293 O,i the Refraction of Sound by the Atmosphere. [Apr. 23 



elevation of 30 feet, although it could be heard distinctly at this latter 

 point a few feet higher. 



As might be expected, the effect of raising the bell was to extend its 

 range to windward, to even a greater extent than was obtained by an 

 equal elevation of the observer. 



These results agree so well with what might be expected from the 

 theory as to place its truth and completeness beyond question. 



It is thus argued that, since the wind raises the sound so that it cannot 

 be heard at the ground, by causing it to move faster below than above, 

 any other cause which produces such a difference in velocity will lift the 

 sound in the same way ; and therefore that an upward diminution in the 

 temperature of the air must produce this effect ; for every degree of tem- 

 perature between 32 and 70 adds nearly one foot per second to the 

 velocity of sound. Mr. Glaisher's balloon observations * show that when 

 the sun is shining with a clear sky, the variation from the surface is 1 

 for every hundred feet, and that with a cloudy sky 0'5, or half what it 

 is with a clear sky. Hence it is shown that " rays " of sound, otherwise 

 horizontal, will be refracted upwards in the form of circles, the radii of 

 which are 110,000 feet with a clear sky, and 220,000 with a cloudy sky 

 that is to say, the refraction on bright hot days will be double what it 

 is on dull days, and still more under exceptional circumstances, and com- 

 paring day with night. 



It is then shown by calculation that the greatest refraction (110,000 

 radius) is sufficient to render sound, from a cliff 235 feet high, inaudible 

 on the deck of a ship at 1^ mile, except such sound as might reach the 

 observer by divergence from the waves passing over his head ; whereas, 

 when the refraction is least (220,000 radius), that is, when the sky is 

 cloudy, the range would be extended to 2| miles, with a similar extension 

 for the diverging waves, and under exceptional circumstances the exten- 

 sion would be much greater. It is hence inferred that the phenomenon 

 which Prof. Tyndall observed on the 3rd of July and other days (namely, 

 that when the air was still and the sun was hot he could not hear guns 

 and other sounds from the cliffs 235 feet high more than 2 miles, 

 whereas when the sky clouded the range of the sounds was extended to 

 3 miles, and, as evening approached, much further) was due, not to the 

 stoppage or reflection of the sound by clouds of invisible vapour, as Prof. 

 Tyndall has supposed, but to the sounds being lifted over his head by 

 refraction in the manner described ; and that, had he been able to ascend 

 30 feet up the mast, he might at any time have extended the range of the 

 sounds by a quarter of a mile at least. 



* Brit, ASBOC. Eeport, 1862, p, 462. 



