history of physical astronomy.

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of the Optica Promota*. Whatever development the method acquiredfrom him, cannot affect the merits of the original discoverer. CaptainSmyth, while vindicating the just claims of G regory, has truly remarked thatHailey needs no borrowed plumes. Even iu the present instance itmust be admitted, that the ability with which he expounded the pe-culiar advantages attending the determination of the solar parallax byobservations of the transits of Venus , the earnestness with which he re-commended the practical application of the method, and the weight' ofhis authority on questions relating to astronomical science, were mainlyinstrumental iu inducing the different governments of Europe to adoptthose liberal proceedings for observing the transits of 1761 and 17G9,which have led to a more accurate knowledge of the dimensions of thesolar system than could otherwise be hoped for.

The physical appearances noticed during the transits of Venus whichhappened in the last century have given rise to a good deal of speculation,but it must be admitted that the conclusions arrived at upon this pointcannot be regarded as altogether satisfactory. The most remarkable ofsuch appearances was that witnessed when the planet was just whollywithin the sun’s limb. It was found on the occasion of each of the tran-sits of 1701 and 1769, that the interior contact of the planet with the sundid not take place regularly at the ingress, the planet appearing, for sometime after it had wholly entered upon the solar disk, to be connected withthe sun’s limb by a dark ligament. A similar phenomenon was observedat the egress of the planet. It was also found that, even after the planethad wholly separated from the sun’s limb, it did not acquire its round formtill after the lapse of several seconds. In order that the reader may forma more accurate conception of these phenomena, it may not, perhaps, beunacceptable to cite in detail a few of the observations relating to them.

Mr. Hirst, who observed the transit of 1761 at Madras, states that “ atthe total immersion, the planet, instead of appearing truly circular, re-sembled more the form of a bergamot pear, or, as Governor Pigott thenexpressed it, looked like a nine-pin: yet the preceding limb of Venus was extremely well defined.” With respect to the end of the transit heremarked, “ that the planet was as black as ink, and the body trulycircular, just before the beginning of the egress, yet it was no sooner incontact with the sun’s preceding limb, than it assumed the same figureas before at the sun’s subsequent limb: the subsequent limb of Venus keeping well defined and truly circular.” f

A similar appearance was observed by Lalande at Paris, by Bergmanat Upsal, and also by several other individuals.

Dr. Maskelyne, who observed the transit of 1769 at Greenwich, givesthe following description of a phenomenon of a similar nature witnessedby him at the ingress of the planet:—

“The irregularity of Venus ’s circular figure was disturbed towards theplace where the internal contact should happen, by the addition of a pro-

* James Gregory , one of the most eminent mathematicians of the seventeenth century,was born at Aberdeen in 1639, and died at Edinburgh in 1675, at the early age ofthirty-six years. He was only twenty-four years old when he published the OpticaPromota , which, besides the important remark alluded to in the text, contained alsothe original explanation of the principle of the reflecting telescope. Halley’s earliestallusion to the utility of observations of the transits of the inferior planets in determiningthe solar parallax, is contained in his Catalogue Stellarum Australian:, published in1679. He subsequently returned to the subject in the volumes of the PhilosophicalTransactions for 1694 and 1716.

f Phil. Trans., 1761, p. 396.