al pressure of 1,000 feet; but we now see that a thickness of
two and a half inches of cast iron will sustain a vertical pressure of
over 3,400 feet.

There is only one pressure pipe for all the hydraulic pumps. This
extends from the tank on the side of the mountain to the 3,000 foot
level. It is tapped at the points where are situated the several sets
of hydraulic pumps. The water from the pressure pipe enters one part
of the pump, where it moves a piston-back and forth, just as the
piston of a steam engine is moved by steam. This water engine moves a
pump which not only raises to the surface the water which has been
used as driving power, but also a vast quantity of water from the
shaft, all of which is forced up to the Sutro drain tunnel through
what is called a return pipe. Each set of hydraulic pumps has its
return pipe; therefore there are three return pipes--one from the
2,400, one from the 2,600, and another from the 3,000 level.

Some idea may be formed of the great size of these hydraulic engines
when it is known that the stations excavated for them at the several
levels where they are placed are 85 feet long, 28 feet wide, and 12
feet high. All this space is so filled with machinery that only
sufficient room is left to allow of the workmen moving about it. One
of these stations would, on the surface, form a hall large enough for
a ball room, and to those who are unacquainted with the skill of our
miners it must seem wonderful that such great openings can be made and
securely supported far down in the bowels of the earth; yet it is very
effectually done. These great subterranean halls are supported by
timbers 14x16 inches square set along the walls three feet apart, from
center to center, and the caps or joists passing overhead are timbers
of the same size. The timber used is mountain spruce. Not one of these
huge stations has thus far cost one dollar for repairs. The station at
the 2,400 level has been in use five years, that at the 2,600 three
years, and the one at the 3,000 level eight months. Room for
ventilation is left behind the timbers, and all are still sound.
Timbers of the same kind are used in the shaft, and all are sound. The
shaft has cost nothing for repairs. Being in hard andesite rock from
top to bottom, the ground does not swell and crowd upon the timbers.

If it shall be thought advisable to go to a greater depth than 3,200
feet, a station of large size will be made on the east side of the