The great thing to guard against is leakage.  If the pipes were simply buried in the ground, it would be almost impossible to trace leakage, or even to know of its existence.  The income of the company might be wasting away, and the loss never suspected until the quarterly returns from the meters were obtained from the inspectors.  Only then would it be discovered that there must be a great leak (or it might be several leaks) somewhere.  But how would it be possible to trace them among 20 or 30 miles of buried pipes?  We cannot break up the public streets.  The very existence of the concern depends upon (1) the daily checking of the meter returns, and comparison with the output from the air compressors, so as to ascertain the amount of leakage; (2) facility for tracing the locality of a leak; and (3) easy access to the mains with 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 Figs. 5 and 6, by which a current is sent round the electro-magnet,