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Of Matter and Motion.
TATTER is an extended folid fubftance; which being comprehended under distinct furfaces, makes fo many particular diftinct bodies.
Motion is fo well known by the fight and touch, that to use words to give a clear 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 reft; as there is to fet a body, which is at reft, into motion.
No parcel of matter can give itfelf either motion or reft, and therefore a body at reft will remain fo eternally, except fome external caufe 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 ; becaufe a body can no more alter the determination of its motion, than it can begin it, alter or ftop its motion itself.
The fwitness 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 fame time; their motions are equally swift; but if A moves two inches,
in the time whilft B is moving one inch; the motion of A is twice as fwift 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 inftance, 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 fwift, it hath double the quantity of motion; and fo in proportion.
It appears, as far as human obfervation reaches, to be a fettled law of nature, that all bodies have a tendency, attraction, or gravitation towards one another.
The fame force, applied to two different bodies, produces always the fame 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 veffel, which with its lading is twenty-fix tons; if the rope that ties them together be pulled, either in the lefs or bigger of thefe veffels, the lefs of the two, in their approach one to another, will move twenty-fix feet, while the other moves but one foot.
Wherefore the quantity of matter in the earth being twenty-fix 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-fix times as faft as in the earth; that is, the moon will move twenty-fix miles towards the earth, for every mile the earth moves towards the
Hence it is, that, in this natural tendency of bodies towards one another, that in the leffer is confidered as gravitation; and that in the bigger as attraction; because the motion of the leffer body (by reason of its much greater fwiftnefs) is alone taken notice of.
This attraction is the strongest, the nearer the attracting bodies are to each other; and, in different diftances of the fame bodies, is reciprocally in the duplicate proportion of those distances. For inftance, 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 fo 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 fo to be taken as a principle in natural philosophy.
Suppofing then the earth the fole body in the universe, and at reft; if God fhould create the moon, at the fame distance that it is now from the earth; the earth and the moon would prefently 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 cafes it will be fuch a curve as ends where it began, or recurs into itself; that is, makes up a circle, or an ellipfis or oval very little differing from a circle.
CHA P. II.
Of the Universe.
O any one, who looks about him in the world, there are obvious feveral diftinct maffes of matter, feparate from one another; fome whereof have difcernible motions. These are the fun, the fixt stars, the comets and the planets, amongst which this earth, which we inhabit, is one. All these are vifible to our naked eyes.
Besides these, telescopes have discovered several fixt ftars, invifible to the naked eye; and feveral other bodies moving about fome of the planets; all which were invifible and unknown, before the use of perspectiveglaffes were found.
The vaft diftances between thefe great bodies, are called intermundane fpaces; in which though there may be fome Auid matter, yet it is fo thin and fubtile, and there is fo little of that in refpect of the great maffes that move in those spaces, that it is as much as nothing. Thefe maffes of matter are either luminous, or opake or dark.
Luminous bodies, are fuch as give light of themfelves; and fuch are the fun and the fixt stars.
Dark or opake bodies, are fuch as emit no light of themselves, though they are capable of reflecting of it, when it is caft upon them from other bodies; and fuch are the planets.
There are fome opake bodies, as for inftance the comets, which, befides the light that they may have from the fun, feem to fhine with a light that is nothing else but an accenfion, which they receive from the fun, in their near approaches to it, in their respective revolu
The fixt stars are called fixt, because they always keep the fame diftance one from another.
The fun, at the fame diftance from us that the fixt ftars are, would have the appearance of one of the fixt ftars.
CHA P. III.
Of our Solar System.
UR folar fyftem confifts of the fun, 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 fhine by reflecting the light of the fun.
They are called planets from a Greek word, which fignifies wandering; because they change their places, and do not always keep the fame diftance with one another, nor with the fixt ftars, as the fixt stars do.
The planets are either primary, or fecondary.
There are fix primary planets, viz. Mercury, Venus, the Earth, Mars, Jupiter, and Saturn.
All these move round the fun, which is, as it were, the centre of their motions.
The fecondary planets move round about other planets. Befides the moon, which moves about the earth; four moons move about Jupiter, and five about Saturn, which are called their fatellites.
The middle distances of the primary planets from the fun are as follows:
The orbits of the planets, and their respective diftances from the fun, and from one another, together with the orbit of a comet, may be seen in the figure of the folar fyftem hereunto annexed,
The periodical times of each planet's revolution about the fun are as follows:
The planets move round about the fun from weft to east in the zodiac; or, to speak plainer, are always found amongst fome of the stars of those conftellations, which make the twelve figns of the zodiac.
The motion of the planets about the fun is not perfectly circular, but rather elliptical.
The reason of their motions in curve lines, is the attraction of the fun, or their gravitations towards the fun,
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