16. The POLAR CIRCLES are two small circles, parallel to the equator, at the distance of 66° 32′ from it, or 23° 28′ from each pole. The northern is called the arctic, the southern the antarctic circle.

17. 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 generally only sixteen parallels of latitude drawn on the terrestrial globe.

18. The HOUR CIRCLE on the artificial globes is a small circle of brass, with an index or pointer fixed to the north pole. The hour circle is divided into 24* equal parts correspondent to the hours of the day; and these are again subdivided into halves and quarters.

19. The HORIZON is a great circle which separates the visible half of the heavens from the invisible. This horizon, when applied to the earth, is distinguished by the sensible and rational horizon.

20. THE SENSIBLE, or visible horizon, is that which terminates our view, and is represented by that circle which we see in a clear day where the earth, or sea, and the sky seem to meet.t

21. THE RATIONAL, or true horizon, is an imaginary plane, passing through the centre of the earth, parallel to the sensible horizon. It determines the rising and setting of the sun, stars, and planets.

one.

* Some globes have two rows of figures on the index, others but On Bardin's new British Globes there is an hour circle at each pole numbered with two rows of figures. On Adam's common globes there is but one index; and on his improved globes the hours are counted by a brass wire with two indexes standing over the equa

tor.

Mr. William Jones has made an hour circle to slide on the brass meridian of many of the globes fitted up by him; it is likewise ment to shew the bearing of places. The form of the hour circle is, however, a matter of little consequence, (provided it be placed under the brass meridian), as the equator will answer every purpose to which a circle of this kind can be applied.

The sensible horizon extends only a few miles; for example, if a man of 6 feet high were to stand on a large plane, or on the surface of the sea; the utmost extent of his view, upon the earth or the sea, would be about three miles. Thus, if h be the height of the eye above the surface of the sea, and d the diameter of the earth in feet, then -h, will shew the distance which a person will be able to see, straight forward. Keith's Trigonometry, Example XLI, page 83.

22. The WOODEN HORIZON circumscribing the artificial globe, represents the rational horizon on the real globe. This horizon is divided into several concentric circles. On Bardin's New British Globes 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 numbered from the north point of the horizon towards the east and west, from 0 to 90 degrees; and from the south point of the horizon, towards the east and west in the same manner.

The Third Circle contains the thirty-two points of the compass, divided into half and quarter points. The degrees in each point are to be found in the azimuth circle.

The Fourth Circle contains the twelve 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.

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 difference of time, shewn 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, has the sign + before it; when the clock or watch ought to be slower, the number of minutes in the difference has the sign - before it. This Circle is peculiar to the New British Globes.

The Eighth Circle contains the twelve calendar months of the year, &c.

23. The CARDINAL POINTS OF THE HORIZON are east, west, north, and south.

24. The CARDINAL POINTS IN THE HEAVENS are the zenith, the nadir, and the points where the sun. rises and sets.

52. The CARDINAL POINTS OF THE ECLIPTIC are the equinoctial and solstitial points, which mark out the four seasons of the year; and the Cardinal Signs are q Aries, Cancer, Libra, and y Capricorn.

26. The ZENITH is a point in the heavens exactly over our heads, and is the elevated pole of our horizon.

27. The NADIR is apoint in the heavens exactly under our feet, being the depressed pole of our horizon, and the zenith, or elevated pole, of the horizon of our antipodes.

28. The POLE of any circle is a point on the surface of the globe, 90 degrees distant from every part of that circle of which it is the pole. Thus the poles of the world are 90 degrees from every part of the equator; the poles of the ecliptic (on the celestial globe) are 90 degrees from every part of the ecliptic.-Every circle on the globe, whether real or imaginary, has two poles diametrically opposite to each other.

29. The EQUINOCTIAL POINTs are Aries and Libra, where the ecliptic cuts the equinoctial. The point Aries is called the vernal equinox, and the point Libra the autumnal equinox. When the sun is in either of these points, the days and nights on every part of the globe are equal to each other.

