ON MODES OF CHANGiyG THE FORMS OF BODIES. 231 



vided throughout the surface, it is allowed to revolve on an axis hy means 

 of 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 of 

 their hardness, as intermediate materials, for grinding and poHshing others. 

 These are diamond dust, corundum, emery, tripoli, putty, glass, sand, flint, 

 red oxid of iron, or crocus martis, and prepared chalk; they are sometimes 

 applied in loose powder, and sometimes fixed on leather, wood, or paper. 

 Cuttle fish bone, and seal skin, are furnished by the animal kingdom, and 

 Dutch rushes by the vegetable; these are employed chiefly in polishing wood 

 or 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 drawn 

 at the same time gradually to and from the centre ; or the slab itself, with the 

 frame on which it rests, is drawn slowly backwards and forwards, while the 

 blocks are working on it. Granite is polished with iron rubbers, by means of 

 sand, emery, and putty; it is necessary to take care during the operation 

 that the water, which trickles down from the rubbers, and carries with 

 it some of the iron, may not collect below the columns, and stain them; 

 but this inconvenience may be wholly avoided by employing rubbers of 

 glass. 



Optical lenses are fixed on blocks by means of a cement, and ground with 

 emery, by a tool of proper convexity or concavity: if they are small, a large 

 number is fixed on the blocks at the same time. Tlie tool is sometimes first 

 turned round its axis by machinery, and when the lenses are to be finished, a 

 compound motion is given to it by means of a crank; and in order to make 

 it more smooth, the wheels turn each other by brushes instead of cogs. The 

 point of the lens where its two surfaces are parallel, is determined by looking 

 through it at a minute object, while it is fixed on a wheel with a tubular axis, 

 and shifting it, until the object no longer appears to move; a circle is then 

 described, as it revolves, in order to mark its outline. 



4 



