OF NATURAL PHILOSOPHY. 213 



assumed, is involved in the general verification of the 

 whole theory, of which, when once assumed, they 

 form a part ; and the same comparison with ob- 

 servation which enables us to decide on the truth of 

 the abstract principle, enables us, at the same time, 

 to ascertain whether we have fixed the values of our 

 data in accordance with the actual state of nature. 

 If not, it becomes an important question, whether 

 the assumed values can be corrected, so as to bring 

 the results of theory to agree with facts ? Thus it 

 happens, that as theories approach to their perfec- 

 tion, a more and more exact determination of data 

 becomes requisite. Deviations from observed fact, 

 which, in a first or approximative verification, may 

 be disregarded as trifling, become important when 

 a high degree of precision is attained. A difference 

 between the calculated and observed places of a 

 planet, which would have been disregarded by 

 Kepler in his verification of the law of elliptic 

 motion, would now be considered fatal to the theory 

 of gravity, unless it could be shown to arise from an 

 erroneous assumption of some of the numerical data 

 of our system. 



(225.) The observations most appropriate for the 

 ready and exact determination of physical data are, 

 therefore, those which it is most necessary to have 

 performed with exactness and perseverance. Hence 

 it is, that their performance, in many cases, becomes 

 a national concern, and observatories are erected and 

 maintained, and expeditions despatched to distant 

 regions, at an expense which, to a superficial view, 

 would appear most disproportioned to their objects. 

 But it may very reasonably be asked why the direct 

 p 3 



