34 



again be seen after his death. " Wherefore, if it should 

 return according to our prediction about the year 1758, 

 impartial posterity will not refuse to acknowledge that this 

 was first discovered by an Englishman." When only 21 

 years of age he undertook a voyage to St. Helena in order 

 to determine the places of the stars of the southern hemisphere. 

 He remained there a year and a half, and, though the weather 

 was very unfavourable, he succeeded in making a catalogue 

 of 341 stars. Terrestrial magnetism had early attracted 

 his attention, and he constructed the first magnetic chart 

 of the world, on which he joined together, by smooth curves, 

 places of equal magnetic variation. To carry out this work 

 he made a couple of long voyages, besides several shorter ones, 

 in which he made numerous hydrographic surveys and tidal 

 observations. His work as Astronomer Royal was begun 

 too late in life to be of much moment, nor was he, as a practical 

 astronomical observer, the equal of his predecessor. Never- 

 theless he determined the positions of the moon throughout 

 a complete ig-year cycle. 



Halley died on January 14, 1742, in the 86th year of his 

 age, and was succeeded in a few days by JAMES BRADLEY 

 (1693-1762), who was clearly marked out for the appointment 

 by the high reputation which he had already earned for 

 himself as a practical and theoretical astronomer, and by the 

 great help which he had given to Halley during his last years, 

 Bradley had already furnished an explanation of the curious 

 fact that the stars appear to move in orbits which they com- 

 plete in the course of a year. This is due to the fact that the 

 velocity of light is about 10,000 times that of the velocity 

 of the earth in her orbit. The maximum apparent displace- 

 ment of the position of a star is, therefore, equal to the 

 apparent diameter of an object seen from a distance of 10,000 

 times that diameter. As Astronomer Royal, Bradley 's 

 great achievement was that he raised the practical work of 

 observation to a far higher standard than any one had attained 

 before him. The great catalogue of the places of more than 

 3,200 stars, prepared by Bessel from his observations, is the 

 earliest which is still used for reference in modern astronomy, 

 and it forms the basis of our knowledge of the actual move- 

 ments of individual stars. Beside his discovery of this 

 " aberration of light " and the determination of its amount, 

 he discovered a second cause of apparent changes in stellar 

 positions, that known as " Nutation," due to the action of the 

 moon, and running its course in a period of 19 years. Further, 

 he determined errors due to his instruments, and to the 



