tion, or
otherwise, the bearing points should be distributed equally over the
surface. Face plates, or planometers, as they are sometimes termed, are
supplied by most of the makers of engineering tools. Every factory should
be abundantly supplied with them, and also with steel straight edges; and
there should be a master face plate, and a master straight edge, for the
sole purpose of testing, from time to time, the accuracy of those in use.

708. _Q._--Is the operation of surfacing, which you have described,
necessary in the case of all slide valves?

_A._--Yes; and in fitting the faces of a D valve, great care must, in
addition, be taken that the valve is not made conical; for unless the back
be exactly parallel with the face, it will be impossible to keep the
packing from being rapidly cut away. When the valve is laid upon the face
plate, the back must be made quite fair along the whole length, by draw
filing, according to the indications of a straight edge; and the distance
from the face to the extreme height of the back must be made identical at
each extremity.

709. _Q._--When you described the operation of boring the cylinder, you
stated that the cylinder, when laid upon its side, became oval; will not
this change of figure distort the cylinder face?

_A._--It is not only in the boring of the cylinder that it is necessary to
be careful that there is no change of figure, for it will be impossible to
face the valves truly in the case of large cylinders, unless the cylinder
be placed on end, or internal props be introduced to prevent the collapse
due to the cylinder's weight. It may be added, that the change of figure is
not instantaneous, but becomes greater after some continuance of the strain
than it was at first, so that in gauging a cylinder to ascertain the
difference of diameter when it is placed on its side, it should have lain
some days upon its side to ensure the accuracy of the operation.

710. _Q._--How is any flaw in the valve or cylinder face remedied?

_A._--Should a hole occur either in the valve, in the cylinder, or any
other part where the surface requires to be smooth, it may be plugged up
with a piece of cast iron, as nearly as possible of the same texture. Bore
out the faulty part, and afterward widen the hole with an eccentric drill,
so that it will be of the least diameter at the mouth. The hole may go more
than half through the iron: fit then a plug of cast iron roughly by filing,
and ham