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fitting in a spiral groove on the spindle, similar to a screw; only that the groovemakes but a quarter of a turn round the spindle, and is very much inclined to it,so as to cause the valve to be sufficiently drawn back by a quarter turn of thehandle. This regulator is very efficient, and acts a little more uniformly than theother, but the friction in it is greater from the spiral motion, and it is more liablethan the other to get out of order and stick fast.
Steam Chests. —'The steam chests or slide valve boxes U U, (Plates XC., XCI., andfig. 4, Plate XCII.,) are made of cast iron half an inch thick; and are bolted down uponthe top of each cylinder, and to the front plate of the smoke box, in which holes are cutof the same size as the steam chests, and closed by cast iron plates termed bonnetsbolted on the outside. The steam chests have the two branches of the steam pipefixed upon them at the back, and opening into them; and a stuffing box, It, is cast uponthe end of each, passing through the tube plate n, the joints being completely andfirmly closed by running melted lead into it all round.
Slide Valves.— In the steam chests are placed the slide valves, V V, shown in sectionlongitudinally and across in Plate XC., and fig. 4, Plate XCII. They are brass boxes11- inch deep inside and three eighths of an inch thick, having flanches five eighths ofan inch thick all round them at the bottom. The slide valves are made to movebackwards and forwards by the spindles which have each a cross piece at the end•that fits into a notch in the back of the valve; the spindle is generally connected tothe valve by means of a rectangular wrought iron frame, called a bridle, droppedover the valve and having the spindle screwed into it. The bridle has an advan-tage in holding the valve very steady, and yet allowing it to drop through readilyas it is worn by friction, and thus keep always in contact with the surface thatit slides upon. The valve spindle l moves steam-tight through the stuffing box It atthe inner end of the steam chest.
Cylinders and Piston,:?— - 1 The cylinders, PI H, where motion is produced bythe pressure of the steam, are made of cast iron, five eighths of an inch thickand 12 inches diameter inside, and are bored out perfectly smooth and cylindrical.A box is cast on the top of each cylinder, running along its whole length and flaton the upper side, containing two hollow rectangular passages m, n, and anotherpassage o', between them; these are separated from each other, and the endsof them are shown coming to the outside and opening into the steam chest at theleft cylinder, (Plate XCI.) The two outer passages m',n, are the steam ports; theyare eight inches long and one inch wide, and open one into each extremity of thecylinder, for the purpose of conveying steam to and from the cylinders; their areais 8 square inches, or rather less than the area of the steam pipes. The other pas-sage o', is the waste steam port, of the same length, and one inch and a half wide on theface; expanding inside, as shown in Plate XC., occupying all the space between the
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