INDUCTIVE EPOCH OF NEWTON. 407 



which produces their proper revolution about Jupiter, the other regu- 

 lates their motion round the sun." And in another place (cap. 20), 

 he attempts to show an effect of this principle upon the inclination of 

 the orbit ; though, as might be expected, without any real result. 



The case which most obviously suggests the notion that the sun 

 exerts a power to disturb tbe motions of secondary planets about pri- 

 mary ones, might seem to be our own moon ; for the great inequalities 

 which had hitherto been discovered, had all, except the first, or ellip- 

 tical anomaly, a reference to the position of the sun. Nevertheless, I 

 do not know that any one had attempted thus to explain the curiously 

 irregular course of the earth's attendant. To calculate, from the dis- 

 turbing agency, the amount of the irregularities, was a problem which 

 could not, at any former period, have been dreamt of as likely to be at 

 any time within the verge of human power. 



Newton both made the step of inferring that there were such forces, 

 and, to a very great extent, calculated the effects of them. The infer- 

 ence is made on mechanical principles, in the sixth Theorem of the 

 third Book of the Principia; — that the moon is attracted by the sun, 

 as the earth is ; — that the satellites of Jupiter and Saturn are attracted 

 as the primaries are; in the same manner, and with the same forces. 

 If this were not so, it is shown that these attendant bodies could not 

 accompany the principal ones in the regular manner in which they do. 

 All those bodies at equal distances from the sun would be equally 

 attracted. 



But the complexity which must occur in tracing the results of this 

 principle will easily be seen. The satellite and the primary, though 

 nearly at the same distance, and in the same direction, from the sun, 

 are not exactly so. Moreover the difference of the distances and of 

 the directions is perpetually changing ; and if the motion of the satel- 

 lite be elliptical, the cycle of change is long and intricate : on this 

 account alone the effects of the sun's action will inevitably follow cycles 

 as long and as perplexed as those of the positions. But on another 

 account they will be still more complicated ; for in the continued 

 action of a force, the effect which takes place at first, modifies and 

 alters the effect afterwards. The result at any moment is the sum of 

 the results in preceding instants : and since the terms, in this series of 

 instantaneous effects, follow very complex rules, the sums of such 

 series will be, it might be expected, utterly incapable of being reduced 

 to any manageable degree of simplicity. 



It certainly does not appear that any one but Newton could make 



