THE STEAM-ENGINE. 



motion of the beam into a rotary motion by 

 means of a crank (both these were invented 

 before 1778, the engine being patented in 1782, 

 but the crank having before that date been pirated 

 and patented by another). In 1784, Watt also 

 patented and published his parallel motion, 

 throttle-valve, governor, and indicator ; all four 

 of which are in substance still used. The prin- 

 cipal improvements since the time of Watt have 

 been either in matters relating to the boiler ; in 

 details of construction consequent on our increased 

 facilities, improved machinery, and greater know- 

 ledge of the strength of materials ; in the enlarged 



application of his principle of expansive working ; 

 or in the application of the steam-engine to the 

 propulsion of carriages and vessels. 



We ^have said that Watt invented the ' double- 

 acting' engine. By this we mean such an engine 

 as was sketched in fig. i, in which the steam acts 

 on both sides of the piston, instead of only on 

 one, as in Newcomen's engine. Watt's engine, 

 although not of the form now generally used, con- 

 tains all the parts now considered essential ; and 

 before describing these parts in detail, we will 

 therefore describe it as it is illustrated in fig. 8. 

 The steam from the boiler passes direct to the 



Fig. 8. 



valve-chest, v, which is simply a long box attached 

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

 which are so regulated as to open communication 

 between the boiler, cylinder, and condenser, in 

 such a way that when the top of the cylinder is 

 open to the boiler, the bottom communicates with 

 the condenser, and vice -versd. When the steam 

 has done its work, it passes out through the bent 

 pipe into the condenser, f, where it is met by a 

 jet of water (not shewn in the engraving), and 

 condensed, as before explained, g is a pump 

 called the air-pump, which continually draws away 

 the contents of the condenser, and discharges 

 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 suction- 

 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, bb. 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 shewn in fig. 7. 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 i 



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 arrangement of rods shewn in fig. 8, 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 furthest from the 

 cylinder is connected by a rod, cc, called a con- 

 necting-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 fly-wheel, ee, will be explained further on. 



Having now given a general idea of the progress 

 of the steam-engine up to the time of Watt, and of 

 its construction and arrangement, we devote the 

 remainder of our space to a detailed description 

 of the construction and uses of its various parts, 

 as they are made in modern engines, and to a 

 glance at the principal modifications of Watt's 

 engine which are now in use. 



The principal materials used in the construc- 

 tion of steam-engines are cast-iron, malleable or 

 wrought iron, and brass. Steel and copper are 

 also used to a small extent. All parts of com- 

 plex form, such as the cylinder, bedplate, &c. are 

 made of cast-iron. The moving parts, such as 

 the piston and connecting rods, shaft, valve-gear, 



435 



