Chat. XXI.

LOCOMOTIVE ENGINES.

1277

chambers, and with one or other of the openings which lead to the front or back of thepiston, giving the first impulse to the moving powers. When the steam is admittedthrough the regulator into the pipes which conduct to the steam chamber or valve-box, itthere presses the sliding valve lightly upon its horizontal plane; this, hpwever, is movedbackwards and forwards by means of a rod, having four sheaves or rings fixed upon theaxles of the driving-wheels, upon false centres or eccentrics, so that at each revolution ofthe axle the slide is moved backwards and forwards, admitting the steam both before and

Fig. 2048.

WATER GAUGE.

Fig.2049 MITRE VALVE.

behind the piston. Thus an alternate motion is kept up by the slide passing, first, over oneopening, then the other; the steam passes by the third aperture into the chimney, where itcauses an additional draught to the fire. The motion given to the slide, however, is alittle in advance of the required stroke of the piston, it being necessary that the steam shouldbe admitted before the piston commences moving, so that it may exert its full powerupon it; and this is effected by giving a lead to the slide The eccentric attached to the

Fig. 20. r )0.

SLIDE VALVE AND ECCENTRIC.