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In the commentaries to this book we try to understand d´Alembert thoughts and how he contrives to translate his ideas on mechanics to the fluid realm with a new and radical point of view; how he arrives at the first two fundamental differential equations among the velocity components; and how he tries to reduce the resistance of a moving body, which is a change of its momentum, to the hydrostatical pressure, which is related to the gravity. All this knowing that his mechanics has no forces and no pressures as well, and that the fluids are aggregates of individual particles.

The essay A New Theory of the Resistance of Fluids was a turning point in Fluid Mechanics because clearly, for the first time, the resistance is shown as the results of a fluid subjected to differential equations in a continuous mode instead of a set of impacts of individual particles. This contribution has been recognized by the scholars. However, only partial attention has been p
aid to this work, which can be justified due to the difficulty in its reading and also because it was eclipsed by the publication, a few years later, of Euler´s three Memoirs that established modern hydrodynamics.

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Julián Simón obtained the degree in Aeronautical Engineering of the Polytechnical University of Madrid in 1966. For more than 40 years, until his retirement in 2007, he worked in the Spanish National Institute for Aerospace Technology (INTA). He passed through all the professional positions of the Institute. However, in those 40 years, his principal tasks were related to rockets, from sounding rockets until launcher vehicles. He worked in aerodynamics, flight dynamics, solid motors and project management. He also participated in several international committees as the Spanish representative.

From the beginning of his professional life he was involved in the Spanish National Aeronautical Association, being elected member of the board several times.

In 1986 he obtained the degree in Philosophy in the Spanish Open University (UNED) and in 1992 the doctorate. Even when his main job was always rather intense, he tried to combine it with the History of Science, especially the
origin of the fluid mechanics, given his knowledge of that subject. He has produced two books and several articles on this matter.

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