[Illustration: FIG. 15.--THE DOUGLASS ARGAND. (_A A, Focal Plane, or Belt of Strongest Light._)]
The multiple or concentric Argand invented by Mr. (now Sir) J. N. Douglass, the Engineer to the Trinity House, may be mentioned here. This burner is of the type of those last noticed, but possesses certain peculiar features which give it a distinct claim to novelty. As will be seen by the accompanying illustration, the concentric cylinders of which the burner is composed terminate at different heights; their tops forming a regular gradation of steps, of which the innermost is the highest. These cylinders are of considerable depth, permitting the gas and air to be heated by contact with their surfaces before the point of ignition is reached. The essential feature of the invention, however, is a series of deflectors of peculiar shape, which, in addition to directing air on to the surfaces of the flames, are so formed "as to force the outer flame or flames on to the inner flame or flames in the manner illustrated." By this means the flames are concentrated and united into one, and combustion is quickened; and, a greater intensity of heat being thus attained, the illuminating power is much augmented. When this burner was first brought into notice, in 1881, high hopes were entertained as to its future. The results which it was said to afford, being far in advance of anything previously obtained from a simple Argand, seemed to promise for the burner a speedy and unequivocal success. At the North-East Coast Marine Exhibition, held in 1882, a burner with ten rings was exhibited, which was reported to develop, from 16-candle gas, 6 candles per cubic foot--a truly remarkable result to be given by so simple a burner. But, notwithstanding its apparently successful introduction, the burner has made little or no headway in the direction of its practical application. Indeed, it may almost be said to have faded altogether out of public view. This would seem to imply that there are difficulties in the way of its successful working, when brought under ordinary conditions, which were not foreseen at the time of its invention.
[Sidenote: Effects of excessive pressure with Argand and flat-flame burners.]