466 D'ALEMBEET. 



certain measure of its length and surface. "When we 

 affirm that a body moves in the diagonal when solicited 

 by two impulses along the two sides of a parallelogram, 

 we assume, not merely that there is a body and that 

 there is motion, but that the body has certain qualities 

 and that motion has certain laws, and these are facts 

 which exist, not mere suppositions which we make. 

 D'Alembert has only the merit, and a great one it is, 

 of having, first in his ' Dynainique ' and afterwards in 

 his 'Elemens/ reduced the whole laws of motion and 

 equilibrium to the fewest and simplest possible funda- 

 mental principles, and therefore generalized those prin- 

 ciples. 



All D'Alembert's writings have now passed under our 

 review : it remains to form a more general estimate of his 

 merits in the two capacities with a detailed view of 

 which we have been occupied, his merits as a man of 

 science and a man of letters. And certainly the differ- 

 ence is very wide betM'een his position in these two 

 different classes ; nor can I avoid marvelling, with Sir J. 

 Mackintosh, at the partiality which so far blinded Mr. 

 Stewart, as to make him consider him very eminent in both. 



Among mathematicians he holds a high place indeed, 

 ranking on the very first line. Euler was perhaps a 

 more fertile analyst ; and he gave incomparably greater 

 contributions to the science, than either D'Alembert or 

 indeed any other man. Clairaut was excelled by none 

 in the profoundness of his researches, and the originality 

 of his methods, and he excelled all others in the marvel- 

 lous precocity of his genius as a geometrician. At the 

 same time, we can never forget that D'Alembert's dis- 

 covery of the dynamical theorem, and his most felicitous 



