454 WRITINGS OF JOSEPH HENRY. [1875 



Observations in regard to the Aerial Echo. — This phenom- 

 enon has been frequently observed in the researches of the 

 Light-House Board, in case of powerful sounds from the 

 siren and from the fog-trumpet.* It consists of a distinct 

 reflection of sound as if from a point near the horizon in the 

 prolongation of the axis of the trumpet. The question of 

 the origin of this echo has an important bearing — according 

 to Dr. Tyndall, on the explanation of the abnormal phenom- 

 ena of sound we have mentioned. He refers it to the non- 

 homogeneous condition of portions of the air which reflect 

 back the waves of sound in accordance with the analogy of 

 the reflection of light at the common surface of two media 

 of different densities. We have adopted the provisional 

 hypothesis that it is due to the reflection from the waves and 

 the larger undulations of the surface of the ocean in connec- 

 tion with the divergency of beams of powerful sounds. To 

 bring these hypotheses to the test of a crucial experiment, 

 arrangements were made under the direction of Mr. Brown 

 to change the direction of the axis of one of the sirens from 

 the horizontal to the vertical position. 



August 5, 1875. — The first observations were made August 

 5, with the siren in its usual horizontal position, while the 

 air was so charged with fog as to render the sound of the in- 

 strument necessary for the guidance of the mariner, the 

 image of the sun being obscured and the land invisible 

 from the sea. Under these conditions an echo was heard 

 when the pressure of the steam reached 50 pounds per 

 square inch. The reflection in this case (as usual) was 

 from a point in the sea-horizon in the prolongation of the 

 axis of the trumpet. It was not however heard more dis- 

 tinctly when standing near the origin of the sound than 



* The same phenomenon is mentioned by Froissart in his account of the 

 embarkation of the expedition of the French and English to the coast of 

 Africa to assist the Genoese against the pirates in 1390. "It was a beauti- 

 ful sight," says the chronicler, "to view this fleet, with the emblazoned 

 banners of the different lords fluttering in the wind, and to hear the minstrels 

 and other musicians sounding their pipes, clarions, and trumpets, whose 

 sounds were re-echoed back by the sea." (See Illustrations of Froissart by 

 H. N. Humphrey, Plate iv.) 



