MOTION

, or Local Motion, is a continued and successive change of place. Borelli defines it, the successive passage of a body from one place to another, in a determinate time, by becoming successively contiguous to all the parts of the intermediate space.

Motion is considered as of various kinds; as Natural, Violent, Absolute and Relative, &c, &c.

Natural Motion, is that which has its principle, or actuating force, within the moving body. Such is that of a stone falling towards the earth. And

Violent Motion, is that whose principle is without, and against which the moving body makes a resistance. Such is that of a stone thrown upwards, or of a ball shot off from a gun, &c.

Motion is again divided into Absolute and Relative.

Absolute Motion, is the change of absolute place, in any moving body, considered independently of any other motion; whose celerity therefore will be measured by the quantity of absolute space which the moveable body runs through. And

Relative Motion, is the change of the relative place of a moving body, or considered with respect to the motion of some other body; and has its celerity estimated by the quantity of relative space run through.

As to the Continuation of Motion, or the cause why a body once in Motion comes to persevere in it: this has| been much controverted among physical writers; and yet it follows very evidently from one of the grand Laws of Nature; viz, that all bodies persevere in their present state, whether of rest or motion, unless disturbed by some foreign powers. Motion therefore, once begun, would be continued in infinitum, were it to meet with no interruption from external causes; as the power of gravity, the resistance of the medium, &c.

Nor has the communication of motion, or how a moving body comes to affect another at rest, or how much of its motion is communicated by the first to the last, been less disputed. See the Laws of it under the word Percussion.

Motion is the proper subject of mechanics; and mechanics is the basis of all natural philosophy; which hence becomes denominated Mechanical.

In effect, all the phenomena of nature, all the changes that happen in the system of bodies, are owing to Motion; and are directed according to the laws of it. Hence the modern philosophers have applied themselves with peculiar ardour to consider the doctrine of Motion; to investigate the properties and laws of it; by observation and experiment, joined to the use of geometry. And to this is owing the great advantage of the modern philosophy above that of the Ancients; who were extremely disregardful of the effects of Motion.

Among all the Ancients, there is nothing extant on Motion, excepting some things in Archimedes's books, De Æquiponderantibus. To Galileo is owing a great part of the doctrine of Motion: he first discovered the general laws of it, and particularly of the descent of heavy bodies, both perpendicularly and on inclined planes; the laws of the Motio<*> of projectiles; the vibration of pendulums, and of stretched cords, with the theory of resistances, &c: things which the Ancients had little notion of.

Torricelli polished and improved the discoveries of his master, Galileo; and added many experiments concerning the force of percussion, and the equilibrium of fluids. Huygens improved very considerably on the doctrine of the pendulum; and both he and Borelli on the force of percussion. Lastly, Newton, Leibnitz, Varignon, Mariotte, &c, have brought the doctrine of Motion still much nearer to perfection.

The general laws of Motion were first brought into a system, and analytically- demonstrated together, by Dr. Wallis, Sir Christopher Wren, and M. Huygens, all much about the same time; the first in bodies not elastic, and the two latter in elastic bodies. Lastly, the whole doctrine of Motion, including all the discoveries both of the Ancients and Moderns on that head, was given by Dr. Wallis in his Mechanica, sive De Motu, published in 1670.

Quantity of Motion, is the same as Momentum, which see. It is a principle maintained by the Cartesians, and some others, that the Creator at the beginning impressed a certain Quantity of Motion on bodies; and that under such laws, as that no part of it should be lost, but the same portion of Motion should be constantly preserved in matter: and hence they conclude, that if any moving body strike another body, the former loses no more of its Motion than it communicates to the latter. This position however has been opposed by other philosophers, and perhaps justly, unless the preservation of Motion be understood only of the quantity of it as estimated always in the same direction; for then it seems the principle will hold good. However, the reasoning ought to have proceeded in the contrary order; by first observing from experiment, or otherwise, that when two bodies act upon each other, the one gains exactly the Motion which is lost by the other, in the same direction; and from hence made the inference, that there is therefore the same Quantity of Motion preserved in the universe, as was created by God in the beginning; since no body can act upon another, without being itself equally acted upon in the opposite or contrary direction.

The Continuation of Motion, or the cause why a body once in Motion comes to persevere in it, has been much controverted among physical writers; and yet it follows very evidently from one of the grand Laws of Nature; viz, that all bodies persevere in their present state, whether of Motion or rest, unless they are disturbed by some foreign powers. Motion therefore, once begun, would be continued for ever, were it to meet with no interruption from external causes; as the power of gravity, the resistance of the medium, &c.

