Refraction of Sound by the Atmosphere. 547 



recorded by Prof. Tyiidall with his mind altogether unbiased with any 

 thought of establishing this hypothesis. He was looking for an expla- 

 nation in quite another direction. Had it not been so he would probably 

 have ascended the mast, and thus found whether or not the sound was 

 all the time passing over his head. On the worst day an ascent of 

 30 feet should have extended the range nearly | mile. 



The height of the sound-producing instruments is apparently treated 

 as a subordinate question by Prof * Tyndall. At the commencement of 

 his lecture, he stated that the instruments were mounted on the top and 

 at the bottom of the cliff ; and he subsequently speaks of their being 

 235 feet above him. He does not, however, take any notice of the com- 

 parative range of those on the top and those at the bottom of the cliff ; 

 but wherever he mentions them he speaks of them as on the cliff, lead- 

 ing me to suppose that for some reason those at the bottom of the cliff 

 had been abandoned, or that they were less efficient than those above. 

 If I am right in this surmise, if the sounds from below did not range so 

 far as those from above, it is a fact in accordance with refraction, but of 

 which, I think, Prof. Tyndall has offered no explanation. 



[Besides the results of Prof. Tyndall's experiments there are many 

 other phenomena which are explained by this refraction. Humboldt 

 could hear the falls of Orinoco three times as loud by night as by day 

 at a distance of one league ; and he states that the same phenomenon has 

 been observed near every waterfall in Europe. And although Humboldt 

 gave another explanation*, which was very reasonable when applied to 

 the particular case at Orinoco t, } r et it must be admitted that the circum- 

 stances were such as would cause great upward refraction ; and hence 

 there can be but little doubt that refraction had a good deal to do with 

 the diminution of the sound by day. 



In fact if this refraction of sound exists, then, according to Mr. 

 Glaisher's observations, it must be seldom that we can hear distant 

 sounds with any thing like their full distinctness, particularly by day ; 

 and any elevation in the observer or the source of the sound above the 



* " That the sun acts upon the propagation and intensity of sound by the obstacles 

 met in currents of air of different density, and by the partial undulations of the atmo- 

 sphere arising from unequal heating of different parts of the soil During the 



day there is a sudden interruption of density wherever small streamlets of air of a high 

 temperature rise over parts of the soil unequally heated. The sonorous undulations are 

 divided, as the rays of light are refracted wherever strata of air of unequal density are 

 contiguous. The propagation of sound is altered when a stratum of hydrogen gas is 

 made to rise over a stratum of atmospheric air in a tube closed at one end ; and M. 

 Biot has well explained, by the interposition of bubbles of carbonic acid gas, why a glass 

 filled with champagne is not sonorous so long as that gas is evolved and passing through 

 the strata of the liquid." Humboldt' s Travels, Bonn's Series, vol. ii. p. 264. 



t The sounds proceeded over a plane covered with rank vegetation interspersed with 

 black rocks. These latter attained a very considerable elevation of temperature under 

 the effects of the tropical sun, as much as 48 C., while the air was only 28; and hence 

 over each rock there would be a column of hot air ascending. 



YOL. XXII. 2 T 



