472 WRITINGS OF JOSEPH HENRY. [1875 



which was heard came from all points of the horizon. On 

 one occasion General Woodruff called attention to a small 

 cloud passing directly over the zenith, from which a few 

 drops of rain fell upon the platform on which the light-house 

 is erected. Advantage was taken of this occurrence to 

 direct strong blasts of the siren toward the cloud, but no 

 perceptible echo was returned. We have failed therefore 

 in this series of investigations to obtain any postive facts in 

 addition to those already known as to the character of the 

 echo. In regard to the hypothesis offered for its explana- 

 tion, if we found little in its support, we have met with noth- 

 ing to invalidate it. But whatever may be the cause of the 

 phenomenon, we do not consider it an important factor in 

 explanation of the results we have obtained, since it was too 

 feeble to produce any effect in the way of absorbing any 

 notable part of the original sound. Its importance from 

 Dr. Tyndall's point of view is its apparent support of the 

 hypothesis of a flocculent condition of the atmosphere. 



Observations on Effect of Elevation on Audibility. — The 

 next class of experiments at Little Gull Island had rela- 

 tion to the effect of elevation on sound. The conditions 

 here however for arriving at definite results on this point 

 were by no means so favorable as those at Block Island. 

 The height which could be commanded was only that of the 

 tower of the light-house, the gallery of which is 74 feet above 

 the platform upon which the buildings are erected, and 92 

 feet above the level of the sea, — much less than that at Block 

 Island. Besides this, the variableness of the wind at the 

 surface of the ocean and at heights above was not favorable 

 for the illustration of the point in question. 



The theoretical conditions in order that the sound may be 

 heard with greater distinctness at an elevation than below, 

 are (as we have said before,) that the wind be moving with 

 a greater velocity in a given direction at an elevation than 

 at the surface of the earth, and that the difference in the 

 velocities may be against the sound-wave, so that its upper 

 part may be more retarded than the lower. In this case the 

 direction of a beam of sound will be curved upward, leaving 

 as it were a vacuum of sound beneath. The distance of the 



