The substances on which he operated
were various, and some of the results were shewn to his friends,
but the mechanism by which they were made has never been
described. More recently, Mr Hawkins, who, nearly at the same
time, had also contrived a similar machine, has placed it in the
hands of an artist, who has made copies in ivory from a variety
of busts. The art of multiplying in different sizes the figures
of the sculptor, aided by that of rendering their acquisition
cheap through the art of casting, promises to give additional
value to his productions, and to diffuse more widely the pleasure
arising from their possession.
153. Screw cutting. When this operation is performed in the
lathe by means of a screw upon the mandril, it is essentially an
art of copying, but it is only the number of threads in a given
length which is copied; the form of the thread, and length as
well as the diameter of the screw to be cut, are entirely
independent of those from which the copy is made. There is
another method of cutting screws in a lathe by means of one
pattern screw, which, being connected by wheels with the mandril,
guides the cutting point.
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