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286. Hence, if the sun illumines half the earth, hé illumines 180 degrees of the earth; or 90 degrees every way, from the place over which he is vertical. Hence, also, it is 180 degrees from the north to the south pole ; and 90 degrees from each pole is the middle of the earth, called
the Equator. 287. Hence, as half the heavens are always visible, 180 degrees are visible; and from the point over head, it will be 90 degrees to that line, where the earth and the heavens appear to the eye to meet, called the horizon. Hence, also, an inhabitant of the equator can see the stars as far as each pole; i. e. he can see 90 degrees each way
288. The inhabitants of the poles can see the stars as far as the equator, and no farther ; i. e. they can see 90 degrees. When the sun is vertical over the equator, he shines as far as each pole : because he shines 90 degrees from the place where he is vertical, or over head.
289. Also, when the sun is vertical 10 degrees north of the equator, he shines 10 degrees beyond the north pole, and his rays do not reach the south pole by 10 degrees; and when he becomes vertical 23 degrees north of the equator, he shines 23 degrees beyond the north pole, and 231 degrees short of the south pole.
290. In its annual orbit, the earth ascends 231 degrees above the level of the equator, and descends 23 degrees below. Hence, when it is at the highest point above, the sun will be vertical over that part of the globe which is 231 below the equator, and when below, the contrary.
Obs.-The terms above and below, up and down, relate merely to the fcelings and sensations of human beings. In nature, there is no up or down, or above or below. The earth is round, and all bodies fall towards its centre; because, according to Sir R. Phillips, they perform rotations inversely as their density, or seek to exhibit equal momenta in re · volving with the mass. All men, and every thing called upright, stand in a straight line towards the contre of the earth;
with the earth beneath their feet, and the heavens, which surround the earth, over their heads. We usually place the south pole downward, but the inhabitant of the south pole, like him at the north pole, stands with his feet towards the centre of the earth, both having the heavens over head. The inhabitants of New South Wales are the Antipodes to the people in England, standing with their feet towards them, and their heads in opposite directions, each of them wondering how the other stands; but the earth is the centre or loadstone of all its inhabitants; and in nature up and down are merely relative terms.
2. It is usual for authors to talk about the inclination of the axis to the orbit, its parallelism, &c. &c. but as I consider the ascent and descent in the plane as more simple, and more accordant with the phenomena, although it is a mere change of terms, I prefer my own explication to that gener. ally adopted, particularly in aid of the tutor, if he should amuse his pupils by passing a globe round a candle, to shew the change of the seasons. The idea of an inclination of the axis I consider a vulgar error. The moon ascends and descends in its orbit about five and a quarter degrees; but no one ever talked of the inclination of the moon's axis to the plane of its orbit!
3. The obliquity or angle of the orbit diminishes at the rate of a minute in 110 years, and a degree in 6,600 years. Observations were made in China 2,900 years ago; by which it appears, that the obliquity was 23o. 54'; but is now only 23° 28'.-a wonderful coincidence, and a proof, at once, of the diminution and the observation.
291. It must be evident, that during all the time the sun is vertical north of the equator he will shine as many degrees beyond the north pole as he is vertical north of the equator; and that, during all the time he is vertical to the south of the equator, he will constantly illumine as many degrees beyond the south pole.
Obs.-As it is ninety degrees from the equator to each pole, and as the sun shines ninety degrees from the part where he is vertical, he must of course shine as many degrees beyond either pole as he is degrees advanced towards it from the equator. If I can read an inscription, ninety yards off, and I advance twenty-three and a half nearer to it, it is evident I could read it if it were now placed twenty three and a half yards further. The understanding of this single proposition, is all that is needful to comprehend the phenomena of the seasons, and the various lengths of day and night.
292. The earth, in its diurnal rotation, carries every place round in a circle which is equi-distant from the equator; and all places which are exactly the same distance from the equator, are carried round in the same circle.
Distance to the north or south of the equator is called latitude; and of course,
if the sun shines vertically at 10 degrees north of the equator, all places having 10 degrees of north latitude will pass exactly under the sun on that day.
