in a horizontal position, to a moveable vertical axis, and, at the instant of firing, it will more or less change its direction, and will cause this axis to revolve.

Preserving the same arrangement, let us suppose that this vertical rotatory axis is hollow, but closed at the upper end; that it terminates below, like a kind of chimney, in a boiler in which steam is generated; that there exists, besides, a free lateral communication between the interior of this axis and the interior of the gun-barrel, so that, after having filled the axis, the steam penetrates into the barrel, and escapes from it by its horizontal opening. If we except the different degree of impetuosity, this steam, as it escapes, will act precisely as the gases disengaged from the powder in the gun-barrel plugged at its extremity, and perforated laterally; excepting that you will not here have only a single shock, as in the case of the sharp and instantaneous discharge of the gun; on the contrary, the rotatory motion will be, like the cause which produces it, uniform and continuous.

In place of a single gun-barrel, or rather in place of a single horizontal tube, let several be adapted to the vertical rotatory tube, and we shall have, with merely some unimportant variations, the ingenious apparatus of Hero of Alexandria.

Here, unquestionably, is a machine in which steam generates motion, and is capable of producing mechanical effects of some importance ;—here is a real steam-engine. Let me hasten to add, that in no particular does it really coincide, either

in its form, or in the mode in which the movingpower acts, with the steam-engines now in use. If ever the reaction of a current of steam should become useful in practice, it would, beyond all doubt, be necessary to refer the original idea of it to Hero; at the present day, the rotatory æolipile can be mentioned here only as wood-engraving would be in the history of printing.*

In the engines used in our manufactories, in our steam-boats, on our railroads, the motion is the direct result of the elasticity of steam. It becomes, then, important to enquire where and in what manner the notion of this force has had its origin.

The Greeks and Romans were not ignorant that steam is capable of acquiring a prodigious mechanical power. They went so far as to explain, by the

* These reflections apply equally to the plan which Branca, an Italian architect, published at Rome in 1629, in a work entitled "Le Machine;" and which consisted in producing a rotatory motion by directing the steam issuing from an æolipile, as a blast or current of wind, on the pinions of a wheel. If, contrary to all probability, steam should ever come to be used with advantage as a direct blast, Branca, or the author, now unknown, from whom he may have borrowed the idea, will take the first place in the history of this new kind of engine. As far as our present steam-engines are concerned, Branca has no real claims whatever.-M. ARAGO.

The complete title of Branca's work alluded to above, is as follows: Le Machine volume nuouo et di molto artificio da fare effetti marauigliosi tanto Spiritali quanto di Animale Operatione arichito di bellissime figure conle dichiarationi a ciascuna di esse in lingua uolgare et latina. Del Sig. Giovanni Branca, cittadino Romano. Ingegniero, et Architetto della Sta. Casa di Loreto. All' Illustriss. Monsignor Tiberio Cenci Vescouo di Jesi. In Roma adistanza di Jacomo Manuci In Piazza Nauona. Con licentia de Superiri. per Jacomo Mascardi. M.DC.XXIX."-TR.

help of the sudden conversion into steam of some great body of water, those frightful earthquakes which, in a few seconds, drive the ocean from its natural boundaries, overthrow from their very foundations the most solid monuments of human industry, cast up dangerous rocks in the midst of unfathomable seas, and raise lofty mountains in the very heart of continents.

Whatever may have been said about it, this theory of earthquakes does not necessarily suppose that its authors had turned their attention to calculations, or experiments, or accurate mensuration. No one at the present day is ignorant that at the instant of the red-hot metal passing into the earthen or stucco moulds used in a foundry, a few drops of moisture contained in these moulds are sufficient to produce dangerous explosions. Notwithstanding the progress of science, metal-founders at the present day do not always avoid these accidents. How, then, was it possible for the ancients to guard completely against them? While they were casting the thousands of statues which formed the splendid ornaments of the temples, the public squares, the gardens, the private dwelling-houses at Athens and Rome, accidents would necessarily happen; the workmen found out their immediate cause, while the philosophers, in obedience to the principle of generalisation, which was the distinguishing characteristic of their school of philosophy, discovered in them miniature, but faithful representations of the eruptions of Etna.

All this may be quite true, and yet be of no

importance whatever to the history which now engages our attention. I confess, indeed, that I have said even thus much on those slight delineations by ancient science of the powers of steam, only that I may, if possible, live at peace with the Daciers, of both sexes, and the Dutens of our days.*

Power, whether natural or artificial, before it is made of real use to man, has almost always been turned to the account of superstition. It will be seen that steam is no exception to the general rule.

Ancient history had informed us, that on the banks of the Weser, the god of the Teutones of old sometimes showed himself unpropitious by a sort of thunder-clap, immediately succeeded by a cloud

* From the same motive, I can hardly allow myself to omit here mentioning an anecdote which, through all that in it is romantic and contrary to our present knowledge of the mode in which steam operates, lets us see the high idea which the ancients formed of the power of this mechanical agent. It is related that Anthemius, Justinian's architect, had a dwelling-house adjoining that of Zeno, and that to play a trick upon that orator, his avowed enemy, he placed in the ground-floor of his own house several caldrons filled with water; that from a hole bored in the lid of each of these caldrons, a flexible tube was conducted to the partition wall, and fitted in under the beams which supported the ceilings, of Zeno's house. The result was, that these ceilings, the moment the fire was lighted beneath the caldrons, shook as if there had been a violent earthquake.—M. Arago.

For a minute and amusing narrative of this achievement of Anthemius, see Agathias, " de Imperio et rebus gestis Justiniani," Lib. v. In the folio Paris edition of 1660, it is to be found at pp. 150, 151. The principal part of the story of Agathias, and the testimony which Procopius (de Ædificiis) bears to the skill of Justinian's architect, are given in Gibbon's Roman Empire, Chap. xl.— TR.

The statue of

which filled the sacred enclosure. the god Busterich, discovered, it is said, in excavations, clearly shows the method by which the pretended miracle was effected.

The god was of metal; the head was hollow and contained an amphora* of water; wooden plugs closed up both the mouth and another opening above the forehead; live coals, dexterously placed in a cavity of the skull, gradually heated the liquid. Very soon, the steam generated forced out the plugs with a loud report; it then escaped with violence in two streams, and raised a thick cloud between the deity and his stupefied worshippers. It would appear that in the middle ages some monks found their account in this invention, and that the head of Busterich has performed its office before other than Teutonic multitudes.†

In order to fall in, after the first faint glimpses by the Greek philosophers, with any useful ideas on the properties of steam, we find ourselves compelled to pass over an interval of nearly twenty centuries. It is true, that then accurate, conclu

* About nine English gallons.-TR.

+ Hero of Alexandria attributed the sounds which have been the subject of so much controversy, which the statue of Memnon emitted when struck by the rays of the rising sun, to the passage, through certain apertures, of a current of steam which the heat of the sun was supposed to produce from water; with which the Egyptian priests, it is said, supplied the interior of the pedestal of the statue. Solomon de Caus, Kircher, and others, have gone so far as to try to discover the particular contrivances, by means of which the cunning of priestcraft thus ensnared the imagination of the credulous; but every thing leads us to believe that their conjectures are not well founded, if, indeed, there is here any thing to be conjectured.-M. ARAGO.