were made with instruments of sizes which, for the most part, cannot be said to have been equal to the task to which they were applied. The truncature of the southern horn first spoken of by Schröter, was thought by Denning, in 1882, to be obvious; and in the same year, by watching the displacement of certain bright and dusky spaces on the disc, the same observer concluded that a rotation period of about 25 hours was indicated. In 1882 Schiaparelli at Milan commenced a prolonged study of Mercury. Believing that it was essential to observe through a good condition of atmosphere, and that this was impossible if the planet were only looked at in twilight, when it was necessarily at a low altitude, Schiaparelli made all his observations with the Sun and planet high up in the heavens. He considered, in effect, that the blaze of the Sun’s light was a lesser evil than the tremors inseparable from observations of the planet, clear it might be in some degree of inconvenient Sun-light, but viewed through the vapours and atmospheric disturbances, which always spoil all observations near the horizon. Schiaparelli’s observations yielded various results, most of them novel, and one of them very startling. He considers Mercury to be a much spotted globe and to be enveloped in a tolerably dense atmosphere. He thought he noticed brownish stripes and streaks (which might be regarded as permanent markings), more clearly visible on some occasions than on others; and that these systematically disappeared near the limb, owing to the increased depth there of the atmosphere through which they had to be looked at. The foregoing observations may be regarded as not unreasonable; they may even be accepted without further question. But what are we to say to Schiaparelli’s conclusions that these markings are so nearly permanent, taking one day with another, that Mercury’s rotation cannot be measured in hours at all, but is a matter of days,—in point of fact, of 88 days; and that in reality Mercury occupies in its rotation on its axis the whole of the 88 days which constitute its sidereal year, or period of revolution round the Sun. The counterpart of this for us would be that, instead of the inhabitants of the earth having a day of 24 hours, they would have only one day and night every 365 days. Astronomers are not at present satisfied to accept this conclusion in regard to Mercury.