X. 



RECENT EXPERIMENTS ON FOG-SIGNALS* 



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

 -L 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 light- 

 house-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 ema- 

 nates 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 sea- 



* A discourse delivered in the Royal Institution, March 2?, 

 1878. 



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