The theorems of hydrostatics are thus true for all stationary fluids, however viscous they may be; it is only when we come to hydrodynamics, the science of the motion of a fluid, that viscosity will make itself felt and modify the theory; unless we begin by postulating the perfect fluid, devoid of viscosity, so that the principle of the _normality of fluid pressure_ is taken to hold when the fluid is in movement. 5. _The Measurement of Fluid Pressure._--The pressure at any point of a plane in the interior of a fluid is the intensity of the normal thrust estimated per unit area of the plane. Thus, if a thrust of P lb. is distributed uniformly over a plane area of A sq. ft., as on the horizontal bottom of the sea or any reservoir, the pressure at any point of the plane is P/A lb. per sq. ft., or P/144A lb. per sq. in. (lb./ft.² and lb./in.², in the Hospitalier notation, to be employed in the sequel). If the distribution of the thrust is not uniform, as, for instance, on a vertical or inclined face or wall of a reservoir, then P/A represents the average pressure over the area; and the actual pressure at any point is the average pressure over a small area enclosing the point. Thus, if a thrust [Delta]P lb. acts on a small plane area [Delta]A ft.² enclosing a point B, the pressure p at B is the limit of [Delta]P/[Delta]A; and p = lt([Delta]P/[Delta]A) = dP/dA, (1) in the notation of the differential calculus. 6. _The Equality of Fluid Pressure in all Directions._--This fundamental principle of hydrostatics follows at once from the principle of the normality of fluid pressure implied in the definition of a fluid in § 4. Take any two arbitrary directions in the plane of the paper, and draw a small isosceles triangle abc, whose sides are perpendicular to the two directions, and consider the equilibrium of a small triangular prism of fluid, of which the triangle is the cross section. Let P, Q denote the normal thrust across the sides bc, ca, and R the normal thrust across the base ab. Then, since these three forces maintain equilibrium, and R makes equal angles with P and Q, therefore P and Q must be equal. But the faces bc, ca, over which P and Q act, are also equal, so that the pressure on each face is equal. A scalene triangle abc might also be employed, or a tetrahedron. [Illustration: FIG. 1a.] It follows that the pressure of a fluid requires to be calculated in one direction only, chosen as the simplest direction for convenience. 7. _The Transmissibility of Fluid Pressure._--Any additional pressure applied to the fluid will be transmitted equally to every point in the case of a liquid; this principle of the _transmissibility of pressure_ was enunciated by Pascal, 1653, and applied by him to the invention of the _hydraulic press_.