MECHANICS 83 



those by which, as we have seen, rotatory movements of 

 water may be produced.* 



As in liquids, so in aeriform bodies, movements may 

 take place in the form of waves. This is shown by the 

 oscillating movements of the atmosphere near the surface 

 of the earth on a hot, sunny day in summer. Such 

 oscillations are made manifest by the apparent twinkling 

 movement of the objects seen through the oscillating 

 waves of air. 



But these subjects, heat and the light which makes 

 things visible to us, are subjects which pertain not to 

 mechanics, but to those sciences, the elements of which 

 will be briefly introduced to the reader's notice in the 

 next chapter. 



Like liquids, aeriform bodies differ much in density. 

 This it is which makes a balloon rise in the air as bubbles 

 of oil will rise in water. The reason why a balloon rises 

 is that it contains an aeriform fluid so much lighter than 

 air that its whole weight is less than that of the bulk of 

 air it displaces, and thus the relatively heavier air 

 descends, and so presses it upwards. 



* See ante, p. 74. 



