VI ArPENDIX, 



only may most of the phenomena we have just mentioned be ac- 

 counted for, but also that other abnormal effects maybe anticipated. 



In critically examining the position of the sounding body in 

 the experiment we have mentioned, in which Sir Frederic 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 gay three miles the 

 sound of the instrument was distinctly heard. 1 doubt not that, 

 on examination, all the cases mentioned by General Duane, with 

 one exception, might be referred to the same principle, the excep- 

 tion being expressed in the following remarkable statement in his 

 report to the Light House Board : " The fog-signals have fre- 

 quently been heard at a distance of twenty miles and as frequently 

 cannot be heard at the distance of two miles, and with no percepti- 

 ble difference in the state of the atmosphere. The signal is often 

 heard at a greater 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 Port- 

 land — a distance of nine miles — during a heavy northeast snow 

 storm, the wind blowing a gale nearly from Portland towards the 

 whistle." 



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

 ceived opinion, that I hesitated to have it published under the 

 authority of the Board until it could be verified and more 

 thoroughly examined. In all the observations that have been 

 made under my immediate supervision, the sound has always- 

 been heard further with the wind than against it. It would 

 appear, therefore, from all the observations that the normal effect 

 of the wind is to diminish the sound in blowing directly against it. 



There is, however, a meteorological condition of the atmosphere 

 during a northeast storm on our coast which appears to me to 

 have a direct bearing on the phenomenon in question. It is this : 

 that, while a violent wind is blowing from the northeast into the 

 interior of the country, a wind of equal intensity is blowing in an 

 opposite direction at au elevation of a mile or two. This is shown 

 by the rapidly eastwardly motion of the upper clouds as occasion- 

 ally seen through breaks in the lower. 



As a further illustration of this principle I may mention that 

 on one occasion (in 1855) I started, on my way to Boston from 

 Albany, in the morning of a clear day with a westerly wind. The 

 weather continued clear and pleasant until after passing the Con- 

 necticut River, and until within fifty miles of Boston. We then 

 encountered a storm of wind and rain which continued until 

 we reached the city. On inquiry I learned that the storm had 

 commenced in Boston the evening before, and, although the wind 



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