a globe, and not a flat circle. When the moon is gibbous or horned, the one side appears very ragged and uneven, but the other pretty well defined and circular. The spots in the moon always keep their places exactly; never vanishing, or going from one side to the other, as those of the sun do. We sometimes see more or less of the northern or southern, the eastern or western part of the disk or face; which is owing to what is called her libration. Plate IV. fig. 1, gives a representation of the full moon in her mean libration, with the principal spots according to Riccioli, Cassini, and Mayer.

110. Mercury, when looked at through telescopes magnifying about 200 or 300 times, appears equally luminous throughout his whole surface, without the least dark spot. He appears to have the same phases with the moon, being sometimes horned, sometimes gibbous, and sometimes shining almost with a round face, though not entirely full, because his enlightened side is never turned directly towards us.

111. Dr. Herschel has frequently examined Mercury with telescopes of highly magnifying powers; but he always appeared equally bright on every part of his disk, without any dark spot or ragged edge. But Schroeter, who has so much distinguished himself in this department of astronomy, affirms that he has not only seen spots, but even mountains in Mercury; and that he has succeeded in measuring the altitude of two of them. He makes the elevation of the higher of these about ten English miles and three-quarters, or about thrice the height of the highest mountain on our earth but where so small an error in the admeasurement of the angle on which the computation is founded would entail so great a mistake in the result, we can only consider this determination of the height of the mountains of Mercury, as a strong evidence that considerable elevations do exist on that planet. By examining the variation on the appearance of Mercury's horns from day to day, Schroeter found the period of his diurnal rotation to be about twenty-four days, five hours, and twenty-eight minutes. Considerable difference of opinion exists respecting the atmosphere of this planet: if it possesses any, it certainly at the centre subtends a very small angle.

112. Venus, when viewed through a telescope, is rarely seen to shine with a full face, but has phases and changes like those of the moon, increasing, decreasing, being horned, gibbous, &c. Her illuminated part is constantly turned towards the sun; being directed towards the east when she is a morning star, and towards the west when an evening star. Her different phases were first discovered by Galileo. Dr. Herschel has published, in the Phil. Trans. for 1793, a long series of observations on this planet, from which he concludes, 1 that the planet revolves about its axis, but that the period, and the position of the axis, are uncertain; 2. that the planet's atmosphere is very considerable; 3. that there are probably hills and inequalities upon its surface, though he has not been able to see much of them, owing, perhaps, to the density of its atmosphere; and, 4. that this planet is somewhat larger than the earth,

instead of being less, as former astronomers have imagined. Schroeter, also, in the Phil. Trans. for 1792, published the result of a series of observations on this planet, which were begun in 1780. He infers from his observations that Venus has an atmosphere of great density and height, and that many of her mountains are five or six times as high as those of the earth.

113. Much larger and more remarkable spots have been perceived on the disk of Mars than on that of any other primary planet. By very accurate observations, Herschel has determined the proportion between the polar and equatorial diameters, and the length of the day in this planet. He has also given some good conjectures on its seasons and its atmosphere: the latter it is now ascertained to have; but though considerable, the atmosphere is not of so great an extent as the conjectures on former observations led astronomers to imagine. By very accurate observations, Dr. Herschel has determined that the proportion of his polar and equinoctial axis is as 1272 to 1355, or nearly as 15 to 16; that its time of rotation on its axis is 24 h. 22 m. and that the inclination of the axis of Mars to the orbit of the earth is 59°42′. From the great obliquity of this planet's axis of rotation, the polar regions of it are alternately presented towards the earth, and a much better opportunity is thereby offered for examining its surface than that of any other planet. This, however, is in some degree counterbalanced by the very dense atmosphere with which this planet is surrounded. It is not a little remarkable, that when either pole emerges into the light of the sun, it exhibits a very striking brilliancy, something like what would arise from its being covered with snow. The analogy between this phenomenon and what annually takes place on our own globe, is too obvious to escape notice.

114. The planet Ceres is of a red color, and appears about the size of a star of the eighth magnitude. It is surrounded by a very dense and extensive atmosphere, in which very great and sudden changes are observed to take place. The estimates that have been made of this planet's diameter are a striking instance of the difficulty of measuring the apparent diameters of such small objects. Herschel makes its diameter about 163 miles; and Schroeter about 1624, or nearly ten times as much. Its periodical revolution round the sun is accomplished in about four years, seven months, and ten days.

115. Pallas is nearly of the same size as Ceres, but not quite of so red an appearance. Its period of revolution has been computed to be about four years, ten months, and eleven days; and its diameter has been estimated at from eighty to upwards of 2000 miles. It has also an atmosphere, but of less extent than that of Ceres; but it differs from that and all other planets in the great inclination of its orbit. The planets generally circulate in planes that do not deviate much from the plane of the ecliptic; but the orbit of Pallas is inclined about thirty-five degrees, nearly five times as much as that of any other planet.

