=44.= Some method must be provided for communicating between the bottom
or any level of a shaft and the top landing or the engine room, also
between the top landing and the engine room, so that the hoisting
engineer may be notified when both the head-man and foot-man are ready
for him to hoist. A common method of signaling is by means of a gong,
bell, or triangle placed in the engine room and connected by a wire
or small wire-rope with the point from which it is desired to signal.
Attempts have been made in different localities and by different
associations to adopt a standard code of hoisting signals, and while it
would be advantageous if this could be done, none of the attempts made
have been entirely successful. Although there is no uniform system of
signals, one bell generally means stop, two bells lower, three bells
hoist, and four bells hoist men.
[Illustration: FIG. 40]
=45. Hammer-and-Plate Signal.=--Fig. 40 shows a hammer-and-plate
signal, the plate being a piece of boiler iron or steel. The hammer is
often located beneath the plate instead of above, as shown. Another
style of hammer-and-plate is shown in Fig. 41. The hammer is made of
2-inch square iron and heavy enough to balance the weight of wire
hanging in the shaft and to take the sag out of the horizontal wire
connecting the top of the shaft with the lever _a_. A simple dial
turned by a ratchet motion attached to the lever _a_ is sometimes used
to show the number of strokes, and thus check the number counted by the
engineer. The dial is reset by the engineer as soon as he understands
the signal.
[Illustration: FIG. 41]
[Illustration: FIG. 42]
=46. Electric Bells.=--Electric bells operated by push buttons are
rapidly coming into use for mine signaling on account of the ease and
completeness with which such a system can be installed. Electric flash
lights are also extensively used for signaling purposes. The principle
of action and details of the wiring for electric signals and flash
lights have been described in _Transmission, Signaling, and Lighting_.
=47. Speaking Tubes.=--The laws of certain states require speaking
tubes, in addition to the ordinary means of signaling. These speaking
tubes are generally made of 2-inch iron pipe and are from 300 to 1,500
feet long, and are often provided with whistles at the end of the pipe
and at each level of the mine, by which the attention of persons at any
level can be attracted or the whistle may be omitted and the attention
of persons attracted merely by rapping on the pipe with a piece of iron.
[Illustration: FIG. 43]
=48. Pneumatic Gong Signal.=--Fig. 42 shows an attachment that can be
connected to a speaking tube and that is widely used for signaling. It
consists of a brass cylinder _a_ fitted with a piston _b_ containing
valves _c_. The gong _d_ is attached to the cylinder _e_ inside of
which the clapper _f_ fits loosely. When the piston is pushed inwards,
as shown by the arrow, by means of the handle, the air in the cylinder
and in the pipe _h_ is compressed and forces the clapper _f_ upwards
against the gong _d_. The arrangement of these gongs in the mine is
shown in Fig. 43. A cylinder and whistle are usually placed at each
landing and a gong and whistle in the engine room, though, if desired,
a cylinder, whistle, and gong may be placed at each landing and in the
engine room.
=49. Telephones.=--Telephones connecting the different levels with the
top and the engine room are now frequently used in connection with
other signal systems, but they are not as well adapted as bells or
gongs for rapid-hoisting signaling.