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 22, 1878. 
