284 Professor Powell on the Nature of the 



the principles of theory would require the specific nature of the 

 relation to be more closely examined. This is what I have en- 

 deavoured to do in some investigations in which I have been re- 

 cently engaged. In the London and Edinburgh Journal of 

 Science, No. 31, and continued in No. 32, 33, and 34, I have 

 given an abstract of M. Cauchy's views, to which is annexed tlie 

 deduction of that more specific relation. I have also adverted 

 to the necessity of an exact comparison of the results of expe- 

 riment with the formula thus derived from theory. The test of 

 numerical comparison is that by which alone any theory can, in 

 the present state of science, be substantiated. Such a compari- 

 son I have carried on to a very considerable extent, and with 

 results eminently favourable to the theory : they wilLshortly be 

 laid before the public *. 



Oxford, Feb. 1?. 1835. 



" The following extracts will illustrate the views above taken : — 



" Supposing," says Dr Hartley, " the existence of the ether to be destitute 

 of all direct evidence, still if it serves to explain and account for a variety 

 of phenomena, it will, by this means, have an indirect argument in its fa- 

 vour. Thus we admit the key of a cypher to be a true one when it explains 

 the cypher completely : and the decypherer judges himself to approach to 

 the true key in proportion as he advances in the explanation of the cypher ; 

 and this without any direct evidence at all." — (Observ. on Man, vol. i. p. 15» 

 4th ed.) 



Again, " Philosophy is the art of decyphering the mysteries of nature : and 

 every thing which can explain all the phenomena has the same evidence in 

 its favour, that it is possible the key of a cypher can have from its explaining 

 the cypher."--(Ibid. p. 350.) 



Le Sage (Opuscules relatifs a la Methode) has supported the same view of 

 the subject, and Gravesande, in his Introductio ad Philosophiam, to a chapter 

 on the use of hypotheses, joins another on decyphering. 



Mr Dugald Stewart makes some judicious remarks on the subject (Phil, 

 of Human Mind, vol. ii. p. 444, &c.) The analogy of the cypher, he observes, 

 supposes that we have all the facts before us. " In our physical researches, on 

 the other hand, we are admitted to use only a few detached sentences extract- 

 ed from a volume, of the size of which we are entirely ignorant. No hypo- 

 thesis, therefore, how numerous soever the facts may be with which it tallies, 

 can completely exclude the possibility of exceptions or limitations hitherto 

 undiscovered." 



Again, he observes, there are few, if any, physical hypotheses which afford 

 the ordy way of explaining the phenomena to wliich they are applied, and' 

 therefore, admitting them to be perfectly consistent with all the known facts, 

 thev leave us in the same state of uncertainty in wliich the decypherer would 



