heat as the Earth. The truth of this estimate, of course, depends upon the supposition that the intensity of solar light and heat at the planets, varies inversely as the squares of their distances from the Sun.

In this diagram the light is seen passing in right lines, from the sun on the left toward the several planets on the right. It is also shown that the surfaces A, B, and C receive equal quantities of light, though B is four times, and C nine times as large as A; and as the light falling upon A is spread over four times as much surface at B, and nine times as much at C, it follows that it is only one-ninth as intense at C, and one-fourth at R, as it is at A. Hence the rule, that the light and heat of the planet is, inversely, as the squares of their respective distances,

The student may not exactly understand this last statement. The square of any number is its product, when multiplied by itself. Now suppose we call the distances A, B, and C, 1, 2, and 3 miles. Then the square of 1 is 1; the square of 2 is 4; and the square of 8 is 9. The light and heat, then, would be in inverse proportion at these three points, as 1, 4, and 9; that is, four times less at B than at A, and nine times less at C. These amounts we should state as 1, 4, and one-ninth.

344. This law of analogy, did it exist with rigorous identity at all the planets, would be no argument against their being inhabited; because we are bound to presume that the All-wise Creator has attempered every dwelling-place in his empire to the physical constitution of the beings which he has placed in it.

From a variety of facts which have been observed in relation to the production of caloric, it does not appear probable, that the degree of heat on the surface of the different planets depends on their respective distances from the Sun. It is more probable, that it depends chiefly on the distribution of the substance of caloric on the surfaces, and throughout the atmospheres of these bodies, in different quantities, according to the different situations which they occupy in the solar system; and that these different quantities of caloric are put into action by the influence of the solar rays, so as to produce that degree of sensible heat requisite to the wants, and to the greatest benefit of each of the planets. On this hypothesis, which is corroborated by a great variety of facts and experiments, there may be no more sensible heat experienced on the planet Mercury, than on the surface of Herschel, which is fifty times farther removed from the Sun.

345. The rotation of Mercury on its axis, was determined from the daily position of its horns, by M. Schroeter, who not only discovered spots upon its surface, but several mountains in its southern hemisphere, one of which was 10 miles highnearly three times as high as Chimborazo, in South America.

844.

848. His density, and light and heat? Upon what rule is this estimate based? Would not this law of analogy make against the doctrine that the planets are inhab ited? Is it probable that this law does prevail? Upon what may the relative heat of the planets depend? 345. How was his diurnal revolution determined, and by whom? What said of his surface? What observation respecting mountains in general?

It is worthy of observation, that the highest mountains which have been discovered in. Mercury, Venus, the Moon, and perhaps we may add the Earth, are all situated in their Eouthern hemispheres.

346. During a few days in March and April, August and September, Mercury may be seen for several minutes, in the morning or evening twilight, when its greatest elongations happen in those months; in all other parts of its orbit, it is too near the Sun to be seen by the naked eye. The greatest distance that it ever departs from the Sun, on either side, varies from 16° 12', to 28° 20', alternately.

The distance of a planet from the Sun, as seen from the Earth (measured in degrees), is called its elongation. The greatest absolute distance of a planet from the Sun is denominated its aphelion, and the least its perihelion.

347. The revolution of Mercury about the Sun, like that of all the planets, is performed from west to east, in an orbit which is nearly circular. Its apparent motion, as seen from the Earth, is, alternately, from west to east, and from east to west, nearly in straight lines; sometimes directly across the disc of the Sun, but at all other times either a little above or a little below it.

Were the orbits of Mercury and Venus in the same plane with that of the Earth, they would cross the Sun's disc at every revolution; but as one-half of each of their orbits is bove, and the other half below the ecliptic, they generally appear to pass either above or below the Sun.

B

-A.

Let the right line A, joining the Earth and the Sun in the above diagram, represent the plane of the ecliptic. Now when an interior planet is in this plane, as shown at A, it may appear to be upon the Sun's disc; but if it is either above or below the ecliptic, as shown at B and C, it will appear to pass either above or below the Sun, as shown at D) and E.

For the relative position of the planets' orbits, and their inclination to the plane of the ecliptic, see 1, of the Atlas. Here the dotted lines continued from the dark lines, denote the inclination of the orbits to the plane of the ecliptic, which inclination is marked in figures on them. Let the student fancy as many circular pieces of paper intersecting each other at the several angles of inclination marked on the Map, and be will be enabled to understand more easily what is meant by the "inclination of the planets' orbits."

348. Being commonly immersed in the Sun's rays in the evening, and thus continuing invisible till it emerges from them in the morning, Mercury appeared to the ancients like two distinct A long series of observations was requisite, before they

stars.

846. When may Mercury be seen? Why not at other times? How far does it depart from the Sun on either side? What is meant by the elongation of a planet? Its aphelion and perihelion? 347. In what direction do the planets revolve around the Sun? What is the apparent motion of Mercury? Do they ever cross the Sun's disc? Wy not at every revolution? 348. How was Mercury regarded by the ancients?

recognized the identity of the star which was seen to recede from the Sun in the morning with that which approached it ir the evening. But as the one was never seen until the other disappeared, both were at last found to be the same planet, which thus oscillated on each side of the Sun.

349. Mercury's oscillation from west to east, cr from east to west, is really accomplished in just half the time of its revolution, which is about 44 days; but as the Earth, in the mean time, follows the Sun in the same direction, the apparent elongations will be prolonged to between 55 and 65 days.

