138 GRAVEL AND PLACER MINING IN ALASKA. [bull. 263. 
PI. XV, B (p. 92), shows hydraulic operations in Silver Bow basin, 
Alaska, and illustrates the use of the bank-head water. 
DUTY OF A MINER'S INCH. 
This term is applied to the quantity of material moved by 1 inch of 
water in twenty-four hours. It depends on quantity of water, char- 
acter of material washed, height of bank, size and grade of sluice, and 
kind of riffle. h\ many mines the gravel ma}- be easily broken down 
and carried to the sluice, but may be very hard to move through the 
sluice on account of a light grade, disproportionate width of box, or 
the use of obstructive riffles. Thus, according to Bowie, in the North 
Bloomfield mine the dut}^ varied from 3.86 to 4.8 cubic yards, withi 
100 to 265 feet of bank, sluice 6 feet wide by 32 inches deep, grade 
6i inches in 12 feet. At La Grange mines, on the other hand, the duty 
was from 1.08 to 1.82 cubic yards, height of bank 50 to 80 feet, sluice 1 
feet wide, 30 inches deep, and grade 3 inches in 12 feet. In both case* 
the riffle pavement was principally blocks. 
Table 11 (p. 139) is instructive as showing the variation in duty of 
the miner's inch under the different governing conditions in the North. 
The duty of the miner's inch in the Klondike is large, estimated at 8^ 
cubic yards in twenty-four hours in the operation described on page 
137, with water under 130-foot head and a grade of 12 inches to 12 feett 
in the sluice boxes, a variable amount of bank-head water from 25 to 
100 miner's inches being used. The high duty is accounted for by the 
fact that the material washed is well rounded, by the absence of large 
stones, heavy grades to sluices, and the fact that block riffles are gen- 
erally employed. 
The low duties at Nome are accounted for principally by the fact 
that one-half to two- thirds of the water is generally diverted for use 
in the hydraulic tailings lifts, and partly by the fact that the gravel is 
flat and rough. Iron riffles are generally used, but this factor playl 
little part on account of the short sluices in use. 
