Hammered riveting is much more expensive than machine or snapped
riveting, and has a tendency to crystallize the iron in the rivets,
causing brittleness.

In the present state of the arts all the best machine riveters do
their work by pressure, and not by impact or blow.

The best machines are those of the hydraulic riveting system, which
combines all of the advantages and avoids all the difficulties which
have characterized previous machine systems; that is to say, the
machine compresses without a blow, and with a uniform pressure at
will; each rivet is driven with a single progressive movement,
controlled at will. The pressure upon the rivet after it is driven is
maintained, or the die is retracted at will.

[Illustration: FIG. 17.]

Hydraulic riveting has demonstrated not only that the work could be as
well done without 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.