d been endeavoured to alter the speed of the
lathe-mandrill, according to the hardness of the metal and the diameter
of the thing to be turned, were effectually disposed of. These, though
answering very well where cylinders of equal diameter had to be bored,
and a uniform motion was all that was required, were found very
inefficient where a Plane surface had to be turned; and it was in such
cases that Mr. Clement's lathe was found so valuable. By its means
surfaces of unrivalled correctness were produced, and the slide-lathe,
so improved, became recognised and adopted as the most accurate and
extensively applicable of all machine-tools.

The year after Mr. Clement brought out his improved turning-lathe, he
added to it his self-adjusting double driving centre-chuck, for which
the Society of Arts awarded him their silver medal in 1828. In
introducing this invention to the notice of the Society, Mr. Clement
said, "Although I have been in the habit of turning and making
turning-lathes and other machinery for upwards of thirty-five years,
and have examined the best turning-lathes in the principal
manufactories throughout Great Britain, I find it universally regretted
by all practical men that they cannot turn anything perfectly true
between the centres of the lathe." It was found by experience, that
there was a degree of eccentricity, and consequently of imperfection,
in the figure of any long cylinder turned while suspended between the
centres of the lathe, and made to revolve by the action of a single
driver. Under such circumstances the pressure of the tool tended to
force the work out of the right line and to distribute the strain
between the driver and the adjacent centre, so that one end of the
cylinder became eccentric with respect to the other. By Mr. Clement's
invention of the two-armed driver, which was self-adjusting, the strain
was taken from the centre and divided between the two arms, which being
equidistant from the centre, effectually corrected all eccentricity in
the work. This invention was found of great importance in ensuring the
true turning of large machinery, which before had been found a matter
of considerable difficulty.

In the same year (1828) Mr. Clement began the making of fluted taps and
dies, and he established a mechanical practice with reference to the
pitch of the screw, which proved of the greatest importance in the
economics of manufacture. Before his time, each mechanical enginee