mposition of temper, or bending
tests, both hot and cold, provide for the second.

The following are the leading features of present-day methods.

It might be hastily supposed that, because plates, angles, tees,
channels and joist sections are rolled ready for use, little work
could be left for the plater and boilermaker. But actually so much is
involved that subdivisions of tasks are numerous; the operations of
templet-making, rolling, planing, punching and shearing, bending,
welding and forging, flanging, drilling, riveting, caulking, and
tubing require the labours of several groups of machine attendants,
and of gangs of unskilled labourers or helpers. Some operations also
have to be done at a red or white heat, others cold. To the first
belong flanging and welding, to the latter generally all the other
operations. Heating is necessary for the rolling of tubes of small
diameter; bending is done cold or hot according to circumstances.

[Illustration: FIG. 20.--Thornycroft-Schulz Water-tube Boiler.]

The fact that some kinds of treatment, as shearing and punching,
flanging and bending, are of a very violent character explains why
practice has changed radically in regard to the method of performing
these operations in cases where safety is a cardinal matter. Shearing
and punching are both severely detrusive operations performed on cold
metal; both leave jagged edges and, as experience has proved, very
minute cracks, the tendency of which is to extend under subsequent
stress, with liability to produce fracture. But it has been found
that, when a shorn edge is planed and a punched hole enlarged by
reamering, no harm results, provided not less than about 1/16 in. is
removed. A great advance was therefore made when specifications first
insisted on the removal of the rough edges before the parts were
united.

In the work of riveting another evil long existed. When holes are
punched it is practically impossible to ensure the exact coincidence
of holes in different plates which have to be brought together for the
purpose of riveting. From this followed the use of the drift,--a
tapered rod driven forcibly by hammer blows through corresponding
holes in adjacent plates, by which violent treatment the holes were
forcibly drawn into alignment. This drifting stressed the plates,
setting up permanent strains and enlarging incipient cracks, and many
boiler explo