422 Dr. J. R. Mayer on Celestial Dynamics. 



astronomers be compared with those of our present time, the 

 least alteration in the absolute length of a day may be detected 

 by a change in this ratio, or in a disturbance in the lunar revo- 

 lution. The most perfect agreement of ancient records on the 

 movements of the moon and the planets, on the eclipses of the 

 sun, &c, revealed to Laplace the remarkable fact that in the 

 course of 25 centuries, the time in which our earth revolves on 

 its axis has not altered j ^th part of a sexagesimal second ; and 

 the length of a day therefore may be considered to have been 

 constant during; historic times. 



This result, as important as it was convenient for astronomy, 

 was nevertheless of a nature to create some difficulties for the 

 physicist. With apparently good reason it was concluded that, 

 if the velocity of rotation had remained constant, the volume of 

 the earth could have undergone no change. The earth completes 

 one revolution on its axis in 86,400 sidereal seconds ; it conse- 

 quently appears, if this time has not altered during 2500 years 

 to the extent of jj^th of a second, or 43 q 000 th part of a day, 

 that during this long space of time the radius of the earth also 

 cannot have altered more than this fraction of its length. The 

 earth's radius measures 6,369,800 metres, and therefore its 

 length ought not to have diminished more than 15 centimetres 

 in 25 centuries. 



The diminution in volume, as a result of the cooling-process, 

 is, however, closely connected with the changes on the earth's sur- 

 face. When we consider that scarcely a day passes without the 

 occurrence of an earthquake or shock in one place or another, and 

 that of the 300 active volcanos some are always in action, it would 

 appear that such a lively reaction of the interior of the earth 

 against the crust is incompatible with the constancy of its 

 volume. 



This apparent discrepancy between Cordier's theory of the 

 connexion between the cooling of the earth and the reaction of 

 the interior on the exterior parts, and Laplace's calculation 

 showing the constancy of the length of the day, a calculation 

 which is undoubtedly correct, has induced most scientific men 

 to abandon Cordier's theory, and thus to deprive themselves of 

 any tenable explanation of volcanic activity. 



The continued cooling of the earth cannot be denied, for it 

 takes place according to the laws of nature ; in this respect the 

 earth cannot comport itself differently from any other mass, 

 however small it be. In spite of the heat which it receives 

 from the sun, the earth will have a tendency to cool so long as 

 the temperature of its interior is higher than the mean tem- 

 perature of its surface. Between the tropics the mean tempe- 

 rature produced by the sun is about 28°, and the sun therefore 



