516 RESEAECHES IN SOUND. 



versify, Washington, D. C.5 Mr. T. Brown, of Few York, patentee of 

 tlie siren; Mr. Edw. Woodruff, assistant superintendent of construction ; 

 and Captain Keeney, commander of tlie liglit-liouse steamer Mistletoe. 

 They arrived at Block Island on the afternoon of the 4th of August, 

 1875. This place was chosen as the site of the experiments, first, on 

 account of its insular position, being as it were in the i^rolongation of 

 the axis of Long Island, distant fifteen miles from the most easterly 

 part of the latter, and entirely exposed to the winds and waves of the 

 Atlantic Ocean; and, secondly, because there are on Block Island two 

 light-houses, one of which is of the first order, and connected with it are 

 two fog-signals, one of them with the latest improvements. (See Fig. 4.) 



Observations in kegaed to the Aerial Echo. 



This phenomenon has been frequently observed in the researches of 

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

 from the fog-trumi)et.* It consists of a distinct reflection of sound as if 

 from a point near the horizon in the i^rolongation of the axis of the trum- 

 pet. 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. Vv^c have adopted, as a pro- 

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

 the larger undulations of the surfjxce of the ocean, in connection with 

 the divergenc}^ of beams of powerful sounds. To bring these hypotheses 

 to the test of a crucial exi^eriment, 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. 



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 instrument 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 iirolongation of the 

 axis of the trumi^et. It was not, however, heard more distinctly when 

 standing near the origin of the sound than at several hundred feet on 

 either side of it. The interval between the cassation of the original 

 sound and the commencement of the echo was not as marked as in some 

 previous observations, not being more than four or five seconds. The 



* The same pliciioinenon is mentioued by Froissart iu his-account of tlie enibarkatioii 

 of tlic expedition of the French and English to the coast of Africa to assist the Genoese 

 against the pirates in 1390, " It was a beautifnl sij^ht," says the chronicler, " to view 

 this fleet, with the emblazoned banners of the dili'erent lords liuttering iu the wind, 

 and to hear the minstrels and other mnsieiaus sounding their pipes, clari(uis, and trum- 

 pets, whose sounds were xe-echoed back by the sea." (See Illustrations of Froissart 

 by H. N. Humphrey, Plate IV.) 



