ion, and
installation of the signal apparatus, so as to insure reliability of
operation under the most adverse conditions, and to provide for
accessibility to all the parts for convenience in maintenance. The
system for furnishing power to operate and control the signals
consists of the following:

Two 500-volt alternating current feed mains run the entire length of
the signal system. These mains are fed by seven direct-current
motor-driven generators operated in multiple located in the various
sub-power stations. Any four of these machines are sufficient to
supply the necessary current for operating the system. Across these
alternating mains are connected the primary coils of track
transformers located at each signal, the secondaries of which supply
current of about 10 volts to the rails of the track sections. Across
the rails at the opposite end of the section is connected the track
relay, the moving element of which operates a contact. This contact
controls a local direct-current circuit operating, by compressed air,
the signal and automatic train stop.

Direct current is furnished by two mains extending the length of the
system, which are fed by eight sets of 16-volt storage batteries in
duplicate. These batteries are located in the subway at the various
interlocking towers, and are charged by motor generators, one of which
is placed at each set of batteries. These motor generators are driven
by direct current from the third rail and deliver direct current of 25
volts.

The compressed air is supplied by six air compressors, one located at
each of the following sub-stations: Nos. 11, 12, 13, 14, 16, and 17.
Three of these are reserve compressors. They are motor-driven by
direct-current motors, taking current from the direct-current buss
bars at sub-stations at from 400 to 700 volts. The capacity of each
compressor is 230 cubic feet.

[Illustration: MAIN LINE, PIPING AND WIRING FOR BLOCK AND INTERLOCKING
SYSTEM, SHOWING JUNCTION BOX ON COLUMN]

The motor-driven air compressors are controlled by a governor which
responds to a variation of air pressure of five pounds or less. When
the pressure has reached a predetermined point the machine is stopped
and the supply of cooling water shut off. When the pressure has fallen
a given amount, the machine is started light, and when at full speed
the load is thrown on and the cooling water circulation reestablished.
Oiling of cylinders and bearings is automatic, being supp