[6] There is little difference between the action of an oar in sculling and that of the modern screw-propeller, which is fast superseding the paddle-wheel in all ocean-going steamers: the one has an alternate lateral motion, like the tail of a fish; the other is rotatory, but with the same effect. It may be added that fishes often have the power of “feathering” their tails, by puckering their lobes in their forward motion, and expanding them on their return, so as to displace as little water as possible, while they, at the same time, rely for their advancement on the reaction of the water in the direction of their body. These points have been carefully considered in the construction and arrangement of the blades of the screw, as well as the important fact that the tail of the fish or the sweep of an oar in their motions displace a quantity of water, great in proportion to the length of the instruments employed; and further, that it is by the resistance the water makes to this displacement by the oar or tail, in their continued oscillation, coming as these do from their extreme sweep to the axis of the boat or fish, that either is urged forward. [7] An edition of Hero’s “Pneumatics” has been published by Mr. Woodcroft. Lond., 4th ed. 1851. His second experiment is referred to in Muirhead’s “Life of James Watt,” 2nd ed. p. 107. [8] 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.”