that the Greeks knew that space could be deprived, partially at least, of air, and that they had observed one effect of atmospheric pressure, if they were not acquainted with the fact that air has weight. The experiment encourages the belief that external pressure was recognised as a cause, and if so, they were in possession of a principle, the knowledge of which was afterwards lost, and rediscovered in the seventeenth century by Torricelli, an Italian philosopher.

"In this manner," he says, in another place, "air may be rarified by heat, even as other substances are; for water is changed by fire into air: the vapours from boiling caldrons are nothing else than expanded water taking the form of air, and mists and clouds are nothing else than water raised in the atmosphere by heat, which are partly afterwards converted into air, while portions again descend in rain. Although an exception may be taken to the philosophy here announced, in some particulars, it recognises the formation of steam by heat, and the property of expansion. In another part of Hero's Pneumatics, elasticity is admitted, as it is also by Aristotle and other authors.

The Grecian theory of earthquakes is a sufficient proof that the force of steam was not undervalued, and so plausible is the hypothesis, that some writers of our own times have adopted it. The ancients supposed earthquakes to be produced by steam generated from water, by subterranean heat. Well knowing, as they must have done, the intensity of the volcanic force in rending

asunder the solid masses of the globe, raising mountains, and forming islands in the midst of the sea, the philosophers of antiquity must have had an adequate notion of the power of steam, or otherwise they would not have attributed such violent effects to its presence. But while this is sufficiently evident, there is no reason to believe that any attempt was made to measure its power, or to ascertain the law of its increased expansive force under pressure. Its gigantic energy was known; and without any such reasoning as would now be considered necessary for the advance, not to say the reception of a theory, its presence was admitted to be sufficient to produce earthquake—the most violent phenomenon in nature.

That the Greeks should have had such a vivid conception of the power of steam is so strange that we are led to inquire from what source they obtained it. The question is answered by an author intimately acquainted with the application of steam and the construction of engines. Adopting the hint of M. Arago, he says, "Such notions resulted, not as consequences of any exact or scientific principles, but from vague analogies, derived from effects which could not fail to have been manifested in the arts, such as those which commonly occurred in the process of casting in metal the splendid statues which adorned the temples, gardens, and public places in Rome and Athens. The artisan was liable to the same accidents to which modern founders are exposed, produced by the

casual presence of a little water in the mould into which the inolten metal is poured. Under such circumstances, the sudden formation of steam under an extreme pressure produces, as is well known, explosions attended with destructive effects. The Grecian and Roman artisans were subject to such accidents, and the philosophers, generalizing such a fact, would arrive at a solution of the grander class of phenomena of earthquakes and volcanoes." The supposition is ingenious, and by no means an improbable reason why the ancients, as we are accustomed to call them, formed so large an estimate of the energetic power of steam.

The idea of the application of steam as a motive power was also known to the Greeks, as we learn from Hero. There is a pretty experiment, familiar to most of our readers, which is exhibited by lecturers to show the expansive force of air, and which is also not unfrequently displayed in druggists' shops, as a pleasing ornament or useful fountain. As exhibited in the class-room, it consists of a copper vessel, about half filled with water, the space above the liquid being occupied with highly compressed air. A tube is carried through the vessel, reaching at one end to nearly the bottom of the water, and at the other rising about two feet above the vessel, and terminating in a hemispherical cup. By means of a stop-cock, a passage is opened through the tube, and the expansive force of the compressed air drives the water upward, producing a fountain, while a

ball previously placed in the hemispherical cup, is supported by the jet. Now this is Hero's experiment, with only a variation in the agent employed, for he used the expansive force of steam, instead of compressed air. The vessel containing the water was placed over a fire, and steam was generated, which, issuing from a pipe screwed into the neck of the vessel, rushed out of the aperture thus provided, and supported the ball in the manner already described. Of the two experiments, Hero's will probably be preferred by our readers.

We have on record an instance of the employment of steam many years afterwards, for a vindictive purpose. Anthemius, the architect of St. Sophia at Constantinople, lived next door to Zeno, and they were at enmity. To annoy and injure his neighbour, Anthemius connected some boilers in the ground-floor of his house with the space between the floors in Zeno's dwelling, and when in an angry and vindictive humour, he lighted his furnaces, boiled the water, and raised such a force of steam, that his neighbour's floors heaved and cracked under the pressure, to the great alarm and dismay of all who witnessed the scene.

Although Hero's discoveries, if they were his, were only employed in what would now be called philosophical toys, he really constructed a steam engine, and upon a principle thought worthy of being adopted as a working machine in modern times. A hollow metallic globe was suspended between two pivots, through one of

which its interior was connected with a boiler containing water. From the circumference of the globe, upon a circle, two or more hollow tubes were attached, closed at the end, but having a small opening at one side, the opening in each being so made that when the vessel revolved they followed each other in the same direction. The steam entering the ball escaped through the small openings in the arms, and a rapid rotatory motion was produced, but from what cause may not be at once evident. There is a law in mechanics which says that reaction must always be equal and in a contrary direction to action. When a man runs against another walking in an opposite direction, he drives him forward, but recoils or falls backward himself. So steam, issuing from an aperture, strikes against the atmosphere, and the vessel from which it flows, if capable of motion, is driven in the direction opposite to the course of the steam. If there be many jets of steam, all projected in the same direction, the velocity and force will be proportionately increased.

M. Arago ingeniously explains action and reaction by the selection of a gun as an example. When a fowling-piece is discharged, the rapid vaporization of the powder propels the missile, but the gun itself re-bounds and strikes the shoulder of the sportsman. Now here, it will be observed, the reaction is in a direction opposite that of the motion. But if the mouth of the gun were closed, and an opening were made at the side, the reaction would be side