Cutlers employ grindstones which are roughly mounted but which act well, being driven at a much higher speed by a strap from a large wheel or pulley, and they carefully preserve the truth of the cylindrical face. And in many manufacturing processes the surface speed of the face is still further increased by employing very large stones, and giving them the greatest number of revolutions per minute that is compatible with safety,—this limit even being sometimes exceeded, when the centrifugal force overcomes the rather slight cohesive strength of the stones, and breaks them up into fragments which fly to great distances with disastrous results. Sandstone suitable for grindstones of various degrees of hardness and fineness is found in the coal districts of the north of England, and also in those of the midland counties. A favourite stone for tool-grinding at a low speed is quarried at Bilston in Staffordshire. The neighbourhood of Sheffield also affords some useful qualities of grindstone.

Artificial grindstones closely resembling the natural stones, but of perfectly uniform texture, which the natural ones frequently are not, were made a few years ago by Mr Ransome's ingenious process. Their manufacture has been discontinued, but artificial grindstones of another kind, made with emery instead of sand, are now effecting a complete revolution in the art of the machinist. By means of these emery grinders, to which great variety of size and form can be given so as to suit the particular purposes for which they are intended, the operation of trueing up metal surfaces by hand, whether they are large or small, curved or flat, can in very many kinds of work be entirely dispensed with,—the results being superior in truth of figure, uniform in all cases, and obtained in a mere fraction of the time which the most skilful workman would require. The extensive use of these grinders in America (where the importance of labour-saving machines is properly appreciated) renders it certain that the system will gradually make its way also in England, and that hand-filing will thus to a large extent be superseded,—files being costly instruments in themselves, and many times more costly in their use, owing to the skill which they demand on the part of the workman who handles them.

Artificial wheels made with emery are by no means a new invention. In India and China they have been used for centuries; but, being made with lac and similar fusible materials, such wheels are not capable of being run at the high rate of speed which is a first essential to their efficiency. Others, however, which were not liable to these objections were made and patented in England more than thirty years ago; and it is surprising that these have not been more generally used. The chief advantage of those now made,—some of which are manufactured in the United States, some on the continent of Europe, and some in England,—is that they can be obtained of large size—up to about 3 feet in diameter—and that they are strong enough to be driven at a very high speed without breaking. At a surface speed of 5000 or 6000 feet per minute these wheels cut tempered steel readily, when used either wet or dry, and by their means it can be shaped, if necessary, in its hardest condition, with a facility previously unattainable. As in the case of common grindstones, truth of form is most important to their efficient working, and it is therefore desirable that the work under treatment should be held perfectly rigid, by means of some form of sliding rest, or otherwise. When so used the wear of the emery wheel, which is exceedingly slow, keeps it constantly true without attention. If from any cause this truth of figure be lost, or if it be desired to alter the form of the face, recourse must be had to turning it up with a diamond, nothing else in nature being sufficiently hard for the purpose.