352 MEMORIAL OF JOSEPH HENRY. 



angles to the direction of the wind, gave a very close equality for 

 the reciprocal durations of the sound. In the following month, 

 similar observations were made at Little Gull island, which were 

 very accordant with those made at the former station. As a result 

 of plotting the ranges of audibility in different directions from a 

 given point, producing a series of circular figures (more or less 

 distorted) of very different sizes, Henry was inclined to believe 

 that the whole area of audition is less in high winds than in gentle 

 winds. These investigations as their author well remarks, 

 "though simple in their conception, have been difficult and laborious 

 in their execution. To be of the greatest practical value they 

 were required to be made on the ocean under the conditions in 

 which the results are to be applied to the use of the mariner, and 

 therefore they could only be conducted by means of steam vessels 

 of sufficient power to withstand the force of rough seas, and at 

 times when these vessels could be spared from other duty. They 

 also required a number of intelligent assistants skilled in observa- 

 tion and faithful in recording results." * 



In the summer of last year, 1877, with undiminished ardor, he 

 continued his observations on sound; selecting this time Portland 

 harbor, Monhegan island, and Whitehead light station, on the coast 

 of Maine. At the latter station, the abnormal phenomenon of a 

 region of inaudibility near the fog-signal, and extending outward 

 for two or three miles, (beyond which distance the signal is again 

 very distinctly heard,) had for several years been frequently 

 observed. This singular effect is noticed only in the case of a 

 southerly wind when the vessel is approaching the signal from 

 the same quarter, and consequently with the wind adverse to the 

 direction of the sound-beams, a condition of the wind which is 

 the usual accompaniment of a fog. The observation showed this 

 intermediate "belt of silence" to be well marked on board the 

 steamer both on approaching the station and on receding from it 

 by retracing the same line of travel. Meanwhile the intermittent 

 signal whistle from the steamer was distinctly heard at the station 

 on both the outward and homeward trips of the vessel, throughout 

 its course. The next set of observations was made on the opposite 



* Report of the Light-House Board for 1875, p. 107. 



