and rest-holder firmly on to the bed in a convenient position; b, b are the two standards or supports for the lathe-bed; m is the back-board, or shelf, for placing tools and instruments upon within reach of the workman's hand.

In these lathes, whether driven by the foot or by

steam, it is impossible to produce work requiring much force. Although ample power may be available, the strap is insufficient to transmit it to the lathe-spindle, and the capability of varying the speed is but small, and unsuitable for any but small variations in the diameter of the work. Lathes are therefore fitted with double or back gearing, which enables them to be used upon much heavier work, and of course renders the lathe much more serviceable. This double gear accomplishes its object by diminishing the speed and

increasing the power in a corresponding proportion. It is variously arranged, but usually it is set at the back of the lathe-spindle, as shown in several of the lathes illustrated. The neatest arrangement is that in which the wheels are enclosed in the cone-pulley; but this is not so powerful, nor is it so accessible as outside gearing.

The capabilities of these lathes can be still further increased by the addition of a slide-rest, but then they can scarcely be considered 'hand-lathes,' as that term is used to denote those lathes with which the work is performed by hand-tools, or tools held in the hand. The slide-rest, Fig. 3, is a sort of mechanical hand,

FIG. 3.

g

or instrument to be used instead of the hand, for holding the tools and applying them to the work. The example given in the figure is of plain but convenient construction. The part a bolts down upon the bed with the projecting piece between the bars of the bed; b is a handle, by moving which the slide c is caused to move upon a; d is well fitted upon c, and is moved around a central pin, and adjusted at any required

handle f the

angle, by the screw e; by moving the slide g is caused to traverse along d. The tool is placed in a convenient position on g under the screws, two of which are tightened down to hold the tool firmly in place. By the use of the slide-rest, the operator is enabled to turn plane surfaces, parallel bars, articles of regular taper shape, true curves, &c., far better and faster than when the tool is held in the hands. Some varieties of work, however, can be turned better by hand than by slide-rest tools.

When using these mechanical hands, it is only necessary-after they are properly adjusted-to turn a small handle, and the tool is caused to move along in the required direction. Even this small amount of hand-labour may be rendered unnecessary by making the slide self-acting-that is, causing the motion of the lathe itself to move the handle and work the slide, so as to cause the tool to move. This can be done in many different ways. The slide-rest, when in use, is fastened on the lathe-bed, just opposite that portion of the work it is desired to turn. A finger is fastened to the work, a star-wheel to the handle of the screw, and the work is set in motion. This finger, in revolving, strikes against the teeth of the wheel, and causes it and the screw to revolve slowly; and, consequently, the slide, upon which the tool is mounted, is caused to advance. This, however, is a clumsy method, but seldom used. The most convenient plan is by means of an eccentric on the lathe-spindle, the motion of which is communicated by a wire or cord passing over-head to a lever and ratchet-wheel, on the screw of the sliderest. The neatest, but not so convenient a plan as the