212 FRAGMENTS OF SCIENCE. 



It was this. The rocket being sent up, it exploded at a 

 great height; the echoes retreated in their usual fashion, 

 becoming less and less intense as the distances of the 

 invisible surfaces of reflection from the observers increased. 

 About five seconds after the explosion, a single loud shock 

 was sent back to us from the side of the vessel lying be- 

 tween us and the land. Obliterated for a moment by this 

 more intense echo, the aerial reverberation continued its 

 retreat, dying away into silence in two or three seconds 

 afterward.* 



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 ex- 

 periments 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 toward 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 temperature bet ween 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 floc- 



culence. 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 or.ly to see but to vary such an experiment 



is a necessary prelnoe to grasping its full significance. 



* 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. 



