288 THE WORLD MACHINE 



utilise, Delambre found that the agreement with Bradley's esti- 

 mates is very close. Other measures have confirmed the result, 

 correcting it a very little. It is now possible to effect the 

 measure by mechanical means with revolving mirrors, for ex- 

 ample along something of the same lines as the futile experi- 

 ment of Galileo. 



Bradley's discovery was a great step forward. It added a 

 new and undreamt-of accuracy to astronomical observations ; 

 it proved a great impetus in the study of optical phenomena of 

 light. But it had a further and distinctly greater import : it 

 demonstrated unequivocally the motion of the earth. The aberra- 

 tion of the stars could be explained upon no other supposition. 

 The accuracy of the observations was unquestionable. It was 

 inconceivable that one section of the heavens should show this 

 foolish little wobble while another section did not, on the sup- 

 position that the earth was the fixed centre of the universe. 



It was given to Bradley to make yet another noteworthy dis- 

 covery. That was the nutation or nodding of the earth's axis. 

 Coppernicus, it will be remembered, had pointed out a conical 

 movement of the earth's axis, extending through a period of about 

 twenty-six thousand years. Newton had given the explanation 

 of this movement in the Principia. Bradley discovered a dis- 

 placement of some of the stars which could be explained neither 

 from this motion of precession nor of aberration. He must have 

 been a tenacious, patient sort of a man, for he watched the 

 displacement through a period of nineteen years, and thus found 

 it to be periodic. Of course, this could not be a movement 

 of the stars themselves. His discovery was that, superimposed 

 upon the conical motion was a secondary motion or inclination ; 

 it caused the line marked out by the extension of the earth's 

 axis upon the heavens to pursue a wavy and scollopy sort 01 

 motion instead of being a perfect circle. This motion, like the 

 other, is the result of the varying pull of the moon upon the 

 oblate spheroid of the earth. It was predictable from Newton's 

 theory ; he even suggested that it might exist, though it was 

 a solar rather than a lunar disturbance which he had in mind. 



These discoveries of Bradley were, as we see, made quite 

 by the way. What he set out for he never found. Not even 

 with his wonderful zenith sextant, which could enable him to 

 plot stellar changes whose maximum was but 20" of arc, 



