ELEMENTS OF

NATURAL PHILOSOPHY.

[I AM not acquainted with a better compendium of natural philosophy than this. The science, no doubt, has received very great improvements since the time of Locke, but his exposition of it is still sufficiently exact for all practical purposes. The explanations of terms are brief, correct, and intelligible; and the accounts of the grander phenomena of the universe, though designed only as incentives to inquiry, are such as to open up very magnificent prospects before the mind. As it would be prepos terous to render that long by annotation which the author expressly made short and simple, that it might be the more easily comprehended and the substance of it lodged firmly in the memory, I shall trouble the reader with very few notes.-ED.]

CHAPTER I.

OF MATTER AND MOTION.

MATTER is an extended solid substance; which being comprehended under distinct surfaces, makes so many particular distinct bodies.

Motion is so well known by the sight and touch, that to use words to give a clearer idea of it would be in vain.

Matter, or body, is indifferent to motion or rest.

There is as much force required to put a body, which is in motion, at rest; as there is to set a body, which is at rest, into motion.

No parcel of matter can give itself either motion or rest, and therefore a body at rest will remain so eternally, except some external cause puts it in motion; and a body in motion will move eternally, unless some external cause stops it.

A body in motion will always move on in a straight line, unless it be turned out of it by some external cause, because a body can no more alter the determination of its motion than it can begin, alter, or stop, its motion itself.

The swiftness of motion is measured by distance of place and length of time wherein it is performed. For instance, if A and B, bodies of equal or different bigness, move each

of them an inch in the same time, their motions are equally swift; but if A moves two inches in the time whilst B is moving one inch, the motion of A is twice as swift as that of B.

The quantity of motion is measured by the swiftness of the motion,* and the quantity of the matter moved, taken together. For instance, if A, a body equal to B, moves as swift as B, then it hath an equal quantity of motion. If A hath twice as much matter as B, and moves equally as swift, it hath double the quantity of motion, and so in proportion.

It appears, as far as human observation reaches, to be a settled law of nature, that all bodies have a tendency, attraction, or gravitation towards one another.

The same force, applied to two different bodies, produces always the same quantity of motion in each of them. For instance, let a boat which with its lading is one ton, be tied at a distance to another vessel, which with its lading is twenty-six tons; if the rope that ties them together be pulled, either, in the less or bigger of these vessels, the less of the two, in their approach one to another, will move twenty-six feet, while the other moves but one foot.

Wherefore the quantity of matter in the earth being twenty-six times more than in the moon, the motion in the moon towards the earth, by the common force of attraction, by which they are impelled towards one another, will be twenty-six times as fast as in the earth; that is, the moon will move twenty-six miles towards the earth, for every mile the earth moves towards the moon.

Hence it is, that, in this natural tendency of bodies towards one another, that in the lesser is considered as gravitation, and that in the bigger as attraction,t because the motion

*Whether this be consistent with the received theory of motion is more than I can say, but it appears to me to be a fallacy; for motion having reference to the space traversed, and the time in which the transit is performed, there is as much motion in an ounce ball which traverses five hundred yards in a given number of seconds as in a pound ball which traverses the same distance in the same time, though the motive power which set the matter in motion must be evidently greater than that which imparted motion to the former. Locke, therefore, appears here to confound motion with the motive power; that is, if I apprehend his meaning exactly.-ED.

+ Besides the works of Sir Isaac Newton and the more modern phi

of the lesser body (by reason of its much greater swiftness) is alone taken notice of.

This attraction is the strongest the nearer the attracting bodies are to each other; and, in different distances of the same bodies, is reciprocally in the duplicate proportion of those distances. For instance, if two bodies, at a given distance, attract each other with a certain force, at half the distance they will attract each other with four times that force; at one third of the distance, with nine times that force; and

so on.

Two bodies at a distance will put one another into motion by the force of attraction; which is inexplicable by us, though made evident to us by experience, and so to be taken as a principle in natural philosophy.

Supposing then the earth the sole body in the universe, and at rest; if God should create the moon, at the same distance that it is now from the earth, the earth and the moon would presently begin to move one towards another in a straight line by this motion of attraction or gravitation.

