6. In working in very hard wood and in ivory, the grooving tool, a sharp pointed instrument somewhat similar to the graver, is used in the first part of the operation; and by this the grain of the substance is cut into contiguous grooves, and prepared for an easy reduction by the chisel. The instruments for turning metals are numerous, but they differ in some respects from those for cutting wood.
7. In almost every kind of turning, a tool called the calipers is necessary for measuring the diameters of the work. In its form, it bears some resemblance to the compasses or dividers. One or both of the legs, however, are curved; and one kind of this instrument has four legs, two curved, or two straight, at each end, with a pivot in the centre, on which it is opened and shut. The former of these is employed in measuring the dimensions of outside work, and the latter, for that on the inside. This kind is called the in-and-out calipers; and it is especially useful in turning a cylinder, or pin, which shall exactly fit an internal cylinder already made, and vice versâ.
8. There is but little difference in the management of turning different substances. The principal thing to be attended to is to adapt the velocity of the motion to the nature of the material; thus wood will work best with the greatest velocity that can be given to it. Brass should have a motion about half as quick as wood, and iron and steel still less; for, in operating on metallic substances, the tool is liable to become hot, and lose its temper; besides which, a certain time is requisite for the act of cutting to take place.
9. When compared with many other mechanical operations, the art of turning may be considered as perfect in its accuracy and expedition. The lathe is, therefore, resorted to for the performance of every work of which it is capable; nor is its use confined to the production of forms perfectly cylindrical, for it can be easily made to produce figures of irregular shape, such as lasts, gunstocks, &c.
10. The lathe was well known to the Greeks and Romans, as well as to many other nations of antiquity. Diodorus Siculus, who wrote in the time of Julius Cæsar and Augustus, says that it was invented by one Talus, a nephew of Dædalus. Pliny ascribes it to Theodore, of Samos, and mentions one Thericles, who had rendered himself very famous by his dexterity in managing the lathe. The Greek and Latin authors frequently mention this instrument; and, among the ancients, it was customary to express the accuracy and nicety of a thing by saying, it was formed in a lathe.
