DIKE

When ama of unstratified or ig neo us rock, such as granite, trap, or lava, appears as if injected into rents and fi ures in the stratified rock, so as tc intersect the strata, it is called a. dyke. Dyke is al name given to a mound of earth, stones, or other male rials, intended to prevent lowland from being inn: by the sea, .; as the dykes of Holland. DYNAMICS (Gr. ivvajiis, force, or potcer), signifies literally the doctrine of force or power; but as fol power is known to us in no other way than by its effect, that is, by the motion which it produces in the boi which it acts, and is measured by that motion, dynamics may be defined to be the science which treats of the tno tion of bodies. It is, however, usually restricted to those circumstances of motion in which the moving bodies are at liberty to obey the impulses communicated to them: the opposite cases, or those in which the bodies, whether by external circumstances, or by their connection wit! another, are not at liberty to obey the impulses given, being comprehended in the science of mechanics. Thus, the motion of a stone falling freely to the ground, or of a celestial body in its orbit, belongs to dynamics; while that of a body descending an inclined plane would properly be long to mechanics. The theory of varied motions, and of the accelerating forces which produce them, is founded on two general laws: 1st, That all motion impre ed on a body is by its nature uniform and rectilinear; and 2nd. That different motions impre ed either simultaneously or succe ively on the same body, are compounded In such a manner that the body is found at every instant in the same point of space in which it would have been found in c< of the combination of the motions had they existed rately. Thus, if a body be acted on by two forces at the same Instant, one of winch acting alone would cause it to move uniformly over A B, and the other acting alone would cause it to move over A C at right angles to A B in the lime; the Telocity of the body in the one of these directions will not be changed by the force impelling it In the other. With regard to the first of these laws, it is evi dent that a body in motion, not subjected to the action o' any new force, will continue lo move in the prolongation of the straight line in which it moves at any given instant. since there is no reason why it should deviate from its rectilinear direction rather to one side than the other; but the uniformity of Its motion cannot be a erted h prior i: it is only by induction and experience that we come to be per suaded that the velocity with which any body is impre ed will not diminish of itself, nor the body finally come to rest unle it is impeded by some external causes. The second law, which involves the theory of the composition of forces, has been demonstrated by several m matic'ians, particularly Daniel Bernoulli, DAlernbert, Laplace in the Me can i que Celeste, and Poiinon in his excellent Traite de Me can i que; but all these demonstrations are too difficult to be accounted elementary. The science of dynamics is due entirely to the moderns, and its foundations were laid by the celebrated Galileo Before him no one had considered the forces which act on bodies, excepting in the case of equilibrium; anil although the acceleration of falling bodies, and the curvilinear motion of projectiles, had been attributed to the constant action of terrestrial gravity, no one had yet succeeded in determining the laws of these common phenomena. Galileo first made this important step in advance, and thereby opened a new and boundle field for the pro| of mechanics. Huygens added to Galileo's theory of tin acceleration of falling bodies the theories of the motion o( pendulums and of centrifugal forces, and thus prepared the way for the great discovery of universal gravitation In the hands of Newton mechanics became a new science, and the discovery of the infinitesimal calculus at length enabled geometers to expre all the laws and circumstances of the motion of bodies by analytical equations The investigation of the forces which produce the phenomena of the material world, or of the mutual action of the different parts of matter on each other, now form Indeed the principal object of mathematical studies. The best systematic treatises on Dynamics are to be found in Lit grange's Me can i que Analylique, and Pois.son's Trail p. d< Me can i que. Of late years several valuable elementary treatises have appeared in our own language. See Foi.ce, Mechanics, Motion. [s. 387]