Several steam-boilers are described, usually simple pipes or cylindrical vessels, and the steam generated in them by the heat of the fire on the altar forms a steam-blast. This blast is either directed into the fire, or it "makes a blackbird sing," blows a horn for a triton, or does other equally useless work. In one device, No. 70, the steam issues from a reaction-wheel revolving in the horizontal plane, and causes dancing images to circle about the altar. A more mechanical and more generally-known form of this device is that which is frequently described as the "First Steam Engine." The sketch from Stuart is similar in general form, but more elaborate in detail, than that copied by Greenwood, which is here also reproduced, as representing more accurately the simple form which the mechanism of the "Æolipile," or Ball of Æolus, assumed in those early times. [Illustration: FIG. 3.--Hero's Engine, B. C. 200.] The cauldron, _A B_, contains water, and is covered by the steam-tight cover, _C D_. A globe is supported above the cauldron by a pair of tubes, terminating, the one, _C M_, in a pivot, _L_, and the other, _E F_, opening directly into the sphere at _G_. Short, bent pipes, _H_ and _K_, issue from points diametrically opposite each other, and are open at their extremities. A fire being made beneath the cauldron, steam is formed and finds exit through the pipe, _E F G_, into the globe, and thence rushes out of the pipes, _H K_, turning the globe on its axis, _G L_, by the unbalanced pressure thus produced. The more elaborate sketch which forms the frontispiece represents a machine of similar character. Its design and ornamentation illustrate well the characteristics of ancient art, and the Greek idea of the steam-engine. This "Æolipile" consisted of a globe, _X_, suspended between trunnions, _O S_, through one of which steam enters from the boiler, _P_, below. The hollow, bent arms, _W_ and _Z_, cause the vapor to issue in such directions that the reaction produces a rotary movement of the globe, just as the rotation of reaction water-wheels is produced by the outflowing water. It is quite uncertain whether this machine was ever more than a toy, although it has been supposed by some authorities that it was actually used by the Greek priests for the purpose of producing motion of apparatus in their temples. It seems sufficiently remarkable that, while the power of steam had been, during all the many centuries that man has existed upon the globe, so universally displayed in so many of the phenomena of natural change, that mankind lived almost up to the Christian era without making it useful in giving motion even to a toy; but it excites still greater surprise that, from the time of Hero, we meet with no good evidence of its application to practical purposes for many hundreds of years.