422 WRITINGS OF JOSEPH HENRY, [1874 



By the application of the principle we have stated and the 

 effect of the wind in connection with the peculiarities of the 

 topography of a region and the position of the sounding 

 body, we think that not only may most of the phenomena 

 we have just mentioned be accounted for, but also that other 

 abnormal effects may be anticipated. 



In critically examining the position of the sounding body 

 in the experiment we have mentioned, in which Sir Fred- 

 erick Arrow and Captain Webb assisted, it was found that 

 the signal was placed on the side of a bank with a large 

 house directly in the rear, the roof of which tended to deflect 

 the sound upwards so as to produce in the rear a shadow, 

 but on account of the divergency of the beam this shadow 

 vanished at the distance of a mile and a half or two miles, 

 and at the distance of say three miles the sound of the in- 

 strument was distinctly heard. I doubt not that on exami- 

 nation all the cases mentioned by General Duane, with one 

 exception, might be referred to the same principle, the 

 exception being expressed in the following remarkable state- 

 ment in his report to the Light-House Board : "The fog-sig- 

 nals have frequently been heard at a distance of twenty miles, 

 and as frequently cannot be heard at the distance of two 

 miles, and with no perceptible difference in the state of the 

 atmosphere. The signal is often heard at a great distance 

 in one direction, while in another it will be scarcely audible 

 at the distance of a mile. For example, the whistle at Cape 

 Elizabeth can always be distinctly heard in Portland, — a 

 distance of nine miles, during a heavy north-east snow- 

 storm, the wind blowing a gale directly from Portland to- 

 ward the whistle." 



This is so abnormal a case, and so contrary to generally 

 received opinion, that I hesitated to have it published under 

 the authority of the board until it could be verified and 

 more thoroughly examined. In the observations that have 

 been made under my immediate supervision the sound has 

 always been heard farther with the wind than against it. 

 It would appear therefore from all the observations that the 

 normal effect of the wind when blowing directly against 

 the sound, is to greatly diminish it. 



