1820.] Dr. James Bradley. 325 



of different positions, or, which comes to the same thing, an 

 apparent motion in the heavens, which will make it describe in 

 a year eUipses more or less elongated according to the position 

 of the star. 



Such is the theory of the aberration of hght which Dr. Brad- 

 ley pubhshed in 1727, and which was received by astronomers 

 with that applause which it deserved. M. Clairaut made it the 

 subject of an excellent memoir printed in the Memoirs of the 

 French Academy of Sciences for 1737, in which he examined 

 the theory of aberration to the bottom, and gave the rules neces- 

 sary to apply it to practice. The result of his calcalation is, 

 that the velocity of light deduced from the aberrations observed 

 in the stars is absolutely the same as that assigned it by the 

 ingenious explanation which Roemer had given of the retarda- 

 tion of the echpses of the first satelhte of Jupiter. This is a new 

 proof of the accuracy of the hypothesis, if it stood in need of 

 being proved. 



Three years after this glorious epoch in the life of Dr. Brad- 

 ley, the place of Reader in Astronomy and Physics in the 

 Museum at Oxford became vacant. It was bestowed upon him, 

 and certainly no individual in the university was better qualified 

 for the task thus assigned him. 



Dr. Bradley's diUgence as an observer was redoubled by the 

 increase of his reputation. He gradually discovered that the 

 inclination of the axis of the earth on the plane of the ecliptic 

 was not constant ; but underwent a variation amounting to some 

 seconds, the period of which was nine years. This period 

 seemed at first to bid defiance to all explanation. What could a 

 period of nine years have in common with tlie revolution of the 

 earth round the sun which is completed in one year ? Dr. Brad- 

 ley was, however, fortunate enough to find the true cause in the 

 Newtonian theory of attraction. 



The first principle of this theory, it is well known, is, that all 

 bodies attract each other mutually directly as their mass, and 

 inversely as the square of their distances. From this attraction 

 combined with rectihneal motion, Newton deduced the orbits of 

 the planets, and, in particular, the orbit of the earth. If that 

 oibit were a circle, and if the globe of the earth were exactly 

 spherical, the attraction of the sun would act only to keep it in 

 its orbit, and would not derange the position of its axis. But 

 neither of these suppositions is true. The earth is sensibly 

 greatest at the equator, and its orbit is an ellipse in one of the 

 foci of which the sun is placed. When the position of the earth 

 is such that the plane of its equator passes through the centre of 

 the sun, then the sun has no other action but that of drawing 

 the globe towards itself; but always parallel to itself, and without 

 deranging the position of its axis. This happens at the two 

 equinoxes. As the earth recedes from these two points, the sun 

 leaves the plane of the equator, and approaches to one or other 



