150 LAPLACE. 



coiiiiiiry, lodiico the axis to a pormanent condition iu C(»)sequence of 

 tlie mobility of the waters and the resistance which their osciUations 

 experience. 



The remarks which I have just made with respect to the position of 

 the terrestrial axis are equally applicable to the time of the earth's ro- 

 tation, which is the unit, the true standard of time. The importance of 

 this element induced Laplace to exaftiine whether its numerical value 

 might not be liable to vary from internal causes, such as earthquakes 

 and volcanoes. It is hardly necessary for me to state that the result 

 obtained was negative. 



The admirable memoir of Lagrange upon the libratiou of the moon 

 seemed to have exhausted the subject. This, however, was not the 

 case. 



Tlie motion of revolution of our satellite around the earth is subject 

 to perturbations, technically teimed secular, which were either unknown 

 to Lagrange or which he neglected. Tbese inequalities eventually phice 

 the body, not to speak of entire circumferences, at angular distances of 

 a semicircle, a circle and a half, &c.. from the position which it would 

 otherwise occupy. If the movement of rotation did not participate in 

 such perturbations, the moon in the lapse* of ages would present iu suc- 

 cession all the parts of its surface to the earth. 



This event wdl not occur. The hemisphere of the moon which is 

 actually invisible will remain invisible forever. Laplace, in fact, has 

 shown that the attraction of the earth introduces into the rotatory mo- 

 tion of the lunar s[)lieroid the secular inequalities which exist in the 

 movement of revolution. 



liesearches of this nature exhibit in full relief the power of mathemat- 

 ical analysis. It would have been very difiicult to have discovered by 

 synthesis truths so profoundly enveloped in the complex action of a 

 multitude of forces. 



We should be inexcusable if we omitted to notice the high impor- 

 tance of the labors of Laplace on the improvement of the lunar tables. 

 The immediate object of this improvement was, in effect, the [)romo 

 tion of maritime intercourse between distant countries; and, what was 

 indeed far sui)erior to all considerations of mercantile interest, th(! pres- 

 ervation of the lives of mariners. 



Thanks to a sagacity without parallel, to a perseverance which knew 

 no limits, to an ardor always youthful, and which communicated itself 

 to able co-adjutors, La[)lace solved the celebrated problem of the longi- 

 tude more comi)letely than could have been ho[)ed for in a scientilic 

 poiut of view, with greater precision than the art of navigation, in its 

 utmost relinement, demanded. The ship, the sport of the winds and 

 tempests, has no occasion, in the present day, to be afraid of losing 

 itself in the immensity of the ocean. An intelligent glance at the starry 

 vault indicates to the pilot, iu every place and at every time, his dis- 

 tance from the meridian of Paris. The extreme perfection of the ex- 



