Vacuum, pressure, and the first hydrodynamic devices

The writings of our 3rd century BC authors have unfortunately been lost for the most part. We know them primarily through citations and references contained in the writings of the subsequent period, in particular those of the Roman Vitruvius (about 25 BC) and of Heron of Alexandria (around 60 BC). It is thought that Straton of Lampasaque con­ducted the first studies of a vacuum and may be at the origin of the concept reported by Heron that “an absolute vacuum does not exist, but one can artificially produce vacuum in opposition to nature.” Straton was the private tutor of the future Ptolemy II Philadelph in about 290 BC, and, after 286 BC, he was the successor of Theophraste as the head of the Academy of Athens. One of the experiments reported by Heron, but very likely inspired by Straton, consisted in blowing air into a hermetically sealed metallic sphere through a small tube: “This shows clearly that the compression of bodies contained in the sphere enables them to reside in the dispersed pockets of the vacuum.” Inversely, one can suck out the air contained in the sphere through the same tube, “a considerable quantity can be withdrawn, without any substance taking its place inside the sphere.”[172] [173] Vitruvius credits Ctesibios of Alexandria with the invention of the “fire pump” (Figure 5.5). This would appear to be the first hydrodynamic device in which the flow of water is not driven by gravity, but by the action of an artificially induced pressure. Philon of Byzantium appears to have pursued this interest in flow devices “under pres­sure” through his interest in connected vases and siphons, and perhaps also under the influence of the work of Archimedes that we will now discuss.