450 PHILOSOPHICAL TRANSACTIONS. [aNNO IOQI. 



That is in time O' 8" 12'" O' 



By which the sun is advanced O* 0° 20' 41" 



This arch seen from the earth gives O O Q 36 



The base answering thereto O O Q 30 



The time in which Mercury runs over the base, is o" l"36"' O' 



But 46 sidereal years exceed as many Julian with 1 1 intercala- 

 tions, by , 019 3 O 



Therefore Mercury returns to the sun after 46 Julian years and 1 4 51 O 

 Or, with 12 intercalations, when the preceding year is either the 



2d or 3d after bissextile O 4 51 O 



But the perpendicular, by which Mercury advances to the north, is O' 0° l' 22" 

 But the most accurate period of Mercury about the sun, is in 



263 sidereal years and O'* l" 1 r 30' 



And these sidereal years exceed as many Julian, with 66 interca- 

 lations, by O 10 20 a 



Hence after 263 Julian years Mercury revolves to the sun, but 



later by O 11 31 30 



But if the preceding year be bissextile, add 1 1 1 31 30 



After this interval it advances more northerly by O O 10 



The other wider periods are easily discovered from what has been just now 

 found, and they are either of 6 or 7 years. That of 7 years depresses Mer- 

 cury 22' 47" to the south ; and 7 whole days, less by Qm. sooner, if there have 

 been two intercalations ; but with one intercalation, viz. when the former year 

 is bissextile, 6 days are to be subducted, adding only 9m. as before. It is more 

 rarely that after 6 years this planet is seen again in the sun's disk ; after that 

 period it passes 30' 50' more northerly ; and later by 8d. 17h. 25m. if the pre- 

 ceding year be the 2d or 3d after bissextile, otherwise Qd. I7h. 25m. are to be 

 added. 



In like manner, if the conjunction happen at the descending node in April. 



The longitude of the sun from the first star of Aries O' 1 5° 44' O" 



Longitude of Mercury seen from the sun 6 15 44 



Distance of the planet from the sun 45308. 



Its distance from the e^rth 55699. 



Mercury's motion, seen from the sun, in 6 hours O 



The sun's motion in the same time O 



The motion of Mercury from the sun O 



Hence the angle of Mercury's visible way with the ecliptic, seen 



in the sun, is O 



And the visible motion from the earth in 6 hours 



