On Lighthouse Characteristics

By Sir William Thomson, D.C.L., F.R.S.

Paper read at the Naval and Marine Exhibition, Glasgow, February 11th, 1881. [Scanned source of exhibit catalogue.]
Reprinted in Popular Lectures and Addresses, Vol. iii, pp. 389-421. [Scanned source.]

For a lighthouse to fulfil the reason of its existence, it must not only be seen, it must be recognised when seen. If seen, and not known, a lighthouse is of no use; if not seen, it certainly could not be of use. There has been much of discussion as to what is the primary and most important quality of a lighthouse. Penetrative power—to allow the light to be seen in thick weather at as great a distance as possible—is, of course, the first object to be striven for. The next question is—How to make use of a lighthouse when seen? If a sailor descrying a lighthouse from a great distance is in doubt whether the light is on a fishing-boat a mile off, or on the masthead of a steamer three miles off, or on a lighthouse six miles off, it is obvious that the lighthouse in merely letting its light be seen, had achieved but a small part of the task to be achieved. I do not want to take the ungracious part—of criticising or saying anything has been done less well than it should be done; nor do I want to be behind in expressing my cordial and most sincere admiration of the great work which has been done for the world by the lighthouse boards of this country—by the Trinity Board, the Board of Northern Lights, the Commissioners of Irish Lights, and,—not least in intensity, if not so great as the others in quantity, of good done,—by the Clyde Navigation Trustees. But I must say that there has not been among lighthouse authorities hitherto quite enough of determination to make the very most of the distinctive character and the possibilities of giving a distinctive character, to their lights that science and common-sense placed before them. There is too much, perhaps, of the idea of saving oil, or of making a certain quantity of oil go a great way, and not quite enough of the idea that the object of the lighthouse after all is to be known, and that to be seen without being known is not enough. The question to be considered is how to know one light from another—how to know a light descried just above the horizon, and dipping now below the horizon, lost sight of for a quarter of a minute, again seen, lost for a little time, and again seen continuously—to recognise it with certainty, and without loss of time, in such circumstances. The Holywood Bank Light in Belfast Lough, the leading light for vessels entering the Lough, is so recognised, being a short-short-long eclipsing light. The Copeland Light off the south entrance of Belfast Lough is not recognisable by any distinguishing characteristic, being merely a fixed light. It has, however, I am informed, been determined by the Commissioners of Irish Lights to alter it, and give it a distinctive character. I take those two cases because, when a celebrated lighthouse engineer was with me on one occasion in my yacht, approaching Belfast in the small hours of a summer morning, we had just that experience of them both. I said to him, "Look at that light and tell me what it is; is it a masthead light, or what is it?" He could not tell. It was the Copeland Light, as we learned soon afterwards from our position. My friend fully admitted after that, what he never admitted before—namely, that it was possible to confound a lighthouse light with a light on a steamer's masthead; and soon after, the Holywood Bank, barely visible ten miles off, was recognised by its short-short-long within a quarter of a minute of its being first seen, and gave a triumphant proof of the practical value of its distinctive character.

With reference to the description of lights and their distinctions in lighthouses, there is, in the first place, to be considered the character of the light, and the appliances for economizing of it. The old coal fire on the cliff, which was the first lighthouse, was a relic of the past, which would never now be set up for the purpose of marking a point on the coast; yet, practically, where there are blazing furnaces at ironworks, as on the Ayrshire coast in the neighbourhood of Ardrossan, these same fires do constitute very important, though undesigned marks by which a mariner discovers his position. The substitution of economical lamps, in which a great deal of light was given with a moderate consumption of fuel, took the place of the coal fires on the cliffs. Then reflectors were introduced. A great invention was made early this century, which led to the now prevailing dioptric system. It is perfectly clear that the great brilliance and success in economizing the fuel of the flames in the lighthouses of the present day is directly due to the invention of the dioptric system; and has been largely promoted by the great use made of it, and the great improvements effected on it, by Messrs. Stevenson, the engineers of Northern Lights. Then came the question of how to economize light when not wanted to show all round, as, for instance, in the case of the Lamlash Light, which shows a brilliant light seaward, and a moderately-bright light over the Bay of Lamlash. The occulting light shown by the Messrs. Stevenson in our present Exhibition is a light fulfilling one of the conditions of characteristic quality, with very perfect economy of light. The very principle by which light was economized has given one of the first lighthouse characteristics in the ordinary revolving light. Not content with condensing the light to the horizon so as to shed itself out in all horizontal directions, engineers condensed it into certain fixed directions for special reasons. Sometimes they condensed the light into a ray, for the reason of sending it in the direction of a particular channel: sometimes for the sake of giving greater intensity than they could practically attain otherwise, and then they made the ray revolve so as to shed its brightness all round the horizon in the period of its revolution. A policeman's bull's-eye lantern is an instance in point. There is a greater intensity of light in a ray from an ordinary bull's-eye lantern than a light of anything like the same power could give without that optical appliance, or something equivalent to it, or more perfect than it.

