SEQUEL TO THE EPOCH OF NEWTON. 433 



CHAPTER IV. 



Sequel to the Epoch of Newton, continued. — Verification and 

 Completion of the Newtonian Theory. 



Sect. 1. — Division of the Subject. 



THE verification of the Law of Universal Gravitation as the govern- 

 ing principle of all cosmical phenomena, led, as -we have already 

 stated, to a number of different lines of research, all long and difficult. 

 Of these we may treat successively, the motions of the Moon, of the 

 Sun, of the Planets, of the Satellites, of Comets ; we may also con- 

 sider separately the Secular Inequalities, which at first sight appear to 

 follow a different law from the other changes ; we may then speak of 

 the results of the principle as they affect this Earth, in its Figure, in 

 the amount of Gravity at different places, and in the phenomena of the 

 Tides. Each of these subjects has lent its aid to confirm the general 

 law : but in each the confirmation has had its peculiar difficulties, and 

 has its separate history. Our sketch of this history must be very rapid, 

 for our aim is only to show what is the kind and course of the con- 

 firmation which such a theory demands and receives. 



For the same reason we pass over many events of this period which 

 are highly important in the history of astronomy. They have lost 

 much of their interest for us, and even for common readers, because 

 they are of a class with which we are already familiar, truths included 

 in more general truths to which our eyes now most readily turn. Thus, 

 the discovery of new satellites and planets is but a repetition of what 

 was done by Galileo : the determination of their nodes and apses, the 

 reduction of their motions to the law of the ellipse, is but a fresh ex- 

 emplification of the discoveries of Kepler. Otherwise, the formation 

 of Tables of the satellites of Jupiter and Saturn, the discovery of the 

 eccentricities of the orbits, and of the motions of the nodes and apses, 

 by Cassini, Halley, and others, would rank with the great achievements 

 in astronomy. Newton's peculiar advance in the Tables of the celestial 

 motions is the introduction of Perturbations. To these motions, so 

 affected, we now proceed. 



Vol. I.— 28 



