A Treatise On The Principles And Practice Of Harbour Engineering

CHANNEL DEMARCATION. 257 range are matters of fundamental importance. In leading and harbour lights, a much lower degree of illuminating power is all that is necessary or désir- able. Few channels present any lengthy stretch of straight fairway ; most are characterised by sharp bends and intricate windings, necessitating the employment of numerous lights, which need not have more than a very moderate range, say 5 or 6 miles at the outside. Only in times of fog and squalls is there any real necessity for a penetrat- ing light of high calibre, and under these adventitious circumstances, it is possible to bring into action some special reinforcement, and so temporarily intensify the normal power of the light. Channel lighting is effected by buoys, beacons, lightships, and lighthouses. Luminous Buoys.—Buoys are lighted by means of oil or gas. The former is most usually petroleum; the latter almost invariably vaporised paraffin. Coal gas is unsuitable. One of the first necessities of buoyage illumination is compact storage of the illuminant ; and coal gas, when subjected to the pressure which is necessary for this purpose, loses a very considerable proportion of its illuminating power, and bums with the bluish flame of the Bunsen burner. Moreover, it distils a tarry liquid, which obviously causes obstruction and inconvenience in pipes and tubes. Oil gas is not only free from this defect, but it is also relatively a very powerful illuminant at high pressures. Its candle-power, when compressed to 150 Ibs. per square inch, is from 40 to 45, comparable with 55 to 60 at ordinary pressure. The difficulty of using petroleum oil as a direct illuminant lies in the fact that, in the course of combustion, wicks become rapidly charred or coated with carbon, to such an extent as to obstruct and ultimately arrest the capillary attraction necessary for raising the oil from the reservoir to the burning point. This means that the light will be extinguished unless there be someone at hand to dress the wick. Now constant, to say nothing of skilled attention, is impracticable in the case of buoys. They have neces- sarily to be left to take care of themselves between certain times of inspection, which can only be frequent at the expense of economy ; and whether the period be long or short, there is the same risk of failure of the light. In France, carbonised wicks, i.e. wicks specially prepared by a uniform deposit of carbon upon them, have been introduced. But these, while satis- factory in achieving their special purpose, entail corresponding difficulties of another kind. The wicks not only require careful preparation, but they call for adjustment of the utmost nicety, and they are used with burners of a very complex character. Constant watching is therefore still an essential feature of the system, and this fact discounts their use in connection with buoys. An English burner, however, known as the Wig’hain burner, has been contrived to meet the conditions of the case, and the following is a brief description of its mode of action. In an ordinary petroleum lamp, the wick is set perpendicularly to the oil reservoir from which it draws its supplies, and there would be considerable 17