2 ELEMENTARY PRINCIPLES OF FLUID PRESSURE. 



2. A Fluid is a body so constituted, that its parts are all ready to yield 

 to the action of the smallest force or pressure, in whatsoever direction 

 it may be exerted. The following are some of the simplest and most 

 obvious properties of fluids.* 



3. Every particle of a fluid presses equally in all directions, whether 

 it be upwards or downwards, laterally or obliquely; consequently, the 

 lateral pressure of a fluid is equal to its perpendicular pressure. The 

 converse of this is equally obvious, and is thus expressed. 



4. Every particle of a fluid in a state of quiescence, is pressed equally 

 in all directions. 



5. When a fluid is m a state of rest, the pressure exerted against the 

 surface of the vessel which contains it, is perpendicular to that surface. 



6. When a mass of fluid is in a state of rest, its surface is horizontal, 

 or perpendicular to the direction of gravity. 



7. If two fluids which do not mix, are poured into the same vessel, 

 and suffered to subside, their common surface is parallel to the horizon ; 

 consequently the surfaces of fluids continue horizontal, when sub- 

 jected to the pressure of the atmosphere. 



8. The particles of a fluid, situated at the same perpendicular depth 

 below the surface, are equally pressed. 



9. When a fluid is in a state of rest, the pressure upon any of its 

 constituent elements, wheresoever situated, is equal to the weight of 

 a column of fluid particles, whose length is equal to the perpendicular 

 depth of the particle or element pressed ; consequently, the pressure 

 on any particle varies ^s its perpendicular depth, and in any vessel 

 containing a fluid in a state of rest, the parts that are deepest sustain 

 the greatest pressure. 



These principles, which flow immediately from the conditions of 

 fluidity, are too simple and obvious to require demonstration, yet 

 nevertheless, the writers on hydrostatical science generally accompany 

 them with a sort of popular proof, which may be found in almost 

 every treatise that has appeared on the subject. But our immediate 

 object being to unfold the more important elementary principles, by 

 the resolution of a series of examples dependent upon one general 

 proposition, we have thought it unnecessary to exhibit the demonstra- 

 tions here (Note A). The general proposition is as follows : 



* Fluids are generally divided into two sorts, compressible and incompressible, or 

 elastic and non-elastic ; the latter of which, or incompressible and non-elastic fluids, 

 such as water, mercury, wine, &c., form the subject of the present article ; the dis- 

 cussion of the compressible and elastic fluids, is reserved for another place. The 

 compressibility of water is so small, that in all practical operations in mechanics 

 its bulk or mass may generally be considered unalterable : for at a thousand fathoms 

 depth it can only be compressed one-twentieth of its bulk at the surface. 



