102 



DISSERTATION SECOND. 



[part tl. 



ly or inaccurately, and to propose as a mere conjecture. In 

 the history of human knowledge, there is hardly any discove- 

 ry to which some gradual approaches had not been made 

 before it was completely brought to light. To have found 

 out the means of giving certainty to the thing asserted, or of 

 disproving it entirely; and, when the reality of the principle 

 was found out, to measure its quantity, to ascertain its laws, 

 and to trace their consequences with mathematical precision, 

 —in this consists the great difficulty and the great merit of 

 such a discovery as that which is now before us. In this 

 Newton had no competitor : envy was forced to acknowledge 

 that he had no rival, and consoled itself with supposing that 

 he had no judge. 



Of all the physical principles that have yet been made 

 known, there is none so fruitful in consequences as that of 

 gravitation ; but the same skill that had directed Newton to 

 the discovery, was necessary to enable him to trace its conse- 

 quences. 



The mutual gravitation of all bodies being admitted, it was" 

 evident, that while the planets were describing their orbits 

 round the greatest and most powerful body in the system, 

 they must mutually attract one another, and thence, in their 

 revolutions, some irregularities, some deviations from the 

 description of equal areas in equal times, and from the laws 

 of the elliptic motion, might be expected. Such irregulari- 

 ties, however, had not been observed at that time in the mo- 

 tion of any of the planets, except the moon, where some of 

 them were so conspicuous as to have been known to Hippar- 

 chus and Ptolemy. Newton, therefore, was very natural- 

 ly led to inquire what the different forces were, which, ac- 

 cording to the laws just established, could produce irregulari- 

 ties in the case of the moon's motion. Beside the force of 

 the earth, or rather of the mutual gravitation of the moon 

 and earth, the moon must be acted on by the sun ; and the 



