Leonardo’s studio was like his childhood room in one respect—it was still filled with all the different things that had aroused his curiosity. Books were everywhere—on his tables and shelves and piled on the floor—books by Ptolemy, Pliny, and Strabo on geography and natural history, by Aristotle on physics, even one by Guido, a tenth-century monk, who has been called the father of modern music. In addition, there were books on arithmetic, agriculture, geometry, grammar, philosophy, fables, poetry and even one containing jokes. A map of the world hung on the wall, together with his drawings; and, scattered throughout the whole studio were the plants, fossils, rocks and animal skeletons he was still collecting from his trips into the country. There was also a huge table extending down the middle of Leonardo’s studio upon which were many drawings and instruments for working geometrical problems. His easel near the window supported a painting—a study for his commission in the Palazzo. And on his desk was a confusion of papers containing notes all written in his “secret” writing. At twenty-six Leonardo was deep in the study of mechanical law, geometry, and botany. For example, he had observed the rings in trees and their relationship to the age of the trees. In mechanics, he was absorbed in drawing models of a “variable speed drive.” By meshing three cogged wheels of different diameters to a common lantern wheel, Leonardo saw that different speeds of rotation could be obtained at the same time. This same principle is used in the gear shift of modern automobiles. About mechanics Leonardo wrote that it was “the paradise of the mathematical sciences because by means of it one comes to the fruit of mathematics.” Now, too, he was starting to write about his observations on the flight of birds, the formations of clouds and the behavior of smoke in the air. He compared the flying of birds to the swimming of fish in the sea, and the flow of air to the flow of water. Two hundred years before Newton, Leonardo would define the principles of aerodynamic reciprocity, as contained in Newton’s Third Law of Motion. At this time, Leonardo had an idea for making the Arno river navigable all the way from Florence to Pisa by the addition of canals, thus giving Florence an outlet to the sea. He also had thoughts for the improvement of irrigation in order to make use of land that did not have enough water. Nothing that Leonardo saw in his day’s activities was too small to pass unnoticed and unquestioned. The flight of a butterfly, the stratification of rock in a cliffside, the shape of a mighty cumulus cloud, the turning of a carriage wheel on a bumpy road, the play of muscles in a farmer’s back, the curling of water around a rock in a stream—all of these aroused Leonardo’s curiosity. Continually, he studied these things and painstakingly drew them and wrote about them in his notebooks.