interval of time, presenting the same appearance, and occupying the same place. But nothing of this kind is to be found in the history of these appearances.

There was a remarkable spot in 1769, which appeared to be depressed below the surface of the sun. As it approached the limb, the umbra, or shadowy margin nearest the centre of the sun, disappeared first. On its return to the other limb, the other margin, being nearest the centre, was invisible. It was hid apparently by the intervening portion of the sun's body. As the spot advanced upon the disc, it came into view. Sir W. Herschel observed many appearances of this kind, and was fully persuaded, that these dark specks were below the surface of the sun. He has given views and observations, which tend very much to confirm this opinion. He undertook to measure the sides of the depression, and to determine its depth. He thought that the phenomena of the spots arise, not from excavations in the body of the sun, but from openings in his atmosphere; that the solid substance of the sun is opaque, like the planets, and that, like the planets, it is surrounded by an atmosphere; that this atmosphere is transparent to the height of about 2000 miles, and gives support at this elevation to a stratum of dark clouds, on which, as the outer substance of the sun, rests the flood of luminous matter, which presents itself to the surrounding planets. This light is supposed to be produced by the combustion of gases, which are generated below, and which in their ascent drive away the clouds and billows of flame that float upon them, and thus give us a glimpse both of the opaque surface of the sun, and of the cloudy stratum above it, and that these form the nucleus and umbra of a spot. Our earth probably presents similar appearances to the inhabitants of the moon. Where it is covered with clouds, it will exhibit a uniform brightness; where there are breaks and interruptions, the naked body of the earth will be seen of a darker shade, on account of those clouds, which intercept much of the light. The sides of the opening also would present themselves alternately, as the earth revolved on its axis, by which means their depth might be ascertained, as Herschel ascertained the depth of the openings in the solar clouds.

The sun's spots, then, according to this hypothesis, are chasms in his atmosphere, occasioned by ascending currents of gaseous fuel, and they are succeeded by faculæ, as they are called, or bright spots, on account of this additional supply of combustible matter, which, it may be supposed, is most completely on fire soon after the opening has closed. But how is the sun ordinarily furnished when there are no spots? The gas may be more diffused, and by ascending in smaller quantities, may produce no sensible disturbance of the luminous fluid. Besides, there are probably openings, that are too small to be seen, and the sun may never be free from them. It is only when their absolute magnitude is very great, that they become an object of any attention.

Herschel observed, that the luminous matter of the sun, when viewed with his best telescopes, is far from preserving always the same aspect. It is sometimes even and tranquil, and sometimes it is thrown up into ridges, and appears to be agitated, like the sea in a storm. The changes, when in this state, are often very rapid, small openings are formed and closed in a few minutes, and clouds are seen passing with a rapidity, that considerably alters their situation in the course of an hour. These fluctuations are more particularly observed during the time of large and frequent spots. Hence Herschel inferred, that there is a variable emission of light and heat, intimately connected with the appearance and disappearance of spots, and that seasons of uncommon heat and cold, of fertility and barrenness, so far as they depend upon the supply of heat, are to be traced not so much to accidental causes near at hand, as to the inconstancy of the fountain. We are like plants in a green-house, that are healthy and vigorous, or chilled with the frost, according as the flues are well or ill attended to. We depend for the very means of subsistence, as well as for all the comforts of climate, upon operations, that are going on ninety millions of miles off; upon the more or less rapid compositions and decompositions, that are taking place in this great laboratory of nature.

As the sun's spots move across the disc all in the same direction, and in the same time, and are as long behind the sun as they are before it, we infer that they belong to the surface of the sun, and are carried about by the sun's rotation. From the position of the paths described by the spots, we are able, moreover, to calculate the position of the axis on which the sun turns. It is not exactly perpendicular to the ecliptic, but inclined about 6o. As the spots are about 14 days in passing across the disc, it will be readily seen that the time of an apparent rotation is about 28 days. We say apparent, because, as the earth is carried round the sun in the same direction, a spot must describe as much more than a real revolution, as the earth has moved in the same time. Whence, to a spectator at rest, the sun would complete a revolution in between 25 and 26 days.

X. ROTATION &c. OF THE PLANETS.

It is a curious fact that the motions of the planets, both in their annual course round the sun, and in their rotations on their axes, are nearly in the same direction, namely, from west to east. The same remark is applicable, also, to the satellites, so far as our knowledge extends, with the exception of those of Uranus or Herschel.*

* Herschel's six satellites have their orbits nearly perpendicular to the ecliptic. As to their rotation, and that of six of Saturn's satellites, and that of the new planets, we have no information whatever.

It may be remarked further, that the smaller plan to complete their rotations. An attempt has been the supposition that the motion in the orbit, and t result of a single impulse. In this case the larger direction more favorable to a rotatory motion; jus likely to turn on its axis, while it is impelled forwar Time of Rotation.

XI. ORBITS OF THE PLANI

A fruitful source ef error in the ancient systems supposition that the motions of the planets took Kepler laid the foundation of the present improved establishing the following propositions known by Laws.

1. The planets revolve in ovals, or elliptical cu the focus of the ellipse.

2. The spaces passed over by a line drawn f volving body, in different parts of the orbit, are eq are allowed. In other words, A planet, as it ap

sun, moves just so much faster, as to make the space passed over by the revolving radius, in a day, for instance, always the same.

3. If we take the times in which any two planets complete their revolution, and multiply each by itself; and then take their mean distances from the sun, and multiply each by itself twice; the two first results will have the same proportion to each other, as the two last; in other words, the squares of the times are as the cubes of the distances.

There are several particulars respecting a planet, given in the following tables, which being known, we are able to calculate its position or place at any time. These are termed the elements of a planet's orbit, and are as follows.

1. The time employed in making a complete circuit of the heavens, which is called the sidereal revolution.

2. The average distance of the planet from the sun, which is half the greatest diameter of the elliptical orbit. This is called the mean distance. 3. The eccentricity, or the proportion which the distance from the focus to the centre, bears to the mean distance. This is subject to a slight change, the amount of which, for a century, is put down under the title of secular variation.

4. The position or mean longitude of the planet, for any given time, as the beginning of the century. This longitude is reckoned from the vernal equinox, or the 1st of Aries, as longitude on the earth is reckoned from any assumed meridian, as that of Greenwich. But celestial longitude is counted on the ecliptic, and only in one direction, namely, from west to east, in the direction of the planetary motions.

5. The position of the point of nearest approach of the planet to the sun, is called the perihelion (from two Greek words, which signify about the sun.) This point is referred, also, to the 1st of Aries, and its position is determined, like that of the planet itself, by its distance from the vernal equinox, reckoned on the ecliptic. This distance is denominated the longitude of the perihelion. As the perihelion has a slow motion, its position is given for a particular time, or epoch, and also its change of position in a century, termed the secular variation.

6. The orbits of the planets not being coincident with, or parallel to the ecliptic, their oblique position with respect to the ecliptic, is of great importance in calculating their places. This is called the inclination of the orbit. Like the other elements, it is subject to a slight change; hence the inclination is given for a particular epoch, together with the alteration in a century.

7. The two points in which the orbit of a planet cuts the ecliptic are called its nodes. That node through which the planet passes in coming from the south to the north, is distinguished as the ascending node. The position of this point is determined, like that of other points, by its distance

in like manner, has a slight motion; accordingly i a particular epoch, together with the sidereal and change with respect to a star in a century.

(3.) Ratio of the Eccentricity to the Mean Dist of 1801, with the Secular Variation. The S

(4.) Mean Longitude, for the Minute which sep cember, 1800, from the 1st of January, 1801,

Mercury

Venus

The Earth