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 cast 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?