MINING. 271 



riferous districts are very dry in summer, and in some places 

 tliere is not a spring nor a brook within many miles. The 

 artificial ditch supplies the want. The ditches are made by 

 large companies, which sell the water by the "inch." An 

 inch of water is as much as will run out of an orifice an inch 

 square, with the water standing six or seven inches deep in 

 the flume over the orifice. The depth of water over the orifice 

 is called the " head." The orifice is usually two inches high, 

 and as long as necessary to give the amount of water desired. 

 Nobody wants less than ten or twelve inches for mining : a 

 "sluice-head" is about eighteen inches; a "hydraulic head" is 

 from forty to two hundred inches. The water, however, is 

 not measured accurately. Of course, the amount which runs 

 through the orifice will depend to a considerable extent upon 

 the " head," which is usually greater in the morning than at 

 night. At sunrise there may be fifteen inches head, and at 

 sunset only three. The water collects during the night, and 

 is exhausted during the day. The price of water is in no 

 place less than ten cents an inch per day ; in some places it is 

 forty cents ; the average is about twenty cents. 



Many of these ditches are extensive enterprises, and have 

 cost hundreds of thousands of dollars. When they cross ra- 

 vines and valleys, large flumes — wonders of carpentry — must 

 be built. Some of these are two hundred feet high and a mile 

 long, and so large that a horse and wagon can be driven 

 through them. In all, save length and durability, they are as 

 wonderful as the great Roman aqueducts, whose tall ruins still 

 stand in the Campagna, near the Eternal City. In some cases 

 iron tubes have been used, and, although they are very expen- 

 sive, yet they may pay for themselves, by preventing evapora- 

 tion, leaking, and soaking, which take away much of the water 

 from flumes and ditches. 



§ 202. Prospecting, — "Prospecting" is the search for gold. 

 The instruments used by the prospector for placer-mines are 

 usually the pan, pick, and shovel. He should be familiar with 

 the general laws of the distribution of gold, and then try the 



