Obrazy na stronie
PDF
ePub

diurnal motion of the Sun westward, and bringing about the alternate returns of day and night.

urnal mo

121. As to the common objections against the objecEarth's motion on its axis, they are all easily an-against the swered, and set aside. That it may turn without be. Earth’s diing seen or felt by us to do so, has been already tion anshewn, $ 119. But some are apt to imagine that is swered the Earth turns eastward (as it certainly does, if it turns at all) a ball fired perpendicularly upward in the air must fall considerably westward of the place it was projected from. This objection, which at first seems to have some weight, will be found to have none at all, when we consider that the gun and ball partake of the Earth's motion; and therefore the ball being carried forward with the air as quick as the Earth and air turn, must fall down on the same place. A stone let fall from the top of a main-mast, if it meet with no obstacle, falls on the deck as near the foot of the mast when the ship sails as when it does not.

If an inverted bottle full of liquor, be hung up to the ceiling of the cabin, and a small hole be made in the cork to let the liquor drop through on the floor, the drops will fall just as far forward on the floor when the ship sails as when it is at rest. And gnats or flies can as easily dance among one another in a moving cabin, as in a fixed chamber. As for those scripture-expressions which seem to contradict the Earth's motion, the following reply may be made to them all: It is plain, from many instances, that the Scriptures were never intended to instruct us in philosophy or astronomy; and therefore, on those subjects, expressions are not always to be taken in the literal sense; but for the most part as accommodated to the common apprehensions of mankind. Men of sense in all ages, when not treating of the sciences purposely, have followed this method : and it would be in vain to follow any other in addressing ourselves to the vulgar, or bulk of any

community. Moses calls the Moon a GREAT LUMINARY (as it is in the Hebrew) as well as the Sun: but the Moon is known to be an opaque body, and the smallest that astronomers have observ. ed in the heavens; and that it shines upon us, not by any inherent light of its own, but by reflecting the light of the Sun. Moses might know this; but had he told the Israelites so, they would have stared at him; and considered him rather as a madman, than as a person commissioned by the Almighty to be their leader.

CHAP. IV.

The Phenomena of the Heavens as seen from different

Parts of the Earth.

kept to the 122.

[ocr errors]

We are

E are kept to the Earth's surface, on Earth by

all sides, by the power of its central gravity. attraction; which laying hold of all bodies accord

ing to their densities or quantities of matter, with. out regard to their buiks, constitutes what we call their weight. And having the sky over our heads, go where we will, and our feet toward the centre of the Earth; we call it up over our heads, and down under our feet: although the same right line which is down to us, if continued through and be.

yond the opposite side of the Earth, would be up to Plate II. the inhabitants on the opposite side. For, the inFig. 1.

habitants n, i, e, m, 4, 0, 9. I stand with their feet toward the Earth's centre C; and have the same figure of sky N, I, E, M, S, O, Q, L, over their heads. Therefore, the point S is as directly upward to the inhabitant s on the south pole, as N is to the inhabitant n on the north pole : so is E to the inhabitant e supposed to be on the north end of Peru; and Q to the opposite inhabitant q on the mid

dle of the island Sumatra. Each of these observers podes.

is surprised that his opposite or antipode can stand with his head hanging downward. But let either

Anti

go to the other, and he will tell him that he stood as Plate II. upright and firm on the place where he was, as he now stands where he is. To all these observers, the Sun, Moon, and stars, seem to turn round the points N and S, as the poles of the fixed axis NCS; Axis of because the Earth does really turn round the mathe- the world. matical line n C s as round an axis of which n is the

Its poles. north pole, and s the south pole. The inhabitant U (Fig. II.) affirms that he is on the uppermost side of Fig. 11. the Earth, and wonders how another at L can stand at the undermost side, with his head hanging downwards. But U in the mean time forgets, that in twelve hours time he will be carried half round with the Earth, and then be in the very situation that L now is; although as far from him as before; and yet, when U comes there, he will find no difference as to his manner of standing; only he will see the opposite half of the heavens, and imagine the heavens to have gone half round the Earth.

