76 PHYSICAL SCIENCE BK. n 



clouds collide, the sounds may be expected to 

 correspond in volume to the greatness of the 

 encountering bodies. 



XXVIII 



1 BUT clouds, says some one, are seen striking upon 

 mountains without causing any sound. How is 

 that consistent with your theory? Well, in the 

 first place, a sound is not caused by any and every 

 method of cloud collision, but only when there is 

 an arrangement of their position suitable for pro 

 ducing a sound. Striking the backs of the hands 

 does not produce clapping, but the contact of palm 

 with palm does. It makes a great difference, too, 

 whether the clouds that strike are hollow, or flat 

 and extended. In the second place, the clouds 

 must not merely drift, as against a mountain, but 



2 be driven with great tempestuous violence. Besides, 

 a mountain does not cut through a cloud, it merely 

 disperses it by displacing the successive front layers 

 of it. Even a bladder does not give a report 

 irrespectively of the method in which it emits the 

 air in it ; it depends on the way in which the air 

 escapes. If the bladder is cut with a knife, the air 

 is emitted without the ear perceiving it. It must 

 be burst, not cut, in order to give a report. The 

 same, I assert, holds in regard to the clouds : they 

 emit no peal unless broken up with great violence. 



3 Besides, clouds driven against a mountain are not 

 broken up, but merely pour round certain parts 

 of the mountain, tree branches, shrubs, and rough 

 projecting boulders. They are rent thereby, and 

 emit by numerous exits whatever air they may 

 contain ; but there is no rattle unless the air all 



