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planets in straight lines from those parts of their orbits 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 resistance 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 heavens; and the best modern observers find that our 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 conwhich in the present age shew the times of solar and sequences
thereof. 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
* 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.
jectile force at the beginning, adds a little more to it
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
Of Light. Its proportional Quantities on the different
Planets. Its Refractions in Water and Air. The
IGHT consists of exceeding small par-
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. ENTytt 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. II. p. 65.
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; effects and if they were as large as the smallest particles of that would matter discernible by our best microscopes, instead ensue,
from their of being serviceable to us, they would soon deprive being us of sight, by the force arising from their immense larger. 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. by reflected to our eyes, they excite in us the idea of ble to us. that body, by forming its picture on the retinaf. 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.
one ano. ther'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
* Book ). Art. 57.
thinner in every part of that space; at a treble dis. Plate II. tance, it will be nine times thinner; and at a quadruple distance, sixteen times thinner, than it was at first; and so on, according to the increase of the square surfaces B, C, D, E, described upon the distances AB, AC, AD, AE. Consequently, the quantities of this rarefied light received upon a surface of any given size and shape whatever, removed successively to these several distances, will be but one-fourth, one-ninth, one-sixteenth, respectively, of the whole quantity received by it at the first distance
Or, in general words, the densities and quantities of light, received upon any given plane, are diminished in the same proportion, as the squares of the distances of that plane, from the luminous body, are increased : and on the contrary, are increased in the same proportion as these squares are diminished.
170. The more a telescope magnifies the discs of Why the the Moon and planets, so much the dimmer they planets appear
than to the bare eye; because the telescope dimmer cannot magnify the quantity of light as it does the when surface; and, by spreading the same quantity of light through over a surface so much larger than the naked eye telescopes
than by beheld, just so much dimmer must it appear when viewed by a telescope, than by the bare eye.
171. When a ray of light passes out of one medium* into another, it is refracted, or turned out of its first course, more or less, as it falls more or less obliquely on the refracting surface which divides the two mediums. This may be proved by several experiments ; of which we shall only give three for example's sake.
1. In a bason, FGH, put a piece of Fig. VIII. money, as DB, and then retire from it to A ; that is, till the edge of the bason at E just hides the money from your sight; then keeping your head
the bare eye.
A medium, in this sense, is any transparent body, or that through which the rays of light can pass; as water, glass, diamond, air; and even a vacuum is sometimes called a medium.