14 



that were there any such upon record, they would have beea 

 deposited in the celebrated library of Alexandria, and would, 

 of course, have been noticed in his great work. This disco- 

 ver}^ however, was "very early attempted by two eminent 

 Astronomers of the Greek School, who, each of them, sug- 

 gested a different method of determining the distance of the 

 earth from the sun, both highly ingenious and strictly geome- 

 trical, but practically insufficient, from their requiring ob- 

 servations of such minuteness and accuracy, that not even 

 the improved state of modern optical instruments can enable 

 us to attain to the precision they demand. A necessary step 

 to the most direct and certain solution, of this nice question, 

 is to determine the angular quantity of the difference of the ' 

 sun's apparent place in the Heavens, as seen at the same in- 

 stant, from the centre and the surface of the earth, usually 

 called the angle of the sun's parallax; for it is well known to ' 

 Astronomers, that this quantity is equal to the angle under 

 which the earth's semidiameter is seen from the sun; and that, 

 were this angle, and the measure of the mean semidiameter 

 of the earth, also given; from these data, by the help of plain 

 trigonometry, the distance of the sun, and, of course, that of 

 every primitive planet belonging to our system, may be rea- 

 dily determined. But this angle is too small for direct ob- 

 servation; a very close approximation to the discovery of it, 

 has been made by various acute and well-conceived, though 

 indirect methods. The parallactic angle of the planet Mars, 

 the theory of gravitation, and, above all, the two recent ob- 

 servations of the transits of the planet Venus. over the sun's 



disk, 



