288 Prof. Challis on Newton's "Foundation of all Philosophy' 9 



gress in theoretical physics, I have given special attention to 

 the application of partial differential equations in hydro- 

 dynamics, and, after a long-sustained intellectual effort, suc- 

 ceeded at length in demonstrating certain propositions which 

 hold the same place relatively to general theoretical physics 

 that the dynamical problems which Newton first solved held 

 with respect to physical astronomy. Much more, I am fully 

 aware, is required to be effected in the same direction. But I 

 claim to have done for general physics what Newton did for 

 physical astronomy ; that is, to have entered upon the appro- 

 priate course, and to have overcome the initial difficulties. 

 Those who would dispute this claim, must either call in question 

 the truth of the hydrodynamical theorems, or contest their 

 general applicability to theoretical physics. I am prepared to 

 maintain both these positions. 



A few more general remarks respecting the relation of theory 

 to experiment remain to be made. It is admitted that scientific 

 knowledge does not consist merely in acquaintance with the 

 facts and laws which experiment reveals; to be complete, it 

 must embrace the results of experiment by a theory — that is, as 

 the term implies, by a perception of their causes resting on an 

 intelligible basis. For proof that theory is recognized to be the 

 end of experiment, I may appeal to the attempts which are 

 constantly being made by experimentalists to theorize — that is, 

 to assign causes. " Felix qui potuit rerum cognoscere causas." 

 But, because experiment is thus subordinate to the establishment 

 of theory, those experiments are the best which are best adapted 

 to this purpose, — on which account correct theoretical notions 

 are of much importance to the experimentalist, as serving to 

 control and guide the course of his experiments. In this 

 respect I venture to affirm, from acquaintance with the theore- 

 tical applications of experiments, that experimental research 

 might be greatly aided by the recognition of the fundamental 

 ideas respecting matter and force which I have so much insisted 

 upon in this communication, and by the acceptance of the 

 mathematical results which I have obtained relative to the laws 

 of the motion and pressure of the setherial medium. 



The theoretical principles which it is the object of the fore- 

 going discussion to explain and to establish, are precisely those 

 on which are founded the theories of light, heat, gravity, elec- 

 tricity, galvanism, and magnetism which I have proposed in 

 various communications to this Journal. I might appeal to the 

 large number of successful explanations of phenomena which 

 those theories exhibit, for evidence of the truth of the principles 

 and the correctness of the deductive reasoning applied to them. 

 But I feel it to be vain to expect that the appeal will be attended 



