of inhabitants, which is 280 inhabitants for every square mile. The surface of Mercury contains 32,000,000 of square miles, which is not much less than all the habitable parts of our globe. At the rate of population now stated, it is sufficiently ample to contain 8,960, 000,000, or eight thousand nine hundred and sixty millions of inhabitants, which is more than eleven times the present population of our globe. And although the one half of the surface of this planet were to be considered as covered with water, it would still contain nearly six times the population of the earth. Hence it appears, that small as this planet may be considered when compared with others, and seldom as it is noticed by the vulgar eye, it in all probability holds a far more distinguished rank in the intellectual and social system under the moral government of God, than this terrestial world of which we are so proud, and all the living beings which traverse its surface. Frivalous corjectime.

I shall only mention further the following particulars in reference to this planet. In its revolution round the sun, its motion is swifter than that of any other planet yet discovered; it is no less than at the rate of 109,800 miles every hour at an average, although in some parts of its course it is slower, and in other parts swifter, since it moves in an elliptical orbit. Of course it flies 1830 miles every minute, and more than thirty miles during every beat of our pulse. The density of this planet is found by certain physical calculations and investigations, founded on the laws of universal gravitation to be nine times that of water, or equal to that of lead; so that a ball of lead 3200 miles in diameter would exactly poise the planet Mercury. This density is greater than that of any of the other planets, and nearly twice the density of the earth. The mass of this planet, or the quantity of matter it contains, when compared with the mass of the sun, is, according to LaPlace, as 1 to 2,025,810, or about the two millionth part; that is, it would require two millions of globes of the size and density of Mercury to weigh one of the size and density of the sun. But as Mercury contains a much greater quantity of matter in the same bulk than the sun, in point of size it would require 22,000,000 of globes of the bulk of Mercury to compose a body equal to that of the sun. In consequence of the great density of this planet, bodies will have a greater weight on its surface than on the earth. It has been computed, that a body weighing one pound on the earth's surface would weigh one pound eight and a half drachms on the surface of Mercury. If the centrifugal force of this planet were suspended, and its motion in a circular course stopped, it would fall towards

the sun, as a stone when thrown upward falls to the ground, by the force of gravity, with a velocity continually increasing as the square of the distance from the sun diminished. The time in which Mercury or any other planet would fall to the sun by the centripetal force, or the sun's attraction, is equal to its periodic time divided by the square root of thirty-two; a principle deduced from physical and mathematical investigation. Mercury would therefore fall to the sun in 15 days, 13 hours; Venus in 39 days, 17 hours; the earth in 64 days, 13 hours; Mars in 121 days, 10 hours; Vesta in 205 days; Ceres in 297 days, 6 hours; Fallas in 301 days, 4 hours; Juno in 354 days, 19 hours; Jupiter in 765 days, 19 hours, or above two years; Saturn in 1901 days, or about five years; Uranus in 5425 days, or nearly fifteen years; and the Moon would fall to the earth, were its centrifugal force destroyed, in 4 days, 20 hours. Some of the deductions stated above may be apt to startle some readers as beyond the powers of limited intellects, and above the range of human investigation. The discoveries of Newton, however, have now taught us the laws by which these bodies act upon one another; and as the effects they produce depend very much upon the quantities of matter they contain, by observing these effects we able, by the aid of mathematical reasoning, to determine the quantities of matter in most of the planets with considerable certainty. But to enter on the demonstration of such points would require a considerable share of attention and of mathematical knowledge, and would probably prove tedious and uninteresting to the general reader.

Mercury revolves in an orbit which is elliptical, and more eccentric than the orbits of most of the other planets, except Juno, Ceres, and Pallas. Its eccentricity, or the distance of the sun from the centre of its orbit, is above 7,000,000 of miles. The time between its greatest elongations from the sun varies from 106 to 130 days. Its orbit is inclined to the ecliptic, or the plane of the earth's orbit, in an angle of seven degrees, which is more thaň double the inclination of the orbit of Venus.

II. OF THE PLANET VENUS.

Of all the luminaries of heaven, the sun and moon excepted, the planet Venus is the most conspicuous and splendid. She appears like a brilliant lamp amid the lesser orbs of night, and alternately anticipates the morning dawn and ushers in the evening twilight. When she is to the westward of the sun, in winter, she cheers our mornings with her vivid light, and is a prelude of the near approach of the break of day and the rising sun. When

she is eastward of that luminary, her light bursts upon us after sunset, before any of the other twinkling orbs of heaven make their appearance; and she discharges, in some measure, the functions of the absent moon. The brilliancy of this planet has been noticed in all ages, and has been frequently the subject of description and admiration both by shepherds and by poets. The Greek poets distinguished it by the name of Phosphor when it rose before the sun, and Hesperus when it appeared in the evening after the sun retired; and it is now generally distinguished by the name of the Morning and Evening Star.

