CHAPTER XX. 



METHOD OF VARIATIONS. 



EXPERIMENTS may be of two kinds, experiments of 

 simple foot, and experiments of quantity. In the first 

 class of experiments we combine certain conditions, and 

 wish to ascertain whether or not a certain effect of any 

 quantity exists. Hooke wished to ascertain whether or 

 not there was any difference in the force of gravity at the 

 top and bottom of St. Paul's Cathedral. The chemist 

 continually performs analyses for the purpose of ascertaining 

 whether or not a given element exists in a particular mi- 

 neral or mixture; all such experiments and analyses are 

 qualitative rather than quantitative, because though the 

 result may be more or less, the particular amount of the 

 result is not the object of the inquiry. 



So soon, however, as a result is known to be discoverable, 

 the scientific man ought to proceed to the quantitative 

 inquiry, how great a result follows from a certain amount 

 of the conditions which are supposed to constitute the 

 cause ? The possible numbers of experiments are now in- 

 finitely great, for every variation in a quantitative condition 

 will usually produce a variation in the amount of the effect. 

 The method of variation which thus arises is no narrow or 

 special method, but it is the general application of experi- 

 ment to phenomena capable of continuous variation. As 

 Mr. Fowler has well remarked, 1 the observation of variations 

 is really an integration of a supposed infinite number of 

 applications of the so-called method of difference, that is 

 * sxperiment in its perfect form. 



1 Elements of Inductive liogic^ ist edit. p. 175. 



