504 WRITINGS OF JOSEPH HENRY. [1877 



1. As the steamer advanced, and the distance from the 

 trumpet was increased, the loudness of the echo diminished, 

 contrary to the effect of an echo from a plane surface, since 

 in the latter case the echo would have increased in loudness 

 as the reflecting surface was approached, because the whole 

 distance travelled by the sound to and from the reflector 

 would have been lessened. The efiect however is in accord- 

 ance with the supposition that the echo is a multiple sound, 

 the several parts of which proceed from different points at 

 different distances of the space in front of the trumpet, and 

 that as the steamer advances towards the verge of the hori- 

 zon it leaves behind it a number of the points from which 

 the louder ones proceed, and thus the effect upon the ear is 

 diminished as the distance from the trumpet is increased. 



2. The duration of the echo was manifestly increased, in 

 one instance, from five seconds, as heard at the mouth of the 

 trumpet, to twenty seconds. This would also indicate that 

 the echo is a multiple re-action of varying intensities from 

 different points, and that at the place of the steamer, the 

 fainter ones from a greater distance would be heard, which 

 would be inaudible near the trumpet. 



3. The arc of the horizon, from which the echo appeared 

 to come, was also increased in some cases to more than three 

 times that subtended by the echo at the place of the trumpet. 

 This fact again indicates that the echo consists of multiple 

 sounds from various points at or near the surface of the sea, 

 the angle which the aggregate of these points subtend 

 necessarily becoming greater as the steamer advances. 



But perhaps the most important facts in regard to the echo 

 are those derived from the series of observations on the sub- 

 ject made by Mr. Henry W. Clark, the intelligent keeper of 

 the principal light-house station on Block Island, and by 

 Mr. Joseph Whaley, keeper of the Point Judith light-house. 

 Mr. Clark was furnished with a time-marker to observe the 

 duration of the echo, and both were directed to sound the 

 trumpets every Monday morning for half an hour, noting 

 the temperature, the height of the barometer, the state of 



