ECONOMICAL CONSIDERATIONS. 



°73 



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, a chance 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 canons, with which its western slope is farrowed, 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 I he 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 ;>f 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. 



