ADDRESS. Ixxiii 



its conformity to known or ascertainable facts, — as when Kepler determined 

 the elliptic orbit of Mars; or by the fulfilment of the predictions it has 

 sanctioned, — as in the occurrence of an Eclipse or an Occultation at the 

 precise moment specified many years previonsly ; or, still more emphatically, 

 by the actual discovery of phenomena till then unrecognized, — as when the ' 

 Perturbations of the planets, shown by Newton to be the necessary results of 

 their mutual attraction, were proved by observation to have a real existence ; 

 or as when the unknown disturber of Uranus was found in the place assigned 

 to him bj^ the computations of Adams and Le Verrier. 



We are accustomed, and I think most rightly, to speak of these achieve- 

 ments as triiimphs of the Human Intellect. But the very phrase implies that 

 the work is done by Mental Agency. And even in the very first stage of the 

 process — the interpretation of observations — there is often a liability to serious 

 error. Of this we have a most noteworthy example in the fact that the esti- 

 mated distance of the Earth from the Sun, deduced from observations of the 

 last Transit of Yenus, is now pretty certainly known to be about three 

 millions of miles too great ; the strong indications of such an excess afi'orded 

 by the nearly coincident results of other modes of inquiry having led to a 

 reexamination of the record, which was found, when fairly interpreted, 

 fully to justify — if not even to require — the reduction. Even the veri- 

 fication of the prediction is far from proving the Intellectual process by 

 which it was made to have been correct. For we learn from the honest 

 confessions of Kepler, that he was led to the discovery of the Elliptic orbit of 

 Mars by a series of happy accidents, which turned his erroneous guesses into 

 the right direction ; and to that of the passage of the Eadius Vector over equal 

 areas in equal times, by the notion of a whirling force emanating from the Sun, 

 which we now regard as an entirely wrong conception of the cause of orbital 

 revolution *. It should always be remembered, moreover, that the Ptolemaic 

 system of Astronomy, Avith all its cumbrous ideal mechanism of " Centric and 

 Excentric, Cycle and Epicycle, Orb in Orb," did intellectually represent all that 

 the Astronomer, prior to the invention of the Telescope, could see from his actual 

 standpoint, the Earth, with an accuracy which was proved by the fulfilment 

 of his predictions. And in that last and most memorable anticipation which 

 has given an imperishable fame to our two illustrious contemporaries, the 

 inadequacy of the basis afforded by actual observation of the perturbations 

 of Uranus, required that it should be supplemented by an assumption of the 

 probable distance of the disturbing Planet beyond, which has been shown by 

 subsequent observation to have been only an approximation to the truth. 



Even in this most exact of Sciences, therefore, wo cannot proceed a step, 

 without translating the actual Phenomena of Natiire into Intellectual Eepre- 

 scntations of those phenomena ; and it is because the Newtonian conception 

 is not only the most simple, but is also, up to the extent of our present 

 knowledge, universal in its conformity to the facts of observation, that we 

 accept it as the only Scheme of the Universe yet promulgated, which satisfies 

 our Intellectual requirements. 



When, under the reign of the Ptolemaic System, any new inequality was 

 discovered in the motion of a Planet, a new wheel had to be added to the 

 ideal Mechanism, — as Ptolemy said, " to save appearances." If it should 

 prove, a century hence, that the motion of Neptune himself is disturbed 

 liy some other attraction than that exerted by the interior Planets, we 

 should confidently expect that not an ideal but a real cause for that dis- 

 turbance will be found in the existence of another Planet beyond. But 

 * See Drinkwater's ' Life of Kepler,' in the Library of Useful Knowledge, pp. 26-35. 



