Electric light has been adopted in one or two cases; but while the foremost authorities agree that it throws the best, most brilliant and most powerful beam of light, the system is generally impracticable on account of its great cost. When tests with this light were made some years ago in comparison with the light thrown from oil burners, it was claimed that the latter, owing to its reddish-yellow tinge, was the most suitable from the all-round point of view, and that it could penetrate to a greater distance in foggy weather. I have been informed by several authorities, who have gone more deeply into this question since, that this is a fallacy, and that the advantage rests completely with electric light. Experience in Germany, which has two magnificent electric lighthouses, and in Scotland, certainly supports this contention, and I have been assured that the sole reason why electric lighting has not been adopted more widely is the heavy cost, both of installation and of maintenance. When electric lighting is rendered cheaper and is brought more to the level of existing lighting arrangements, one may expect another complete change in lighthouse practice. In this direction, as explained in another chapter, the Germans have carried out practical experiments in their characteristic manner, and have brought the cost of maintaining a most powerful electric light to the minimum.
One very great advantage of the electric light is the ease with which the power of the beam may be increased during thick weather, so as to secure penetration to the greatest distance, and decreased to suit easier conditions in clear weather.
This point raises the question, “From how far can a light be seen out at sea?” This factor is influenced by climatic conditions, and also by the curvature of the earth. The higher the light, or the spectator, or both, is elevated above the water, the greater the distance from which the light can be seen. The table on p. [52], prepared by Mr. Alan Stevenson, the eminent Scottish lighthouse engineer, gives the distances at which objects can be seen at sea, according to the respective elevations of the object and the eye of the observer.
For instance, the passenger on a liner the boat-deck of which is 40 feet above the water, approaching the English Channel, will sight the Bishop Rock light from a distance of about 22 miles, because the focal plane—that is, the bull’s-eye of the lens—is 163 feet above the water, which, according to the following table, equals about 14½ miles, to which must be added the height of the boat’s deck, 40 feet representing 7·25 miles. Similarly, the ray of the Belle Ile light will come into view when the vessel is 32½ miles distant—height of focal plane of light, 470 feet = 25 miles, + eye of observer on board the liner, 45 feet = 7·69 miles; while the Navesink light, being 246 feet above the water, may be picked up by the captain of a liner from a distance of 28 miles. The range of many lights, however, owing to the curvature of the earth, is greatly in excess of their geographical range, and with the most powerful lights the glare of the luminous beams sweeping the clouds overhead may be seen for a full hour or more before the ray itself comes into view.
TABLE OF DISTANCES AT WHICH OBJECTS CAN BE SEEN AT SEA, ACCORDING TO THEIR RESPECTIVE ELEVATIONS AND THE ELEVATION OF THE EYE OF THE OBSERVER.
| Heights in Feet. | Distances in Statute or English Miles. | Distances in Geographical or Nautical Miles. |
|---|---|---|
| 5 | 2·958 | 2·565 |
| 10 | 4·184 | 3·628 |
| 15 | 5·123 | 4·443 |
| 20 | 5·916 | 5·130 |
| 25 | 6·614 | 5·736 |
| 30 | 7·245 | 6·283 |
| 35 | 7·826 | 6·787 |
| 40 | 8·366 | 7·255 |
| 45 | 8·874 | 7·696 |
| 50 | 9·354 | 8·112 |
| 55 | 9·811 | 8·509 |
| 60 | 10·246 | 8·886 |
| 65 | 10·665 | 9·249 |
| 70 | 11·067 | 9·598 |
| 75 | 11·456 | 9·935 |
| 80 | 11·832 | 10·260 |
| 85 | 12·196 | 10·570 |
| 90 | 12·549 | 10·880 |
| 95 | 12·893 | 11·180 |
| 100 | 13·228 | 11·470 |
| 110 | 13·874 | 12·030 |
| 120 | 14·490 | 12·560 |
| 130 | 15·083 | 13·080 |
| 140 | 15·652 | 13·570 |
| 150 | 16·201 | 14·220 |
| 200 | 18·708 | 16·220 |
| 250 | 20·916 | 18·14 |
| 300 | 22·912 | 19·87 |
| 350 | 24·748 | 21·46 |
| 400 | 26·457 | 22·94 |
| 450 | 28·062 | 24·30 |
| 500 | 29·580 | 25·65 |
| 550 | 31·024 | 26·90 |
| 600 | 32·403 | 28·10 |
| 650 | 33·726 | 29·25 |
| 700 | 35·000 | 30·28 |
| 800 | 37·416 | 32·45 |
| 900 | 39·836 | 34·54 |
| 1,000 | 41·833 | 36·28 |
By permission of the “Syren and Shipping.”
LOOKING UP THE LANTERN OF THE NEEDLES LIGHTHOUSE.