Page 931 - calmining1890

MINING OF GOLD ORES IN CALIFORNIA. 867

also much safer than under-hand sloping, where the walls are bad, but
not so safe where there is much loosened ground in the vein. Under-
hand sloping is sometimes preferably adopted where the ore is very
friable and very rich, because there is less loss of rich pieces in break-
ing the ore, as the broken ore falls on solid ground in this method,
while it falls upon waste in the method of over-hand stoping and may
get lost. But this loss may, to a great extent, be obviated by laying
boards near the face to be blasted.
Where the width and character of the ground to be stoped renders
the employment of the system of over-hand or under-hand stoping
impracticable, recourse is had to the system known as square sets, or a
modification of this system. This is the well known method of timbering
employed on the Comstock, at Eureka, and in other places where local
circumstances require it. In California the nature of the ore bodies
require but infrequent adoption of square sets, as compared with the
use of this system in silver mining, where the ore bodies are usually
wider and more irregular in shape.
Where timber is expensive the walls of the vein are sometimes sup-
ported by filling in waste, instead of by using square sets.
This waste is obtained from other portions of the mine where dead
work is being carried on, or by crosscutting into the foot or hanging
wall.
HOISTING.

In the " prospect " stage of the mine's history, the hoisting is done in
buckets or Cornish kibbles by windlasses, to the depth of about from
one hundred to one hundred and fifty feet. When these depths are
reached a horse whim is generally introduced. The horse whim is good
for prospecting to a depth of from two hundred to three hundred feet,
if the water is not troublesome. Below these depths a small portable
hoist belonging to the class known as " baby hoist," or " pony hoist,"
is often used in doing further work of a prospecting character. For
depths of from three hundred to five hundred feet such a plant is very
useful, provided the output of the property is small. These hoists are
provided with a friction drum, or link motion, and have an adjustable
band foot-break. The boiler and fixtures are complete upon one bed plate.
These hoists are made with either double or single cylinders; they are
made from five up to twenty horse-power. Hoists of twelve horse-power
are of a useful size. Such a hoist, with double cylinders, will raise about
one and three fourths tons, at a speed of two hundred and fifty feet per
minute.
The weight of this hoist complete, with double cylinders and a boiler
of commensurate capacity, is about four and one third tons.
The introduction of a mill, and a consequent increased output, requires
the installation of a hoisting plant of commensurate size. Where water
is not available for power, large engines of various differences in detail
of construction are used. Geared hoists are far more generally used than
direct acting hoists. Friction or spur gearing are used. These engines
have various devices to effect the saving of steam by the use of cut-offs,
automatic adjustables, etc.
For rapid hoisting, which is an essential feature where extensive
developments are being carried on at great depth, the direct acting hoist-
ing engine is required.