Besides its light, a modern lighthouse generally contains also, for use in such thick or foggy weather that the light cannot be seen, a sound-making appliance, the object of which is not only to be heard, but when heard to be immediately recognised to be itself and nothing else. Mr. Price Edwards, in his communication to the Society of Arts, of 15th December last, on "Signalling by means of Sound," gave an interesting and clear description of the chief practical methods hitherto in use for this exceedingly important addition to the efficiency of lighthouses; and I shall have occasion to return to the subject of characteristic sounds in relation to the several methods which have been adopted to give characteristic qualities to the light itself of a lighthouse.

Setting aside colour—now generally admitted to be indefensible, as a distinction for lighthouse lights, except in the proper use of it, which is to distinguish different directions of the light by coloured sectors to mark rocks or other dangers, or the safe limits of navigable channels—we find all the characteristic qualities of lighthouses to come under one or other of the following three descriptions:—

I. Flashing lights.
II. Fixed lights.
III. Occulting or eclipsing lights.

The well-known name "Revolving lights" is habitually limited to flashing lights; but it is liable to ambiguity, because the same revolving mechanism is also applied in many cases to produce the eclipses of "Occulting or eclipsing lights." The official description of the revolving light in the "Admiralty List of Lights," is as follows:—

"Rev.—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.]"

This, in fact, includes the description of the flashing light:—

"Fl.—Flashing—showing flashes at short intervals, or groups of flashes at regular intervals."

A combination of the fixed and flashing qualities, though comparatively rare, constitutes an important characteristic light, described in the Admiralty list as follows:—

"F. and Fl.—Fixed light with addition of white or coloured flashes, preceded and followed by a short eclipse."

Thus we have really very little of complexity in the fundamental classification into the three descriptions of Flashing Fixed, and Occulting.

In the flashing light, the light is visible for only a short time—a fraction of a second, or from that to five or six seconds—and then disappears; and, for a much longer time than the duration of the flash, it remains invisible, until it again flashes out as before. In the fixed light there is no distinguishing characteristic whatever, but merely a light seen shining continuously and uniformly, The occulting light is visible during the greater part of its time like a fixed light, shining continuously and uniformly. Characteristic distinction is given by a short eclipse, or by a very rapid group of two or three short eclipses, or of short and longer eclipses recurring at regular periods, "flashes of darkness," as they have been called, cutting out, as it were from the light its mark, by which it may be distinguished and recognised to be itself and nothing else, in the very short time (from half-second at the least, to seven seconds at the most) occupied by the group of eclipses.


Six years ago, in every flashing light there was just one flash in the period, and thus the length of the period was the sole distinction between one flashing light and another. Thus, in the "Admiralty List of Lights for the British Islands" for 1875, we find about 100 flashing lights of different periods, from the four-seconds' period of Ardrossan Breakwater Light to the two-minutes' period of the upper light of Lundy Island, of the South Stack, Holyhead, and of one of the lights on Slyne Head, off the west coast of Ireland; and the distinction of each one of these 100 lights was solely its period as a simple flashing light, except in cases in which the objectionable distinction by colour was put in requisition. When it was determined to choose periods the same, or nearly the same, for neighbouring lights, it was found necessary to add distinction by colour, objectionable as this is if not enforced by necessity. Thus, for example, the Gull Stream lightship, in the fairway between the Goodwin Sands and the Kentish Coast, is a revolving light of twenty seconds' period, while the East Goodwin lightship, about six miles from it, is a revolving light of fifteen seconds' period. Without greater accuracy than is generally to be found in the time-keeping of flashing lights, even on shore, the distinction between fifteen and twenty seconds could scarcely be relied upon as given by the mechanism; and even if given trustworthily by the mechanism, the distinction could only be discovered by the sailor with certainty by the aid of a chronometer, the use of which is out of the question as a practical means for recognising a light when seen. To give sufficient distinction between these two lights, therefore, it was found necessary to use colour; the East Goodwin was made green, the Gull Stream white. Again, the St. Agnes Light, Scilly, and the light on the Wolf Rock two far outlying lights, about twenty miles asunder, are each of them of half a minute period from flash to flash, and the sole distinction between them is that the flashes of the Wolf Light are alternately white and red, while those of the St. Agnes' Light are all white.

