EFFECTS OF ATMOSPHERIC TRANSPARENCY. 219 



same effect is perceived when we look at objects 

 through spirits and water that are not perfectly 

 mixed, or when we view distant objects over a 

 red-hot poker or over a flame. In all these 

 cases the light suffers refraction in passing from 

 a medium of one density into a medium of a 

 different density, and the refracted rays are con- 

 stantly changing their direction as the different 

 currents rise in succession. Analogous effects 

 are produced when sound passes through a mixed 

 medium, whether it consists of two different 

 mediums or of one medium where portions of it 

 have different densities. As sound moves with 

 different velocities through media of different 

 densities, the wave which produces the sound will 

 be partly reflected in passing from one medium 

 to the other, and the direction of the transmission 

 wave changed : and hence in passing through 

 such media different portions of the wave will 

 reach the ear at different times, and thus destroy 

 the sharpness and distinctness of the sound. 

 This may be proved by many striking facts. 

 If we put a bell in a receiver containing a mix- 

 ture of hydrogen gas and atmospheric air, the 

 sound of the bell can scarcely be heard. During 

 a shower of rain or of snow, noises are greatly 

 deadened ; and when sound is transmitted along 

 an iron wire or an iron pipe of sufficient length, 

 we actually hear two sounds, one transmitted 

 more rapidly through the solid, and the other 

 more slowly through the air. The same property 

 is well illustrated by an elegant and easily repeated 

 experiment of Chladni's. When sparkling cham- 

 pagne is poured into a tall glass till it is half full, 

 the glass loses its power of ringing by a stroke 



