Earth’s rotation by tidal friction. This brings us to the subject of celestial evolution, which is discussed in another chapter. CHAPTER V. THE INNER PLANETS. Much progress has been made during the last hundred years in our knowledge of the planets. In fact, the study of Mercury only dates from the commencement of the nineteenth century. Our knowledge of the vicinity of the Sun is very limited, and Mercury is difficult of observation. So limited, in fact, is our knowledge of the Sun’s surroundings, that it is not yet known for certain whether there is a planet, or planets, between Mercury and the Sun. Perturbations in the motion of the perihelion of Mercury’s orbit led Le Verrier in 1859 to the belief that a planet of about the size of Mercury, or else a zone of asteroids, existed between Mercury and the Sun. It was, however, obvious that such a planet could only be seen when in transit across the Sun’s disc, or during a total eclipse. Meanwhile a French doctor, Lescarbault, informed Le Verrier that he had seen a round object in transit over the Sun’s disc. Le Verrier, certain that this was the missing planet, named it “Vulcan,” and calculated its orbit, assigning it a revolution period of twenty days. But it was never seen again. Transits of “Vulcan” were fixed for 1877 and 1882, but nothing was seen on these dates. During the total eclipse of July 29, 1878, two observers—_James Watson_ (1838-1880), the well-known astronomer, and _Lewis Swift_ (born 1820)—believed themselves to have discovered two separate planets, and ultimately claimed two planets each, which were never heard of again. During the total eclipse of 1883 an active watch for “suspicious objects” was kept, but with no result. At the eclipses of 1900 and 1901 respectively, photographs were exposed by the American astronomers, W. H. Pickering and _Charles Dillon Perrine_ (born 1867), but on none of these plates could any trace of “Vulcan” be found. At the total eclipse of August 30, 1905, plates were again exposed, but no announcement has been made of an intra-Mercurial planet; and the prevalent opinion among astronomers is that no planet comparable with Mercury in size exists between that planet and the Sun. The study of the physical appearance of Mercury was inaugurated by Schröter, who in 1800 noticed that the southern horn of the crescent presented a blunted appearance, which he attributed to the existence of a mountain eleven miles in height. From observations of this mountain he came to the conclusion that the planet rotated in 24 hours 4 minutes. This was afterwards reduced by _Friedrich Wilhelm Bessel_ (1784-1846) to 24 hours 53 seconds.