CHAPTER XX. 



METHOD OF VARIATIONS. 



EXPERIMENTS may be of two kinds : experiments of 

 simple fact, and experiments of quantity. In the first 

 class of experiments we combine certain conditions, and I 

 wish to ascertain whether or not a certain effect of any 

 quantity exists. Thus Hooke, as before described, wished 

 to ascertain whether or not there was any difference in the 

 force of gravity at the top and bottom of St. Paul's Cathe- 

 dral. The chemist continually performs analyses for the 

 purpose of ascertaining whether or not a given element 

 exists in a particular mineral or mixture ; ah 1 such experi- 

 ments and analyses are qualitative rather than quantita- 

 tive, because though the result may be more or less, and is 

 necessarily quantitative, the particular amount of the result 

 is not the immediate object of the enquiry. 



So soon, however, as a result is known to be dis- 

 coverable, the scientific man ought to proceed to the 

 strictly quantitative enquiry, how great a result follows 

 from a certain amount of the conditions which are sup- 

 posed to constitute the aggregate cause ? The possible 

 numbers of experiments are now indefinitely great, for 

 every variation in a necessary condition will usually pro- 

 duce 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 experiment to 

 phenomena capable of continuous quantity. As Professor 



