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wedge is withdrawn, the segments collapse sufficiently to enable them to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contract the segments when the central wedge is withdrawn.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.

421. _Q._--Will you explain the means that are adopted to regulate the
admission of steam to the cylinders?

_A._--In locomotives, the admission of the steam from the boiler to the
cylinders is regulated by a valve called the regulator, which is generally
placed immediately above the internal fire box, and is connected with two
copper pipes;--one conducting steam from the highest point of the dome down
to it, and the other conducting the steam that has passed through it along
the boiler to the upper part of the smoke box. Regulators may be divided
into two sorts, viz., those with, sl