_Hydra_ is a long constellation, which occupies a quarter of the horizon, under Cancer, the Lion, and the Virgin. The head, formed of four stars of the fourth magnitude, is to the left of Procyon, on the prolongation of a line drawn from that star to Betelgeuse. The western side of the great trapezium of the Lion, like the line from Castor and Pollux, points to α, of the second magnitude. This is the Heart of Hydra; we remark the asterisms of the second class, Corvus the Crow, and Crater the Cup. _Eridanus_, _Cetus_, _Piscis Australis_, and the _Centaur_ are the only important constellations which remain to be described. We find them, in the order which we have indicated, to the right of Orion. Eridanus is a river composed of a train of stars winding from the left foot of Orion and losing itself below the horizon. After following long windings, it ends with a fine star of the first magnitude, α Eridani, or Achernar. This is the river into which Phaeton fell when he unskilfully directed the Chariot of the Sun. It was placed in the sky to console Apollo for the death of his son. To find the Whale (Cetus), we may notice below the Ram a star of the second magnitude which forms an equilateral triangle with the Ram and the Pleiades; this is α of Cetus, or the Jaw; α, μ, ξ, and γ form a parallelogram which represents the head. The base, α, γ, may be produced to a star of the third magnitude, δ, and to a star of the neck marked ο. This star is one of the most curious in the heavens. It is named the Wonderful, _Mira Ceti_. It belongs to the class of variable stars. Sometimes it equals in brightness stars of the second magnitude, sometimes it becomes completely invisible.[11] Its variations have been followed since the end of the Sixteenth Century, and it has been found that they are reproduced periodically every 331 days on the average. The study of these singular stars presents us with curious phenomena. Lastly, the constellation of the Centaur is situated below Spica of the Virgin. The star θ, of the second magnitude, and the star ι, of the third, mark the head and the shoulder. This is the only part of this figure which rises above our horizon. The Centaur contains the _nearest star_ to us (α) of the first magnitude, the distance of which is about twenty-five billions of miles. The feet of the Centaur touch the _Southern Cross_, formed of four stars of the second magnitude, always hidden below our horizon. It reigns in silence above the icy solitudes of the Southern Pole, where ships proceed only with difficulty. Further on, at the centre of the other hemisphere, is the southern celestial pole, which is not marked by any remarkable star. It was from this region, Dante relates, that, having visited hell, inclosed in the centre of the earth, he went to the Mountain of Purgatory, and from there to the Heights of Paradise. These beautiful dreams have disappeared in the sunshine of modern astronomy.