VOL. XXXI.] PHILOSOPHICAL TRANSACTIONS. 443 



Some Remarks on a late Essay of Mr. Cassini, wherein he proposes to Jind, by 

 Observation, the Parallax and Magnitude of Sirius. By Edmund Halley^ 

 LL.D. R. S. S. N° 364, p. 1. Anno \7'10. Fol. XXXI, 



In the Memoirs of the Royal Academy of Paris, for the year 17 J 7, but 

 lately published, there is a remarkable Essay, by Mr. Cassini, concerning the 

 annual parallax of the fixed stars, and particularly of Sirius; in which he de- 

 termines the diameter of Sirins to be as much larger than that of the sun, as 

 the sun's is than that of the earth, which he supposes to be 100 times : and the 

 distance from the sun to the earth being certainly about 100 diameters of the 

 sun, it will follow, that the globe of Sirius must have its diameter equal to the 

 distance between the earth and sun. 



To prove this, he says he made use of an excellent telescope, of 34 French 

 feet, or 36 English, leaving an aperture of only an inch and half, to take off 

 the spurious rays of the star, which then appeared round, and sufficiently well 

 defined; and comparing its body with that of Jupiter, which he says was then 

 50' diameter, he found that the apparent diameter of Jupiter was 10 times 

 greater than that of the star, which by consequence was seen under an angle 

 of about 5" ; which is his first position. 



He then says, that to make the observations of the parallax of this star with 

 all the exactness possible, he employed a telescope of 3 feet, in a copper tube, 

 having fixed in the common focus of the two glasses, 4 threads crossing each 

 other in the centre, under angles of 45°. This tube he firmly fixed to the plain 

 of a mural arch, which had been for above 30 years immoveably cemented to 

 the wall of the Royal Observatory, to which he chose to fix it, because of its 

 solidity, and less liable to shake ; and that after having stood 30 years, there 

 was no fear of its settling any further in the space of one year ; besides, that it 

 was easy to perceive if any such alteration should happen to it. 



Having therefore fixed his 3 feet tube as above, so that, about the beginning 

 of April, 1714, New Stile, the star, being exactly in the meridian, passed over 

 the centre of the tube, he observed that on the 20th of April the star touched 

 the horizontal thread with its under edge, being apparently all above it, in the 

 inverting tube, but really below. On May 15 and June 6, it passed again by 

 the centre. On June 27 it appeared a little under, and on July 9 it was found 

 to touch the under part of the thread. October 5 it again passed by the centre; 

 but on December 29 it touched the upper part of the thread. January 18, 

 1715, being the coldest day of that winter, it passed exactly by the centre; 

 and on March 27 and April i , it almost touched the upper side of the hori- 

 zontal thread, from which it seemed a little separated. But on June 7 it passed 



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