148 



DRESSING OF ORES 



748 



The stuff to bo washed is placed in the chest a, into which a current of water 

 runs, The ore, floated onwards by the water, is carried through a sieve on a 



small sloping table <r, under 

 which is concealed the higher 

 end of the moveable table 

 d b c u ; and it thence falls 

 on this table, diffusing it- 

 self uniformly over its sur- 

 face. The particles deposited 

 on this table form an oblong 

 slope upon it ; the successive 

 percussions that it receives 

 determine the weightier mat- 

 ters, and consequently those 

 richest in metal, to accumu- 

 late towards its upper end at 

 u. Now the workman by 

 means of the lever p, raises 

 the lower end d a little in 

 order to preserve the same 

 degree of inclination to the 

 surface on which the deposit 

 is strewed. According as the 

 substances are swept along by 

 the water, he is careful to re- 

 move them from the middle 

 of the table towards the top, 

 by means of a wooden rake. 

 With this intent, he walks on 

 the table dbcu, where the 

 sandy sediment has sufficient 

 consistence to bear him. 

 When the table is abundantly 

 charged with the washed ore, 

 the deposit is divided into 

 three bands or segments db, 

 bc,cu. Each of these bands 

 is removed separately and 

 thrown into the particular 

 heap assigned to it. Every 

 one of the heaps thus formed 

 becomes afterwards the object 

 of a separate manipulation on a percussion table, but always according to the samfo 

 procedure. It is sufficient in general to pass twice over this table the matters con- 

 tained in the heap, proceeding from the superior band c u, in order to obtain a pure 

 slime ; but the heap proceeding from the intermediate belt b c, requires always a 

 greater number of manipulations, and the lower band db still more. These successive 

 manipulations are so associated that eventually each heap furnishes pure slime, which 

 is obtained from the superior band cu. As to the lightest particles which the -\vMcr 

 sweeps away beyond the lower end of the percussion table, they fall into launders, 

 whence they are removed to undergo a new manipulation. 



Fig. 749 is a profile of a plan which has been advantageously siibstituted, in llio 

 Hartz, for that part of the preceding apparatus which causes the jolt of the piece o u 



against the table dbcu. By means of this 

 749 plan, it is easy to vary, according to the circum- 



stances of a manipulation always delicate, the 

 force of percussion which a bar xy, ought to 

 '* communicate by its extremity y. With this 



view a slender piece of wood u is made to slide 

 in an upright piece, v x, adjusted upon an axis 

 at v. To the piece u a rod of iron is con- 

 nected, by means of a hinge z; this rod is 

 capable of entering more or less into a case or 

 sheath in the middle of the piece v x, and of 

 being stopped at the proper point, by a setscrew which prossos ngainst this piece. 

 If it be wished to increase the force of percussion, wu must lower the point z ; if 

 to diminish it, we must raise it, In the first case, the extremity of the piece u, ad- 



