ON MODES OF CHANGING THE FOllMS OF BODIES.

231

' Med throughout the surface, it is allowed to revolve on an axis hy means°f the friction; its motion being confined to one direction by the action of a

catch.

Various substances, chiefly of mineral origin, are also used, on account oftheir hardness, as intermediate materials, for grinding and polishing others.-Ihese are diamond dust, corundum, emery, tripoli, putty, glass, sand, flint,re <l oxid of iron, or crocus martis, and prepared chalk; they are sometimesa pplied in loose powder, and sometimes fixed on leather, wood, or paper.Vuttle fish bone, and seal skin, are furnished by the animal kingdom, andDutch rushes by the vegetable; these are employed chiefly in polishing woodor ivory.

Marble is made smooth by rubbing one piece on another, with the interposi-tion of sand; the polishing blocks are sometimes caused to revolve by machi-nery in a trough, in which the marble is placed under water, and are drawnat the same time gradually to and from the centre ; or the slab itself, with theh'ame on which it rests, is drawn slowly backwards and forwards, while theblocks are working on it. Granite is polished with iron rubbers, by means ofSa ud, emery, and putty; it is necessary to take care during the operationthat the water, which trickles down from the rubbers, and carries withJ t some of the iron, may not collect below the columns, and stain them;hut this inconvenience may be wholly avoided by employing rubbers of8'lass.

Optical lenses arc fixed on blocks by means of a cement, and ground withtJrier y, by a tool of proper convexity or concavity: if they are small, a largedumber is fixed on the blocks at the same time. The tool is sometimes firstturned round its axis by machinery, and when the lenses are to be finished, a^ OT npound motion is given to it by means ot a crank; and in order to makemore smooth, the wheels turn each other by brushes instead of cogs. ThePmnt of the lens where its two surfaces are parallel, is determined by lookingl0u gh it at a minute object, while it is fixed on a wheel with a tubular axis,lln d shifting it, until the object no longer appears to move; a circle is thenes cribed, as it revolves, in order to mark its outline.

4