7. A clock that is so regulated as to move through twenty-four hours in the course of a sidereal day, is said to be regulated to sidereal time.

Astronomers have found, by comparing a certain number of solar and sidereal days, that a mean solar day is 24 hours, 3 minutes, and 56.55 seconds of sidereal time, so that the excess of a mean solar day above a sidereal day, is 3 minutes, 56.55 seconds in sidereal time.

8. An hour is a certain determined part of a day, and is equal or unequal. An equal hour is the 24th part of a mean solar day, as shown by well regulated clocks; unequal hours are those measured by the returns of the sun to the meridian, or those shown by a correct sun-dial.

Hours are divided into 60 equal parts, called minutes, a minute into 60 equal parts called seconds, a second into 60 equal parts called thirds, &c.

9. A sidereal hour is the 24th part of a sidereal day, and is therefore less than an hour of mean solar time.

For, a mean solar day is to a sidereal day, as 24 hours is to 23h. 56m. 4.1 seconds in mean solar time; or, as 24h. 3m. 56.555 seconds is to 24 hours in sidereal time: And, consequently, if the length of a solar hour be taken equal to unity or 1, a sidereal hour will be equal to .99727 in mean solar time; or, if a sidereal hour be taken equal to unity, or 1, an hour of mean solar time will be equal to 1.0027379 in sidereal time. Hence, by multiplying any given portion of sidereal time by .99727, we shall have the corresponding mean solar time; and, on the contrary, to reduce mean solar time, to sidereal time, we must multiply by 1.0027379.

10. A year, in the general extent of the word, is a period or space of time, measured by the revolution of some celestial body in its orbit.

As year denoted originally a revolution, and was not limited to that of the sun; accordingly, we find by the oldest accounts, that people have, at different times, expressed other revolutions by it, particularly that of the moon; and consequently that the years of some accounts are to be reckoned only months, and sometimes periods of 2, or 3, or 4 months. This will assist us greatly in understanding the accounts that certain nations give of their own antiquity, and perhaps also of the age of men. We read expressly, in several of the old Greek writers, that the Egyptian year, at one period, was only a month; and we are farther told that at other periods it was 3 months, or 4 months.

The Egyptians boasted, almost 2000 years ago, of having accounts of events 48000 years distance. A great deal must be allowed to fallacy on the above account; but besides this, the Egyptians had, in the time of the Greeks, the same ambition which the Chinese have at present, and wanted to pass themselves on that people, as these do upon us, for the oldest inhabitants of the earth. They had also recourse to the same means, and both the present and the early impostors have pretended to ancient observations of the heavenly bodies, and recounted eclipses in particular, to vouch for the truth of their accounts.

Since the time in which the solar year, or period of the earth's revolution round the sun, has been received, we may account with certainty; but for those remote ages, in which we do not precisely know what is meant by the term year, it is impossible to form any satisfactory conjecture of the duration of time in the accounts.

11. The returns of the sun to the same equinox mark the years, in the same manner as its returns to the meridian mark the days. The solar year is either astronomical or civil.

12. The astronomical solar year is that which is determined precisely by astronomical observations; and is of two kinds, tropical and sidereal, or astral.

13. It is found by observation that the sun, in consequence of its annual motion in the ecliptic, employs three hundred and sixty-five days, five hours, forty-eight minutes, and fifty-one seconds, in moving from one equinox to the same again. This period of time is called the tropical year.

This is the only proper or natural year, because it always keeps the same seasons to the same months.

14. Observation also shows us that the sun em ploys 365 days, 6 hours, 9 minutes, and 11 seconds, in passing from any fixed star, till it returns to the same again. This period is called the sidereal year.

Hence, the sidereal year is 20 minutes and 20 seconds longer than a tropical year; and it likewise follows that the equinoctial points must have a motion along the ecliptic in a direction contrary to the order of the signs, amounting to 50" 1, in a year: for, as the sun describes the whole ecliptic, or 360° in a year, 365d. 5h. 48m. 51sec.: 360°:: Id: 59' 8" 2, the daily mean motion of the earth, or the apparent mean motion of the sun in a day; and therefore Id: 59′ 8′′:: 20′20′′: 50" 1. This retrograde motion of the equinoctial points is called the recession of the equinoctial points.

15. That form of year which a nation has adopted for computing their time by, is called a civil year. The American civil year is a period of 365 days, 6 hours, which is either common or bissextile. The common civil year is that consisting of 365 days; having seven months of 31 days each; four of 30 days, and one of 28 days: the bissextile, usually called the leap year, consists of 366 days, having one day extraordinary, called the intercalary, or bissextile day; and takes place every 4th year. In this year February contains 29 days.

What is time?

QUESTIONS.

How is time measured?

What is a true solar day?

What is a mean solar day?

What is a civil day?

What is a sidereal day, and what is its duration

in mean solar time?

What is an hour?

What is a sidereal hour?

What is a year?

What is a true solar year?

What is an astronomical year, and how is it di

vided?

What is a tropical year, and what is the length of it?

What is a sidereal year, and what is its duration?
What is a civil year, and how is the American

civil year divided?

How many days does the common civil year consist of, and what is the duration of the bissextile or leap year?

CHAPTER VIII.

Positions of the Sphere. Names assigned to Persons from their different situations on the Globe, &c.

1. Position of the sphere, is its situation with respect to certain circles on the surface of the earth and the horizon. There are principally three positions of the sphere; right, parallel, and oblique.

2. A right sphere is that position of the earth where the equator passes through the zenith and nadir, the poles being in the rational horizon.

The inhabitants who have this position of the sphere live at the equator; they have therefore no latitude, nor no elevation of the pole. All the heavenly bodies will appear to revolve round the earth from east to west, in circles parallel to the equinoctial, according to their different declinations; one half of the starry heavens will be constantly above the horizon, and the other half below; and the sun always rises at right angles to their horizon, making their days and nights of equal length at all times of the year, because the horizon bisects the circle of diurnal revolution; so that the stars will be visible for twelve hours, and invisible for the same space of time.

3. A parallel sphere is that position of the earth where the equator coincides with, and all its parallels are parallel to the horizon.

Hence, the poles of the world are in the zenith and nadir, while all the meridians cut the horizon at right angles. The inhabitants of a sphere in this position, if there are any, live at the poles; they have the greatest possible latitude; and the stars, which are situated in the hemisphere to which the inhabitants belong, never set, but describe circles parałlel to the horizon; while those stars of the contrary hemisphere never rise.

During the time that the sun is describing the northern signs, the inhabitants of the north pole have continual day, and those of the south pole continual night; and while he is describing the southern signs, the inhabitants of the north pole have continual night, and those of the south pole continual day.

4. An oblique sphere is that position of the earth in which the equator and all its parallels are unequally divided by the horizon.

This is the most common position of the sphere, or it is the situation which the earth has with respect to all its inhabitants, except those at the equator and poles.

To the inhabitants of an oblique sphere, the pole of their hemisphere is elevated above the horizon as many degrees as are equal to the latitude, and the opposite pole is depressed as much below the horizon; so that the stars only, at the former, are seen. The sun and all the heavenly bodies rise and set obliquely; the seasons are variable, and the days and nights are unequal.

5. The inhabitants of the earth have different names assigned to them by geographers, according to the several meridians and parallels of latitude they lie under, and are called antæci, periæci, and antipodes.

6. The antæci, or antecians, are those who live under the same meridian, or line of longitude, and have the same degrees of latitude, but the one has