Here we take leave of the planet Mercury--an interesting and beautiful object, which stimulates our intellectual curiosity, while at the same time it eludes our attempts to make a closer acquaintance. There is, however, one point of attainable knowledge which we must mention in conclusion. It is a difficult, but not by any means an impossible, task to weigh Mercury in the celestial balance, and determine his mass in comparison with the other globes of our system. This is a delicate operation, but it leads us through some of the most interesting paths of astronomical discovery. The weight of the planet, as recently determined by Von Asten, is about one twenty-fourth part of the weight of the earth, but the result is more uncertain than the determinations of the mass of any of the other larger planets. CHAPTER VIII. VENUS. Interest attaching to this Planet--The Unexpectedness of its Appearance--The Evening Star--Visibility in Daylight--Lighted only by the Sun--The Phases of Venus--Why the Crescent is not Visible to the Unaided Eye--Variations in the Apparent Size of the Planet--The Rotation of Venus--Resemblance of Venus to the Earth--The Transit of Venus--Why of such Especial Interest--The Scale of the Solar System--Orbits of the Earth and Venus not in the same Plane--Recurrence of the Transits in Pairs--Appearance of Venus in Transit--Transits of 1874 and 1882--The Early Transits of 1631 and 1639--The Observations of Horrocks and Crabtree--The Announcement of Halley--How the Track of the Planet differs from Different Places--Illustrations of Parallax--Voyage to Otaheite--The Result of Encke--Probable Value of the Sun's Distance--Observations at Dunsink of the Last Transit of Venus--The Question of an Atmosphere to Venus--Other Determinations of the Sun's Distance--Statistics about Venus. It might, for one reason, have been not inappropriate to have commenced our review of the planetary system by the description of the planet Venus. This body is not especially remarkable for its size, for there are other planets hundreds of times larger. The orbit of Venus is no doubt larger than that of Mercury, but it is much smaller than that of the outer planets. Venus has not even the splendid retinue of minor attendants which gives such dignity and such interest to the mighty planets of our system. Yet the fact still remains that Venus is peerless among the planetary host. We speak not now of celestial bodies only seen in the telescope; we refer to the ordinary observation which detected Venus ages before telescopes were invented.