D'ALEMBERT. 401 



tion, or too much depending upon various circumstances 

 that furnish no precise data, to be capable of analytical 

 investigation. The Memoir consists of three parts. In 

 the first he calculates the oscillations caused by the 

 two heavenly bodies supposing them at rest, or the 

 earth at rest in respect of them. In the second, he 

 investigates their operation on the supposition of their 

 motion. In the third, he endeavours to trace the effects 

 produced upon the oscillations by terrestrial objects. 

 The paper is closed with remarks upon the effects of 

 temperature. The whole inquiry is conducted with 

 reference to the general dynamical principle which he 

 had so happily applied to the equilibrium and pressure 

 of fluids, in his first work upon that difficult subject. 



The subject of fluids was, perhaps, the one which 

 most occupied D'Alembert's attention, and for the 

 greatest number of years. His ' Opuscula' contain 

 several interesting tracts upon its various depart- 

 ments, especially the first and fifth volumes, which 

 were published in 1761 and 1768 respectively. But 

 above half the eighth volume relates to the same sub- 

 ject, and it appeared as late as 1780, so that this 

 inquiry had retained its hold on his mind for a period 

 of nearly forty years.* 



* The readers of D'Alembert's papers on these subjects will have real 

 obligations to Bossut, if they read with D'Alembert that great didactic 

 writer's admirable treatise, ' Hydrodynamique,' second edition. He was 

 an intimate friend and, indeed, may be said to have been a pupil, of 

 D'Alembert and of Condorcet. His ' Calcul Integral et Differentiel,' is 

 also a truly excellent and useful work. Of the four great elementary 

 treatises on this subject, Lacroix's, Bougainville's, Cousin's, and Bossut's, 

 the last appears to me the best ; but I am aware of the high opinion which 

 D'Alembert entertained of Bougainville's. He was accustomed to refer to 

 Bossut those who applied to him for explanations of his writings, as New- 

 ton did to Demoivre Why, may it be permitted us respectfully to ask, 



why will so many mathematicians fancy it beneath them to write clearly, 

 simply, and, as didactic matter should be written, intelligibly and always 

 proceeding from what is known and explained to what is not, without an- 

 ticipation? Surely Bossut was as great a geometrician as themselves, 

 and he condescended to write as if he were teaching and not commenting, 

 alluding, or referring. 



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