Lightships and Lighthouses. Frederick Arthur Ambrose Talbot
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.
So far as the candle-power of any light is concerned, the method of determining this factor, varying according to the calculating methods adopted, is somewhat misleading. So far as Great Britain is concerned, the practice of setting out the candle-power of any light in the official list has been abandoned, the authorities merely stating that such and such a light is of great power. The United States and Canada, on the other hand, indicate the approximate candle-power.
By courtesy of Messrs. Chance Bros. & Co., Ltd.
FIXED APPARATUS OF THE FOURTH ORDER FOR SARAWAK.
The focal distance is 250 millimetres, and the diameter of lantern inside glazing 6 feet 7¾ inches.
By combining and arranging the integral parts of the optical apparatus, the lighthouse engineer is able to accomplish many astonishing results. Thus, while the various types generally follow accepted broad lines, coinciding with the order which they represent, here and there some very striking divergences are made. The Bell Rock light is perhaps the most interesting example in this direction. It was designed by Messrs. D. and T. Stevenson, and built by Messrs. Chance Brothers and Co. The light is alternating, the colours being white and red. Externally the optical apparatus appears to be bizarre, yet it is one of the most perfect which has ever been installed. In its design and construction almost all the known lighthouse optical elements are incorporated, including the equiangular refractor, the reflecting