1875] WRITINGS OF JOSEPH HENRY. 457 



zenith in which the sonorous impulse was strongest, there 

 must have been reverberations from the surface of the land 

 and the ocean. This will be evident when we consider the 

 great divergency of sound by which sonorous waves from a 

 vertical trumpet are thrown down to the plane of the hori- 

 zon on every side, some of which meeting oblique surfaces 

 must be reflected back to the ear of the observer near the 

 source of the sound. This inference will be more evident 

 when it is recollected that the reflected rays of sound diverge 

 as well as those of the original impulse. Hence reflection from 

 the surface of the sea is a true cause of the echo, but whether 

 it be a sufficient one may require further investigation. For 

 this explanation it is not necessary that the sea should be 

 covered with crested waves; a similar effect would take place 

 were the surface perfectly smooth but in the form of long 

 swells, which in places exposed to an open sea are scarcely 

 ever absent. Moreover the increased loudness of the echo is 

 a fact in accordance with the same view. 



The observations were repeated with the same effect on 

 succeeding days, until this class of experiments was ended by 

 the bursting of the India-rubber tube. Had a distinct echo 

 been heard from the zenith the result would have been de- 

 cidedly in favor of the hypothesis of a reflection from the 

 air; but as this was not the case the question still remained 

 undetermined, especially since the atmosphere during these 

 experiments was evidently in a homogeneous condition. We 

 do not agree however in the position taken in the report of 

 the Trinity Board, that on the origin of this echo depends 

 the whole solution of the problem as to the efficient cause of 

 the abnormal phenomena of sound. The ingenious experi- 

 mental illustrations of the reflection of sound from a flame 

 or heated air establish clearly the possibility of such reflec- 

 tion; but it must be remembered that they were made under 

 exaggerated conditions, the atmosphere being in a state of 

 extreme rarefaction in a limited space, and the sound of a 

 feeble character, while the phenomena in nature are pro- 

 duced with a comparatively small difference of temperature 

 and with powerful sounds. 



