It is not only as a prime mover that the importance of the steam-engine is to be regarded ; collateral branches of mechanical art have grown out of it, which, in their reciprocating effects, have contributed much to its influence ; and thus so illimitable is its adaptability, and so obedient to control, that it can be brought alike to rend rocks asunder, or to weave into delicate fabrics the fragile thread of the silk-worm ; accomplishing marvels in the diversified ranges of manufacture, with such ease, precision, and celerity, that the very fictions of old enchantment seem but the type and symbol of its potency. As the im provements made from time to time in its structure and operations have led to imptovernents in its application, and as it is daily advancing in simplicity and perfection by the contributions of the learned theorist and the practical artisan no limits can be assigned to the sphere of its usefulneu, nor can conjecture measure the amount of benefit which, directly and indirectly, must accrue to society from its extended employment,—at least until some greater or cheaper power be discovered, whenever that may happen.
The power of the skatn-engine is derived from two causes : first, from the property of water to expand itself in bulk under the action of heat ; and, secondly, from the sudden reduction (whilst in this expanded state) to its original bulk upon the application of cold.
Water, when heated to the boiling point (212° of Fahrenheit's thermometer), remains no longer liquid, but assumes the aeriform and highly elastic state of steam (see article STEAM]; and if whilst in this state, and contained in a closed vessel, it is submitted to the action of still more highly increased heat, it becomes yet more rarefied, and exerts an increased pressure on the sides of the vessel cor responding to the degree of heat applied to it. This degree of heat may be augmented until the pressure of the steam overcomes the strength or resistance of the vessel, and bursts it in pieces. A power is thus positively obtained by the force of pressure from within.
The application of any cold body to steam again restores it to its original condition of water by condensation ; and a power is negatively obtained from this property by the force of pressure from without. If a small quantity of water be brought to the boiling point in a weak vessel, and the air which it contained be allowed to escape, so that its capacity shall be entirely filled with steam alone, the condensation of steam by cooling will reduce the water to its original volume, and leave a void in the vessel equal to the space previously filled with air. The pressure of the atmosphere (very nearly pounds to the square inch) will then exert itself upon the external sides of the vessel, which, if too weak to resist such a force, will become necessarily crushed together. Again : if the vessel be of a cylindrical figure, closed at the bottom, and its open top fitted with a movable piston (in contact with the water partially filling the cylinder), the piston will be raised as the steam becomes generated ; now, if before the piston arrives at the top, the fire be removed, and cold be applied to the sides of the cylinder, the steam, becoming thereby condensed, leaves a void which is instantly filled up by the descent of the piston acted upon by the atmosphere pressing upon its exposed surface without.
Whatever may be the construction of a steam-engine, every modification of it derives its power from one or other of these two principles, or from both in combination.
The two tables given for greater perspicuity in the next page—the first being the result of a series of experiments made by Dr. Dalton, the second supplied by the Royal Academy of France, in their report upon the comparative degrees of safety between high and low pressure engmes—are inserted, as being not only essential to the working engineer, but interesting to the general inquirer : they differ in no very material point with other calculations that have been made, and are quite near enough to be adopted as a standard for guidance in practical operations.
The origin of this invention became, long ago, a matter of earnest inquiry ; the conception and contrivance of a machine of such rare importance becoming points of great interest to the scientific world. Posthumous fame, though it may be a stirring incentive to the living, is of little consequence to the dead ; still it is a creditable sentiment to desire to render homage where it is truly due, and to invest with honour this names of those who, by their illus trious acts, really deserve it. But where nations themselves become exalted by the achievement of an individual, that sentiment is often accompanied by into coming prejudices, which warp the judgment, and convert the enquirer nto 'a jealous partisan. Sharp controversies are originated and maintained in an unbrotherly spirit, and truth, already mystified by the veil of time, becomes more difficult of attainment than ever by the very efforts of the disputants themselves.
French writers affirm that to their country belongs the glory of first inven tion ; English authorities deny the claim, and insist that it belongs to theirs. Others, on the contrary, refuse it to both; and, referring to the disclosures made in the works of scientific men who existed in remote times, pronounce judg ment, some in favour of Egypt, and some of Italy ; whilst, in reality, the steam engine, growing, as it has done, in gradual formation and perfection, under the eontributions of men of genius living in different countries and at different periods, may in strictness be regarded rather as the elaboration of an age than as the sole product of any one master-mind.