distance, but with a force which, according to Weber, depends on their
relative velocity, and according to a theory hinted at by Gauss, and
developed by Riemann, Lorenz, and Neumann, acts not instantaneously,
but after a time depending on the distance. The power with which this
theory, in the hands of these eminent men, explains every kind of
electrical phenomena must be studied in order to be appreciated.

Another theory of electricity, which I prefer, denies action at a
distance and attributes electric action to tensions and pressures in
an all-pervading medium, these stresses being the same in kind with
those familiar to engineers, and the medium being identical with that
in which light is supposed to be propagated.

Both these theories are found to explain not only the phenomena by the
aid of which they were originally constructed, but other phenomena,
which were not thought of or perhaps not known at the time; and both
have independently arrived at the same numerical result, which gives
the absolute velocity of light in terms of electrical quantities.

That theories apparently so fundamentally opposed should have so large
a field of truth common to both is a fact the philosophical importance
of which we cannot fully appreciate till we have reached a scientific
altitude from which the true relation between hypotheses so different
can be seen.

I shall only make one more remark on the relation between Mathematics
and Physics. In themselves, one is an operation of the mind, the
other is a dance of molecules. The molecules have laws of their own,
some of which we select as most intelligible to us and most amenable
to our calculation. We form a theory from these partial data, and we