complications to which it has given rise, the difficulty of finding a standard which should be more reliable, and yet be as convenient to employ, has been so great that the candle still survives as the only legal unit of light, although many attempts have been made to replace it by something more satisfactory. Another reason for the survival of the candle is that the attempted improvements in the manufacture of sperm candles have in all probability resulted in their emitting a small fraction less light than was the case with those in use at the time of the passing of the Act; and the vested interests of the gas companies is so strong that they are able to successfully oppose any alteration of the standard which would in any way tell against them.

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Attempts have from time to time been made by the Gas Referees to render the method of using the candles more accurate. In their earliest Notification,' some eight years after the sperm candle was legalized, they prescribed that the candles are to be lighted at least ten minutes before the beginning of each experiment, so as to have arrived at their normal rate of burning, which is shown when the wick is slightly bent and the tip glowing.' In 1877 this direction was altered so as to read: The candles are to be lighted at least ten minutes before the beginning of each testing, so as to have attained their normal rate of burning;' and it was also ordered that all testings were to be rejected where the candles burnt more than 126 or less than 114 grains per hour. No further alteration took place until 1889, when, the Gas Referees' attention having been drawn to the great differences ascribed to the illuminating value of the gas by variations in the position of the wicks, the relative positions of the candles and wicks were for the first time defined; and it was also prescribed that the gas examiner should use a different candle for each test made. All this, however,

has proved perfectly inadequate to make the candle into a satisfactory standard.

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The most convenient standard of light, and the one most used for practical testing in gas-works, is the Methven screen, first introduced in 1878, which consists of an upright rectangular metallic plate fixed in front of a 'London' Argand burner. The upright plate has a slot which is covered by a thin silver plate, having a vertical slot of such dimensions as to allow of the passage of as much light as equals that afforded by two average standard sperm candles, when the Argand burner is provided with sufficient gas to give a flame 3 inches in height. It is manifest that, when used with coal-gas, it is only within certain limits that one could rely upon a small portion cut out of the original flame emitting a practically constant light; and that, although these limits are well within the illuminating value of the London gas supply, it is still very advisable to obtain greater constancy in result, which is done by using an aperture of smaller area, and carburetting the coal-gas in the burner with pentane.

"Perhaps the most important of all the proposed standards was Mr. Vernon Harcourt's 1-candle pentane unit, first described by him at the meeting of the British Association for the Advancement of Science in 1877. The gas used for this standard is made by bringing together, in a gas-holder, air and liquid pentane, which evaporates and mixes with the air, in the proportion of 1 cubic foot of air and 3 cubic inches of pentane. The pentane to be used is a mixture of pentane with some of the paraffins of lower and higher boiling-points, which is prepared by distilling the light petroleum at 60° C., at 55° C., and twice at 50° C. The pentane thus prepared must satisfy the following tests :--On agitation with one-twentieth of its bulk of fuming sulphuric acid for five minutes, it must impart to the acid only a faint brown colour. Its liquid density

must be between 0·62 and 0·63 at 62° Fahr. The liquid must evaporate at the ordinary temperature absolutely without residue when the tension of its vapour is not less than 7.5 inches of mercury. The density of the vapour compared with air must not be less than 2.47, nor greater than 2.53.

“The standard 1-candle pentane unit burner consists of a brass tube 4 inches in length and 1 inch in diameter, which the gas enters towards the bottom. The upper end of the tube is closed by a brass plug, half an inch in thickness, in the middle of which is a round hole, a quarter of an inch in diameter. Around the burner is placed a glass cylinder, 6 inches by 2 inches, the top of which is level with the top of the burner-air entering through the gallery on which the chimney stands. Above the burner is supported, at a height of 62-5 mm., a piece of platinum wire about 0.6 mm. in diameter, and from 2 to 3 inches in length. The air gas passes through a small meter delivering at each revolution one-sixtieth of a cubic foot, and then through a small governor fitted to regulate the flow to half a cubic foot an hour. The height of the flame is adjusted by means of a delicate stopcock, until the top of the flame appears to touch, but not to pass, the horizontal platinum wire, which is adjusted so as to be exactly over the flame, and to extend not less than half an inch beyond it.

"In practice, the great drawback to this unit is that it is very difficult to get two observers to adjust the height of the flame exactly alike, and also that it is very easily affected by draughts and by vibrations; and it also appears preferable to use a standard which shall be more nearly that of the light which is to be tested. To fulfil these conditions, Mr. Dibdin has proposed a 10-candle pentane Argand, burning fully pentanized gas or air. It consists of a specially constructed Argand burner, giving a slightly

bellied flame, the top of which is cut off by a screen exposing a fixed height of flame; and this burner, from the constancy of the results obtained, the ease of manipulation, and the small extent to which it is affected by external causes, seems likely to take the place of candles should these unreliable standards ever be discarded.

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The Harcourt screened lamp consists of a vessel, somewhat like an ordinary spirit-lamp, which contains the oil or liquid with which the wick is fed. This liquid is again pentane, derived from petroleum by repeated rectification. Fastened to the vessel containing the pentane is a tube, through which the wick is passed, and which is fitted with an ordinary double-spiked wheel turned with a handle for regulating its height. Another tube surrounds the wicktube, and answers the purpose of keeping the latter cool, and also of steadying the current of air supplied to the flame. A metal chimney surrounds the burner and the lower part of the flame, and is connected with the upper chimney by curbed metal bands, sufficiently removed from the flame as not to affect it. Through the space thus left

between the upper and lower chimneys, the central part, of the long flame is alone visible; and the opening can be regulated so as to yield a light equal to 1⁄2, 1, or 1 candle as desired. The lamp is in reality an improved amylacetate lamp, but burning pentane; and only a portion of the flame is taken for the standard.”

CHAPTER XV.

THE METHODS EMPLOYED FOR CONTROLLING PRESSURE AT THE WORKS SO AS TO SECURE AN ADEQUATE SUPPLY OF GAS AT THE VARIOUS POINTS OF CONSUMPTION, WITH A DUE REGARD FOR ECONOMICAL EFFECT.

IN

N order that the gas may be delivered on the district from the works at a uniform pressure at all periods, and independent of any fluctuations in the consumption, it is necessary to employ the instrument known as the station governor. The instrument is placed between the gasholder and the distributing main, and serves, in the first instance, to reduce the pressure thrown by the various gasholders (which is generally in excess of the requirements for the proper supply of the district) to the pressure which is just sufficient for such supply, and also to keep the pressure required at any particular period constant, automatically adapting itself, when once set to a stated pressure, to the varying fluctuations of consumption on the district, in other words it "governs " the pressure, hence its name of "governor."

The governor as frequently met with is shown in fig. 40, and consists of a tinned iron gas-holder, floating freely in a cast-iron tank, and having suspended from its crown a parabolic plug. The inlet pipe for the gas is exactly in the centre of the tank, and finishes at the top in a conical-seated flange which exactly fits the parabolic