early modes of illumination—facet reflectors and lamps—silvered copper reflectors and argand lamps—isle of may coal light—improvements in catoptric lights—distinctions for lighthouses invented by mr. stevenson, viz., flashing, intermittent, and double lights—floating light lantern—lighting of stage of covent garden theatre—dioptric system of lighthouse illumination.
seeing that, for reasons stated in the last chapter, i was led to give up the idea of attempting to follow any chronological sequence in this memoir, it may perhaps be convenient, before speaking of my father’s general practice as a civil engineer, that i should supplement the sketch i have given of the bell rock lighthouse by some account of the other important duties he performed as engineer to the commissioners of northern lighthouses—an office which, as we have seen, he held for so long a period.
the lighthouse towers of the last century, though useful as beacons by day, were after all most imperfect guides by night. indeed, the rude expedients adopted at that early period to give light to the sailor in a dark and moonless sky, present a very curious contrast to the modern system of lighthouse illumination—the result of careful study by modern philosophers and engineers. if49 proof of this be wanted, we have only to refer to the twenty-four miserable candles, unaided by reflectors or any other optical contrivance, which shed their dim and uncertain light from smeaton’s famous eddystone for nearly half a century after it was built.
but indeed at that early time all lights had not even the advantage of the glazed lantern which protected the candles of the eddystone from the winter’s blast and summer’s breeze; the grand tour de cordouan on the coast of france was then lighted by blazing fagots of wood burned in an open chauffer, and many of the early lighthouses were open coal fires.
when mr. smith, however, was appointed engineer to the scotch lighthouse board, he, as has been already said, came forward as the advocate of lamps aided by reflectors, a system which he introduced at kinnaird head in 1787; so that the lighthouse board of scotland never employed any less perfect mode of illumination. these early reflectors, which had been in use in england, consisted of small pieces or facets of common mirror glass arranged in a hollow mould and fixed in their places by plaster of paris; but soon afterwards the facets of mirror glass, though forming good instruments for their day, and of their kind, were discarded, and the reflectors were thereafter made of copper, plated with silver, and brightly polished.
i am not in a position to say when or by whom these metallic reflectors were first introduced, or what was their exact form, the question being invested in some degree of doubt; but it was to the perfecting of these optical50 instruments and adapting them to practical use in a lighthouse that mr. stevenson’s attention was early directed. thus we find him in 1805 reporting as follows:—
“the operations at the start point were this season begun upon monday the 27th of may, and the lighthouse was finished upon saturday the 17th october and the light advertised to be lighted upon the night of wednesday the 1st of january 1806. some nights before i left sanday i had the light set in motion, when the effect appeared to be most excellent; indeed, it must be equal to the scilly or cromer lights, and superior to the revolving light at tinmouth: at the former there are twenty-one reflectors, and at the latter there are fifteen, whereas at the start point lighthouse i only use seven reflectors, but by altering the motion of the machinery and construction of the revolving part, i produce the desired effect.”
and again in 1806:—
“i was late in the season for making all the observations i could have wished upon the start point and north ronaldsay lights, and was not very well appointed in a vessel for keeping the sea in bad weather. i however made a cruise for this purpose, and stood towards the fair isle in a heavy gale of wind, with an intention to run for shetland, but the wind shifted, and i stretched towards copinshaw, at the distance of about ten or twelve miles to the westward of orkney, with both lights in view. the second night i went through north ronaldsay firth to have a west view of the lights. i put about off westra, and stood northward with both lights in view, when it came to blow with great violence from the s.w., and it was with much difficulty we could regain the coast. although on this trip i had rather bad weather, with a heavy swell of sea, yet it was very answerable for my purpose, and i was upon the whole much pleased with the51 appearance of the new light; but i find, when at the distance of ten or twelve miles, with the sea running high, the light is seen for rather too short a period, so that it would be proper to place other seven reflectors upon the frame at an angle of about 40° to the present reflectors, in the event of removing north ronaldsay light.”
