182 Prof. Challis on the Hydro dynamical Theory of the 



properly so called. The antecedent steps of theory ought, for 

 distinction, to be called empirical or provisional, as resting on 

 arbitrary hypotheses, and as subsidiary to true and complete 

 theory. True theory rests on hypotheses that are not only com- 

 prehensible, but also ultimate and necessary — that is, such as do 

 not admit of being accounted for by any ulterior hypotheses. 

 This will be proved to be the specific quality of the hypotheses 

 stated above (art. 1), if they should be shown to be adequate to 

 the explanation of the nature, laws, and consequences of the 

 operation of all the different kinds of physical force. To de- 

 monstrate their adequacy for this purpose has been the express 

 object of the many theoretical researches I have been occupied 

 with relative to the modus operandi of physical force generally. 

 This course of philosophy I propose to call Neivtonian, its 

 " foundation " having been indicated by Newton in the Third 

 Book of the Principia. (See the discussion of this view in the 

 Philosophical Magazine for October 1863, p. 280.) 



5. Having thus pointed out that a distinction is to be made 

 between empirical theory resting on arbitrary hypotheses and 

 ultimate theory resting on strictly a priori hypotheses, I have 

 further to state in what respect the two kinds of theory may be 

 considered to be related to each other. Let it be supposed that by 

 means of a theory depending on certain ascertained facts, and on 

 hypotheses thereby suggested, a true mathematical expression 

 of the laws of the phenomena proposed to be accounted for has 

 been obtained. According to views entertained by some theo- 

 rists of the present day, natural philosophy consists in thus 

 arriving at phenomenal laws, and there is no occasion for any 

 further investigation. But the principles of the philosophy I 

 call " Newtonian " demand that the explanations of all phe- 

 nomena and their laws should be inferred by mathematical rea- 

 soning from the before-mentioned fundamental hypotheses. 

 Now this may be done in two ways — either directly, by in- 

 dependent deductions from the a priori hypotheses, or inter- 

 mediately, by deducing from the same hypotheses explana- 

 tions of the facts and hypotheses which form the basis of a 

 true empirical theory. It is evident that in either way the phe- 

 nomena are shown to be consequences of the operation of intel- 

 ligible causes, and are completely explained. It appears thus 

 that the empirical method is subsidiary to the a priori method 

 whenever the explanation of phenomena is effected by the aid 

 and intervention of the former, and that in this respect the two 

 methods may be mutually related. These remarks will receive 

 elucidation in the course of the subsequent discussions. 



6. I propose, in the first instance, to give an a priori expla- 

 nation of the facts relating to the action of a large magnet on a 