"Next Mercury, Venus runs her larger round,
With softer beams and milder glory crown'd;
Friend to mankind, she glitters from afar,
Now the bright evening, now the morning star.
From realms remote she darts her pleasing ray,
Now leading on, now closing up the day;
Term'd Phosphor when her morning beams she
yields,

And Hesp'rus when her ray the evening gilds."

Before proceeding to a more particular description of this planet, I shall lay before the reader a brief explanation of the nature of the planetary orbits, as I may have occasion to refer to certain particulars connected with them in the following descriptions. All the planets and their satellites move in elliptical orbits, more or less eccentric. The following figure exhibits the form of these orbits.

The figure A D B E represents the form of a planetary orbit, which is that of an oval or ellipse. The longest diameter is A B; the shorter diameter D E. The two points Fand G are called the foci of the ellipse, around which, as two central points, the ellipse is formed. The sun is not placed in C, the centre of the orbit, but at F, one of the foci of the ellipse. When the planet, therefore, is at A, it is nearest the sun, and is said to be in its perihelion; its distance from the sun gradually increases till it reaches the opposite point, B, when it is at its greatest distance from the sun, and is said to be in its aphelion;

when it arrives at the points D and E of its orbit, it is said to be at the mean distance. The line A B, which joins the perihelion and aphelion, is called the line of the apsides, and also the greater axis or the transverse axis of the orbit; D E is the lesser or conjugate axis F D, the mean distance of the planet from the sun; F C, or G C, the eccentricity of the orbit, or the distance of the sun from its centre; F is the lower focus, or that in which the sun is placed; G the higher focus; A the lower apsis, and B the higher apsis. The orbits of some of the planets are more elliptical than others. The eccentricity of the orbit of Mercury is above 7,000,000 miles; that is, the distance from the point F, where the sun is placed, to the centre, C, measures that number of miles; while the eccentricity of Venus is only about 490,000 miles, or less than half a million. Most of the planetary orbits, except those of some of the new planets, approach very nearly to the circular form.

The orbits of the different planets do not all lie in the same plane, as they appear to do m orreries and in the representations generally given of the solar system. If we suppose a plane to pass through the earth's orbit, and to be extended in every direction, it will trace a line in the starry heavens which is called the ecliptic, and the plane itself is called the plane of the ecliptic. The orbits of all the other planets do not lie in this plane, one half of each orbit rising above it, while the other half falls below it. This may be illustrated by supposing a large bowl or concave vessel to be nearly filled with water; the surface of the water will trace a circular line round the inner surface of the bowl, which may represent the ecliptic, while the surface of the water itself is the plane of the ecliptic, and the bowl is the one half of the concave sky. If we now immerse in the bowl a large circular ring obliquely, so that one half of it is above the surface of the water and the other half below, this ring will represent the orbit of a planet inclined to the ecliptic or to the fluid surface; or if we take two large rings or hoops of nearly equal size, and place the one within the other obliquely, so that the half of the one hoop may be above, and the opposite half below the other hoop, it will convey an idea of the inclination of a planet's orbit to the plane of the ecliptic. Thus, if the circle E F G H (Fig. 10) represent the plane of the earth's orbit or the ecliptic, the circle A B C D may represent the orbit of a planet which is inclined to it; the semicircle I A B K being below the level of the ecliptic, and the other half or semicircle being above it. The points of intersection at I and K, where the circles cut one another, are called the nodes. If the planet is moving in the direction A I D, the

point, K, the descending node. The line I K, which joins the nodes, is called the line of the nodes, which, in the different planetary orbits, points to different parts of the heavens. It is when Mercury and Venus are at or near the line of the nodes that they appear to make a transit across the sun's disk. The moon's orbit is inclined to the plane of the earth's orbit in an angle of about five degrees; and it is only when the full moon or change happens at or near the nodes that an eclipse can take place, because the sun, moon, and earth are then nearly in the same plane; at all other times of full or change, the shadow of the moon falls either above or below the earth, and the shadow of the earth either above or below the moon. The ecliptic is supposed to be divided into twelve signs, or 360 degrees, which have received the following names:Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces. Each of these signs is divided into thirty equal parts, called degrees; each degree into sixty parts, or minutes; each minute into sixty parts, or seconds, &c.

Having stated the above definitions, which it may be useful to keep in mind in our further discussions, I shall proceed to a particular description of the motions and other phenomena of Venus.

