CHAPTEE II. 



Hydrostatics Principles Pressure Intensity Transmissibility of Pressure Atmospheric 

 Pressure Pressure Gauges- Resultant Pressure Centre of Pressure Equilibrium of 

 Floating Bodies Metacentric Height Oscillations of Ships Strength of Pipes and 

 Cylinders. 



ART. "5. HYDROSTATICS. 



COMMONLY, any substance which, at ordinary temperatures, possesses 

 in a marked degree the property of accommodating itself to the shape of 

 any vessel into which it may be placed, is termed a fluid. 



Fluids may be divided into two classes ; gases and liquids ; according as 

 they are easily, or with difficulty, compressible. 



Definition of a Perfect Fluid. By a perfect fluid we mean a substance 

 such that the pressure exerted by it on any surface with which it may be 

 in contact is everywhere normal to that surface. 



The laws governing the action and reaction, and generally the statical 

 equilibrium, of such a fluid may be easily deduced from theoretical con- 

 siderations. Before, however, extending these laws to the case of such a 

 fluid as water, it becomes necessary to determine to what extent this 

 differs, in its essentials, from our conceptions of a perfect fluid. 



From the above definition it follows that with a perfect fluid any action 

 of the nature of friction between solid and fluid is impossible, since this 

 would necessitate the action of some force tangential to the surface. It 

 follows that there can be no frictional resistance to the motion of a solid 

 body through the fluid, or to the steady motion of the fluid through any 

 pipe or channel having solid boundaries, and that in consequence any 

 portion of the fluid may be separated from any other portion by a force 

 however small, if applied for a sufficient length of time. Further, the 

 perfect fluid is incapable of existing in a state of tension. 



Such a conception is useful, although no such substance as a perfect 

 fluid is known in Nature. All known fluids, in virtue of their properties 

 of cohesion and viscosity, offer some resistance of the nature of friction 

 to the motion of any solid surface with which they may be in contact. 



Evidently then the laws governing the behaviour of a perfect fluid when 

 in motion, are not applicable, without some modification, to water ; nor 

 are the laws governing its statical equilibrium applicable to that of water 



