THE FOUR EXPERIMENTAL METHODS. 289 



power over the fortunes of mankind, it is implied in the very terms that 

 the conjunctions or oppositions of different stars can have no such power. 

 Although the most striking applications of the Method of Concomitant 

 Variations take place in the cases in which the Method of Difference, 

 strictly so called, is impossible, its use is not confined to those cases ; it \^ 

 may often usefully follow after the Method of Difference, to give addition- 7|^ 

 al precision to a solution which that has found. When by the Method of *" 

 Difference it has first been ascertained that a certain object produces a 

 certain effect, the Method of Concomitant Variations may be usefully call- 

 ed in, to determine according to what law the quantity or the different re- 

 lations of the effect follow those of the cause. 



§ Y. The case in which this method admits of the most extensive em- 

 ployment, is that in which the variations of the cause are variations of 

 quantity. Of such variations we may in general affirm with safety, that 

 they will be attended not only with variations, but with similar variations, 

 of the effect : the proposition that more of the cause is followed by more 

 of the effect, being a corollary from the principle of the Composition of 

 Causes, which, as we have seen, is the general rule of causation ; cases of 

 the opposite description, in which causes change their properties on being 

 conjoined with one another, being, on the contrary, special and exceptional. 

 Suppose, then, that when A changes in quantity, a also changes in quantity, 

 and in such a manner that we can trace the numerical relation which the 

 changes of the one bear to such changes of the other as take place within 

 our limits of observation. We may then, with certain precautions, safely 

 conclude that the same numerical relation will hold beyond those limits. 

 If, for instance, we find that when A is double, a is double ; that when A is 

 treble or quadruple, a is treble or quadruple ; we may conclude that if A 

 were a half or a third, a would be a half or a third, and finally, that if A 

 were annihilated, a would be annihilated ; and that a is wholly the effect of 

 A, or wholly the effect of the same cause with A. And so with any other 

 numerical relation according to which A and a would vanish simultaneous- 

 ly ; as, for instance, if a were proportional to the square of A. If, on the 

 other hand, a is not wholly the effect of A, but yet varies when A varies, it 

 is probably a mathematical function not of A alone, but of A and something 

 else : its changes, for example, may be such as would occur if part of it re- 

 mained constant, or varied on some other principle, and the remainder va- 

 ried in some numerical relations to the variations of A. In that case, when 

 A diminishes, a will be seen to approach not toward zero, but toward some 

 other limit ; and when the series of variations is such as to indicate what 

 that limit is, if constant, or the law of its variation, if variable, the limit 

 will exactly measure how much of a is the effect of some other and inde- 

 pendent cause, and the remainder will be the effect of A (or of the cause 

 of A). 



These conclusions, however, must not be drawn without certain precau- ;^^ 

 tions. In the fii'st place, the possibility of drawing them at all, manifestly ,/i 

 supposes that we are acquainted not only with the variations, but with the ^, ^^u 

 absolute quantities both of A and a. If we do not know the total quan- , 

 titles, we can not, of course, determine the real numerical relation according . 

 to which those quantities vary. It is, therefore, an error to conclude, as 

 some have concluded, that because increase of heat expands bodies, that 

 is, increases the distance between their particles, therefore the distance is 

 wholly the effect of heat, and that if we could entirely exhaust the body of 



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