566 PHILOSOPHICAL TRANSACTIONS. [anNO I76I. 



If the sun's lower limb had been made to run down the fixed wire, and the 

 moveable wire brought to the planet, and the difference of the time of passage 

 had been observed between it and the sun's consequent limb, on the supposition 

 that the wire was not exactly parallel to the diurnal motion, it would have caused 

 a considerable error in the difference of right ascension, observed at the distance 

 of the sun's semidiameter. But the method made use of requires some calcula- 

 tion, to determine the position of Venus on the sun's disk. 



Let therefore the circle EDaB represent the 

 sun's disk, in which let Ea be parallel to the ^/'^'^ 



equator, and bd an hour circle; let the dotted A 



line FG represent the fixed wire of the micro- / j 



meter, hi the moveable wire, and kl the per- ^ — \ 



pendicular or horary wire. The difference of \ '• /" 



right ascension hv, and of declination ch, will \-X 



be determined in the following manner: so, or ^ sN^ 



CD, the semidiameter of the sun is given, and H |. :>r.^ 



CD — DH = CH, the difference of declination: =L "" 



and sc and ch being given, sh may be found; and then the observed difference 



of right ascension sv being diminished in the ratio of radius to the sine of the 



polar distance of Venus, will give hv the difference of right ascension. 



The sun's horizontal diameter, as measured by the micrometer, was 31' 33^, 

 and that of Venus by several observers 58"; the following observations were 

 therefore deduced by assuming the semidiameter of the sun = 13' 46^5, and 

 that of Venus = 29'. 



Here are then given a number of observations of the kind above described, 

 both those made at Shirburn castle and those at Greenwich ; from each of which, 

 calculated as above, the corresponding difference of right ascension and declina- 

 tion between the centres of the sun and Venus are obtained. 



In order to determine more exactly the time of the ecliptic conjunction, with 

 the latitude of Venus then, with the time of the middle of the transit, and the 

 nearest approach of the centres, and thence the true place of her node, Mr. B. 

 has carefully computed the following numbers fi*om theory : because, as Dr. 

 Halley has observed, in the Philos. Trans., N° 386, " there is always an una- 

 voidable, though small uncertainty in what we observe, yet greater than there 

 can be in the theory, especially now it is so very near the truth." The solar 

 numbers were computed from new tables, not yet published, corrected by the 

 small equations, occasioned by the influence of the moon and planet Jupiter, 

 and also the nutation of the earth's axis. The sun's place was very well observed 

 on the meridian, both at Greenwich and Shirburn, the day of the transit; which, 

 allowing for the difference of longitude of those places, agreed to a surprizing 



