 In Paris, a small scientific circle of mathematicians and physicists gradually formed around Mersenne. From 1633 or so, meetings were organized on Thursdays at the houses of one or another of his friends or protectors. This “Academia Parisiensis”, as it came to be called, brought together men such as Étienne Pascal, Mydorge, Hardy, Roberval, Fermat. It was later to welcome the young Blaise Pascal, and Jacques Le Pailleur who was to become the group’s leader when Mersenne died in 1648. Discussions were mostly “mathematical” (MC V: 209). Mersenne used to submit problems of his own invention to his friends, and some were often remarkably stimulating and the origin of important discoveries such as the “roulette” problem, solved by Roberval in 1634; and then more indirectly by Fermat and Descartes. Mersenne himself undertook to establish a first list of what will eventually be called the “Mersenne primes”, prime numbers that can be written in the form 2n − 1 (for some integer n), whose properties and determination are still topical questions in contemporary mathematics. Mersenne envisaged his meeting group, and his own net of correspondents, as the prototype for a larger institution that would bring scholars together in a pacified Europe, foreshadowing the foundation of the later Académie des sciences in 1666, or the Royal Society in England (MC V: 301–302, and Goldstein 2013)