Ludovico was very much interested in the things Leonardo had showed him. Although he was a man of limited imagination and was not able to grasp the scope of Leonardo’s proposals, he was nevertheless involved in a war. Since Ludovico’s aging military engineer was to be replaced, Leonardo left the forbidding castle of the Sforzas with high hopes of getting the position. In the meantime, he was commissioned to paint the portrait of a young girl from a noble family in Milan. At the same time, he began the bronze equestrian statue of Ludovico’s father, Francesco Sforza. For this work, he began an intensive study of horses. Since hunting was the popular sport at the court of the Sforzas, Ludovico owned a stable of the finest Arabian horses, and here Leonardo commenced his drawings. Again, his research for a work of art led him beyond just making preparatory sketches. His studies developed into notes, and his notes into a planned book on the anatomy of the horse. During these months of waiting for the appointment as military engineer, Leonardo furthered his experiments with cannon. In the course of these experiments, he came across a power that would later revolutionize all industry—steam. He devised—although he attributed the original idea to Archimedes—a water vessel connected to a copper tube which was heated by a fire. The water when flowing into the red-hot tube changed into steam and the pressure of the steam blew out a ball at the mouth of the tube with great force. Leonardo experimented with steam in other ways. He built an apparatus for measuring the transformation of water into vapor. It consisted of a metal box in which was a thin animal bladder partly filled with water. Resting on the top of the bladder was a flat lid attached by a cord hung from two pulleys to a counterweight on the outside. As the water was heated, the steam in the bladder pushed up the lid. As the lid rose both the volume and the pressure could be measured. There were distillation experiments with various condensers, one in particular that anticipated the modern condenser of Leibig, introducing double walls that formed a complete jacket for cooling with water in continual circulation. Not content with having an idle moment, Leonardo again turned to searching out books that he had not read and trying to fill the gaps in his education. He became especially interested in the German philosopher, Cardinal Cusanus. Cusanus, like himself, had been influenced by Toscanelli and was a man devoted to the natural sciences. Leonardo also studied the philosophy of Aristotle and the writings of St. Augustine. Throughout his life Leonardo believed in an active mind for, as “iron rusts from disuse, stagnant water loses its purity and in cold weather becomes frozen, even so does inaction sap the vigor of the mind.”