1889.] on Beacon Lights and Fog Signals. 427 



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 maxi- 

 mum 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 aj)paratus. I have found by experi- 

 ment that the aggregate intensity of the beam from the 24 candles 

 was G7 candle units nearly. 1 he 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 convenient plat- 

 form for the light-keejjers, and adds very considerably to their 

 opportunities for landing and relief. The Town Council and inhabi- 

 tants 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 available for such a purpose, undertook to 

 deliver to the authorities at Plymouth, at actual cost for labour, the 

 lanterns and the four rooms of the tower. These have been re-erected 

 by public subscription, on a foundation of granite, corresponding 

 nearly with the lower portions of Smeaton's tower, and it is to be 

 hoped that it will be preserved by the town of Plymouth as a monu- 

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

 most successful and beneficent works in civil engineering. 



It is extremely difiicult to estimate with a fair degree of accuracy 

 the maximum 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, carried out a 

 long series of experiments with a self-registering instrument he 

 devised for determining the force of sea waves on exposed structures. 

 He found at the Skerry vore rock lighthouse the Atlantic waves there 

 gave an average force for five of the summer months, in 1843-4, of 

 611 lbs. per square foot. The average result for the six winter 

 months of the same year was 2086 lbs. per square foot, or three 

 times as great as in the summer months. The greatest force registered 

 was on the 29th March, 1845, during a westerly gale, when a pressure 

 Vol XII. (No. 83.) 2 g 



