First Edition of one of Galileo's rarest and most significant works, covering both dynamics and hydraulics. This work contains what is perhaps the first printed account of the famous experiment conducted at the Tower of Pisa, in which it was shown that the force of gravity is a constant. Though at times catalogued under Benedetto Castelli as the author, the work is accepted by scholars of science as having been written by Galileo himself, although he is listed as the translator on the title-page. Castelli did write the introduction. The manuscript, in the Biblioteca Nazionale Centrale (Florence), is largely in Galileo's hand. This copy has been expertly restored, with the vellum covers repaired, the contents washed and resewn, new endpapers. There is an old monastical library stamp to the title-page, and the trace from another which has been removed. *2 misbound after A8; it contains the dedication. Several leaves in signature G misbound but all present. There are a number of errors in the pagination.