The Communication of Motion, or the manner in which a moving body comes to affect another at rest, or how much of its Motion is communicated by the sirst to the last, has also been the subject of much discussion and controversy. See the Laws of it under the word Percussion.

Motion may be considered either as Equable, and Uniform; or as Accelerated, and Retarded. Equable Motion, again, may be considered either as Simple, or as Compound; and Compound Motion either as Rectilinear, or as Curvilinear.

And all these again may be considered either with regard to themselves, or with regard to the manner of their production, and communication, by percussion, &c.

Equable Motion, is that by which the moving body proceeds with exactly the same velocity or celerity; passing always over equal spaces in equal times.

The Laws of Uniform Motion, are these: 1. The spaces described, or passed over, are in the compound ratio of the velocities, and the times of describing those spaces. So that, if V and v be any two uniform velocities, S and s the spaces described or passed over by them, in the respective times T and t: then is , or ; taking T = 4, t = 3, V = 5, and v = 4.

2. In Uniform Motions, the time is as the space directly, and as the velocity reciprocally; or as the space divided by the velocity. So that .

3. The velocity is as the space directly, and the time reciprocally; or as the space divided by the time. That is, .

Accelerated Motion, is that which continually receives fresh accessions of velocity. And it is said to be| uniformly accelerated, when its accessions of velocity are equal in <*>equal times; such as that which is produced by the continual action of one and the same force, like the force of gravity, &c.

Retarded Motion, is that whose velocity continually decreases. And it is said to be uniformly Retarded, when its decrease is continually proportional to the time, or by equal quantities in equal times; like that which is produced by the continual opposition of one and the same force; such as the force of gravity, in uniformly retarding the Motion of a body that is thrown upwards.

The Laws of Motion, uniformly accelerated or retarded, are these:

1. In uniformly varied motions, the space, S or s, is as the square of the time, or as the square of the greatest velocity, or as the rectangle or product of the time and velocity.

That is, .

2. The velocity is the time, or as the space divided by the the time, or as the square root of the space.

That is, .

3. The time is as the velocity, or as the space divided by the velocity, or as the square root of the space.

That is, .

4. When a space is described, or passed over, by an uniformly varied Motion, the velocity either beginning at nothing, and continually accelerated; or else beginning at some determinate velocity, and continually retarded till the velocity be reduced to nothing; then the space, so run over by the variable Motion, will be exactly equal to half the space that would be run over in the same time by the greatest velocity if uniformly continued for that time. So, for instance, if g denote the space run over in one second, or any other time, by such a variable Motion; then 2g would be the space that would be run over in one second, or the same time, by the greatest velocity uniformly continued for the same time; or 2g would be the greatest velocity per second which the moving body had. Consequently, if t be any other time, s the space run over in that time, and v the greatest velocity attained in it; then, from the foregoing articles, it will be the velocity, and the space. And hence, for any such uniformly varied Motions, the relations among the several quantities concerned, will be expressed by the following equations: viz, , , , . And these equations will hold good in the Motion either generated or destroyed by the force of gravity, or by any other uniform force whatever. See also the articles Gravity, Acceleration, Retardation, &c. Again,

Simple Motion, is that which is produced by some one power or force only, and is always rectilinear, or in one direction, whether the force be only momentary or continued. And

Compound Motion, is that which is produced by two or more powers acting in different directions. See Compound, and Composition of Motion.

If a moving body be acted on by a double power; the one according to the direction AB, the other according to AC; with the Compound Motion, or that which is compounded of these two together, it will describe the diagonal AD of the parallelogram, whose sides AB and AC it would have described in the same time with each of the respective powers apart.

And if the radius of a circle be carried round upon the centre C, while a point in the radius sets off from A, and keeps moving along the radius towards the centre; then, by this Compound Motion, the path of the point will be a kind of a spiral ABC.

For the Particular Laws of Motion, arising from the Collision of bodies, both Elastic and Non-elastic, and that where the directions are both Perpendicular and Oblique, see Percussion.

For Circular Motion, and the Laws of PROJECTILES, see the respective words.

For the Motion of Pendulums, and the Laws of Oscillation, see Pendulum.

Perpetual Motion, is a Motion which is supplied and renewed from itself, without the intervention of any external cause.

The celebrated problem of a Perpetual Motion, consists in the inventing a machine, which has the principle of its Motion within itself; and is a problem that has employed the mathematicians for 2000 years; though none perhaps have prosecuted it with attention and earnestness equal to those of the present age. Infinite are the schemes, designs, plans, engines, wheels, &c, to which this long-desired Perpetual Motion has given birth.

