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.