Vol. liiJ J'htlosophical transactions. 505 



a phenomenon ; more particularly too, as the advantages resulting from the ob- 

 servations of this transit, are probably of the greatest moment. The first and 

 only observation of this kind was made by our ingenious countryman the Rev. 

 Jeremiah Horrox, a young gentleman of very distinguished abilities, who, by 

 his own observations, with instruments constructed under his own inspection, 

 and finished by his own hands, was enabled to correct the so much boasted tables 

 of Lansberg, and to predict, with a degree of precision unknown to those times, 

 a phenomenon, which he himself thought to be of great consequence. He im- 

 mediately communicated this important discovery to his friend and companion 

 in his astronomical studies, Mr. Wm. Crabtree, and earnestly exhorted him to 

 prepare for the observation. The state of the heavens on that day was not very 

 favourable : however, both Mr. Horrox and his friend were lucky enough to 

 observe it ; the former, at a time when the limbs of the sun and Venus were in 

 the point of contact, viz. on the 24th of November 1639, o.s. And these two 

 were the first and only persons that ever saw Venus in the sun before the present 

 year. 



By the Rudolphine tables, constructed from the observations of Tycho Brahe, 

 Kepler was enabled to predict, in the year 1629, that Venus would pass over the 

 sun's disk in the year 1761. Dr. Halley, in a memoir published in the Phil. 

 Trans. N° 348, exhorted the astronomers of all countries to attend to this rare 

 phenomenon with all possible diligence; as it would furnish them with the best 

 means of determining the parallax and distance of the sun, and consequently the 

 dimensions of the whole solar system. How far the method proposed by him 

 will enable us to solve this difficult problem, must be left to time to discover, 

 when the observations, made in places properly situated, can be compared with 

 those made here, and in other famous observatories. But as the tables which 

 Dr. Halley made use of were very imperfect, his own not being then constructed, 

 and did not represent the place of Venus on the sun with that accuracy which 

 the method in this case required : and as that eminent philosopher committed a 

 small mistake in his calculations, by placing the axis of Venus's path, and the 

 axis of the equator, on the same side of the axis of the ecliptic; a mistake which 

 the most accurate calculator might easily fall into : from these considerations, 

 the honour of determining the sun's true parallax is probably reserved for the 

 present occasion. 



The method of determining the right ascension and declination of the centre 

 of Venus from that of the sun, in his late observations, was the same which Dr. 

 Bradley used in observing a former transit of Mercury. The planet was made to 

 run down the fixed wire of the micrometer, and the difference of the time of 

 passage was observed between it and that part of the sun's limb which was cut 

 by that wire; and the moveable wire was brought to touch the sun's lower limb. 



