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. If 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 optical
And again in 1806:—
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 and 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 the 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. 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 so 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. 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, 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 this
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 effects
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.” * * * * * 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, “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. |