i find from his correspondence that my father consulted sir john leslie, the distinguished professor of natural philosophy, and alexander adie, the well-known optician, as to the best mode of procuring a true parabolic form for the construction of his reflectors, and having introduced a simple means of withdrawing the lamp from the reflector, his new catoptric apparatus may be said to have been completed.
fig. 4.
fig. 5.
the bell rock was the first lighthouse that was illuminated by mr. stevenson’s improved apparatus (shown in section in fig. 4), where a is the fountain for the oil, b the burner, and the directions of the incident and52 reflected rays are represented by dotted lines. in fig. 5 the reflector is shown in elevation; the lamp is represented as lowered down from the reflector, which is effected by a sliding arrangement controlled by a guide,—the object being to allow the lamp to be removed while the reflector is being polished, and to insure its being returned to its exact position in the true focus of the reflector. perhaps the most valuable opinion that can be quoted as to the utility of this arrangement is that of mr. airy, the astronomer-royal, who, after the apparatus had been in use fifty years, and after having inspected the lighthouses both of britain and france, says—“this lighthouse” (girdleness, in aberdeenshire) “contains two systems of lights. the lower, at about two-fifths of the height of the building, consists of thirteen parabolic reflectors of the usual form. i remarked in these, that by a simple construction, which i have not seen elsewhere, great facility is given for the53 withdrawal and safe return of the lamps, for adjusting the lamps, and for cleaning the mirrors;” and in closing his report he adds, “it is the best lighthouse that i have seen.”6
notwithstanding the introduction of this improved apparatus at the bell rock in 1811, a coal-fire, which had existed for the long period of 181 years on the isle of may, at the entrance to the firth of forth, still continued, in 1816, to send forth its feeble and misleading light, and as it was one of the best specimens of the lighthouses of days now passed away, it may not be uninteresting to give a short account of it.
the may light was at that period what is called a “private light”—the right of levying dues on shipping being vested in the duke of portland, who was owner of the island. there were many private lights in england, but the isle of may was the only one that still remained in scotland, and the commissioners of northern lighthouses, believing it to be advantageous that so important a light should be placed under public management, so as to secure for the shipping a better light, and exemption from the high passing tolls charged by the proprietor, entered into treaty with the duke of portland for the purchase of his rights. this negotiation resulted in the introduction of a bill into parliament in 1814, authorising the purchase of the isle of may, with the right of levying toll, for the sum of ￡60,000.
so soon as the property came into the hands of the commissioners they erected a new lighthouse, and on the54 1st of february 1816 the old coal chauffer was discontinued, and a light from oil with reflectors was exhibited in its stead. i am enabled from an old plan in my possession to present the reader with two sketches of the original chauffer light of the isle of may.
fig. 6.
fig. 6 is an elevation of the building, with the tackle for raising the fuel to the top, and its inscription stone over the door bearing the date 1635. fig. 7 shows the building in section, with its stone winding staircase and vaulted chambers, the whole structure apparently being so designed as to be perfectly proof against fire—a precaution very necessary for a building dedicated to such a purpose, for it is recorded that no fewer than 400 tons of coal were annually consumed in the open chauffer on its top.
55
fig. 7.
it was, as i have said, one of the best coal-fires in the kingdom, and three men were employed to keep the bonfire burning, so that its inefficiency as a light was not due to any want of outlay in its support. but its appearance was ever varying, now shooting up in high flames, again enveloped in dense smoke, and never well seen when most required. when mr. stevenson visited the island, with a view to its purchase by the commissioners, he was told by the keeper, that in violent gales the fire only kindled on the leeward side, and that he was in the habit of putting his hand through the windward bars of the chauffer to steady himself while he supplied the fire with coals, so that in the direction in which it was most wanted hardly any light was visible. nothing can be worse than any variableness or uncertainty in the appearance of a light. better far not to exhibit it at all than to show it irregularly; and the coal lights were so56 changeable and destitute of characteristic appearance as to be positively dangerous. this indeed was too sadly proved by the loss of h. m. ships ‘nymphen’ and ‘pallas,’ which on the 19th december 1810 were wrecked near dunbar, the light of a limekiln, on the coast of haddington, having been mistaken for the coal light of the isle of may. fortunately only nine of their combined crews of 600 men perished; but the vessels, valued at not less than ￡100,000, became total wrecks.
