RESEARCHES IN SOUND. 555 



increased, the loudness of the echo diminished, contrary to the effect of 

 an echo from, a i^lane 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-wave 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 

 l)arts of which proceed from different jjoints at different distances of the 

 space in front of the trumpet, and that as the steamer advances toward 

 the verge of the horizon, it leaves behind it a number of the i)oiuts from 

 which the louder ones proceed, and thus the eflfect ujjon the ear is dimin- 

 ished 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 ti'umijet, to twenty 

 seconds. 



This would also indicate that the echo is a multiple reaction of vary- 

 ing intensities from difl'erent points, and that at the i^lace 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 multijde sounds from various points at or near the sur- 

 face of the sea, the angle which the aggregate of these points subtend 

 necessarily becoming greater as the steamer advances. 



But perhaps the most imi)ortant facts in regard to the echo are those 

 derived from the series of observations on the subject, made by Mr. 

 Henry W. Clark, the intelligent keeper of the principal light-house sta- 

 tion on Block Island, and by Joseph Whaley, keeper of the Point Judith 

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

 diu'ation of the echo, and both were directed to sound the trumpets every 

 Monday morning for half an hour, noting the temi)erature, the height of 

 the barometer, the state of the weather as to clearness or fog, the direc- 

 tion and intensity of the wind, and the surface of the ocean. 



From the observations made at these two points, for more than two 

 years at one station and over a year at the other, the echo may be con- 

 sidered as produced constantly under all conditions of weather, even 

 during dense fogs, since at Block Island it was heard lOG times out of 

 113, and at Point Judith 50 times out of 57, and on the occasions when 

 it was not heard the wind was blowing a gale, making a noise sulhciently 

 intense to drown the sound of the echo. These results appear to be 

 sufficient to disprove the hypothesis that the phenomenon is produced 

 by an acoustic cloud accidentally situated in the prolongation of the axis 

 of the trumpet. It must be due to something more permanent in its 

 effects than that from a portion of air differing from that of the general 

 atmosphere in temi^erature or density, since such a condition cannot 

 exist in a dense fog embracing all the region of the locality of the phe- 



