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ECONOMICAL CONSIDERATIONS. 
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carry into the sluices from two to ten cubic yards of gravel, according to the 
character of the material. This, with the data previously given, will give an 
idea of the amount of water required in hydraulic mining operations. The 
use of powder as an auxiliary is not at all uncommon, and the loosening of 
the compacted mass by immense blasts previous to employing the water is, 
in some localities, a matter of absolute necessity.* 
Another important matter must be kept in view, as one of the conditions 
on which the success of hydraulic mining, on a large scale, depends: this is 
the topography of the region where the work is to be carried on. There 
must be, in the first place, room for sluices of very considerable length, with 
a suitable grade ; and, furthermore, achance for the discharge of the tailings, 
which, of course, accumulate with a rapidity corresponding to the bulk of 
the gravel washed. From the descriptions of the topographical character 
of the Sierra Nevada, given in the preceding pages, it must have been clearly 
seen that the deep cations, with which its western slope is furrowed, are an 
essential element in the hydraulic mining business: their existence is as 
important to its prosperity as is an abundance of water. 
The use of a large quantity of water implies the handling of a correspond- 
ingly large amount of gravel, and this again connects itself with the low 
tenor in gold of the material. It is this which is the essential feature of the 
hydraulic system: a very large quantity of gravel can be handled, provided 
water is abundant and cheap, without any considerable increase of expense 
beyond what would be required for working a small amount of material. 
The poverty of the gravel makes the business unremunerative unless prose- 
cuted on an immense scale; and this again is impossible without cheap and 
* For information in regard to the use of powder in the hydraulic mines, see Mr. Bowie’s paper in Vol. VI. 
of the Transactions of the American Institute of Mining Engineers, to which valuable contribution to the technical 
‘side of the hydraulic mining business reference has already been made. Reference may also be made to Mr. 
Waldeyer’s paper in the Fifth Report of the United States Commissioner of Mining Statistics (1873). In this 
communication an excellent description is given of the hydraulic mining process, and of the various kinds of 
fixtures (they can hardly be called tools or machines) which go to make up the plant required for carrying 
on that kind of business. One of the most important recent improvements in the process is the employment 
of so-called ‘‘under-currents.”” These are boxes, from ten to twenty feet wide, and from thirty to fifty long, 
the bottoms of which are lined with riffles, and which are placed on one side of the sluices, and a little below 
them, at intervals varying according to the topographical and other conditions. These boxes receive from the 
bottom of the main sluices, through openings between steel bars, a certain portion of the finer washings, gold 
and gold amalgam, which then have a broad surface over which to spread themselves, and the velocity of the 
shallow stream being checked, these finer particles have an opportunity to become caught in the riffles ; what is 
not thus detained goes back, lower down, into the main sluices again. This device is thought to be especially 
effective in saving the so-called ‘‘rusty gold,” which will not readily amalgamate, on account of its coating 
of oxide of iron. 
