Many of Leonardo da Vinci's manuscripts were intended as preparatory sketches for future published works, and the Codex Leicester is one of the most advanced. Written densely on 18 sheets illustrated with geometrical diagrams and illustrations of experiments imagined and real, it is a complex and fascinating meditation around a subject that enthralled Leonardo for much of his career: water. Named after the 1st Earl of Leicester who purchased it in 1717, it was later renamed after the wealthy industrialist, art collector and philanthropist Armand Hammer. In 1994 it was bought at auction for $30.8 million by Bill Gates who renamed it once again Codex Leicester . It will be shown at a special exhibition at the Chester Beatty Library, Dublin in June. The accompanying catalogue reproduces the Codex with informative captions, comparative material, scholarly essays and a contribution from Dorothy Cross, one of Ireland's most important contemporary artists. With a foreword by Bill and Melinda Gates, i