To Leonardo, water was also a phenomenon that from his youth never failed to excite his curiosity. The use of water power to run machines, to irrigate fields and to carry boats inland was a subject that he never ceased investigating. Out of his experiments at this time he constructed a device for raising water to high levels. It was based on the geometric spiral of Archimedes. He took a piece of gut, inflated it, and let it dry. Then, covering it with a coat of wax to make it waterproof, he wound it around a thin staff in a spiral. He put one end in a stream and attached it by gears to a cogged water wheel; this set the long screw to turning, and he was able to raise water from a low level to any height he desired. With a multiple system of these screws he could raise water in continuous circulation to the reservoirs on the highest towers. In the year 1494, King Charles VIII of France crossed the Alps at the head of an army of twenty-five thousand men. Now Ludovico, by a series of diplomatic maneuvers, had allied himself with Charles and had, by secret negotiation, actually invited the invasion. By such an alliance he hoped to use Charles’ army to overcome the forces of the Pope which stood in the path of Ludovico’s ambition to become the most powerful ruler in Italy. Outwardly Charles was asserting his rights to the Kingdom of Naples, but inwardly he dreamt of leading a crusade against the infidels in the Holy Land. At the same time young Gian Galeazzo Sforza, Duke of Milan, was dying. Ludovico desired this title for himself; however, until Galeazzo was out of the way, he could not have it. There were ugly rumors that young Sforza had been poisoned. Moreover, in 1494, the Medicis—another powerful obstacle—were expelled from Florence, and a republic was established. Soon young Gian Galeazzo died, leaving a son, Francesco. This son was the rightful heir to the Dukedom of Milan but Ludovico usurped the boy’s claim and declared himself Duke of Milan. Now Ludovico was in a position to await the impending battle between Charles and the Pope. With such military and political ambitions in mind, Duke Ludovico now assigned Leonardo the task of reviewing Milan’s defenses. Again Leonardo submitted to Ludovico his plans for strengthening fortresses and designs for new ones. The great architect Bramante was also assigned the task of seeing to the city’s defenses, and for some time the two brilliant men worked together. Then, in the spring of 1494, Leonardo was sent to Vigevano where Ludovico’s young wife was staying. This town was also the birthplace of Ludovico, and Leonardo was given the job of designing and building a small summer house and garden there for Beatrice. In addition, Leonardo built a kind of “air conditioner” for her bedroom. It consisted of a large waterwheel that cooled the air circulated into her room. Although this ancient device had long been known to the Greeks and Romans, Leonardo was the one who succeeded in perfecting it.