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Bat our readers will probably ask : What does the present system consist of? To this we answer that it appears to be a combination of many systems by which an effective variety of distinctions is obtained. But one great merit pervades the present heterogeneous collection of distinguishing characteristics. Every light says at once what it is; there is no code of symbols or letters to be interpreted—the Wolf Rock Light is revolving, showing a red and a white flash alternately every half minute; the light on the Mull of Galloway in Scotland is intermittent or occulting, showing a fixed light for half-a-minate then eclipsed for fifteen seconds; the Calf Rock Light on S.W. Coast of Ireland is flashing, showing a flash every quarter of a minute ; then there are fixed lights, red and white; revolving, white or red or green ; double lights shown from one tower; or from two lighthouses a short distance from each other; electric lights which are very distinctive ; sundry combinations of fixed and flashing; and a system of recent introduction known as group flashing, whereby two or more flashes in quick succession are repeated at certain periods. The above characteristics and their combinations, varied by time periods ranging from ten seconds to two minutes, afford a much larger number of distinctions than could be obtained by the dot-and-dash system alone, with the advantage that the seaman knows at once what he is looking at.
We do not pretend to affirm that the lighting arrangements on oar coasts are perfect; there are doubtless many improvements which may be made, particularly in the general direction indicated by Sir William Thomson in regard to shortening the period of darkness in revolving lights. But, in the absence of complaints from seafaring men, it may fairly be assumed that the arrangements are generally effective and do not call for any revolutionary change. The steady progress made during the past twenty years in all that relates to lighthouse illumination, the increased efficiency and number of lights, the development of sound-signalling for foggy weather, the improvements in and increase in the number of buoys, afford sufficient reasons for the nautical community to be satisfied with what is being done for them, and explain the apathy with which they have regarded the efforts of even a distinguished man of science like Sir William Thomson to bring about a scheme which does not
appear to them to be necessary, and does not commend itself to their practical judgment.
But in one respect we think there is undoubtedly room for im provement. The nomenclature of the different kinds of light: requires to be made more intelligible and more definite than it now is. In the Admiralty List of Lights the following summary is giver of the various characteristics employed :
“ (1.) Fixed, or Steady.
“(2.) Flashing.–Showing flashes at short intervals, or group of flashes at regular intervals.
“(3.) Revolving.–Light gradually increasing to fall effect, the decreasing to eclipse. [At short distances and in clear weather faint continuous light may be observed.]
“ (4.) Fixed and Flashing.–Fixed light with addition of whit or coloured flashes preceded and followed by a short eclipse.
• (5.) Intermittent, or Occulting.--A light suddenly and totall eclipsed. When light between eclipses visible less than thirt: seconds term occulting applied. When light visible longer thai half-minute, term intermittent applied.
“ (6.) Alternating.-Red and white light alternately at equa intervals, without any intervening eclipse."
The terms employed above no doubt express very clearly to th authorities the different characteristics of lights; but the majorit of nautical men would have some difficulty in accurately definin each term. Some of the expressions are entirely unknown t many mariners, and, as regards the terms "revolving," “flashing, “ intermittent," “occulting," and “alternating," there are, we ar sure, few sailors who could tell the difference between them. far as we can judge, it seems that all lights which come and g are among mariners known indiscriminately as “revolving," an are distinguished merely by the length of the duration of the inter val of darkness.
If an authoritative announcement were put forth, giving thre or four plain names, which would generically comprehend all th different distinctions employed, and explaining each of the varietie of these divisions in a perfectly simple manner, we think it woul be of the greatest benefit to navigation. It would unquestionabl
remove some of the obscurity and uncertainty attendant upon the nse of the present terms, and render still more unnecessary any such complicated code of signals as that advocated by Sir William Thomson.
As regards the third of Sir William Thomson's propositions, viz., the abolition of colour as a distinction of lighthouse lights, we do not think the use of colour should be entirely discarded at present. Some recently established revolving green lights have proved very effective for marking narrow channels, and Sir William Thomson admits that the intensities of the red and white flashes of the Wolf revolving light are very perfectly equalised, “which is quite a triumph of optical science and skill." Now in these cases colour appears to be quite successfully utilised and in such a manner as not to be liable to be confounded with the port and starboard lights of vessels under way. As regards the use of colour for fixed lights, the question deserves consideration as to how far the coloured media adopted robs the light of its penetrating power, and on the other hand to what extent the coloured light resists the obscuring influence of a misty or hazy atmosphere. It is believed by some, as stated in an article * in this journal in May last on the subject of the Electric light, that the passage of red rays through aqueous vapour is not so much obstructed as is the transmission of yellow, green, violet and blue rays, and Sir William Thomson alludes to a reported case of the white flashes of the Wolf light having been rendered ineffectual in a haze, while the red rays were visible.
