r
adopted a thread of his own; so that when a piece of work came under
repair, the screw-hob had usually to be drilled out, and a new thread
was introduced according to the usage which prevailed in the shop in
which the work was executed. Mr. Clement saw a great waste of labour
in this practice, and he promulgated the idea that every screw of a
particular length ought to be furnished with its appointed number of
threads of a settled pitch. Taking the inch as the basis of his
calculations, he determined the number of threads in each case; and the
practice thus initiated by him, recommended as it was by convenience
and economy, was very shortly adopted throughout the trade. It may be
mentioned that one of Clement's ablest journeymen, Mr. Whitworth, has,
since his time, been mainly instrumental in establishing the settled
practice; and Whitworth's thread (initiated by Clement) has become
recognised throughout the mechanical world. To carry out his idea,
Clement invented his screw-engine lathe, with gearing, mandrill, and
sliding-table wheel-work, by means of which he first cut the inside
screw-tools from the left-handed hobs--the reverse mode having before
been adopted,--while in shaping machines he was the first to use the
revolving cutter attached to the slide rest. Then, in 1828, he fluted
the taps for the first time with a revolving cutter,--other makers
having up to that time only notched them. Among his other inventions
in screws may be mentioned his headless tap, which, according to Mr.
Nasmyth, is so valuable an invention, that, "if he had done nothing
else, it ought to immortalize him among mechanics. It passed right
through the hole to be tapped, and was thus enabled to do the duty of
three ordinary screws." By these improvements much greater precision
was secured in the manufacture of tools and machinery, accompanied by a
greatly reduced cost of production; the results of which are felt to
this day.

Another of Mr. Clement's ingenious inventions was his Planing Machine,
by means of which metal plates of large dimensions were planed with
perfect truth and finished with beautiful accuracy. There is perhaps
scarcely a machine about which there has been more controversy than
this; and we do not pretend to be able to determine the respective
merits of the many able mechanics who have had a hand in its invention.
It is exceedingly probable that others besides Clement worked out the
problem in their own way,