n
the circuit, as two bodies of the same kind and acting upon the same
principles, as far as the first inductive phenomena are concerned,
notwithstanding the different forms of discharge which ultimately
follow[A], as we may compare, according to Coulomb's investigations[B]
_different lengths_ of different insulating bodies required to produce the
same amount of insulating effect.

[A] These will be examined hereafter (1348. &c.).

[B] Memoires de l'Academie, 1785, p. 612. or Ency. Britann. First
Supp. vol. i. p. 614.

1332. This comparison is still more striking when we take into
consideration the experiment of Mr. Harris, in which he stretched a fine
wire across a glass globe, the air within being rarefied[A]. On sending a
charge through the joint arrangement of metal and rare air, as much, if not
more, electricity passed by the latter as by the former. In the air,
rarefied as it was, there can be no doubt the discharge was preceded by
induction (1284.); and to my mind all the circumstances indicate that the
same was the case with the metal; that, in fact, both substances are
dielectrics, exhibiting the same effects in consequence of the action of
the same causes, the only variation being one of degree in the different
substances employed.

[A] Philosophical Transactions, 1834, p, 212.

1333. Judging on these principles, velocity of discharge through the _same
wire_ may be varied greatly by attending to the circumstances which cause
variations of discharge through spermaceti or sulphur. Thus, for instance,
it must vary with the tension or intensity of the first urging force (1234.
1240.), which tension is charge and induction. So if the two ends of the
wire, in Professor Wheatstone's experiment, were immediately connected with
two large insulated metallic surfaces exposed to the air, so that the
primary act of induction, after making the contact for discharge, might be
in part removed from the internal portion of the wire at the first instant,
and disposed for the moment on its surface jointly with the air and
surrounding conductors, then I venture to anticipate that the middle spark
would be more retarded than before; and if these two plates were the inner
and outer coating of a large jar or a Leyden battery, then the retardation
of that spark would be still greater.

1334. Cavendish was perhaps the first to show distinctly that discharge was
not always by one channel[A], but, if several are present, by