STEAM-ENGINE 



703 



attached to the cylinder, a. In this chest are placed 

 valves, which are so regulated as to open com- 

 munication between the boiler, cylinder, and con- 

 denser, in such a way that when the top of the 

 cylinder is open to the boiler the bottom communi- 

 cates with the condenser, and vice vend. When 

 the steam has done its work it passes out through 



Fi. a 



the bent pipe into the condenser, f, where it is 

 met by a jet of water (not shown in the engrav- 

 ing), and condensed, as before explained ; g is a 

 pump called the air-pump, which continually draws 

 away the contents of the condenser, and dis- 

 charges them into a cistern, h, called the hot well. 

 A small force-pump, /, draws part of the water from 

 this cistern, and sends it back again to the boiler, 

 there to be reconverted into steam, while the rest 

 of the water is allowed to run to waste. A suc- 

 tion-pump, k, supplies water to the large tank 

 round the condenser, and also for the condensing 

 jet. Inside the cylinder are the piston and the 

 rod (called the piston-rod) connecting it with the 

 beam, 66. In Newcomen's engine the rod had only 

 to pull the beam down, and not to push it up ; it 

 could, therefore, be connected to it by a chain, as 

 shown in fig. 1. In the double-acting engine the 

 piston-rod is required both to pull and to push the 

 beam, so that the chain is no longer admissible. 

 It is obvious that as the head of the rod must 

 move in a straight line, while every point in the 

 beam describes an arc of a circle, the two cannot be 

 rigidly connected. Watt invented the arrange- 

 ment of rods shown in fig. 3, by which the piston- 

 rod head is always guided in a straight line, while 

 the end of the beam is left free to pursue its own 

 course. This is called a 'parallel motion.' The 

 end of the beam farthest from the cylinder is con- 

 nected by a rod, cc, called a connecting-rod, to the 

 crank, /, which is firmly fixed on the shaft ; and by 

 this means the reciprocating motion of the beam is 

 converted into the rotary motion of the 'crank- 

 shaft,' r. The governor, m, and the flywheel, ee, 

 will be explained further on. 



The cylinder and its piston are both made of 

 cast-iron. The former is very accurately bored 

 in a spfrin] machine, and ought always to be 

 covered outside with non-conducting material to 

 prevent radiation of heat. It is frequently en- 



closed in another cylinder, and the annular space 

 or ' jacket ' between them filled with steam from 

 the boiler, principally with the object of prevent- 

 ing liquefaction in tne cylinder, which is fatal to 

 economical working. The openings for the en- 

 trance and discharge of the steam ( shown at C and 

 D in fig. 2) are both called ports. 



The valve or valves which 

 regulate the admission of 

 steam to the cylinder vary 

 very much in construction and 

 design. In ordinary engines 

 one valve, called a slide-valve, 

 does the whole work for each 

 cylinder in a way which we 

 shall explain by the aid of 

 fig. 4. This figure shows the 

 valve in two positions viz. 

 those corresponding to the 

 times when the piston is at 

 the middle of its stroke, going 

 in the two different directions; 

 c and (I are the ports, the ends 

 of which are denoted by the 

 same letters in fig. 2 ; 6 is 

 the 'exhaust port,' or open- 

 ing through which the steam 

 passes to the condenser; and 

 a is the slide-valve working 

 inside the steam-chest (the 

 latter not shown). The 

 sketch to the left shows the 

 position of the valve when 

 the piston is moving upwards. 

 The steam enters the cylinder 

 through d, as shown oy the 

 arrows, while the steam in 

 the other end is free to rush 



out by c under the valve, and through 6 into the 

 condenser. By the time the piston has reached 

 the same position, going in the opposite direc- 

 tion, the valve is in the position shown in the 

 right-hand sketch, and the motion of the steam is 

 exactly reversed. The valve in fig. 4 opens one 

 port at the same moment as it closes the other. 

 This corresponds to entirely non-expansion work- 

 ing. In order to ' cut off' the steam before the end 

 of the stroke the 

 breadth of the ends of 

 the valve must be in- 

 creased. This is called 

 giving ' tap ' to the 

 valve. When it is de- 

 sired to 'cut off' the 

 steam earlier than half- 

 stroke, a separate valve, 

 called an expansion 

 valve (of which there 

 are innumerable varie- 

 ties), is generally used. 

 The rod to which the 

 piston is attached is 

 called the piston-rod, 

 and the rod whicli actu- 

 ally drives the crank 

 the connecting-rod. In 

 Watt's engine and similar machines these are con- 

 nected to opposite ends of a beam, but in the com- 

 mon type of engine shown in fig. 6 (below) the 

 two rods are directly attached. The flywheel is a 

 large wheel fixed on the crank-shaft, and having 

 a very heavy rim. As it revolves this contains, 

 stored up in itself, a great quantity of energy, and 

 so equalises the motion of tne shaft, and by restor- 

 ing some of the energy enables the engine to pass 

 the 'dead-points,' or points at which the connect- 

 ing-rod and crank are in a line. The condenser is 

 simply a cast-iron box of any convenient shape. 



Fig. 4. 



