CHAMBERS'S INFORMATION FOR THE PEOPLE. 



&c. or any pieces subjected to tensile strain, and 

 all bolts, nuts, and other fastenings, are made of 

 wrought-iron, forged in the smithy into the desired 

 shape, and afterwards finished in the machine- 

 shop. Brass or gun-metal is used generally for 

 parts working under water, as well as for most of 

 the wearing surfaces about the engine. Steel is 

 sometimes used for rods and bolts, when great 

 strength has to be combined with lightness. 

 Copper is used largely for the pipes where light- 

 ness is any object, but cast-iron is the common 

 material in other cases. 



The Cylinder. This is always made of cast- 

 iron, and is generally covered outside with non- 

 conducting material, to prevent radiation of heat, 

 and often inclosed in a jacket supplied with steam 

 from the boiler, in order to prevent loss from seve- 

 ral causes principally from internal condensation. 

 In fig. 9, AA is the cylinder, and B the jacket. 

 The piston-rod, E, is made to work through a 

 ' stuffing-box,' F, in order to prevent the leakage 

 of steam. C and D shew the ports, or openings 

 for the entrance and discharge of the steam. They 

 lead from the cylinder into a valve-chest or box 

 in which the valves (to be afterwards described) 

 work. 



Fig. 9. 



The Piston (P, fig. 9) is also made of cast-iron. 

 It is obviously necessary that its circumference 

 should fit exactly the bore of the cylinder, even 

 after long working, and it is made to do so by 

 means of springs or other contrivances. 



The Valve or valves that regulate the admission 

 of steam to the cylinder are very various in con- 

 struction and design. Sometimes they are four in 

 number, each end of the cylinder having both an 

 induction and an eduction valve. Sometimes two 

 valves are used ; but in ordinary modern engines, 

 one valve (called a slide-valve) is made to do the 

 whole work for each cylinder, in a way which we 

 will explain by the aid of fig. 10. This figure 

 shews the valve in two positions namely, those 

 corresponding to the times when the piston is at 

 the middle of its stroke, going in the two different 

 directions c and d are the ports, the ends of 

 which are denoted by the same letters in fig. i ; b 

 is the 'exhaust port,' or opening through which 

 the steam passes to the condenser ; and a is the 

 slide-valve working inside the steam-chest (not 

 shewn). The sketch to the left shews the position 

 of the valve when the piston is moving up- 

 wards. The steam enters the cylinder through 

 d, as shewn by the arrows, while the steam in the 

 other end is free to rush out by c, under the valve, 

 and through b into the condenser. By the time 

 the piston has' reached the same position, going 

 in the opposite direction, the valve is in the posi- 



426 



tion shewn in the right-hand sketch, and the 

 motion of the steam is exactly reversed. 



Fig. 10. 



By the very simple expedient of lengthening the 

 ends of the valve, the engine can be worked ex- 

 pansively. The amount by which each end is 

 longer than the width of the port is called the lap 

 of the valve ; and the greater the amount of the 

 lap, the earlier in the stroke is the steam cut off. 

 We have not space to explain the reason of this, 

 but it will be found fully gone into in most of the 

 popular treatises on the steam-engine. Various 

 practical considerations make it unadvisable to 

 cut off the steam much before half-stroke by 

 means of lap, and therefore, when it is desired to- 

 expand the steam into more than twice its original 

 volume, a separate valve is employed, called an 

 expansion valve. Numberless varieties of these 

 valves are used, some working on the back of the 

 slide-valve, and some in a separate chamber ; the 

 principle of them all is that they merely affect the 

 entrance of steam into the cylinder, while its exit 

 is controlled by the slide-valve. Expansion valves 

 are generally so arranged that the engineer can 

 alter the degree of expansion at will, by moving a 

 wheel or lever. 



It will be seen that if the valve only commenced 

 to open the port at the same instant that the 

 piston was ready to commence its stroke, the 

 engine could not work, unless the momentum of 

 the moving parts was sufficient to keep the shaft 

 in motion, and so carry the piston on, until the 

 valve was sufficiently open to admit the steam. 

 Although this would generally be the case, yet it 

 is of great importance that the admission of steam 

 should be as prompt and quick as possible ; and 

 with this object engineers always allow the valve 

 what is called lead. This simply means that the 

 slide-valve is allowed to have opened the port 

 about one-eighth of an inch before the piston has 

 really arrived at the end of its stroke. 



The valves are almost invariably worked by 

 eccentrics, through the intervention of suitable rods 

 and levers called the valve-gear. An eccentric is 

 an ingenious substitute for a crank. It consists 

 essentially of a circular disc of metal with a hole 

 in it. The centres of the disc and hole do not 

 coincide, and hence its name of eccentric. The 

 shaft of the engine passes through the hole, and 

 the shaft and eccentric are so fixed together that 

 the latter must revolve with the former. A ring 

 called a ' strap ' is placed round the eccentric, and 

 connected by a rod to the spindle of the valve. 



