xx.] METHOD OF VARIATIONS. 44& 



gas it increases in bulk or pressure, and as far as we can go 

 the higher the temperature the higher the pressure. Our 

 experiments are of course restricted in temperature both 

 above and below, but there is every reason to believe that 

 the bulk being the same, the pressure would never return 

 to the same point at any two different temperatures. We 

 may of course repeatedly raise and lower the temperature 

 at regular or irregular intervals entirely at our will, and 

 the pressure of the gas will vary in like manner and 

 exactly at the same intervals, but such an arbitrary series 

 of changes would not constitute Periodic Variation. It 

 would constitute a succession of distinct experiments, 

 which would place beyond reasonable doubt the connexion 

 of cause and effect. 



Whenever a phenomenon recurs at equal or nearly 

 equal intervals, there is, according to the theory of proba- 

 bility, considerable evidence of connexion, because if the 

 recurrences were entirely casual it is unlikely that they 

 would happen at equal intervals. The fact that a brilliant 

 comet had appeared in the years 1301, 1378, 1456, I53 1 , 

 1607, and 1682 gave considerable presumption in favour 

 of the identity of the body, apart from similarity of the 

 orbit. There is nothing which so fascinates the attention 

 of men as the recurrence time after time of some unusual 

 event. Things and appearances which remain ever the 

 same, like mountains and valleys, fail to excite the curiosity 

 of a primitive people. It has been remarked by Laplace 

 that even in his day the rising of Venus in its brightest 

 phase never failed to excite surprise and interest. So 

 there is little doubt that the first germ of science arose 

 in the attention given by Eastern people to the changes 

 of the moon and the motions of the planets. Perhaps the 

 earliest astronomical discovery consisted in proving the 

 identity of the morning and evening stars, on the grounds 

 of their similarity of aspect and invariable alternation. 1 

 Periodical changes of a somewhat complicated kind must 

 have been understood by the Chaldeans, because they were 

 aware of the cycle of 6585 days or 19 years which brings 

 round the new and full moon upon the same days, hours, 

 and even minutes of the year. The earliest efforts of 



1 Laplace, System of the World, vot i. pp. 50, 54, &c. 



G G 