293. As the sun, when vertical 10 degrees north of the equator, shines 10 degrees beyond the north pole, and 10 degrees short of the south pole, it is evident that during that rotation of the earth, no place within 10 degrees of the north pole can turn out of the sun-shine, so that to them it will be all day; and that no place within 10 degrees of the south pole, can turn into the sun-shine, so that to them it will be all night.
294. When the sun is vertical over the equator, he will then, of course, shine exactly as far as each polu; and the boundary of day and night will cut all the circles made by the diurnal rotation of every place, into two equal parts; so that the day part of the circle being equal to the night part, the days and nights will then be equal all over the world.
295. The sun is vertical over the equator on the 21st of March; and the earth descends in its annual orbit for 91 days, till the 21st of June, when the sun is vertical over all places 23; degrees north of the equator; so that, during the 91 days, the sun gradually gains the 23 degrees, and has been successively vertical over all countries within that distance of the equator.
296. During the same time, he has successively shone, by similar gradual advances, as many degrees beyond the north pole, and afforded to those countries perpetual day; and, of course, an increased length
of day, to all places in the northern hemisphere, in proportion to their proximity to the pole, in consequence of his shining over the larger part of their diurnal circles.
297. The opposite phenomena will, necessarily, take place in the southern hemisphere of the earth. The sun's rays will fall short of the south pole as many degrees as he advances above the equator, and the country round that pole will be involved in darkness ; and the nights will increase in length, in the same proportion as the days increase in the northern hemisphere.
Obs.-All this will be made evident, by hanging any round body somewhat below the level of a fire or candle. It will be seen, that the light shines over one pole, and does not reach the other. If then, the ball be turned round, it will be observed, that the circles performed by any parts of the surfaee are unequally divided by the light; and that it will be constant day near the north pole, and that all the phenomena will be reversed in the other, or lower hemisphere.
298. When the earth has so ascended in its orbit, as to render the sun vertical 23 degrees north of the equator, it then begins to descend again after the same rate; and in 91 days, viz. the 21st of September, the sun becomes vertical again over the equator.
The earth, however, continues to ascend in its orbit by successive gradations; till, on the 21st of December, the sun becomes vertical at 23, degrees south of the equator, rendering it constant day at the south pole, and reversing all the phenomena which were experienced, when he was vertical 23 degrees to the north.
299. The earth then gradually descends in its orbit till the 21st of March, when the sun being vertical over the equator, and inclining towards neither of the poles, the days, of course, are equal in each hemisphere; and spring is beginning in the northern hemisphere, and autumn in the southern.
Look nature through, 'tis revolution all;
The dying day. Stars rise, and set, and rise.
300. The heat of summer is produced by two causes ; first, by the greater intensity of the sun's rays, owing to their falling with increased perpendicularity and density; and secondly, by the excess of the length of the days over that of the nights ; so that the heat imbibed in the day is not wholly parted with during the night.
Obs. The effect of obliquity, in regard to rays, will be evident, if a board be held perpendicularly before a fire. It will then receive a body of rays equal to its breadth. But if it be placed obliquely, at an angle of 45 degrees, then only half the rays will fall on its surface, and the other half will pass over it. So it is with the surface of the earth in summer and winter.
2. The increased heat in the polar regions is amazingly great, owing to six months' continued day: which melts, or nearly, the ice and snow, produced during the six months' night. Hence, in northern countries, where the day lasts 18 and 20 hours, or where the sun is above the horizon for any number of days together, the heat of the summer is equal to that of any part of the world.
301. The air which we breathe, and in which we live, just as fishes live in water, surrounds the whole earth, to the height, as is supposed, of 40 miles. It is of a blue colour, and therefore gives that colour to the heavens. The passage of the sun's rays through it, is supposed to generate part of their heat, as well as their action on the ground and on various bodies.
302. The solar heat is supposed, indeed, to be excited, chiefly, by its action on terrestrial bodies; it being found, that in the hottest climates, snow and ice lie unthawed all the year on the tops of mountains; and