e length of the rivet
to a cherry red. Particular attention must be given to the thickness
of the fire in which they are heated.

Steel, of whatever kind, should never be heated in a thin fire,
especially in one having a forced blast, such as an ordinary
blacksmith's or iron rivet furnace fire. The reason for this is that
more air passes through the fire than is needed for combustion, and in
consequence there is a considerable quantity of free oxygen in the
fire which will oxidize the steel, or in other words, burn it. If free
oxygen is excluded steel cannot burn; if the temperature is high
enough it can be melted and will run down through the fire, but
burning is impossible in a thick fire with a moderate draught.

This is an important matter in using steel rivets and should not be
overlooked; the same principle applies to the heating of steel plates
for flanging.

_Riveting._--There are four descriptions of riveting, namely:

(1) Hammered or hand riveting.

(2) Snapped or set.

(3) Countersunk.

(4) Machine.

For good, sound work, machine riveting is the best.

Snapped riveting is next in quality to machine riveting.

Countersunk riveting is generally tighter than snapped, because
countersinking the hole is really facing it; and the countersunk rivet
is, in point of fact, made on a face joint. But countersinking the
hole also weakens the plate, inasmuch as it takes away a portion of
the metal, and should only be resorted to where necessary, such as
around the front of furnaces, steam chests or an odd hole here and
there to clear a flange, or something of that sort.

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 with