Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/650

 628 called an occulting light. (6) The intermittent light of unequal 2&amp;gt;criods, first proposed by Mr II. L. Stevenson in 1871, showing from the same apparatus different durations of the dark periods with equal periods of light, such as fixed for 2&quot; eclipsed 5, fixed 2&quot; eclipsed for 2&quot;, and then, as at first, fixed for 2&quot;. (7) Group flashing light. This distinction consists in giving a series of groups of two or more flashes separated by short eclipses, the groups being separated from one another by a larger eclipse. Mr &quot;Wigham of Dublin first proposed this distinction, and applied it to a dioptric revolving apparatus illuminated by gas, the effect being produced by raising and lowering the gas as Mr Wilson of Troon had done in 1827. Dr Hopkinson produced the same effect optically and for any illuminant by altering the arrangement of the ordinary revolving apparatus. (8) Fixed lights illuminating the whole horizon, but showing revolving or intermittent characteristics over small arcs. These can be produced by masks arranged like Venetian house blinds placed opposite the arc where the character istic is to be different from the main light. This arrangement and No. 9 were lately proposed by Mr T. Stevenson. (9) Intermittent flashing light. This is produced by condensing prisms revolving round a fixed light apparatus. The flashing light continues giving its rapidly recurring flashes for a certain period, which is succeeded by a dark period. The light which is not required during the dark periods is condensed by the prisms so as to strengthen the light periods. Uniform System of Characteristics. Various modes of distinguish ing lights have been at different times proposed, such as Babbage s, which represented the stations by numbers, and Sir W. Thomson s by the Morse alphabet, or dots and dashes ; but the following results seem to show that the present system is satisfactory, while it is free from many objections to which the other modes are liable. The Board of Trade records of shipwrecks show that during the space of twenty years (1857 to 1876)2716 vessels were wrecked by stranding on the coasts and islands of Scotland and the Isle of Man, and that of all these there were only twenty-six, and the great majority of them were small vessels, in which it was even alleged that the cause was a failure in identifying the distinctive characteristic of some one of the Northern lighthouses, and in one-third of these cases the sailors were foreigners, and made such glaring mistakes as taking a fixed for a revolving light. It is surely more probable that in many of these cases they did not confound the characteristics at all, but really did not know what they were. But, waiving all these con siderations, there were after all only about 1 per cent, of all the wrecks which were alleged to have been caused by mistaking one light for another. Power of Coloured Lights. Experiments, and particularly those of M. Eeynaud, have shown that red of the same initial power as white penetrates a fog to a greater distance than the white, owing to the red rays not being so much dispersed by refraction. Experi ments made at Edinburgh in 1868 show that a light should, before passing through red glass, have four and a half times the initial power of a white light. The same results have since been obtained by M. Allard of Paris. An objection to all coloured lights is the fact that some persons are unable to detect any difference between certain colours. Accord ing to the experiments of Professor Holmgren on from sixty to seventy thousand persons in Europe and America, about 4 2 per cent, are congenitally colour-blind, and the commonest form of this defect is inability to distinguish between red and green, which arc the only tints employed for lighthouse distinctions. This objection, however, must gradually lose its weight, as the Board of Trade has now adapted the &quot;colours test&quot; as part of their examinations for certificates in navigation. Distribu- Distribution of Lights on a Coast. Mr Alan Stevenson gives the tion of following as a few of the chief considerations which should guide light*. us in the selection of sites and characteristic appearance of the lighthouses to be placed on a line of coast. &quot; 1. The most prominent points of a line of coast, or those first made on over-sea voyages, should be first lighted ; and the most powerful lights should be adapted to them, so that they may be dis covered by the mariner as long as possible before his reaching land. &quot;2. So far as is consistent with a due attention to distinction, revolving lights of some description, which are necessarily more powerful than fixed lights, should be employed at the outposts on a line of coast. &quot; 3. Lights of precisely identical character and appearance should not, if possible, occur within a less distance than 100 miles of each other on the same line of coast which is made by over-sea vessels.
