that which is already in it becomes 

 expanded in volume, and proportionally dimi- 

 nished in density and pressure. Moreover, when 

 the double eccentric and link motion are employed 

 to work the slide, or when a special valve is 

 provided for cutting off or arresting the ingress of 

 steam to the cylinder at an earlier period of the 

 stroke than that which is determined from the* 

 motion of the slide alone, the engine is said to 

 be worked expansively, and the gradual diminution of 

 pressure during the stroke becomes still more marked. 



Again, after the piston has completed its stroke, the 

 cylinder being filled with steam of such final pressure as 

 may result from the causes we have named, has to empty 

 itself on the return of the piston, through the ports and 

 waste-pipes. To move the steam through these passages 

 demands some force, which acts as a back pressure or 

 resistance on the piston, and thereby diminishes its effec- 

 tive working force. 



Having in view, then, all these causes of change in the 

 steam-pressure during each stroke of the piston, it be- 

 comes important to ascertain what is the mean or aver- 

 ri','i! [pressure throughout, which may be reckoned as the 

 working pressure, or the actual force applied to each part 

 of the surface of the piston to move it against the resist- 

 ance of the machinery on which it acts. 



11 1 E INDICATOR. The indicator, to which we have 

 already referred as having been invented by Watt (see 

 p. 847), is a simple and beautiful instrument, by which 

 this element of power can be ascertained with the greatest 

 accuracy. A (Fig. 108), is a small cylinder open at top, 

 fitted with a piston, and communicating by a pipe and 



We ohall afterwards have occasion to describe the moans of reme- 

 dying thi* evil by super-heating the steam before it puages to the 



stop-cock, with either the upper or lower part of the 

 main cylinder. The piston is pressed down into the 

 cylinder by a nicely adjusted spiral spring, and a pencil 

 B is fixed to the piston-rod. C is a roller, round which 

 Fig. 193. 



a piece of paper is wound ; on the 

 axis of this roller is fitted a pul- 

 ley D, connected by a string with 

 some of the moving parts of the 

 engine. The roller is also fitted 

 with a spring, like the main- 

 spring of a watch, in such a man 

 ner that, after being pulled round 

 in one direction by the motion of 

 the engine communicated through 

 the string, it is made to recoil 

 by the spring. If we suppose 

 the stop-cock closed, the piston, 

 being pressed on by the spring and 

 the atmosphere, will remain at 

 the bottom of the cylinder ; and 

 the pencil being stationary at its 

 lowest point E, while the roller is 

 made to rotate backwards and 

 forwards, will describe a line on 

 the paper which would appear 

 straight on its being unfolded 

 from the roller. But if, while 

 the roller continues its motion, 

 the stop-cock be opened, then the 

 piston will be subjected to the 

 pressure of steam in the main 

 cylinder, and will be forced up- 

 wards in opposition to the pres- 

 sure of the spring and of the atmosphere, and the 

 pencil will trace a line on the paper, varying in 

 height as the piston rises and falls. But farther, 

 if the spiral spring be so adjusted that we know 

 exactly how many pounds will compress it an inch, and 

 if we know the area of the piston, we can, exactly, 

 measure the amount of pressure on it by the height to 

 which the pencil is raised above the neutral line E, 

 where it remains when subjected to no upward pressure. 

 And thus the position of the pencil on the paper, or the 

 mark left by it at any point, furnishes the measure of the 

 pressure on the main piston of the engine at the cor- 

 responding point of 

 its stroke. On un- 

 folding the paper 

 from the roller, we 

 should find a figure 

 (Fig. 199) described 

 on it by the pencil, 

 which, when properly 

 analysed, gives us the 

 means of reckoning 

 the varying pressure 

 on the piston, and * 

 often points out defects in some of the adjustments, and 

 suggests modes of remedying them. 



Fig. 199. 



