equal thickness throughout, it follows that the slit is of equal depth throughout its length, so that the gas issues with equal force from every portion of the slit. The slit varies in width to suit various qualities of gas.

A batswing burner requires less pressure than a fishtail, and consequently the gas issues with less directive force, and the flame is not so stiff as with a fishtail; it is consequently necessary to protect it from draughts, which in the case of Sugg's table-top burner is effected by providing the burner with a rim just below the tip. The large batswing burners, which are used in street lighting, are provided with side wings or lugs for the same purpose. As previously remarked, some flat-flame burners are provided with means for checking the pressure of gas, but the ordinary means of effecting this object, viz., by the plugging of the burner case with cotton-wool or other material, is not to be commended. On the other hand, the burners manufactured by Mr. George Bray, of Leeds, and known as Bray's "Special" burner, are so constructed that they fulfil this object admirably.

In this description of burner a piece of enamel is introduced into the lower part of the burner case, pierced by a small circular hole for the admission of gas into the burner, the diameter of this hole determining the quantity of gas which at any particular pressure is admitted into the burner.

Placed just above the enamel washer is a layer of muslin, for the purpose of lessening the "swirl" which is given to the current of gas by passing through the narrow opening in the enamel. These burners are designed so as to enable the gas to be consumed at the lowest pressure which will maintain a firm flame, and with the least amount of swirl in the gas current as it issues from the burner. The former object is attained by diminishing the area of the opening which admits gas to the burner,

without a corresponding decrease of the orifices through which the gas issues into the atmosphere, and the latter by interposing the layer of muslin which is immediately above the diminishing arrangement, and also by the enlargement of the gas chamber in the upper part of the burner.

The Argand burner is shown in fig. 62, and may be said to consist of an annular steatite ring or chamber, provided with a series of tubes or connections for communicating between the interior of the chamber and the gas supply, and perforated on its upper surface with a number of holes for the emission of the gas. Through these holes the gas issues in a series of jets, which immediately coalesce, to form one cylindrical sheet of flame. The burner is surmounted and the flame enclosed by a glass chimney, which is supported on a light gallery connected with the lower portion of the burner, which is made of brass. The chimney serves the double purpose of shielding the flame from air currents, and of drawing upon the surface of the flame the supply of air necessary for its proper and complete combustion; and this is further assisted by placing between the chimney and the burner orifices a small metallic cone, which directs the air supply upon the surface of the flame.

The combined sectional area of the three tubes which supply the gas to the annular chamber is less than the aggregate area of the burner orifices from which the gas issues, so that the gas is delivered from the latter at a considerably less pressure than that at which it enters the burner.

It is necessary to note that in the Argand type of burner the air supply is produced under conditions totally different from those which govern its production in the case of flatflame burners. In flat-flame burners the quantity of air supplied to the flame is determined by the pressure of the

gas, or, in other words, the velocity with which it issues from the burner. In Argand burners, on the contrary, the air supply is obtained quite independently of the pressure at which the gas issues; and the conditions most effective for the economical combustion of the gas, and the development from it of the highest illuminating power attainable, are only secured when the pressure of gas is reduced to a minimum.

Within the last few years various attempts have been made to develop more illuminating power from a given quantity of gas than was capable of being obtained from the burners of the Argand and flat-flame type. These attempts have resulted in the introduction of the so-called Regenerative "burners," and the Incandescent gas-burner known as the Welsbach, from the last-mentioned of which Gas Companies will undoubtedly reap enormous advantages.

FIG. 62.

The so-called Regenerative burners utilize the waste products of combustion from the gas-flame to raise the temperature of the gas before it is ignited, and likewise of the air necessary for its combustion; and it is by the adoption of this principle that the results yielded by the use of burners of this type are obtained. The effect of heating the gas and air before combustion may be briefly said to be the liberation of the carbon particle at an early stage in the flame; the raising of such particles to a higher temperature than is possible with the ordinary form of burner, and also that such particles are kept in the incandescent condition for a longer period.

There are various descriptions of regenerative burners in use, amongst the most popular being those of Mr. Sugg, known as the Cromartie" and the "Wenham." In the

66

latter form of burner the flame, which is circular, somewhat resembling an inverted Argand, is deflected by a plate of porcelain, and passes upwards towards the interior of the burner, the gas and air supplying the same being contained in separate pipes, which are heated by the products of combustion and the flame itself in the chamber below, from which any other air is prevented from having access by means of an air-tight glass shade which surrounds the flame.

Fig. 63 shows a section of the Wenham lamp; A is the air-inlet, B, the regenerator, which is made of iron, and serves to absorb the heat of the gas-flame burning beneath it, and to communicate it to the incoming air. A cylinder, c, conveys the heated air to the burner through the perforated discs; D is the burner; E, the flame; F, the reflector; &, the fastener of the ring, J, which carries the lamp-glass, K, and has a hinge at H; L is the chimney; м, the gas supply; and N, the heat disperser.

The following table, by Professor Lewes, shows the amount of light obtained from burners of different types:

The most notable advance, however, in the means for developing the maximum amount of light obtainable from

B

E

K

FIG. 63.

coal-gas is by the employment of the incandescent gasburner of Dr. Welsbach.

"mantle" of

In this system, what is known as a cone or incombustible material is dipped in a solution of the rare