But M. De la Hire has proved the impossibility of any such machine, and sinds that it amounts to this; viz, to find a body which is both heavier and lighter at the same time; or to find a body which is heavier than itself. Indeed there seems but little in nature to countenance all this assiduity and expectation: among all the laws of matter and Motion, we know of none yet that seem likely to furnish any principle or foundation for such an effect.|

Action and reaction it is allowed are always equal; and a body that gives any quantity of Motion to another, always loses just so much of its own; but under the present state of things, the resistance of the air, the friction of the parts of machines, &c, do necessarily retard every Motion.

To continue the Motion therefore either, first, there must be a supply from some foreign cause; which in a Perpetual Motion is excluded.

Or, 2dly, all resistance from the friction of the parts of matter must be removed; which necessarily implies a change in the nature of things.

Or, 3dly and lastly, there must be some method of gaining a force equivalent to what is lost, by the artful disposition and combination of mechanic powers; to which last point then all endeavours are to be directed: but how, or by what means, such force should be gained, is still a mystery.

The multiplication of powers or forces, it is certain, avails nothing; for what is gained in power is lost in time, so that the quantity of Motion still remains the same. This is an inviolable law of nature; by which nothing is left to art, but the choice of the several combinations that may produce the same effect.

There are various ways by which absolute force may be gained; but since there is always an equal gain in opposite directions, and no increase obtained in the same direction; in the circle of actions necessary to make a perpetual movement, this gain must be presently lost, and will not serve for the necessary expence of force employed in overcoming friction, and the resistance of the medium. And therefore, though it could be shewn, that in an infinite number of bodies, or in an infinite machine, there could be a gain of force for ever, and a Motion continued to infinity, it does not follow that a perpetual movement can be made. That which was proposed by M. Leibnitz in the Leipsic Acts of 1690, as a consequence of the common effimation of the forces of bodies in Motion, is of this kind, and for this and other reasons ought to be rejected. See Perpetual Motion; also Orffyreus's Wheel, &c.

Animal Motion, is that by which the situation, figure, magnitude, &c, of the parts and members of animals are changed. Under these Motions, come all the animal functions; as respiration, circulation of the blood, excretion, walking, running, &c.

Animal Motions are usually divided into two species; viz, Natural and Spontaneous.

Natural Motion, is that involuntary one which is effected without the command of the will, by the mere mechanism of the parts. Such as the Motion of the heart and pulse; the Peristaltic Motion of the intestines, &c. But

Spontaneous, or Muscular Motion, is that which is performed by means of the muscles, at the command of the will; which is hence called Voluntary Motion. Borelli has a celebrated treatise on this subject, entitled De Motu Animalium.

Intestine Motion, denotes an agitation of the particles of which a body consists.—Some philosophers will have every body, and every particle of a body, in continual Motion. As for fluids, it is the definition they give of them, that their parts are in continual Motion. And as to solids, they infer the like Motion from the effluvia continually emitted through their pores. Hence Intestine Motion is represented to be a Motion of the internal and smaller parts of matter, continually excited by some external, latent agent, which of itself is insensible, and only discovers itself by its effects; appointed by Nature to be the great instrument of the changes in bodies.

, in Astronomy, is peculiarly applied to the orderly courses of the heavenly bodies.

Mean Motion. See Mean.

The Motions of the celestial luminaries are of two kinds: Diurnal, or Common; and Secondary, or Proper.

Diurnal, or Primary Motion, is that with which all the heavenly bodies, and the whole mundane sphere, appear to revolve every day round the earth, from east to west. This is also called the Motion of the Primum Mobile, and the Common Motion, to distinguish it from that rotation which is peculiar to each planet, &c.

Secondary, or Proper Motion, is that with which a star, planet, or the like, advances a certain space every day from the west towards the east. See the several Motions of each luminary, with the irregularities, &c. of them, under the proper articles, Earth, Moon, Star, &c.

Angular Motion, is that by which the angular position of any thing varics. See Angular.

Horary Motion, is the Motion during each hour. See Horary.

Paracentric Motion of Impetus. See PARACENTRIC.

Motion of Trepidation, &c. See Trepidation and Libration.

MOTIVE Power or Force, is the whole power or force acting upon any body, or quantity of matter, to move it; and is proportional to the momentum or quantity of motion it can produce in a given time. To distinguish it from the Accelerative force, which is considered as affecting the celerity only.