796 EEPORT— 1886. 



Trinity House, who accepted the responsibility of carrying out the investiga- 

 tion. 



A Committee was formed of members of the Corporation, who secured the 

 friendly co-operation of the Scotch aod Irish Lii^hthouse Boards, and many dis- 

 tinguished scientific men at home and abroad. I had the honour of acting, in my 

 official capacity as Engineer-in-Cliief to the Trinity House, in making the arrange- 

 ments for exhibiting the experimental lights, and in reporting to the Board from 

 time to time, as in all other matters referred to me professionally. 



These investigations were carried out in full view of all who were in any way 

 interested in the subject. The whole arrangements were open to public inspection, 

 and, in their desire to arrive at a wise and just decision on so important a question, 

 the Trinity House Committee courted the fullest inquiry. Many members of 

 scientific societies, especially those connected with engineering, were invited, and 

 visited the station. The French lighthouse authorities, who rendered much kind 

 assistance in obtaining observations, sent tlieir representatives to view the arrange- 

 ments, and ofiicers from the lighthouse services of Germany, Denmark, Norway aud 

 Sweden, Russia, Italy, Spaiu, Brazil, the United States, and Canada visited the 

 station and witnessed the experiments. 



In order to obtain, with uniformity and method, a consensus of comparative 

 eye-measurements — in addition to the measurements of the Committee and their 

 officers at their different stations ashore and afloat, to those of the coastguard men 

 at nine stations between Dungeness and the North Foreland, and to the more pre- 

 cise scientific measurements of the experts — special observation books were prepared 

 and widely distributed to shipping associations and port authorities, with a view to 

 their securing the co-operation of masters of vessels, pilots, and others navigating in 

 the vicinity of the South Foreland. 



The South Foreland Station is especially adapted for lighthouse experiments 

 generally, because of the existing facilities for observations on land and sea. The 

 land in the neighbourhood has no hedges and few trees, and affords facilities for 

 observations at distances of between two and three miles. The station is provided 

 with surplus steam power for driving experimental machines for electric lights, aud 

 it is easily accessible from Loudon. 



Three I'ough timber towers of su^fficient strength to withstand, witliont tremor, 

 the effects of heavy gales were erected at the rear of the High Lighthouse, 150 feet 

 apart. These towers were marked in large letters A, B, and C. A tower was 

 devoted to electricity, B to the gas system of Mr. Wigham, and C to such gas or 

 oil lamps as might be proposed to, and approved by, the Committee for trial 

 during the experiments. A lantern of the usual first-order dimensions, but with 

 an additional height in the glazing for the passage of beams from superposed optical 

 apparatus of the first order, was provided for each tower. The optical apparatus 

 in each lantern was, in the outset, special in relation to the illuminant to be used 

 for producing fixed and flashing lights. For the electric arc lights, optical apparatus 

 of the second order of Fresnel was adopted, the apparatus having a focal distance of 

 700 '"/m. The dimensions of this apparatus are greater than optically required for 

 the largest electric arc light yet tried for lighthouse illumination, but the internal 

 capacity is found to be only just sufficient for the perfect manipulation of the light 

 by a lightkeeper of possibly robust build. For the large gas and oil flames in the 

 A and C lanterns the apparatus adopted was of the usual first-order size, having a 

 focal distance of 920 ™/„. 



The lanterns were partially glazed on opposite sides, north and south, the 

 southern arc being chiefly for observation from the sea. To the northward the 

 land is better adapted for observations on shore, and here three observing huts 

 were erected at the respective distances of 2,144, 6,200, and 12,973 feet ; each hut 

 was provided with accommodation for two watchers, and a chamber fitted with a 

 large plate glass window in the direction of the experimental lights, and special 

 apparatus for their photometric measurement. The third hut proved to be practi- 

 cally of but little value for photometry, the distance being too great ; it, however, 

 afibrded an accurately Icnown distance for eye-measurements, and a barrack and 

 starting-point for watchers endeavouring to determine the vanishing distance of 



