HYDRA.--This is an extraordinary constellation, winding through a vast space from east to west for more than one hundred degrees. It lies south of Cancer, Leo, and Virgo, and reaches from Canis Minor to Libra. It contains sixty stars, principally of the second, third, and fourth magnitudes. The head of Hydra may readily be distinguished by four bright stars south of Acubens, in the Crab. They form a rhomboidal figure. The three upper stars form a beautiful curve, and are too distinct and conspicuous to be forgotten when once seen. Alphard, twenty-three degrees south south-west of Regulus, is a very brilliant star of the second magnitude, and is in the heart of Hydra, and comes to the meridian twenty minutes before nine o'clock on the 1st of April. When the head of the Hydra is on the meridian, its other extremity is many degrees below the horizon, so that its whole length cannot be traced out in the heavens until its centre is on the meridian. "Near the Equator rolls The sparkling Hydra, proudly eminent, To drink the Galaxy's refulgent sea; Nearly a fourth of the encircling curve Which girds the ecliptic his vast folds involve; Yet ten the number of his stars diffused O'er the long track of his enormous spires; Chief beams his heart, sure of the second rank, But emulous to gain the first." According to mythology, the Hydra was a terrible monster that infested the Lake of Lerna, in the Peloponnesus. It was reported to have had a hundred heads, and, as soon as one of these was cut off, two grew in its place, unless the wound was stopped by fire. "Art thou proportioned to the Hydra's length, Who, by his wounds, received augmented strength? He raised a hundred hissing heads in air; When one I lopped, up sprang a dreadful pair." The formidable monster was at last destroyed by Hercules, with the assistance of Iolaus, and who afterwards, dipping his arrows in the gall of the Hydra, rendered every wound inflicted by them incurable and mortal. * * * * * THE CUP.--This small constellation lies south of the Lion, and rests upon the Hydra. Six of the principal stars form a crescent or semi-circle, opening to the west. The crescent of the Cup is so striking and clearly defined, when the moon is absent, that no description is necessary to point it out, as it is the only one of the kind in that part of the heavens. * * * * *