RECENT EXPERIMENTS ON FOG-SIGNALS 293 



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 Jo- 

 seph 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 re- 

 flecting 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 reflect- 

 ing 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 enun- 

 ciate. But, as I have indicated, not only to see but to 

 vary such an experiment is a necessary prelude to grasp- 

 ing its full significance. 



In a paper published in the " Philosophical Transac- 

 tions" for 1876, Professor Osborne Keynolds refers to 

 these echoes in the following terms: "Without attempt- 

 ing to explain the reverberations and echoes which have 

 "been observed, I will merely call attention to the fact that 

 in no case have I heard any attending the reports of the 

 rockets, 1 although they seem to have been invariable with 



1 These carried 12 oz. of gunpowder, which has been found by Colonel Fraser 

 to require an iron case to produce an effective explosion. 



