138 



that's it; 



the locomotive through the at- 

 mosphere. 



The tubes, 10 ? which pass 

 through the boiler, tend greatly 

 to security from bursting, be- 

 cause, as they are much weaker 

 than the external case of the 

 boiler, they are almost certain to 

 give way first, and the burs dug 

 of one or two tubes is rarely 

 productive of more serious 

 consequence than extinguishing 

 the fire. 



The steam chamber, a, is neces- 

 sary, because, owing to the limited 

 size of the boiler, and its motion, 

 the steam in the boiler is mixed 

 with spray from the water. A 

 chamber is therefore added, which 

 serves as a reservoir for the pure 

 steam, from which the cylinders 

 are supplied. 



A blast pipe passes from the 

 cylinders into the chimney for 

 the exit of waste steam, or steam 

 that has been used. The puffing 

 noise of the engine is caused by 

 the entry of steam into this pipe, 

 and its escape therefrom. 



Formerly, hot ashes were 

 thrown out from the chimney, 

 with every escape of smoke and 

 steam, causing injury to passen- 

 gers, and frequent fires along the 

 sides of the rail. To prevent 

 this, a wire net was placed across 

 the top of the chimney, which 

 reduced the evil, but did not re- 

 medy it. The wire grating is 

 now placed at the bottom of the 

 chimney, instead of the top ; and 

 as this catches the ashes before 

 they mingle with the escaping 

 steam, which enters the chimney 

 higher up, the inconvenience and 

 danger are overcome. 



As the piston rods are attached directly to 

 cranks on the axle, each piston makes a double 

 stroke for every revolution of the driving wheels ; 

 consequently, when the engine is running at 

 great speed, the movement of the piston is so 

 rapid that there is neither time for the free 

 emission of the waste steam, nor the full action 

 of the high pressure steam admitted. There is, 

 therefore, a great waste of power, occasioned by 

 the admitted steam having to act against that 

 which is escaping. With a driving wheel six 

 feet in diameter, a locomotive engine will be 

 propelled eighteen feet by each double stroke of 

 the piston, if there be no shifting on the rails ; 

 consequently, in the space of a mile the piston 

 must make 300 double strokes per minute. 



The speed attained by railway 

 travelling is at present so great, 

 that a perfect system of signalling 

 is essential to safety. The sig- 

 nals used are of great variety, 

 and various lines have codes 

 peculiar to themselves. The fol- 

 lowing signals^ taken from those 

 used on several lines, may be 

 referred to as an example of 

 others, though they may not 

 be identical. They consist of 

 police signals, signals shown at 

 intermediate stations, and long* 

 tunnels, and the engine signals. 



When the line is clear, a po- 

 liceman^ tan ds passively with the 

 flags in his hand, 11 ; if it be 



445. 



446. 



desired that the engine should 

 slacken speed, and proceed with 

 caution, from another engine 

 having recently passed, or from 