30. The SOLSTITIAL POINTS are Cancer and Capricorn. When the sun is in, or near, these points, the variation in his greatest altitude is scarcely perceptible for several days; because, the ecliptic near these points is almost parallel to the equinoctial, and, therefore, the sun has nearly the same declination for several days. When the sun enters Cancer it is the longest day to all the inhabitants on the north side of the equator, and the shortest day to those on the south side. When the sun enters Capricorn it is the shortest day to those who live in north latitude, and the longest day to those who live in south latitude.

31. A HEMISPHERE is half the surface of the globe; every great circle divides the globe into two hemispheres. The horizon divides the upper from the lower hemisphere in the heavens; the equator separates the northern from the southern on the earth; and the brass meridian, standing over any place on the terrestrial globe, divides the eastern from the western hemisphere.

32. The MARINER'S COMPASS is a representation of the horizon, and is used by seamen to direct and ascertain the course of their ships. It consists of a circular brass box, which contains a paper card, divided into 32

equal parts, and fixed on a magnetical needle that always turns towards the north. Each point of the compass contains 11° 15′, or 11 degrees, being the 32d part of 360 degrees.

33. The VARIATION OF THE COMPASS is the deviation of its points from the correspondent points in the heavens. When the north point of the compass is to the east of the true north point of the horizon, the variation is east ; if it be to the west, the variation is west.

The learner is to understand, that the compass does not always point directly north, but is subject to a small annual variation. At present, in England, the needie points about 24 degrees to the westward of the north.

The compass is used for setting the artificial globe north and south; but care must be taken to make a proper allowance for the variation.

34. LATITUDE OF A PLACE, on the terrestrial globe, is its distance from the equator in degrees, minutes or geographical miles, &c. and is reckoned on the brass meridian, from the equator towards the north or south pole.

35. LATITUDE OF A STAR OR PLANET, on the celes tial globe, is its distance from the ecliptic, northward or southward, counted towards the pole of the ecliptic, on the quadrant of altitude. The greatest latitude a star can have is 90 degrees, and the greatest latitude of a planet is nearly 8 degrees.* The sun being always in the ecliptic, has no latitude.

36. The QUADRANT OF ALTITUDE is a thin slip of brass divided upwards from 0 to 90 degrees, and downwards from 0 to 18 degrees, and, when used, is generally screwed to the brass meridian. The upper divisions are used to determine the distances of places on the

The newly-discovered planets, or Asteroids, Ceres and Pallas, do not appear to be confined within this limit.

C

earth, the distances of the celestial bodies, their altitudes, &c.; and the lower divisions are applied to finding the beginning, end, and duration of twilight.

37. LONGITUDE OF A PLACE on the terrestrial globe, is the distance of the meridian of that place from the first meridian, reckoned in degrees and parts of a degree on the equator. Longitude is either eastward or westward, according as the place is eastward or westward, of the first meridian. The greatest longitude that a place can have is 180 degrees, or half the circumference of the globe.

38. LONGITUDE OF A STAR, OR PLANET, is reckoned on the ecliptic from the point Aries, eastward, round the celestial globe. The longitude of the sun is what is called the sun's place on the terrestrial globe.

39. ALMACANTARS, OR PARALLELS OF ALTITUDE, are imaginary circles parallel to the horizon, and serve to shew 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.

40. PARALLELS OF CELESTIAL LATITUDE are small circles drawn on the celestial globe, parallel to the ecliptic.

41. PARALLELS OF DECLINATION are small circles parallel to the equinoctial on the celestial globe, and are similar to the parallels of latitude, on the terrestrial globe.

42. The COLURES are two great circles passing through the poles of the world; one of them passes through the equinoctial points, Aries and Libra ;* the other through the solstitial points, Cancer and Capricorn: hence they are called the equinoctial and solstitial colures. They divide the ecliptic into four equal parts, and mark the four seasons of the year.

43. AZIMUTH, OR VERTICAL CIRCLES, are imaginary great circles passing through the zenith and the nadir, cutting the horizon at right angles. The altitudes of the heavenly bodies are measured on these circles, which circles may be represented by screwing the quad

* In the time of Hipparchus the equinoctial colure is supposed to have passed through the middle of the constellation Aries. Hippars chus was a native of Nicæa, a town of Bythinia in Asia Minor, about 75 miles S. E. of Constantinople, now called Isnic; he made his observations between 160 and 135 years before Christ.