138 THUNDER REFRACTION OF SOUND. SECT. XVI. 



tensity, which will be greater still if the surface be spherical and 

 concentric with him. Undulations of sound diverging from one 

 focus of an elliptical shell (N. 180) converge in the other after 

 reflection. Consequently a sound from the one will be heard in 

 the other as if it were close to the ear. The rolling noise of 

 thunder has been attributed to reverberation between different 

 clouds, which may possibly be the case to a certain extent. Sir 

 John Herschel is of opinion that an intensely prolonged peal is 

 probably owing to a combination of sounds, because, the velocity 

 of electricity being incomparably greater than that of sound, the 

 thunder may be regarded as originating in every point of a flash 

 of lightning at the same instant. The sound from the nearest 

 point will arrive first ; and if the flash run in a direct line from 

 a person, the noise will come later and later from the remote 

 points of its path in a continued roar. Should the direction of 

 the flash be inclined, the succession of sounds will be more rapid 

 and Intense : and if the lightning describe a circular curve round 

 a person, the sound will arrive from every point at the same 

 instant with a stunning crash. In like manner the subterranean 

 noises heard during earthquakes like distant thunder may arise 

 from the consecutive arrival at the ear of undulations propagated 

 at the same instant from nearer and more remote points ; or if 

 they originate in the same point, the sound may come by different 

 routes through strata of different densities. 



Sounds under water are heard very distinctly in the air imme- 

 diately above ; but the intensity decays with great rapidity as 

 the observer goes farther off, and is altogether inaudible at the 

 distance of two or three hundred yards. So that waves of sound, 

 like those of light, in passing from a dense to a rare medium, 

 are not only refracted, but suffer total reflection at very oblique 

 incidences (N. 189). 



The laws of interference extend also to sound. It is clear that 

 two equal and similar musical strings will be in unison if they 

 communicate the same number of vibrations to the air in the 

 same time. But if two such strings be so nearly in unison that 

 one performs a hundred vibrations in a second, and the other a 

 hundred and one in the same period during the first few vibra- 

 tions the two resulting sounds will combine to form one of 

 double the intensity of either, because the aerial waves will sen- 

 sibly coincide in time and place ; but one will gradually gain 



