148. By turning. The art of turning might perhaps itself be classed amongst the arts of copying. A steel axis, called a mandril, having a pulley attached to the middle of it, is supported at one end either by a conical point, or by a cylindrical collar, and at the other end by another collar, through which it passes. The extremity which projects beyond this last collar is formed into a screw, by which various instruments, called chucks, can be attached to it. These chucks are intended to hold the various materials to be submitted to the operation of turning, and have a great variety of forms. The mandril with the chuck is made to revolve by a strap which passes over the pulley that is attached to it, and likewise over a larger wheel moved either by the foot, or by its connection with steam or water power. All work which is executed on a mandril partakes in some measure of the irregularities in the form of that mandril; and the perfect circularity of section which ought to exist in every part of the work, can only be ensured by an equal accuracy in the mandril and its collar. 149. Rose engine turning. This elegant art depends in a great measure on copying. Circular plates of metal called rosettes, having various indentations on the surfaces and edges, are fixed on the mandril, which admits of a movement either end-wise or laterally: a fixed obstacle called the 'touch', against which the rosettes are pressed by a spring, obliges the mandril to follow their indentations, and thus causes the cutting tool to trace out the same pattern on the work. The distance of the cutting tool from the centre being usually less than the radius of the rosette, causes the copy to be much diminished. 150. Copying dies. A lathe has been long known in France, and recently been used at the English mint for copying dies. A blunt point is carried by a very slow spiral movement successively over every part of the die to be copied, and is pressed by a weight into all the cavities; while a cutting point connected with it by the machine traverses the face of a piece of soft steel, in which it cuts the device of the original die on the same or on a diminished scale. The degree of excellence of the copy increases in proportion as it is smaller than the original. The die of a crown-piece will furnish by copy a very tolerable die for a sixpence. But the chief use to be expected from this lathe is to prepare all the coarser parts, and leave only the finer and more expressive lines for the skill and genius of the artist.