MEANS OF COMMUNICATION. 25 
plate at the entrance of the main steam-pipe s; when the movable plate is 
turned by the crank so that its openings correspond with those in the 
stationary one, the way is opened for the steam from p tos; if the movable 
plate be turned a quadrant, then the openings are closed and the steam is 
shut off from s. 
The main steam-pipe s passes through the forward end plate of the 
boiler, and is connected with two descending pipes which carry the steam 
to the steam-boxes vu vu, whence it is delivered by the slide-valves to the 
cylinders ; the cast-iron steam-boxes are situated immediately over the 
cylinders, and are screwed fast at their ends to the boiler plates. The slide- 
valves also communicate with the exhaust-pipe r, which conducts the steam 
after it has operated upon the pistons into the chimney «a, as indicated by 
the arrows (pl. 4, jig. 2). This almost interrupted stream of steam into the 
chimney creates a powerful draught, which in stationary engines is accom- 
plished by the height of the chimney. Rods connected with the slide- 
valves, and passing through stuffing-boxes in the steam chambers, which 
are actuated in a manner hereafter explained, serve to bring the valves in 
a proper position to throw the steam alternately upon one side or other of 
the pistons. When the slide-valve is in the position seen in pl. 4, jig. 30, 
the steam is thrown upon the back of the piston, while the space in front 
of the piston is opened to the exhaust-pipe. When a contrary motion takes 
place in the slide-valves, then the space at the back of the piston is thrown 
open to the exhaust-pipe, and the steam rushes out from this side of the 
piston. The piston passes through a stuffing-box in the head of the steam- 
cylinder, to prevent leakage of the steam and the consequent loss of power. 
The end of the piston-rod is attached to a cross-head which runs upon ways, 
and is connected by a joint with a connecting-rod, which embraces at its 
other end the crank upon the shaft of the main driving-wheel, which thus 
receives its motion from the piston. 
We come now to the apparatus which serves to regulate the motion of 
the slide-valves; in general this is accomplished by eccentrics, of which at 
least two are necessary in the locomotive, as there are two cylinders to be 
operated. These eccentrics are seen at E, in jigs. 3, 4, and 5, and give 
motion to the connecting-rods e f, which operate the double-armed levers 
Z ~, connected with the rods, m, which move the slide-valves. The eccen- 
trics are so arranged that they admit the steam to the cylinder just before the 
piston reaches the end of its stroke; the steam thus operates as a cushion to 
receive the blow of the piston, and prevent the injury which would other- 
wise result to the machinery. This is called the lead of the engine. /%2g.6 
shows an arrangement of double eccentrics, for the purpose of reversing the 
motion of the engine. 
The wheels are an important part of the locomotive; they are constructed 
in a great variety of ways. In the machine before us two kinds of wheels 
are used. The middle or driving-wheels have no flange, and a diameter of 6 
feet ; the other two pair, the running-wheels, are 4 feet in diameter, and are 
furnished with flanges, which serve to retain the locomotive upon the rails. 
The rim of the wheels consists of two concentric hoops. The interior, to 
605 
