vertical plane, to which the vertical plane of the flame is at right angles. Neither of these devices singly gave a solution of the difficulty; but by combining the two--the injector and the twin-flame principle--the modern flat-flame acetylene burner has been evolved, and is now met with in two slightly different forms known as the Billwiller and the Naphey respectively. The latter apparently ought to be called the Dolan. [Illustration: FIG. 8.--TYPICAL ACETYLENE BURNERS.] The essential feature of the Naphey burner is the tip, which is shown in longitudinal section at A in Fig. 8. It consists of a mushroom headed cylinder of steatite, drilled centrally with a gas passage, which at its point is of a diameter suited to pass half the quantity of acetylene that the entire burner is intended to consume. The cap is provided with four radial air passages, only two of which are represented in the drawing; these unite in the centre of the head, where they enter into the longitudinal channel, virtually a continuation of the gas-way, leading to the point of combustion by a tube wide enough to pass the introduced air as well as the gas. Being under some pressure, the acetylene issuing from the jet at the end of the cylindrical portion of the tip injects air through the four air passages, and the mixture is finally burnt at the top orifice. As pointed out in Chapter VII., the injector jet is so small in diameter that even if the service-pipes leading to the tip contain an explosive mixture of acetylene and air, the explosion produced locally if a light is applied to the burner cannot pass backwards through that jet, and all danger is obviated. One tip only of this description evidently produces a long, jet-like flame, or a "rat-tail," in which the latent illuminating power of the acetylene is not developed economically. In practice, therefore, two of these tips are employed in unison, one of the commonest methods of holding them being shown at B. From each tip issues a stream of acetylene mixed with air, and to some extent also surrounded by a jacket of air; and at a certain point, which forms the apex of an isosceles right-angled triangle having its other angles at the orifices of the tips, the gas streams impinge, yielding a flat flame, at right- angles, as mentioned before, to the plane of the triangle. If the two tips are three-quarters of an inch apart, and if the angle of impingement is exactly 90°, the distance of each tip from the base of the flame proper will be a trifle over half an inch; and although each stream of gas does take fire and burn somewhat before meeting its neighbour, comparatively little heat is generated near the body of the steatite. Nevertheless, sufficient heat is occasionally communicated to the metal stems of these burners to cause warping, followed by a want of alignment in the gas streams, and this produces distortion of the flame, and possibly smoking. Three methods of overcoming this defect have been used: