Mercury is said to have been occulted by Venus in the year 1737.[21] But whether this was an actual occultation, or merely a near approach does not seem to be certain. The first transit of Mercury across the sun's disc was observed by Gassendi on November 6, 1631, and Halley observed one on November 7, 1677, when in the island of St. Helena. Seen from Mercury, Venus would appear brighter than even we see it, and as it would be at its brightest when in opposition to the sun, and seen on a dark sky with a full face, it must present a magnificent appearance in the midnight sky of Mercury. The earth will also form a brilliant object, and the moon would be distinctly visible. The other planets would appear very much as they do to us, but with somewhat less brilliancy owing to their greater distance. As the existence of an intra-Mercurial planet (that is a planet revolving round the sun within the orbit of Mercury) seems now to be very improbable, Prof. Perrine suggests that possibly "the finely divided matter which produces the zodiacal light when considered in the aggregate may be sufficient to cause the perturbations in the orbit of Mercury."[22] Prof. Newcomb, however, questions the exact accuracy of Newton's law, and seems to adopt Hall's hypothesis that gravity does not act _exactly_ as the inverse square of the distance, and that the exponent of the distance is not 2, but 2·0000001574.[23] Voltaire said, "If Newton had been in Portugal, and any Dominican had discovered a heresy in his inverse ratio of the squares of the distances, he would without hesitation have been clothed in a _san benito_, and burnt as a sacrifice to God at an _auto da fé_."[24] An occultation of Mercury by Venus was observed with a telescope on May 17, 1737.[25] May transits of Mercury across the sun's disc will occur in the years 1924, 1957, and 1970; and November transits in the years 1914, 1927, and 1940.[26] From measurements of the disc of Mercury during the last transit, M. R. Jonckheere concludes that the _polar_ diameter of the planet is greater than the _equatorial_! His result, which is very curious, if true, seems to be supported by the observations of other observers.[27] The rotation period of Mercury, or the length of its day, seems to be still in doubt. From a series of observations made in the years 1896 to 1909, Mr. John McHarg finds a period of 1·0121162 day, or 1{d} 0{h} 17{m} 26{s}·8. He thinks that "the planet possesses a considerable atmosphere not so clear as that of Mars"; that "its axis is very considerably tilted"; and that it "has fairly large sheets of water."[28] CHAPTER III Venus