In the upper part of every retort is an opening from which ascends a vertical pipe three or four inches in diameter. The gas, as it is formed, having no other outlet, ascends this pipe, passes thence to another pipe placed horizontally, and then enters a descending pipe, which dips into a large main fourteen or fifteen inches in diameter. This main is placed horizontally along the whole length of the retort-house, and receives all the gas from the whole range of retorts on one side, there being two mains on oppo site sides of each retort-house. In these mains commences that purification of the gas- which is the object of four successive processes, carried on in four distinct kinds of apparatus, viz, the hydraulic mains, the condensers, the purifiers, and the saturators. As may be readily sup posed, the transference of the various products, such as gas, tar, ammoniaeal liquor, &c., from vessel to vessel, requires a large assemblage of pipes, some of which are carried underground, and others within view.
The retort-houses, such as have just been described, arc four in number ; two situated in the northern quadrangle, and the other two being placed parallel and contiguous in the central building of the southern quadrangle. To these a series of smaller rooms are attached to the southern end of the retort-houses, and within view from the entrance-gates. One of these is the office of the superin tendent of the works, and the other two contain very ingenious specimens of ap paratus whereby he can regulate the sup ply of gas at all hours of the day, calculate how much gas has been made within a certain period, ascertain the rate at which it is being manufactured at any particular time, and keep a cheek over the labors of the men. Ono of these rooms is called the 'valve-room,' and contains the apparatus for regulating the pressure and supply of the gas. To un derstand the use of such apparatus, it is necessary to recall to mind the stnking change which occurs throughout a large city as evening is drawing on. The lamp lighter is seen busily hastening from lamp to lamp, placing his slight ladder against the street lamp-irons, and kind ling the flames which give to our streets no small .share of their evening attrac tions ; the shopkeeper begins to illumi nate his wares, with one blaze if he be a 'humble dealer, with a dozen if his house be a' gin-palate,' with a score or two if he sells unparalleled in linen drapery; the theatres, the club-houses, the evening exhibition-rooms—all begin to display a blaze of light near about one time. Now it must be obvious that the sudden demand thus created is enor mous, and it may be easily conceived that great judgment is required in ad justing the supply. In order that the gas may be propelled through the main pipes from the factory to the remotest point supplied from the works, it is ne cessary to give the gas a pressure or elas tic force greater than that of the atmos phere. If this pressure be too small, the lights at remote places would burn much too faintly ; if too large, the flames, would become so strong as to consume an inor dinate quantity of gas ; if the gas flowed from the gasometers at an hour before dusk at the same rate as at an hour after dusk, the utmost confusion and ir regularity would occur. To obviate these
evils is the object of the pressure appara tus. Around the valve-room are placed valves connected with the great' main. There are several mains branching out from the factory in as many different di rections, for the supply of different parts of the town; and as each main requires a supply of gas proportionate to the nature and extent of the district through which it passes, a pressure-apparatus is attached to it distinct from the others. Directing attention to one main only, it may be stated that after the gas leaves the gaso meters and enters the main, it is placed in communication with a small tube lead ing to a ' pressure-indicator,'- by which the exact pressure at any time of the day or night is determined. So long as the pressure is such as is required, no changes are made ; but when it is either too great or too small, recourse.is had to a valve, whose interior apparatus is in connection with the main.. If the pressure is too great, the valve is drawn partly across the main, by which the sup ply of gas is slackened : if too small, tile valve is opened more than before, to ad mit a greater volume of gas. These ad justments are, as was before observed, made in the 'valve-room,' every main having its own ' pressure-indicator' and its own ' valve.' A room adjacent to the one just men tioned, and called the "meter-room," ex hibits to view a cast-iron ease, about ten feet square, and seven or eight feet high, occupying the centre of the room. On the front are six or eight small dials, like clock-faces, and at the back are two pipes ascending through the floor, and entering the case. All the gas made at the works passes into this case or "meter" by one of the pipes just spoken of, and leaves it by the other. The meter will contain a certain known quantity of gas ; and while this quantity is passing through the ma chine, an index hand is caused, by me chanism within the case, to revolve once round a dial-plate. Every ten revolutions of this hand causes another index to re volve once round another dial-plate ; ten of these latter revolutions cause one revo lution of a third index ; and so on through six successive stages, the last index re volving only once while a million cubic feet of gas are passing through the meter. The superintendent, by looking at the in dications in these six dial-faces, is thus able to tell, even to a single foot, how much gas has passed through the meter to the main pipes. There are two other dials on the front of the meter, one of which is a regular clock, and the other an ingenious arrangement for showing the rate at which the gas ispassing through the meter at any particular time.