286 THE WORLD MACHINE 



Roemer. Not even after Roemer had worked out the velocity 

 of light did the cause of this wandering occur to him ; not to 

 him nor to any one else for half a century. 



Fifteen years after Roemer's death, two years before that 

 of Newton, Samuel Molyneux, a wealthy amateur, had set up 

 at Kew a magnificent sextant of twenty-four feet radius. His 

 intent was to confirm or disprove the parallax of some cf the 

 fixed stars which Hooke had claimed he had observed. He 

 associated with him the young Savilian Professor of Astronomy 

 at Oxford, James Bradley. A little later, Molyneux gave up 

 the work for other duties, and Bradley went on alone. Their 

 idea had been to take a single star, watch it very closely from 

 one season to the other, that is to say, from one side of the 

 sun and the other, thinking that by means of their new instru- 

 ment the question could be settled. 



What they were in search of was some shift in the relative 

 position of two stars very close to each other ; such a shift as 

 one might observe in the relative position of two peaks or two 

 church spires seen from two distantly separated points. What 

 Bradley found was an odd shift in the position of the star itself. 

 He had made the same observation as had Picard, but on 

 another star. It was not in the least what he was looking for, 

 but it was certainly something new, and he followed up his 

 observation with others through three or four years. Wherever 

 he turned his instrument the disclosure was the same, with this 

 difference, that whereas some of the stars seemed to describe 

 a little ellipse, others would wobble in longer and longer ellipses, 

 until it amounted simply to a movement back and forth along 

 a straight line. 



The movement was extremely small, only about 20" at the 

 most ; that is, a little more than the hundredth part of the 

 apparent diameter of the sun and moon. But it was there ; 

 moreover it was universal. So far from being fixed, the whole 

 firmament was in a wobble very minute, indeed, but neverthe- 

 less measurable. What was the explanation ? Obviously, until 

 the matter could be cleared up, fit was no use to think of 

 measuring parallax. 



As the story runs, the explanation came to Bradley one day 

 in a boat on the Thames. Whenever the course of the boat was 

 changed, the apparent direction of the wind changed with it. 

 It flashed across the astronomer's mind that the position of an 



