RECENT EXPERIMENTS ON FOG-SIGNALS 1 



THE care of its sailors is one of the first duties of a 

 maritime people, and one of the sailor's greatest 

 dangers is his proximity to the coast at night. 

 Hence the idea of warning him of such proximity by 

 beacon -fires placed sometimes on natural eminences and 

 sometimes on towers built expressly for the purpose. 

 Close to Dover Castle, for example, stands an ancient 

 Pharos of this description. 



As our marine increased greater skill was invoked, and 

 lamps reinforced by parabolic reflectors poured their light 

 upon the sea. Several of these lamps were sometimes 

 grouped together so as to intensify the light, which at a 

 little distance appeared as if it emanated from a single 

 source. This "catoptric" form of apparatus is still to some 

 extent employed in our lighthouse -service, but for a long 

 time past it has been more and more displaced by the great 

 lenses devised by the illustrious Frenchman, Fresnel. 



In a first-class " dioptric" apparatus the light emanates 

 from a lamp with several concentric wicks, the flame of 

 which, being kindled by a very active draught, attains to 

 great intensity. In fixed lights the lenses refract the rays 

 issuing from the lamp so as to cause them to form a lumi- 

 nous sheet which grazes the sea-horizon. In revolving 



1 A discourse delivered in the Royal Institution, March 22, 1878. 



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