1878.] Prof. Tyndall. Experiments on Fog- Signals. 



257 



more intense eclio, the aerial reverberation 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 the 8th of March, 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 

 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 considered 

 .adverse to the notion of aerial echoes. When the trumpet of a syren 

 is pointed towards the zenith, it is alleged that when the syren 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 tempera- 

 ture 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 interpret 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, 

 Professor Osborne Reynolds refers to these echoes in the following 

 terms : " Without attempting 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,* 

 although they seem to have been invariable with the guns and pistols. 

 These facts suggest that the echoes are in some way connected with 

 the direction given to the sound. They are caused by the voice, 

 trumpets, and the syren, all of which give direction to the sound ; but 

 I am not aware that they have ever been observed in the case of a 

 sound which has no direction of greatest intensity." 



The reference to the voice and other references cause me to think 

 that, in speaking of echoes, Professor Osborne Reynolds and myself 

 are dealing with different phenomena. Be that as it may, the fore- 

 going observations render it perfectly certain that the condition as to 

 direction here laid down is not necessary to the production of the 

 echoes. 



* These carried 12 oz. of gunpowder. 



