run FOUR EXPERIMENTAL METHODS, 233 



The metliotl by whicl'i tliese rosults were oLtaijied^ may he tevmcd 

 the Method of Concomitant Variations : it is regulated by the I'ollow- 

 " ing canon : — 



Fifth Canon. 



JV/iatcver inlienoinenon varies in any manner wJienever another 

 phenotncnoji varies in some particu/ar manner, is cither a cause or an 

 effect of that plienomcnon, or is connected with it tJirous^h some fact 

 of causation. 



The last clause is subjoined, because it by no^means follows when 

 two phenomena accompany each other in their variations, that the 

 one is cause and the other effect. The same thing may, atid indeed 

 must ha^^pen, supposing them to be two diHerent efi'ects of a common 

 cause : and by this method alone it would never be possible to ascer- 

 tain which of the two suppositions is the true one. The only way to 

 solve the doubt would be that which we have so often adverted to, 

 viz., -by endeavoring to ascertain whether we can produce the or;e 

 set of variations by means of the .other. In the case of heat, for 

 example, by increasing the temperature of a body we increase its 

 bulk, but by increasing its bulk we do not increase its temperature ; 

 ou the contrary (as in the rarefaction of air under the receiver of an 

 air-pump), we generally diminish it : therefore heat is not an effect, 

 but a cause, of increase of bulk. If we cannot ourselves produce 

 the variations, \ye must endeavor, though it is an attempt which is 

 seldom successful, to find them produced by nature in some case 

 in which the preexisting circumstances are perfectly known to us. 



It is scarcely necessary to say, that in order to ascertain the uniform 

 concomitance of variations in the effect Avith variations in the caiuse, the 

 same precautions must be used as in any other case of the determina- 

 tion of an invariable sequence. We must endeavor to retain all the 

 other antecedents unclianged, while that particular one is subjected to 

 the requisite series of variations ; or in other words, that we may be 

 warranted in inferring causation from concomitance of variations, the 

 concomitance itself must be proved by the Method of Difference. 



It might at first appear that the Method of Concomitant Variations 

 assumes a new axiom, or law of causation in generp^l, namely, that 

 every modification of the cause is followed by a change in the effect. 

 And it docs usually happen that when a phenomenon A causes a phe- 

 nomenon a, any variation in the quantity or in the various relations of 

 A, is unifoi-mly followed by a variation in the quantity or relations of 

 a. To take a familiar instance, that of gi-avitation. The sun causes a 

 certain tendency to motion in the earth ; here we have cause and effect; 

 but that tendency is towards the sun, and therefore varies in direction 

 as the sun varies in the relation of position; and moreover the tendency 

 varies in intensity, in a certain numerical ratio to the sun's distance 

 fi'om the earth, that is, according to another relation of the sun. 

 Thus we see that there is not only an invariable connexion between 

 the sun and the earth's gi'avitation, but that two of the relations of the 

 sun, its position with re,-4pect to the earth and its distance from the 

 earth, are invariably connected as antecedents with the quantity and 

 direction of the earth's gravitation. The cause of the egp-th's gravita- 

 G G 



