2. The comparative distances of the planets are represented in the cut, page 15, and also in the following:

3. To assist his conception of these vast distances, the student may imagine a rail. road laid down from the sun to the orbit of Neptune. Now if the train proceed from the sun at the rate of thirty miles an hour, without intermission, it will reach Mercury in 152 years; the Earth in 361 years; Jupiter in 1,884 years; Saturn in 3,493 years; Herschel in 6,933; and Neptune in 10,800 years! Such a journey would be equal to riding 900,000 times across the continent, from Boston to Oregon!

4. It is now about 5,850 years since the creation of the world. Had a train of cars started from the sun at that time toward the orbit of Neptune, and traveled day and night ever since, it would still be 284 millions of miles within the orbit of Herschelabout where the head of the locomotive stands, as shown in the cut! To reach even that planet would require over 1,000 years longer; and to arrive at Neptune, nearly 6,000 years to come! Such is the vast area embraced within the orbits of the planets, and the spaces over which the sunlight travels, to warm and enlighten its attendant worlds.

56. The apparent magnitude of the heavenly bodies depends much upon the distance from which they are viewed; the magnitude increasing as the distance is diminished, and diminishing as the distance is increased.

NEAR AND REMOTE VIEWS OF THE SAME OBJECT.

C

Let A represent the position of an observer upon the earth, to whom the sun appears 82', or about half a degree in diameter. Now it is obvious that if the observer advance to B (half way), the object will fill an angle in his eye twice as large as it filled when viewed from A. Again: if he recede from A to C, the object will appear but half as large. Hence the rule, that the apparent magnitude is increased as the distance is diminished, and diminished as the distance is increased.

57. Could a beholder leave the earth, and, descending toward the sun, station himself upon Mercury, he would find the apparent magnitude of the sun vastly increased. Should he then return, and pass outward to Mars or Jupiter, he would observe a corresponding diminution in the sun's magnitude, in proportion as the distance was increased. Hence the apparent magnitude must vary

56. How apparent magnitudes of heavenly bodies modified? (Illustrate by diagram.)

57. Suppose a person to go to Mercury-what effect upon apparent size of the sun?

exceedingly, as viewed from different points in the solar

The above cut represents the relative apparent magnitude of the sun, as seen from the different planets. In angular measurements, its diameter would be as follows:

Let us continue our imaginary journey outward, be yond Neptune, toward the fixed stars, and in a short time the glorious sun, so resplendent and dazzling to our view, will appear only as a sparkling star; and the fixed stars will expand to view as we approach them, till they assume all the magnitude and splendor of the sun himself.

LIGHT AND HEAT OF THE PLANETS.

58. As the distances of the planets, respectively, affect the apparent magnitude of the sun, as viewed from their surfaces, so it must affect the relative amount of light and heat which they respectively receive from this great luminary.

59. The amount of light and heat received from the sun, by the several planets, is in inverse proportion to the square of their respective distances.

58. What effect has the distances of the planets from the sun, respectively upon their relative light and heat?

59. What rule governs the diffusion of light? (Illustrate by a diagram.)

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PHILOSOPHY OF THE DIFFUSION OF LIGHT.

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368 B

MVEM A
A

1. Here 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 B, as it is at A. Hence the rule, that the light and heat of the planets is, inversely, as the squares of their respective distances.

2. 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 3 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 §.

60. The intensity of light and heat received upon the several planets varies, according to their respective distances, from 6 times as much as our globe toth part as much.

1. The comparative light and heat of the planets-the earth being 1-is as follows:

2. From this table it appears that Mercury has 6 times as much light and heat as our globe, Uranus only, and Neptune onlyth part as much. Now if the average temperature of the earth is 50 degrees, the average temperature of Mercury would be 825 degrees; and as water boils at 212, the temperature of Mercury must be 118 degrees above that of boiling water. Venus would have an average temperature of 100 degrees, which would be twice that of the earth. On the other hand, Jupiter, Saturn, Uranus, and Neptune, seem doomed to the rigors of perpetual winter. And what conception can we form of a region 900 times as cold as our globe! Surely,

"Who there inhabit must have other powers,

Juices, and veins, and sense, and life, than ours;

One moment's cold, like theirs, would pierce the bone,
Freeze the heart's blood, and turn us all to stone !"

8. It is not certain, however, that the heat is proportionate to the light received by

60. Between what limits does the light and heat of the several planets vary? (What would that be for Mercury? For Venus? How with the ex terior planets? Poetry? Is it certain that the heat of the planets is in exact proportion to the light they respectively reccive? Why not?)

the respective planets, as various local causes may conspire to modify either extreme of the high or low temperatures. For instance, Mercury may have an atmosphere that arrests the light, and screens the body of the planet from the insupportable rays of the sun; while the atmospheres of Saturn, Herschel, &c., may act as a refracting medium to gather the light for a great distance around them, and concentrate it upon their otherwise cold and dark bosoms.

MAGNITUDE OF THE PLANETS.

61. The planets vary as much in their respective magnitudes, as in their distances. Their several diameters, so far as known, are as follows:

1. The asteroids are so small and so remote, that measurements of their exact diameters are obtained with great difficulty; hence the numerous blanks in the above table. And even when diameters are given, they are somewhat doubtful.

2. In the case of the other planets, we have given their mean or average diameters, according to the best authorities. As most of them are more or less oblate, their polar diameters are less, and their equatorial more, than the amount given in the table.

62. The magnitude of the principal planets, as compared with the earth, is as follows:-Mercury, as large; Venus, Jupiter, 1,400 times as large; Saturn, 1,000 times; Uranus, 90 times; and Neptune, 60.

9

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1. The magnitudes of spherical bodies are to each other as the cubes of their diameters. Thus, 7912×7912×7912=495,289,174,428, the cube of the earth's diameter; and 2950X2950×2950=25,672,375,000, the cube of the diameter of Mercury. Divide the former by the latter, and we have 19 and a fraction as the number of times the bulk of Mercury is contained in the earth.

61. State the diameters of the several planets? (Why blanks in the table? What diameters are given-polar, equatorial, or neither?)

62. Give the magnitude of the principal planets, as compared with the earth. (How ascertain relative magnitudes? How possible that a mere star can be such an immense world?)

2. It may seem almost incredible that what appear only as small stars in the heavens should be larger than the mighty globe upon which we dwell. But when we consider their immense distance, and the effect this must have upon their apparent magnitude, as illustrated at 55, it is evident that the planets could not be seen at all were they not very large bodies. The above cut will give some idea of the magnitude of the several planets, as compared with each other, and also with the sun.

63. The Sun is 1,400,000 times as large as our globe, and,500 times as large as all the other bodies of the solar system put together. It would take one hundred and twelve such worlds as our earth, if laid side by side, to reach across his vast diameter.

DENSITY.

64. The planets differ greatly in their density, or in the compactness of the substances of which they are composed. Mercury is about three times as dense as our globe, or equal to lead. Venus and Mars are about the same as the earth; while Jupiter and Uranus are only th as dense, or about equal to water. Saturn has only th the density of our globe, answering pretty nearly to cork

68. State the magnitude of the sun as compared with the earth. With the rest of the system. Illustration?

64. What meant by density? Do the planets differ in this respect? State and illustrate. (How masses of planets ascertained? How with Mercury?)