General Appearances and apparent Motions of Venus.-This planet, as already noticed, is only seen for a short time, either after sunset in the evening, or in the morning before sunrise. It has been frequently seen by means of the telescope, and sometimes by the naked eye, at noonday, but it was never seen at midnight, as all the other planets may be, with the exception of Mercury. It never appears to recede further from the sun than

forty-seven degrees, or about half the distance from the horizon to the zenith. Of course, it was never seen rising in the east or even shining in the south after the sun had set in the west, as happens in regard to all the other heavenly bodies, with the exception now stated.

When this planet, after emerging from the solar rays, is first seen in the evening, it appears very nea. the horizon about twenty minutes after sunset, and continues visible only for a very short time, and descends below the horizon not far from the point where the sun went down. Every succeeding day its apparent distance from the sun increases; it rises to a higher elevation, and continues a longer time above the horizon. Thus it appears to move gradually eastward from the sun for four or five months, till it arrives at the point of its greatest elongation, which seldom exceeds forty-seven degrees, when it appears for some time stationary; after which it appears to commence a retrograde motion from east to west, but with a much greater degree of apparent velocity; approaching every day nearer the sun, and continuing a shorter time above the horizon, till, in the course of two or three weeks, it appears lost in the splendour of the solar rays, and is no longer seen in the evening sky till more than nine or ten months have elapsed. About eight or ten days after it has disappeared in the evening, if we look at the eastern sky in the morning, a little before sunrise, we shall see a bright star very near the horizon, which was not previously to be seen in that quarter; this is the planet Venus, which has passed its inferior conjunction with the sun, and has now moved to the westward of him, to make its appearance as the morning star. It now appears every succeeding day to move pretty rapidly from the sun to the westward, till it arrives at the point of its greatest elongation, between 45° and 48° distant from the sun, when it again appears stationary; and then returns eastward, with an apparently slow motion, till it is again immersed in the sun's rays, and arrives at its superior conjunction, which happens after the lapse of about nine months from the time of being first seen in the morning. But the planet is not visible to the naked eye all this time on account of its proximity to the sun when slowly approaching its superior conjunction. After passing this conjunction it soon after appears in the evening, and resumes the same course as above stated. During each of the courses now described, when viewed with a telescope, it is seen to pass successively through all the phases of the moon, appearing gibbous or nearly round when it is first seen in the evening; of the form of a half moon when

tion it is just 27 millions of miles from the earth; whereas, at its superior conjunction, it is no less than 163 millions of miles from the earth, for it is then further from us by the whole diameter of its orbit, which is 136 millions of miles. This is the reason why it appears much smaller at its superior conjunction than when near its inferior; although, in the latter case, there is only a small crescent of its light presented to us, while in the former case its full enlightened hemisphere is turned to the earth.

The following figure will exhibit more distinctly the phases of this planet in the different parts of its course, and the reason of the difference of its apparent magnitude in different points of its orbit. At A it is in the superior conjunction, when it presents to our view a round full face. At B it appears as an evening star, and exhibits a gibbous phase, somewhat less than a full moon. At D it approaches somewhat nearer to a half moon At E, near the point of its eastern elongation Fig. 12.

Let the earth be supposed at K; then when Venus is in the position marked A, it is nearly in a line with the sun as seen from the earth, in which position it is said to be in its superior conjunction with the sun, or beyond him, in the remotest part of its orbit from the earth; in which case the body of the sun sometimes interposes between the earth and Venus; at other times it is either a little above or below the sun, according as it happens to be either in north or south latitude. When it is in this position the whole of its enlightened hemisphere is turned towards the earth. As it moves on its orbit from A to B, which is from west to east, and is called its direct motion, it begins to appear in the evening after sunset. When it arrives at B, it is seen among the stars at L, in which position it assumes a gibbous phase, as a portion of its enlightened hemisphere is turned from the earth. When it arrives at C, it appears among the stars at M, at a still greater distance from the sun, and exhibits a less gibbous phase, approaching to that of a half moon. When arrived at D, it is at the point of its greatest eastern elongation, when it appears like a half moon, and is seen among the stars at N; it now appears for some time stationary; after which it appears to move with a rapid course in an opposite direction, or from east to west, during which it presents the form of a crescent, till it approaches so near the sun as to be overpowered with the splendour of his rays. When arrived at E, it is said to be in its inferior conjunction, and, consequently, nearest the earth. In this posi

it appears like a half moon. During all this course it moves from west to east. From F to I it appears to move in a contrary direction, from east to west, during which it assumes the figure of a crescent, gradually diminishing in breadth, but increasing in extent, till it arrives at I, the point of its inferior conjunction, when its dark hemisphere is turned towards the earth, and is consequently invisible, being in a situation similar to that of the moon at the time of change. It is seen no longer in the evenings, but soon appears in the morning under the figure of a slender

