EESEAKCHES IN SOUND. 471 



east storm, which sometimes rages for several days at a given point on 

 the coast without being felt more than a few miles in the interior, the air 

 continuously flowing in below and going out above. Indeed, in such 

 cases a break in the lower clouds reveals the fact of the existence above 

 of a rapid current in the opposite direction. 



The full significance, however, of this idea did not reveal itself to me 

 nntil in searching the bibliography of sound I found an account of the 

 hypothesis of Professor Stokes in the Proceedings of the British Associa- 

 tion for 1850, * in which the effect of an upper current in deflecting the 

 wave of sound so as to throw it down npon the ear of the auditor, or 

 directing it npward far above his head, is fully exijlained. This subject 

 will be referred to in the subsequent parts of the report, in the attempt 

 to explain various abnormal phenomena of sound which have been ob- 

 served during the series of investigations connected with the Light- 

 House Board. 



During these investigations an attempt was made to ascertain the 

 velocity of the wind in an ujiper stratum as compared with that in the 

 lower. The only important result however, was the fact that the veloc- 

 ity of the shadow of a cloud passing over the ground was much greater 

 than that of the air at the surface, the velocity of the latter being de- 

 termined approximately by running a given distance with such speed 

 that a small flag was at rest along the side of its pole. \Yhile this 

 velocity was not i^erhaps greater than six miles per hour, that of the 

 shadow of the cloud was apparently equal to that of a horse at full 

 speed. 



During this and subsequent investigations, inquiries were made in 

 regard to the effect of fog upon sound, it being a subject of considera- 

 ble importance to ascertain whether waves of sound, like the rays of 

 light, are absorbed or stifled by fog. On this point, however, observers 

 disagree. At first sight, from the very striking analogy which exists in 

 many respects between sound and light, the opinion largely prevails 

 that sound is impeded by fog; although observers who have not been 

 influenced by this analogy have, in many instances, adopted the opposite 

 opinion, that sound is better heard during a fog than in clear weather. 

 For instance, the Rev. Peter Ferguson, of Massachusetts, informs me 

 that from his own observations, sound is conveyed farther in a fog than 

 in a clear air. He fonnds this opinion on observations which he has 

 made on the sound of locomotives of several railways in ])assing over 

 bridges at a distance. Unfortunately, the question is a dilflcult one to 

 settle, since the effect of tbe wind, in order to arrive at a true result, 

 must be carefully eliminated. Captain Keeney, who has i)reviously 

 been mentioned, related the following occurrence, in the first part of 

 which he was led to suppose that fog had a very marked influence in 

 deadening sound, thongh in a subsequent part he came to an opposite 

 conclusion : He was sailing during a dense fog, with a slight wind bear- 



* Report of Britisli Association, 1856; Abstracts, p. 22. 



V 



