After Geometrical Similitude has been retained by making two structures, animal, machine, or steamship, from the same drawing but to a different scale, there is still the remaining variable of the material of construction to be taken into account ; and a further question arises of the Mechanical Similitude in a calculation of the relative velocity and stress in the parts.

The first statement of the problem is to be found in the Principia, Book II, Section VII, Prop. XXXII, Theorem XXVI.

In its application to the steamship the principle goes by the name of Froude's Law; it is applied to infer the performance of the full size steamer by calculation on the record of the model in the experimental tank.