THE ATMOSPHERE AND FOG-SIGNALING. 695 



being awoke by them. He does not remember ever before hearing 

 the sounds so loud in Dover ; it seemed as though the observers were 

 close to the instruments. 



Other days of fog preceded this one, and they were all days of 

 acoustio transparency, the day of densest fog being acoustically the 

 clearest of all. 



The results here recorded are of the highest importance, for they 

 bring us face to face with a dense fog and an actual fog-signal, and 

 confirm in the most conclusive manner the previous observations. 

 The fact of Captain Atkins and Mr. Edwards being awakened by the 

 siren proves, beyond all our previous experience, its power during the 

 fog on the 7th of February. 



It is exceedingly interesting to compare the transmission of sound 

 on February 7th with its transmission on October 14th. The wind 

 on both days had the same strength and direction. My notes of the 

 observations show the latter to have been throughout a day of ex- 

 treme optical clearness. The range was 10 miles. During the fog 

 of February 7th, the Argus heard the sound at 11 miles; and it was 

 also heard at the Varne light-vessel, which is 12f miles from the 

 Foi-eland. 



It is also worthy of note that through the same fog the sounds 

 were well heard at the South Sand Head light-vessel, which is in the 

 opposite direction from the South Foreland, and actually behind the 

 siren. For this important circumstance is to be borne in mind : on 

 February 7th the siren happened to be pointed, not toward the Argus, 

 but toward Dover. Had the yacht been in the axis of the instrument, 

 it is highly probable that the sound would have been heard all the 

 way across to the coast of France. 



It is hardly necessary for me to say a word to guard myself against 

 the misconception that I consider sound to be assisted by the fog 

 itself. The fog-particles have no more influence upon the waves of 

 sound than the suspended particles stirred up over the banks of New- 

 foundland have upon the waves of the Atlantic. A homogeneous air 

 is the usual associate of fog, and hence the acoustic clearness of foggy 

 weather. 



Experiments ox Artificial Fogs. These observations are 

 clinched and finished by being brought within the range of laboratory 

 experiment. Here we shall learn incidentally a lesson as to the cau- 

 tion required from an experimenter. 



The smoke from smouldering brown paper was allowed to stream 

 upward into the tunnel abed (see p. 689) ; the action upon the sound- 

 waves was strong, rendering the short and agitated sensitive flame k 

 tall and quiescent. Here the action of the smoke seemed clearly 

 demonstrated. 



Air, first passed through ammonia, then through hydrochloric acid, 