The insufficiency of the distinction of flashing lights, merely by length of period, had come to be felt so strongly that a very important fresh distinction was introduced in 1875, in the lightship then first placed on the Royal Sovereign shoal; the Group Flashing Light of Mr. Hopkinson, in which, instead of just one flash in the period, there are, in the case of the Royal Sovereign Light, three flashes in the period, or, as may be in other cases, two flashes, or four flashes, the interval between the successive flashes of the group being much shorter than the interval from group to group in the whole period. In two cases in the English Channel, the North Sand Head and the Casquets, the new triple flashing light was introduced to replace, by a group of three flashes in rapid succession, three separate lights which had been the characteristic arrangement previously; three fixed lights in the case of the North Sand Head, and three simple flashing lights in the case of the Casquets.

Mr. Preece has imprudently pointed out that Mr. Hopkinson's triple flashing light is the letter S of the Morse-Colomb flashing alphabet. Sailors, we may hope, are happily ignorant of this truth, otherwise the proverbial captain of the collier would be calling out to his chief officer—"Bill, was that a S, or a I, or a H, or a E?" Bill, if he was well up in dramatic literature, would reply, "Captain, them is things as no fellow can understand." I must say, however, that I agree with Mr. Preece, and think that, while many may find memory aided, none can be embarrassed, by an official statement of the Morse letter corresponding to any group of flashes or edipses that may be chosen as the characteristic for any particular light. This, however, is a matter of comparatively small moment at present. The great thing is to find how lights may be most surely and inexpensively rendered distinctive, so that no sailor, educated or uneducated, highly intelligent or only intelligent enough to sail a collier through gales, and snowstorms, and fogs all winter between Newcastle and Plymouth, may know each light as soon as he sees it, without doubt or hesitation.

This object is fully attained by the triple flashing light, if quick enough. The triple flashing light of the Casquets, and of Bull Point (Bristol Channel) which are the quickest of the kind hitherto made, complete their three flashes in twelve seconds, after which there is an interval of eighteen seconds of darkness. These are, no doubt, very admirable and thoroughly distinctive lights; but it would be very much better if they were made three times as fast, which, with the existing machinery, could, I believe, be easily done. If this were done they would show their flashes each in two-thirds of a second, and with only a second of time between. Thus, the three flashes completed in four seconds would be instantly recognised as a group of three, without the necessity of any counting either of flashes or of numbers of seconds of time in the intervals between the flashes; and, instead of having to wait in darkness for eighteen seconds, the sailor would only have to wait six seconds for a repetition of the triple flash.

The Royal Sovereign, the Seven Stones, the Newarp (near Yarmouth, on the east coast), and the Saltees (off the south coast of Ireland), all lightships supplied within the last few years with the Triple Flashing Light, are each of them of one minute period, of which there is thirty-six seconds of darkness, and twenty-four seconds of flashing. These lights are all too slow to do full advantage to the triple flash system. When one of them is first seen, it is very apt to be confounded with an ordinary "revolving light"—that is, a single-flash flashing light. Even somewhat careful watching—at all events if the weather or the distance from the light be such as to leave any room for doubt—does not always immediately resolve the doubt. A sixfold quickening of each of these lights would greatly enhance its distinctive quality, and would make it really fulfil the condition pointed out by the Elder Brethren of the Trinity House, as the object to be aimed at in every modern lighthouse, "That he that runs may read."

