470 Prof. Challis on the Fundamental Ideas of 



sical force, or perhaps resolve it into volition. This, however, is 

 approaching very closely to the region of metaphysics ; and as 

 far as regards physical research, it suffices to take the law of the 

 proportionate variation of the density and pressure of the sether 

 as an ultimate fact. 



All the different kinds of physical force detected by observa- 

 tion and experiment are modifications of pressures of the sether. 

 This is not a distinct hypothesis, but only a necessary sequel of the 

 preceding hypotheses. Here it is proper to remark that although 

 all the hypotheses may have been expressed in intelligible terms, 

 and are such as observation and experience suggest, and are also 

 a proper basis for mathematical reasoning, they are not there- 

 fore necessarily true. Their truth can only be proved by a 

 satisfactory comparison of the mathematical deductions from 

 them with facts. But inasmuch as they satisfy those three 

 essential conditions of hypotheses, this circumstance exempts 

 them from prejudgment, and ought to secure an impartial con- 

 sideration on the part of my contemporaries of the explanations 

 of numerous phenomena which I have derived from their ma- 

 thematical consequences. Further, it is to be said that hypo- 

 theses fulfilling the above-named conditions are such that if the 

 deductions from them admit of satisfactory comparison with 

 facts of observation and experiment, they fully explain the facts, 

 because the explanations rest on an intelligible basis. Hypo- 

 theses not fulfilling the same conditions do not completely 

 explain phenomena. 



With respect to the comparison of mathematical results from 

 the hypotheses with facts, I have to remark that there is an 

 initial difficulty, relating to the appropriate mathematical rea- 

 soning, which must be overcome before the comparison can be 

 effected. In Newton's time, as soon as the calculation proper 

 for finding the motion of a single physical particle acted upon 

 by accelerative forces was discovered, physical astronomy and 

 other branches of mechanics made rapid progress. What is re- 

 quired at the present time is the discovery of the correct pro- 

 cesses for calculating the motions' and pressures, under given 

 circumstances, of a congeries of physical particles forming an 

 elastic fluid, and influencing each other's motions by mutual 

 pressure. Till this be done, the hypothesis of the sether can- 

 not be brought to bear on Theoretical Physics. I take this op- 

 portunity for expressing the conviction that there ought not to 

 be among philosophers such diversity of opinion as that existing 

 in the present day relative to the foundations and the true me- 

 thod of philosophy. The proper principles and the proper 

 course w ere pointed out at the Newtonian epoch of science; and 

 in adopting the foregoing hypotheses, I have only followed out 

 the rules then laid down, with such modifications as the existing 