during the long period he held the office as engineer to the board, mr. stevenson designed and executed eighteen lighthouses in the district of the northern lighthouse commissioners, many of them in situations which called for much forethought and great energy. all his lighthouse works were characterised by sagacity and inventiveness, and exhibit successive stages of improvement, equally indicative of the growing prosperity of the board and of the alacrity and zeal with which their engineer laboured in his vocation. whether we consider the accuracy and beauty of the catoptric apparatus, the arrangements of the buildings, or the discipline observed by the lightkeepers of the northern lighthouses, we cannot fail to recognise the impress of that energetic and comprehensive cast of mind which directed the whole. acting under the direction of an enlightened board of commissioners, my father may, with the strictest propriety, be said to have created the lighthouse system of scotland. his merits indeed in this respect were generally acknowledged in other quarters; and many of the irish lighthouses, and several lighthouses in our colonies, were fitted up with apparatus prepared after his designs.
57 in the course of his labours my father’s attention was much given to the question of distinction among lights—a matter of the utmost importance, especially in narrow seas, where many lights are required; and at his suggestion, the northern lighthouse commissioners fitted up a temporary light-tower on inchkeith, in which numerous experiments having this object in view were made.
fig. 8. fig. 9.
he was the inventor of two useful distinctions—the intermittent and flashing lights. in the intermittent distinction the light is suddenly obscured by the closing of metallic shades which surround the reflector frame, and on their opening, it is as suddenly revealed to sight, in a manner which completely distinguishes it from the ordinary revolving light, which from darkness, gradually increases in power till it reaches its brightest phase, and then gradually declines until it is again obscured; the action of these shades in producing the intermittent effect is illustrated in figs. 8 and 9. the flashing light,58 by a peculiar arrangement of reflectors, and a rapid revolution of the frame which carries them, is made to give a sudden flash of great power, once in five seconds of time, and thus has a distinctive appearance very different from either the revolving or intermittent light. for these distinctions mr. stevenson received from the king of the netherlands a gold medal as a mark of his majesty’s approbation.
fig. 10.
mr. stevenson also, in 1810, gave a design for a double light at the isle of may, as shown in fig. 10, in which all lighthouse engineers will see the embryo of the double light of the present day.
i must not omit to notice his improvement on the lanterns of floating lightships, now universally adopted, which he introduced in 1807. previously to this59 date the lightships exhibited their lights from small lanterns suspended from the yardarms or frames. mr. stevenson realised the inutility of such a mode of exhibition, and conceived the idea of forming a lantern to surround the mast of the vessel, and to be capable of being lowered down to the deck to be trimmed, and raised when required to be exhibited. his plan had the advantage of giving a lantern of much greater size, because it encased the mast of the ship, and with this increase of size it enabled larger and more perfect apparatus to be introduced, as well as gearing for working a revolving light. fig. 11 shows this lantern, and the following is his description of it:—
“the lanterns were so constructed as to clasp round the masts and traverse upon them. this was effected by constructing them with a tube of copper in the centre, capable of receiving the mast, through which it passed. the lanterns were first completely formed, and fitted with brass flanges; they were then cut longitudinally asunder, which conveniently admitted of their being screwed together on the masts after the vessel was fully equipped and moored at her station. letters a a show part of one of the masts, b one of the tackle-hooks for raising and lowering the lanterns, c c the brass flanges with their screw-bolts, by which the body or case of the lantern was ultimately put together. there were holes in the bottom and also at the top connected with the ventilation: the collar-pieces e and g form guards against the effects of the weather. the letter h shows the front of the lantern, which was glazed with plate-glass; i is one of the glass shutters by which the lamps were trimmed, the lower half being raised slides into a groove made for its reception; k shows the range of ten agitable burners or lamps out of which the oil cannot be spilt60 by the rolling motion of the ship. each lamp had a silvered copper reflector l placed behind the flame.”
