This peculiar phenomenon has proved one of the greatest drawbacks to the proper observation of transits, for it is quite impossible to note the exact instant of the planet's entrance upon and departure from the solar disc in conditions such as these. The black drop seems to bear a family resemblance, so to speak, to the phenomenon of Baily's beads. In the latter instance the lunar peaks, as they approach the sun's edge, appear to lengthen out in a similar manner and bridge the intervening space before their time, thus giving prominence to an effect which otherwise should scarcely be noticeable. The last transit of Mercury, which, as has been already stated, took place on November 14, 1907, was not successfully observed by astronomers in England, on account of the cloudiness of the weather. In France, however, Professor Moye, of Montpellier, saw it under good conditions, and mentions that the black drop remained very conspicuous for fully a minute. The transit was also observed in the United States, the reports from which speak of the black drop as very "troublesome." Before leaving the subject of transits it should be mentioned that it was in the capacity of commander of an expedition to Otaheite, in the Pacific, to observe the transit of Venus of June 3, 1769, that Captain Cook embarked upon the first of his celebrated voyages. In studying the surfaces of Venus and Mercury with the telescope, observers are, needless to say, very much hindered by the proximity of the sun. Venus, when at the greatest elongations, certainly draws some distance out of the glare; but her surface is, even then, so dazzlingly bright, that the markings upon it are difficult to see. Mercury, on the other hand, is much duller in contrast, but the disc it shows in the telescope is exceedingly small; and, in addition, when that planet is left above the horizon for a short time after sunset, as necessarily happens after certain intervals, the mists near the earth's surface render observation of it very difficult. Until about twenty-five years ago, it was generally believed that both these planets rotated on their axes in about twenty-four hours, a notion, no doubt, originally founded upon an unconscious desire to bring them into some conformity with our earth. But Schiaparelli, observing in Italy, and Percival Lowell, in the clear skies of Arizona and Mexico, have lately come to the conclusion that both planets rotate upon their axes in the same time as they revolve in their orbits,[12] the result being that they turn one face ever towards the sun in the same manner that the moon turns one face ever towards the earth--a curious state of things, which will be dealt with more fully when we come to treat of our satellite.