420. Q.—­Will you state the dimensions of the tube plate, and the means of securing the tubes in it?

A.—­The tube plates are generally made from five eighths to three fourths of an inch thick, but seven eighths of an inch thick appears to be preferable, as when the plate is thick the holes will not be so liable to change their figure during the process of feruling the tubes:  the distance between the tubes should never be made less than three fourths of an inch, and the holes should be slightly tapered so as to enable the tubes to hold the tube plates together.  The tubes are secured in the tube plates by means of taper ferules driven into the ends of the tubes.  The ferules are for the most part made of steel at the fire box end, and of wrought iron at the smoke box end, though ferules of malleable cast iron have in some cases been used with advantage:  malleable cast iron ferules are almost as easily expanded when hammered cold upon a mandrel, as the common wrought iron ones are at a working heat.  Spring steel, rolled with a feather edge, to facilitate its conversion into ferules, is supplied by some of the steel-makers of Sheffield, and it appears expedient to make use of steel thus prepared when steel ferules are employed.  In cases where ferules are not employed, it may be advisable to set out the tube behind the tube plate by means of an expanding mandrel.  There are various forms of this instrument.  One form is that known as Prosser’s expanding mandrel, in which there are six or eight segments, which are forced out by means of a hexagonal or octagonal wedge, which is forced forward by a screw.  When the wedge is withdrawn, the segments collapse sufficiently to enable them to enter the tube, and there is an annular protuberance on the exterior circle of the segments, which protuberance, when the mandrel is put into the tube, just comes behind the inner edge of the tube plate.  When the wedge is tightened up by the screw, the protuberance on the exterior of the segments composing the mandrel causes a corresponding bulge to take place in the tube, at the back of the tube plate, and the tube is thereby brought into more intimate contact with the tube plate than would otherwise be the case.  There is a steel ring indented into the segments of Prosser’s mandrel, to contract the segments when the central wedge is withdrawn.  A more convenient form of the instrument, however, is obtained by placing the segments in a circular box, with one end projecting; and supporting each segment in the box by a tenon, which fits into a mortise in the cylindrical box.  To expand the segments, a round tapered piece of steel, like a drift, is forced into a central hole, round which the segments are arranged.  A piece of steel tube, also slit up to enable a central drift to expand it, answers very well; but the thickness of that part of the tube in which there requires to be spring enough to let the mandrel expand, requires to be sufficiently reduced to prevent the pieces from cracking when the central drift is driven in by a hammer.  The drift is better when made with a globular head, so that it may be struck back by the hammer, as well as be driven in.  An expanding mandrel, with a central drift, is more rapid in its operation than when the expansion is produced by means of a screw.