Note 8 — Chapter I

The principle of Hero’s steam-machine depends on the physical law that, when any fluid issues from a vessel in which it has been confined, the vessel is acted on by a force equal to that with which the fluid escapes, but in the opposite direction. Thus, if water issues from an orifice, a pressure is produced behind the orifice corresponding to the force with which the water escapes: hence, the recoil of a gun when fired. If the muzzle were turned at right angles to the length of the gun, the explosive gases would escape sideways, and the shooter, instead of being forced back, would spin round. The orifices in each case are exposed to the atmosphere, which tends to rush in with a force of a little less than 15 lbs. on the square inch: the force, therefore, with which the steam escapes represents the excess of its elasticity over that of the atmosphere, which furnishes, as it were, the fulcrum, and thus gives motion to the machine. Mr. Bourne states that the principle of the Æolipile is the same as that embodied in Avery and Ruthven’s engines for the production of rotatory power. “These engines,” he says, “are more expensive in steam than ordinary engines and travel at an inconvenient speed; but in other respects they are quite as effectual, and their construction is extremely simple and inexpensive.”