IXTEODUCTORY AND HISTORICAL. Ixiii 



Cassini,* with the contemporaneous observations! of himself, Picard, and Roemer, at Paris and 

 Brion, in order to deduce a better value for the parallax. The planet had been compared, both 

 at Cayenne and Paris, with * Aquarii, but Cassini did not succeed in obtaining any good value, 

 farther than deducing an upper limit of 9", if the observations were to be trusted. In 1684, 

 however,:]; Cassini published a memoir revising his computation from the materials and from 

 correspondent observations, 1672, September 5, 9, and 24, deduced as the equatorial horizontal 

 parallax of Mars, 25g-" 3", corresponding to a solar parallax of 9". 5 1", or a distance from 

 the earth of 21,600 terrestrial semi-diameters, and with a possible error of 2,000 or 3,000 semi- 

 diameters. From these values he inferred the true diameter of the sun to be just one hundred 

 times that of the earth. 



About the same period, Cassini, with Roemer and Sedileau, tried || the method of parallaxes 

 in right-ascension, proposed by Cassini himself, and employed by him to ascertain the distance 

 of the comet of 1680. He proposed, from a comparison of the observed right-ascensions of the 

 celestial body on opposite sides of the meridian, to deduce the amount of its geocentric parallax! 

 but this attempt, though laboriously carried out, was entirely unproductive of satisfactory 

 results. Observations were made at the same time by La Hire,^f but without convincing him 

 that the parallax was sensible, as Lalande infers,** from the fact that La Hire, in his well 

 known tables,ft published in 1687 and 1702, never employed a larger value than 6". 



Flamsteed, who had been, during the same period, observing in England, fixed the upper 

 imit of the parallax of Mars firstJJ: at 30", and then at 25", so that the solar parallax could 

 not exceed 10", or 21,000 terrestrial semi-diameters. 



The transit of Mercury, 1677, October 28, was observed at St. Helena by Halley, who ob, 

 tained|||| 45" as the resultant parallax, but attributed but little weight to the determination- 

 preferring a value between 10" and 20". The arguments of Streete in favorllf of a value 15" 

 are cited by Halley with apparent approval, and are very curious. In this connexion, Halley 

 again urged the great advantage of the transits of Venus for the determination of this important 

 quantity,*** as he had already doneftt in 1691. 



La Caille's astronomical expedition to the Cape of Good Hope took place in 1740. Animated 

 by the encouraging example of Richer, he founded an astronomical shrine in the southern 

 hemisphere, which has since been rendered even more illustrious by the labors of such men as 

 Henderson, Herschel, Fallows, and Maclear, offering an example well deemed worthy of imita- 

 tion by our younger land. 



La Caille made a large number of observations of the declination of Mars at opposition ; and 

 from a comparison of these with corresponding measurements in the northern hemisphere, 

 extending through about six weeks, deduced the value of the solar parallax as 10". 2. 



In the succeeding year, 1741, he repeated the investigation, and with a result not essentially 

 different. The mean of four correspondent observations of Venus at her inferior conjunction in 

 1751 was similarly computed by La Caille, giving 10". 38 as the mean value, and, from the 

 whole seriesJJJ of investigations, he arrived at the conclusion that the horizontal equatorial 



Recueil d' Observations faitt en plusieurs voyages, SfC., Paris, 1693. Rec. de Mtm. del'Acad. VII, P. 1, pp. 353-6. 

 t Mint, de I'Acad. Roy. da Science*, VII, P. pp. 331, 351. 

 t Mem. de I'Acad. Soy. da Sciences, VIII, p. 55. 



Memoires de I'Acad. Roy. des Sciences, VII, p. 115. || Ibid., pp. 105-7. 



TJ Ibid., p. 111. oo LALANDE, Astron. II, p. 413. 



f\ LA HIKE, Tabula: Astronomic^, p. 6 

 ft Phil. Trans. 1672, No. 89, pp. 5118. 



Phil. Trans. 1673, No. 96, p. 6100, (erroneously printed 6000.) 



Ill Phil. Trans., No. 193, p. 511. I have not access to Halley's Southern Catalogue of Stars, in which arc given his 

 observations of the transit of Mercury. 



lfly See LALANDE'S Astronomie, 1729, II., 409 ; Astron. Carolina, ed. 1661, p. 12 ; ed. 1710, p. 34. 



oo Phil. Trans., 1716, p. 454. 



ttt Phil. Trans. 1673, No. 93, p. 619. 



t# Mtm.de I'Acad. da Scienca, 1748, 601 ; 1761, 310 ; 1760, 73. See also CASSIKI DB TDTOT, 1760, 292. 



