THE FOUR EXPERIMENTAL METHODS. 465 



tions, 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 we 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 separately, 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 

 investigating 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 suffi- 

 ciently conspicuous to attract of themselves the atten- 

 tion 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 &, 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 CANON. 



Subduct from any phenomenon such part as is known 

 by previous inductions to be the effect of certain antece- 

 dents, and the residue of the phenomenon is the effect of 

 the remaining antecedents. 



VOL. I. 2 H 



