ast in the
frames, and are recessed out internally: the bosses encircling the holes
are made quite level across, and made very true with a face plate, and the
pillars which have been turned to a gauge are then inserted. The top frame
is next put on, and must bear upon the collars of the columns so evenly,
that one of the columns will not be bound by it harder than another. If
this point be not attained, the surfaces must be further scraped, until a
perfect fit is established. The whole of the bearings in the best
oscillating engines are fitted by means of scraping, and on no other mode
of fitting can the same reliance be placed for exactitude.

723. _Q._--How do you set out the trunnions of oscillating engines, so that
they shall be at right angles with the interior of the cylinder?

_A._--Having bored the cylinder, faced the flange, and bored out the hole
through which the boring bar passes, put a piece of wood across the mouth
of the cylinder, and jam it in, and put a similar piece in the hole through
the bottom of the cylinder. Mark the centre of the cylinder upon each of
these pieces, and put into the bore of each trunnion an iron plate, with a
small indentation in the middle to receive the centre of a lathe, and
adjusting screws to bring the centre into any required position. The
cylinder must then be set in a lathe, and hung by the centres of the
trunnions, and a straight edge must be put across the cylinder mouth and
levelled, so as to pass through the line in which the centre of the
cylinder lies. Another similar straight edge, and similarly levelled, must
be similarly placed across the cylinder bottom, so as to pass through the
central line of the cylinder; and the cylinder is then to be turned round
in the trunnion centres-the straight edges remaining stationary, which will
at once show whether the trunnions are in the same horizontal plane as the
centre of the cylinder, and if not, the screws of the plates in the
trunnions must be adjusted until the central point of the cylinder just
comes to the straight edge, whichever end of the cylinder is presented. To
ascertain whether the trunnions stand in a transverse plane, parallel to
the cylinder flange, it is only necessary to measure down from the flange
to each trunnion centre; and if both these conditions are satisfied, the
position of the centres may be supposed to be right. The trunnion bearings
are then turned, and are fitted into blocks of wood, in which th