Few people know that Leonardo da Vinci and Niccolo Machiavelli crossed paths when Leonardo worked - ostensibly as an engineer, possibly as a spy - in Cesare Borgia's court and Machiavelli was Florence's ambassador there. Soon thereafter, they formed a friendship and an alliance.

Astonishingly, during the rich first decade of the sixteenth century, the pair joined together under the inspiration of one of Leonardo's most fantastic dreams: to build a system of canals that would make the Arno River navigable from Florence to the sea. Under Machiavelli's supervision, the Florentine government tried - and ultimately failed - to realize a portion of this plan in 1504.

Roger Masters's account of the friendship between two of history's greatest geniuses starts with this tale of a magnificent lost dream and spirals outward to the art, politics, intrigue, and sexual scandals of Florence. Leonardo's preoccupation with the Arno project explains many of the tantalizing mysteries of his work.

It is the reason for the startling bird's-eye view of the valley in the background of the Mona Lisa; it is part and parcel of both his obsession, in the Notebooks, with understanding the dynamics of water, and his work on canals and swamp drainage in Milan, Rome, and France. As for Machiavelli, were it not for his time spent in prison, he might never have been compelled to write The Prince.

Fortune Is a River is at once a study of genius and a rich and delightful introduction to the wonders of the Renaissance.