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The last of Mercury was in 1815, and the next will be in 1892. The greatest heat on the planet Venus probably exceeds the heat of the torrid zone about as much, as that exceeds the average heat of 60 degrees north latitude.
T'ke Earth. The Earth is a spherical body. This is obvious from the following considerations: First, From analogy; as all the other planets and heavenly bodies are spherical. Secondly,
To people on shore, the mast of a ship appears before the huli; but were the earth a plate, the hull would appear long before the mast, by reason of the much greater angle which it subtends. Thirdly, The earth has been sailed round by Magellan, Drake, Dampier, Anson, Cook, and many others. Fourthly, The boundary of the earth's shadow upon the moon, in a lunar eclipse, is always circular; and nothing but a spherical body can, in all situations, prodece a circular shadow. The unevennesses of the earth's surface have no effect opon its shadow on the moon; for the height of the highest mountain on the globe, is only equal to 1500th part of the earth's diameter.
The earth is not a perfect sphere, but an oblate spheroid ; that is, its equatorial diameter is longer than its axis. The difference of these diameters is about 34 miles. The mean diameter of the earth, or the diameter in latitude 45 degrees, is 7928 miles. Of course, the equatorial diameter is 7945 miles, and the length of the earth's axis is 7911. The equatorial circumference of the earth le about 21,970 miles; its mean circumference, in latitude 45 degrees, 24.917; and its meridional circumference, 24,963. The number of square miles on the earth's surface is 197,459,101 ; and 260.909,292,265 is the number of cubic miles contained in the earth. It performs a rotation on its axis once in 24 hours.
The earth is surrounded with a thin, invisible, elastic fluid, called air, the whole body of which forms what is called the atmosphere. The density of the air is not always the same, it being subject to be expanded by heat and contracted by cold. In its mean state it is found to be about 850 times lighter than water.
Notwithstanding the seeming inequality in the distribution of light and darkness, it is certain, that, throughout the whole world, there is nearly an equal proportion of light diffused on every part, if we disregard what is absorbed by clouds, vapors, and the atmosphere itself. The equatorial regions have indeed the most intense light during the day, but the nights are long and dark; while on the other hand, in the northerly and southerly parts, though the sun shines less powerfully, yet the length of time that it appears above the horizon, with the longer duration of twilight, makes up for the seeming deficiency, Mars
. The diameter of Mars is 4189 miles, and its mean distance from the sun is 144,000,023 miles. Its annual revolution occupies 1 year, 321 days, 23 hours, 31 minutes, and its rotation on its axis 21 hours, 39 minutes, 22 seconds. It moves in its orbit at the rate of 56,000 miles an hour. Its bulk, compared with that of the earth, is as 7 to 24; and its density, as 7 to 10. One pound on thé earth would weigh 0.34 in this planet. Mars is of a fiery
red colour. By the telescope, dark spots are discoverable on its surface; but round its poles, particularly the southern, an intense and permanent brightness. Mars is an oblate spheroid. Its axis is to its equatorial diameter, as 98 to 103. It has an atmosphere of considerable extent.
Jupiter. Jupiter, the largest of the planets, is 89,170 miles in diameter, and 491,702,301 miles from the sun.
Its bulk, compared with that of the earth is nearly as 1400 to 1 ; its density as 5 to 22; and its weight as 312 to 1. One pound on the earth would weigh 2.33lbs. in Jupiter. Its shape is that of an oblate spheriod. Its polar diameter is to that of its equatorial, as 12 to 13; and the difference of their lengths, is upwards of 6000 miles. Its ecliptic and equator are nearly coincident; that is, its axis is nearly perpendicular to the plane of its orbit. Hence this planet has no sensible change of seasons. If its axis were inclined any considerable number of degrees towards the plane of its orbit, just so many degrees round each pole would, in their turn, be almost six years together in total darkness. It revolves on its axis in 9 hours, 55 minutes; and round the sun in 11 years, 314 days, 18 hours, 45 minutes. Its hourly motion in its orbit is 30,000 miles. From a comparison of the most ancient, with the modern observations, there is some reason to conclude, that the period of its revolution is decreasing. Jupiter is surrounded by faint substances, called belts. These were discovered in 1665. They are parallel to each other, and to the equator of the planet. The quantity of light and heat enjoyed by Jupiter, is to that enjoyed by the Earth, as 37 to 1000.
