TO W. GARNETT, Esq.
Glenlair, 11th August 1877.
Your
experiments on
electrified paraffin oil
are excellent, and may lead to increase of knowledge.
If the fluid dielectric and
also the air are
perfect insulators, nothing can get electrified, but the equation at
the
surface, instead of being P=
Po, will be
(excluding capillary action)
where
is the resultant electric force normal to the surface and just outside
it.
This causes the surface to
rise wherever the
normal force is great, or close to the electrodes.
· · · · ·
·
The science of displacements
is in Euc. I.
4, etc., and wherever one figure is placed upon another. It belongs
to
the method of contemplating the relations of two figures which may be
supposed
to co-exist, though we may also suppose that they
are
copies
of the same figure in different positions.
But
just as we assume that distance is a continuous
quantity capable
of measurement, though all our attendants
at
measurement
are made with instruments made of non-rigid and discontinuous matter,
so
we may assume that time is a continuously flowing
quantity
capable of measurement, though we have not yet found out any accurate
method
of comparing distant intervals of time.
Now Kinematics requires no
more than this notion
of time, as the common independent variable t. If we suppose
that is that (unknown) which flows uniformly then for kinematical
purposes
it is enough that t is a function of;
but when we come to Kinetics proper we
must have
very small.
Have you read Julius in
Nature, about the beginning
of June? [14th June].
The most
constant things we
know are the properties
of bodies. For instance, water in equilibrium with ice
and
vapour gives us a good deal.
I. A unit
of density (not the
orthodox one) .
II. A
unit of pressure (too
small for practical
use) .
III. A
unit of time (namely,
the time of revolution of a satellite just grazing a sphere of water)
= T.
These
three quantities being independent of each other
give M, L, and
T. [401]
P/D
gives a (velocity) squared
which could also
be got from the P/D of the vapour
(a different one).
Then
this gives also a standard temperature; all that
we want is to
get pure water.