392 THE PRINCIPLES OF SCIENCE. 



which are called errors in one case, may really be most 

 important and interesting phenomena in another inves- 

 tigation. When we speak of eliminating error we really 

 mean disentangling the complicated phenomena of nature. 

 The physicist rightly wishes to treat one thing at a time, 

 but as this object can seldom be rigorously carried into 

 practice, he has to seek some mode of counteracting the 

 tendency to error. 



The general principle of the subject is that a single 

 observation can render known only a single quantity. 

 Hence if several different quantitative effects are known 

 to enter into any investigation, we must have at least 

 as many distinct results of observation as there are 

 quantities to be determined. Every complete experiment 

 will therefore consist in general of several operations. 

 Guided if possible by previous knowledge of the causes 

 in action, we must arrange these determinations, so that 

 by a simple mathematical process we may distinguish the 

 separate quantities. There appear to be five principal 

 methods in which we may accomplish this object ; these 

 methods are specified below and illustrated in the suc- 

 ceeding sections. 



1 i ) The Method of Avoidance. The physicist may seek 

 for some special mode of experiment or opportunity of 

 observation, in which the error is non-existent or inap- 

 preciable. 



(2) The Differential Method. He may find opportunities 

 of observation when all interfering phenomena remain 

 constant, and only the subject of observation is at one time 

 present and another time absent ; the difference between 

 two exact observations then gives its amount. 



(3) TJie Method of Correction. He may endeavour to 

 estimate the amount of the interfering force by the best 

 available mode, and then make a corresponding correction 

 in the results of observation. 



