: MINING. 257 
sluices have paid large profits to their owners. Tail-sluices 
are always large, long, and paved with stones; and sometimes 
they are double, so that one side may be cleaned up while the 
other continues washing. In a branch of the Yuba there is, 
or was not long since, a tail-sluice twenty feet wide. 
§ 188. Tunnel-Sluice—A tunnel-sluice is a sluice in a tun- 
nel. It sometimes happens that a considerable body of water 
runs out through a tunnel; and in such case, a sluice at the 
bottom of the tunnel offers the easiest method of getting out 
and washing the dirt. The tunnels are never cut level, but 
with a slightly-ascending grade, so that the water will always 
run out. The grade is so low, that transverse riffle-bars must 
be used; for with longitudinal riffle-bars or stones, there would 
be'too much danger of choking. These tunnel-sluices, because 
of their low grades, require much more attention than any 
other kind of sluices. 
§ 189. Ground-Slwice.—All the sluices hitherto mentioned 
and described have wooden boxes, but the ground-sluice has no 
box: the water runs on the ground. The place selected for the 
ground-sluice is some spot where there is a considerable supply 
of water, a steep descent for it, and much poor dirt. The stream 
is turned through a little ditch, which the miners labor to deepen 
and enlarge, and when it is deep they prize off the high banks 
so that the dirt may fall down into the ditch. This is a very 
cheap and expeditious way of washing, but it is not applied 
extensively. It is used to the most advantage for washing 
where the water is abundant for only a few weeks after heavy 
rains, and where it would not pay to erect large sluices. A 
few cobble-stones should be left or thrown at intervals in the 
bed of the ground-sluice to arrest the gold, for if the bed were 
smooth clay, the precious metal might all be carried off. 
Quicksilver is not used in the ground-sluice. After the dirt 
has all been put through the ground-sluice, it is cleaned up in a 
short board-sluice, or a tom. 
190. Long Tom.—tThe tom or long tom, an instrument ex- 
tensively used in the Californian mines in 1851 and 1852, but 
wR 
