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 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?