William A. Wallace demonstrates the importance of two early manuscripts of Galileo dismissed by earlier researchers as juvenile exercises. Analyzing all his scientific writings from the late 1580s to 1610 and from 1610 to 1640, this book illuminates both the sources and the evolution of Galileo's thought. Originally published in 1984. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.