the minimum of disturbance to the streets. It will be readily
understood, from the drawings, how this is effected. First, the pipes
are laid in concrete troughs, near the surface of the road, with
removable concrete covers strong enough to stand any overhead traffic.
At intervals there are junctions for service connections, with street
boxes and covers serving as inspection chambers. These chambers are also
provided over the ball-valves, which serve as stop-valves in case of
necessity, and are so arranged that in case of a serious breach in the
portion of main between any two of them, the rush of air to the breach
will blow them up to the corresponding seats and block off the broken
portion of main. The air space around the pipe in the concrete trough
will convey for a long distance the whistling noise of a leak; and the
inspectors, by listening at the inspection openings, will thus be
enabled to rapidly trace their way almost to the exact spot where there
is an escape. They have then only to remove the top surface of road
metal and the concrete cover in order to expose the pipe and get at the
breach. Leaks would mostly be found at joints; and, by measuring from
the nearest street opening, the inspectors would know where to break
open the road to arrive at the probable locality of the leak. A very
slight leak can be heard a long way off by its peculiar whistling sound.

[Illustration: COMPRESSED AIR POWER]

The next point is to obtain a daily report of the condition of the mains
and the amount of leakage. It would be impracticable to employ an army
of meter inspectors to take the records daily from all the meters in the
district. We therefore adopt the method of electric signaling shown in
the second drawing. In the engineer's office, at the central station, is
fixed the dial shown in Fig. 1. Each consumer's meter is fitted with the
contact-making apparatus shown in Pig. 4, and in an enlarged form in