272 THE WORLD MACHINE 



worth while to test out his theory by actual observation. Had 

 he done so he might easily have anticipated Newton. But he 

 reflected upon the fact, noted by Fracastor and others, that 

 the tails of comets point always away from the sun ; he ex- 

 plained this on the supposition that the tail is formed by rays 

 of the sun which penetrate the body of the comet and carry away 

 with them some portion of its substance. 



We do not think of light as a hail or bombardment of ex- 

 tremely minute corpuscles as Kepler did, and Newton too ; but 

 with this difference, his theory was in close coincidence with 

 our modern ideas. Within the last two or three years it has 

 been shown that, as Maxwell predicted, light may exert a 

 pressure, even though it be a form of motion and not a substance. 

 It is apparently this pressure which swings the tail of the comet 

 around so that the body of the comet usually points towards 

 the sun. 



Galileo got no further. But two years after the appearance 

 of the great comet studied by Newton, came another. From 

 careful observations, Halley was able to compute its orbit in 

 accordance with Newton's principles. In 1705 he too published 

 a work on comets, in which no less than twenty-four orbits were 

 calculated. He had been struck by the resemblance between 

 the paths described by the first of these and one of seventy- 

 five years before. Looking through the old records, he found 

 mention of another in 1531 and again in 1456 ; he conjectured 

 that they were perhaps all of them but different appearances 

 of the same comet, revolving round the sun in a period of about 

 seventy-five and a half years. Inspired by this coincidence, 

 he stepped out into space, as it were, and watching its path 

 with the eye of the imagination, predicted its return in 1758. 



Truly a splendid flight of the mind it was, thus to throw off 

 the heavy shackles which tie us down to the here and now, 

 and sweep through the centuries, backwards through history, 

 forward through history yet unmade ; but his faith was justified. 

 Not only did the comet reappear, but long after the soul of 

 Halley had followed that of Caesar, in the common way of 

 all our human kind, the mathematician Clairaut, utilising the 

 materials he had left, calculated its return within a month. It 

 returned again in 1835, its appearance this time being predicted 

 within three days. The existence of the planet Uranus, which 

 had been a disturbing influence in the previous calculations, 



