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HISTORY OF PHYSICAL ASTRONOMY.

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but was for most part cloudy. A little before eight o’clock the sun was visiblethrough the openings of the denser clouds, but a thin veil of nebulousmatter still rendered it impossible to distinguish any minute object, eitherupon the actual disk of the sun or upon his image in the dark chamber.Towards nine o’clock, the sun being distinctly visible, he perceived a blackspot upon the image. It was a little to the east of the vertical diameter,and about a fourth of its length from the lower extremity. He roughlymarked its position, not having the remotest suspicion that it was Mercury on account of its extreme smallness, for its diameter scarcely seemed toexceed the half of one of the minute parts into which he had divided thediameter of the white circle. He was rather inclined to believe that it■was a solar spot, which, although not visible on the preceding day, mighthave been formed during the intermediate interval, as had happened onseveral previous occasions, in the course of his experience. The sun havingagain appeared through the clouds at nine o’clock, he measured the distanceof the spot from the centre of the image, intending to compare its positionwith that of the planet, if the latter should eventually enter upon the sun,for it occurred to him that an observer at some other station might beinduced to do so likewise, and that suitable data would thereby be obtainedfor determining the parallax of the planet. A little afterwards, when thesun again became visible, he measured anew the distance of the spot fromthe centre of the image, and was surprised to find that it had considerablyincreased during the intermediate interval. From its rapid motion, hefelt assured that it could not be one of those ordinary spots that appearfrom time to time on the surface of the sun, and he now began to enter-tain some suspicion of its real nature, but still he could not persuade him-self that it was Mercury , so much was his mind pre-occupied with the ideathat the planet would exhibit a larger apparent magnitude. While hewas pondering whether he had not made a mistake in his first measure-ment, the sun shone forth, whereupon, having again ascertained the posi-tion of the spot, he found its distance from the centre to be even greaterthan on the previous occasion. He now concluded without hesitation, thatit was in reality the planet which the black spot represented. He there-fore immediately gave the preconcerted signal to his assistant in thechamber above him, but unfortunately he had abandoned his post, andfor some time could not be found. He returned, however, before the planethad gone off the sun, and made the necessary observations.

Gassendi states that the planet, as seen depicted on the solar image,was somewhat diluted, and of a ruddy colour around the margin. Itsdiameter seemed to be equal to about two-thirds of the interval betweentwo consecutive points of the diameter of the image. The apparentdiameter of the planet was therefore only 20". This was far below theangular magnitude which astronomers usually assigned to it.

The planet had nearly gone off the sun when Gassendi first saw it.From its observed motion during the short interval of its visibility, hecalculated that it had entered upon the sun’s disk at 5 h 28 m a.m., andthat its final egress took place at 10 h 28 m a.m. The transit had thereforelasted five hours precisely. By reducing the calculations of Kepler to themeridian of Paris , he found that the observed time of the transit was inadvance of the computed time by 4 h 49 m 30 s .

The transit of Mercury observed by Gassendi , took place when theplanet was passing through the ascending node of its orbit. The second