THE ATMOSPHERE AND FOG-SIGNALING. 687 



assistant engineer, walked in the other direction. At 12.50 p. m. the 

 wind blew a gale, and broke into a thunder-storm with violent rain. 

 Inside and outside of the Cornhill Coast-guard Station, a mile from 

 the instruments, in the direction of Dover, Mr. Ayres heard the sound 

 of the siren through the storm ; and, after the rain had ceased, all 

 sounds were heard distinctly louder than before. Mr. Douglas had 

 sent a fly before him to Kingsdown, and the driver had been waiting 

 for fifteen minutes before he arrived. During this time no sound had 

 been heard, though forty blasts had been blown in the interval ; nor 

 had the coast-guard man on duty, a practised observer, heard any of 

 them throughout the day. During the thunder-storm, and while the 

 rain was actually falling with a violence which Mr. Douglas describes 

 as perfectly torrential, the sounds became audible, and were heard 

 by all. 



To rain, in short, I have never been able to trace the slightest 

 deadening influence upon sound. The reputed barrier offered by 

 " thick weather" to the passage of sound was one of the causes which 

 tended to produce hesitation in establishing sound-signals on our 

 coasts. It is to be hoped that the removal of this error may redound 

 to the advantage of coming generations of seafaring men. 



Action of Snow. Falling snow, according to Derham, is the most 

 serious obstacle of all to the transmission of sound. We did not ex- 

 tend our observations at the South Foreland into snowy weather; but 

 a previous observation of my own bears directly upon this point. On 

 Christmas-night, 1859, I arrived at Chamouni, through snow so deep 

 as to obliterate the road-fences, and to render the labor of reaching 

 the village arduous in the extreme. On the 26th and 27th it fell 

 heavily. On the 27th, during a lull in the storm, I reached the Mon- 

 tanvert, sometimes breast-deep in snow. On the 28th, with great 

 difficulty, two lines of stakes were set out across the glacier, with the 

 view of determining its winter motion. On the 29th, the entry in my 

 journal, written in the morning, is, " Snow, heavy snow ; it must 

 have descended through the entire night, the quantity freshly fallen 

 is so great." 



Under these circumstances I planted my theodolite beside the Mer 

 de Glace, having waded to my position through snow which, being 

 dry, reached nearly to my breast. Assistants were sent across the 

 glacier with instructions to measure the displacement of a transverse 

 line of stakes planted previously in the snow. A storm drifted up 

 the valley, darkening the air as it approached. It reached us, the 

 snow falling more heavily than I had ever seen it elsewhere. It soon 

 formed a heap on the theodolite, and thickly covered my own clothes. 

 Here, then, was a combination of snow in the air, and of soft, fresh 

 snow on the ground, such as Derham could hardly have enjoyed ; 

 still through such an atmosphere I was able to make my instructions 



