282 GEOLOGICAL SURVEY 
In this State what is supposed to be an improvement on this, is used. 
Fig. 44 is a sketch of this machine, which consists of four cast-iron wheels, each weighing 
about 1500 pounds. They revolve in a circular trough, composed of four segments, weighing 3600 
pounds. ‘he wheels make from six to eight revolutions round the cirele per minute, and grind, of 
ordinary ore, from fifty to seventy bushels a day. Six to eight pounds of mercury are used at a 
time, in the mill; a small stream of water flows into the trough, which carries off the fine matter, 
as the ore is ground. The ore is fed in bya man with a shovel. Some care is requisite to regulate 
the quantity admitted, and the velocity of the wheels. The water, as it escapes, passes over mer- 
cury, placed in the riffle-box, a, so as to catch any gold not taken up in the mill. Notwithstanding 
this, [ have rarely examined the water, as it escapes from the riffle, without finding both mercury 
and gold. 
In North Carolina I saw another contrivance appended to the mill: it consisted of an inclined 
table, with numerous grooves, containing mercury, over which the ground ore and water were 
made to meander; and finally, to pass over a sheepskin, which detained the last atom. 
A scraper in front of the wheels, to stir up the ore, is a great improvement ; for there is consider- 
able difficulty in grinding some of the ores of Lancaster, in consequence of the fine quartz sand 
