PHILOSOPHICAL SOCIETY OF WASHINGTON. 33 



* " There are six steam fog-whistles on the coast of Maine ; there have been 

 frequently heard at a distance of twenty miles, and as frequently cannot be heard 

 at the distance of two miles, and this with no perceptible difference in the state 

 of the atmosphere. 



" The signal is often heard at a great distance in one direction, while in an- 

 other it will be scarcely audible at the distance of a mile. This is not the effect 

 of wind, as the signal is frequently heard much farther against the wind than 

 with it ; for example, the whistle on Cape Elizabeth can always be distinctly 

 heard in Portland, a distance of nine miles, during a heavy northeast snow-storm 

 the wind blowing a gale directly from Portland toward the whistle." 

 * * * * *■***# t * 



" The most perplexing difficulty, however, arises from the fact that the signal 

 often appears to be surrounded by a belt, varying in radius from one to one and 

 a half miles, from which the sound appears to be entirely absent. Thus, in mov- 

 ing directly from a station, the sound is audible for the distance of a mile, is then 

 lost for about the same distance, after which it is again distinctly heard for a long 

 time. This action is common to all ear-signals, and has been at times observed at 

 all the stations, at one of which the signal is situated on a bare rock twenty miles 

 from the main land, with no surrounding objects to affect the sound." 



Prof. Henry, in considering the results of Gen. Duane's experi- 

 ments, and his own, some of which were made in company with Sir 

 Fred'k Arrow and Capt. Webb, H. B. M. Navy, both of the British 

 Light-House Establishment, who were sent here to study and report 

 on our fog-signal system, formulated these abnormal phenomena. 

 He said they consisted of: 



" I. The audibility of a sound at a distance and its inaudibility nearer the 

 source of sound. 



" 2. The inaudibility of a sound at a given distance in one direction, while a 

 lesser sound is heard at the same distance in another direction. 



" 3. The audibility at one time at a distance of several miles, while at another 

 the sound cannot be heard at more than a fifth of the same distance. 



" 4. While the sound is generally heard further with the wind than against it, 

 in some instances the reverse is the case. 



" 5. The sudden loss of a sound in passing from one locality to another in the 

 same vicinity, the distance from the source of sound being the same." -(• 



These experiments were not confined to our own shores. Dr. 

 Tyndall, the well known English physicist, who stands in the same 

 relation to the British Light-House Establishment that Prof. Henry 

 did to our own, writes thus : 



* Annual Rep't L. H. Board 1874, pp. 99-100. 

 t L. H. B. Annual Rep. 1875, page 106. 



