A different system, which has been adopted widely throughout the East and in Australian waters, is the Wigham petroleum beacon. This system possesses many notable features, the most important being that well-refined petroleum oil is employed. In many parts of the world carbide of calcium is not readily obtainable, and, moreover, is somewhat expensive, whereas, on the other hand, oil is comparatively cheap and available in unlimited quantities. The principle of working is somewhat novel. The wick is not burned in the manner generally followed in regard to lamps--viz., at the end, which within a short time becomes carbonized and brings a marked diminution of the illuminating power--but it is moved so that the same part is not exposed continuously to the action of the heat arising from combustion. It is caused to travel horizontally over a small roller, in a specially-constructed burner, combustion taking place, therefore, on its flat side. It is moved slowly and continuously over this roller, so that it cannot burn through, and in this manner the flame, being constantly emitted from a fresh surface, is of uniform intensity. [Illustration: THE WIGHAM THIRTY-ONE DAY UNATTENDED PETROLEUM LIGHT. The type at left shows the lamp carried upon a cast-iron pillar; while on the right it is mounted upon a lattice tower.] The lamp comprises three main parts. There is the lantern, with the lens and the projecting panes of plate-glass, in the focus of which the burner is fixed. Then there is the burning-oil reservoir, which feeds the wick as it moves towards the burner. This reservoir is circular in shape, somewhat shallow, and serves as a deck on which the lantern is built up. The third part is the float cylinder, made of copper, which is attached to the underside of the oil reservoir. This cylinder is filled with oil, which is kept quite distinct from the burning oil, and thereon floats a weighted copper drum, to which one end of the wick is secured by means of a hook. At the lower end of this cylinder is a micrometer valve, which when opened permits the oil to drip away at a certain speed. This causes the float to fall with the oil in the cylinder, and to drag the wick over the burner roller and down the float cylinder after it, so that a fresh surface of the wick is presented continuously for combustion. The lamps themselves may be divided into two broad classes--the single-wick and the three-wick respectively. The latter obviously emits the more brilliant light, and is the type which is coming into more extensive use at the present time. In the latest type a duplex burner is employed, and this has been found to give a very powerful light with a comparatively low oil consumption.