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yield to any force impressed on it, and by yielding are easily 

 moved among themselves. This is Newton s definition. It 

 includes gases and aeriform bodies, as well as those to 

 which we, in ordinary conversation, apply the terms &quot;fluid&quot; 

 or &quot; liquid.&quot; 



The fundamental idea of a fluid is, that of a body whose 

 particles may be moved amongst each other on the appli 

 cation of the slightest possible force. It is therefore 

 directly opposed to a rigid body, whose definition is that 

 its particles cannot be moved amongst each other, no 

 matter how great a force is applied. It is evident that 

 no substance that we meet with in nature is strictly either 

 a fluid or a rigid body ; but they approach more or less to 

 the one or the other. When they partake more of the 

 fluid than the rigid nature, they are called &quot; viscous ;&quot; 

 when the contrary, they are called (&amp;lt; solid.&quot; These two 

 are therefore indefinite terms, and no clear boundary can 

 be drawn between them. 



2. The science of Hydrostatics is divided into two parts. 

 In one we assume certain general principles as the grounds 

 of all our reasoning. We may consider these as established 

 either by experiment, or as truths which it is the office of 

 the other part of the science to demonstrate. In the other 

 we make certain general assumptions as to the constitution 

 of a fluid, and then we attempt to deduce from these the 

 general principles on which all the rest of hydrostatics is 

 founded. This division occurs in most mechanical sciences. 

 Thus, in Geometrical Optics, we assume the laws of 

 reflection and refraction ; it is the part of Physical Optics 

 to establish their truth. It is not here our office to enter 

 into the science of Molecular Hydrostatics; we must post 

 pone, therefore, such consideration to a future chapter. 



The mathematical theory of Hydrostatics is founded upon 

 two laws. 



