In this process, unless the time of
revolution of the mandril is the same as that of the screw which
guides the cutting point, the number of threads in a given length
will be different. If the mandril move quicker than the cutting
point, the screw which is produced will be finer than the
original; if it move slower, the copy will be more coarse than
the original. The screw thus generated may be finer or coarser--
it may be larger or smaller in diameter--it may have the same or
a greater number of threads than that from which it is copied;
yet all the defects which exist in the original will be
accurately transmitted, under the modified circumstances, to
every individual generated from it.
154. Printing from copper plates with altered dimensions.
Some very singular specimens of an art of copying, not yet made
public, were brought from Paris a few years since. A watchmaker
in that city, of the name of Gonord, had contrived a method by
which he could take from the same copperplate impressions of
different sizes, either larger or smaller than the original
design. Having procured four impressions of a parrot, surrounded
by a circle, executed in this manner, I shewed them to the late
Mr Lowry, an engraver equally distinguished for his skill, and
for the many mechanical contrivances with which he enriched his
art.
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