[6] The British Museum contains four manuscript copies of Hero's "Pneumatics," which were written in the fifteenth and sixteenth centuries. These manuscripts have been examined with great care, and a translation from them prepared by Prof. J. G. Greenwood, and published at the desire of Mr. Bennett Woodcroft, the author of a valuable little treatise on "Steam Navigation." This is, so far as the author is aware, the only existing English translation of any portion of Hero's works. The first part of Hero's work is devoted to applications of the syphon. The 11th proposition is the first application of heat to produce motion of fluids. An altar and its pedestal are hollow and air-tight. A liquid is poured into the pedestal, and a pipe inserted, of which the lower end passes beneath the surface of the liquid, and the upper extremity leads through a figure standing at the altar, and terminates in a vessel inverted above this altar. When a fire is made on the altar, the heat produced expands the confined air, and the liquid is driven up the tube, issuing from the vessel in the hand of the figure standing by the altar, which thus seems to be offering a libation. This toy embodies the essential principle of all modern heat-engines--the change of energy from the form known as heat-energy into mechanical energy, or work. It is not at all improbable that this prototype of the modern wonder-working machine may have been known centuries before the time of Hero. Many forms of hydraulic apparatus, including the hand fire-engine, which is familiar to us, and is still used in many of our smaller cities, are described, the greater number of which are probably attributable to Ctesibius. They demand no description here. A hot-air engine, however, which is the subject of his 37th proposition, is of real interest. Hero sketches and describes a method of opening temple-doors by the action of fire on an altar, which is an ingenious device, and contains all the elements of the machine of the Marquis of Worcester, which is generally considered the first real steam-engine, with the single and vital defect that the expanding fluid is air instead of steam. The sketch, from Greenwood's translation, exhibits the device very plainly. Beneath the temple-doors, in the space _A B C D_, is placed a spherical vessel, _H_, containing water. A pipe, _F G_, connects the upper part of this sphere with the hollow and air-tight shell of the altar above, _D E_. Another pipe, _K L M_, leads from the bottom of the vessel, _H_, over, in syphon-shape, to the bottom of a suspended bucket, _N X_. The suspending cord is carried over a pulley and led around two vertical barrels, _O P_, turning on pivots at their feet, and carrying the doors above. Ropes led over a pulley, _R_, sustain a counterbalance, _W_. [Illustration: FIG. 1.--Opening Temple-Doors by Steam, B. C. 200.]