372 



RESUME AND THEORETICAL DISCUSSION, 



At the " No. 8 " mine of the North Bloomfield Gravel Mining Company,* 

 during the time from Jan. 1 ? 1875 ? to Oct. 13 ? 1877, 7,071,630 cubic yards 

 of gravel were washed with an expenditure of 3,750,797,560 cubic feet of 

 water. This gives an average of 534 cubic feet of water required to wash 

 one cubic yard of gravel ; or, in other words, the gravel at this locality 

 required for moving it an expenditure of water nearly equal to twenty times 

 its bulk. At the Blue Tent Company's mine, where careful record has been 

 kept of the amount of gravel washed, water used, etc., for the past few years, 

 the various kinds of gravel met with were moved at the rate of from 2.38 to 

 10.12 cubic yards per miner's 24-hour inch ; f or, in other words, the gravel 

 required, according to its condition, from eight to thirty-four times its volume 

 of water to disintegrate it and carry it into the sluices. That which demanded 



the largest quantity of water specified is described as being " hard, indurated, 

 and clayey." 



Mr. Hague adopted seven cubic yards as the amount of gravel which, on 

 the average, in the divide between the South and the Middle Yuba, could be 

 moved by a 24-hour inch of water, t and this is said by Professor Pettee to 

 be corroborated by the results obtained at Smartsville. Seven cubic yards 

 to the 24-hour inch gives an amount of gravel not quite one twelfth of the 

 volume of the water used. 



Mr. Ashburner considered that a 24-hour inch of water would move only 

 about three and a half cubic yards of the lower portion of the gravel deposit 

 m Bear River and its tributaries. This gravel may be considered as repre- 

 senting the hardest kind ordinarily worked by the hydraulic method. 



It appears, therefore, that a 24-hour inch of water will disintegrate and 



* The writer is indebted for these statistics to the kindness of Hamilton Smith, Jr., Esq. 



t A miner's inch flowing for twenty-four hours is considered on the San Juan Divide equal to 2,230 cubic feet 

 of water ; in the Bear River mines it is a little less (about 2,200). The difference arises from the different forms 

 of opening for the water to pass through, and the variable number of inches pressure allowed. 



J See ante, p. 207. 



§ The following extract from Mr. Hague's report, to which reference has before been made, states clearly and 

 concisely the main facts relating to the use of water by the hydraulic method : " At the present day the diameters 

 of the nozzles vary from five to eight inches ; the pressures under which they are used, at various places, from 

 150 to 400 feet; the velocity with which the water is discharged may vary from 75 to 150 feet per second, accord- 

 ing to the pressure ; and the quantity of water thus discharged through one nozzle, according to all these varyin 

 conditions, ranges from 300 or 400 to 1,200 or even 1,500 inches. A discharge of 1,000 inches in a single stream 

 is not unusual. The volume of water thus discharged is 1,570 cubic feet per minute, weighing but little less than 

 100,000 pounds. The water used in the actively working mines on the ridge [the San Juan Divide, or the 

 district between the South and the Middle Yuba] at present varies from 500 or 600 inches running ten hours, to 

 3,1)00 inches running twenty-four hours." 



o 



