A.—­The rings are to be fitted laterally to the piston, and to one another, by scraping—­a steady pin being fixed upon the flange of the piston, and fitting into a corresponding hole in the lower ring, to keep the lower ring from turning round; and a similar pin being fixed into the top edge of the lower ring to prevent the upper ring from turning round; but the holes into which these pins fit must be made oblong, to enable the rings to press outward as the rubbing surfaces wear.  In most cases it will be expedient to press the packing rings out with springs where they are not packed behind with hemp, and the springs should be made very strong, as the prevailing fault of springs is their weakness.  Sometimes short bent springs, set round at regular intervals between the packing rings and body of the piston, are employed, the centre of each spring being secured by a steady pin or bolt screwed into the side of the piston; but it will not signify much what kind of springs is used, provided they have sufficient tension.  When pistons are made of a single ring, or of a succession of single rings, the strength of each ring should be tested previously to its introduction into the piston, by means of a lever loaded by a heavy weight.

447. Q.—­What kind of piston is employed by Messrs. Penn?

A.—­Messrs. Penn’s piston for oscillating engines has a single packing ring, with a tongue piece, or mortice end, made in the manner already described.  The ring is packed behind with hemp packing, and the piece of metal which covers the joint is a piece of thick sheet copper or brass, and is indented into the iron of the ring, so as to offer no obstruction to the application of the hemp.  The ring is fitted to the piston only on the under edge; the top edge is rounded to a point from the inside, and the junk ring does not bear upon it, but the junk ring squeezes down the hemp packing between the packing ring and the body of the piston.

448. Q.—­How should the piston rod be secured to the piston?

A.—­The piston rod, where it fits into the piston, should have a good deal of taper; for if the taper be too small the rod will be drawn through the hole, and the piston will be split asunder.  Small grooves are sometimes turned out of the piston rod above and below the cutter hole, and hemp is introduced in order to make the piston eye tight.  Most piston rods are fixed to the piston by means of a gib and cutter, but in some cases the upper portion of the rod within the eye is screwed, and it is fixed into the piston by means of an indented nut.  This nut is in some cases hexagonal, and in other cases the exterior forms a portion of a cone which completely fills a corresponding recess in the piston; but nuts made in this way become rusted into their seat after some time, and cannot be started again without much difficulty.  Messrs. Miller, Ravenhill & Co. fix in their piston rods by means of an indented hexagonal