390 WRITINGS OF JOSEPH HENRY. [1874 



itself to me until in searching the bibliography of sound I 

 found an account of the hypothesis of Professor George G. 

 Stokes, in the proceedings of the British Association for 1856,* 

 in which the effect of an upper current in deflecting the wave 

 of sound so as to throw it down upon the ear of the auditor, 

 or directing it upward far above his head, is fully explained. 

 This subject will be referred to in the subsequent parts of the 

 report, in the attempt to explain various abnormal phenom- 

 ena of sound that have been observed during the series of 

 investigations connected with the Light-House Board. 



During these investigations an attempt was made to ascer- 

 tain the velocity of the wind in an upper stratum as com- 

 pared with that in the lower. The only important result 

 however was the fact that the velocity 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 deter- 

 mined approximately by running a given distance with such 

 speed that a small flag was at rest along the side of its pole. 

 While this velocity was not perhaps 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 considerable importance to ascertain whether 

 waves of sound like the rays of light are absorbed or stifled 

 by fog. On this point however observers disagree. 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 in- 

 stances 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 founds this opinion on observa- 

 tions which he has made on the sound of locomotives of 



*Keport of British Association, 1856; Abstracts, p. 22. 