350. The passage of Mercury or Venus directly between the Earth and the Sun, and apparently over this disc, is called a Transit. A transit can never occur except when the interior planet is in or very near the ecliptic. The Earth and the planet inust be on the same side of the ecliptic; the planet being at one of its nodes, and the Earth on the line of its nodes.

This cut represents the ecliptic and zodiac, with the orbit of an interior planet, his nodes, &c. The line of his nodes is, as shown, in the 16° of 8 and the 16° of m. Now if the earth is in 8, on the line L N, as shown in the cut, when Mercury is at his ascending node (8), he will seem to pass upward over the Sun's face, like a dark spot, as represented in the figure. On the other hand, if Mercury is at his descending node (8), when the earth is in the 16° of M, the former will seem to pass downward across the disc of the Sun.

351. As the nodes of his orbit are on opposite sides of the ecliptic, and are passed by the Earth in May and November, it follows that all transits of Mercury must occur in one or the other of these months. They are, therefore, called the Node months. As is shown in the diagram, the Earth passes the

349. In what time is the oscillation of Mercury from east to west What is the apparent time, and why? 350. What is a transit? What are the nodes of a planet's orbit? The line of the nodes

really accomplished? When do they occur? 851. What are the

ascending Node of Mercury in November, and the descending in May; the former of which is in the 16th degree of Taurus, and the latter in the 16th degree of Scorpio.

All the transits of Mercury ever noticed have occurred in one or the other of these months, and for the reasor already assigned. The first ever observed took place November 6, 1631; since which time there have been 29 others by the same planet-in all 80 -8 in May, and 22 in November.

352. The last transit of Mercury occurred November 11, 1861; and the next will take place November 4, 1868. Besides this, there will be four more during the present century—two in May, and two in November.

The accompanying cut is a delineation of all the transits of Mercury from 1802 to the close of the present century. The dark line running east and west across the San's center represents the plane of the ecliptic, and the dotted lines the apparent paths of Mercury in the several transits. The planet is shown at its nearest point to the Sun's center. Its path in the last transit and in the next will easily be found.

The last transit of Mercury was W observed in this country by Professor Mitchel, at the Cincinnati Observatory, and by many others both in America and in Europe. The editor had made all necessary preparation for observing the phenomenon at his residence, near Oswego, New York; but, unfortunately, his sky was overhung with clouds, which hid the sun from his view, and disappointed all his hopes.

353. By comparing the mean motion of any of the planets with the mean motion of the Earth, we may readily determine the periods in which they will return to the same points of their orbit, and the same positions with respect to the Sun. The knowledge of these periods will enable us to determine the hour when the planets rise, set, and pass the meridian, and in general all the phenomena dependent upon the relative position of the Earth, the planet and the Sun; for at the end of one of these periods they commence again, and all recur in the same order.

We have only to find a number of sidereal years, in which the planet completes xactly, or véry nearly, a certain number of revolutions; that is, to find such a number of planetary revolutions, as, when taken together, shall be exactly equal to one, or any number of revolutions of the Earth. In the case of Mercury this ratio will be as $7.969 s to 365.256. Whence find that,

node months of a planet? The node months of Mercury? transit of Mercury occur? When will the next take place? present century? What said of the last transit of Mercury? mine when transits will occur?

352. When did the last What cthers during the 853. How may we deter. What ratio is found between the revolutions of Mercury

7 periodical revolutions of the Earth are equal to 29 of Mercury. 13 periodical revolutions of the Earth are equal to 54 of Mercury: 83 periodical revolutions of the Earth are equal to 187 of Mercury: 46 periodical revolutions of the Earth are equal to 191 of Mercury.

Therefore, transits of Mercury, at the same node, may happen at intervals of 7, 13, 33, 46 &c. years. Transits of Venus, as well as eclipses of the Sun and Moon, are calculated upon the same principle.

The following is a list of all the Transits of Mercury from the time the first was observed by Gassendi, November 6, 1631, to the end of the present century:

354. The sidereal revolution of a planet respects its absolute motion; and is measured by the time the planet takes to revolve from any fixed star to the same star again. The synodical revolution of a planet respects its relative motion; and is measured by the time that a planet occupies in coming back to the same position with respect to the Earth and the Sun.

In the adjoining cut the revolution of the Earth from A, opposite the star B, around to the same point again, would be a sidereal revolution.

Suppose the Earth and Mercury to start together from the points A C (where Mercury would be in inferior conjunction with the Sun), and to proceed in the direction of the arrows. In 88 days Mercury would come around to the same point again; but as the Earth requires more than four times that number of days for a revolution, she will only have reached the point D when Mercury arrives at C again; so that they will not be in conjunction, and a synodic revolution will not be completed by Mercury. He starts on, however, in his second round, and constantly gaining upon the Earth, till in 27 days from the time he left C the second time, he overtakes the Earth at E and F, and is again in inferior conjunction.

the synodic revolution of a planet must

355. The absolute motion of Mercury in its orbit is 109,757 miles an hour; that of the Earth is 68,288 miles; the difference, 41,469 miles, is the mean relative motion of Mercury, with respect to the Earth.

The sidereal revolution of Mercury is 87d. 23h. 15m. 44s. Its synodical revolution is

and the Earth? 354. What is a sidereal revolution of a planet? A synodical¥ 855. What is the absolute motion of Mercury in his orbit? What is that of the Earth? The difference, or relatice motion of Mercury? What is his sidereal period? His synodis How is the latter ascertained?