WHEELS. - In almost every branch of the manufacturing and mechanical arts, the processes of abrasion are advantageously fulfilled by rotatory motion applied to various grinders and polishers. These are generally circular disks, made of a great variety of substances, and are for the most part fed with abrasive powders. Most of these apparatus, with the exception principally of the grindstone, are known by the cutler and the tool maker as wheels; by the mechanician as laps, by the lapidary as mills, by the optician as tools, and also by many other conventional names; the first name, or Wheels, has been selected for the title of this article as being the most general. A few words will be first offered on the principal modes in which these wheels are employed. 1. - General Mechanical Arrangements. - Cutlers and tool makers place the axes of the wheels horizontally, and employ both for the grindstone and the polishing wheels the same framework or apparatus. Mechanicians frequently employ nearly the same arrangement as cutlers and tool makers, and in some few cases mount the laps and wheels as adjuncts to the lathe.

Seal Engravers always use a small lathe mandrel, to which their delicate grinders are attached. Lapidaries, unlike the above-named artizans, mostly place the axes of their mills vertically, and frequently drive them by the left hand, as will be explained. Opticians fix their spherical tools for grinding and polishing lenses, horizontally, on the top of a fixed post, and rub the lenses or specula upon the same with an elliptical motion given by the hands, and they continually walk round the post, to change the direction in which the grinder and tool successively meet. See Chap. XXXIII.

These and other mechanical arrangements, will however be touched upon in the course of the chapters immediately following, and therefore, it is intended at this place principally to direct attention to the abrasive apparatus, and which will be classed under seven heads namely: -

A. - Wheels of Natural Stone such as Grindstones.

B. - Wheels of Factitious Stone, or Composition Wheels.

C. - Wheels of Metal, or Metallic Laps.

D. - Wheels of Wood, or Glaze Wheels.

E. - Wheels of Leather, or Buff Wheels.

F. - Wheels of Cloth, or Cloth Wheels.

G. - Wheels of Bristles, or Wire, or Brush Wheels.

In every case but the first, the cement, metal, wood, leather, cloth or bristles, are to be viewed solely as the vehicles or carriers by which the abrasive matters or powders are applied. And in speaking of these apparatus, their structure will be first noticed, and some observations on the modes of using them and keeping them in order, will be then subjoined. The first of the seven sections or the natural grindstones will be now considered under their principal although varied features.