This would seem to point to the greater usefulness of red light in thick weather and affords an argument for its retention for lighthouse purposes, as long as it can be effectively differentiated from the coloured lights of vessels under way.
It might be desirable that some definite experiments should be made on the subject of the value of coloured lights and coloured media, so as to enable an accurate judgment to be formed as to the practical efficiency of such means of distinction in varying conditions of the atmosphere. But as far as we can judge at the
* Nautical Magazine, May, 1879, p. 376.
present time, it seems to us that what is really wanted is something by means of which a distinctive element can be imparted to coloured fixed lights when employed for lighthouses, so that mariners may not confound them with ships' lights.
It is somewhat singular that for ships' lights, which are of comparatively recent introduction, claims are asserted for distinctive peculiarities which had been in use for lighthouses long before. We of course do not deny the great value of coloured side-lights, but we do think it a little hard that old established lighthouses and their distinctions should be regarded as matters secondary to modern ships' lights, and that the former should have to give way as regards distinctive features to the latter.
THE NEW NAVIGATION AND SUMNER'S METHOD.
PROBLEM in Nautical Astronomy of no inconsider.
able value is gradually taking its proper place among the various methods used by navigators to determine
the position of a ship at sea. It cannot, however, with any propriety be said to have any claim to novelty ; so far indeed from being a new problem, we are warranted in stating that the principle which underlies the method has been recognised for apwards of a century; but in its old form few were acquainted with it, and amongst those few must be reckoned the officers of the old East India Company's ships, who, we have good reason for stating, had a due appreciation of the extent to which its results could be trusted,-above all its general excellence in determining the latitude, though the imperfection of timekeepers and the uncertainty of lunars rendered the longitude but an indifferent approximation to the ship's true position as referred to the meridian of Greenwich.
In process of time, as soon as the chronometer had come to be so far perfected as a timekeeper that, by careful verification (as opportunity offered) of the error and rate, its degree of dependence, in individual cases, was known, the problem began to assume a new form-projection combined with calculation; and it is on record
that skilful navigators were enabled, by utilising only a part of the method, in connection with a sounding, or bearing of a distant inland object, to identify the ship's place on the chart, and thus shape the course anew, or keep on, as required.
It is, nevertheless, undoubtedly true that this problem, which, for more than half a century had come within the ken of comparatively few navigators, and which was more frequently a matter of computation than of projection, took a new point of departare when, in 1843, Captain THOMAS H. SUMNER, of Boston, U.S., published his work entitled, “ A New and Accurate Method of finding a Ship's Position at Sea, by projection on Mercator's chart. When the Latitude, Longitude, and apparent Time at ship are uncertain, one Altitude of the Sun, with the True Greenwich Time, determines, first, the True Bearing of the Land ; secondly, the Errors of Longitude by Chronometer, consequent to any Error in the Latitude; thirdly, the Sun's True Azimuth. When two Altitudes are observed, and the Elapsed Time noted, the True Latitude is projected; and if the Times be noted by Chronometer, the True Longitude is also projected at the same operation.” We give the title in its entirety, because it fully expresses what the problem is capable of determining, provided always that the data—other than the latitude and longitude—are correct, but those approximately known. It has come, in these latter years, to be generally recognized as “Sumner's Method," and the lines of position are sometimes inappropriately called "Sumner lines ; ” we say inappropriately, because the geometrical name is unquestionably the better. Extracts from Sumner's work were first published in France, by M. Joseph Barthet, in the Annales Maritimes, of 1847. Its progress, as a problem in daily use, has been more rapid with us than with our continental neighbours ; within the last few years, however, it has taken an extraordinary hold on them, and they have developed it in a new form; from the position by dead reckoning, they calculate an approximate point, and then by projection, and the use of several special tables, rectify the position of this point, as that of the ship.
We will briefly explain the principle of the problem.