2. RELATION OF THE SOLAR SYSTEM TO THE UNIVERSE.— The Solar System, or the sun with his accompanying train of planets, satellites, and comets, constitutes but a small portion of the material universe. When we survey the heavens at night, we behold a multitude of luminous objects called stars; and, by the assistance of a good telescope, myriads more become visible. Their apparent magnitudes are very different, and this difference has been made the basis of classification in forming some estimate of their number. Those visible to the naked eye are divided into six classes: the brightest stars are said to be of the first magnitude; those of an inferior degree of brightness, of the second magnitude; and so on, down to the sixth, which comprises the smallest stars visible to the naked eye in the clearest moonless night. The telescope vastly extends the power of vision, and astronomers are familiar with stars of

each other:-" The distance between the orbits of any two planets is nearly twice as great as that between the orbits of the next two nearer the sun." Thus suppose the distance of Mars from the Sun to be represented by 4; then, Venus will be 4+3, or 7 such distances; the Earth 4+twice 3, or 10 such distances; or more generally thus:

the sixteenth degree of magnitude; and there is no reason to suppose that this is the limit to the progression, as every increase in the dimensions and power of the instrument brings into view myriads of stars that were invisible before.

Number of the Stars.-The total number of stars visible to the naked eye in the most favoured localities is about 5000. It is only at the equator, however, that so large a number can be seen; for there only the spectator has the opportunity of seeing the whole heavens, without altering his position. Should he take up his position at either of the poles, no more than half the starry firmament can ever pass in review before him; while at all intermediate positions, the number of stars visible in any one night will depend on the latitude of the place. Argelander of Bonn has classified the number of stars visible to the naked eye as follows:

It thus appears that each inferior class is about three times as numerous as the one preceding it. The whole number of stars already registered, down to the seventh magnitude, is about 15,000; and some eminent astronomers have estimated the total number of stars visible by means of the best telescopes, down to the sixteenth degree of magnitude, at 500,000,000,000! Prodigious as this number is, it will be indefinitely increased as the powers of the telescope are extended; and, for aught we know, the actual number of the stars may be infinite.

Distance of the Stars.-The distance of the fixed stars from the earth is as inconceivable as their number; but, until recently, there were no data from which any probable calculation could be made. In the year 1838, however, the parallax (or angle subtended by the diameter of the earth's orbit, as seen from a star) was measured in the case of three of them. The parallax of a Centauri was ascertained by Professor Henderson of Edinburgh to be 0".9128, or nearly one second; that of 61 Cygni, by Professor Bessel of Königsberg, who found it to be 0.3483; and that of a Lyra, by Otto Struve, who found it to be about 0".25, or a quarter of a second. The major diameter of the earth's orbit being about 195,000,000 of miles, a parallax of one second will give a distance of 20,000,000, 000, 000 (twenty billions) of miles, which is probably the distance from our sun of the nearest fixed star;-a distance so great that light, which travels at the rate of 192,000 miles per second, would require 31 years to traverse it. The distance of the star 61 Cygni, its parallax being only of a second, will be three times this number; and of

a Lyra, whose parallax is of a second, will be four times twenty billions!

Magnitude of the Stars. In the present state of astronomical science, the magnitude of even the nearest of the fixed stars cannot be given with any degree of accuracy. It is certain, however, that, in general, they are greatly larger than our sun; for were the sun to be removed from his present position, where he has an apparent diameter of 32′ 3", and made to occupy the place of a Centauri, which is regarded as the nearest of the fixed stars, his diameter would be reduced to 0".0093, or less than the hundredth part of a second. Here he would fail to be seen by the naked eye, and no telescope ever invented could give us any idea of his size. If, on the other hand, a Centauri were removed from his actual position, and made to occupy the place of our sun, it is calculated that the light which he emits would be 2 times greater than that of the sun; and hence, it is argued, his magnitude must be correspondingly greater. The intrinsic splendour of Sirius is 63 times greater than that of a Centauri, and 147 times greater than that of the sun; and hence it is supposed the magnitude of Sirius is 147 times greater than that of our luminary. Considerable uncertainty, however, attaches to this mode of estimating the magnitude of those distant bodies. The light of the sun is so immensely superior in intensity to that of any star that it is impracticable to obtain any direct comparison between them, and it is only by using the moon as an intermediate term of comparison that any approximation to accuracy can be made. Wollaston, in 1829, found the proportion of the sun's light to that of the full moon to be as 801,072 to 1; while the light of the full moon exceeds that of a Centauri in the proportion of 27,408 to 1. Combining these results, he calculated the light of the sun as exceeding that of the star 21,955,000,000 times. Hence, from the parallax above assigned to the star, it is easy to conclude that its intrinsic splendour is 2.3247 times that of the sun.

