ver in a special machine. A length of flue
is rotated on a table, while the flange is turned over within a minute
between revolving rollers. There is another advantage in the adoption
of machine-flanging, besides the enormous saving of time, namely, that
the material suffers far less injury than it does in hand-flanging.

These differences in practice would not have assumed such magnitude
but for the introduction of mild steel in place of malleable iron.
Iron suffers less from overheating and irregular heating than does
steel. Steel possesses higher ductility, but it is also more liable to
develop cracks if subjected to improper treatment. All this and much
more is writ large in the early testing of steel, and is reflected in
present-day practice.

A feature peculiar to the boiler and plating shops is the enormous
number of rivet holes which have to be made, and of rivets to be
inserted. These requirements are reflected in machine design. To punch
or drill holes singly is too slow a process in the best practice, and
so machines are made for producing many holes simultaneously. Besides
this, the different sections of boilers are drilled in machines of
different types, some for shells, some for furnaces, some peculiar to
the shells or furnaces of one type of boilers, others to those of
another type only. And generally now these machines not only drill,
but can also be adjusted to drill to exact pitch, the necessity thus
being avoided of marking out the holes as guides to the drills.

Hand-riveting has mostly been displaced by hydraulic and pneumatic
machines, with resulting great saving in cost, and the advantage of
more trustworthy and uniform results. For boiler work, machines are
mostly of fixed type; for bridge and girder work they are portable,
being slung from chains and provided with pressure water or compressed
air by systems of flexible pipes.

Welding fills a large place in boiler work, but it is that of the
edges of plates chiefly, predominating over that of the bars and rods
of the smithy. The edges to be united are thin and long, so that short
lengths have to be done in succession at successive "heats." Much of
this is hand work, and "gluts" or insertion pieces are generally
preferred to overlapping joints. But in large shops, steam-driven
power hammers are used for closing the welds. Parts that are commonly
welded are the furnace flue