o be bent to the proper curvature. The
machine by which this bending is effected is seen above, in the
back-ground. It consists of three rollers, placed in such a position in
relation to each other, that the plate, in being forced through between
them, is bent to any required curvature. These rollers are made to
revolve by great wheels at the sides, with handles at the circumference
of them, which handles act as levers, and are worked by men, as seen in
the engraving.

The separate plates of which a boiler is composed are fastened together
by means of massive rivets, and it is necessary, accordingly, to punch
rows of holes along the edges of the plates for the insertion of the
rivets. This process may be seen on the _left_ in the above engraving.
Two men are holding the plate which is to be punched. The punch is
driven through the plate by means of the great lever, which forms the
upper part of the engine. The upright part in front is driven forward by
means of the cam in the large wheel behind, a part of which only is seen
in the engraving. This cam raises the long arm of the lever by means of
the pulley in the end of it, and so drives the point of the punch
through the plate. There is a support for the plate behind it, between
the plate and the man, with a small opening in it, into which the punch
enters, driving before it the round button of iron which it has cut from
the plate.

On the right, in the above engraving, is a punching engine worked by
men, the other being driven by steam power. These machines are
sufficient to make all the ordinary perforations required in
boiler-plates. Larger holes, when required, have to be bored by a drill,
as represented in the following engraving.

[Illustration: THE BORING-ENGINE.]

The view below represents the interior of one of the great boiler rooms
where the boilers are put together by riveting the plates to each other
at their edges. Some men stand inside, holding heavy sledges against the
heads of the rivets, while others on the outside, with other sledges,
beat down the part of the iron which protrudes, so as to form another
head to each rivet, on the outside. This process can be seen distinctly
in the boiler nearest to the observer in the view below. The planks
which are seen crossing each other in the open end, are temporary
braces, put in to preserve the cylindrical form of the mass, to prevent
the iron from bending itself by its own weight, before the iron heads
a