Group Occulting.—A continuous light with, at regular intervals,
groups of two or more sudden and total eclipses.
Alternating.—Lights of different colours (generally red and white) alternately without any intervening eclipse. This characteristic is not to be recommended for reasons which have already been referred to. Many of the permanent and unwatched lights on the coasts of Norway and Sweden are of this description.
Colour.—The colours usually adopted for lights are white, red and green. White is to be preferred whenever possible, owing to the great absorption of light by the use of red or green glass screens.
Fig. 49.—Otter Rock Light-vessel.
Sectors.—Coloured lights are often requisite to distinguish cuts or sectors, and should be shown from fixed or occulting light apparatus and not from flashing apparatus. In marking the passage through a channel, or between sandbanks or other dangers, coloured light sectors are arranged to cover the dangers, white light being shown over the fairway with sufficient margin of safety between 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 similar 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 100 m. In narrow seas the distance between lights of similar characteristic may be less. Landfall lights are, in a sense, the most important of all and the most powerful apparatus available should be installed at such stations. The distinctive characteristic of a light should be such that it may be readily determined by a mariner without the necessity of accurately timing the period or duration of flashes. For landfall and other important coast stations flashing dioptric apparatus of the first order (920 mm. focal distance) with powerful burners are required. In countries where the atmosphere is generally clear and fogs are less prevalent than on the coasts of the United Kingdom, second or third order lights suffice for landfalls having regard to the high intensities available by the use of improved illuminants. Secondary coast lights may be of second, 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 estuaries and narrow seas, may be lighted by flashing lights of fourth order or smaller size. Where sectors are requisite, occulting apparatus 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.
Range of Lights.—The range of a light depends first on its elevation above sea-level and secondly on its intensity. Most important lights are of sufficient power to render them visible at the full geographical range in clear weather. On the other hand there are many harbour and other lights which do not meet this condition.
The distances given in lists of lights from which lights are visible—except in the cases of lights of low power for the reason given above—are usually calculated in nautical miles as seen from a height of 15 ft. above sea-level, the elevation of the lights being taken as above high water. Under certain atmospheric conditions, and especially with the more powerful lights, the glare of the light may be visible considerably beyond the calculated range.
Table III.—Distances at which Objects can be seen at Sea,
according to their Respective Elevations and the Elevation
of the Eye of the Observer. (A. Stevenson.)
| Heights in Feet. | Distances in Geographical or Nautical Miles. | Heights in Feet. | Distances in Geographical or Nautical Miles. |
| 5 | 2·565 | 110 | 12·03 |
| 10 | 3·628 | 120 | 12·56 |
| 15 | 4·443 | 130 | 13·08 |
| 20 | 5·130 | 140 | 13·57 |
| 25 | 5·736 | 150 | 14·02 |
| 30 | 6·283 | 200 | 16·22 |
| 35 | 6·787 | 250 | 18·14 |
| 40 | 7·255 | 300 | 19·87 |
| 45 | 7·696 | 350 | 21·46 |
| 50 | 8·112 | 400 | 22·94 |
| 55 | 8·509 | 450 | 24·33 |
| 60 | 8·886 | 500 | 25·65 |
| 65 | 9·249 | 550 | 26·90 |
| 70 | 9·598 | 600 | 28·10 |
| 75 | 9·935 | 650 | 29·25 |
| 80 | 10·26 | 700 | 30·28 |
| 85 | 10·57 | 800 | 32·45 |
| 90 | 10·88 | 900 | 34·54 |
| 95 | 11·18 | 1000 | 36·28 |
| 100 | 11·47 |
Example: A tower 200 ft. high will be visible 20·66 nautical miles to an observer, whose eye is elevated 15 ft. above the water; thus, from the table:
| 15 | ft. elevation, | distance visible | 4·44 | nautical miles |
| 200 | ” | ” | 16·22 | ” |
| 20·66 | ” |
Elevation of Lights.—The elevation of the light above sea-level need not, in the case of landfall lights, exceed 200 ft., which is sufficient to give a range of over 20 nautical miles. One hundred and fifty feet is usually sufficient for coast lights. Lights placed on high headlands are liable to be enveloped in banks of fog at times when at a lower level the atmosphere is comparatively clear (e.g. Beachy Head). No definite rule can, however, be laid down, and local circumstances, such as configuration of the coast line, must be taken into consideration in every case.
Choice of Site.—“Landfall” stations should receive first consideration and the choice of location for such a light ought never to be made subservient to the lighting of the approaches to a port. Subsidiary lights are available for the latter purpose. Lights installed to guard shoals, reefs or other dangers should, when practicable, be placed seaward of the danger itself, as it is desirable that seamen should be
able to “make” the light with confidence. Sectors marking dangers