Article 41. The Miner’s Inch
The miner’s inch may be roughly defined to be the quantity of water which will flow from a vertical standard orifice one inch square, when the head on the center of the orifice is 6½ inches. The coefficient of discharge is about 0.623, and accordingly the actual discharge from the orifice in cubic feet per second is
and the discharge in one minute is
60 × 0.0255 = 1.53 cubic feet.
The mean value of one miner’s inch is therefore about 1.5 cubic feet per minute.
The actual value of the miner’s inch, however, differs considerably in different localities. Bowie states that in different counties of California it ranges from 1.20 to 1.76 cubic feet per minute.
The reason for these variations is due to the fact that when water is bought for mining or irrigating purposes, a much larger quantity than one miner’s inch is required, and hence larger orifices than one square inch are needed. Thus at Smartsville, a vertical orifice or module, 4 inches deep and 250 inches long, with a head of 7 inches above the top edge, is said to furnish 1000 miner’s inches. Again at Columbia Hill, a module 12 inches deep and 12¾ inches wide, with a head of 6 inches above the upper edge, is said to furnish 200 miner’s inches. In Montana the customary method of measurement is through a vertical rectangle, one inch deep, with a head on the centre of the orifice of 4 inches, and the number of miner’s inches is said to be the same as the number of linear inches in the rectangle; thus under the given head an orifice one inch deep and 60 inches long would furnish 60 miner’s inches. The discharge of this is said to be about 1.25 cubic feet per minute, or 75 cubic feet per hour.
A module is an orifice which is used in selling water, and which under a constant head is to furnish a given number of miner’s inches, or a given quantity per second. The sizes and proportions of modules vary greatly in different localities, but in all cases the important feature to be observed is that the head should be maintained nearly constant in order that the consumer may receive the amount of water for which he bargains and no more.
The simplest method of maintaining a constant head is by placing the module in a chamber which is provided with a gate that regulates the entrance of water from the main reservoir or canal. This gate is raised or lowered by an inspector once or twice a day so as to keep the surface of the water in the chamber at a given mark. This plan, though simple, is costly, except in works where many modules are used and where a daily inspection is necessary in any event, and it is not well adapted to cases where there are frequent and considerable fluctuations in the surface of the water in the feeding canal.
Numerous methods have been devised to secure a constant head by automatic appliances; for instance, the gate which admits water into the chamber may be made to rise and fall by means of a float upon the surface; the module itself may be made to decrease in size when the water rises, and to increase when it falls, by a gate or by a tapering plug which moves in and out and whose motion is controlled by a float. These self-acting contrivances, however, are liable to get out of order, and require to be inspected more or less frequently.*
The use of the miner’s inch, or of a module, as a standard for selling water, may be said to have a certain advantage in simplicity, as it depends merely upon an arbitrary definition. It is, however, greatly to be desired for the sake of uniformity that water should be bought and sold by the cubic foot. Only in this way can comparisons readily be made, and the consumer be sure of obtaining exact value for his money.
* A cheap and simple method of maintaining a nearly constant head by means of an excess weir is described by Foote in the Transactions American Society of Civil Engineers for March 1887.
The following are the values of the miner’s inch in different parts of the United States. In California and Montana it is established by law that 40 miner’s inches shall be the equivalent of one cubic foot per second, and in Colorado 38.4 miner’s inches is the equivalent. In other States and Territories there is no legal value, but by common agreement 50 miner’s inches is the equivalent of one cubic foot per second in Arizona, Idaho, Nevada, and Utah; this makes the miner’s inch equal to 1.2 cubic feet per minute.
The cut, Fig. 129, shows the form of measuring-box ordinarily used, and the following table gives the discharge in cubic feet per minute of a miner’s inch of water, as measured under the various heads and different lengths and heights of apertures used in California.
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Last revised: 30 December 2010.