RECENT EXPERIMENTS ON FOG-SIGNALS. 377 



About five seconds after the explosion, a single loud 

 shock was sent back to us from the side of the vessel 

 lying between us and the land. Obliterated for a mo- 

 ment by this more intense echo, the aerial reverbera- 

 tion continued its retreat, dying away into silence in 

 two or three seconds afterwards.* 



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 high- 

 est degree probable. 



To attempt to interpret an experiment which I 

 have not had an opportunity of repeating, is an opera- 

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

 of this that I refer here to a result announced by Pro- 

 fessor 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 



* 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. 



