FEBRVAEy 8, 1895.] 



SCIENCE. 



149 



others are those of Laplace and Poisson. 

 It was the life-work of Laplace to deduce 

 the consequences of the law of gravitation 

 as applied to the solar system. No problem 

 of equal magnitude has ever been attacked 

 and treated single-handed with such con- 

 sunmiate skill and success as sho^\ni by La- 

 place in his .l/c'wnuV/i/e Celeste. The five vol- 

 umes of this work, together with the popular 

 exposition contained in his Systhne clu Maude, 

 constitute, I think, the greatest systematic 

 treatise ever written. Think, for a moment, 

 of the mental eciuipment essential to begin 

 such an investigation. Copernicus and 

 Kepler had discovered by observation the 

 salient features of the motions of the planets 

 about the sun. Newton showed that these 

 features were immediately and easil}' de- 

 rived results of the law of gra^^tation. But 

 these were the salient features only. Had 

 our planet been the sole one of the sj-stem, 

 had it been moonless and devoid of rotation, 

 the task of Laplace would have been easy. 

 But instead of a single planet, there is a 

 crowd of them, each rotating on its axis 

 while traveling about the sun, and most of 

 them accompanied bj^ lunar attendants. 

 When this array of facts is considered, the 

 simple law of gn-avitation leads to gi-eat 

 complication. The motion of our planet at 

 any time depends not only on its position 

 relatively to the sun, but on its position 

 relatively to the m^ighboring planets. Our 

 moon also plays an important role in the 

 motions of the earth. By reason of these 

 interactions the earth's axis of rotation, 

 whicli is the principle line of reference 

 for astronomical observations, pursues a de- 

 vious course in the lieavens. Add to these 

 difficulties those arising from the facts that 

 our {)lanet is surrounded by an atmosphere 

 which prevents us from ol)serving our true 

 relative position, and that light travels with 

 a finite tliough gi-eat speed, and the mag- 

 nitude of the ta.sk Laplace set for himself 

 is in some degree apparent. A complete 



mastery of every branch of the mathematics 

 and physics of his day and a capacity to en- 

 large the boundaries of either were the in- 

 dispen.sable prerequisites, which, .sujiple- 

 meuted by a boundless genius for industry, 

 enabled him to make dj'namical astronomy 

 the most perfect of the aj^plied sciences. 

 His conception of the magnitude and im- 

 portance of the work he undertook is clearly 

 but modestly set forth in the preface to the 

 Mccaniqiie Celeste. " Astronomy," he says, 

 " considered in the most general mannei- is a 

 grand problem of mechanics, whose solution 

 depends on the precision of observations and 

 on the perfection of mathematical analysis. 

 It is extremelj' desirable to avoid all em- 

 pu'icism in our treatment of this problem 

 and to draw on observation for indispensable 

 data only. The present work is destined to 

 accomplish, as far as I am able, this inter- 

 esting object. I trust that, in consideration 

 of the difficulties of the subject, mathema- 

 ticians and astronomers will receive the 

 work with indulgence." 



Not less important than the contributions 

 of Lagrange and Laplace to pure mechanics 

 and dynamical astronomy were the volumi- 

 nous and luminous wTitings of Poisson dur- 

 ing the same periotl. Equally at home with 

 Lagrange and Laplace in their favorite re- 

 searches, man}' of which he corrected and 

 extended, he explored the additional fields 

 of heat, light, elasticity, electricity, and mag- 

 netism. To his penetrating insight into 

 these abstruse subjects and to the wealth of 

 analytical resources he developed are due 

 more than to any other single source the 

 subsequent developments of mathematical 

 physics, by which is meant the application 

 of mechanics to physical questions. His 

 discoveries and researches are scarcely less 

 brilliant than tho.se of his two eminent con- 

 temporaries, while he outstripped both of 

 them in his range and grasp of matliematical 

 and physical principles. Moreover, he 

 was the prince of expositors of mathematic-jxl 



