Leonardo also painted two fine portraits of the lawful duke, Gian Galeazzo Sforza, and his wife, Isabella of Aragon, the latter picture "beyond all description beautiful and charming," now preserved in the Ambrosian Library. When these persons were married, Leonardo invented for the entertainment of the guests at the wedding feast a mechanical device called "The Paradise," a representation of the heavens and the revolving planets, which opened as the bride and bridegroom approached, while a person in imitation of the Deity recited complimentary verses. Leonardo now began on the great equestrian statue of the warrior Francesco Sforza. He studied ancient works of art, especially the equestrian statue of Marcus Aurelius in Rome, made almost countless drawings of horses in repose or on the battle-field, many of which are still preserved at Windsor Castle, studied every movement of live horses and every muscle of dead ones, and did not complete his clay model for ten long years. A genius like Da Vinci spends ten years on the model of an equestrian statue, and yet some artists of the present day, men and women, paint and mould horses or human beings after a few weeks or months of study, and expect to win fame! When the clay model was exhibited in public at the royal wedding of the sister of Gian Galeazzo to the Emperor Maximilian, the enthusiasm was very great. All Italy talked of it, and poets and critics extolled it as beyond the works of Greece or Rome. Unfortunately the ensuing wars depleted the treasury of Milan, and prevented the work from being cast in bronze. When the French entered Milan in 1499, it became a target for the archers. Two years later the Duke of Ferrara asked the use of the model that a bronze horse with a statue of himself might be made; but the King of France refused, and the model finally disappeared. During these years Leonardo founded the Milan Academy. Probably many of the manuscript volumes which he left were notes of lectures delivered to the students. He must have spoken to them on botany, optics, mechanics, astronomy, hydrostatics, anatomy, perspective, proportion, and other matters. He wrote a book on the anatomy of the horse. "He also," says Vasari, "filled a book with drawings in red crayons, outlined with the pen, all copies made with the utmost care from bodies dissected by his own hand. In this book he set forth the entire structure, arrangement and disposition of the bones, to which he afterwards added all the nerves, in their due order, and next supplied the muscles, of which the first are affixed to the bones, the second give the power of cohesion or holding firmly, and the third impart the motion."