DISTINCTION AND DISTRIBUTION OF LIGHTS, ETC.
The following are the various light characteristics which may be exhibited to the mariner : Fixed.—Showing a continuous or steady light. Seldom used in modern lighthouses and generally restricted to small port or harbour lights. A fixed light is liable to be confused with lights of shipping or neighbouring shore lights.
Flashing.'—Showing a single flash, the duration of darkness 'For the purposes of the mariner a light is classed as flashing or occulting solely according to the duration of light and darkness and without any reference to the apparatus employed. Thus the light shown by a fixed apparatus, in which the light source is mechanically eclipsed but yet the period of darkness is greater than that of light, is classed in the Admiralty List of Lights as a "flashing" light.
always being greater than that of light. This characteristic or that immediately following is generally adopted for important lights. The French authorities have given the name Feux-Eclair to flashing lights of short duration.
Group-flashing.—Showing groups of two or more flashes in quick succession separated by short eclipses with a larger interval of darkness between the groups.
Fixed and Flashing.—Fixed light varied by a single flash, which may be preceded and followed by a short eclipse. This type of light, in consequence of the unequal intensities of the beams, is unreliable.
Fixed and Group-flashing.—Similar to the preceding and open to the same objections.
Occulting.—A continuous light eclipsed at regular intervals, the duration of light being equal to or greater than that of dark ness.
Group Occulting.—A continuous light with groups of two or more occultations.
Alternating.—Lights having any of the foregoing characteris tics and which alter in colour are indicated by the addition of the word "alternating" to the appropriate description. When used alone in describing a light the word indicates two distinct colours alternating without any intervening eclipse. Alternating lights are not to be recommended for reasons which have already been referred to.
Colour.—The colours usually adopted for lights are red and green. A white light is to be preferred whenever possible, owing to the great absorption of light by the use of red or green glass screens.
Sectors.—Where coloured lights are employed to distinguish cuts or sectors, they should be shown from apparatus of fixed sec tion and not from revolving apparatus. In marking the passage
through a channel, or between sandbanks or other dangers, col oured-light sectors are arranged to cover the dangers, white light being shown over the fairway with sufficient margin of safety be tween the edges of the coloured sectors next the fairway and the dangers.
Choice of Characteristic and Description of Apparatus.— In determining the choice of characteristic for a light, due regard must be paid to existing lights in the vicinity. No light should be placed on a coast line having a characteristic the same as, or simi lar to, another in its neighbourhood unless one or more lights of dissimilar characteristic, and at least as high power and range, intervene. In the case of landfall lights the characteristic should differ from any other within a range of zoo m. In narrow seas the distance between lights of similar characteristic may be less. Landfall lights are the most important of all and the most power ful apparatus available should be installed at such stations. The distinctive characteristic of a light should be such that it may readily be recognized by a mariner without the necessity of ac curately timing the period or duration of flashes. For landfall and other important coast stations flashing dioptric-apparatus of the first-order (92o mm. focal distance), or its equivalent in power, with powerful burners are required. In countries where the at mosphere is generally clear and fogs are less prevalent than on the coasts of Britain, second- or third-order lights suffice for land falls, having regard to the high intensities available by the use of improved illuminants. Secondary coast lights may be of sec ond-, third- or fourth-order of flashing character, and important harbour lights of third- or fourth-order. Less important harbours and places where considerable range is not required, as in estu aries and narrow seas, may be lighted by flashing lights of fourth order or smaller size. Where sectors are requisite, occulting ap paratus should be adopted for the main light : or subsidiary lights, fixed or occulting, may be exhibited from the same tower as the main light but at a lower level. In such cases the vertical distance between the high and the low light must be sufficient to avoid commingling of the two beams at any range at which both lights are visible. Such commingling or blending is due to atmospheric aberration.