Imágenes de páginas

middle, between these times, viz. on the 22d of Winter December, the north pole is as far as it can be in the solstice. dark, which is 234 degrees, equal to the inclination of the Earth's axis from a perpendicular to its orbit: and then the northern parallels are as much in the dark as they were in the light on the 21st of June; the winter nights being as long as the summer days, and the winter days as short as the summer nights. It is needless to enlarge farther on this subject, as we shall have occasión to mention the seasons again in describing the Orrery, § 397. Only this must be noted, that whatever has been said of the northern hemisphere, the contrary must be understood of the southern; for on different sides of the equator the seasons are contrary; because, when the northern hemisphere inclines toward the Sun, the southern declines from him.


204. As Saturn goes round the Sun, his oblique- The phely-posited ring, like our Earth's axis, keeps parallel of Saturn's to itself, and is therefore turned edgewise to the Sun ring. twice in a Saturnian year; which is almost as long as 30 of our years, § 81. But the ring, though considerably broad, is too thin to be seen by us when it is turned edgewise to the Sun, at which time it is also edgewise to the Earth; and therefore it disappears once in every fifteen years to us. As the Sun shines half a year together on the north pole of our Earth, then disappears to it, and shines as long on the south pole; so, during one half of Saturn's year, the Sun shines on the north side of his ring, then disappears to it, and shines as long on the south side. When the Earth's axis inclines neither to nor from the Sun, but is sidewise to him, he then ceases to shine on one pole, and begins to enlighten the other; and when Saturn's ring inclines neither to nor from the Sun, but is edgewise to him,

Plate V.

Fig. III.

he ceases to shine on the one side of it, and begins to shine upon the other.

Let S be the Sun, ABCDEFGH Saturn's orbit, and IKLMNO the Earth's orbit. Both Saturn and the Earth move according to the order of the letters: when Saturn is at A his ring is turned edgewise to the Sun S, and he is then seen from the Earth as if he had lost his ring, let the Earth be in any part of its orbit whatever, except between N and 0; for while it describes that space, Saturn is apparently so near the Sun as to be hid in his beams. As Saturn goes from A to C, his ring appears more and more open to the Earth: at Cthe ring appears most open of all; and seems to grow narrower and narrower, as Saturn goes from C'to E, and when he comes to E, the ring is again turned edgewise both to the Sun and Earth; and as neither of its sides are illuminated, it is invisible to us, because its edge is too thin to be perceptible; and Saturn appears again as if he had lost his ring. But as he goes from E to G, his ring opens more and more to our view on the under side; and seems just as open at G as it was at C; and may be seen in the night time from the Earth in any part of its orbit, except about M, when the Sun hides the planet from our view. As Saturn goes from G to A, his ring turns more and more edgewise to us, and therefore it seems to grow narrower and narrower; and at A, it disappears as before. Herce, while Saturn goes from A to E, the Sun shines on the upper side of his ring, and the under side is dark; and while he goes from E to A, the Sun shines on the under side of his ring, and the upper side is dark.

It may perhaps be imagined that this article might have been placed more properly after § 81, than here; but when the candid reader considers Fig. I. and that all the various phenomena of Saturn's ring depend upon a cause similar to that of our Earth's


seasons, he will readily allow that they are best ex- Plate VI. plained together; and that the two figures serve to

illustrate each other.