The satisfactory distinctions of group-flashing lights are exhausted in the groups of two or three or four flashes; because, to count five or six, or more, would be embarrassing and liable to mistake at sea. It has been proposed to obtain further distinction by using groups of longer and shorter flashes, as in Captain Colomb's Flashing Telegraph, now in general use, and very thoroughly appreciated both in the Navy and in the Army; but there are optical difficulties in the way of making, with satisfactory economy, groups of long and short flashes, separated by short intervals of darkness in the group, and comparatively long intervals of darkness between successive groups; and considering how very much more useful and satisfactory at sea is a lighthouse showing long light with short intervals of darkness than even the quickest of flashing lights, it does not seem desirable to push the distinctions of flashing lights further than the double, triple, and quadruple groups. The periods for these lights which seem best adapted for practical purposes, all things considered, but most particularly their value to the sailor, are as follows:—

Number of flashes
in period.
Duration of
each flash.
Duration of
One ¼ sec. ¼ sec. 2 sec.
One 1 " 1 " 8 "
Two ½ " 2 " 6 "
Two 1 " 4 " 12 "
Three ½ " 3½ " 9 "
Four ¼ " 2½ " 8 "

It may be objected to the suggestions of the preceding table, that the quarter-second flashes are too short to be perceived with the same certainty as flashes of five or six seconds' duration. Experiment alone can answer decisively the question whether, with equal maximum brilliancy in each flash, a flash of quarter-second duration recurring every two seconds, or one of half-second recurring every four seconds, or one of one second recurring every eight seconds, is the most easily to be seen at a great distance or in hazy weather. From physiological experiments already made, it has been concluded that one-tenth of a second is a long enough time to fully excite the sensibility and perceptive power of the eye, and it seems probable that rapidity of recurrence of the contrasts between light and darkness will give a positive advantage to the quicker flash in respect to perceptibility, even when the observer knows in what direction to look for the light; and when he does not know exactly in what direction to look, which is the practical case of a sailor at sea trying to pick up a light, shortness of the time of invisibility is of supreme importance. All things considered, it seems most probable that the quarter-second flash recurring every two seconds will be very much more easily and surely picked up practically at sea than a flash of one second recurring every eight seconds.

Before passing from this subject of flashing lights, I may be allowed to say that I first received my impression of the vital importance of quickness in a light from a very practical man—the man who, in 1866, showed us within a quarter of a mile, in mid-ocean, where to find the cable which had been laid and lost in 1865—Captain Moriarty, R.N. I well remember when on one occasion, either in 1858 or 1865, I do not know which, in making the Irish coast in dirty weather, he said—

"Those lighthouses should flash out their characters like your electric signals; every lighthouse should flash, and flash, and flash, many times in a minute, showing you which lighthouse it is every time. That long minute of the revolving light has often seemed to me like an age, when I have been anxious to make out where we were in a gale of wind and rain."


Of the 623 lights numbered in the "Admiralty List of Lights for the British Islands for 1881," 49O are fixed, 112 are flashing, and 21 are occulting (or "eclipsing," or "intermittent"); and similar proportions are to be found in the official list of lights for other parts of the world. Thus it appears that fixed lights constitute the great majority. The fixed light has a great advantage in respect to practical usefulness over the flashing light, in being always visible. The superior brilliancy produced by optical condensation of the revolving light is, in some respects, dearly bought economy, when the great diminution of usefulness to the sailor, in its comparatively long periods of darkness, is taken into account. Theorists who praise the revolving light unqualifiedly for its superior penetrative power seem to forget the counterpart in optics to the great principle in dynamics—that which is gained in power is lost in speed: in flashing lights, what is gained in brilliancy is lost in time of visibility. The painfully anxious scanning of the horizon for a one-minute flashing light, is known to every one who has ever had occasion to look for one in practical navigation; and the comparative ease of picking up a fixed light, and keeping sight of it when it is found, in difficult circumstances, is thoroughly appreciated at sea by sailors. Still, if the revolving light can be seen at all, whatever be the difficulty in picking it up, and whatever the annoyance of losing sight of it and having to pick it up again, it has fulfilled the object of a lighthouse. All are agreed in the maxim that "the grand requisite of all sea lights is penetrative power;" and if the fixed light cannot be seen at a distance, or in weather in which the revolving light is seen, the fixed light has failed, and the revolving light has done its work for the occasion. It depends very much on the special circumstances whether the same quantity of light, given out uniformly as a fixed light, or condensed and given out in flashes, with comparatively long intervals of darkness, as in the revolving light, is better in respect to being seen. In stormy or variable weather, with heavy showers of rain or snow, the fixed light is much safer than a one-minute revolving light of much greater absolute brilliancy; as several successive flashes of the revolving light may be lost through passing showers, while the fixed light loses no chance of being seen in intervals between the showers. On the other hand, in hazy or foggy weather of tolerably steady character, a revolving light can be seen efficiently at a greater distance than the same absolute quantity of light, given out uniformly as a fixed light.

