(3.) The discovery of the fact, that a mechanical force is produced when water is evaporated by the application of heat, [Pg013] must be considered as the first capital step in the invention of the steam engine. It is recorded in a work entitled _Spiritalia seu Pneumatica_, that Hero of Alexandria contrived a machine, 120 years before the Christian era, which was moved by the mechanical force of the vapour of water. The principle of this machine admits of easy explanation: When a fluid issues from any vessel in which it is confined, that vessel suffers a force equal to that with which the fluid escapes from it, and in the opposite direction. If water issues from an orifice, a pressure is produced behind the orifice corresponding to the force with which the water escapes. If a man discharge a gun, the gases produced by the explosion of the powder issue with a certain force from the muzzle, and his shoulder is driven backwards by the recoil with a corresponding force. If the muzzle, instead of being presented forwards, were turned at right angles to the length of the gun, then, as the gases of explosion would escape sideways, the recoil would likewise take place sideways, and the shooter, instead of being driven backward, would be made to spin round as a dancer pirouettes. This was the principle of Hero's steam engine. A small globe or ball was placed on pivots at A and B (_fig._ 1.), on which it was capable of revolving: steam was supplied through one of these pivots from one of the tubes D C E F, which communicated with the boiler. This steam filled the globe A I B K, and also the arms I H and K G. A lateral orifice, represented at G, near the end of these arms, allowed the steam to escape in a jet, and the reaction, producing a recoil, had a tendency to drive the arm round. A small orifice at H, on the other side of the tube, produced a like effect. In the same manner, any convenient number of arms might be provided, surrounding the globe and communicating with its interior like the spokes of a wheel. Thus these arms, having lateral orifices for the escape of the steam, all placed so that the recoil may [Pg014] tend to turn the globe in the same direction, a rotatory motion might be communicated to any machinery which it was desired to move. After having been allowed to slumber for nearly two thousand years, this machine has recently been revived, and engines constructed similar to it are now working in these countries. In the proper place we shall describe Avery's Rotatory Engine, which it will be seen is, not only in its principle, but almost in its details, the machine of HERO OF ALEXANDRIA.