THE ATMOSPHERE AND FOG-SIGNALING. 6 99 



us with surprising power, piercing through the whistling and moaning 

 of the wind, which blew through Dover toward Folkestone. The 

 sounds w r ere heard at 6 miles from the Foreland on the Folkestone 

 road, and, had the instruments not then ceased sounding, they might 

 have been heard much farther. At the South Sand Head light-vessel, 

 3f miles on the opposite side, no sound had been heard throughout 

 the day. On the 28th, the wind being north by east, the sounds were 

 heard in the middle of Folkestone, 8 miles off, while in the opposite 

 direction they failed to reach 3f miles. On the 29th the limits of 

 range were Eastware Bay on the one side, and Ivingsdown on the 

 other ; on the 30th the limits were Kingsdown on the one hand, and 

 Folkestone Pier on the other. With a wind having a force of 4 or 5, 

 it was a very common observation to hear the sound in one direction 

 three times as far as in the other. 



This well-known effect of the wind is exceedingly difficult to ex- 

 plain. Indeed, the only explanation worthy of the name is one offered 

 by Prof. Stokes, and suggested by some remarkable observations by 

 De la Roche. In vol. i. of the " Annales de Chemie" for 1816, p. 1*76, 

 Arago introduces De la Roche's memoir in these words : " L'auteur 

 arrive a des conclusions, qui d'abord pourront paraitre paradoxales, 

 mais ceux qui savent combien il mettait de soins et d'exactitude dans 

 toutes ses recherches se garderont sans doute d'opposer une opinion 

 populaire a des experiences positives." The strangeness of De la 

 Roche's results consisted in his establishing, by quantitative measure- 

 ments, not only that sound has a greater range in the direction of the 

 wind than in the opposite direction, but that the range at right angles 

 to the wind is the maximum. 



In a short but exceedingly able communication presented to the 

 British Association in 1857, the eminent physicist above-mentioned 

 points out a cause which, if sufficient, would account for the results 

 referred to. The lower atmospheric strata are retarded by friction 

 against the earth, and the upper ones by those immediately below 

 them ; the velocity of translation, therefore, in the case of wind, in- 

 creases from the ground upward. This difference of velocity tilts the 

 sound-wave upward in a direction opposed to, and downward in a 

 direction coincident with, the wind. In this latter case the direct 

 wave is reenforced by the wave reflected from the earth. Now, the 

 reenforcement is greatest in the direction in which the direct and re- 

 flected waves inclose the smallest angle, and this is at right angles to 

 the direction of the wind. Hence the greater range in this direction. 

 It is not, therefore, according to Prof. Stokes, a stifling of the sound 

 to windward, but a tilting of the sound-wave over the heads of the 

 observers that defeats the propagation in that direction. 



This explanation calls for verification, and I wished much to test 

 it by means of a captive balloon rising high enough to catch the de- 

 flected wave ; but, on communicating with Mr. Cox well, who has 



