DEVELOPMENT OF MATHEMATICAL THOUGHT. 695 



verse of differentiation, — led early to investigations of 

 the kind just mentioned. The experience that ordinary 

 fractions might be expressed by decimal fractions — i.e., 

 by finite or infinite series — led to the inverse problem 

 of finding the smn of such series and many other an- 

 swerable and apparently unanswerable problems. The 

 older method of research consisted in treating these 

 problems when and as they arose : new chapters were 

 accordingly added to the existing chapters of the 

 text -books, dealing with special functions or mathe- 

 matical expressions. It was only towards the end of 

 the eighteenth century, and at the beginning of the 

 nineteenth, that Lagrange, Gauss, and Cauchy felt and 

 proclaimed the necessity of attacking the question gener- 

 ally and systematically ; the labours of Euler having 

 accumulated an enormous mass of analytical knowledge, 

 a great array of useful formulte, and amongst them not 

 a few paradoxes which demanded special attention. I 

 have already had occasion to refer to the problem of 

 the general solution of equations as an instance where, 

 in the hands of Abel, the tentative and highly ingenious 

 attempts of earlier analysts were replaced by a method- 

 ical and general treatment of the whole question. An- 

 other chapter of higher mathematics, the investigation of 

 expressions which presented themselves in the problems 

 of finding the length of the arc of an ellipse, and which 

 opened the view into the large province of the so-called 

 higher transcendents, gave Abel further occasion of lay- 

 ing new foundations and of creating a general theory of 

 equations or of forms. 



But yet another interest operated powerfully in the 



