570 MmmG mDiJSTBT. 



at the upper end and near one side, move downward with the stream, but as 

 the percussion is applied at the opposite side, thej obtain, at each throw of 

 the table, a lateral motion, which varies in amount according to the density of 

 the particles, so that the heaviest, the grains of ore, move entirely across the 

 table to the side opposite to that at which they entered ; the lighter particles, 

 or grains of ore and gangue combined, move part way across, while the light- 

 est, or grains of earthy character, move downward in a nearly straight line, 

 describing curves such as are shown in Fig, 2, at a, «\ a^, c?, «*. By this 

 means a separation of the particles is effected according to their density, and 

 as they are discharged at different parts of the front edge of the table, they 

 may be received there in separate vessels or troughs, provided for the several 

 classes, the first, consisting of nearly pure ore, being ready for smelting, or 

 other metallurgical treatment; the second, consisting of mingled ore and 

 gangue, may be returned for repeated dressing; and the third, nearly pure 

 gangue, is allowed to run to waste. 



Fig. 4 shows the construction of the frame of a double table, consisting 

 of two cross-pieces, y^ and five longitudinal pieces, g. This frame is covered 

 by hard-wood plank or boards which are smoothly dressed and carefully fitted 

 together, forming the surface of the table over which the material for concen- 

 tration is allowed to pass.^ The sides and upper end of this surface are fur- 

 nished with bordering strips of wood, 7^, and a similar strip divides the surface 

 longitudinally in the middle, thus forming a double table. The lower end of 

 the table is also furnished with short strips, ^, which may be moved on a pivot^ 



1 As the top of the table, hitherto constructed of maple boards, soon begins to rot or 

 wear upon the surface, and thus to lose its desired smoothness, it is better to cover it with 

 a stout, water-proof gum cloth, which must possess the requisite degree of smoothness 

 or polish, so that the fine particles of slime may not adhere to it, and should be white or 

 of light color, so that the dark streaks of ore may be clearly distinguished. The cloth 

 should be applied to the table when warm, so that it may bo well stretched under ordi- 

 nary temperature. When stretched and nailed upon the table, the edges of the cloth 

 are covered with narrow strips of leather, in order to prevent tearing; and at the upper 

 end it is covered with a strip of zinc, 10 or 12 inches wide, upon which the water and 

 solid material fall from the distributing boards, passing thence quietly on to the cloth. 

 Such a cloth-covering is said to last over a year, and to be especially well adapted to the 

 treatment of the finest material ; only the number of shocks must be increased for such 

 to 120 or 150 per minute. 



^ These pointed strips may also be fixed in any desired position or place by driving 

 wooden wedges between them and a transverse piece of wood that crosses the table 



H 



