THE FOUR EXPERIMENTAL METHODS. 



261 



Vented from co-existing. In respect, 

 indeed, to most of the permanent 

 causes, no such difficulty exists ; since, 

 though wecannoteliminate them as co- 

 existing facts, we can eliminate them 

 as influencing agents, by simply trying 

 our experiment in a local situation 

 beyond the limits of their influence. 

 The pendulum, for example, has its 

 oscillations disturbed by the vicinity 

 of a mountain : we remove the pen- 

 dulum to a sufficient distance from 

 the mountain, and the disturbance 

 ceases : from these data we can de- 

 termine by the Method of Difference 

 the amount of effect due to the moun- 

 tain ; and beyond a certain distance 

 everything goes on precisely as it 

 would do if the mountain exercised 

 no influence whatever, which, accord- 

 ingly, we, with sufficient reason, con- 

 clude to be the fact. 



The difficulty, therefore, in apply- 

 ing the methods already treated of to 

 determine the effects of Permanent 

 Causes, is confined to the cases in 

 which it is impossible for us to get 

 out of the local limits of their influ- 

 ence. The pendulum can be removed 

 from the influence of the mountain, 

 but it cannot be removed from the 

 influence of the earth : we cannot 

 take away the earth from the pen- 

 dulum, nor the pendulum from the 

 earth, to ascertain whether it would 

 continue to vibrate if the action which 

 the earth exerts upon it were with- 

 drawn. On what evidence, then, do 

 we ascribe its vibrations to the earth's 

 influence? Not on any sanctioned 

 by the Method of Difference ; for 

 one of the two instances, the negative 

 instance, is wanting. Nor by the 

 Method of Agreement ; for though 

 all pendulums agree in this, that dur- 

 ing their oscillations the earth is al- 

 ways present, why may we not as 

 well ascribe the phenomenon to the 

 sun, which is equally a co-existent 

 fact in all the experiments? It is 

 evident that to establish even so simple 

 a fact of causation as this, there was 

 required some method over and above 

 those which we have yet examined. 



As another example, let us take 

 the phenomenon Heat. Indepen- 

 dently of all hypothesis as to the real 

 nature of the agency so called, this 

 fact is certain, that we are unable 

 to exhaust any body of the whole of 

 its heat. It is equally certain that 

 no one ever perceived heat not eman- 

 ating from a body. Being unable, 

 then, to separate Body and Heat, we 

 cannot effect such a variation of cir- 

 cumstances as the foregoing three 

 methods require ; we cannot ascer- 

 tain, by those methods, what portion 

 of the phenomena exhibited by any 

 body is due to the heat contained in 

 it. If we could observe a body with 

 its heat, and the same body entirely 

 divested of heat, the Method of 

 Difference would show the effect due 

 to the heat, apart from that due to 

 the body. If we could observe heat 

 under circumstances agreeing in no- 

 thing but heat, and therefore not char- 

 acterised also by the presence of a 

 bod}', we could ascertain the effects 

 ci heat, from an instance of heat with 

 p body and an instance of heat with- 

 out a body, by the Method of Agree- 

 ment ; or we could determine by the 

 Method of Difference what effect was 

 due to the body, when the remainder 

 which was due to the heat would be 

 given by the Method of Residues. 

 But we can do none of these things ; 

 and without them the application of 

 any of the three methods to the solu- 

 tion of this problem would be illusory. 

 It would be idle, for instance, to at- 

 tempt to ascertain the effect of heat 

 by subtracting from the phenomena 

 exhibited by a body all that is due 

 to its other properties ; for as we 

 have never been able to observe any 

 bodies without a portion of heat in 

 them, effects due to that heat might 

 form a part of the very results which 

 we were affecting to subtract in order 

 that the effect of heat might be shown 

 by the residue. 



If, therefore, there were no other 

 methods of experimental investiga- 

 tion than these three, we should be 

 unable to determine the effects due 



