Page:The American Cyclopædia (1879) Volume XI.djvu/328

 316 MECHANICS the word mechanics signifies the inventing of machines (Gr. jto^avcurifa*, to in vent or construct), or at most consideration of the action of forces upon them, and this is the sense in which the ancients used it ; but the science has long since passed beyond such limits, and comprehends the laws by which the motions of the heavenly bodies are governed, as well as those which affect their form, and also the action of gravi- tation upon bodies on the earth. Hydrostatics, hydrodynamics, and even sometimes pneumat- ics, are considered branches of the general science of mechanics. (See HYDROMECHANICS, and PNEUMATICS.) The invention of simple machines for moving large masses of bodies is older than history. The lever and inclined plane were probably the first simple powers used, the construction of the latter being natu- rally suggested by the advantages offered by the natural slopes of hills. It is generally be- lieved that the vast blocks of stone which are found in elevated positions in ancient Egyp- tian structures were raised to their places by inclined planes formed of earth on the exte- rior of the walls and afterward removed. The successive steps in the invention of machines have not been recorded, the work of Vitruvius on architecture, written under Augustus, being the principal source of information of those which were in use at and before his time. From his descriptions there were then in use the lever, the wheel and axle, the simple and compound pulley, and a forcing pump for sup- plying the public fountains, whose invention he ascribes to Ctesibius of Alexandria, who flourished in the latter half of the 3d cen- tury B. C. Among other machines this mech- anician also invented the clepsydra or water clock. (See CLEPSYDRA.) Vitruvius also de- scribes a complex machine in which wheels acted upon each other by means of cogs, and which was used for measuring distances trav- elled by carriages or ships ; and he describes at considerable length military engines for throw- ing masses of stone. Water wheels were used for grinding corn, and water was raised by buckets which were moved by wheels, the power being supplied by men walking on them. It is probable that most of the engines described by him were in use by the Greeks before the erection of the Parthenon. There is no positive evidence of the employment of the expansive force of steam as a moving pow- er before the latter part of the 17th century, and then it was only used to raise water ; and its general application to machinery dates only from the year 1768. Among the ancients, Archimedes seems to stand almost or quite alone in the power of conceiving in any great degree true theoretical ideas of mechanics. He explained the theory of the lever and some important properties of the centre of gravity, and also the fundamental doctrines of hydro- statics, particularly the equilibrium of floating bodies. The idea that the particles of a fluid have the power to communicate force with- out loss, in consequence of being perfectly free to move among one another, seems nev- er to have been clearly comprehended before his time. Archimedes was so far ahead of his age that it was only within a comparatively recent period (the epoch of Galileo) that his doctrines became established. Before Archi- medes the doctrines of Aristotle had been generally adopted in physics, and for many centuries afterward were accepted by the sci- entific world. Aristotle taught that all motion was naturally circular, an. idea which lasted till the first law of motion was established. He divided motion into natural and violent, and accounted for the fact that a body thrown in a horizontal direction diminishes in velocity, while a falling body increases, by saying that the former is a violent and the latter a natu- ral motion. In accounting for the continu- ance in motion of a stone thrown by the hand, he asks: "If the hand was the cause of the motion, how could the stone move at all when left to itself? If not, why does it ever stop ? " He answers by saying : " There is a motion communicated to the air, the successive parts of which urge the stone onward, and each part of this medium continues to act for some while after it has been acted upon; and the motion ceases when it comes to a particle which cannot act after it has ceased to be act- ed on." He attempted the discussion of the properties of some of the simple machines, and propounded some theories in regard to the le- ver, but even these have more the nature of mere suggestions. He asks why small forces may move great weights by means of a lever, and answers by asking if it is because the great- er radius moves the faster ; and he also queries whether a wedge affords power because it is composed of two opposite levers. As to the reason why a person when rising from a chair bends his legs and body to acute angles with his thigh, he suggests that it is because the right angle is connected with equality and rest. Here he again departs from the more philo- sophical method he had pursued with the arms of the lever, and returns to his usual meta- physical ones ; and this is the tendency of most of the mechanical ideas of the ancients, with the exception, as has been observed, of those of Archimedes, which were unappreciated by his contemporaries. The latter, in his in- vestigations concerning the properties of the lever, commences with the axiom that two equal weights balance each other on a lever of equal arms, and proceeds by employing the mathematical methods then in use to show that equilibrium always will exist in a lever when the bodies supported by it are inversely proportional to their distances from the ful- crum. Proceeding in his reasoning, he also concludes that there must be in every body a centre of force corresponding to the fulcrum in the lever. The principles which were so long ago clearly conceived by him were how- ever not only undeveloped, but were not un-