With reference to the difficulty of seeing Mercury, owing to its proximity to the sun, Admiral Smyth says, "Although Mercury is never in _opposition_ to the earth, he was, when in the house of Mars, always viewed by astrologers as a most malignant planet, and one full of evil influences. The sages stigmatized him as a false deceitful star (_sidus dolosum_), the eternal torment of astronomers, eluding them as much as terrestrial mercury did the alchemists; and Goad, who in 1686 published a whole folio volume full of astro-meteorological aphorisms, unveiling the choicest secrets of nature, contemptuously calls Mercury a 'squinting lacquey of the sun, who seldom shows his head in these parts, as if he was in debt.' His extreme mobility is so striking that chemists adopted his symbol to denote quicksilver."[16] Prof. W. H. Pickering thinks that the shortness of the cusps (or "horns") of Mercury's disc indicates that the planet's atmosphere is of small density--even rarer than that of Mars. The diameter of Mercury is usually stated at about 3000 miles; but a long series of measures made by Prof. See in the year 1901 make the real diameter about 2702 miles. This would make the planet smaller than some of the satellites of the large planets, probably smaller than satellites III. and IV. of Jupiter, less than Saturn's satellite Titan, and possibly inferior in size to the satellite of Neptune. Prof. Pickering thinks that the density of Mercury is about 3 (water = 1). Dr. See's observations show "no noticeable falling off in the brightness of Mercury near the limb." There is therefore no evidence of any kind of atmospheric absorption in Mercury, and the observer "gets the impression that the physical condition of the planet is very similar to that of our moon."[17] Schröter (1780-1815) observed markings on Mercury, from which he inferred that the planet's surface was mountainous, and one of these mountains he estimated at about 11 miles in height![18] But this seems very doubtful. To account for the observed irregularities in the motion of Mercury in its orbit, Prof. Newcomb thinks it possible that there may exist a ring or zone of "asteroids" a little "outside the orbit of Mercury" and having a combined mass of "one-fiftieth to one-three-hundredth of the mass of Venus, according to its distance from Mercury." Prof. Newcomb, however, considers that the existence of such a ring is extremely improbable, and regards it "more as a curiosity than a reality."[19] M. Léo Brenner thinks that he has seen the dark side of Mercury, in the same way that the dark side of Venus has been seen by many observers. In the case of Mercury the dark side appeared _darker_ than the background of the sky. Perhaps this may be due to its being projected on the zodiacal light, or outer envelope of the sun.[20]