RECENT EXPERIMENTS ON FOG SIGNALS. 193 



I will here record a few other measurements executed on 

 the Rosegg glacier: the line was staked out across the 

 trunk formed by the junction of the Rosegg proper with 

 the Tschierva glacier, a short distance below the rocky 

 promontory called Agaliogs. 



Rosegg Glacier. 



No. of Stake. Hourly Motion. 



1 0.01 inch. 



2 0.05 



3 0.07 



4 , 0.10 



5 0.11 



6 0.13 



7 0.14 



8 0.18 



9 0.24 



10 0.23 



11 0.24 



This is an extremely slowly moving glacier; the maxi- 

 mum motion hardly amounts to seven inches a day. 

 Crevasses prevented us from continuing the line quite 

 across the glacier. 



CHAPTER X. 



RECENT EXPERIMENTS ON FOG SIGNALS.* 



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. 



* A discourse delivered in the Royal Institution, March 32, 1878. 



