That such was the case may be reasonably inferred from the ancient stone sculptures exhumed by Layard and others, showing as these do beyond question that the people of Egypt, Assyria and Babylonia, when floating on bundles of reeds or on inflated skins, propelled them by the motion of their legs,[4] just as an animal swims by using its limbs for the purpose of propulsion in the water which supports it. In aquatic animals may be seen the types of almost every kind of machinery now adopted by man to lessen bodily toil. The cuttle-fish moves forward by fins, and backwards by ejecting water from a tube; whelks suggest the art of punting and towing; the value of paddles may be learned from ducks or other aquatic birds in their motion through the water, and the use of a folding feather from the lobster; while the combined action of the paddle-wheel and screw-propeller will be found in the microscopic insects “Paramacium caudatum” and “Paramacium compressum.” The marine animals “Vebella” and “Physalia,” familiarly known as the “Portuguese men-of-war,” whose bodies resemble an inflated bladder, float on the water and are propelled by the wind acting on their extended membranes. Swans extend their feathers to sail with the wind; and, though that fairy-looking, fragile thing, the paper-nautilus, seems to be the sport alike of the gale and of the most gentle breeze, it possesses in itself the power of propulsion by projecting water.[5] [Illustration] But the common fish of every sea would have suggested to man, in the most remote ages, a mode of supplementing manual labour: the fin giving him the idea of a paddle or of an oar, and the tail teaching him the art of sculling, the principle in each case being the same: the tail, moving from side to side, by oblique pressure on the water, propels the fish forward along a diagonal line, the resultant of the forces acting from the right and the left sides of the fish, and is, thus, the chief instrument of motion, while the fins serve to direct and steady it.[6] [Sidenote: Hero of Alexandria, B.C. 120.] Nor, indeed, is there much doubt that the ancients were acquainted with the power of steam, though they cannot be said to have applied this knowledge to any useful purposes. A treatise is still in existence “On Pneumatics,” by Hero,[7] a philosophic mathematician who lived at Alexandria about B.C. 120, in which he gives an account of seventy-eight miscellaneous experiments, most of them probably adapted for the superstitious purposes of the heathen priesthood, but some also as certainly foreshadowing the definite application of steam as a motive force. The following, we notice as, in themselves, of considerable interest. [Illustration] [Sidenote: Dancing steam ball.] [Sidenote: Æolipile.]