In the question of economy, the great first cost of the optical apparatus, special to the revolving light, must be set against the greater consumption of oil, or gas, or fuel to obtain in a fixed light, whether it be an oil or gas lamp, or an electric light, the same brilliancy. In many cases, indeed, the interest of the money spent on prisms, and lenses, and mechanism in the revolving light, and in some of the most beautiful and perfect of the appliances for the azimuthal condensation of fixed lights, would supply the oil required to give the same, or nearly the same, brilliancy all round the horizon. These circumstances are, of course, all to be taken into account by the proper authorities in respect to every project for a new lighthouse. But we have actually at present the great fact of 490 fixed lights on the coasts of the British Islands; and when it is considered desirable or necessary to give more brilliancy to any of them, this may not best be done by converting it into a flashing light, but by making it a more powerful oil or gas light, or converting it into an electric light. Indeed, after Mr. Douglas's communication of two years ago (March 25th, 1879) to the Institution of Civil Engineers, on "The Electric Light Applied to Lighthouse Illumination," and the discussion which followed upon it, and considering the great progress which has been made since that time in lighting by electricity, we can scarcely doubt that, in the course of a few years, nothing but the electric light will be thought of for any new lighthouse of the greatest importance.

The great defect of fixed lights at present is the want of characteristic quality by which the sailor, when he sees a light which really is a lighthouse light, may immediately feel sure that it is so, and not a steamer's mast-head light, nor a trawler's or fishing-boat's light, nor a light on shore other than a lighthouse light; and that knowing it to be a lighthouse, he may know exactly which of two or more possible lighthouses it is. The need for thorough-going remedial measures to remove this defect has been more and more felt of late years, and is now very generally admitted. Unless a second light is to be added, or the generally objectionable expedient of colour for distinction is in any particular case to be admitted, the only systematic means of giving characteristic quality to a fixed light is by means of occultations or eclipses; and hence the origin of the "Occulting" or "Eclipsing light." We may accordingly look forward to all, or nearly all, the important fixed lights of our coast being, without any very long delay, converted into lights of this class. It is satisfactory to find that during the last year the Elder Brethren of the Trinity House converted one most important light, that of the North Foreland, and another very important one, the light on the west end of Plymouth Breakwater, into eclipsing lights, and that a similar improvement has been promised for five more of the fixed lights under their charge (Mucking, Lowestoft, Chapman, Flatholm, and Evan) within the official year 1880-1.

Occulting Lights of the British Islands, 1881.
No. Name. Place. Period. Remarks.
12 Plymouth On W. end of breakwater Half-minute The light suddenly disappears for 3 seconds every half minute.
107 North Foreland On head " The light suddenly disappears for 5 seconds every half minute.
282 Tarbet Ness 430 yards from the extremity of the point 3 minutes Visible 2½ minutes, eclipsed ½ minute.
305 Ru Stoer South ear of Ru Stoer " 1 " " ½ "
315 Hebridies, Barra Head Highest part of Bernera Island, South point of the Hebrides 3 " " 2½ " " ½ "
339A Craigmore, Firth of Clyde End of pier, Bogany point, Bute Island 11 seconds Five seconds of light, followed by four eclipses, long-short-long-short.
347 Greenock Garvel point 8 " Light for four seconds, with two short eclipses in the next four seconds.
361 Troon Harbour Inner end of pier 1 minute Visible 40 seconds, exlipsed 20 seconds.
373 Galloway Mull S.E. extreme 3/4 " " 30 " " 15 "
418 Ribble River S.E. of Stanner point, N. side of enterance 4 " " 3½ minutes, " ½ minute.
442 Lynus On the point 10 seconds " 8 seconds, "2 seconds.
454 St. Tudwall West Island 10 " " 8 " " 2 "
476 Cardrose Pillar bank, N. side of Channel 5 " Single exlipse evey five seconds.
494 Bristol Channel
E. side of enterance of Parret River 4 minutes White with red sectors, visible 3½ minutes, eclipsed ½ minute.
512 Cork Harbour Roche point, E. side of enterance 20 seconds " 15 seconds, " 5 seconds
521 Mine Head S. side of head 1 minute " 50 " " 10 "
536 Wicklow On the head 13 seconds " 10 " " 3 "
542A North Bull, Dublin Bay End of North Bull wall 14 " " 10 " " 4 "
555 Dundrum Bay St. John's point 1 minute " 45 " " 15 "
562 Belfast Bay On elbow of Holywood bank in 8 feet water 12 seconds Eight seconds of light, followed by two short and one long eclipse.
566 Rathlin Altacarry head, N.E. point of Island 1 minute White with red sector, visible 50 seconds, eclipsed 10 seconds.
600 Loop Head 500 yards, E. by S., from extremity of head 24 seconds " 20 " " 40 "


