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.

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

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.