CÖ>I!WENTATIO ASTRONOMlCA. f-SS 



« dually, as they must do , were Ihcy of any sensible magnitude , that Ihey vanish at 

 c once with their utmost lustre ; and emerge likewise in a moment, not small at fiist, 

 « but al once appear witli their fuUlight, even Iho' the emersion happen very near Ihe 

 « cusp ; where, if thcy were four seconJs in diameler , they wouM be raany seeonds of 

 « lime in getting entirely separated from Ihe limb. But the contrary appears to all 

 (c Ihose , that have observed the occultalions of these bright stars. And tho' Sirius be 

 « bigger than either of them , yet he is by far less than two of Ihem ; and consequently 

 •« hJs diameter to theirs is less than the Square root of 2 fo I , or than 14 lo 10; 

 « whence, in Mr. Gas sin {'s excellent 36 foot glass, those stars ought to be aboul four 

 « seeonds in diameter ; and they wohld undoubtedly appear so, if viewed after the same 

 ■« tnanber ; whereas we are aliunde certain, that they are less than one Single second in 

 « diameter. The great strength of their native light , forming the resembrance of a ba- 

 « dy, when il is nothing eise but the spissitude of their rays. " 



« As to the other part of the argument , that the alteration of the declmation of 

 t Sirius , on the score of the access of the earth in Decembre , and its recess in June , 

 « amounts to 6 seeonds; I can only remark , that besides that a radius of 3 feet, as it 

 « seems that made use of was no more , is somewhat too small for so extremely nice an 

 « Observation, 6" being subtended by the y— pä""' "^ ^^ inch, some of the observa» 

 « tions before recited do plainly shew , that the refraction of the medium did inlcrmix 

 « with those differences that might be occasioned by the parallax. " 



«But the principal objections against the conclusion of this argument, seems fo he, 

 « that the meridian altitude o( Sirius at Paris being under 25 degrees the ordinary refrac- 

 « tion of the star is 1' 55" or 115 seeonds; and the baromeler rising and fatling above 

 « two inches in thirty , shews that the density of the air, on that score, may be a 15 th 

 « part more at one time than another. Whence the refractions being always proportio- 

 « nai to the density of the medium , as we have all seen it often demonstrated by Mr. 

 « Hawksbee bothin vacuo and in a doubly and trebly Condensed air, it is piain that 

 « in that altityde the refraction of a star may dilTer about 7 or 8 seeonds , or the 15 th 

 « part of 115'', which is more than the whole parallax supposed to have been ob- 

 « served. 



« It were to be wish'd that Mr. Cassini would please lo try this matter by the 

 « Lunida Lyrae , instead of Sirius , which , tho' somewhat less than him , is as near 

 « to the solstitial colure , and has much greater latitude , being but 28" from the pole 

 « of the ecliptick, whence its parallax would be sö much greater; and being at Paris 

 « within 10° of the zenith , the graad objection of the diflerence of refraction, would 

 « be almost wholly removed^" 



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