152.  Engine for copying busts.  Many years since, the late Mr Watt amused himself with constructing an engine to produce copies of busts or statues, either of the same size as the original, or in a diminished proportion.  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.  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.  The relative dimensions of the several impressions were 5.5, 6.3, 8.4, 15.0, so that the largest was nearly three times the linear size of the smallest; and Mr Lowry assured me, that he was unable to detect any lines in one which had not corresponding lines in the others.  There appeared to be a difference in the quantity of ink, but none in the traces of the engraving; and, from the general appearance, it was conjectured that the largest but one was the original impression from the copperplate.