ment of that machine from its early rude and imperfect state to its present comparatively perfect form, it will be necessary to advert to various physical phenomena and mechanical principles, which, however obvious to those who are conversant with matters of science, must necessarily be at least imperfectly known by the great majority of our readers. To refer for information on such topics to other works on Mechanics and general Physics, would be with most readers ineffectual, and with all unsatisfactory. In former editions of the present work, we consigned these necessary general principles of physics and mechanics to a preliminary chapter; but it appears, on the whole, more convenient not to remove the exposition of the principle from the place where its application is required. We shall therefore pause as we proceed, where these difficulties occur, to give such explanation and illustration as may seem best suited to render them intelligible and interesting to the unscientific reader.

The history of the arts and manufactures affords no example of any invention the credit for which has been claimed by so many different nations and individuals as that of the steam engine. The advocates of the competitors for this honour have urged their pretensions, and pressed their claims, with a zeal which has occasionally outstripped the bounds of discretion, and the contest has not unfrequently been tinged with prejudices, national and personal, and characterised by a degree of asperity altogether unworthy of so noble a cause, and beneath the dignity of science.

"When a question is clearly proposed, it is already half resolved." Let us see whether a careful attention to this maxim will aid us in the investigation of the origin of the steam engine. The source of the power of that machine is found in the following natural phenomena.

First. When fire is applied to water, the liquid swells into vapour, and in undergoing this change exerts, as has been already stated, a considerable amount of mechanical force. This force may, by proper means, be rendered applicable to any purpose for which labour or power is needful. Second. The vapour so produced is endowed with the property of elasticity, in virtue of which it is capable of

swelling or expanding into increased dimensions, exerting, as it expands, a force, the energy of which is gradually diminished as the dimensions of the vapour are increased. This mechanical force is likewise capable of being applied to any useful purpose for which labour or power is necessary.

Third. This vapour is capable, by proper means, of being reconverted into water; and when so reconverted, it shrinks into its original dimensions, deserting the large space which it occupied as vapour, and leaving that space a vacuum. It is known in physics, that when a vacuum is produced, surrounding bodies have a tendency to rush into it with a definite amount of force. Consequently any agent which produces a vacuum, becomes a source of a considerable amount of mechanical power. By its reconversion into water, therefore, steam again becomes a mechanical agent.

Such are the natural phenomena in which are found the original sources of all steam power. In some forms of steam engine one of these is used, and in some another, and in some the application of all of them is combined; but in no existing form of steam engine whatever is there any other source of mechanical power.

Neither these nor any other natural forces can be applied immediately to any useful purpose. The interposition of mechanism is indispensable; on the invention and contrivance of that mechanism depends altogether the useful application of these natural forces.

The world owes the steam engine then partly to discovery, and partly to invention.

He that discovered the fact, that mechanical force was produced in the conversion of water into steam, must be justly held to be a sharer in the merit of the steam engine, even though he should never have practically applied his discovery. The like may be said of him who first discovered the source of the mechanical power arising from the expansion of steam.

The discoverer of the fact, that steam being reconverted into water greatly contracted its dimensions, and thereby produced a vacuum, is likewise entitled to a share of the

The mechanism by which these natural forces have been rendered so universally available as a moving power, is very various and complicated, and cannot be traced to one inventor." If a watchmaker," says M. Arago, "well instructed in the history of his art, were required to give a categorical answer to the question, Who has invented watches? he would remain mute; but the question would be divested of much of its difficulty if he were required separately to declare who discovered the use of the main spring, the different forms of escapement, or the balance wheel." So it is with the steam engine. It is a combination of a great variety of contrivances, distinct from each other, which are the production of several inventors. If, however, one name more than the rest be entitled to special notice; if he is entitled to the chief credit of the invention who by the powers of his mechanical genius has imparted to the steam engine that form, and conferred upon it those qualities, on which mainly depends its present extensive utility, and by which it has become an agent of transcendant power, spreading its beneficial effects throughout every part of the civilised globe, then the universal voice will, as it were by acclamation, award the honour to one individual, whose pre-eminent genius places him far above all other competitors, and from the application of whose mental energies to this machine may be dated those grand effects which render it a topic of interest to all for whom the progress of civilisation has any attractions. Before the era rendered memorable by the discoveries of JAMES WATT, the steam engine, which has since become an object of such universal interest, was a machine of extremely limited power, inferior in importance and usefulness to most other mechanical agents used as prime movers; but, from that epoch, it is scarcely necessary here to state, that it became a subject not of British interest only, but one having an important connection with the progress of the human race.

HERO OF ALEXANDRIA, 120 B. C.

(3.) The discovery of the fact, that a mechanical force is produced when water is evaporated by the application of heat,

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.

H

D

C

Fig. 1.

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 DCEF, 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

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.

Although the elastic force of steam was not reduced to numerical measure by the ancients, nor brought under control, nor applied to any useful purpose, yet it appears to have been recognised in vague and general terms. Aristotle, Seneca, and other ancient writers, accounted for earthquakes by the sudden conversion of water into steam within the earth. This change, according to them, was effected by subterranean heat. Such tremendous effects being ascribed to steam, it can scarcely be doubted that the Greeks and Romans were acquainted with the fact, that water in passing into vapour exercises considerable mechanical power. They were aware that the earthquakes, which they ascribed to this cause, exerted forces sufficiently powerful to extend the natural limits of the ocean; to overturn from their foundations the most massive monuments of human labour; to raise islands in the midst of seas; and to heave up the surface of the land of level continents so as to form lofty mountains.

Such notions, however, 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 of 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 molten metal is poured. Under such circumstances, the sudden formation of steam of an extreme pressure produces, as is well known, explosions attended with destructive effects. The Grecian