 * 4. In all cases the distinction of colour should never be adopted

except from absolute necessity. &quot;5. Fixed lights and others of less power may be more readily adopted in narrow seas, because the range of the lights in such situations is generally less than that of open sea-lights. &quot;6. In narrow seas also the distance between lights of the same appearance may often be safely reduced within much lower limits than is desirable for the greater sea-lights. Thus there are many instances in which the distance separating lights of the same character need not exceed 50 miles ; and peculiar cases occur in which even a much less separation between similar lights may be sufficient. &quot; 7. Lights intended to guard vessels from reefs, shoals, or other dangers should, in every case where it is practicable, be placed sea ward of the danger itself, as it is desirable that seamen be enabled to make the lights with confidence. &quot;8. Views of economy in the first cost of a lighthouse should never be permitted to interfere with placing it in the best possible position ; and, when funds are deficient, it will generally be found that the wise course is to delay the work until a sum shall have been obtained sufficient for the erection of the lighthouse on the best site. &quot;9. The elevation of the lantern above the sea should not, if possible, for sea-lights, exceed 200 feet ; and about 150 feet is suffi cient, under almost any circumstances, to give the range which is required. Lights placed on high headlands are subject to be fre quently wrapped in fog, and are often thereby rendered useless, at times when lights on a lower level might be perfectly efficient. But this rule must not, and indeed cannot, be strictly followed, especially on the British coast, where there are so many projecting cliffs, which, while they subject the lights placed on them to occa sional obscuration by fog, would also entirely and permanently hide from view lights placed on the lower land adjoining them. &quot; 10. The best position for a sea-light ought rarely to be neglected for the sake of the more immediate benefit of some neighbouring port, however important or influential ; and the interests of naviga tion, as well as the true welfare of the port itself, will generally be much better served by placing the sea-light where it ought to be, and adding, on a smaller scale, such subsidiary lights as the channel leading to the entrance of the port may require. &quot; 11. It may be held as a general maxim that the fewer lights that can be employed in the illumination of a coast the better, not only on the score of economy, but also of real efficiency. Every light needlessly erected may, in certain circumstances, become a source of confusion to the mariner ; and in the event of another light being required in the neighbourhood, it becomes a deduction from the means of distinguishing it from the lights which existed previous to its establishment. &quot;12. Distinctions of lights, founded upon the minute estimation of intervals of time between flashes, and especially on the measure ment of the duration of light and dark periods, are less satisfactory to the great majority of coasting seamen, and are more liable to derangement by atmospheric changes, than those distinctions which are founded on what may more properly be called the characteristic appearance of the lights, in which the times for the recurrence of certain appearances differ so widely from each other as not to require for their detection any very minute observation in a stonny night. Thus, for example, flashing lights of five seconds interval, and revolving lights of half a minute, one minute, and two minutes, are much more characteristic than those which are distinguished from each other by intervals varying according to a slower series of 5&quot;, 10&quot;, 20&quot;, 40&quot;, &c. &quot; 13. Harbour and local lights, which havs a circumscribed range, should generally be fixed instead of revolving ; and may often, for the same reason, be safely distinguished by coloured media. In many cases also, where they are to serve as guides into a narrow channel, the leading lights which are used should, at the same time, be so arranged as to serve for a distinction from any neighbouring lights.&quot; Table of Distances at winch Objects can be seen at Sea, according Distanc&amp;gt; to their rcs2&amp;gt;cctive derations, and the elevation of the eye of the at wind observer. Distances in Distances in Distances in Heights in Feet. Geographical or Nautical Heights in Feet. Geographical or Nautical Heights in Feet. Geographical or Nautical Miles. Miles. Miles. ]i) 2-565 3-628 70 9-598 9-935 250 300 18-14 1.9-87 ]r, 4-443 81) 10-20 350 21 4G 2f 5-130 65 10 57 400 22-9 . &quot; 5-731; 90 1088 450 24-33, ! 6 283 95 11-18 500 25-G5 0-787 100 11-47 550 2G-90 41 7-255 110 12-03 600 28-10 45 7 G9G 120 12-5(5 650 29-25 50 8-112 130 13-08 700 30-28 65 8-50!) 140 13-57 800 32-45 00 S-8Sfi 150 14-22 900 34-54 C5 9-249 200 10-22 1000 3G-28 objects are visi ble. Example. A tower 200 feet high will be visible 20 C6 nautical miles to an observer whose eye is elevated 15 feet above the water ; thus, from the table : 15 feet elevation, distance visible 4 f 44 nautical miles. 200 16-22 20-66
 * .&quot;