Proper Motion of the Sun and Stars.-The so-called "fixed stars" are, in reality, all in motion: and no fixed point-no object absolutely at rest, is to be met with in the whole universe. The power of gravitation, which binds together the numerous members of the solar system, appears to be equally operative among the most distant objects in space. The relative distances of the fixed stars, and even the configuration of the constellations, are imperceptibly altering. Of all the bright stars observed by the ancients, not one has kept its place unchanged. In the case of Arcturus, for example, of μ Cassiopeia, and of a double star in Cygnus, this change of position has, in 2000 years, amounted to 24, 34, and 6 moon's diameters, respectively. While some vary only the twentieth part of a second annually, others vary 7.7 seconds, showing a ratio in their proper motions of 1:154. The Southern Cross will not always shine in the heavens in its present form, for the four stars of which it consists are moving in different directions. Even our own sun, so long regarded as stationary in the centre of the system, is found to be in rapid motion through space, and daily traversing a distance of

422,000 miles,—a space nearly equal to his own radius. Sir W. Herschel arrived at the conclusion, three quarters of a century ago, that he was moving in the direction of a Herculis,-a point in right ascension 260° 34', and north polar distance 63° 43', for the year 1790. Otto Struve, from a very elaborate discussion of the proper motion of 392 stars, determined the point, for 1850, to be in right ascension 261° 52'; declination 37° 33'. It will probably, however, be a long time yet before astronomers are in a position to determine whether this motion of our system through space is in a right line or curvilinear; and, if the latter, what that point is around which it is revolving. Dr Mädler of Dorpat has, indeed, hazarded the conjecture that our sun is only one of the millions of stars of the well-known Milky Way, which consists of a mighty ring, or wheel of stars, greatly crowded together at the circumference, but comparatively few towards the centre. The central group of this grand system, which composes our firmament, is, he thinks, the Pleiades, which revolves around Alcyone, the brightest orb of that beautiful constellation. The distance of our sun from that centre of force he calculates at 31,500,000 times the distance of the earth from the sun,-a distance so great that light could not traverse it in less than 500 years, and requiring 18,200,000 years for our sun to complete one revolution! But however lofty such conceptions of genius may be, they are not to be regarded as established scientific truths.

3. FORM, SIZE, AND MOTIONS OF THE EARTH.-Having thus traced the relation of the earth to surrounding worlds, we now return to examine itself more minutely. Its form is that which a perfect sphere * of semi-fluid consistency would assume, were it made to revolve around its axis with the same rapidity as the earth does. Such a form is called an oblate-spheroid,+-that is, a sphere somewhat flattened or compressed at the poles, like an orange. The larger or equatorial diameter exceeds the polar diameter by 26 miles-the former being 7924, and the latter 7898 miles. In round numbers, the diameter may be stated at 8000

Among the numerous proofs of the spherical form of the earth, the following may be mentioned:

1. A much greater extent of the earth's surface is visible from the top of a mountain than from a plain near the level of the sea.

2. As the mariner nears the land, he first sees the tops of the mountains; and on approaching nearer, the lower grounds become visible.

3. In cutting for a canal, it is found that allowance must be made for a dip of about 8 inches per mile, in order to keep the water at a uniform level.

4. In travelling to any considerable distance, either north or south, new stars come to view in the direction in which the traveller is advancing, while others disappear in the direction from which he is receding.

5. Many navigators, who have sailed constantly in one direction, whether due east or due west, have returned to the port from which they set out.

6. The shadow which the earth casts on the moon, during an eclipse, is always circular.

7. All the other members of the solar system are spherical.

A prolate spheroid, on the contrary, is a sphere somewhat elongated in the direction of its poles, forming a body shaped like a lemon.

miles; the radius, or semi-diameter, at 4000; the circumference at 25,000; the area, or superficial content, at 197,000,000 square miles; and the volume, or solid content, at 260,775,000,000 cubic miles.*

Motions of the Earth.-The earth has three motions: first, that referred to above, in accompanying the sun through space (see p. 2); second, an annual or orbitual motion round the sun, which it performs in 365.26 mean solar days; and the third, called its diurnal motion, round its own axis, in 1 day, or 24 solar hours. The axis is an imaginary line passing through the earth's centre, and inclined to the plane of its orbit at an angle of 23° 27′ 56.5". This imaginary line remains always parallel to itself; or, what is the same thing, its extremities, which are called its poles, always point to the same fixed stars, and present themselves alternately to the sun,-thus giving rise to the variety of the seasons, as the diurnal motion, which is from west to east, causes the alternations of day and night, and of the rising, southing, and setting of the heavenly bodies. If the axis on which the earth performs her daily rotation were exactly perpendicular to the plane of her path round the sun, one constant climate would characterise the same parallel of latitude at all times of the year, and all the benefits which result to mankind from the regular succession of the seasons would have been wanting; but by the simple arrangement of the axis being inclined 234° from the perpendicular, the All-Wise Creator has made perpetual provision for the regular recurrence of summer and winter, of seed-time and harvest. §

4. MATHEMATICAL DIVISIONS OF THE EARTH.-In order to describe with precision the position of places on the earth's surface, and the effects that result from its orbitual and diurnal

3.1416,

* Tyros in mathematics may be reminded of the following facts:1. The circumference of a circle, or sphere, whose diameter is 1, is or 3 nearly. Hence, to find the circumference of any other circle, or sphere, inultiply its diameter by 34.

2. The area of a circle is found by multiplying its radius by half its circumference. Thus the area of the circle AD is equal to the area of the triangle A B C, the base of which, A B, is the circumference В х С

of the circle. More briefly, A = 2

A

D

-B

3. The area of a sphere is equal to the convex area of the circumscribing cylinder; and its solid content is equal to of the solid con

tent of the circumscribing cylinder. Or, Area D 2 x 3.1416.

Solid content=

rad. x area
3

+ Strictly speaking, not around the sun, but around the com

non centre of gravity of the sun and earth, which is a point A

267 miles from the sun's centre.

B

But a sidereal day, or one measured by the stars, consists of 23 h. 56 m. 4 s. of solar time.

§ For beautiful illustrations of the seasons, and of the changes of day and night, see A. K. JOHNSTON'S School Atlas of Astronomy, Plate XI.