xv.] ANALYSIS OF QUANTITATIVE PHENOMENA. 339 



Methods of Eliminating Error. 



In any particular experiment it is the object of the ex- 

 perimentalist to measure a single effect only, and he 

 endeavours to obtain that effect free from interfering 

 effects. If this cannot be, as it seldom or never can 

 really be, he makes the effect as considerable as possible 

 compared with the other effects, which he reduces to a 

 minimum, and treats as noxious errors. Those quantities, 

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

 important and interesting phenomena in another investiga- 

 tion. 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 

 irrelevant and interfering causes. 



The general principle 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 ob- 

 servations as there are quantities to be determined. Every 

 complete experiment will therefore consist in general of 

 several operations. Guided if possible by previous know- 

 ledge of the causes in action, we must arrange the deter- 

 minations, so that by a simple mathematical process we 

 may distinguish the separate quantities. There appear to 

 be five principal methods by which we may accomplish 

 this object ; these methods are specified below and illus- 

 trated in the succeeding sectioiis. 



(1) The, Method of Avoidance. The physicist may seek 

 for some special mode of experiment or opportunity of obser- 

 vation, in which the error is non-existent or inappreciable. 



(2) The Differential Method. He may find opportunities 

 of observation when all interfering phenomena remain con- 

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

 present and another time absent ; the difference between 

 two observations then gives its amount. 



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

 estimate the amount of the interfering effect by the best 

 available mode, and then make a corresponding correction 

 in the lesults of observation. 



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