PURINGTON.] 
HYDRAULIC MINING. 
137 
In piping against a 40-foot bank of frozen gravel, us here illustrated, 
it has been found good practice to use the giant against a certain part 
of the bank from two to six hours a day, and allowing the gravel to 
thaw the remainder of the twenty-four hours. Piping continuously 
against a frozen bank is awasteof water and power. (See PI. XXIV, 
B.) The use of powder is of no avail. Even if the gravel is broken 
off and moved to the sluice in frozen chunks it can not be washed nor 
can the gold be extracted from it. A combination of water under 
pressure and of the action of the sun, rightly adjusted, is most effective 
in thawing frozen gravel. Mr. J. P. Hutchins says that for 200 to 250 
miner's inches of water a face covering 50,000 square feet should be 
allowed on the shady side of hills, while a much less surface will 
suffice on a sunny slope. 
Tail race 
Fig. 29. — Method of hydraulieking frozen gravel, Klondike benches. 
It will be noticed that in the Klondike operations here figured deep 
trenches or "ground sluices " are cut in the bed rock leading from the 
face to the head of the tail sluice. The cutting and constant extension 
of such trenches is a necessity in almost any hydraulic operation, to a 
greater or less extent, and forms a considerable item of expense. At 
the time here figured four men in twenty-four hours were employed 
all the time in blasting out the bed-rock cuts, the expense amounting 
to not less than $50 a day. In some cases the bench miners have 
sunk a shaft at a central part of the ground and run a long tunnel on 
the grade of the sluice, to which all the gravel is moved. Although 
such tunnels carrying the tail sluice will be from 200 to 500 feet in 
length, and will cost from $3,000 to $10,000, they will generally be 
found less expensive than the constant cutting of trenches to connect 
with the main sluice carried as shown in the plant. 
BANK-HEAD WAT Kit. 
In no case seen in the North has the amount of bank-head water been 
excessive. This is a useful accessory to the mining operations, and 
where it is available should be used. The width of tail sluices should 
be great enough to allow for excess water, as at times of local rains 
such water can frequently be obtained and a larger amount of gravel 
thereby assisted in reaching the sluice. 