The 22 eclipsing lights at present existing in the British Islands are described in the preceding Table (see page 412), extracted from the Admiralty List of Lights for 1881.[1]

To these is to be added the Cardross Light on the Clyde, at present a red light, but which, before the end of next month, is to be converted into a white eclipsing light of the same character as the Craigmore light in Rothesay Bay, long-short-long-short. It was judged by the Trustees of the Clyde Navigation, under whose charge this light is, that the long-short-long-short would be thoroughly free from liability to be mistaken for the occulting light (short-short) off Garvel point, three miles from it, and would, in the circumstances, give it a more telling characteristic quality than a single eclipse in the period, or than any group of three eclipses.

It will be seen, from the preceding Table of occulting lights that, with the exceptions of Holywood Bank, Craigmore, Garvel Point, Chapman, and Cardross, the distinction in each case is only a single eclipse in the period, and that, except in nine of them, the period is one minute or upwards, but in all, except five, the duration of the eclipse is less than half a minute. In all the more recent eclipsing lights the period is half a minute or less, and the duration of the eclipse is at most five seconds. The tendency, undoubtedly, is to quicken the action still further, following the example of the old Point Lynus Light, with its eight seconds of visibility and two seconds of eclipse.

The necessity for a very short period is not so urgent in the case of eclipsing lights as it is in the case of flashing lights. A long period in the case of a flashing light means a long period of darkness, throughout which the light is lost sight of. The inconvenience of a long period in an eclipsing light is merely the length of time during which the sailor may have to wait to know which light it is; he never loses sight of the light except for the two or three seconds' duration of one of the eclipses. But quickness of each group is just as important to allow ready and sure identification of its character as is the quickness of a group of flashes in the group-flashing lights of which I have already spoken.

The important question is now to be met—How may eclipses be best arranged to give the requisite number of characteristic distinctions, for the large number of fixed lights on our coasts which need distinction, with as little as may be of interference with the valuable quality of fixity? The answer, I believe, is by groups of eclipses described as follows:—First—one, two, three, or four very short eclipses say of not more than one second each separated by equal intervals of light in the group, and the groups of eclipses following one another after intervals of not less than eight seconds of undisturbed bright light; next groups of two or three short and long eclipses, the short eclipse one second, the long eclipse three seconds, the interval of light between the eclipses of a group one second, and the interval of undisturbed light between the groups of eclipses not less than eight seconds. I fixed upon the time one second, because, after many trials of mechanisms to produce the eclipses I found that I could produce all the groups of eclipses at the rate corresponding to one second for the short eclipse by a simple and inexpensive machine applicable to any lighthouse, large or small, and of any variety of optical arrangement, whether merely with condensation to the horizon, or with the additional appliances required to condense into a particular azimuth.