fig. 11.
the reputation of my father’s catoptric apparatus was not, it appears, confined to those interested in the welfare of the seaman. in 1819, mr. stevenson was waited on by a gentleman passing hurriedly through edinburgh, who came on behalf of mr. harris, the manager of covent garden theatre, who was desirous to try catoptric apparatus for certain stage effects61 which he intended to introduce in london. the proposal seems rather to have taken the lighthouse engineer by surprise, but on learning that the gentleman who had favoured him with a call was mr. benson, the famous singer of the day, he wrote the following letter to mr. harris:—
“i had some conversation with mr. benson of your theatre on the day he proposed to leave this for london. the purpose of his visit to me was to inquire about the reflectors we used in the lighthouses upon this coast, which are under my direction, as he had some plan in view for dispensing with the footlights on the stage by the introduction of reflected light.
“being desirous to give every facility to mr. benson’s views, i offered him the loan of a reflector, which i showed him; but from his being on the eve of setting off, and wishing to keep the discovery, if practicable, for your theatre, i agreed to send it to you at covent garden, and this letter is to acquaint you that a case containing the reflector and its burner was shipped to your address.
“you are to understand that there is no charge whatever to be made; i only request that the reflector may be returned when you have made your trials. i no sooner learned that i conversed with the gentleman who sings so delightfully in ‘rob roy’ than i felt an irresistible inclination to oblige him.
“wishing you every success in the projected improvement in lighting the stage, i remain,” etc.
the reflector was duly returned by mr. harris. the note intimating its shipment says—“it is an excellent reflector, but it collects the light too much in one spot for our use; i mean, it does not spread the light sufficiently about.”
62 i mention this small matter, not so much because the manager of covent garden theatre came to edinburgh to get his information, but to show that mr. harris’s experiment, made in 1819, foretold the result of all trials that have since been made to light railway stations, public gardens, and parks, by using lighthouse apparatus, which is designed to condense the rays of light, and not to diffuse them, and is therefore inapplicable for such purposes.
* * * * *
the remarks i have made on lighthouse illumination refer to what is known as the catoptric system, whereby the light is acted on by reflection alone. the invention of the dioptric, system by fresnel was first communicated to mr. stevenson in a letter received from colonel colby of the royal engineers, who had an opportunity of knowing the benefit of fresnel’s dioptric light in making certain trigonometrical observations for connecting the government surveys of the shores of england and france across the english channel. the letter is in the following terms:—
“tower, 1st nov. 1821.
“my dear sir,—i am quite ashamed of having delayed answering your letter, and thanking you for the communications you sent me for so long a time. in regard to the lamps, an account will be given of them in the annales de chimie for the next month. the lens is composed of pieces of glass forming a circle three feet in diameter, ground to three feet focal length. the lamp is similar to an argand lamp, having hardly any other difference,63 except four concentric circular wicks instead of one. the external wick is about three inches in diameter. the light given by the lens is remarkably brilliant. when we were at folkestone hill, the lamp at blancnez appeared to give about four times the light of the dungeness lighthouse, though the distance of the lamp was nearly double that of the lighthouse. the only difficulty which occurs to me in their employment in lighthouses is the small angle to which a single lens gives light. i think one lens is brilliant for seven degrees, and could not answer for more than eight or nine degrees.
“the cordouan lighthouse is to be fitted up with ten lenses round one lamp.
“with best wishes to mrs. s. and your family, ever yours,
“thos. colby.”
the merits of the dioptric system of illumination were brought before the commissioners of northern lighthouses in mr. stevenson’s report of december 1821, and, as is well known, it has, with various extensions and important improvements, been very generally adopted in all cases where it is applicable to lighthouse illumination.
