CHAP, ix.] THE LOCOMOTIVK. 460 



The locomotive is in reality, so far as regards the driving machinery, 

 formed of two steam-engines coupled together. There are two cylin- 

 ders, each provided with its piston and its slide-valve, and each 

 piston-rod acts, by a connecting-rod, on the crank or knee of the axle 

 which carries the pair of driving-wheels. There are even in some 

 kinds of locomotives four cylinders and four engines, working two by 

 two, on two different axles. There is nothing special, except in 

 the working and the details, to distinguish the driving-machinery 

 from that we have seen at work in fixed engines, whether on land 

 or sea. The drawings given show the arrangement of the cylinders, 

 which are generally placed in front, sometimes horizontal, 'sometimes 

 slightly inclined, sometimes placed outside the framework containing 

 the boiler and engine, sometimes inside. In the figure the cylinders 

 are inside and horizontal. This is the arrangement generally preferred 

 in England. 



Our longitudinal and transverse sections of a locomotive show its 

 arrangements clearly. In Fig. 314 the distribution and escape of the 

 steam are shown. The steam, which is brought by the pipe ss as far 

 as the space called the smoke-box, finds there two conduits, uu, which 

 end, after making a turn, in the steam-chests of the two cylinders ; 

 after having acted on the pistons, it crosses the pipes v v, and by the 

 vertical pipe v, which opens at the base of the chimney, it escapes 

 and produces a sudden puff, which one always hears in a moving 

 locomotive. 



The rapidity with which these noises, produced by the escape of 

 the steam, succeed each other when the train is at full speed indicates 

 the number of strokes of the piston in each cylinder. The number 

 may be calculated according to the velocity of the train. In express 

 trains this velocity reaches forty miles an hour, and if we suppose 

 this distance run by a passenger-engine whose driving-wheel is 7 ft. 

 9 in. diameter, or 24 feet circumference, the engine has made 8800 

 turns of the wheel, each of which corresponds to a double stroke of 

 the pistons. This is 2 J double, or 5 single strokes per second. 



