Circles at the distance of 23° 28' from the poles, are called the Polar Circles. The northern is denominated the Arctic Circle, and the southern, the Antarctic Circle.

The Tropics and Polar Circles divide the earth into Zones. That part of the earth's surface, lying between the tropics, is called the Torrid or Burning Zone; the parts between the Tropics and Polar Circles, are called the Temperate Zones; and those parts, which lie beyond the Polar Circles, are denominated the Frigid or Frozen Zones.

A Climate is a tract of the earth's surface, included between the equator and a parallel of latitude, or between two parallels of such a breadth, as that the length of the day in one shall be an half hour longer than in the other. Within the polar circles, however, the breadth of a climate is such that the length of a day, or the time of the sun's continuance above the horizon, is a month longer in one parallel, than in another. The breadth of the climates continually decrease, as they approach the poles; the first climate, reckoning from the equator, being about 8 degrees broad, while that which is nearest the poles, is scarcely four miles. The word Climate is also used to denote the difference in the temperature of the air, in different places.

The Latitude of a place is its distance from the equator, reckoned in degrees, north or south. If a place be situated between the Equator and the North pole, it is said to be in north latitude; if it lie between the Equator and the South pole, it is in south latitude.

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The Longitude of a place is its distance, east or west, from some fixed meridian, measured on the equator.

On the equator, a degree of longitude is equal to 60 geographical miles; and consequently, a minute on the equator is equal to a mile. But as all meridians cut the equator at right angles, and approach nearer and nearer to each other, till they cross at the poles; it is obvious, that the degrees of longitude will continually decrease, as they proceed from the equator to either pole; so that in the 60th degree of latitude, a degree of longitude is but 30 miles, or half as long as a degree on the equator.

THE GLOBES.

The Globes are two spherical bodies, whose convex surfaces are supposed to give a true representation of the

earth and heavens, as visible by observation.

One of these

is called the Terrestrial, the other the Celestial globe. On the convex surface of the Terrestrial globe, all the parts of the earth and sea are delineated in their relative form, size, and situation.

On the surface of the Celestial Globe, the images of the several constellations, and the unformed stars are delineated; and the relative magnitude and position, which the stars are observed to have in the heavens, carefully preserved.

In order to render these globular bodies more useful, they are fitted up with certain appurtenances, whereby a great variety of useful problems are solved in a very easy and expeditious manner.

The Brazen Meridian is that ring or hoop in which the globe hangs on its axis, which is represented by two wires passing through the poles. The circle is divided into four quarters, of 90 degrees each; in one semicircle, the divisions begin at each pole, and end at 90 degrees where they meet. In the other semicircle, the divisions begin at the middle, and proceed thence towards each pole, where there are 90 degrees. The graduated side of this brazen circle serves as a meridian for any point on the surface of the earth, the globe being turned about till that point comes under the circle.

The Hour Circle is a small circle of brass, divided into twenty four hours, the quarters and half quarters. It is fixed to the brazen meridian, with its centre over the north pole; to the axis is fixed an index, that points out the division of the hour circle, as the globe is turned round on its axis.

The Horizon is represented by the upper surface of the wooden circular frame, encompassing the globe about its middle. On this wooden frame is a kind of perpetual calendar, contained in several concentric circles; the inner one is divided into four quarters, of 90 degrees each; the next circle is divided into the twelve months, with the days in each, according to the new style; the next contains the twelve equal signs of the ecliptic, each being divided into thirty degrees; the next, the twelve months and days, according to the old style; and there is another circle, containing the thirty-two points of the compass, with their

halves and quarters. Although these circles are on all horizons, yet they were not always placed in the same order.

The Quadrant of Altitude is a thin slip of brass, one edge of which is graduated into 90 degrees and their quarters, equal to those in the meridian To one end of this is fixed a brass nut and screw, by which it is put on and fastened to the meridian; if it be fixed to the zenith or pole of the horizon, then the graduated edge represents a vertical circle, passing through any point.

