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the medium to be removed. Hence, all the heavenly bodies appear higher than they really are, and the nearer they are to the horizon the greater the refraction, or difference between their apparent and true altitudes will be ; at noon the refraction is the least. The sun and moon appear of an oval figure sometimes, near the horizon, by reason of refraction; for the under side being more refracted than the upper, the perpendicular diameter will be less than the horizontal one, which is not affected by refraction.

Refraction is variable according to the different density of the air; hence it happens that we sometimes are able to see the tops of mountains, towers, or spires of churches, which at other times are invisible, though we stand in the same place. The ancients knew nothing of refraction, the first who composed a table thereof was Tycho Brahe.

The sun's meridian altitude on the longest day decreases from the tropic of Cancer to the north pole ; and in the torrid zone when the sun is vertical there is no refraction; hence, the refraction is the least in the torrid zone, and greatest at the poles. Varenius, in his geography, speaking of the wintering of the Dutch in Nova Zembla, latitude 76° north,in the year 1596, says,they saw the sun in the year 1597 six days sooner than they would have seen him, had there been no refraction. 86. ANGLE OF POSITION between two places on the terrestrial globe, is an angle at the zenith of one of the places, formed by the brass meridian, and the quadrant of altitude passing through the other place, and is measured on the horizon.

87. RHUMBS are the divisions of the horizon into 32 parts, called the points of the compass. The ancients* were acquainted only with the four cardinal points, and the wind was said to blow from that point to which it

was nearest.

A Rhumb line, geometerically speaking, is a loxodromic or spiral curve, drawn, or supposed to be drawn upon the earth, so as to cut each meridian at the same angle, called the proper angle of the thumb. If this line be continued, it will never return into itself so as

* Pliny's Nat. Hist. Lib. I. chap. 47.

to form a circle, except it happen to be due east and west, or due north and south; and it can never be a right line upon any map, except the meridians be parallel to each other, as in Mercator's and the plane chart. Hence, the difficulty of finding the true bearing between two places on the terrestrial globe, or on any map but those above mentioned. The bearing found by a quadrant of altitude on a globe, is the only measure of a spherical angle upon the surface of that globe, as defined by the angle of position, and not the real bearing of the rhumb, as shewn by the compass: for, by the compass, if a place A bear due east from a place B, the place B will bear due west from the place A; but this is not the case when measured with a quadrant of altitude.

88. THE FIXED STARS are so called, because they have usually been observed to keep the same distance with respect to each other. The stars have an apparent motion from east to west,in circles parallel to the equinoctial, arising from the revolution of the earth on its axis, from west to east; and, on account of the precession of the equinoxes, their longitudes increase about 501 seconds in a year; this likewise causes a variation in their declinations and right ascensions: their latitudes are also subject to a small variation.

89. THE POETICAL RISING AND SETTING OF THE STARS, SO called, because, they are taken notice of by the ancient poets, who referred the rising and setting of the stars to the sun. Thus, when a star rose with the sun, or set when the sun rose, it was said to rise and set Cosmically. When a star rose at sun-setting, or set with the sun, it was said to rise and set Achronically. When a star first became visible in the morning, after having been so near the sun as to be hid by the splendor of his rays, it was said to rise Heliacally; and when a star first became invisible in the evening, on account of its nearness to the sun, it was said to set Heliacally.

90. A CONSTELLATION is an assemblage of stars on the surface of the celestial globe, circumscribed by the outlines of some assumed figure, as a ram, a dragon, a bear, &c. This division of stars into constellations is necessary, in order to direct a person to any part of the heavens where a particular star is situated.

The following Tables contain all the constellations on the New British Globes. The northern constellations are in number 34; the southern 47; the zodiacal constellations are 12; amounting in the whole to 93. The largest stars are called stars of the first magnitude; those of the sixth magnitude are the smallest that can be seen by the naked eye. The number of stars in each constellation, except those marked with asterisks, are taken from Flamstead.

CONSTELLATIONS IN THE Zo- Num-Names of the princi DIAC, generally called the Signs of the Zodiac.

pal Stars, and their Magnitudes.

ber of

Stars.

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THE SOUTHERN CONSTELLATIONS.

V. CONSTELLATIONS SOUTH OF Num-
THE ZODIAC, that rise nearly in the ber of
east,and set nearly in the west, at stars.
London.

1. Cetus, The Whale,

Names of the principa Stars, and their Magnitude.

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