274 LECTURE XXIX. 



[It will be seen, that by far the larger part of the machine is the boiling 

 apparatus; the working machinery occupying only the comparative!} 

 small space below. That portion of the boiler which contains water is 

 shaded in the figure ; and the tubes are seen at E, by which the flame 

 penetrates the whole body of the boiler from the fire-box C to the 

 smoke-box F. From every part of the surface of the water, steam is rapidly 

 and constantly emitted; but it has no way of escape from the boiler, 

 except tjy ascending the steam dome T, in which the mouth of the steam 

 pipe d f Is situated. After entering the steam pipe, it has to traverse the 

 whole length of the boiler d'SS before it reaches the cylinder. The .object 

 of this arrangement is to separate from the steam a quantity of water, 

 which, being raised by the violence of ebullition, would otherwise be 

 carried along to the cylinder. The same arrangement facilitates the 

 regulation of the steam, by bringing it into the immediate neighbourhood 

 of the engine driver, who is enabled to increase or diminish the supply which 

 is furnished to the cylinder, by means of a winch h f acting on a valve e'. 

 As the steam pipe is everywhere inclosed in steam, there is no loss of tem- 

 perature on this account, except a very trifling amount due to the time 

 which elapses between the production of the steam, and its application to do 

 its work. Two safety valves are placed in the upper part of the boiler ; one 

 at 0, loaded with a constant weight, and out of the reach of the conductor ; 

 the other at N. 



In the steam chest at U is the D valve, admitting steam to the front or 

 back of the cylinders W, which are horizontal, and alternately suffering it 

 to escape by the waste port up the blast pipe />, to increase the draught of 

 the chimney, as already mentioned. The construction of the working 

 machinery is of the most simple kind. An axle, bent so as to form two 

 cranks, at right angles to each other, is attached to the two driving wheels. 

 These are larger than the other wheels, of which there are usually two 

 pair, provided with flanges or rims on the inside of their circumference, for 

 the purpose of retaining the machine on the rail. Thus the axle and 

 driving wheels of this engine are analogous to the shaft and paddle wheels 

 of the marine engine. There is no beam, but the piston rods Y being con- 

 fined by guide bars, which allow them to play backwards and forwards 

 through the space of about 18 inches, are attached immediately at their 

 extremities to the connecting rods which act on the cranks. These being 

 at right angles to each other, the force is equalised as in the marine engine. 

 The valve machinery consists, as usual, of an eccentric and levers, but in 

 the locomotive, each cylinder is provided with two sets of eccentrics, the 

 one being the reverse of the other, that is, tending to move the valve back 

 when the other would move it forwards. A lever is in the direction of the 

 driver, by means of which one set may be thrown out of gear, whilst 

 the other is thrown in. The rod of the eccentric which is not in gear is 

 seen at/". It terminates in a Y, so that when raised it will readily catch 

 the working levers (at M, fig. 3). By this means the action of the engine 

 can be instantaneously reversed. 



It will be seen from this description that the locomotive is by far the 

 most simple form of the steam engine. As, however, it is applied to per- 

 form work in which great speed is necessary ; so much so as to require that] 



