11. It is proper to observe that, though the meridians completely invest the globe; they are usually, and very properly called only semicircles, which is the property, of the meridian of any place, the other half of the same circle being called the opposite meridian.

Meridians, according to this acceptation, are also called lines of longitude.

12. To supply the place of other meridians, the globe is hung in a large brass circle, which is usually called the brazen meridian, or sometimes, by way of distinction, the universal meridian, or only the meridian.

The brazen meridian is divided into 360 degrees. On one half of the meridian, these degrees are numbered from 0 to 90, from the equator towards the poles, and are used for finding the latitudes of places. On the other half of the meridian they are numbered from the poles towards the equator, and are used in the elevation of the poles.

13. The brazen meridian, which may be made to coincide with the meridian of any place, divides the globe into two equal hemispheres, called the eastern

and western.

Hence, east and west are only relative terms with respect to places situated on the earth: for instance, London is east of New-York, and west of Paris; consequently, with respect to the meridian of New-York, London is in the eastern hemisphere; and with respect to the meridian of Paris, London is in the western hemisphere.

14. When the sun comes to the meridian of any place, not within the polar circles, it is noon or mid-day at that place.

15. The first meridian is that from which geographers begin to reckon the longitudes of places. 16. The English and Americans count the lo

gitudes of places from the meridian of London; and the French from the meridian of Paris.

Hence, in American, as well as in English maps and globes, the first meridian is a semicircle supposed to pass through London, or the observatory at Greenwich.

17. The meridional circles that pass through the equinoctial and solstitial points, are called colures ; the former being called the equinoctial, and the latter the solstitial colure.

The first determines the equinoxes, the second shows the solstices; and by dividing the ecliptic into four equal parts, they also designate the four seasons of the year.

QUESTIONS.

What is a great circle?

What are the poles of a great circle ?

How many great circles are there drawn on the

globe, and what are they called?

What is the equator, and what is its use?

Why is the equator, when referred to the heavens, called the equinoctial?

What is the ecliptic, and in what angle does it intersect the equator?

What are the principal points of the ecliptic, and what are they called?

What are the meridians, and in what angle do

they cut the equator?

What are the meridians, when considered as lines

of longitude?

What is the brazen meridian?

What is the first meridian ?

From what meridian do the Americans count the

longitude?

What are the colures, and into how many parts do they divide the ecliptic ?

CHAPTER III..

Of the small circles on the terrestrial globe.

1. Circles which divide a globe into two unequal parts, are called small circles.

2. The small circles on the terrestrial globe, which divide it into two unequal parts, are the tropics, polar circles, parallels of latitude, &c.

3. The tropics are two small circles parallel to the equator at the distance of 23° 28' from it; the northern is called the tropic of Cancer, and the southern the tropic of Capricorn.

Two planes are parallel when, being produced ever so far, they do not meet.

4. The polar circles are two small circles parallel to the equator, at the distance of 66° 32′ from it, οι 23° 28' from each pole.

5. Parallels of latitude are small circles drawn through every ten degrees of latitude, on the terrestrial globe, parallel to the equator.

Every place on the globe is supposed to have a parallel of latitude drawn through it, though there are only sixteen drawn on Wilson's terrestrial globe. When the parallels of latitude are referred to the heavens, they correspond to the parallels of declination.

6. Parallels of altitude, commonly called Almacanters, are imaginary circles parallel to the horizon, and serve to show the height of the sun, moon,

or stars.

These circles are not drawn on the globe, but they may be described for any latitude by the quadrant of altitude.

QUESTIONS.

What is a small circle, and how many are usually drawn on the globe ?

What are the tropics, and how far do they extend from the equator?

What distance are the polar circles from the poles, and what distance are they from the equator ? What are the parallels of latitude, and how many are generally drawn on the globe ?

What circles are called almacanters, and what do the parallels of altitude show ?

CHAPTER IV.

Of the wooden horizon, and other appendages to the terrestrial globe.

1. The horizon is a great circle which separates the visible half of the heavens from the invisible; the earth being considered as a point in the centre of the sphere of the fixed stars.

2. Horizon, when applied to the earth, is either sensible or rational.

3. The sensible, or visible, horizon is the circle which bounds our view, where the sky appears to touch the earth or sea.

The sensible horizon extends only a few miles; for instance, the mean diameter of the earth being, (according to D. Adrain's computation,) 7920 miles, and the circumference 24880 miles; if a man of 6 feet high were to stand on the earth where the surface is spherical, or on the surface of the sea, the utmost extent of his view on the earth or sea, would be 3 miles nearly. Thus, 7920 miles is equal to 41817600 feet, to which add 6 feet, and the sum will be 41817606, this multiplied by 6, gives 250905636; then by extracting the square root of this last number, we shall find

15840 very nearly; which is equal to 3 miles, the distance which a man 6 feet high can see straight forward, admitting there is nothing to intercept his view.

4. The rational, or true horizon, is an imaginary plane passing through the centre of the earth, parallel to the sensible horizon.

The rational horizon determines the rising of the sun, stars, and planets.

5. The wooden horizon, circumscribing the terrestrial globė, represents the rational horizon on the real globe of the earth.

6. The wooden horizon is divided into several concentric circles: On Wilson's terrestrial globe they are arranged in the following order :

The first circle is marked amplitude, and is numbered from the east towards the north and south, from 0 to 90 degrees; and from the west towards the north and south in the

same manner.

The second circle is marked azimuth, and is reckoned from the north and south points of the horizon towards the east and west from 0 to 90 degrees.

The third circle contains the thirty-two points of the compass, divided into half and quarter points.

The fourth circle contains the twelve signs of the ecliptic, (usually called the signs of the Zodiac,) with the figure and character of each sign.

The fifth circle contains the degrees of the signs, each sign comprehending 30 degrees, as has been already observed. The sixth circle contains the days of the month answering to each degree of the sun's place in the ecliptic.

The seventh circle contains the equation of time, or the difference of time, shown by a well regulated clock and a correct sun-dial. When the clock ought to be faster than the dial, the number of minutes expressing the difference at noon, has the sign+, which is read plus, or more, before it; and when the clock or watch ought to be slower, the number of minutes in the difference has the sign -, less, or minus, before it.

The eighth circle contains the 12 calender months of the year