ng rod is the distance
from the centre of the beam when level, or the plane of the main centre, to
the centre of the paddle shaft. The length of the side rods is the distance
from the centre line of the beam when level, to the centre of the cross
head when the piston is at half stroke. The length of the radius rods of
the parallel motion is the distance from the point of attachment on the
cross head or side rod, when the piston is at half stroke, to the extremity
of the radius crank when the crank is horizontal; or in engines with the
parallel motion attached to the cross head, it is the distance from the
centre of the pin of the radius crank when horizontal to the centre of the
cylinder. Having fixed the centre of the parallel motion shaft in the
manner just described, it only remains to put the parts together when the
motion is attached to the cross head; but when the motion is attached to
the side rod, the end of the parallel bar must not move in a perpendicular
line, but in an arc, the versed sine of which bears the same ratio to that
of the side lever, that the distance from the top of the side rod to the
point of attachment bears to the total length of the side rod.

721. _Q._--How do you ascertain the accuracy of the parallel motion?

_A._--The parallel motion when put in its place should be tested by raising
and lowering the piston by means of the crane. First, set the beams level,
and shift in or out the motion shaft plummer blocks or bearings, until the
piston rod is upright. Then move the piston to the two extremes of its
motion. If at both ends the cross head is thrown too much out, the stud in
the beam to which the motion side rod is attached is too far out, and must
be shifted nearer to the main centre; if at the extremities the cross head
is thrown too far in, the stud in the beam is not out far enough. If the
cross head be thrown in at the one end, and out equally at the other, the
fault is in the motion side rod, which must be lengthened or shortened to
remedy the defect.

722. _Q._--Will you describe the method pursued in erecting oscillating
engines?

_A._--The columns here are of wrought iron, and in the case of small
engines there is a template made of wood and sheet iron, in which the holes
are set in the proper positions, by which the upper and lower frames are
adjusted; but in the case of large engines, the holes are set off by means
of trammels. The holes for the reception of the columns are c