VOL. LIII.] PHILOSOPHICAL TRANSACTIONS. 45 



and practical part of astronomy, that many methods of determining it have been 

 employed by the astronomers of every age, Mr. Flamsteed informs us, in the 

 92d and 96th numbers of the Philos. Trans., that from some observations made 

 on the planet Mars, he had found the sun's parallax not to exceed 10 seconds ; 

 and Dr. Halley, in a memoir written expressly with a view to ascertain the 

 exact quantity of it, supposes it not to be greater than 12-i^''. When we con- 

 sider the imperfect state of astronomy at the time when Mr. Horrox lived, we 

 cannot sufficiently admire the wonderful genius of that young gentleman, who 

 at the age of 24 could collect, from his own observations, that the parallax of 

 the sun did not exceed 1 4 seconds ; while many celebrated astronomers, whose 

 tables were then in the greatest repute, had assigned a parallax of more than 2 

 minutes to the sun, which Kepler had supposed could not be less than 5Q 

 seconds, and which Hevelius, who published the admirable treatise of Mr. 

 Horrox, entitled, Venus in Sole visa, fixed at 41 seconds. 



In the year 1719j Dr. Pound and his nephew, that illustrious astronomer, 

 Mr. Bradley, did, when Mars was in opposition to the sun, demonstrate (to 

 use the words of Dr. Halley, Phil. Trans., N" 366,) the extreme minuteness of 

 the sun's parallax, and that it was not more than 12", nor less than g", on 

 many repeated trials. At the same time, and by the same kind of observations, 

 Maraldi determined this parallax to be 10", the result of his observations agree- 

 ing exactly with those deduced from the correspondent observations by Richer at 

 Cayenne, and by Cassini at Paris, in the year 1 672. 



The voyage which the Abbe de la Caille undertook, to perfect a catalogue of 

 some of the principal fixed stars, furnished the astronomers with the means of 

 determining the sun's parallax by corresponding altitudes of the planets Mars 

 and Veims, to be observed on each side of the equator, with all the accuracy of 

 which that method is capable. The astronomers here in Europe were invited to 

 determine the distances of the planets from particular stars on stated days, 

 while the Abbe himself proposed to make the corresponding observations on 

 the southernmost part of Africa at the Cape of Good Hope. By the differences 

 of the altitudes of the northern limb of Mars, and of such stars as were nearly 

 in the same parallel, observed on the same day at the Cape with a sextant of 

 6 f radius ; at Greenwich by Dr. Bradley with a mural quadrant of 8 f. ; at 

 Bologna in Italy by M. Zanotti with a similar instrument of 5 f. ; at the Royal 

 Observatoi-y at Paris by Messieurs Cassini de Thury and Gentil, with a move- 

 able quadrant of 6 f. ; and in Sweden by Messieurs Wargentin, Stronmer and 

 Schemmark, with telescopes of 7 and 8 f. armed with micrometers; it was 

 found, when every reduction is made, that according to each observation, the 

 dates of which are given below, the horizontal parallax of the sun, when at its 

 mean distance from the earth, was as is represented in the following table. 



->4 



