to the reverberatory furnace. These
advantages compensate for the wear and tear and the cost of moving the
heavy dead-weight.

Shaft calcining furnaces are available for fine ores and permit the
recovery of the sulphur. They are square, oblong or circular in section,
and the interior is fitted with horizontal or inclined plates or prisms,
which regulate the fall of the ore. In the Gerstenhoffer and
Hasenclever-Helbig furnaces the fall is retarded by prisms and inclined
plates. In other furnaces the ore rests on a series of horizontal
plates, and either remains on the same plate throughout the operation
(Ollivier and Perret furnace), or is passed from plate to plate by hand
(Maletra), or by mechanical means (Spence and M'Dougall).

The M'Dougall furnace is turret-shaped, and consists of a series of
circular hearths, on which the ore is agitated by rakes attached to
revolving arms and made to fall from hearth to hearth. It has been
modified by Herreshoff, who uses a large hollow revolving central shaft
cooled by a current of air. The shaft is provided with sockets, into
which movable arms with their rakes are readily dropped. The Peter
Spence type of calcining furnace has been followed in a large number of
inventions. In some the rakes are attached to rigid frames, with a
reciprocating motion, in others to cross-bars moved by revolving chains.
Some of these furnaces are straight, others circular. Some have only
one hearth, others three. This and the previous type of furnace, owing
to their large capacity, are at present in greatest favour. The
M'Dougall-Herreshoff, working on ores of over 30% of sulphur, requires
no fuel; but in furnaces of the reverberatory type fuel must be used, as
an excess of air enters through the slotted sides and the hinged doors
which open and shut frequently to permit of the passage of the rakes.
The consumption of fuel, however, does not exceed 1 of coal to 10 of
ore. The quantity of ore which these large furnaces, with a hearth area
as great as 2000 ft. and over, will roast varies from 40 to 60 tons a
day. Shaft calcining furnaces like the Gerstenhoffer, Hasenclever, and
others designed for burning pyrites fines have not found favour in
modern copper works.

_The Fusion of Ores in Reverberatory and Cupola Furnaces._--After the
ore has been partially calcined, it is smelted to extract its earthy
matter and to concentrate the copper with part of its iron and sulphur
into a matte. In reverbera