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 | ” |
| Fig. 50.—Spar Gas Buoy. |
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 seaward of the light should not be employed except when the danger is in the near vicinity of the light. Outlying dangers require marking by a light placed on the danger or by a floating light in its vicinity.
9. Illuminated Buoys.—Gas Buoys. Pintsch’s oil gas has been in use for the illumination of buoys since 1878. In 1883 an automatic occulter was perfected, worked by the gas passing from the reservoir to the burner. The lights placed on these buoys burn continuously for three or more months. The buoys and lanterns are made in various forms and sizes. The spar buoy (fig. 50) may be adopted for situations where strong tides or currents prevail. Oil gas lights are frequently fitted to Courtenay whistling (fig. 51) and bell buoys.
In the ordinary type of gas buoy lantern the burner employed is of the multiple-jet, Argand ring, or incandescent type. Incandescent mantles have been applied to buoy lights in France with successful results. Since 1906, and more recently the same system of illumination has been adopted in England and other countries. The lenses employed are of cylindrical dioptric fixed-light form, usually 100 mm. to 300 mm. diameter. Some of the largest types of gas-buoy in use on the French coast have an elevation from water level to the focal plane of over 26 ft. with a beam intensity of more than 1000 candles. A large gas-buoy with an elevation of 34 ft. to the focal plane was placed at the entrance to the Gironde in 1907. It has an incandescent burner and exhibits a light of over 1500 candles. Oil gas forms the most trustworthy and efficient illuminant for buoy purposes yet introduced, and the system has been largely adopted by lighthouse and harbour authorities.
There are now over 2000 buoys fitted with oil gas apparatus, in addition to 600 beacons, light-vessels and boats.