out a blow, but that it could be _better done without a
blow_, and that the riveted material was stronger when so secured than
when subjected to the more severe treatment under impact.

What is manifestly required in perfect riveting is that the metal of
the rivet while hot and plastic shall be made to flow into all the
irregularities of the rivet holes in the boiler sheets; that the
surplus metal be formed into heads as large as need be, and that the
pressure used to produce these results should not be in excess of what
the metal forming the boiler shall be capable of resisting.

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,
while the hand rivet is very imperfect. This experiment was tried
several times, with similar results each time.

The hand rivet, it will be observed, filled up the hole very well
immediately under the head formed by the hammer; but sufficient