232 LECTURE XIX. 



Machines for trituration, by means of which the larger niasses-of matter are 

 crushed, broken, or ground, into smaller parts, are in general comprehended 

 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 with iron 

 or steel. But barks and seeds are more usually ground by the repeated pres- 

 sure of two wheels of stone, rolling on an axis which is forced in a hori- 

 zontal direction round a fixed point. A nobleman of distinguished rank and 

 talents has lately employed for a moj^ar 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 revolving 

 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 making 

 butter. The purpose of agitation is perhaps more effectually answered by an 

 alternate motion, which has sometimes been produced in a barrel churn, by 

 means of a cord attached to a heavy pendulum. 



Threshing machines are of two kinds ; the one consists of a number of flails, 

 beating the corn nearly in the same manner as they are used by labourers ; 

 in the other, which is more commonly employed in this countr}', the corn is 

 drawn along by two revolving rollers, and caused to pass between a cylinder, 

 and its concave cover, while a number of blocks, projecting from the surface 

 of the cylinder, beat or rub out the grains very effectually from the ears ; the corn 

 falls out at the lower part, and is winnowed by a fan which the machine turns 

 at the same time. In this manner it is said that a horse will thresh about 100 



