applies the
principle of Mr. Wilde. A small machine furnishes a direct current,
which is carried round the electro-magnets of a second and larger
machine. Wilde's principle, it may be added, is also applied on the
Thames Embankment and the Holborn Viaduct; a small Gramme machine
being used in each case to excite the electro-magnets of the large
one.

The Farmer-Wallace machine is also an apparatus of great power. It
consists of a combination of bobbins for induced currents, and of
inducing electro-magnets, the latter being excited by the method
discovered by Siemens and Wheatstone. In the machines intended for
the production of the electric light, the electromotive force is so
great as to permit of the introduction of several lights in the same
circuit. A peculiarly novel feature of the Farmer-Wallace system is
the shape of the carbons. Instead of rods, two large plates of
carbons with bevelled edges are employed, one above the other. The
electric discharge passes from edge to edge, and shifts its position
according as the carbon is dissipated. The duration of the light in
this case far exceeds that obtainable with rods. I have myself seen
four of these lights in the same circuit in Mr. Ladd's workshop in the
City, and they are now, I believe, employed at the Liverpool Street
Station of the Metropolitan Railway. The Farmer-Wallace 'quantity
machine' pours forth a flood of electricity of low tension. It is
unable to cross the interval necessary for the production of the
electric light, but it can fuse thick copper wires. When sent through
a short bar of iridium, this refractory metal emits a light of
extraordinary splendour. [Footnote: The iridium light was shown by Mr.
Ladd. It brilliantly illuminated the theatre of the Royal
Institution.]

The machine of M. de Meritens, which he has generously brought over
from Paris for our instruction, is the newest of all. In its
construction he falls back upon the principle of the magneto-electric
machine, employing permanent magnets as the exciters of the induced
currents. Using the magnets of the Alliance Company, by a skilful
disposition of his bobbins, M. de Meritens produces with eight magnets
a light equal to that produced by forty magnets in the Alliance
machines. While the space occupied is only one-fifth, the cost is
little more than one-fourth of the latter. In the de Meritens machine
the commutator is abolished. The internal heat is hardly sensible,