EXAMPLES OF THE FOUR METHODS. 301 



ing Dew formed in clear intervals will often even evaporate again 



when the sky becomes thickly overcast." The proof, therefore, is complete, 

 that the presence or absence of an uninterrupted communication with the 

 sky causes the deposition or non-deposition of dew. Now, since a clear sky 

 is nothing but the absence of clouds, and it is a known property of clouds, as 

 of all other bodies between which and any given object nothing intervenes 

 but an elastic fluid, that they tend to raise or keep up the superficial tem- 

 pei'ature of the object by radiating heat to it, we see at once that the dis- 

 appearance of clouds will cause the surface to cool ; so that nature, in this 

 case, produces a change in the antecedent by definite and known means, 

 and the consequent follows accordingly : a natural experiment which satis- 

 fies the requisitions of the Method of Difference.* 



The accumulated proof of which the Theory of Dew has been found 

 susceptible, is a striking instance of the fullness of assurance which the in- 

 ductive evidence of laws of causation may attain, in cases in which the in- 

 variable sequence is by no means obvious to a superficial view. 



§ 4. The admirable physiological investigations of Dr. Brown-Sequard 

 afford brilliant examples of the application of the Inductive Methods to a 

 class of inquiries in which, for reasons which will presently be given, di- 

 rect induction takes place under peculiar difiiculties and disadvantages. 

 As one of the most apt instances, I select liis speculation (in the proceed- 

 ings of the Royal Society for May 16, 1861) on the relations between mus- 

 cular irritability, cadaveric rigidity, and putrefaction. 



The law which Dr. Brown-Sequard's investigation tends to establish, is 

 the following : " The greater the degree of muscular irritability at the time of 

 death, the later the cadaveric rigidity sets in, and the longer it lasts, and the 

 later also putrefaction appears, and the slower it progresses." One would 

 say at first sight that the method here required must be that of Concomi- 

 tant Variations. But this is a delusive appearance, arising from the circum- 

 stance that the conclusion to be tested is itself a fact of concomitant varia- 

 tions. For the establishment of that fact any of the Methods may be put 

 in requisition, and it will be found that the fourth Method, though really 

 employed, has only a subordinate place in this particular investigation. 



The evidences by which Dr. Brown-Sequard establishes the law may be 

 enumerated as follows : 



1st. Paralyzed muscles have greater irritability than healthy muscles. 

 Now, paralyzed muscles are later in assuming the cadaveric rigidity than 

 healthy muscles, the rigidity lasts longer, and putrefaction sets in later, and 

 proceeds more slowly. 



* I must, however, remark, that this example, which seems to militate against the assertion 

 we made of the comparative inapplicability of the Method of Difference to cases of pure ob- 

 servation, is really one of those exceptions which, according to a proverbial expression, prove 

 the general rule. For in this case, in which Nature, in her experiment, seems to have imi- 

 tated the type of the experiments made by man, she has only succeeded in producing the 

 likeness of man's most imperfect experiments ; namely, those in which, though he succeeds 

 in producing the phenomenon, he does so by employing complex means, which he is unable 

 perfectly to analyze, and can form, therefore, no sufficient judgment what portion of the effects 

 may be due, not to the supposed cause, but to some unknown agency of the means by which 

 that cause was produced. In the natural experiment which we are speaking of, the means 

 used was the clearing off a canopy of clouds ; and we certainly do not know sufficiently in 

 what this process consists, or on what it depends, to be certain a priori that it might not oper- 

 ate upon the deposition of dew independently of any thermometric effect at the earth's surface. 

 Even, therefore, in a case so favorable as this to Nature's experimental talents, her experiment 

 is of little value except in conoboration of a conclusion already attained through other means. 



