§ 9. _Relation of Pressure, Volume, Temperature and Density in
Compressible Fluids._--Certain problems on the flow of air and steam
are so similar to those relating to the flow of water that they are
conveniently treated together. It is necessary, therefore, to state as
briefly as possible the properties of compressible fluids so far as
knowledge of them is requisite in the solution of these problems. Air
may be taken as a type of these fluids, and the numerical data here
given will relate to air.
[Illustration: FIG. 3.]
_Relation of Pressure and Volume at Constant Temperature._--At
constant temperature the product of the pressure p and volume V of a
given quantity of air is a constant (Boyle's law).
Let p0 be mean atmospheric pressure (2116.8 lb. per sq. ft.), V0 the
volume of 1 lb. of air at 32° Fahr. under the pressure p0. Then
p0V0 = 26214. (1)
If G0 is the weight per cubic foot of air in the same conditions,