crescent, and passes through all the other phases represented in the diagram, at M, N, O, &c., till it arrives again at A, its superior conjunction. The earth is here supposed to be placed at K; and if it were at rest in that position, all the changes now stated would happen in the course of 224 days. But as the earth is moving forward in the same direction as the planet, it requires some considerable time before Venus can overtake the earth, so as to be in the same position as before with respect to the earth and the sun. The time, therefore, that intervenes between the superior conjunction and the same conjunction again is nearly 584 days, during which period Venus passes through all the variety of its motions and phases as a morning and evening star.

This diversity of motions and phases, as formerly stated, serves to prove the truth of the system, now universally received, which places the sun in the centre, and the earth beyond the orbit of Venus. In order to illustrate this point to the astronomical tyro in the most convincing manner, I have frequently used the following plan. With the aid of a planetarium, and by means of an ephemeris or a nautical almanac, I place the earth and Venus in their true positions on the planetarium, and then desire the learner to place his eye in a line with the balls representing Venus and the earth, and to mark the phase of Venus, as seen from the earth, whether gibbous, a half moon, or a crescent. I then adjust an equatorial telescope (if the observation be in the daytime), and, pointing it to Venus, show him this planet with the same phase in the heavens; an experiment which never fails to please and to produce conviction.

It has generally been asserted by astronomers that it is impossible to see Venus at the time of its superior conjunction with the sun. Mr. Benjamin Martin, in his "Gentleman and Lady's Philosophy," vol. i., says, "At and about her upper conjunction Venus cannot be seen, by reason of her nearness to the sun." And in his "Philosophia Britannica," vol. iii., the same opinion is expressed: "At her superior conjunction Venus would appear a full enlightened hemisphere, were it not that she is then lost in the sun's blaze, or hidden behind his body." Dr. Long, in his Astronomy,” vol. i., says, "Venus, in her superior conjunction, if she could be seen, would appear round like the full moon." Dr. Brewster, in the article of Astronomy in the 'Edinburgh Encyclopædia," when describing the phases of Mercury and Venus, says, "Their luminous side is completely turned to the earth at the time of their superior conjunction, when they would appear like the full moon, if they were not then eclipsed by

the rays of the sun. The same opinion is expressed in similar phrases by Ferguson, Gregory, Adams, Gravesend, and most other writers on the science of astronomy, and has been copied by all subsequent compilers of treatises on this subject. In order to determine this point, along with several others, I commenced, in 1813, a series of observations on the celestial bodies in the daytime, by means of an equatorial instrument. On the 5th of June that year, a little before midday, when the sun was shining bright, I saw Venus distinctly with a magnifying power of sixty times, and a few minutes afterward with a power of thirty, and even with a power of fifteen times. At this time the planet was just 3° in longitude and about 13' in time east of the sun's centre, and, of course, only 23° from the sun's limb. Cloudy weather prevented observations when Venus was nearer the sun.* On the 16th of October, 1819, an observation was made, in which Venus was seen when only six days and nineteen hours past the time of her superior conjunction. Her distance from the sun's eastern limb was then only 1° 28′ 42′′. A subsequent observation proved that she could be seen when only 1° 27′ from the sun's margin, which approximates to the nearest distance from the sun at which Venus is distinctly visible. About the tenth of March, 1826, I had a glimpse of this planet within a few hours of its superior conjunction, but the interposition of clouds prevented any particular or continued observations. It was then about 1° 25 the sun's centre. Observations were likewise made to determine how near its inferior conjunction this planet may be seen. The following is the observation in which it was seen nearest to the sun. On March 11th, 1822, at thirty minutes past twelve, noon, the planet being only thirty-five hours past the point of its inferior conjunction, I perceived the crescent of Venus by means of an equatorial telescope, magnifying about seventy times. It appeared extremely slender, but distinct and well-defined, and apparently of a larger curve than that of the lunar crescent when

from

*The particulars connected with this observa tion, and with those made on the other planets, and on stars of the first and second magnitudes, together with a description of the instrument, and the manner of making day observations, are recorded in Nicholson's "Journal of Natural Philosophy," &c.. for October, 1813, vol. xxxvi. p. 109 to 128, in a communication which occupies about twenty pages; and also, in an abridged form, in the "Monthly Magazine," "Annals of Philosophy," and other periodical journals of that period. During the succeeding winter the celebrated Mr. Playfair, professor of natural philosophy in the university of Edinburgh, communicated, in his lectures to the students, the principal details contained in that communication as new facts in astronomical science.