MINING. 271 
riferous districts are very dry in summer, and in some places 
there 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 
