May 23, 1889] 



NATURE 



87 



BEACON LIGHTS AND FOG SIGNALS} 

 I. 



TT is stated by Samviel Smiles in his " Lives of Engineers " 

 ■*• that, "with Winstanley's structure on the Eddystone in 

 1696, may be said to have commenced the modern engineering 

 efforts in directing the great sources of power in Nature for 

 the use and convenience of man," efforts which', followed up by 

 Rudyerd, Smeaton, and others, have been so successful in con- 

 verting hidden dangers into sources of safety, and insuring the 

 beneficent guidance of the mariner in his trackless path. 



The famous structure of Smeaton, which had withstood the 

 storms of more than half a century with incalculable advantage 

 to mankind, became in course of time a matter of anxiety and 

 watchful care to the Corporation of Trinity House, owing to the 

 great tremor of the building with each wave stroke, during heavy 

 westerly storms. The joints of the masonry frequently yielded to 

 the heavy strains, and the sea-water was driven through them to 

 the interior of the building. The upper part of the structure was 

 strengthened wiih internal ironwork in 1839, and again in 1865. 

 On the last occasion it was found that the chief mischief was 

 caused by the upward stroke of the heavy seas against the pro- 

 jecting cornice of the lantern gallery, thus lifting this portion of 

 the masonry, together with the lantern above it. Unfortunately, 

 the portion of the gneiss rock on which the lighthouse was 

 founded had become seriously shaken by the heavy sea strokes 

 on the tower, and the rock had thus been seriously undermined 

 at its base. The waves rose during storms considerably above 

 the summit of the lantern, thus frequently eclipsing the light, 

 and altering its distinctive character from a fixed light to an 

 occulting. This matter of distinctive character in a beacon light 

 was one of little importance'at the date of the erection of Smea- 

 ton's lighthouse, when coal fires were the only illuminating 

 agents along the coasts ; but with the rapid development of our 

 commerce, and the great increase in the number of coast lights, 

 it has become an absolute necessity that each light maintain a 

 clearly distinctive character. It was therefore determined by the 

 Trinity House, in 1877, to erect a new lighthouse at a distance 

 of 120 feet from Smeaton's tower, where a safe and permanent 

 foundation was found, but at a much lower level, which 

 necessitated the laying of a large portion of the foundation 

 masonry below low water. The foundation-stone of this work 

 was laid on August 19, 1879, ^y H-R-H. the Duke of Edinburgh, 

 Master of the Trinity House ; assisted by H. R. H. the Prince of 

 Wales, an honorary Elder Brother of the Corporation. 



On June i, 1881, H.R.H. the Master, when passing up 

 the Channel in H.M.S. Lively, landed at the rock and laid 

 the last stone of the tower ; and on May 18 of the following 

 year H.R.H. lighted the lamps, and formally opened the light- 

 house. The edifice was thus completed within four years from 

 its commencement, at a cost of ;if59,255. The work was 

 executed under the immediate direc'ion of the Trinity House and 

 their Engineer, and with a saving of ;^24,ooo on the lowest sum 

 at which it had been found that it could be executed by contract. 

 Every block of granite in the structure is dovetailed together 

 both vertically and horizontally, on a system devised by my 

 father, and first adopted at the Hanois Rock Lighthouse off the 

 west coast of Guernsey. The illuminating apparatus consists of 

 two superposed oil lamps, each of six concentric wicks, and of 

 two drums of lenses of 920 millimetres focal distance, twelve 

 lenses in each drum. The optical apparatus is specially designed 

 on the system of Dr. John Hopkinson, F. R.S., for a double 

 flashing light, and shows two flashes in quick succession, at 

 intervals ol half a minute. Attention has of late been directed 

 to the subject of superposed lights in lighthouses, which became 

 a necessity when several small luminaries had to be substituted 

 for the large coal or wood fire of our early lighthouses. The 

 credit of first superposing lighthouse luminaries is doubtless due 

 to Smeaton, who lighted his lantern, in 1759, with twenty-four 

 large tallow candles in two tiers. The idea was followed in 

 1790 with the first revolving light, established at the St. Agnes 

 Lighthouse, Scilly Islands, which consisted of fifteen oil lamps 

 and reflectors, arran^^ed in three groups, and in three tiers. The 

 number of the lamps and reflectors at this and other first class 

 lights was afterwards extended to thirty, and in four tiers. In 

 1859, Mr. J. W. D. Brown, of Lewisham, proposed superposed 

 lenses for signal and lighthouse lanterns, with a separate light for 

 each tier of lenses. In 1872, Mr. John Wigham, of Dublin, pro- 



' Friday evening discourse delivered at the -Royal Institution by Sir 

 James N. Douglass, F.R.S., on March 15. We are indebted to the Editor 

 of the Engineer (or the use of the woodcuti illustrating this discourse. 



