MAN THE INTERPRETER OF NATURE. 191 



commonly known as the " exact " sciences, of which astronomy 

 may be taken as the type, the data afiforded by precise methods 

 of observation can be made the basis of reasoning, in every step 

 of which the mathematician feels the fullest assurance of certainty ; 

 and the final deduction is justified either by 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 previously; 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 by the computations of Adams and Le Verrier. 



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

 achievements as triumphs 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 exam.ple in tlie fact that the estimated distance of the 

 earth from the sun, deduced from observations of the last transit 

 of Venus, is now pretty certainly known to be about three millions 

 of miles too great ; the strong indications of such an excess afforded 

 by the nearly coincident results of other modes of inquiry having 

 led to a re-examination of the record, which was found, when fairly 

 interpreted, fully to justify — if not even to require — the reduc- 

 tion. Even the verification 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 radius vector over 

 equal areas in equal times, by the notion of a whirling force ema- 

 nating from the sun, which we now regard as an entirely wrong 

 conception of the cause of orbital revolution.* It should always 



♦ See Drinkwater's " Life of Kepler," in the Library of Useful Knowledge, 

 pp. 26-35. 



