< Page:Popular Science Monthly Volume 7.djvu
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THE POPULAR SCIENCE MONTHLY.
a long time. This action is common to all ear-signals, and has been at times observed at all the stations, at one of which the signal is situated on a bare rock twenty miles from the main-land, with no surrounding objects to affect the sound."

It is not necessary to assume here the existence of a "belt," at some distance from the station. The passage of an acoustic cloud over the station itself would produce the observed phenomenon.

Passing over the record of many other valuable observations, in the Report of General Duane, I come to a few very important remarks which have a direct bearing upon the present question:

"From an attentive observation," writes the general, "during three years, of the fog-signals on this coast, and from the reports received from the captains and pilots of coasting-vessels, I am convinced that, in some conditions of the atmosphere, the most powerful signals will be at times unreliable. [1]
"Now it frequently occurs that a signal, which under ordinary circumstances would be audible at a distance of fifteen miles, cannot be heard from a vessel at the distance of a single mile. This is probably due to the reflexion mentioned by Humboldt.
"The temperature of the air over the land where the fog-signal is located being very different from that over the sea, the sound, in passing from the former to the latter, undergoes reflexion at their surface of contact. The correctness of this view is rendered more probable by the fact that, when the sound is thus impeded in the direction of the sea, it has been observed to be much stronger inland.
"Experiments and observation lead to the conclusion that these anomalies in the penetration and direction of sound from fog-signals are to be attributed mainly to the want of uniformity in the surrounding atmosphere, and that snow, rain, and fog, and the direction of the wind, have much less influence than has been generally supposed."

The Report of General Duane is marked throughout by fidelity to facts, rare sagacity, and soberness of speculation. The last three of the paragraphs just quoted exhibit, in my opinion, the only approach to a true explanation of the phenomena which the Washington Report reveals. At this point, however, the eminent chairman of the Lighthouse Board strikes in with the following criticism:

"In the foregoing I differ entirely in opinion from General Duane, as to the cause of extinction of powerful sounds being due to the unequal density of the atmosphere. The velocity of sound is not at all affected by barometric pressure; but if the difference in pressure is caused by a difference in heat, or by the expansive power of vapor mingled with the air, a slight degree of obstruction of sound may be observed. But this effect we think i-s entirely too minute to produce the results noted by General Duane and Dr. Tyndall, while we shall find in the action of currents above and below a true and efficient cause."

I have already cited the remarkable observation of General Duane, that, with a snow-storm from the northeast blowing against the sound,

  1. ↑ Had I been aware of its existence I might have used the language of General Duane to express my views on the point here adverted to. (See chapter vii., pp. 319, 320.)
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