posed superposed lenses for lighthouses, in conjunction with his 

 large gas flames, and the firtt application of these was made in 

 1877 at the Galley Head Lighthouse, County Cork. In 1876 

 Messrs. Lepaute and Sons, the eminent lighth)use optical, 

 engineers of Paris, made successful experiments with superposed 

 lenses and mineral oil flames, and one of their apparatus was 

 exhibited at the Paris International Exhibition of 1878. The 

 results of these experiments were given by M. Henry Lepaute, 

 in a paper contributed to the Congress at Havre in 1877, of the 

 French Association for the Advancement of Science. The Eddy- 

 stone represents the first practical application of superposed 

 lenses of the first order, with oil as the illuminant. 



The apparatus at the Eddystone is provided with two six- 

 wick burners of the Trinity House improved type, and has a 

 minimum intensity for clear weather of about 38,000 candle 

 units, and a maximum intensity of about 160,000 candle units 

 for atmosphere impaired for the transmission of light. The 

 chandelier light in Smeaton's lantern was unaided by optical 

 apparatus, 1 have found by experiment that the aggregate 

 intensity of the beam from the twenty-four candles was 67 candl^ 

 units nearly. The maximum intensity of the flashes now sent to 

 the mariner is about 2380 times that of the candle beam, while the 

 annual cost for the mineral oil illuminant is about 82 per cent. less. 



The sound signal for foggy weather consists of two bells of 

 40 cwt. each, mounted on the lantern gallery, and rung by 

 machinery. If any wind occurs with the fog, the windward 

 bell is sounded. The distinctive character of the signal is two 

 sounds of the bell in quick succession every half-minute, thus 

 corresponding with the character of the light signal. 



The tendency of the curvilinear outline near the base of 

 Smeaton's and of other 'similar sea towers that have followed 

 it, to elevate the centre of force of heavy waves on 

 the structure, induced me to adopt a cylindrical base 

 for the new lighthouse, which is found to retard 

 the rise of waves on the structure, while it affords a con- 

 venient platform for the lijjhtkeepers, and adds very consider- 

 ably to their opportunities for landing and relief. The Town 

 Council and inhabitants of Plymouth having expressed a desire 

 that Smeaton's lighthouse should be re-erected on Plymouth 

 Hoe, in lieu of the Trinity House sea mark thereat, the Trinity 

 House, who, as custodians of public money, had no funds avail- 

 able for such a purpose, undertook to deliver to the authorities 

 at Plymouth, at actual cost for labour, the lantern, and the four 

 rooms of the tower. These have been re-erected by public sub- 

 scription on a frustrum of granite, corresponding nearly with 

 the lower portion of Smeaton's to.ver, and it is to be hoped that 

 it will be preserved by the town of Plymouth as a monument to 

 the genius of Smeaton, and in commemoration of one of the 

 most successful and beneficent works in civil engineering. 



It is extremely difficult to estimate, with a fair degree of 

 accuracy, the maxijaum force of the waves with which some of 

 the most exposed of these sea structures may occasionally have 

 to contend. The late eminent lighthouse engineer, Mr. Thomas 

 Stevenson, F.R. S.E., carried out a long series of experiments 

 with a self-re.:jistering instrument he devised for determining 

 the force of sea-waves on exposed structures. He found at the 

 Skerry vore Kock Lighthouse the Atlantic waves there gave 

 an average force for five of the summer months in 1843-44 

 of 611 pounds per square foot. The average result for the 

 six winter months of the same year was 2086 pounds per 

 square foot, or three times as great as in the summer months. 

 The greatest force registered was on March 29, 1845, during a 

 westerly gale, when a pressure of 6083 pounds, or 2j tons nearly, 

 per square foot was recorded. After Smeaton had carefully con- 

 sidered the great defect of the building of Rudyerd at the 

 Eddystone, viz. want of weight, he replied that, " if the light- 

 hou e was to be so contrived as not to give way to the sea, it 

 must be made so strong as that the sea must be compelled to 

 give way to the building." Smeaton also had regard to durability 

 as an important element in the structure, for he adds : "In con- 

 templating the use and benefit of such a structure as this, my 

 ideas of what its duration and continued existence ought to be 

 were not confined within the boundary of one nge or two, but 

 extended themselves to look towards a possible perpetuity." 

 Thus Smeaton soon arrived at the firm conviction that his light- 

 house must be built of granite, and of this material nearly all 

 lighthouses on exposed tidal rocks have since been constructed, 

 while those on submerged sandbanks are open structures of iron, 

 erected on screw piles or iron cylinders. The screw pile was the 

 invention of the late Mr. Alexander Mitchell, of Belfast. 



We have here a molel of the first lighthouse, erected in 1838, 



