226 SCIENCE FROM AN EASY CHAIR 



seen at the rooms of the Royal Astronomical Society) 

 show the most wonderful changes of form of its tail, so 

 that they look more like the record of the changes of 

 some living creature than those of a heavenly body. 

 Already, in October 1909, Halley's comet, which has been 

 anxiously awaited, has been seen, though it is not expected 

 to be bright and visible to all until May 1910. Comets 

 are among the exceptional delights of the astronomer 

 that is to say, big comets, for two or three small comets 

 visible only by a telescope or by photography turn up 

 every year. Some comets are expected visitors, others 

 make their appearance quite casually, some because they 

 apparently have no regular period, some because that 

 period is as yet undiscovered. Edmund Halley was the 

 first to discover the law of movement of a comet and to 

 predict the return in 1758 of that seen in 1682. He did 

 not live to witness the verification of his prediction. This 

 comet, now called Halley's comet, was, he conjectured, 

 the same which had appeared in 1531 and in 1607. His 

 prediction of its return proved to be a year out (owing to 

 perturbations caused by Neptune and Uranus, two planets 

 undiscovered in his day), but it appeared in 1759, and 

 went round once again and reappeared in 1835, and now 

 is eagerly expected by astronomers to appear in full 

 brilliancy in 1910. Its period is about seventy-five or 

 seventy-six years. 



