X. 



RECENT EXPERIMENTS ON FOG-SIGNALS. 1 



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 some- 

 times 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 appa- 

 ratus 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 illus- 

 trious 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 luminous sheet which grazes the 



1 A discourse delivered in the Royal Institution, March 22, 1378, 