A machine fulfilling these conditions is now at work in the college tower of the University of Glasgow, performing the short-long-short of the following table for four hours every evening. It has been doing this for a month, and shows no signs of wear. Indeed, there is no part of the machine which is liable to wear in the course of years regular service in a lighthouse. I refer to this machine at present, because it has been supposed that the plan of mechanism used in the Holywood Bank, the Garvel Point, and Cardross Lights—that is, a mechanism producing eclipses by revolving screens, and therefore applicable only to light without azimuthal condensation—is the only mechanism which can practically produce the groups of eclipses at the speed necessary to carry out this method of giving characteristic qualities to fixed lights. The use of gas in lighthouses, whether for smaller lights visible to a distance of four or five miles, or for any more powerful lights such as the splendid lighthouses of Tuskar and Houth Bailey, is admirably adapted for the quickest systems of eclipses of from one half to three seconds' duration for giving distinctive character, although it has not been taken advantage of in this respect except in the small Craigmore Light in Rothesay Bay by Mr. Mortimer Evans.

My proposal for giving character to fixed lights is at present definitely limited to the ten varieties shown in the following table—the short eclipse being one second, the long three seconds, in every case, except the one-short and the long-short-long-short. In these the short eclipse is a half-second; and the long eclipse is three half-seconds.

Number of eclipses in each group. Description of the eclipses. Time from beginning to end of each group of eclipses. Period of time from beginning of one group to beginning of the next.
One Long 3 seconds 12 seconds
One Short ½ second 10 "
Two Short-short 3 seconds 12 "
Two Short-long 5 " 15 "
Two Long-short 5 " 15 "
Three Short-short-short 5 " 15 "
Three Short-short-long 7 " 20 "
Three Short-long-short 7 " 20 "
Three Long-short-short 7 " 20 "
Four[2] Long-short-long-short 5½ " 15 "

It is to be remarked that the times stated in the third and fourth columns need not be known or noted to let the light be recognised. The description in the second column, "short-short," for instance —or "short-long-short"—or "one long "—or "one short" as the case may be, suffices, and is intelligible to every one, learned or unlearned, and lets the light be recognised with the greatest ease. As to the distinction between "long" and "short," the contrast between the two, following one of them instantly after the other, is unmistakable. The only cases of the preceding table in which there is not this contrast to show the distinction are the first and second; but the half-second eclipse of case 2 cannot in practice be ever mistaken for the three-seconds eclipse of case 1, which is six times as long.

It is obvious this plan may be understood immediately by any person learned or unlearned, reading the description, or being told it by word of mouth, and that no knowledge of the Morse letters corresponding to the several groups of eclipses is needed. Indeed, if Mr. Preece and others had not let out the secret, I might have brought forward this proposal without any acknowledgment of indebtedness to Morse or to Captain Colomb, had I been disposed to omit to give credit where credit is due for very brilliant and valuable inventions, and had I thought only of the very best way of putting forward my little suggestion in the manner most likely to promote its early adoption by the lighthouse authorities.

I have only to add, in conclusion, that the highly important suggestion of Sir Richard Collinson, to use a high and a low note in direct contrast, to give characteristic sounds for lighthouses, may be worked out systematically in a very convenient manner by using the combinations of the preceding table; with a high note instead of the short eclipse, and a low note instead of the long eclipse; the low note of the same duration as the high note; the interval between the notes of each group about the same as the time of each blast; and the interval of silence between the group of blasts much longer than the whole time of each group. When the fog-siren is used there is no difficulty in making the blasts as short as we please, and they certainly ought not to be longer than a half-second or three-quarters of a second. Quickness is here, as in many other nautical matters, of vital importance. Let any one try for himself, sounding a high and a low note in rapid succession, or two high notes and a low, or any other of the combinations of the preceding table, and he cannot fail to be convinced there is in each case a characteristic sound, which needs no musical ear for its appreciation, and which cannot be misunderstood by any one who has heard it, or has read it as the description of the sound of such and such a lighthouse, or has been told of it by word of mouth. The distinction between long and short blasts, as Mr. Price Edwards pointed out in his communication to the Society of Arts already referred to, has not proved satisfactory in experience; and I believe this will generally be admitted to be the case by those who have experience of the working of the Morse system of long and short blasts of the steam whistle or siren at sea. There is an uncertainty as to the instant when the sound ceases, prolonged as it often is by echoes, and in the case of the steam whistle an uncertainty also as to when it begins, which is very distressing to any one trying to understand Morse signals by long and short sounds. But corresponding signals by very short high and low notes following one another very quickly, with ample times of silence between the groups of sounds, are exceedingly clear and may easily be distinguished, even when the sounds are barely audible.


Related References:

Also see Kelvin's "Lighthouses of the Future", Good Words, 1873, pp. 217-224.