nearer the


weather is


205. The Earth's orbit being elliptical, and the The Earth Sun keeping constantly in its lower focus, which is Sun in 1,377,000 miles from the middle point of the longer winter axis, the Earth comes twice so much, or 2,754,000 than in miles, nearer the Sun at one time of the year than at another: for the Sun appearing to us under a larger angle in winter than in summer, proves that the Earth is nearest the Sun in winter (see the Note on Article 185). But here this natural ques- Why the tion will arise: Why have we not the hottest weather coldest when the Earth is nearest the Sun? In answer it when the must be observed, that the eccentricity of the Earth's Earth is orbit, or 1,377,000 miles, bears no greater propor- the Sun. tion to the Earth's mean distance from the Sun, than 17 does to 1000; and therefore this small difference of distance cannot occasion any sensible difference of heat or cold. But the principal cause of this difference is, that in winter the Sun's rays fall so obliquely upon us, that any given number of them is spread over a much greater portion of the Earth's surface where we live, and therefore each point must then have fewer rays than in summer. Moreover, there comes a greater degree of cold in the long winter nights, than there can return of heat in so short days; and on both these accounts the cold must increase. But in summer the Sun's rays fall more perpendicularly upon us, and therefore come with greater force, and in greater numbers on the same place; and by their long continuance, a much greater degree of heat is imparted by day than can fly off by night.

206. That a greater number of rays fall on the same place, when they come perpendicularly, than when they come obliquely on it, will appear by the figure. For, let AB be a certain number of the Fig. IL Sun's rays falling on CD (which let us suppose to

be London) on the 21st of June: but, on the 22d of December, the line CD, or London, has the oblique position CD to the same rays; and therefore scarce a third part of them falls upon it, or only those between A and e; all the rest, c B, being expended on the space d P, which is more than double the length of CD or Cd. Besides, those parts which are once heated, retain the heat for some time; which, with the additional heat daily imparted, makes it continue to increase, though the Sun declines toward the south; and this is the reason why July is hotter than June, although the Sun has withdrawn from the summer tropic; as we find it is generally hotter at three in the afternoon, when the Sun has gone toward the west, than at noon when he is on the meridian. Likewise, those places which are well cooled require time to be heated again; for the Sun's rays do not heat even the surface of any body till they have been some time upon it. therefore we find January, for the most part, colder than December, although the Sun has withdrawn from the winter tropic, and begins to dart his beams more perpendicularly upon us, when we have the position CF. An iron bar is not heated immediately upon being put into the fire, nor grows cold till some time after it has been taken out.



The Method of finding the Longitude by the Eclips es of Jupiter's Satellites: the amazing Velocity of Light demonstrated by these Eclipses.

First me- 207. GEOGRAPHERS arbitrarily choose to


of some remarkable and lon- place the first meridian. There they begin their gitude of places, reckoning; and just so many degrees and minutes as any other place is to the eastward or westward of that meridian, so much east or west longitude they say it has. A degree is the 360th part of a circle,


be it great or small, and a minute the 60th part of a Piat: V. degree. The English geographers reckon the longitude from the meridian of the Royal Observatory at Greenwich, and the French from the meridian of Paris.

208. If we imagine two great circles, one of Fig. II. which is the meridian of any given place, to inter- Hour cir sect each other in the two poles of the Earth, and to cles. cut the equator E at every 15th degree, they will be divided by the poles into 24 semi-circles, which divide the equator into 24 equal parts; and as the Earth turns on its axis, the planes of these semicircles come successively one after another every hour to the Sun. As in an hour of time there is a revo- An hour lution of fifteen degrees of the equator, in a minute of time equal to of time there will be a revolution of 15 minutes of 15 dethe equator, and in a second of time a revolution of grees of 15 seconds. There are two tables annexed to this chapter, for reducing mean solar time into degrees and minutes of the terrestrial equator; and also for converting degrees and parts of the equator into mean solar time.

209. Because the Sun enlightens only one half of the Earth at once, as it turns round its axis, he rises to some places at the same moment of absolute time that he sets at to others; and when it is mid-day to some places, it is mid-night to others. The XII on the middle of the Earth's enlightened side, next the Sun, stands for mid-day; and the opposite XII, on the middle of the dark side for midnight. If we suppose this circle of hours to be fixed in the plane of the equinoctial, and the Earth to turn round within it, any particular meridian will come to the different hours so as to shew the true time of the day or night at all places on that meridian. Therefore,

210. To every place 15 degrees eastward from any given meridian, it is noon an hour sooner than on that meridian; because their meridian comes


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