It is well known that metals, when subjected, either cold or hot, to
sufficient pressure, will obey almost exactly the same laws as fluids
under similar conditions, and will flow into and fill all the crevices
of the chamber or cavity in which they are contained. If, therefore, a
hot rivet is inserted into the holes made in a boiler to receive it,
and is then subjected to a sufficient pressure, it will fill every
irregularity of the holes, and thus fulfill one of the conditions of
perfect riveting. This result it is impossible to accomplish with
perfection or certainty by ordinary hand riveting, in doing which the
intermittent blows of an ordinary hammer are used to force the metal
into the holes. With a hydraulic riveting machine, however, an
absolutely uniform and continuous pressure can be imparted to each
rivet, so as to force the hot metal of the rivet into all the
irregularities of the holes in the same way as a hydraulic ram will
cause water to fill any cavity, however irregular.

[Illustration: FIG. 18.]

In order to illustrate the relative advantages of machine over hand
riveting, two plates were riveted together, the holes of which were
purposely made so as not to match perfectly. These plates were then
planed through the center of the rivets, so as to expose a section of
both the plates and rivets. From this an impression was taken with
printer's ink on paper and then transferred to a wooden block, from
which Figs. 17 and 18 were made.

The machine-driven rivet is marked _a_, and _b_ represents the
hammered rivet.

It will be observed that the machine rivet fills the hole completely,