CAUSE. 327 



to the facility with which its particles, like 

 those of all gases, move over or between each 

 other, at the application of the least force. If we 

 do but gently breathe upon the thick and num- 

 berless particles forming 



" The gay motes that people the sunbeams," 



they are instantly thrown into the most violent 

 commotion, and the disturbance does not cease, 

 in consequence of the small amount of friction, 

 for a very long period. 



Part also are due to the effect of heat upon 

 air. A body heated, whether naturally or arti- 

 ficially, instantly causes motion in the particles 

 of air by which it is surrounded. If, for ex- 

 ample, a cannon ball were heated to redness 

 and placed on a tripod, as in the cut, it would 

 cause the surrounding air 

 to be in rapid motion. A 

 stream of hot air, upon the 

 principles before alluded to, 

 would rise up, and a stream 

 of colder air would be con- 

 tinually called to occupy its 

 place. In this manner a 

 current would be set up 

 which would somewhat re- 

 semble, could its direction be rendered visible, 

 the appearance shown in the cut. Therefore 



