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MATHEMATICAL PAPERS. 4J 



Mr. JOSEPH pope's defcript'ion of an Orrery of I 



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conflniSlion. 



S the number of appearances, explained by an 

 Orrery, depends upon the niunber and exa<5tnefs of its mo- 

 tions, the following defcription of the performances of the 

 Orrery completed by me in the year 178 8, will fhew its 



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^On the large circle of this machine, reprefenting the eclip- 

 tic, are engraven the ligns and degrees of the Z<4diac, with 

 the days and months of the year ; in the centre of which, is 

 a brafs ball, reprefentihg the Sun, turning on its axis, in 25^ 

 diurnal rotations of the earth ; its axis declines from the axis 

 of the ecliptic circle, 8 degrees, and its north pole tends to- 

 wards the 8th degree of Pifces. 



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The planets reprefented by ivory balls, are as near their 

 relative diameters and diftances as the convenience of the ma- 

 chine will permit ; they revolve round the fun in their vari- 

 ous eccentric and inclined orbits. Mercury, the nearefl to the 

 Sun, being the firft in order, with his axis perpendicular to 

 the ecliptic, performs his motion round the Siiii ill 87H diiir- 



of the earth, or iri $7 days, 23' hours 





The next is Venus, fhe performs her annual courfe in 224 

 days 17 hours, of our time, and her' diHrnal indtidn, m 24 

 days 8 hours ; her axis declines 75' degrees ffom that of the 

 eciijptic, keeping parallel to xtfelf ; the North Pole tends' to- 

 wards the 20th degree of Aquarius.. 



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