THE LAW OF ERROR. 459 



are in any case only approximate, it is usually quite un- 

 necessary to make the computations with any great 

 degree of accuracy, and places of decimals may therefore 

 oe freely cut off to save arithmetical work. The mean 

 equations having been computed, their solution by the 

 ordinary methods of algebra gives the most probable 

 values of the unknown quantities. 



Works upon the Theory of Probability and the Law 



of Error. 



Regarding the Theory of Probability and the Law of 

 Error as constituting, perhaps, the most important subjects 

 of study for any one who desires to obtain a complete 

 comprehension of logical and scientific method as actually 

 applied in physical investigations, I will briefly indicate 

 the works in one or other of which the reader will best 

 pursue the study. 



The best popular, and at the same time profound 

 English work on the subject is De Morgan's ' Essay on 

 Probabilities and on their Application to Life Contin- 

 gencies and Insurance Offices/ published in the ' Cabinet 

 Cyclopaedia,' and to be obtained from Messrs. Longman. 

 No mathematical knowledge beyond that of common 

 arithmetic is required in reading this work. Quetelet's 

 ' Letters,' already often referred to, also form a most inter- 

 esting and excellent popular introduction to the subject, 

 and the mathematical notes are also of value. Sir George 

 Airy's brief treatise ' On the Algebraical and Numerical 

 Theory of Errors of Observation and the Combination of 

 Observations,' contains a complete explanation of the Law of 

 Error and its practical applications. De Morgan's treatise 

 * On the Theory of Probabilities' in the ' Encyclopaedia Me- 

 tropolitana,' presents an abstract of the more abstruse in- 

 vestigations of Laplace, together with a multitude of pro- 



