RECENT EXPERIMENTS ON FOG-SIGNALS. 329 



the aerial reverberation continued its retreat, dying 

 away into silence in two or three seconds afterwards. 1 



I have referred to the firing of an 8-oz. rocket from 

 the deck of the Galatea' on March 8, 1877, stating the 

 duration of its echoes to be seven seconds. Mr. Prentice, 

 who was present at the time, assured me that in his 

 experiments similar echoes had been frequently heard 

 of more than twice this duration. The ranges of his 

 sounds alone would render this result in the highest 

 degree probable. 



To attempt to interpret an experiment which I have 

 not had an opportunity of repeating, is an operation of 

 some risk ; and it is not without a consciousness of this 

 that I refer here to a result announced by Professor 

 Joseph Henry, which he considers adverse to the notion 

 of aerial echoes. He took the trouble to point the 

 trumpet of a syren towards the zenith, and found that 

 when the syren was sounded no echo was returned. 

 Now the reflecting surfaces which give rise to these 

 echoes are for the most part due to differences of tem- 

 perature between sea and air. If, through any cause, 

 the air above be chilled, we have descending streams 

 if the air below be warmed, we have ascending streams 

 as the initial cause of atmospheric flocculence. A 

 sound proceeding vertically does not cross the streams, 

 nor impinge upon the reflecting surfaces, as does a 

 sound proceeding horizontally across them. Aerial 

 echoes, therefore, will not accompany the vertical sound 

 as they accompany the horizontal one. The experiment, 

 as I interpret it, is not opposed to the theory of these 

 echoes which I have ventured to enunciate. But, as I 

 have indicated, not only to see but to vary such an 



1 The echoes of the gun fired on shore this day were very brief ; 

 those of the 12-oz. gun-cotton rocket were 12" and those of the 8-oz. 

 cotton-powder rocket 11" in duration. 



