mer it into the hole, whereby the plug will become riveted in it,
and its surface may then be filed smooth. Square pieces may be let in after
the same fashion, the hole being made dovetailed, and the pieces thus
fitted will never come out.

711. _Q._--When cylinders are faced with brass, how is the face attached to
the cylinder?

_A._--Brass faces are put upon valves or cylinders by means of small brass
screws tapped into the iron, with conical necks for the retention of the
brass: they are screwed by means of a square head, which, when the screw is
in its place, is cut off and filed smooth. In some cases the face is made
of extra thickness, and a rim not so thick runs round it, forming a step or
recess for the reception of brass rivets, the heads of which are clear of
the face.

712. _Q._--What is the best material for valve faces?

_A._--Much trouble is experienced with every modification of valve face;
but cast iron working upon cast iron is, perhaps, the best combination yet
introduced. A usual practice is to pin brass faces on the cylinder,
allowing the valve to retain its cast iron face. Some makers employ brass
valves, and others pin brass on the valves, leaving the cylinder with a
cast iron face. If brass valves are used, it is advisable to plane out two
grooves across the face, and to fill them up with hard cast iron to prevent
rutting. Speculum metal and steel have been tried for the cylinder faces,
but only with moderate success. In some cases the brass gets into ruts; but
the most prevalent affection is a degradation of the iron, owing to the
action of the steam, and the face assuming a granular appearance, something
like loaf sugar. This action shows itself only at particular spots, and
chiefly about the angles of the port or valve face. At first the action is
slow; but when once the steam has worked a passage for itself, the cutting
away becomes very rapid, and, in a short time, it will be impossible to
prevent the engine from heating when stopped, owing to the leakage of steam
through the valve into the condenser. Copper steam pipes seem to have some
galvanic action on valve faces, and malleable iron pipes have sometimes
been substituted; but they are speedily worn out by oxidation, and the
scales of rust which are carried on by the steam scratch the valves and
cylinders, so that the use of copper pipes is the least evil.

713. _Q._--Will you explain in what manner the joints of an engine are
made?