Page:Popular Science Monthly Volume 6.djvu/707
THE ATMOSPHERE AND FOG-SIGNALING.
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 extend 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 Montanvert, 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