If a body, that by the attraction of another would move in a straight line towards it, receives a new motion any ways oblique to the first, it will no longer move in a straight line, according to either of those directions, but in a curve that will partake of both. And this curve will differ, according to the nature and quantity of the forces that concurred to produce it; as, for instance, in many cases it will be such a curve as ends where it began, or recurs into itself: that is, makes up a circle, or an ellipsis* or oval very little differing from a circle,

losophers, to which the reader will refer on this subject, it may be worth while to examine the previous speculation of Hobbes, in which the same theory is developed, though with less method and completeness. (Elements of Philosophy, Part IV. c. xxx. § 2. See also Lord Bacon, Sylva Sylvarum, 703-4.)-ED.

* Kepler seems to have been the first who observed that the planets may move in ellipses; but it was reserved for Sir Isaac Newton to demonstrate the truth of this observation. The reader will find this demonstration in Lord King's Life of Locke, vol. i. p. 389, et seq., "where," in the opinion of his Lordship, "the lemmas which are prefixed are expressed in a more explanatory form than those of the Principia usually are."-ED.

CHAPTER II.

OF THE UNIVERSE.

To any one, who looks about him in the world, there are obvious several distinct masses of matter, separate from one another; some whereof have discernible motions. These are the sun, the fixed stars, the comets and the planets, amongst which this earth, which we inhabit, is one. All these are visible to our naked eyes.

Besides these, telescopes have discovered several fixed stars, invisible to the naked eye; and several other bodies moving about some of the planets; all which were invisible and unknown, before the use of perspective glasses were found.

The vast distances between these great bodies are called intermundane spaces; in which though there may be some fluid matter, yet it is so thin and subtile, and there is so little of that in respect of the great masses that move in those spaces, that it is as much as nothing.

These masses of matter are either luminous, or opaque or dark.

Luminous bodies, are such as give light of themselves; and such are the sun and the fixed stars.

Dark or opaque bodies are such as emit no light of themselves, though they are capable of reflecting of it, when it is cast upon them from other bodies; and such are the planets.

There are some opaque bodies, as for instance the comets, which, besides the light that they may have from the sun, seem to shine with a light that is nothing else but an accension, which they receive from the sun, in their near approaches to it, in their respective revolutions.

The fixed stars are called fixed, because they always keep the same distance one from another.

The sun, at the same distance from us that the fixed stars are, would have the appearance of one of the fixed stars.

CHAPTER III.

OF OUR SOLAR SYSTEM.

OUR solar system consists of the sun, and the planets and comets moving about it.

The planets are bodies. which appear to us like stars; not that they are luminous bodies, that is, have light in themselves; but they shine by reflecting the light of the sun.

They are called planets from a Greek word, which signifies wandering; because they change their places, and do not always keep the same distance with one another, nor with the fixed stars, as the fixed stars do.

The planets are either primary, or secondary.

There are six primary planets,* viz., Mercury, Venus, the Earth, Mars, Jupiter, and Saturn.

All these move round the sun, which is, as it were, the centre of their motions.

The secondary planets move round about other planets. Besides the moon, which moves about the earth, four moons move about Jupiter, and five about Saturn,† which are called their satellites.

The middle distances of the primary planets from the sun are as follows:

The orbits of the planets, and their respective distances from the sun and from one another, together with the orbit of a comet, may be seen in the figure of the solar system hereunto annexed.‡

The periodical tines of each planet's revolution about the sun are as follows:

The number now discovered amounts to twenty-three. Of these, twelve have been discovered since the year 1845, eleven of them rotating between the orbits of Mars and Jupiter; the remaining one is the planet Neptune, exterior to all the rest, and whose discovery is one of the greatest intellectual triumphs of the present age.—ED.

+ Saturn has been found by modern astronomers to possess eight moons, besides his luminous belts; and Uranus certainly has four moons, if not more. -ED.

The engraving alluded to, being now commonly found in all elementary treatises on the subject, has been omitted.--ED.