In turning a piece of work on the old-fashioned lathe, the workman applied and guided his tool by means of muscular strength. The work was made to revolve, and the turner, holding the cutting tool firmly upon the long, straight, guiding edge of the rest, along which he carried it, and pressing its point firmly against the article to be turned, was thus enabled to reduce its surface to the required size and shape. Some dexterous turners were able, with practice and carefulness, to execute very clever pieces of work by this simple means. But when the article to be turned was of considerable size, and especially when it was of metal, the expenditure of muscular strength was so great that the workman soon became exhausted. The slightest variation in the pressure of the tool led to an irregularity of surface; and with the utmost care on the workman's part, he could not avoid occasionally cutting a little too deep, in consequence of which he must necessarily go over the surface again, to reduce the whole to the level of that accidentally cut too deep; and thus possibly the job would be altogether spoiled by the diameter of the article under operation being made too small for its intended purpose. The introduction of the slide rest furnished a complete remedy for this source of imperfection. The principle of the invention consists in constructing and fitting the rest so that, instead of being screwed down to one place, and the tool in the hands of the workman travelling over it, the rest shall itself hold the cutting tool firmly fixed in it, and slide along the surface of the bench in a direction exactly parallel with the axis of the work. Before its invention various methods had been tried with the object of enabling the work to be turned true independent of the dexterity of the workman. Thus, a square steel cutter used to be firmly fixed in a bed, along which it was wedged from point to point of the work, and tolerable accuracy was in this way secured. But the slide rest was much more easily managed, and the result was much more satisfactory. All that the workman had to do, after the tool was firmly fitted into the rest, was merely to turn a screw-handle, and thus advance the cutter along the face of the work as required, with an expenditure of strength so slight as scarcely to be appreciable. And even this labour has now been got rid of; for, by an arrangement of the gearing, the slide itself has been made self-acting, and advances with the revolution of the work in the lathe, which thus supplies the place of the workman's hand. The accuracy of the turning done by this beautiful yet simple arrangement is as mechanically perfect as work can be. The pair of steel fingers which hold the cutting tool firmly in their grasp never tire, and it moves along the metal to be cut with an accuracy and precision which the human hand, however skilled, could never equal.