Saturn. The diameter of Saturn is 79,042 miles, and its distance from the sun is 901,668,908 miles. Its bulk is proportioned to that of the earth nearly, as 1000 to 1. Its density, as 26 to 288, and its weight, as 98 to 1. A body weighing ilb. on the earth, would weigh 1.02 on this planet. It is an oblate spheriod, its axis being to its equatorial diameter, as 10 to 11. It rerolves on its axis in 10 hours, 16 minutes, 2 seconds, and round the sun in 29 years, 164 days, 7 hours, 21 minutes. Its hourly motion in its orbit is about 22,000 miles. The intensity of the sun's light and heat, is about 95 times greater at the Earth, than at Saturn. This planet has belts discoverable on its disc; but they are not so large or numerous as the belts of Jupiter. The most remarkable appearance, however, is a large ring, entirely separated from the planet itself, and yet completely surrounding it. The plane of the ring coincides with the plane of Saturn's equator, 40 that the axis of the planet makes a right angle with it. When the outer edge of the ring is turned towards the earth, it is invisible, except with telescopes of very grcat powers; either on account of its thinness, or of its almost total incapacity to reflect light. The ring is double, or is composed of two rings, having the same plane and the same centre. The outside diameter of the larger ring is 204,883 miles, and its inner diameter 190,248 miles; so that the breadth is 7318 miles. The outside diameter of the smaller ring is 134,393 miles, its inner diameter 146,345, and its breadth 13,024. The space between the rings is 2,977 miles. There is no visible connection
between the two rings. They both, however, revolve on a comcon axis, in 10 hours, 32 minutes, 15 seconds; a period longer than that of Saturn's rotation by 16 minutes, 13 seconds. The ring is doubtless no less solid than the planet; and it is observed to cast a strong shadow upon it. Its light is also generally brighter than that of the planet. The thickness of the ring is probably less than 1000 miles, and its outer edge is not flat, but spherical. As the planet revolves round the sun, the plane of the ring is always parallel with itself, so that in each Saturnian year, it is twice turned edgewise towards the sun,
Herschel. This planet is called in England Georgium sidus, on the continent of Europe, Uranius, and generally, in this country, Herschel. There is no reason to believe that it had ever been ob served by any inhabitant of the Earth before the 13th of March, 1781, when it was discovered by Dr. Herschel. Its diameter is 35,112 miles, and its distance from the sun, is 1,803,534,392. Its hourly motion in its orbit is 16,000 miles. Its bulk, compared with that of the Earth, is nearly as 90 to 1, and its weight as 16.84 to 1. A body on the Earth weighing llb. would weigh 0.93lb, in this planet. The period of its revolution round the sun is 83 years, 150 days, 18 hours. It has not yet been determined whether it revolves on an axis. Yet there can be no doubt of this fact, as its shape is that of an oblate spheroid. The quantity of light and heat, communicated to the Earth by the sun, is at least 360 times as great, as that enjoyed by Herschel; and the diameter of the sun, as seen from it, is not more than twice the apparent diameter of the planet Venus, as seen from the Earth. The plane of its orbit is nearly coincident with the plane of the ecliptic. Owing to its immense distance few discoveries have been made respecting it.
Satellites. A satellite, or moon, is a body revolving round a planet, and, in company with the planet, round the sun. Of these there are 18 in our system, distributed in the following manner: 1 to the Earth ; 4 to Jupiter; 7 to Saturn; and 6 to Herschel.
The Moon. The moon's diameter is 2180 miles. This is to the diameter of the Earth nearly as 20 to 73. Its surface is to that of the Earth as 1 to 13; its bulk as 1 to 49; its density as 5 to 4 nearly; and its weight as 1 to 39. Its mean distance from the Earth is 239,029 miles, which is to the sun's mean distance nearly as 1 to 390. The angle which its orbit makes with the ecliptic varies from 5 degrees to 5 18. The moon revolves round the Earth in 27 days, 7 hours, 43 minutes. The interval of time between one new moon and the next, is 29 days, 12 hours, 44 minutes. If the Earth stood still, or had no revolution round the sun, every month would be of the former length; but as the Earth, during a lunary revolution, materially alters its place, it takes the moon 2 days 5 hours to regain what it has lost by the earth's motion. The moon's orbit, to a spectator on the sun, always appears concave. In differeat parts of its orbit the apparent size of the moon is found to vary. This is owing to the elliptical shape of the orbit. It is found by