460 THE PRINCIPLES OF SCIENCE. 



found and original remarks concerning the theory generally. 

 In Lubbock and Drink water's work on ' Probability,' in the 

 Library of Useful Knowledge, we have a very concise but 

 good statement of a number of important problems. The 

 Rev. W. A. Whitworth has given, in an interesting little 

 work entitled ' Choice and Chance,' a number of good illus- 

 trations of the calculations both in the theories of Com- 

 binations and Probabilities. In Mr. Todhunter's admirable 

 History we have an exhaustive critical account of almost all 

 writings upon the subject of probability down, to the cul- 

 mination of the theory in Laplace's works. In spite of the 

 existence of these and some other good English works, there 

 seems to be a want of an easy and yet pretty complete 

 introduction to the study of the theory of probabilities. 



Among French works the ' Traite Elementaire du Cal- 

 cul des Probabilites/ by S. F. Lacroix, of which several 

 editions have been published, and which is not difficult 

 to obtain, forms probably the best elementary treatise. 

 Poisson's ' Eecherches sur la Probabilite des Jugements,' 

 (Paris, 1837), commences with an admirable investigation 

 of the grounds and methods of the theory. While La- 

 place's great ' Theorie Analytique des Probabilites ' is of 

 course the ' Principia' of the subject, his 'Essai Philo- 

 sophique sur les Probabilites ' is a popular discourse, and 

 is one of the most profound and interesting essays ever 

 published. It should be familiar to every student of 

 logical method, and has lost little or none of its import- 

 ance by lapse of time. 



Detection of Constant Errors. 



The Method of Means is absolutely incapable of elimi- 

 nating any error which is always the same, and which 

 always lies in one direction. We sometimes require to be 

 aroused from a false feeling of security, and to be urged 



