called Arided. Ovid, in Met. vi. 115, among other changes, alluded to Jupiter's assumption of the Swan's form to betray Leda: Fecit et Asterien aquila luctante teneri ; The principal stars of Cygnus are Arided in his body, Albireo near his bill. DIFFERENTIAL REFRACTION, is the difference between the refractions of different stars produced by the respective differences in their light. See Phil. Mag. for April, 1824, and Perennial Calendar. For example, the red stars are less refrangible than the white, or even those which have much blue light; as Aldebaran, Betalgeus, and Arcturus, for instance, are less refrangible than Sirius, Lyra, or Aquila. DELPHINUS the Dolphin, situated near to Aquila, is well known by its four principal stars of the third magnitude, placed, as it were, in the corners of a lozenge. Ovid makes the Dolphin rise heliacally Jan. 9th, which would make it rise now at the end of that month; in fact, it does now rise about the 24th of January. Ovid's words, in Fasti, lib. i. are, Interea Delphin clarum super aequora sidus Tollitur, et patriis exerit ora vadis. On the 10th June, that is in Fasti, vi. he describes its acronycal rising, which now takes place in July, See Account of Risings, under ARCTURUS: Navita puppe sedens Delphina videbimus, inquit, Again, of the heliacal setting in autumn: Quem modo coelatum stellis Delphina videbas, DOUBLE STARS.-Of these Herschel has found about 700, of these about 40 had been observed before. The following will serve as a specimen, and afford the observer a few objects for his attention. a Herculis, is a beautiful double star; the two bodies are apparently unequal; the largest is red, and the smallest of a bluish colour inclining to green. y Andromedae, double, very unequal; the larger of a reddish white colour; the smaller a fine bright sky blue, inclining to green. B Lyrae, quadruple, unequal white, but three out of the four inclined to red. e Boötis, double, very unequal, larger, of a reddish colour; the smaller is blue, or of a faint lilac colour. a Lyrae, double, very unequal; the larger is a fine brilliant white, the smaller dusky. Polaris, Castor, and many others, might be enumerated; two stars that are only very close, particularly when of different magnitudes, are not considered as double stars; as, for instance, Little Alcor and Great Mizar, in the Great Bear. DRACO the Dragon, a constellation lying round the pole of the ecliptic, may always be seen on a clear night. This Dragon, in fable, is the hundred headed Dragon which guarded the golden apples in the garden of the Hesperides, and which was slain by Hercules: it is a fable confounded with the apple offered to Eve by the serpent. This constellation seems to be the one alluded to by Virgil in the well known lines, Geor. i. 244: Maximus hic flexu sinuoso elabitur Anguis, There are two other serpent constellations. See HYDRA and Serpent. It is probably this constellation to which Ovid alludes in the directions which he has made Phoebus give to Phaeton; he here calls it by the name of Anguis: Altius egressus, coelestia tecta cremabis; Neu te dexterior tortum declinet in Anguem, In another passage, however, the Dragon is called Tum primum radiis gelidi caluere Triones, EARTH.-The Earth is the next planet above Venus in the system. It is eighty two millions of miles from the Sun, and goes round him in 365 days 5 hours and 49 minutes, from any equinox or solstice to the same again; but from any fixed star to the same again, as seen from the Sun, in 365 days 6 hours and 9 minutes; the former being the length of the tropical year, and the latter the length of the sidereal. It travels at the rate of fifty-eight thousand miles every hour; which motion, though 120 times swifter than that of a cannon ball, is little more than half as swift as Mer cury's motion in his orbit. The Earth's diameter is 7970 miles; and by turning round its axis every 24 hours from west to east, it causes an apparent diurnal motion of all the heavenly bodies from east to west. By this rapid motion of the Earth on its axis, the inhabitants about the equator are carried 1042 miles every hour, whilst those on the parallel of London are carried only about 580, besides the fifty eight thousand miles by the annual motion above mentioned, which is common to all places whatever. The Earth is round like a globe; as appears, 1. By its shadow in eclipses of the Moon; which shadow is always bounded by a circular line. 2. By our seeing the masts of a ship whilst the hull is hid by the convexity of the water. 3. By its having been sailed round by many navigators. The hills take off no more from the roundness of the Earth, in comparison, than grains of dust do from the roundness of a common globe. The seas and unknown parts of the Earth contain 160,522,026 square miles; the inhabited parts 38,990,569; Europe 4,456,065; Asia 10,768,823; Africa 9,654,807; America 14,110,874. In all 199,512,595; which is the number of square miles on the whole surface of our globe. EQUATION OF TIME.-The annexed Table shews how much the Sun is faster or slower than the clock ought to be, so far as the difference depends upon the obliquity of the ecliptic; of which thẻ signs of the first and third quadrants are at the head of the Table, and their degrees at the left hand; and in these the Sun is faster than the clock; the signs of the second and fourth quadrants are at the foot of the Table, and their degrees at the right hand; in all which the Sun is slower than the clock: so that entering the Table with the given sign of the Sun's place at the head of the Table, and the degree of his place in that sign at the left hand; or with the given sign at the foot of the table, and degree at the right hand; in the angle of meeting, is the number of minutes and seconds that the Sun is faster or slower than the clock or in other words, the quantity of time in which the real Sun, when in that part of the ecliptic, comes sooner or later to the meridian than the fictitious Sun in the equator. Thus, when the Sun's place is Ŏ Taurus 12 degrees, he is 9 minutes 49 seconds faster than the clock; and when his place is Cancer, 18 degrees, he is 6 minutes 2 seconds slower. This part of the Equation of Time may, perhaps, be somewhat difficult to understand by a figure, because both halves of the ecliptic seem to be on the same side of the globe; but it may be made very easy to any person who has a real globe before him, by putting small patches on every tenth or fifteenth degree both of the equator and ecliptic, beginning at Aries r; and then, turning the ball slowly round westward, he will see all the patches from Aries to Cancer come to the brazen meridian sooner than the corresponding patches on the equator; all those from Cancer to Libra will come later to the meridian than their corresponding patches on the equator; those from Libra to Capricorn sooner, and those from Capricorn to Aries later: and the patches at the beginnings of Aries, Cancer, Libra, and Capricorn, being either on, or even with those on the equator, shew that the two Suns either meet there, or are even with one another, and so come to the meridian at the same moment. |