in the sound produced by the explosion. Hence in the most careful
experiments, where the vacuum was made as perfect as possible, and the
explosion correspondingly the more violent, the results were actually
the worst. With his explanations, the theory of the subject was rendered
quite complete.

Space fails, or I should mention in detail Mr. Joule's experiments on
magnetism and electro-magnets, referred to at the commencement of this
sketch. He discovered the now celebrated change of dimensions produced
by the magnetization of soft iron by the current. The peculiar noise
which accompanies the magnetization of an iron bar by the current,
sometimes called the "magnetic tick," was thus explained.

Mr. Joule's improvements in galvanometers have already been incidentally
mentioned, and the construction by him of accurate thermometers has been
referred to. It should never be forgotten that _he first_ used small
enough needles in tangent galvanometers to practically annul error from
want of uniformity of the magnetic field. Of other improvements and
additions to philosophical instruments may be mentioned a thermometer,
unaffected by radiation, for measuring the temperature of the
atmosphere, an improved barometer, a mercurial vacuum pump, one of the
very first of the species which is now doing such valuable work, not
only in scientific laboratories, but in the manufacture of incandescent
electric lamps, and an apparatus for determining the earth's horizontal
magnetic force in absolute measure.

Here this imperfect sketch must close. My limits are already passed. Mr.
Joule has never been in any sense a public man; and, of those who know
his name as that of the discoverer who has given the experimental basis
for the grandest generalization in the whole of physical science, very