xvn.] THE LAW OF EKROK. 395 



basis of probability and of related questions concerning 

 causation, belief, design, testimony, &c. ; but I cannot 

 always agree with Mr. Venn's opinions. No mathematical 

 knowledge beyond that of common arithmetic is required 

 in reading these works. Quetelet's Letters form a good 

 introduction to the subject, and the mathematical notes 

 are of value. Sir George Airy's brief treatise On the 

 Algebraical and Numerical Theory of Errors of Observa- 

 tions and the Combination of Observations, contains a 

 complete explanation of the Law of Error and its prac- 

 tical applications. De Morgan's treatise " On the Theory 

 of Probabilities" ic the Encyclopaedia Metropolitana, 

 presents an abstract of the more abstruse investigations 

 of Laplace, together with a multitude of profound and 

 original remarks concerning the theory generally. In 

 Lubbock and Drinkwater's work on Probability, in the 

 Library of Useful Knowledge, we have a concise but 

 good statement of a number of important problems. The 

 Rev. W. A. "Whitworth has given, in a work entitled 

 Choice and Chance, a number of good illustrations of 

 calculations both in combinations 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 culmination of the theory in 

 Laplace's works. The Memoir of Mr. J. W. L. Glaisher 

 has already been mentioned (p. 375). 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 com- 

 plete mathematical introduction to the study of the theory. 

 Among French works the Traite EUmentaire du Calcul 

 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 

 Recherches sur la Probabilite des Jugements (Paris 1837), 

 commence with an admirable investigation of the grounds 

 and methods of the theory. While Laplace's great Theorie 

 Analytique des Probability is of course the " Principia " 

 of the subject ; his Essai Philosophique sur les Probability 

 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 importance by lapse of time. 



