a malleable iron hoop around
the cylinder to take the strain, and this will in all cases be the
preferable expedient, where from any peculiarities of structure there is a
difficulty in introducing bolts and keys of sufficient strength.

445. _Q._--Which is the most eligible species of piston?

_A._--For large engines, pistons with a metallic packing, consisting of a
single ring, with the ends morticed into one another, and a piece of metal
let in flush over the joint and riveted to one end of the ring, appears to
be the best species of piston; and if the cylinder be oscillating, it will
be expedient to chamfer off the upper edge of the ring on the inner side,
and to pack it at the back with hemp. If the cylinder be a stationary one,
springs may be substituted for the hemp packing, but in any case it will be
expedient to make the vertical joints of the ends of the ring run a little
obliquely, so as to prevent the joint forming a ridge in the cylinder. For
small pistons two rings may be employed, made somewhat eccentric internally
to give a greater thickness of metal in the centre of the ring; these rings
must be set one above the other in the cylinder, and the joints, which are
oblique, must be set at right angles with one another, so as to obviate any
disposition of the rings, in their expansion, to wear the cylinder oval.
The rings must first be turned a little larger than the diameter of the
cylinder, and a piece is then to be cut out, so that when the ends are
brought together the ring will just enter within the cylinder. The ring,
while retained in a state of compression, is then to be put in the lathe
and turned very truly, and finally it is to be hammered on the inside with
the small end of the hammer, to expand the metal, and thus increase the
elasticity.

446. _Q._--The rings should be carefully fitted to one another laterally?

_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