purington.] HYDKAULIC MINING. Ill 
Long conduits are used in order that the difference in elevation 
between the source of the water and the point where it is used may 
afford the pressure. The length of the conduit will depend on the 
slope or grade of the surface. In regions of low mountains and gentle 
slopes, long ditches or flume lines are necessary. In steep mountain 
regions the same results are attainable with short conduits. 
In the gravel-mining districts of Alaska examples of the two 
extremes are found. It is an unfortunate, but at the same time 
natural, result of the governing geological conditions, that to obtain 
the use of water under head ditches must be the largest in the richest 
and most promising placer districts of Alaska. A long head ditch is 
expensive both to construct and to maintain. That the expense of 
such undertakings has not deterred miners from attempting them is 
evidenced by early California operators, where single companies some- 
times built more than 100 miles of ditch line. Each mile of ditch 
line or other form of conduit adds from $2,000 to $15,000 to the initial 
capital necessary to start the hydraulicking operation, and increases 
the annual cost of ditch maintenance. 
Some of the precautions which apply equally well to an} r country 
regarding the construction of water conduits, found in the text-books 
on hydraulic mining, may well be repeated here. 
Van Wagenen" says: 
When the miner has measured the stream from which he is to draw his water 
supply, and has determined the point where he will tap it, he is prepared to con- 
rider the question of water channels. These may be of three kinds — the ditch, the 
wooden flume, and the iron pipe. * * * It is generally desirable to have the 
east possible fall in a water channel, or, in other words, to bring the water to as high 
i point of the ground to be worked as circumstances will allow. As the friction of 
;he sides and bottom of a channel retards the flow, and necessitates a higher grade 
;han would be necessary if there were none, it becomes of importance to decrease 
his element as much as possible. On this score wood and iron waterways present 
lecided advantages, owing to their comparative smoothness. In any case, how- 
ever, the quantity of friction developed depends upon the wet perimeter of the 
hannel used. The following law will therefore be found to be of service: 
The least wet perimeter that will hold or carry a given volume is attained when the width 
f bottom is from one arid three-fourths to two and. one-fourth times the depth of the sides. 
For example, a channel having a cross section of 510 square inches will develop the 
>ast amount of friction when its dimensions are 15 by 34 or 17 by 30, or somewhere 
x'tween these measurements. A knowledge of this fact will be found serviceable in 
^instructing flumes. The least perimeter, of course, requires the least lumber, and 
nan v thousand or million feet may be saved in a long flume by building in the cor- 
ed proportions. 
I Bowie 6 says: 
| All water courses on the line of the ditch should be secured. Their supply par- 
iallv counteracts the loss by evaporation, leakage, and absorption, and frequently 
prnishes an additional quantum of water during several months of the year. 
a Van \\ a^enen, T. F., Manual of Hydraulic Mining, 1880, p. 51. 
i> Bowie, A. J., jr., A Practical Treatise on Hydraulic Mining in California, 1885, p. 135. 
