A
check valve placed at the delivery pipe regulates the emission of the
compressed air into a receiver constructed of boiler-iron, and a "main"
thence serves for the reticulation of the principal underground
workings, where service pipes eventually connect with the rock-boring,
windÂing, ventilating, and pumping machines so frequently worked in
California and Nevada. The distance travelled by the air so compressed
does not appear to diminish the pressure very much, for I found in
Grass Valley that the pressure registered at the surface receiver had
barely decreased i lb. at the second receiver placed in the
800-foot level, or a total distance of 1,800 feet from the compressor;
and at the Sutro tunnel, Nevada, one per cent, of pressure was only
lost in a distance of over 13,000 feet, through leakage and friction in
the air pipes. The compression of air causes the evolution of heat, and
when in that state it loses its valuable expansive power as a motor.
The engines, or compressors, also become disordered on account of this
heat expanding the ends of the air cylinders, leaving the central
portions cool, thereby depriving the piston of its necessary perfect
fit, whereby the air streams through the crevices around the piston,
thus disabling the check valves as well as injuring the packing, and
becoming quite useless as a motive-power. Some compressors have
therefore their pistons working in water; their suction valves are
likewise made of brass, to better withstand the heat, but those are
only temporary preventatives ; and the idea of the National Company to
cast their air cylinders with double walls and forming a spiral jacket
at the outside of these cylinders, surrounding the inner one, by means
of which an open passage is supplied with regular flow of cold water,
has been very effective in keeping both the cylinder and the piston
cool, and in a fit state for heavy work, and likewise in preventing the
injurious mixing of air with water so heated. The mains are simply gas
pipes, the same as the service pipes ; but for a length of from 30 to
40 feet next to the machines to be worked by compressed air five-ply
canvas vulcanized rubber hoses wrapped with niarlin are used, in order
to save them from injury during blasting or other rough usage.
It
may be stated here that the American miners look upon manual labor with
the greatest possible aversion, and, as they are a very inventive
people, machines of various kinds are made to take the place of the
laborer, whose functions are much relieved thereby, and a greater
number are found employment for.
