422 THE POPULAR SCIENCE MONTHLY. 



a long time. This action is common to all ear-signals, and has been at times ob- 

 served 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 at- 

 mosi>here, the most powerful signals will be at times unreliable.' 



" 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 tlieir surface of contact. The cor- 

 rectness 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 lieport of General Duane is marked throughout by fidelity to 

 facts, rare sagacitj^, 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 "Wasliington Re- 

 port reveals. At this point, however, the eminent chairman of the 

 Lio-hthouse Board strikes in with the following criticism : 



" In the foregoing I differ entirely in opinion fi'om 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 ex- 

 pansive 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 pro- 

 duce the results noted by General Duane and Dr. Tyndall, while we shall find 

 in the action of currents above and below a true and etflcient cause." 



I have already cited the remarkable observation of General Duane, 

 that, with a snow-storm from the northeast blowing against the sound, 



' 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.) 



