SCIENCE OF COMMON THINGS. 49 



Laws and phenomena of fluids. Water in motion and at rest. 



PAET III. 



THE LAWS AND PHENOMENA OF FLUIDS, 



S91 Into what two classes may all fluid substances "be divided? 



Into liquids, as water, oil, molasses, etc. ; and into 

 gases, as common air, carbonic acid gas, oxygen, and 

 others. 



J393 What designation do we give to those branches of science, which 

 treat of the laws and phenomena of liquids ? 



Hydrostatics, which considers the laws and pheno- 

 mena of water and other liquids in a state of rest ; and 

 hydraulics, which considers the laws and phenomena 

 of liquids in motion. 



S93 What designation do we give to that department of science which 

 treats of the laws and phenomena of gases, and other substances resembling 

 air ? 



We apply the term Pneumatics to that department 

 of science which explains and illustrates those pheno- 

 mena which arise from the weight, pressure, or motion 

 of common air and other gaseous bodies. 



CHAPTEE I. 



WATER IN MOTION AND AT REST. 



S94 When water or any other fluid is at rest, in what condition is its 

 surface ? 



The surface of water at rest is always perfectly level. 



