the 3d of November, the absolute equation of time, resulting from both these causes, will be the greatest; the time shown by a regular going clock, being then about 164 minutes slower than the time shown by the Sun.

10. The velocity of the Earth, like all the other planets, varies in different parts of its orbit, it being most rapid in the perihelion, about January the 1st, and slowest when in aphelion about July 1st. The daily motion in the perihelion is 62' 12", and in the aphelion 59' 12".

11. This unequal motion of the Earth causes the summer half year, north of the equator, to be about 8 days longer than the winter half year. Or the interval between the vernal and autumnal equinoxes, is about 8 days longer than the interval between the autumnal and vernal equinoxes.

From the spring equinox to the summer solstice

From the summer solstice to the autumnal equinox

From the autumnal equinox to the winter solstice

From the winter solstice to the spring

days. hrs. min.

92 21 36

93 13.58.

89 16 51

89 1 24

equinox Hence, from the spring equinox to the autumnal equinox is 186 days, 11 hours, 34 minutes; and from the autumnal equinox to the spring equinox is 178 days, 18 hours, 15 minutes, making a difference of 7 days, 17 hours, 29 minutes.

12. The velocity of light is to that of the Earth in its orbit as 10313 to 1; and it is found by observation to be 8 minutes 7 seconds in coming from the Sun to the Earth.

When the Earth is in its perihelion, light takes about 7 minutes, 59 seconds in passing from the Sun to the Earth; at the mean distance of the Earth from the Sun, S minutes, 71⁄2 seconds; and at the greatest distance of the Earth from the Sun, 8 minutes, 15 seconds.

TABLE.

Showing the mean longitude of the Earth, reckoning from the mean equinox, at the epoch of mean noon, at Paris, January 1st, 1801; longitude of the perihelion, &c.

Eccentricity in miles

1,618,000.

Its sidereal revolution is performed in 365 days, 6 hours,

9 minutes, 11 seconds.

Its tropical revolution, or tropical year, 365 days, 5 hours, 48 minutes, 51 seconds.

The sidereal motion of the apsides is direct 19′ 40′′; but the tropical motion, is direct 1' 2" nearly in a year, or 10 43' 10' in one hundred years; making the length of the year to consist of 365 days, 6 hours, 14 minutes, 2 seconds; this is called the Anomalistic year. A complete tropical revolution of the apsides is performed in 20,931 years.

As the centrifugal force is greater at the equator than near the poles, the weight of bodies are increased as we proceed from the equator to the poles. If the gravity of a body at the equator be unity or 1, at or near the poles it will be 1.00569. This variation of the action of gravity in different latitudes, also causes the same pendulum, as has already been remarked, to vibrate slower at the equator than at or near the poles. For a pendulum to vibrate seconds at the equator, it must be 39 inches in length, and at or near the poles 39.206 inches. The density of the Earth is to that of water as 11 to 2.

The Earth is surrounded by a rare and elastic fluid, which is called the atmosphere; neither the temperature nor the density of this fluid is uniform, but diminishes in proportion to its distance from the surface of the Earth; the height of the atmosphere is supposed to be about 45 miles. If the density of the atmosphere were every where the same, at its temperature at 55 degrees, and the height of the barometer at 30 inches, the height of the atmosphere would be 27100 feet. The weight of the atmosphere upon every ot on the Earth's surface is about 2160 pounds.

What is the 'rth?

What is the figne of the Earth, and what is its

mean diameter?

What is the ratio of

'he Earth's axis to its equa

torial diameter, according

putation?

to Dr. Adrain's com

Is the axis of the Earth pe. "pendicular to the plane of the equinoctial ?

Have the equinoctial points a re ograde mo

tion?

'me?

ich

What are the causes of the equation of t When is that part of the equation of time, w depends upon the obliquity of the ecliptic greatest > When is that part depending upon the unequal motion of the Sun greatest?

How much longer is the summer half year, in northern latitude, than the winter half?

How much greater is the velocity of light than that of the earth in its orbit?

CHAPTER IX.

Of Mars. 8

1. Mars is the next planet, after the Earth, in the order of distance from the Sun; it performs its sidereal revolution from west to east, or in the order of the signs, round the Sun in 686 days, 23 hours, 30 minutes and 36 seconds, at the mean rate of about 55,166 miles per hour.

As the orbit of this planet includes that of the Earth, it seems to move from west to east round the Earth. Its apparent motion is, however, very unequal; when it begins to be visible in the morning, a little after the conjunction, its motion is direct and most rapid; it becomes gradually slower, and the planet when it arrives at about 136° 48′ from the Sun, is stationary; the motion then becomes retrograde, increasing in velocity till Mars is 1800 distant from the Sun, or in opposition, so as to be on the meridian at midnight. This velocity ther becomes a maximum, diminishes, and again becomes noning, when Mars, approaching the Sun, is distant from 136° 48'. Its motion then becomes again direct, after having been retrograde during 73 days, and in this interval the planet describes an arc of retrogradation of abeat 16° 12', continuing to approach the Sun, it finishesy immerging in the evening into the Sun's rays. These singular phenomena are renewed at every opposition of Mars, but with a considerable difference as to the extent and duration of his retrogradations. See La Place's System of

of the World. The period in which all those changes take place, or the interval between two successive conjunctions, or oppositions, is about 780 days, which is the length of the synodical revolution of this planet.

The irregularities of Mars in its orbit, being the most considerable of all the primary planets, Kepler fixed upon it as the first object of his investigations respecting the nature of the planetary orbits; and after extraordinary labour, he at last discovered that the orbit of this planet was elliptical; that the Sun is placed in one of the foci; and that there is point round which the angular motion is uniform. In the pursuit of this inquiry he found the same thing of the Earth's orbit; hence, by analogy, it was reasonable to think that all the planetary orbits are elliptical, having the Sun in one of the foci.

no

2. Continued and accurate observations show that the figure of Mars is not an exact sphere, but an oblate spheroid, whose axis or polar diameter is to its equatorial one as 1272 to 1355, or as 15 16 nearly. The mean diameter of Mars is about 4200 miles, and this planet revolves on its axis, from west to east in 24 hours, 39 minutes, and 213 seconds, which is the length of its day.