24 The Extraction of Gold. 
by means of four chains leading back, and with its head — 
resting against a block of timber. It is pushed forward by 
-means 3 of a lever, and when released swings back against 
the block with a smart blow, making from. twenty to fifty 
blows per minute. The sand and water is run on to the head 
of the table and flows down it, carrying off the lighter ma- 
terial, the heavier being retained on the table and gradually 
brought up to the head by the force of the percussion blows. 
In dressing ordinary ores, a table of the size mentioned will 
put through from one to one and a half tons in twelve hours, 
and the material retained on it is still mixed with such a 
proportion of the poor waste as to require a second and some- 
times a third dressing. These known defects evidently ren- 
dered the percussion table inapplicable to the concentration 
of the sulphides in this colony, where material and labour 
are so costly, however useful it may be under more favourable 
conditions in this respect. 
The object in trying the table was therefore to see if its 
defects could not be remedied, or the percussion principle ap- 
plied to more advantage. Careful observation of the working 
of a small percussion table soon led to the conciusion that 
the cause of its imperfect action was the hard bank formed 
upon it by the sand, which prevented the blow from pro- 
ducing its full effect on the heavier particles; and it was 
evident that the action would be much improved if the sand 
on the table could be kept loose, in a semi-fluid state,.so as 
to allow the biow to produce a maximum effect. When 
finely ground ore is suspended in disturbed water, a blow 
given to the side of the vessel containing the mixture will 
momentarily check the current and tend to throw down 
‘the materials in suspension in the order of their specifie 
gravity, the heavier particles falling first ; and even where 
gold or any of the sulphides are in such a fine state of 
division as to float on the surface of the water, a similar 
blow will at once cause them to sink, and at the same time 
draw them towards the point where the blow is applied. 
This is the action of the percussion-table, and when the 
sand on the table is kept loose the sulphides, however finely 
crushed, are thrown down by the sudden check given to the 
current of water by the percussion blow, drawn below the | 
surface of the sand on the table where they are protected 
from the action of the water, and gradually accumulated 
towards the head, the point where the blow is given. To 
apply this principle with success several details require to 
