This mechanical axiom of the **normality** of fluid pressure is the foundation of the mathematical theory of hydrostatics.

Standard solutions are prepared by weighing out the exact amount of the pure substance and dissolving it in water, or by forming a solution of approximate **normality**, determining its exact strength by gravimetric or other means, and then correcting it for any divergence.

In considering the motion of a fluid we shall suppose it non-viscous, so that whatever the state of motion the stress across any section is normal, and the principle of the **normality** and thence of the equality of fluid pressure can be employed, as in hydrostatics.

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.