when the collapsed bags are withdrawn through the small holes; the bag at one end being withdrawn first, and the gas thus admitted allowed to force out the air contained in the main, before withdrawing the bag at the other end, the air thus forced out being got rid of at a stand-pipe placed near the bag at the other end of the main. When it is considered that all the air has been expelled the remaining bag can be taken out, plugs being screwed in the holes through which the bags were inserted.

A useful apparatus in the relaying of mains is the patent pipe extractor, invented by Mr. Helps, of the Gas-Works, Croydon.

It is frequently required to test a live main for soundness, in order to trace out leakages, and various methods have been devised for effecting this object.

The late Mr. Brothers devised an apparatus for this purpose which simply consists of two 30-light meters, one of which registers the passage of the gas in the usual way, while the other is made to act as an exhauster, either by continuing the spindle of the drum through the meter case, and attaching a handle to it, or by means of a small wheel geared into a larger one on the periphery of the drum, the former being worked by a handle from the outside. The main having been severed, and the two ends carefully

plugged, the exhauster inlet is connected to the live main, and the meter outlet to the dead section of main, the exhauster and meter also being joined. On the exhauster being gently turned, gas is drawn from the live main and forced through the meter into the length of main under test, thus indicating the amount of loss in a certain time and under a given pressure. The main is divided into sections, the supply to each being cut off in rotation.

The laying of mains and services embraces both the laying of mains on the district and the mains on the gasworks itself. It will therefore be necessary to say a few

words concerning the latter.

Works mains are usually jointed together either by means of the open socket joint or by the flange system, which consists in having projecting flanges cast on either end of the pipes which require connecting, the flanges being either faced and bolted together, or a piece of millboard is inserted between the two flanges. Flange joints. are most frequently employed when it is required to connect a main to a valve, or in a position above the ground, as, being rigid, the flange joint is apt to be fractured should any settlement of the ground occur. For this reason, flange joints are not employed in the laying of mains on the district.

As mentioned under condensation, the gas, as it leaves the hydraulic main, carries in suspension a considerable quantity of aqueous vapour, but as the temperature of the gas during its passage through the various apparatus from the retort-house to the outlet of the scrubbing plant becomes reduced, the greater proportion of this aqueous vapour becomes condensed to the liquid form; but even after leaving the last-mentioned apparatus, a certain quantity of aqueous vapour still remains, the quantity mainly depending upon the atmospheric temperature prevailing at any particular time, as the warmer

the temperature the more aqueous vapour the gas is capable of holding in suspension; so that although the greater part of the aqueous vapour is deposited during condensation, etc., a portion remains which condenses afterwards, and is met with at all points of the works' mains and on the district mains also, as previously described. It is necessary, therefore, to get rid of this deposited water which otherwise would block the mains and obstruct the flow of gas; for this reason the mains should be laid with a slight fall in order that the liquid may run to suitable points. The amount of fall, as a general rule, should not be less than half an inch in each 9 feet length of pipe, and, if possible, it is advisable to give an inch fall in the same distance, more especially if much tar deposits in the main.

At the various lowest points, such as at the inlets to the exhauster, the washer and scrubber, and the purifiers, large receivers should be provided, into which all the products of condensation may drain, the accumulated liquid being periodically pumped out. This periodical pumping must be strictly attended to, otherwise the mains will become logged; it will be necessary therefore at some points of the mains to pump these receivers several times in the course of twenty-four hours.

It is very important to arrange the mains so that the gas can slip past any of the apparatus on the works. This is effected by means of bye-pass connections, the "byepass" being a main connecting the inlet and outlet of an apparatus, valves being fixed on the inlet, outlet, and on the main between. When an apparatus is working normally, the valve on the bye-pass is shut, and those on the inlet and outlet are open, but should a stoppage take place, then, by opening the bye-pass valve, the gas has a free course past the apparatus, and, on closing the inlet and outlet valves, the obstruction can be cleared.

The various changes required in altering the direction of

flow of gas are made by means of valves of which there are various descriptions, hydraulic and slide, the latter, also known as the disc valve, being the most common and convenient. As generally employed, they are worked by means of a rack and pinion, which raises or lowers a disc which closes or opens the gas-way. Figs. 51, 52 show such a valve when shut and also when partly open. The gas-way of the valve is of the same size as the main to which it is attached,

and that part of the valve against which the disc slides is accurately planed and faced, the disc being pressed against the face of the valve by means of a strong spring and a series of wedges, so as to keep it perfectly gas-tight; the valve being fixed so that the pressure of the gas will be at the back of the disc. When the disc is being opened it slides into the side chamber shown in the figure, the pinion wheel which works the rack being also inside the valve-case, so that the spindle of the pinion which projects through a gland outside the valve-case is the only portion exposed.

SERVICES.

The material generally used for services from the mains to the consumer's premises or public lamps is wroughtiron tubing, which should always be of the best quality procurable. The different parts which are employed in the various descriptions of service laying are shown in

Plate IV., from which it will be seen that the connections are all made by means of male and female screws.

Lead service pipes are also employed in some instances, but these require to be carefully supported throughout their length in order to prevent sagging and the resulting accumulation of water.

In order to preserve the iron tube used in service laying, the pipes may either be coated by the Bower-Barff process, or be heated and afterwards well coated with tar, or where the soil is very destructive the system employed by the