The cylindrical vessel P, of fig. 3, surrounding the orifice of the pipe O, which delivers the tar into the tar cistern, fig. 3, serves to keep this pipe constantly immersed into a portion of tar, so that the contents of the cistern may be drawn off by the cock without admitting air into any part of the apparatus. The tar cistern has a small hole at the top, to allow the air which it encloses to escape, as it becomes filled with tar and ammoniacal liquor. The main condenser L, is placed, as shown in the drawing, higher than the level of the water in the gasometer cistern, to allow a free descent of the distillatory liquids as they pass from this vessel along into the pipes M, N, O, &c. The cistern of the gasometer, as well as the lime machine, and tar cistern, are constructed of cast iron plates, bolted and cemented together with iron cement. The gasometer is made of sheet iron plates rivetted together—E, E, are two iron stays—G, G, are friction wheels.

METHOD of correcting the relative pressure of the Gasometer, so as to cause the gas which it contains to be uniformly of an equal density.[42]

We have mentioned already that the pressure of the gas in the gasometer should be invariable, for it is obvious that the weight of the gasometer is constantly increasing in proportion as it fills with gas, and rises out of the water—see p. 88, and 167. To render its pressure uniform, we first take the absolute weight of that part of the gasometer which becomes immersed in the water, and knowing the specific weight of the substance of which it is composed, we divide its absolute weight by the specific weight of the substance of which it is composed; and this being done, we make part of the chain, (measured at right angles from the axis of the wheels over which it passes downwards towards the top of the gasometer,) which is equal to the length of that part of the gasometer which becomes immersed in water, equal in weight to the specific gravity of the substance of which the gasometer is composed. For example, let us suppose that the part of the gasometer which becomes immersed in water weighs 861 lb. and that it is composed of sheet iron, the specific gravity of which, in round numbers, we will take to be 7. It is then evident, that the part of the chain of the gasometer measured downward from the axis of the wheel over which it passes, and which is equal in length to the height of the gasometer, must be loaded with a weight of, or must itself weigh, 123lb. for this would be the weight of the water displaced by the gasometer; or let us suppose the gasometer to be made of sheet copper, the specific weight of which (omitting decimals) is 8; and that the absolute weight of the gasometer is 1792lbs. then the chain of the gasometer equal in length to the height of the gasometer, immersed into the water must weigh 224lb. for this would be the weight of the quantity of water which the gasometer displaces. This being accomplished by then adding or diminishing the absolute or balance weight of the gasometer, any desired uniform pressure may be effected, and the same bulk of gas will always be of the same specific gravity.

DIRECTIONS TO WORKMEN
ATTENDING
THE GAS-LIGHT APPARATUS[43].

Particular care must be taken to make the joints of the mouth-pieces of the retorts perfectly air tight, which may be done in the following manner:—Take some common clay, dry, pulverize, and sift it, then add as much water as will make it into the consistency of treacle; make the mouth-piece and the lid of the retort clean, lay this luting thinly over the turned part of the lid, press the lid so luted gently to the mouth-piece, and then secure it moderately, by means of the iron wedge: if the workman observes this rule, he will never fail to make good joints; but if, on the other hand, the operator is careless and neglects to remove the old luting, &c. from the turned or smooth part of the mouth of the retort, and thereby cause a bad joint, the consequence will be the loss of a considerable quantity of gas, and a very disagreeable smell and smoke.

The bridge or row of bricks of the flue C, of the retorts, should never be made hotter than a bright red, which may be regulated by the door of the ash-pit being kept close shut when the fire is getting too hot. If the operator neglects this, and suffers the fire-bricks to arrive at a bright white heat the retorts will soon be destroyed, and bad gas be produced.

The gasometer should be well examined, at least once a week, to see if it leaks, by the following method, viz. Let the main stop-cock be shut, then make a mark on the gasometer at the water’s edge when it is full or nearly of gas, there being no gas coming from the retorts at the time, and if the mark sinks in the water, the gasometer leaks; to find out the place, walk slowly round it, and you may perceive the leak by the smell, apply a lighted candle to the part suspected, and if there be gas issuing from it, it will take fire, and perhaps appear like a small blue flame—blow it out, and mark the place: thus proceed round the gasometer till you have found all the places; if you perceive a smell, and yet cannot produce a flame in the part suspected, take a brush with a little thin white-lead paint, and lay it on the part where you think the leak is, and, if it be there, the gas which escapes from the leak, will immediately turn the paint brown. After the sides of the gasometer have been well examined, and secured by dipping a piece of cloth about the size of a shilling, into some melted pitch, tempered with a little bees-wax and tar, apply the cloth whilst hot to the place with the end of your finger, rubbing it till it is quite cold; next examine the top of the gasometer in the same manner,—when it is about two feet high in the cistern, it will then be better to get at. The water in the cistern should always be kept within 3 or four inches of the top, if suffered to sink much lower without replenishing, the gas will not pass through a sufficient quantity of water, and oily particles will be apt to condense in the pipes, to their great detriment.

The only thing to be observed in the place lighted is, that the lamps and pipes are not suffered to be touched on any pretence whatever, but by the person entrusted with their care. When a lamp is not wanted, it must be completely shut off from the pipe which supplies it, by a stop-cock provided for the purpose, and not opened again but when a flame is held over it; not a lighted candle, as the tallow is liable to drop into the lamps; lighted paper is better.

ESTIMATE
OF
THE PRICE
OF
A GAS-LIGHT APPARATUS,
IF ERECTED IN LONDON,

Capable of affording, every 24 hours, Light equal to 40,000 Tallow Candles, six in the pound, burning one hour.

A gas-light apparatus complete for work, capable of affording every twenty-four hours a quantity of light equal to 1,400 Argand’s Lamps, each lamp equal in intensity to six candles, six in the pound, burning for five hours, will cost 3,500l. if erected in this metropolis.

LONDON Price List of the most essential articles[44] employed in the erection of a Gas-Light apparatus.

FINIS.