THE FOUR EXPERIMENTAL METHODS. 



263 



fancied by some, to a fixed point of 

 space. 



The method by which these re- 

 su^s were obtained may be termed 

 the Method of Concomitant Varia- 

 tions : it is regulated by the following 

 canor. : — 



Fifth Canon. 

 Whaiever phenomenon varies in any 

 manner whenever another phenomenon 

 varies i% some particular manner, is 

 either a cause or an effect of that pheno- 

 menon, 07 is connected with it through 

 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, and indeed must happen, sup- 

 posing them to be two different effects 

 of a common cause : and by this 

 method alone it would never be pos- 

 sible to ascertain which of the sup- 

 positions is the true one. The only 

 way to solve the doubt would be that 

 which we have so often adverted to, 

 viz. by endeavouring to ascertain 

 whether we can produce the one set 

 of variations by means of the other. 

 In the case of heat, for example, bj' 

 increasing the temperature of a body 

 we increase its bulk, but by increas- 

 ing its bulk we do not increase its 

 temperature ; on the contrary, (as in 

 the rarefaction of air under the re- 

 ceiver 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, we must endeavour, though 

 it is an attempt which is seldom suc- 

 cessful, to find them produced by 

 nature in some case in which the pre- 

 existing circumstances are perfectly 

 known to us. 



It is scarcely necessary to say, that 

 in order to ascertain the uniform con- 

 comitants of variations in the effect 

 with variations in the cause, the same 

 precautions must be used as in any 

 other case of the determination of 

 an invariable sequence. We must 



endeavour to i-etain all the other 

 antecedents unchanged, while that 

 particular one is subjected to the 

 requisite series of variations ; or, in 

 other words, that we may be war- 

 ranted in inferring causation from 

 concomitance of variations, the con- 

 comitance 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 general, namely, that 

 every modification of the cause is 

 followed by a change in the effect. 

 And it does usually happen that when 

 a phenomenon A causes a phenomenon 

 a, any variation in the quantity or in 

 the various relations of A is uniformly 

 followed by a variation in the quan- 

 tity or relations of a. To take a 

 familiar instance, that of gravitation. 

 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 more- 

 over the tendency varies in intensity, 

 in a certain numerical correspondence 

 to the sun's distance from the earth, 

 that is, according to another relation 

 of the sun. Thus we see that there 

 is not only an invariable connection 

 between the sun and the earth's 

 gravitation, but that two of the rela- 

 tions of the sun, its position with 

 respect to the earth and its distance 

 from the earth, are invariably con- 

 nected as antecedents with the quan- 

 tity and direction of the earth's gravi- 

 tation. The cause of the earth's 

 gravitating at aU is simply the sun ; 

 but the cause of its gravitating with 

 a given intensity and in a given 

 direction is the existence of the sun 

 in a given direction and at a given 

 distance. It is not strange that a 

 modified cause, which is in truth a 

 different cause, should produce a dif- 

 ferent effect. 



Although it is for the most part 

 true that a modification of the cause 

 is followed by a modification of the 