116. Juno is of a reddish color, and is surrounded by an atmosphere of considerable den

sity. Its diameter is allowed by all observers to be less than that of either Ceres or Pallas. It differs from all other planets in the eccentricity of its orbit; being, when at its greatest distance from the sun, at double the least distance. period of its revolution is about four years and 128 days.

The

117. Vesta appears like a star of the sixth magnitude, and may on a clear night be sometimes seen with the naked eye. Its light is whiter and more intense than any of the other three small planets. Its apparent diameter has been estimated at about half that of the fourth satellite of Saturn; and yet it is very remarkable that its light is so intense, that Schroeter saw it several times with his naked eye, while it requires a telescope of considerable power to see the fourth or indeed any satellite of Saturn. This planet revolves in about three years, sixty-six days, and four hours. The orbits of all these four little planets (which from their smallness have been called Asteroids) intersect each other in various places; and the points of intersection are continually varying from the changes in the places of their aphelia.

118. Jupiter has the same general appearance with Mars, only that the belts on his surface are much larger and more permanent. Their number is very variable, as sometimes only one, and at other times no fewer than eight, may be perceived. They are generally parallel to one another, but not always so; and their breadth is likewise variable, one belt having been observed to grow narrow, while another in its neighbourhood has increased in breadth, as if the one had flowed into the other. The time of their continuance is very uncertain, sometimes remaining unchanged for three months; at others, new belts have been formed in an hour or two. In some of these belts large black spots have appeared, which moved swiftly over the disk from east to west, and returned in a short time to the same place; from whence the rotation of this planet about its axis has been determined.

119. The figure of Jupiter is evidently an oblate spheroid, the longest diameter of his disk being to the shortest as thirteen to twelve. His rotation is from west to east, like that of the sun, and the plane of his equator is very nearly coincident with that of his orbit; so that there can scarcely be any difference of seasons in that planet. His rotation has been observed to be somewhat quicker in his aphelion than his perihelion.

120. The most remarkable circumstance attending this planet, is his having four moons or satellites, which constantly revolve round him at different distances. These are all supposed to move in ellipses; though the eccentricities of all of them are too small to be measured, excepting that of the fourth; and even this amounts to no more than 00-07 of its mean distance from the primary.

121. The periodic times and distances of these satellites, in semidiameters of Jupiter, as well as in English miles, the angles under which their orbits appear, as seen from the earth, at its mean distance from Jupiter, taken from the latest and most exact observations, are as follow:

122. The nodes of these satellites are not in the same place. All of them, by reason of their immense distance, seem to keep near their primary, and their apparent motion is a kind of oscillation like that of a pendulum, going alternately from their greatest distance on one side to the greatest distance on the other, sometimes in a straight line, and sometimes in an elliptic curve. When a satellite is in its superior semicircle, or that half of its orbit which is more distant from the earth than Jupiter is, its motion appears to us direct, according to the order of the signs; but in its inferior semicircle, when it is nearer to us than Jupiter, its motion appears retrograde; and both these motions seem quicker the nearer the satellites are to the centre of the primary, slower the more distant they are, and, at the greatest distance of all, they appear for a short time to be stationary.

123. It is evident, from this account of the system of Jupiter and his satellites, that occultations of them must frequently happen by their going behind their primary, or by coming in betwixt us and it. The former takes place when they proceed towards the middle of their upper semicircle; the latter when they pass through the same part of their inferior semicircle. Occultations of the former kind happen to the first and second satellite; at every revolution, the third very rarely escapes an occultation, but the fourth more frequently by reason of its greater distance. It is seldom that a satellite can be discovered upon the disk of Jupiter, even by the best telescopes, excepting at its first entrance, when, by reason of its being more directly illuminated by the rays of the sun than the planet itself, it appears like a lucid spot upon it. Sometimes, however, a satellite, in passing over the disk, appears like a dark spot, and is easily to be distinguished. This is supposed to be owing to spots on the body of these secondary planets; and it is remarkable, that the same satellite has been known to pass over the disk at one time as a dark spot, and at another so luminous that it could not be distinguished from Jupiter himself, except at its coming on and going off.

124. To account for this phenomenon, we must say that either the spots are subject to change, or, if they be permanent like those of our moon, that the satellites at different times turn different parts of their globes towards us. Possibly both these causes may contribute to produce the phenomena just mentioned. For these reasons also both the light and apparent magnitude of the satellites are variable; for the fewer spots there are upon that side which is turned towards us, the brighter it will appear; and, as the bright side

only can be seen, a satellite must appear larger the more of its bright side it turns towards the earth, and the less so the more it happens to be covered with spots. The fourth satellite, though generally the smallest, sometimes appears bigger than any of the rest; the third sometimes seems least, though usually the largest; nay, a satellite may be so covered with spots as to appear less than its shadow passing over the disk of the primary, though we are certain that the shadow must be smaller than the body which casts it. To a spectator placed on the surface of Jupiter, each of these satellites would put on the various appearances of the moon; but they appear to us always round, having constantly their enlightened half turned towards the earth.

