HISTORY OF PHYSICAL ASTRONOMY.
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planet during the transit of 1832, it is quite clear from the drawing he hasgiven of its position, as well as from the terms in which he alludes to it,that it was not situate in the centre of the disk. The explanation of Prof.Powell, however, requires that the spot should be exactly in the centre.Again, we have seen that, on the occasion of the transit of 1799, two spotswere seen on the disk of the planet, but, according to the diffraction theory,there should be only one visible. It is manifest that the observations offuture transits of the planet can alone throw additional light upon thesubject.
On the occasion of the transit of Mercury in November, 1848, the limbof the planet was seen by one of the observers at Greenwich to be con-nected for some time with the sun’s limb by black lines, similar to thosenoticed by Mr. Baily during the annular eclipse of 1830. It is worthyof remark, however, that although seven other individuals observed thetransit with separate telescopes, no such phenomenon was seen by eitherof them*. These dark lines seem much more difficult to explain thaneither the dark ligament or the protuberance observable when the moonor planet appears upon the solar disk wholly detached from the sun’slimb jv
The transits of Venus across the sun’s disk are phenomena of muchrarer occurrence than those of Mercury . It has been already mentioned,that in 1029, Kepler predicted the occurrence of a transit of this planeton Dec. 6, 1631. According to his calculation, it appeared that the planetwould not enter upon the sun’s disk till towards sunset; it was not im-probable, therefore, that the phenomenon would be altogether invisible inEurope. Gassendi , however, was not without hopes that in the presentinstance, as had previously happened with respect to Mercury , there mightbe a considerable error in the calculated result, and that in reality theplanet would begin to be visible on the sun’s disk several hours earlierthan the time assigned by the tables. Tie accordingly made arrangementsto observe the transit similar to those which he had employed so success-fully in the previous month on the occasion of the transit of Mercury .He has given an account of his fruitless observations on this occasion ina second letter to Schiekliard. He intended to watch the appearance of thesun on the 4th and 5th of the month, but an impetuous storm of windand rain rendered the face of the heavens invisible on both of those days.On the 6tli he continued to obtain occasional glimpses of the sun, till alittle past three o’clock in the afternoon, but no indication of the planetcould be discerned upon his disk as depicted upon the white circle. Onthe 7th, he saw the sun during the whole forenoon, but he looked invain for any trace of the planet. It is now well known that the transitof the planet took place during the night between the 6th and 7th ofDecember j.
It has been mentioned that Kepler had announced that, after the transitof 1631, Venus would not again be seen upon the sun’s disk previous tothe year 1761. Astronomers were therefore under the impression, after
* Monthly Proc. Ast. Soc., vol. ix., p. 69.
f Dr. Foster, who observed this transit at Bruges , states that the planet, when seen onthe sun, had rather the appearance of a glole than a disk. (Monthly I’roc. Ast. Soc., vol.ix., p. 4.) A similar remark has been made on the occasion of solar eclipses, see p. 401,(Note).
t The two letters of Gassendi to Sehickhard respecting the transits of Mercury and Venus were published under the title of “Mercurius in Sole Visus ct Venus invisa.” (OperaOmnia, tom. iv., p. 537, et seq.)
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