 The five treatises published in a row in 1634 are representative of the kind of intellectual productions Mersenne would henceforth offer. Two of them (Questions inouyes, and Questions théologiques…) are eclectic lists of “curious” questions, intended for entertainment (récréation) as well as for instruction of the learned: can one walk on water without miracle? How many grain of sands can be contained in the sphere of the earth ? From where does the satisfaction come when one believes one has found a new demonstration or truth? In these questions metaphysical, apologetic and even theological considerations are (in spite of the title of the second treatise) barely present. The Questions harmoniques and the Préludes to the harmonie universelle prepare the way for the publication of Mersenne’s great achievement on musical theory, The Harmonie Universelle (1636–7), where he offers what is still considered a major contribution to the scientific understanding of “consonance” and acoustics. Finally the Mechaniques de Galilée illustrate Mersenne’s constant interest in physico-mathematical questions of Galilean type, that is Archimedean mechanics stricto sensu, dealing with basic machines, such as the lever, the wheel, the pulley, but also the sciences of motion, free fall, ballistics, pneumatics, hydrostatics, and optics. Mersenne pursued these interests in the 1640s, when he started to make his own campaigns of experiments in order to verify Galilean numerical data on free fall and on the velocity of a cannon ball, especially during his journey to Italy (1644–1645) where he met Torricelli. These researches resulted in the publication of his Cogitate physico-mathematica (1644) and his Novarum observum physico-matheaticarum (1647), which includes in its first part a treatise by Roberval on the system of the world. A book on optics was to be published after Mersenne’s death (L’Optique et la catoptrique, 1651).