Although it appears from the writings of the Greek physician, Hero, who flourished at Alexandria more than a century before Christ, that steam was well known to the ancients, it was employed by them merely as a toy, or as a means of exciting the wonder of the credulous. In his treatise on Pneumatics, Hero gives descriptions of various methods of employing steam or heated air for the purpose of producing apparently magical effects; from which we infer that the agency of heat was employed by the heathen priests in the performance of their rites. By one of the devices which he describes, water was apparently changed into wine; by another, the temple doors were opened by fire placed on the sacrificial altar; while by a third, the sacrificial vessel was so contrived as to flow only when the money of the votary was cast into it. Another ingenious device consisted in the method employed to pour out libations. Upon the altar-fire being kindled, the air in the interior became expanded and, pressing upon the surface of the liquid which it contained, forced it up a connecting-pipe, and so out of the sacrificial cup. The libation was made, and the people cried, “A miracle!” But Hero knew the trick, and explained the arrangement by which it was accomplished: it forms the subject of his eleventh theorem. The most interesting of the other devices described by Hero is the whirling Æolipile, or ball of Æolus, which, though but a toy, possessed the properties of a true steam-engine, and was most probably the first ever invented. As Hero’s book professes to be, for the most part, but a collection of the devices handed down by former writers, and as he does not lay claim to its invention, it is probable the Æolipile may have been known long before his time. The machine consisted of a hollow globe of metal, moving on its axis, and communicating with a caldron of water placed underneath. The globe was provided with one or more tubes projecting from it, closed at the ends, but open on one side. When a fire was lit under the caldron, and the steam was raised, it filled the globe, and, projecting itself against the air through the openings in the tubes, the reactive force thus produced caused the globe to spin round upon its axis “as if it were animated from within by a living spirit.”[2] The mechanical means by which these various objects were accomplished, as explained by Hero, show that the ancients were acquainted with the ordinary expedients for communicating motion, such as the wheel and axle, spur-wheels, toothed pinions and sectors, the lever-beam, and other well-known expedients; while they also knew of the cylinder and piston, the three-way cock, slide-valves and valve-clacks,[3] and many other ingenious mechanical details which have been reinvented in modern times.