THE FOUR EXPERIMENTAL METHODS. 285 



arately the causes A and B, and to infer from the effects which they pro- 

 duce 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 negative 

 one, or that in which the given phenomenon is absent, is not the dii'ect re- 

 sult 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 obtained by the same infallible 

 method, and provided we are certain that C is the only antecedent to 

 which the residual phenomenon c can be referred; the only agent of which 

 M^e had not already calculated and subducted the effect. But as we can 

 never be quite certain of this, the evidence derived from the Method of 

 Residues is not complete unless we can obtain C artificially, and try it sep- 

 arately, or unless its agency, when once suggested, can be accounted for, 

 and proved deductively from known 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 in- 

 vestigating laws of nature, this is the most fertile in unexpected results : 

 often informing us of sequences in which neither the cause nor the effect 

 were sufficiently conspicuous to attract of themselves the attention of ob- 

 servers. 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 in- 

 termixture with a and 5, that it would scarcely have presented itself spon- 

 taneously 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 Canon. 



Suhduct from any phenomenon such part as is known by previous in- 

 ductions to be the effect of certain antecedents, and the residue of the phe- 

 nomenon 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 indestructible natural 

 agents, which it is impossible either to exclude or to isolate ; which we can 

 neither hinder from being present, nor contrive that they shall be present 

 alone. It would appear at first sight that we could by no means separate 

 the effects of these agents from the effects of those other phenomena with 

 which they can not be prevented from co-existing. In respect, indeed, to 

 most of the permanent causes, no such difficulty exists ; since, though we 

 can not eliminate them as co-existing facts, we can eliminate them as influ- 

 encing agents, by simply trying our experiment in a local situation beyond 

 the limits of their influence. The pendulum, for example, has its oscilla- 

 tions 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 ef- 

 fect due to the mountain ; and beyond a certain distance every thing goes 

 on precisely as it Avould do if the mountain exercised no influence what- 

 ever, which, accordingly, we, with sufficient reason, conclude to be the fact. 



