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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 improvement. The nomenclature of the different kinds of lights requires to be made more intelligible and more definite than it now is. In the Admiralty List of Lights the following summary is given of the various characteristics employed :

"(1.) Fixed, or Steady.

"(2.) Flashing.-Showing flashes at short intervals, or groups of flashes at regular intervals.

"(3.) Revolving.-Light gradually increasing to full effect, then decreasing to eclipse. [At short distances and in clear weather a faint continuous light may be observed.]

"(4.) Fixed and Flashing.-Fixed light with addition of white or coloured flashes preceded and followed by a short eclipse.

"(5.) Intermittent, or Occulting.-A light suddenly and totally eclipsed. When light between eclipses visible less than thirty seconds term occulting applied. When light visible longer than half-minute, term intermittent applied.

"(6.) Alternating.-Red and white light alternately at equal intervals, without any intervening eclipse."

The terms employed above no doubt express very clearly to the authorities the different characteristics of lights; but the majority of nautical men would have some difficulty in accurately defining each term. Some of the expressions are entirely unknown to many mariners, and, as regards the terms "revolving," "flashing," "intermittent," "occulting," and "alternating," there are, we are sure, few sailors who could tell the difference between them. As far as we can judge, it seems that all lights which come and go are among mariners known indiscriminately as "revolving," and are distinguished merely by the length of the duration of the interval of darkness.

If an authoritative announcement were put forth, giving three or four plain names, which would generically comprehend all the different distinctions employed, and explaining each of the varieties of these divisions in a perfectly simple manner, we think it would be of the greatest_benefit to navigation. It would unquestionably

remove some of the obscurity and uncertainty attendant upon the use 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.

A

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 upwards 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 departure 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.

Latitude alone, or longitude alone, does not indicate the position.

of a place on the globe. The first merely shows that the place is somewhere on a small circle (a parallel) at a definite distance from the equator; the second merely shows that the place is somewhere on a great circle (a meridian) that makes a definite angle with another great circle which passes through a fixed conventional place of reference. To know the position of a place the point of the intersection of these two circles-of the meridian with the parallel-must be determined; but this cannot always be done at sea, at any given or required instant, by any of the ordinary rules of nautical astronomy; though it may be done by a combination of rules; or partly by computation and partly by projection; or where a good point cannot be ascertained as that on which the ship is, a line may be found on or near which she is known to be, and this at the time may be priceless. The position of the ship is thus determined by a method of utilising parts of circles which, in their completeness, would be oblique to the parallels and meridians.

When the declination of a celestial object coincides in amount and name with the latitude of a place on the terrestrial sphere, it must, at some time during the earth's rotation on its axis, appear on the zenith of that place; it will do so when the object's hourangle for the place is 0 h., that is, when it is on the meridian. When this occurs, the Greenwich time by chronometer being known, let it be taken as granted that the object is above the horizon of another place; that its altitude is observed, and its zenith-distance consequently known. In Fig. 1 the object is vertical to the point S of the globe; with S as the pole, and the observed zenith-distance, Sa, as a polar-distance, describe the small circle a a' a" a": this is a circle of position, on some point of which the observation

[graphic]

has been made, for from

FIG. 1.

every point within or without this small circle, a less or greater

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