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 nut, which may be
started by means of an open box key. The thread of the screw is made flat
upon the one side a