MARINE ENGINES.

523

shaft, and that of the bottom one at the cylinder is one-fourth lessin depth; the limb for bolting to the condenser may be made alittle more than one-half of the diameter of the shaft. The variousforms of framing have been already noticed. The thickness of thebrasses at the bottom is one-seventh of the diameter of the journal.The caps when of wrought iron are one-third of the diameter ofthe journal, and when of cast iron one-half. The combined areaof the cap bolts is found by multiplying the combined area of thecylinders by the total pressure of the piston, as for the piston rods,&c., dividing the result by 4000. The brasses are recessed aboutx /± inch in depth, and filled in with white metal, the brass surfacebeing about 1 % inch all round for large shafts, with a bottom striprunning lengthways.

Piston .—The piston has generally a depth of from one-fifth toone-sixth of the diameter of the cylinder. The packing ring shouldnot be less than one-ninth of the diameter of the cylinder; itsgreatest thickness one-fifth of its breadth, diminishing to the partwhere it is cut through. The rubbing surfaces of the junk andbottom rings can be made of about the same collective breadth asthe packing ring, with projections formed on the junk ring andbottom of piston projecting from their respective thicknesses; thistends to make the wear of the spring ring and piston more equal.The ends of the piston may be somewhat less in thickness than thecylinder; they are strongly ribbed, and have all the necessary bossescast for the piston rods. The bolts for the junk ring vary fromy to 1 y inch in diameter, according to the size; in some casesthey are merely tapped into the cast iron, in others nuts are insertedin the body of the piston. The spring ring is kept up to thecylinder face with flat or U-shaped springs, recessed into the bodyof the piston. Two pieces of cast iron are inserted between thepacking ring and the piston, which serve to keep the body andpacking ring central with each other; the pitch of these blocks isabout one-fifth of the circumference of the ring inside. (See p. 403.)

A ir pump, condenser, &c .—The capacity of the air pump equalsone-eleventh part of the cubic contents of the cylinder for doubleaction, and double of that quantity for single action; the length ofthe stroke being the same as for the steam cylinder. Thus thearea of the cylinder is divided by 11 or 5" 5 > as the case may be, toobtain the area of the air pump. The lining of the air pump variesfrom y, to y inch in thickness. The depth of the air pump piston