, Mayer fell back upon
reflection, selecting with marvellous sagacity, from existing physical
data, the single result on which could be founded a calculation of the
mechanical equivalent of heat. In the midst of mechanical appliances,
Joule resorted to experiment, and laid the broad and firm foundation
which has secured for the mechanical theory the acceptance it now
enjoys. A great portion of Joule's time was occupied in actual
manipulation; freed from this, Mayer had time to follow the theory
into its most abstruse and impressive applications. With their places
reversed, however, Joule might have become Mayer, and Mayer might have
become Joule.

It does not lie within the scope of these brief articles to enter upon
the developments of the Dynamical Theory accomplished since Joule and
Mayer executed their memorable labours.

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XXI. DEATH BY LIGHTNING.

PEOPLE in general imagine, when they think at all about the matter,
that an impression upon the nerves--a blow, for example, or the prick
of a pin--is felt at the moment it is inflicted. But this is not the
case. The seat of sensation being the brain, to it the intelligence
of any impression made upon the nerves has to be transmitted before
this impression can become manifest as consciousness. The
transmission, moreover, requires time, and the consequence is, that a
wound inflicted on a portion of the body distant from the brain is
more tardily appreciated than one inflicted adjacent to the brain. By
an extremely ingenious experimental arrangement, Helmholtz has
determined the velocity of this nervous transmission, and finds it to
be about eighty feet a second, or less than one-thirteenth of the
velocity of sound in air. If therefore, a whale forty feet long were
wounded in the tail, it would not be conscious of the injury till half
a second after the wound had been inflicted. [Footnote: A most
admirable lecture on the velocity of nervous transmission has been
published by Dr. Du Bois Reymond in the 'Proceedings of the Royal
Institution' for 1866, vol. iv. p. 575.] But this is not the only
ingredient in the delay. There can scarcely be a doubt that to every
act of consciousness belongs a determinate molecular arrangement of
the brain--that every thought or feeling has its physical correlative
in that organ; and nothing can be more certain than that every
physical change, whether molecular or mechanical, requires time for
its accomplis