It is almost as plausible to regard Leonardo as primarily an engineer as primarily a painter. He offered his services as a military engineer and architect to the Duke of Milan and set forth at length his manifold claims which include, one may note, the ability to construct what we should now, without hesitation, describe as “tanks.” At a later period he actually was appointed architect and engineer-general to Cæsar Borgia, and in this capacity was engaged on a variety of works. He has, indeed, been described as the founder of professional engineering. He was the seer of coming steam engines and of steam navigation and transportation. He was, again, the inventor of innumerable varieties of ballistic machines and ordnance, of steam guns and breech-loading arms with screw breech-lock. His science always tended to become applied science. Experience shows the road to practice, he said, science is the guide to art. Thus he saw every problem in the world as in the wide sense a problem in engineering. All nature was a dynamic process of forces beautifully effecting work, and it is this as it were distinctive vision of the world as a whole which seems to give Leonardo that marvellous flair for detecting vital mechanism in every field. It is impossible even to indicate summarily the vast extent of the region in which he was creating a new world, from the statement, which he set down in large letters, “The sun does not move,” the earth being, he said, a star, “much like the moon,” down to such ingenious original devices as the construction of a diving-bell, a swimming-belt, and a parachute of adequate dimensions, while, as is now well known, Leonardo not only meditated with concentrated attention on the problem of flight, but realised scientifically the difficulties to be encountered, and made ingenious attempts to overcome them in the designing of flying-machines. It is enough—following expert scientific guidance—to enumerate a few points: he studied botany in the biological spirit; he was a founder of geology, discovering the significance of fossils and realising the importance of river erosion; by his studies in the theories of mechanics and their utilization in peace and war he made himself the prototype of the modern man of science. He was in turn biologist in every field of vital mechanism, and the inaugurator before Vesalius (who, however, knew nothing of his predecessor’s work) of the minute study of anatomy by direct investigation (after he had found that Galen could not be relied on) and _post-mortem_ dissections; he nearly anticipated Harvey’s conception of the circulation of the blood by studying the nature of the heart as a pump. He was hydraulician, hydrographer, geometrician, algebraist, mechanician, optician.[45] These are but a few of the fields in which Leonardo’s marvellous insight into the nature of the forces that make the world and his divining art of the methods of employing them to human use have of late years been revealed.