Chap. XX.

GAS LIGHTING.

1239

Fig. 1948. GAS GOVERNOR.

the gas enters: from the gasometer it is distributed into themains, by means of the pressure exerted upon it, andbv the proper regulation of the valves, which commandthe apertures connected with them. The pressure bywhich motion is given to the gas in the mains is measuredby the outside height of water in the cistern, into which thegas-holder is plunged ; its velocity in its passage througha main increases in the ratio of the square root of the press-ing column of water upon the gasometer ; so that byadding to this it may be made to pass either mere rapidlyor to a greater distance. In laying down the iv.ains, thatthey may distribute it as equally as possible, it is neces-sary to take into consideration the velocity of discharge atthe several distances.

Laying down the Mains. — All the pipes employed whosediameter exceeds 1^ inch in the interior, arc made ofcast-iron, in lengths of from 6 to 8 feet; their endsfit into a wide socket joint, the end of one pipe beingcast with a nozzle, inserted into a socket at the other :after these are inserted, hemp soaked in tar is driven into the

joint around the nozzle, and over this is put a luting of clay, within which is poured hotlead, which forms a ring in the socket; this is tightly driven with a blunt chisel, andtrimmed carefully. The pipes or tubes, for the distribution of the gas to the severalhouses or manufactories, are of smaller diameter, though never less than J inch, and madecither of tin, iron, copper, lead, pewter, or other compounds; they should, however, allbe subjected to proof before they are employed by means of a force-pump.

A pipe 1 inch in diameter and 250 feet* in lJngth is sufficient under ordinary circum-stances to transmit 200 cubic feet of gas in an hour ; and as the volume dischargedat the end of a pipe is directly proportionate to the square of its diameter, and inversely asthe square root of its length, other calculations may be founded upon it: it is highlynecessary that the engineer should, previously to his laying down the mains in a town orcity, correctly ascertain the difference of level of every part to be lighted, as well as thequantity of gas to be distributed in the different quarters ; whenever streets are wide, itis preferable to lay down a main on each side, as it is far more convenient to attach theservice pipes as well as for their repair. The pipes should all be laid, if possible, ina straight line, and with a gradual slope of about 1 inch to every 30 or -10 feet; this slope,after it has continued for about 3C0 feet, may then, if it be on the ascent, descend the samedistance, so that the pipes alternately rise and fall, and are not laid level; otherwisethey will not drain off the water that may be condensed within them.

Where the mains are required to be laid at right angles, curved pipes cast for the purposeare used for rounding the corners of the streets, as it is desirable to avoid any line or formthat may obstruct the free passage of the gas through them, which is so very subtilethat it is necessary to have the pipes cast internally as clean as possible, to preventfriction. At the end of every fall of main, or where two descending lines meet, areservoir should be formed, to receive all that drains off by them ; this reservoir, or siphonas it is usually denominated, must beproportionate to the duty it has to per-form ; it is made of cast-iron, of acylindrical form, its diameter being equalto double that of the mains which fallinto it, and the depth is made twice ormore that of the diameter; the topisprovided with a cast-iron cover, intowhich is fixed a wrought-iron pipe thatdescends nearly to the bottom ; on thetop of this pipe, which rises above thecover some inches, is a screw for thepurpose of attaching a small pump, thatis applied whenever the condensed waterhas accumulated, and requires to bedrawn out.

Sliding or Hydraulic Valves for passingoff the gas into the mains : this operationis sometimes ingeniously performed bymeans of a small gasometer or cast-ironvessel, which has on one side a pipe communicating with the main, and on the other a com-

4 k 4

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Fig. 1949.

SLIDING OR HYDRAULIC VALVE.