CHAPTER XXVII 
HYDROSTATICS 
$60. Fluids.—Fluids are of two kinds, Liquids and Gases. A fluid 
is not capable of resisting change of shape or volume unless it is con- 
strained by the sides of a vessel surrounding it. 
For instance, if a cylinder (Fig. 700), closed at one 
end and fitted with a frictionless piston, have the 
_ space between the piston and the closed end of the 
cylinder full of fluid, the piston will support a 
_ force P tending to push it further into the cylinder, 
but if a hole be made in the side of the lide FER: Dn FP TOK: 
: (Fig. 701), a force P, however small, will be sufficient to push the piston 
in and cause the escape of the fluid through the hole. 
, If a definite volume of a liquid be placed in a vessel of greater volume 
the liquid will only occupy a portion of the vessel equal to the original 
 yolume of the liquid, but if a quantity of a gas, however small, be 
introduced into a vessel, however large, it will expand and fill the whole 
of the vessel. 
A liquid when confined, as in Fig. 700, offers a very great resistance 
to decrease in volume ; in fact, for the purposes of the engineer, a liquid 
can in general be taken as incompressible. Water, for example, loses 
- 0:007 of its original volume for each ton of pressure per square inch 
_ applied to it, and the compressibility of mercury is only about one-tenth 
that of water. A gas, on the other hand, is, within certain wide limits, 
_ readily compressible. 
A fluid is said to be more fluid or more like a perfect fluid the less 
_ the friction of its particles on one another, or the less the resistance which 
it offers to a body moving through it. A fluid is said to be more viscous 
_ the greater the friction of its particles on one another, or the greater the 
resistance which it offers to a body moving through it. The viscosity of 
a fluid does not affect its equilibrium, but it does affect its motion. 
The branch of mechanics which treats of the equilibrium of fluids and 
the forces acting on them when at rest is called hydrostatics. That part 
of hydrostatics which deals with gases is called pnewmatics. 
_ 361. Direction of Fluid Pressure on a Surface-—The pressure of 
a fluid on a surface is perpendicular to that surface at 
every point (Fig. 702). This is true for viscous as well 
as for non-viscous fluids at rest. A viscous fluid in 
motion exerts a slight tangential force on a surface over 
_ which it is moving, and the pressure of the fluid on 
~ the surface will therefore not be perpendicular to the 
_ surface in this case. . Fre. 702. 
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