$12 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 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 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 only to see but to vary such an experiment 
is a necessary prelude 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. 
