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But, on the other hand, when we consider that the degree or force of gravity is exactly in proportion to the quantities of matter in those bodies, without any regard to their bulk or quantity of surface, acting as freely on their internal as external parts, it seems to surpass the power of mechanism, and to be either the immediate agency of the Deity, or effected by a law originally established and imprest on all matter by him. But some affirm that matter, being altogether inert, cannot be impressed with any law, even by Almighty power: and that the Deity, or some subordinate intelligence, must therefore be constantly impelling the planets towards the Sun, and moving them with the same irregularities and disturbances which gravity would cause, if it could be supposed to exist. But, if a man may venture to publish his own thoughts, it seems to me no more an absurdity, to suppose the Deity capable of infusing a law, or what law he pleases, into matter, than to suppose him capable of giving it existence at first. The manner of both is equally inconceivable to us; but neither of them imply a contradiction in our ideas: and what implies po contradiction is within the power of Omnipotence.

161. That the projectile force was at first given by the Deity is evident. For matter can never put itself in motion, and all bodies may be moved in any direction whatever; and yet the planets, both primary and secondary, move from west to east, in planes nearly coincident; while the comets move in all directions, and in planes very different from one another; these motions can therefore be owing to no mechanical cause or necessity, but to the free will and power of an intelligent Being.

162. Whatever gravity be, it is plain that it acts every moment of time: for if its action should cease, the projectile force would instantly carry off the

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planets in straight lines from those parts of their or-
bits where gravity left them. But, the planets being
once put into motion, there is no occasion for any
new projectile force, unless they meet with some re-
sistance in their orbits ; nor for any mending hand,
unless they disturb one another too much by their
mutual attractions.

163. It is found that there are disturbances among the plathe planets in their motions, arising from their mutual nets dis

turb one attractions, when they are in the same quarter of the another's heavens; and the best modern observers find that cur motions. years are not always precisely of the same length*. Besides, there is reason to believe that the Moon is somewhat nearer the Earth now than she was formerly; her periodical month being shorter than it was in former ages. For our astronomical tables, The con. which in the present age shew the times of solar and sequences lunar eclipses to great precision, do not answer so well for very ancient eclipses. Hence it appears, that the Moon does not move in a medium void of all resistance, \ 174: and therefore her projectile force being a little weakened, while there is nothing to diminish her gravity, she must be gradually approaching nearer the Earth, describing smaller and smaller circles round it in every revolution, and finishing her period sooner, although her absolute motion with regard to space be not so quick now as it was formerly : and, therefore, she must come to the Earth at last; unless that Being, which gave her a sufficent pro

thereof.

of the

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* If the planets did not mutually attract one another, the areas described by them would be exactly proportionate to the times of description, $ 153. But observations prove that these areas are not in such exact proportion, and are most varied when the greatest number of planets are in any particular quarter of the heavens. When any two planets are in conjunction, their mutual attractions, which tend to bring them nearer to one another, draw the inferior one a little farther from the Sun, and the superior one a little nearer to him; by which means, the figure of their orbits is somewhat altered ; but this alteration is too small to be discovered in several ages.

it at I ceas

jectile force at the beginning, adds a little more to it in due time. And, as all the planets move in spaces full of ether and light, which are material substances, they too must meet with some resistance. And, therefore, if their gravities be not diminished, nor their projectile forces increased, they must necessarily approach nearer and nearer the Sun, and at length

fall upon and unite with him. The world

164. Here we have a strong philosophical argunot eter. ment against the eternity of the World. For, had it

existed from eternity, and been left by the Deity to be governed by the combined actions of the above forces or powers, generally called laws, it had been at an end long ago. And if it be left to them, it must come to an end. But we may be certain, that it will last as long as was intended by its Author, who ought no more to be found fault with for framing so perishable a work, than for making man mortal*.

nal.

CHAP. VIII.

L

Of Light. Its proportional Quantities on the different

Planets. Its Refractions in Water and Air. The
Atmosphere; its Weight and Properties. The
Horizontal moon.

