This chapter is devoted entirely to Evangelista Torricelli who formulated a principle of equilibrium without referring explicitly to dynamic aspects. In the first part some elements of centrobarica are introduced. In the central part Torricelli’s principle is introduced: The centre of gravity of an aggregate of heavy bodies cannot lift by itself. Following this law, which in a first reading does not seem to be a VWL, Torricelli and his successors derived the VWL based on virtual displacements for which any force that can lift a weight p to a height h can raise p/n of nh. In the final part generalization and simplification of Torricelli’s principle are presented.
In the common interpretation, Torricelli’s principle is a criterion of statics which claims that it is impossible for the centre of gravity of a system of bodies in equilibrium to sink from any virtual movement of the bodies. This criterion had a vital role in the history of mechanics. It represents a generalisation of the ancient principle that a single body is in equilibrium if its centre of gravity cannot sink. The generalisation devised by Torricelli states that if the centre of gravity of an aggregate of rigid bodies, considering the aggregate as a whole, is evaluated according to Archimedean rules, then this point has effectively the physical meaning of a centre of gravity.