125. When these moons pass through their inferior semicircles, they cast a shadow upon Jupiter, and thus cause an eclipse of the sun to his inhabitants; and in some situations this shadow may be observed going before or following the satellite. Herschel says, April 6th, 1780, I had a fine view of Jupiter, and saw, as soon as I looked into the telescope, without any previous notice of it, the shadow of the third satellite, and the satellite itself on the lower part of the disk. The shadow was so black and well defined, that I attempted to measure it, and found its diameter, by the micrometer, to be 1" 562.' See plate XI. fig. 2. On the other hand, in passing through their superior semicircles, the satellites may be eclipsed in the same manner as our moon is to us, by passing through the shadow of Jupiter; and this is actually the case with the first, second, and third of these bodies; but the fourth, by reason of the largeness of its orbit, passes sometimes above or below the shadow, as is the case with our moon. The beginnings and endings of these eclipses are easily seen by a telescope when the earth is in a proper situation with regard to Jupiter and the sun; but when this or any other planet is in conjunction with the sun, the superior brightness of that luminary renders both it and the satellites invisible. From the time of its first appearing after a conjunction until near the opposition, only the immersions of the satellites into his shadow, or the beginnings of the eclipses are visible; at the opposition, only the occultations of the satellites, by going behind or coming before their primary, are observable; and from the opposition to the conjunction, only the immersions, or end of the eclipses are to be seen. For let S, plate VI. fig. 8, be the sun; I Jupiter and its shadow; A and P the earth, before and after the opposition of Jupiter; Sp the path of the first satellite in the shadow; At a tangent to Jupiter. When the first satellite enters the shadow, the apparent distance of it from the body of Jupiter is tAs; but at its emersion, the line pA passes through Jupiter, and therefore the emersion is not visible; but after opposition, the earth being at P, the emersion, and not the immersion, will be seen. The same things take place with respect to the second satellite. If muw be the path of the third satellite, m A frequently lies without the body of Jupiter, and therefore both the immersion and eniersion will be visible; the satellite disappears

and re-appears again at a distance from the body of Jupiter, and on the same side.

126. This is exactly true in the first satellite, of which we can never see an immersion with its immediately subsequent emersion: and it is but rarely that they can be both seen in the second; as in order to their being so, that satellite must be near one of its limits, at the same time that the planet is near its perihelion and quadrature with the sun. With regard to the third, when Jupiter is more than 46° from conjunction with, or opposition to, the sun, both its immersions and immediately subsequent emersions are visible; as they likewise are in the fourth, when the distance of Jupiter from conjunction or opposition is 24°. It had long been suspected that the satellites of this planet revolved on their axis; and Dr. Herschel has discovered that each of them revolves about its axis in the time of its revolution round its primary; thus furnishing another striking correspondence between the satellites of the other planets and the moori, the satellite of the earth. They must be very magnificent objects to the inhabitants of Jupiter. The first of them appears to them four times larger than our moon does to us, and goes through all the changes of the moon in the short space of forty-two hours, within which period it is itself eclipsed, and causes an eclipse of the sun on the surface of Jupiter.

127. When Jupiter is in quadrature with the sun, the earth is farthest out of the line that passes through the centres of the sun and Jupiter, and therefore the shadow of the planet is then most exposed to our view: but even then the body of the planet will hide from us one side of that part of the shadow which is nearest to it, through which the first satellite passes; which is the reason that, though we see the entrance of that satellite into the shadow, or its coming out from thence, as the earth is situate on the east or west side thereof, we cannot see them both; whereas the other satellites, going through the shadow at a greater distance from Jupiter, their ingress and egress are both visible. The orbits of the satellites are inclined to the plane of Jupiter's orbit, as is evident from the unequal duration of the eclipses of the same satellite. The fourth satellite, like our own moon, is sometimes in opposition to the sun, without being eclipsed. The third and fourth satellites often disappear in the shadow, and re-appear again on the same side of Jupiter; but only the beginnings or the endings of the eclipses of the first and second satellites are visible. The relative distances of these moons from their primary, are shown in plate VII. fig. 13.

127*. We cannot close this account of Jupiter without noticing two curious results obtained by La Place, with respect to the satellites of Jupiter; results which agree with observation in a remarkable manner. The first is, that if m', m", m", represent the mean motions of the first, second, and third satellites respectively, the m'′ + 2 m′′ — 3m", is always equal to nothing. The second is, that if l', l", and "" represent the mean longitudes of the satellites, as seen from the centre of Jupiter, then l— 3′′ +2¿””