IGHT consists of exceeding small par165.

ticles of matter issuing from a luminous body; as, from a lighted candle such particles of

matter constantly flow in all directions. Dr. NiewThe amaz. EN TYTf computes, that in one second of time there ing small- flow 418,660,000,000,000,000,000,000,000,000, particles000,000,000,000,000, particles of light out of a of light. burning candle ; which number contains at least

• M. de la Grange has demonstrated, on the soundest principles of philosophy, that the solar system is not necessarily perishable; but that the seeming irregularities in the planetary motions oscillate, as it were, within narrow limits; and that the world, according to the present constitution of nature, may be permanent,

+ Religious Philosopher, Vol. III. p. 65.

from their

6,337,242,000,000 times the number of grains of sand in the whole Earth ; supposing 100 grains of sand to be equal in length to an inch, and consequently, every cubit inch of the Earth to contain one million of such grains.

166. These amazingly small particles, by striking The upon our eyes, excite in our minds the idea of light; dreadful and if they were as large as the smallest particles of that would matter discernible by our best microscopes, instead ensue, of being serviceable to us, they would soon deprive being us of sight, by the force arising from their immense lárger. velocity ; which is above 164 thousand miles

every second*, or 1,230,000 times swifter than the motion of a cannon bullet. And, therefore, if the particles of light were so large, that a million of them were equal in bulk to an ordinary grain of sand, we durst no more open our eyes to the light, than suffer sand to be shot point blank against them.

167. When these small particles, flowing from the How obSun or from a candle, fall upon bodies, and are there-jects be

come visi, by reflected to our eyes, they excite in us the idea of that body, by forming its picture on the retina f. And since bodies are visible on all sides, light must be reflected from them in all directions.

168. A ray of light is a continued stream of these The rays particles, flowing from any visible body in a straight of light line. That the rays move in straight, and not in move in crooked lines, unless they be refracted, is evident straight from bodies not being visible if we endeavour to look at them through the bore of a bended pipe; and from their ceasing to be seen on the interposition of other bodies, as the fixed stars by the interposition of the Moon and planets, and the Sun wholly or in part by the interposition of the Moon, Mercury, or Venus. A proof And that these rays do not interfere, or jostle one that they

hinder not * This will be demonstrated in the eleventh chapter. + A fine net-work membrane in the bottom of the eye.

motions.

ble to us.

lines.

one another's

Plate II. another out of their ways, in flowing from different

bodies all around, is plain from the following experiment. Make a little hole in a thin plate of metal, and set the plate upright on a table, facing a row of lighted candles standing by one another; then place a sheet of paper or pasteboard at a little distance from the other side of the plate, and the rays of all the candles, flowing through the hole, will form as many specks of light on the paper as there are candles before the plate; each speck as distinct and large, as if there were only one candle to cast one speck; which shews that the rays are no bindrance to each other in their motions, although they all cross in the hole.

169. Light, and therefore heat, so far as it depends on the Sun's rays, (1 85, toward the end,) decreases in the inverse proportion of the squares of the distances of the planets from the Sun. This is easily demon

strated by a figure; which, together with its de. Fig. XI. scription, I have taken from Dr. Smith's Optics*.

Let the light which flows from a point A, and passes

through a square hole B, be received upon a plane C, In what parallel to the plane of the hole; or, if you please, let proportion the figure C be the shadow of the plane B; and when beat de. the distance C' is double of B, the length and breadth crease at of the shadow C will be each double of the length distance and breadth of the plane B; and treble when AD is from the treble of AB; and so on : which may be easily

examined by the light of a candle placed at A. Therefore the surface of the shadow C, at the distance AC double of AB, is divisible into four squares, and at a treble distance, into nine squares, severally equal to the square B, as represented in the figure. The light, then, which falls upon the plane B, being suffered to pass to double that distance, will be uniformly spread over four times the space, and consequently will be four times

Sun.

* Book ). Art. 57.

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