In the original Billwiller burner, the injector gas orifice was brought centrally under a somewhat larger hole drilled in a separate sheet of platinum, the metal being so carried as to permit entry of air. In order to avoid the expense of the platinum, the same principle was afterwards used in the design of an all-steatite head, which is represented at D in Fig. 8. The two holes there visible are the orifices for the emission of the mixture of acetylene with indrawn air, the proper acetylene jets lying concentrically below these in the thicker portions of the heads. These two types of burner have been modified in a large number of ways, some of which are shown at C, E, and F; the air entering through saw- cuts, lateral holes, or an annular channel. Burners resembling F in outward form are made with a pair of injector jets and corresponding air orifices on each head, so as to produce a pair of names lying in the same plane, "end-on" to one another, and projecting at either side considerably beyond the body of the burner; these have the advantage of yielding no shadow directly underneath. A burner of this pattern, viz., the "Wonder," which is sold in this country by Hannam's, Ltd., is shown in Fig. 10, alongside the single-flame "Wonder" burner, which is largely used, especially in the United States. Another two-flame burner, made of steatite, by J. von Schwarz of Nuremberg, and sold by L. Wiener of London, is shown in Fig. 11. Burners of the Argand type have also been manufactured, but have been unsuccessful. There are, of course, endless modifications of flat-flame burners to be found on the markets, but only a few need be described. A device, which should prove useful where it may be convenient to be able to turn one or more burners up or down from the same common distant spot, has been patented by Forbes. It consists of the usual twin-injector burner fitted with a small central pinhole jet; and inside the casing is a receptacle containing a little mercury, the level of which is moved by the gas pressure by an adaptation of the displacement principle. When the main is carrying full pressure, both of the jets proper are alight, and the burner behaves normally, but if the pressure is reduced to a certain point, the movement of the mercury seals the tubes leading to the main jets, and opens that of the pilot flame, which alone remains alight till the pressure is increased again. Bray has patented a modification of the Naphey injector tip, which is shown in Fig. 12. It will be observed that the four air inlets are at right-angles to the gas-way; but the essential feature of the device is the conical orifice. By this arrangement it is claimed that firing back never occurs, and that the burner can be turned down and left to give a small flame for considerable periods of time without fear of the apertures becoming choked or distorted. As a rule burners of the ordinary type do not well