494 EESEAECHES IN SOUND. 



erally as it passes from its source, is much greater than has been sup- 

 posed from experiments on a small scale. The idea we wish to convey 

 by this is that a beam of sound issuing through an orifice, although at 

 first i)roceeding like a beam of light in paralled rays, soon begins to 

 diverge and spread out into a cone, and at a sufficient distance may in- 

 clude even the entire horizon. 



We may mention also in this connection that from the general fact 

 expressed by the divergence of the rays of sound, the application of 

 reflection as a means of reinforcing sound must in a considerable degree 

 of necessity be a failure. 



By tlie 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 eft'ects may be anticipated. 



In critically examining the position ofthe sounding body in the experi- 

 ment we have mentioned, in which Sir Frederick Arrow and Captain Webb 

 assisted, it was found that the signal was placed on the side of a bank 

 with a large house dkectly 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 instrument was distmctly heard. I doubt not that, on 

 examination, all the cases mentioned by General Duane, with one ex- 

 ception, might be referred to the same principle, the exception being 

 expressed in the following remarkable statement in his report to the 

 Light-House Board: "The fog-signals have frequently been beard at a 

 distance of ticenty miles and as frequently cannot be heard at the distance 

 of two miles, and with no perceptible difference in the state of the atmos- 

 phere. 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 Portland — 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 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 all the 

 observcitions that have been made under my immediate supervision, the 

 sound has always been heard ftirther tvith the wind than against it. It 

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

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



There is however a meteorological condition of the atmosphere dur- 

 ing a northeast storm on our coast which api)ears 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 ofthe country, 

 a wind of equal intensity is blowing in an opposite direction at an ele- 



