502 



WRITINGS OP JOSEPH HENRY. [1877 



The opportunity was embraced while at the mouth of the 

 tunnel to make some observations which might have a bear- 

 ing upon the phenomena of the aerial echo. For this purpose 

 advantage was taken of a large tool-chest which happened 

 to be placed about twenty or thirty feet within the western 

 mouth of the tunnel. By slamming down violently the 

 cover of this chest, a loud sound of an explosive character 

 was produced, from which a prolonged echo was returned 

 from the interior of the tunnel. This echo was slightly in- 

 termittent, suddenly increasing in loudness at intervals for 

 a moment, and again resuming its uniform intensity. This 

 effect was attributed to projecting pieces of rock in that part 

 of the tunnel which had not been lined with brick. An 

 echo was however evidently returned from that portion of 

 which the sides were not projecting, which I would consider 

 an effect of the same cause which produces the aerial echo. 



Aerial Echoes. 



During the year 1877 (as also in 1876) series of experi- 

 ments were made on the aerial echo, in which I was assisted 

 in the first series by General Woodruff, engineer of the third 

 light-house district, and in the second series by Edward 

 Woodruff, assistant engineer of the same district. These expe- 

 riments were made principally at Block Island, though some 

 were also made at Little Gull Island. Especial attention has 

 been given to this phenomenon, (which consists in a distinct 

 echo from the verge of the horizon in the direction of the pro- 

 longation of the axis of the trumpet of the siren,) because the 

 study of it has been considered to offer the easiest access to 

 the solution of the question as to the cause of all the abnor- 

 mal phenomena of sound, and also because it is in itself an 

 object of much scientific interest. 



In my previous notice of this phenomenon, in the report 

 of the Light-House Board for 1874, I suggested that it might 

 be due to the reflection from the crests of the waves of the 

 ocean; but as the phenomenon has been observed during all 

 conditions of the surface of the water this explanation is not 

 tenable. 



