the water from the water-loading tank, when reducing the pressure, are shown in fig. 43: в being the pipe conveying the water for loading'; a the water-loading tank; and c the syphon for the removal of water. Messrs. Cowan have also devised a method for automatically regulating the pressure on a district by means of their Automatic Pressure Changer," shown in fig. 44.

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The apparatus consists of a clock, having geared to it a disc, which it revolves once in twenty-four hours. Attached to this disc are two series of tappets, which can be so fixed as to come into operation at any desired period of the disc's daily revolution. One set of tappets controls a cock supplying water to the loading tanks of the governor bell, the other, a similar cock, withdrawing water therefrom. Shutting off the flow of water when the required addition or reduction of pressure has been attained is effected by means of two small gas-holders placed in communication with the outlet gas. One of these in rising causes the water supply cock to close, the other in falling, the syphon cock.

The weighting of these holders is arranged so as to increase or diminish in steps, as they rise or fall, the pressure required to actuate them during each stage of their travel corresponding to that which the governor is desired to give. The various tappets do not directly actuate the. cocks, this duty being performed by means of suitable weights, which are wound with the clock once a week.

In places where the district supplied varies in level, it is often necessary to place governors at various points, in order to check the increase of pressure which would otherwise take place at parts elevated some distance above the works. The increase of pressure which occurs from a rise in elevation is usually reckoned as a tenth of an inch of water for every 10 feet, while on the other hand, where the gas has to travel to a lower level, a loss in initial pressure

takes place in the proportion of a tenth less pressure for every descent of 10 feet. District governors for the automatic regulation of the pressure in the mains at high altitudes are made both in the wet and dry form. Fig. 45 shows the arrangement adopted by Messrs. D. Bruce, Peebles and Co.

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The operation of controlling the pressure by means of the governor, to the lowest amount consistent with the granting of a sufficiency for the adequate supply of the district, is one of the greatest importance, as upon this depends in a great measure the amount of unaccounted-for gas through leakage from the mains and service pipes, a pressure greatly in excess of that necessary for the requirements of the district, involving a heavy leakage account, as "by doubling the amount of pressure, the consumption of gas is increased by about one-half. Leakage from a pipe is, of course, increased in the same ratio, i.e., in the proportion of the square root of the pressure (Newbigging).

FIG. 45.

TH

CHAPTER XVI.

THE LAYING OF MAINS AND SERVICE PIPES.

HE pipes which convey the gas through the various streets and thoroughfares are known as main pipes or mains, the smaller pipes leading from the main pipes, and which convey the gas to the consumers' premises, etc., being known as services.

The material of which gas mains are made is usually cast iron, although in some instances, as in the laying of a trunk main, they are constructed of riveted wrought-iron or steel plates.

As the leakage which occurs in the distribution of gas is caused in a great measure by defective and badly-jointed mains, it is necessary to see that the pipes employed are made from iron of good quality, that the metal possesses a close grain, but is not too brittle, and that the sectional thickness of the pipe is equal throughout its whole length, and generally, that the pipes are free from defects of every description.

With this aim, all cast-iron pipes having an internal diameter of 5 inches and upwards should be cast vertically in dry sand moulds.

All pipes should be tested by hydrostatic pressure equal to 80 lbs. per square inch, and whilst under this pressure they should be rapped from end to end with a light hammer, which will often detect faults. Should there be any cracks or flaws, these will be shown by the water