76 PHILOSOPHICAL TRANSACTIONS. [aNNO J725. 



Hence, doubling the interval, in 92 Julian years 1'^ Q^ 42"' 20^, the mean 

 motion of Mercury from the equinox is O' 1° 51' 40", from whicli taking 

 5" 44' 50", the motion in I"* Q*' 42"^ 20% we have his motion in 92 Julian years 

 1 1* 26° 6' 50", and in 100 years, 2^ 14° 2' 13", which is but 20' more than the 

 Dr. had some years since printed it, in his astronomical tables, shortly to be 

 published, and differs but one hour's motion from them in 3000 years. 



The forementioned proportion of the distances, viz. 313 to 676, is also 

 between the latitudes seen from the earth and the inclinations, or heliocentric 

 latitudes of the planet : so that 2' 20", at the ingress of l677, gives 5' 2"; 

 and 3' JO" in 1723, becomes 7' 55" for the latitudes at the sun. And the in- 

 clination of the orbit of Mercury to the plane of the ecliptic, determined by 

 accurate observations near his northern limit, being 6° SQ' 20", we compute 

 the distance of the planet from his node, in the former 0^41' 7", and in the 

 latter, 1° 4' 37"; which being deducted from his heliocentric places respectively, 

 leave the place of the ascending node, in 1677, y 14° 21' 3"; and in 1723, 

 y 15° O 53": so that in 46 years, the node is found 39' 50" forwarder in the 

 ecliptic; which is but 1' 30' more than the precession of the equinox in the 

 same time. We may therefore safely assume the plane of the orbit of Mercury 

 to be immoveable in the sphere of fixed stars, and its ascending node to be 

 0^ 15° 41' from the first star of Aries. Nor can so very slow a motion (sup- 

 posing such to be) be fully defined, but by the utmost care and diligence of 

 future astronomers, after the observation of many ages. 



As to the rest of the theory of this planet's motion, the Dr. makes his mean 

 distance from the sun, 38710 such parts as the mean distance of the sun and 

 earth is 100,000; and his greatest equation 23° 42' 37". The epocha of his 

 middle motion, ineunte Anno 1723, vet. styl. from the equinoctial point, he 

 makes | 19° 9' 31"; and that of his aphelion to the same time / 13° 3' 34": 

 the aphelion moving according to the order of the signs, 7 minutes in 8 years. 

 And these numbers may represent the motion of Mercury, with an exactness 

 equal to that of the other planets; perhaps as near as the sun's place by any 

 tables, or those of the fixed stars by any catalogue yet extant. 



It were to be wished, that some good observation, like this, had been made 

 of the like transit of Mercury at his other node in April, where he was seen 

 indeed April '23, 1661, but so imperfectly, that neither ingress nor egress was 

 any where observed; and though it be certain that he traversed the sun on 

 April 26, 1674; and again April 24, 1707; yet we were so unfortunate, that 

 the conjunction in both happened so near midnight, that he escaped unseen by 

 all the astronomers of Europe, excepting singly Mr. Roemer at Copenhagen, 

 whose observation, received by the favour of Mr. de I'lsle the astronomer. 