have an

123. When we see a globe hung up in a room, How our we cannot help imagining it to have an upper and an Earth under side, and immediately form a like idea of the

might Earth; from whence we conclude, that it is as im. upper possible for people to stand on the under side of the and an Earth, as for pebbles to lie on the under side of a side. common globe, which instantly fall down from it to the ground; and well they may, because the attraction of the Earth being greater than the attraction of the globe, pulls them away. Just so would it be with our Earth, if it were fixed near a globe much big. ger than itself, such as Jupiter : for then, it would really have an upper and an under side with respect to that large globe; which, by its attraction, would pull away every thing from the side of the Earth next to it; and only those bodies on its surface, at the opposite side, could remain upon it. But there is no larger globe near enough our Earth to overcome its

M

Plate II. central attraction, and therefore it has no such thing

as an upper and an under side; for all bodies on or near its surface, even to the Moon, gravitate toward its centre.

124. Let any man imagine the Earth, and every thing but himself, to be taken away, and he left alone in the midst of indefinite space; he could then have no idea of up or down; and were his pockets full of gold, he might take the pieces one by one, and throw them away on all sides of him, without any danger of losing them; for the attraction of his body would bring them all back by the ways they went, and he would be down to every one of them. But then, if a sun, or any other large body, were created and placed in any part of space, several millions of miles from him, he would be attracted toward it, and could not save himself from falling down to it.

Fig. I.

125. The Earth's bulk is but a point, as that at C, compared to the heavens; and therefore every inhabitant upon it, let him be where he will, as at n, e, m, s, &c. sees half of the heavens. The inha. bitant n, on the north pole of the Earth, constantly sees the hemisphere E NQ; and having the north pole N of the heavens just over his head, his hori.

zon coincides with the celestial equator EC Q. Half of Therefore all the stars in the northern hemisphere the hea- EN Q, between the equator and north pole, appear ble to an to turn round the line N C, moving parallel to the inhabitant horizon. The equatorial stars keep in the horizon, part of the and all those in the southern hemisphere E SQ are

invisible. The like phenomena are seen by the observer s on the south pole, with respect to the hemi. sphere ES Q; and to him the opposite hemisphere is always invisible. Hence, under either pole, only

Earth.

one half of the heavens is seen; for those parts which are once visible never set, and those which are once invisible never rise. But the ecliptic Y C X, or orbit which the Sun appears to describe once a year by the Earth's annual motion, has the half Y C' constantly above the horizon E C Q of the north pole n; and the other half C X always below it. There- Phenofore while the Sun describes the northern half y cmena at

the poies. of the ecliptic, he neither sets to the north pole, nor rises to the south; and while he describes the southern half C X, he neither sets to the south pole, nor rises to the north. The same things are true with respect to the Moon; only with this difference, that as the Sun describes the ecliptic but once a year, he is for half that time visible to each pole in its turn, and as long invisible; but as the Moon goes round the ecliptic in 27 days 8 hours, she is only visible for 13 days 16 hours, and as long invisible to each pole by turns. All the planets likewise rise and set to the poles, because their orbits are cut obliquely in halves by the horizon of the poles. When the Sun (in his apparent way from X) arrives at C, which is on the 20th of March, he is just rising to an observer at n, on the north pole, and setting to another at s, on the south pole. From Che rises higher and higher in every apparent diurnal revolution, till he comes to the highest point of the ecliptic y, on the 21st of June; when he is at his greatest altitude, which is 23, degrees, or the arc E y, equal to his greatest north declination; and from thience he scems to descend gradually in every apparent circumvolution, till he sets at C' on the 23d of September; and then he goes to exhibit the like appearances at the south pole for the other half of the year. Hence the Sun's apparent motion round the Earth is not in parallel circles, but in spirals; such as might be represented by a thread wound round a globe from tropic to tropic; the spirals being at some distance from one an

« PoprzedniaDalej »