94

MODERN STEAM PRACTICE.

thereby tending to injure the plates, by a thick coating, throughwhich the heat cannot effectually act on the water, or, as it were, thewater cannot keep the plates sufficiently cool and in proper workingcondition. This heating of the plates, whether from violent priming,or from carelessness in the attendant allowing the water to fall belowthe crowns of the furnaces, is the main cause why efficient steam-boiiers at times explode. As already stated at page 32, Steel platesare now being used for boilers, a reduction in thickness beingthereby effected of about 20 per cent., and on the whole weight ofthe boiler about 10 per cent.

THE REGULATION OF STEAM BY THE SLIDE-VALVE,

AS APPLIED TO LAND, LOCOMOTIVE , AND MARINE ENGINES.

The reciprocating motion imparted to the piston of the steam-engine is caused by the steam acting alternately on the top andbottom of the piston; passages, or ports, as they are technicallytermed, being formed in the cylinder to admit the steam: these portshaving a valve so arranged as to admit the steam above and belowthe piston alternately, with means of allowing it afterwards to escapeinto the atmosphere, or into the condenser, as the case may be. Thevalve in its original form was simply an oblong box of cast-iron,open on the front or face, having a flange all round, this face slidingon a corresponding part on the cylinder, both being accurately facedup, and made perfectly steam-tight, reciprocating motion being im-parted to the valve by a simple arrangement similar to the crankand connecting-rod for the piston. This valve, from its peculiarsliding action, rubbing against a corresponding face on the cylinder,is termed the slide-valve. The older valve arrangements admittedthe steam during the entire travel or stroke of the piston'; there werethree ports, two in the cylinder, one at the top and bottom to admitthe steam into the cylinder, and a central one outside the cylinderfor the exhaust or waste steam from it to escape into the atmosphere,or into the condenser, if so fitted. When the valve was at halfstroke, the steam-ports were covered, the valve face for doing sobeing the exact width of the ports, that is to say, the steam ones.It is quite evident that at this position the piston must be either atthe top or the bottom of the cylinder, and as the crank-pin for thepiston rotates round a fixed point, similar to the crank centre for the