230 INDUCTION. 



separately, what effect they must 'produce in the case ABC where 

 they act together. Of the two instances, therefore, which the Method 

 of Difference requires — the one positive, the other negative — the nega- 

 tive one, or that in which the given phenomenon is absent, is jiot the 

 direct result of observation and experiment, but has been arrived at 

 by deduction. As one, of the forms of the Method of Difference, the 

 Method of Residues partakes of its rigorous certainty, provided the 

 previous inductions, those which gave the effects of A and B, were ob- 

 tained by the same infallible method, and provided we are certain that 

 C is the only antecedent to which the residual phenom.enon c-can be 

 refeiTed ; the only agent of which we had not already calculated and 

 subducted the effect. But as we can never be quite certain of this, 

 the e\Hdence derived from the Method of Resiciues is not complete, 

 unless we can obtain C ai-tificially and try it separately, or unless its 

 agency, when once suggested, can be accounted for, and proved de- 

 ductively, fi'om kno^vn laws. 



Even with these reservations, the- Method of Residues is one of the 

 most important among our instruments of discovery. Of all the methods 

 of investigating laws of nature, this is the most fertile in unexpected 

 results ; often infonning us of sequences in which neither the cause nor 

 the effect were su^ciently conspicuous to attract of themselves the 

 attention of observers. The agent C may be an obscure circumstance, 

 not likely to have been perceived unless sought for, nor likely to have 

 been sought for until attention had been awakened by the insufficiency 

 of the obvious causes to account for the whole of the effect. And c 

 may be so disguised by its intermixture with a and b, that it would 

 scarcely have presented itself spontaneously as a subject of separate 

 study. Of these uses of the method, we shall presently cite some 

 remarkable examples. The canon of the Method of Residues is as 

 follows : — 



Fourth Canox. 



Subduct from any phenomenon such part as is hnoicn by previous 

 inductions to be the effect of certain antecedents^ aoid the residue of the 

 phenomenon is the effect of the remaining antecedents. 



§ 6. There remains a class of laws which it is impracticable to 

 ascertain by any of the three methods which I have attempted to 

 characterize ; namely, the laws of those Permanent Causes, or inde- 

 structible natural agents, which it is impossible either to exclude or to 

 isolate : which we can neither hinder from being present, nor contrive 

 that they should be present alone. It would appear at first sight that 

 we could by no Tneans sepai-ate the effects of these agents fi-om the 

 effects of those other phenomena with which they cannot be prevented 

 from coexisting. In respect, indeed, to most of -the permanent causes, 

 no such difficulty exists; since, though we cannot eliminate them as 

 coexisting facts, we can eliminate them as influencing agents, by 

 simply trying our experiment in a local situation beyond the limits of 

 their influence. The pendulum, for example, has its oscillations 

 disturbed by the vicinity of a mountain ; we remove the pendulum to 

 a sufficient distance from the mountain, and the disturbance ceases: 

 from these data we can determine by the Method of Difference, the 

 amount of effect really due to the mountain; and beyond a certain 



