llitherto we have spoken of plain turn ing, which is indeed the most useful and most universally practised. But many other nice and very curious opefations are • perfinnied by this art. if the poppet heads, supporting the mandrel, be made regularly to move from side to side, dur ing the rotation, or the rest be made to approach to, and recede from, the work, any number of times in a turn, the cuts will not be circular, but undulating, in dented or waved in any curve that may be required. Work of this kind, which is chiefly done in watch cases, snuff boxes, and trinkets, is called rose-work. The motion is commonly regulated by certain round plates of brass fixed on the man drel, called roves, which have their edges waved, and are called roses.
Another deviation from regular turning is effected by causing the chuck, which carries the work, to recede crosswise from the centre of the mandrel, back and fbrward during the rotation. The effect of this is, that the diameters of the work are not all equal to each other. It is practicable to produce• a variety of curves in this way, but in our art the pro cess is confined to turning ovals ; and the chuck, by which the work is made thus to slide back and forward, is called an oval chuck.
Numerous geometrical figures are pro duced by turning, by an apparatus upon the principle of the geometrical pen of Suardi, in engines which have been made for curiosity, and at great expense, Medallions, and other similar pieces, are produced by regulating the action of the tool in its advance to, or recess from, the face of a piece exposed to its ac tion.
If the mandrel of a lathe be made to advance and recede in the line of the ax is, once in each turn, the cut will not be in a plane at right angles to the axis of the work, and the line traced upon the work will be an ellipsis, produced by the oblique section of a cylinder. This kind of work is called sWash-work, and may be seen in some old balustrades, where its effect is far from being pleasing. The nature of the curve thus described, which we have called an ellipsis, will manifestly vary according to the law of the alternate motion in the mandrel. When the man drel moves uniformly forward, the cut will be the common helix or screw ; and the motion is used to make screws, though not very frequently, because good turners can easily make them by a notch ed cutting tool, called the screw.
The act of turning is so extensively ap plicable, that it would require a volume to describe its uses, and the methods of practising it. Every round thing which is made by human hands may be referred to this art, es one of its products. The largest columns, the most ponderous ar tillery, and the minutest pivots of watch work, with all wheel-work, rotatory ma chines, vessels, &c. are worked in this method.