2. - The reader is referred to the article Gritstone, for the description of the principal varieties of the sandstones or gritstones used in the mechanical arts for various purposes, the most important of which uses is the grinding of various cutting tools; indeed the removal of the grindstones from our workshops, would be an almost insuperable loss. The principal modes of employing grindstones will be now described.

3. - Grindstones used by Hand. - In the most primitive method the tools to be ground are simply rubbed on the quiescent stone, as stonemasons and others whet their chisels on the foot pavement, after the manner of sharpening a tool upon a hone; or smaller slabs of gritstones are employed after the manner of the butcher's steel, in fact as in whetting a scythe with the rubstone.

It is however very far more usual to fashion the grindstone as a thick disk, or very short cylinder, and to perforate it with a square central hole or eye, for the iron axis upon which the stone is mounted and put in rotation, as described in the succeeding paragraphs.

4. - Grindstones moved by Winch-Handles. - These present the greatest degree of simplicity of all rotary grindstones, and must be familiar to almost every one; when the stone does not exceed about one foot in diameter, it is commonly mounted on the upper edges of the little wooden box or trough, which serves both to support the pivots of the axis on which the stone revolves, and to contain the water with which it is moistened. The one extremity of the spindle is squared for the winch-handle, the central part is squared for the convenience of wedging on the stone with wooden wedges, and there are cylindrical necks or pivots on the axis, the bearings for which are sometimes of hard wood such as lignum vitse, or far better of metal.

In the most common form, two iron staples which surround the pivots are simply driven into the top edges of the wooden trough; in the best form, the trough and bearings are both in metal, and there is a small bar or rest parallel with the axis for supporting the tool which is held in the right hand, whilst the stone is turned with the left. The stones thus mounted sometimes measure nearly as much as 20 inches in diameter, and are used by general artizans for small tools and also by opticians for fitting in the lenses of spectacles.

5. - Ordinary Grindstones used by Carpenters, Smiths, and many others, and which stones vary from about two to four feet in diameter, are often mounted very nearly the same as the last, so as to be worked with a winch-handle, which is then however turned by an assistant, but the frames for these larger stones are continued to the ground, or are sometimes let into the ground, and between the four legs of the frame is placed the water trough.

In these cases, there is no objection to the stone dipping a little way into the water whilst it revolves, as the surface velocity of the stone can be scarcely so great as to cause the water to be thrown off by the centrifugal motion; but the stone should not be allowed to remain immersed at one particular part, or it will be there softened and become more disposed to wear irregularly; the trough is consequently often suspended on a hinge or joint at the one extremity, and hung up by a chain at the other, so that it may be occasionally raised for moistening the stone.

6. - Grindstones moved with Treadles. - For stones from about 20 to 40 inches diameter, this method is highly to be commended, as the stone whilst in rotation, then supplies enough momentum to act as a fly-wheel, and in such cases it is only needful that some part of the iron axis for the stone should be formed as a crank of three or four inches radius, from which a connecting rod, or crank hook should descend to the treadle, jointed to the two back feet of the framework, nearly as in a turning lathe. A higher velocity may be thus given to the grindstone than with a winch-handle, and the workman does not require an assistant to put the stone in motion as when a winch-handle is used. The employment of the treadle, is even now far from being so general as it deserves to be, notwithstanding that it was known and published so long as three centuries back.

7. - Grinding Lathes, or small Grindstones driven by Footwheels and Treadles. - Stones not exceeding a foot to a foot and a half diameter, do not present sufficient momentum to admit of their being driven as in the last example, unless a foot wheel of moderate weight is added to the lower part of the frame, the upper part of which then carries the grindstone spindle fitted with a pulley, so that a leather strap or a catgut band may communicate the motion of the foot-wheel to the spindle, in a manner analogous to that employed in foot-lathes; whence this arrangement has been called the Grinding Lathe. The same frame or lathe is commonly fitted with buff and brush wheels, and is then much used by cutlers for many parts of their works, that require but secondary care; this apparatus is also used by many of the workers in horn, tortoiseshell, ivory and other materials; but cutlers always polish the blades and superior parts of cutlery, upon the apparatus next to be described. 8. - Cutlers' Grindstone driven by the Fly Wheel. - Cutlers' grindstones range from about 6 to 24 inches diameter, and are fixed upon square iron spindles from 12 to 30 inches long, terminating in steel pointed centers; the stone is wedged fast near the right hand extremity of the spindle, and near the left is fixed the pulley for a leather strap which usually measures from 1 to 2 inches wide. The strap commonly proceeds from a hand-wheel of about 5 or 6 feet diameter, turned by a labourer who is situated at the back of the grinder, and the entire arrangement, from the length of space occupied is familiarly termed the long wheel, but in large establishments, the stones are generally driven by steam or other power.

The framework for supporting the grindstone spindle, usually consists of two long pieces or sleepers that lie on the ground and are united at their extremities, they have near the one end two perpendicular posts or standards, at the upper parts of which are placed the hollow centers for the spindle to run in; lignum vitae is the material preferred for the centers, horn is sometimes used, and in a few cases screws with steel centers are employed. The center block on the left hand for the centers near the leather strap, is usually pierced with three or four holes on a horizontal line, for the convenience of making the strap more or less tight, and also for adapting it to pulleys differing somewhat in diameter, without shifting the wheel. Sometimes the post for the center on the right hand, is fitted between two transverse pieces or bearers, and secured by a wedge, like the popit heads of very common turning lathes in order to serve for spindles of various lengths; because the same frame-work is commonly used by the cutler not only for grindstones, but also various laps, buffs and glaze wheels.