ASTRONOMY. 



This was first remarked by Dr H ALLEY, and since 

 that time two transits of Venus over the Sun 

 have happened, which astronomers have taken 

 the greatest pains to observe with accuracy. The 

 general principle which connects this method of 

 finding the parallax with the more elementary me- 

 thods already explained, is all that can be given 

 here. The details of the calculations must be left 

 to the Treatises and M&noires which treat of them 

 particularly. 



. Let S be the Sun (fig, 21,) and E the 

 Earth, both supposed at rest, while Venus at V 

 moves westward in her orbit, with the sum of 

 her own angular velocity and that of the Sun. 

 Let O and O' be the stations of two observers 

 on the surface of the Earth, who see the transit 

 begin when Venus is at the points V and V of 

 her orbit. If the difference of the longitude 

 of the two observers be known, the time that Ve- 

 nus has taken to move over the arch V V is also 

 known, and therefore the arch VV is given, or 

 the angle which the line OO' subtends at the 

 distance of the Sun. But OO' itself is given, 

 and its ratio to the radius of the Earth ; there- 

 fore the angle which that radius subtends at the 

 Sun, or the horizontal parallax of the Sun, is 

 given. 



This 



