DRESSING OF ORES 



81 



driving shaft. During this movement the two lower ends A, B, are raised ; and when 

 the cam of the ehaft quits the rod which it had depressed, the swing chests fall by 

 their own weight. Thus they are made to vibrate alternately upon their axes. The 

 small ore is put into the upper part of the chest A, over which a stream of water falls 

 651 652 



from an adjoining launder. The fragments which cannot pass through a cast-iron 

 griddle in the bottom of the chest are sorted by hand upon a table in front of A, and 

 are classed by the workman, either among the ores to be stamped, whether dry or wet, 

 among the rubbish to be thrown away, or among the ores to be smelted by themselves. 

 As to the small particles which fall through the griddle upon the chest B, supplied also 

 with a stream of water, they descend successively upon two other brass wire sieves, and 

 also through the iron wire r, in the bottom of B. 



Fig. 652 shows the circular hand-riddle employed in the mines of Cornwall. 

 Although this is in advance of hand riddling, yet it is by no means equal to the large 

 sizing trommels employed in Germany. 



The ore is thrown in it at A, the coarser pieces passing longitudinally through the 

 riddle into the shoot B. The riddle is turned by a hookhandle , as shown in the illus- 



653 



tration ; the meshes of the sieve vary from ^ths ef an inch to 1 inch, square, according 

 to the character and quality of the vein-stuff to be operated upon. 



Figs. 653, 654 show an elevation and ground plan of a series of flat separating 



VOL. II. G 