Besides these, there are several circles described on the surface of both globes. Such as the equinoxial, or ecliptic, circles of longitude and right ascension, the tropics, polar circles, parallels of latitude and declination, on the celestial globe; and on the terrestrial, the equator, the eclip. tic, tropics, polar circles, parallels of latitude, hour circles, or meridians, to every fifteen degrees; and on some globes, the spiral rhumbs, flowing from the several centres, called flies.

In using the globes, keep the east side of the horizon towards you, unless the problem require the turning it, which side you may know by the word East, on the horizon ; for then you have the graduated meridian towards you, the quadrant of altitude before you, and the globe divided exactly into two equal parts, by the graduated side of the meridian,

The following problems, as being most useful and entertaining, are selected from a great variety of others, which are easily solved with a globe, fitted up with the aforementioned appertenances.

1. The latitude of a place being given, to rectify the globe for that place.

Let it be required to rectify the globe for the latitude of Boston, 42 degrees 23 minutes north.

Elevate the north pole, till the horizon cuts the brazen meridian in 42° 23′ and the globe is then rectified for the latitude of Boston. Bring Boston to the meridian, and you will find it in the zenith, or directly on the top of the globe. And so for any other place.

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II. To find the latitude and longitude of any place on the terrestrial globe.

Bring the given place under that side of the graduated brazen meridian where the degrees begin at the equator, then the degree of the meridian over it shows the latitude, and the degree of the equator, under the meridian, shows the longitude.

Thus Boston will be found to lie in 42° 23′ north latitude, and 70o 58' west longitude from London, or 3° 10′ east longitude from Philadelphia.

III. To find any place on the globe, whose latitude and longitude are given.

Bring the given longitude, found on the equator, to the meridian, and under the given latitude, found on the me. ridian, is the place sought.

IV. To find the distance and bearing of any two given places, on the globe.

Lay the graduated edge of the quadrant of altitude over both places, the beginning, or 0 degrees, being on one of them, and the degrees between them show their distance; these degrees, multiplied by 60, give the distance in En

lish miles.

V. To find the sun's place in the ecliptic.

Look at the day of the month in the outer calendar upon the horizon, (if the globe was made before the alteration of the style) and opposite to it you will find the sign and degree the sun is in that day. Thus on the 25th of March, the sun's place is 4 degrees in Aries. Then look for that sign and degree in the ecliptic line, marked on the globe, and you will find the sun's place; there fix on a small black patch, so it is prepared for the solution of the following problems.

VI. To find the sun's declination, that is, his distance from the equinoxial line, either northward or southward.

Bring his place to the meridian, observe what degree of the meridian lies over it, and that is his declination. If the sun lies on the north side of the line, he is said to have north declination, but if on the south side, he has south declination.

Note. The greatest declination can never be more than 23o 28' either north or south; that being the distance of the tropics from the equinoxial, beyond which the sun nev. er goes.

VII. To find where the sun is vertical on any day; that is, to find over whose heads the sun will pass that day.

Bring the sun's place to the meridian, observe his declination, or hold a pen or wire over it; then turn the globe round, and all those countries which pass under the wire, will have the sun over their heads that day at noon.

Note. This appearance can only happen to those who live in the torrid zone, because the sun never goes farther from the equinoxial, northward or southward, than the two tropics, from whence he turns again. ·

VIII. To find over whose heads the sun is, at any hour, or at what place the sun is vertical.

Bring the place where you are, (suppose at Boston,) to the meridian; set the index to the given hour by your watch; then turn the globe till the index points to the upper 12, or noon; look under the degree of declination for that day, and you will find the place to which the sun is vertical, or over whose heads it is at that time.

IX. To find, at any hour of the day, what o'clock it is at any place in the world.

Bring the place where you are to the brass meridian set the index to the hour by the watch, turn the globe till the place you are looking for come under the meridian, and the index will point out the time required.

X. To find at what hour the sun rises and sets any day in the year; and also upon what point of the compass.

Rectify the globe for the latitude of the place you are in; bring the sun's place to the meridian, and set the index to 12; then turn the sun's place to the eastern edge of the horizon, and the index will point out the hour of rising; if you bring it to the western edge of the horizon, the index will show the hour of setting.