_Visibility of Mercury._—Copernicus, amid the fogs of the Vistula, looked for Mercury in vain, and complained in his last hours that he had never seen it. Tycho Brahe, in the Island of Hueen, appears to have been far more successful. The planet is extremely fugitive in his appearances, but is not nearly so difficult to find as many suppose. Whenever the horizon is very clear, and the planet well placed, a small sparkling object, looking more like a scintillating star than a planetary body, will be detected at a low altitude and may be followed to the horizon. _Period &c._—Mercury revolves round the Sun in 87^d 23^h 15^m 44^s in an eccentric orbit, so that his distance from that luminary varies from 43,350,000 to 28,570,000 miles. When in superior conjunction the apparent diameter of the planet is 4″·5; at inferior, conjunction it is 12″·9, and at elongation 7″. His real diameter is 3000 miles. _Elongations._—Being situated so near to the Sun, it is obvious that to an observer on the Earth he must always remain in the same general region of the firmament as that body. His orbital motion enables him to successively assume positions to the E. and W. of the Sun, and these are known as his elongations, which vary in distance from 18° to 28°. He becomes visible at these periods either in the morning or evening twilight, and under the best circumstances may remain above the horizon two hours in the absence of the Sun. The best times to observe the planet are at his E. elongations during the first half of the year, or at his W. elongations in the last half; for his position at such times being N. of the Sun’s place, he remains a long while in view. It is unfortunate that when the elongation approaches its extreme limits of 28° the planet is situated S. of the Sun, and therefore not nearly so favourably visible as at an elongation of only 18° or 20°, when his position is N. of the Sun. I have seen Mercury on about sixty-five occasions with the naked eye. In May 1876 I noticed the planet on thirteen different evenings, and between April 22 and May 11, 1890, I succeeded on ten evenings. I believe that anyone who made it a practice to obtain naked-eye views of this object would succeed from about twelve to fifteen times in a year. In a finer climate, of course, Mercury may be distinguished more frequently. Occasionally he presents quite a conspicuous aspect on the horizon, as in February 1868, when I thought his lustre vied with that of Jupiter, and in November 1882, when he shone brighter than Sirius. The planet is generally most conspicuous _a few mornings after his W. elongations_ and _a few evenings before his E. elongations_.