es the
cylinder is cracked by the great strain thus introduced. The chest of the
blow-through valve is very commonly cast upon the valve casing; and in
engines where the cylinders are stationary this is the most convenient
practice. All engines, where the valve is not of such a construction as to
leave the face when a pressure exceeding that of the steam is created in
the cylinder by priming or otherwise, should be provided with an escape
valve to let out the water, and such valve should be so constructed that
the water cannot fly out with violence over the attendants; but it should
be conducted away by a suitable pipe, to a place where its discharge can
occasion no inconvenience. The stuffing boxes of all engines which cannot
be stopped frequently to be repacked, should be made very deep; metallic
packing in the stuffing box has been used in some engines, consisting in
most instances of one or more rings, cut, sprung, and slipped upon the
piston rod before the cross head is put on, and packed with hemp behind.
This species of packing answers very well when the parallel motion is true,
and the piston rod free from scratches, and it accomplishes a material
saving of tallow. In some cases a piece of sheet brass, packed behind with
hemp, has been introduced with good effect, a flange being turned over on
the under edge of the brass to prevent it from slipping up or down with the
motion of the rod. The sheet brass speedily puts an excellent polish upon
the rod, and such a packing is more easily kept, and requires less tallow
than where hemp alone is employed. In side lever marine engines the
attachments of the cylinder to the diagonal stay are generally made of too
small an area, and the flanges are made too thick. A very thick flange cast
on any part of a cylinder endangers the soundness of the cylinder, by
inducing an unequal contraction of the metal; and it is a preferable course
to make the flange for the attachment or the framing thin, and the surface
large--the bolts being turned bolts and nicely fitted. If from malformation
in this part the framing works to an inconvenient extent, the best
expedient appears to be the introduction of a number of steel tapered
bolts, the holes having been previously bored out; and if the flanges be
thick enough, square keys may also be introduced, half into one flange and
half into the other, so as to receive the strain. If the jaw cracks or
breaks away, however, it will be best to apply