

THE SENSE OF HEARING. 237 



thirty feet in a second. This explains why, when a gun is 

 fired at a distance over the surface of a frozen lake, we hear 

 two reports after we see the flash ; first a sharp and loud 

 one, transmitted by the solid ice, and then a weaker and 

 duller one, through the air. Franklin, however, found by 

 his experiments, that, after travelling about a mile through 

 the water, sound lost some of its intensity, which indeed 

 might be expected. Musical tones are said to be acute when 

 the intervals between the vibrations are short, and grave 

 when they are long. Thus a flute called an octave produces 

 a shriller sound than the common flute ; a fiddle than a bass 

 viol. The strings, too, of a violin, which are designed for 

 high or acute notes, are smaller than the others, that their 

 vibrations may be more rapid ; while those which make the 

 grave tones are large, and wound round sometimes with fine 

 wire, to increase the weight, and make them vibrate more 

 slowly. It is the quality and variety of the sounds which, 

 in musical tones, gives the hearer so much pleasure. 



21. In the lower order of animals, hearing is performed 

 by means of an apparatus, much more simple than in man. 

 Some of them have merely a membranous sac, supplied with 

 nervous threads. This is even the case in fishes, which re- 

 quire neither tympanum nor bones, nor any of the accessory 

 parts found in land animals, as the undulations of water strike 

 with greater force upon the organ of hearing than those of 

 air. The apparatus of hearing in the frog is very singular, 

 and designed so as to enable the animal to hear both in air 

 and water. 



22. /Animals with long ears are able to move them by 

 musclos for that purpose, and turn them to the point whence 

 the sound proceeds. This may be seen in the horse, which 

 turns his ear always in the direction of the sound. In stage 

 horses, we often see the leaders turn their ears forward, 

 while those behind turn theirs backward. Some men also 

 have the power of moving their ears! 



23. Like all our other senses, that of hearing is capable 



