RECENT EXPERIMENTS ON FOG-SIGNALS. 287 



stances, come from the glassy sea ; while both their variation of direc- 

 tion and their perfectly continuous fall into silence are irreconcilable 

 with the notion that they came from fixed objects on the land. They 

 came from that portion of the atmosphere into which the trumpet 

 poured its maximum sound, and fell in intensity as the direct sound 

 penetrated to greater atmospheric distances. 



The day on which our latest observations were made was particu- 

 larly fine. Before reaching Dungeness, the smoothness of the sea and 

 the serenity of the air caused me to test the echoing power of the atmos- 

 phere. A single ship lay about half a mile distant between us and the 

 land. The result of the proposed experiment was clearly foreseen. 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 distance of the 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 between us and the land. 

 Obliterated for a moment by this more intense echo, the aerial rever- 

 beration continued its retreat, dying away into silence in two or three 

 seconds afterward. 



I have referred to the firing of an 8-ounce 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 with rockets, 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 op- 

 portunity of repeating is an operation of some risk ; and it is not with- 

 out a consciousness of this that I refer here to a result considered ad- 

 verse to the notion of aerial echoes. When the trumpet of a siren ia 

 pointed toward the zenith, it is alleged that when the siren is sounded 

 no echo is 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 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 in- 

 terpret it, is not opposed to the theory of aerial 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. 



In a paper published in the "Philosophical Transactions" for 1876, 

 Prof. Osborne Reynolds refers to these echoes in the following terms : 

 " Without attempting to explain the reverberations and echoes which 



