There is another branch of fluid mechanics that sees some early development in this period: this is the knowledge of blood circulation. Whereas it was believed that the arteries contained only air prior to this period, Galien of Pergamon (129 — 200? AD)[202] is the first to describe arterial circulation and to study seriously the circulation of blood in the heart. For this he relies on an intense practice of dissection.[203] His only error is in believing that the blood passes directly from the right ventricle to the left ventricle.
The first discovery of the resistance to motion through the air
We mentioned in Chapter 4 the dominant theory of Aristotelian Greek science on the movement of objects in air. This theory held that air actually entrains the movement of a body (a thrown spear or an arrow) rather than slowing down this movement. In this theory, the air displaced by the front of the projectile comes back to the rear and pushes the object in its flight. Jean Philopon of Alexandria (in the 6th or beginning of the 7th century BC), in his Critique of the Physics of Aristotle, strongly rejects this theory:
”How could it be that the air, pushed by the arrow, does not move in the direction of the impulse that has been given to it, but instead does an about-face, as if ordered to do so, and backtracks? Moreover, how could it be that this air, in this about-face, does not disperse into space, but instead returns to strike precisely the notched end of the arrow, continuing to push it and stick to it? Such a conception totally lacks plausibility, and smacks of fiction. z
Later in the same work, Jean Philopon suggests that it is indeed the thrower who “provides the motive force for the rock” (which is what will later be called the kinetic energy or momentum). He also says that “if one imparts an unnatural movement, or a forced motion, upon an arrow or a stone, the same degree of motion will be attained more easily in a vacuum.” Continuing his discourse through the description of experiments with falling bodies, Jean Philopon observes that the time of fall depends very little on the weight. He shows finally that the air does indeed exert a resistance to the advancement of the body in motion.
