To recapitulate, during these years, on the one hand we find that experi- ments with discharges are only partially reconciled with their theoretical predic- tions, and on the other hand that there appears to be a proportionality of the resistance with the square of the impinging velocity, and with the density of the medium. Besides (and this is an important matter) the need was already felt to establish a theoretical basis enabling these phenomena to be explained, a need that Mariotte had anticipated. It is in this context that the first edition of Newton’s Principia saw the light. While Books I and III, respectively titled ‘The Motion of Bodies’ and ‘The System of the World’, are well known, Book II is not. It is also named ‘The Motion of Bodies’ although it deals with fluids, while the first deals with rigid bodies. The coincidence in the titles indicates that Newton put both solids and fluids under the same umbrella. However, Book I is structured as a closed set, characterised by an attempt to derive all motions from a few axioms or laws of motion. This is not the case in Book II, where he has to use several different additional hypotheses, which at the very least complement the others. In this respect, insufficiently justified suppositions abound, together with some small fudges and implausible constructions. In spite of this