ON MODES OF CHANGING THE FORMS OF BODIES. 177 



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 in- 

 terposition of sand ; the polishing blocks are sometimes caused to revolve 

 by machinery 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. The tool is some- 

 times 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 ap- 

 pears to move ; a circle is then described, as it revolves, in order to mark 

 its outline. 



Machines for trituration, by means of which the larger masses of matter 

 are crushed, broken, or ground, into smaller parts, are in general compre- 

 hended under the denomination of mills. After the pestle and mortar, 

 the simplest machine of this kind appears to be the stamping mill ; the 

 stampers resemble the hammers of the mill employed in the extraction of 

 oils from seeds, and the machine is used for reducing to powder the ores of 

 metals, and sometimes also barks, and linseed ; the surface of the stampers 

 being armed Avith iron or steel. But barks and seeds are more usually 

 ground by the repeated pressure of two wheels of stone, rolling on an axis 

 which is forced in a horizontal direction round a fixed point. A noble- 

 man of distinguished rank and talents has lately employed for a mortar 

 mill, a wheel of cast iron, formed of two portions of cones, joined at their 

 bases : after thirty revolutions, the mortar being sufficiently ground, a bell 

 rings, and the horse stops. 



The materials for making gunpowder are also ground by a wheel re- 

 volving in a trough : in order to corn them, they are moistened, and put 

 into boxes with a number of holes in their bottoms, and these boxes being 

 placed side by side, in a circular frame, suspended by cords, the frame is 

 agitated by a crank revolving horizontally, and the paste shaken through 

 the holes : the corns are polished by causing them to revolve rapidly within 

 a barrel. 



A revolving barrel is used for forming and polishing small round bodies 

 of different kinds, and it is often employed in agriculture as a churn for 



