HYPOTHESES. 19 



respecting the law of what we already know to be the cause, 

 but an hypothesis respecting the cause itself. It is allowable, 

 useful, and often even necessary, to begin by asking ourselves 

 what cause may have produced the effect, in order that we may 

 know in what direction to look out for evidence to determine 

 whether it actually did. The vortices of Descartes would have 

 been a perfectly legitimate hypothesis, if it had been possible, 

 by any mode of exploration which we could entertain the hope 

 of ever possessing, to bring the reality of the vortices, as a fact 

 in nature, conclusively to the test of observation. The hypo- 

 thesis was vicious, simply because it could not lead to any 

 course of investigation capable of converting it from an hypo- 

 thesis into a proved fact. It might chance to be disproved, 

 either by some want of correspondence with the phenomena it 

 purported to explain, or (as actually happened) by some ex- 

 traneous fact. " The free passage of comets through the spaces 

 in which these vortices should have been, convinced men that 

 these vortices did not exist."* But the hypothesis would have 

 been false, though no such direct evidence of its falsity had 

 been procurable. Direct evidence of its truth there could 

 not be. 



The prevailing hypothesis of a luminiferous ether, in other 

 respects not without analogy to that of Descartes, is not in its 

 own nature entirely cut off from the possibility of direct 

 evidence in its favour. It is well known that the difference 

 between the calculated and the observed times of the periodical 

 return of Encke's comet, has led to a conjecture that a medium 

 capable of opposing resistance to motion is diffused through 

 space. If this surmise should be confirmed, in the course of 

 ages, by the gradual accumulation of a similar variance in the 

 case of the other bodies of the solar system, the luminiferous 

 ether would have made a considerable advance towards the 

 character of a vera causa, since the existence would have been 

 ascertained of a great cosmical agent, possessing some of the 

 attributes which the hypothesis assumes ; though there would 

 still remain many difficulties, and the identification of the 



* Whewell's Phil, of Discovery, pp. 275, 276. 

 22 



