294 THE WORLD MACHINE 



determine changes of climate, for example and so men came 

 afterwards to perceive. But they were not permanent. It was 

 clear that some time or other the earth would return to pre- 

 cisely the same conditions as that which had once prevailed ; 

 it was merely making a round. 



But the fact that there could be any such deviation from 

 seeming regularity must have very soon suggested that there 

 might be other changes whose effect was progressive and 

 cumulative. Men came to wonder whether the seeming per- 

 manent arrangement of the planets formed a stable system, or 

 whether it might one day go all awry. Has this little patch 

 of the universe which we call the solar system always existed 

 more or less in its present form ? Were the planets always 

 arranged in the same order and same distance and with the 

 same speeds as we perceive them now, or is there underneath 

 all this apparent regularity, this machine-like motion, a subtle 

 deviation which in the course of uncounted ages will change 

 it utterly. 



There was a curious fact turned up by Halley. Studying 

 the records of some oi the old eclipses, he found that the cal- 

 culation, so far as he could perceive, of the times in which they 

 should have occurred did not agree with the records at all. 

 In the pages of Ptolemy and others he found accurate descrip- 

 tions of eclipses which had taken place far back in the old 

 Babylonian and Chaldean days. They did not tally with his 

 computations by nearly two hours. This in a period of twenty- 

 four hundred years is not a great variation ; there was always 

 a chance, of course, that his calculations, or the ideas upon 

 which he based them, were wrong. But the new methods 

 brought in by Newton had reduced planetary motion to a 

 science of marvellous accuracy. So, when the estimates had 

 been carefully checked up and found flawless, astronomers were 

 able to lay hold of any such discrepancy as a distinct and signifi- 

 cant fact ; they could go hunting for the reason, rather than 

 put the matter aside as something inexplicable or the mere 

 product of error. 



Halley did not find the solution of the problem he had posed. 

 Before it could come it was needful to clear up a difficult and 

 highly recondite problem, the pull of the planets on each other, 

 their mutual perturbations, as it is known in more technical 

 phrase, the problem of three bodies. For this, in turn, mathe- 



