On the Figure of the Earth . 113 



radius of that spheroid/is equal to the product of that depth, by 

 the elevation of the temperature of the surface of the earth, above 

 the state of vjrhich I have just spoken, and by a factor indepen- 

 dentof the dimensions of the earth, and which depends only on the 

 qualities of its first stratum relative to heat. From what we 

 know of these qualities we find that if this elevation was many 

 degrees, the increase of heat would be very sensible at depths to 

 which we have penetrated and where nevertheless it has not 

 been observed. 



Note by the Editor of the Annates de Chimie, Sfc. 



We have thought that our readers would not be displeased to 

 meet here with some details of the method by which M. de 

 Laplace, has established the invariability of the duration of the 

 day. 



A mean solar day, is equal to the time occupied by one revolu- 

 tion of the earth on its own axis, increased by the mean apparent 

 motion of the sun, in the same interval. Theory has proved 

 that the mean apparent motion of the sun, like that of all the 

 planets, is constant; the duration of a solar day, therefore 

 can only vary by a change in the velocity of the rotation of the 

 earth. 



The time in which the moon returns to the same position, re- 

 lative to the sun, its conjunction for instance, is called a lunar 

 month. This interval is evidently independent of the velocity 

 of the earth's rotation. Our globe might even cease to turn on its 

 centre, without the moon's advancement in its orbit suffer- 

 ing any alteration. From hence results a very simple method 

 of discovering if the duration of the solar day has changed. 



Suppose that at present, the duration of a lunar month be as- 

 certained by direct observation ; that is, how many days, and 

 fractions of day, the moon occupies in returning to its conjunc- 

 tion with the sun. It is evident that on repeating this observa- 

 tion at another time, a different result will be found, if the length 

 of the day has clianged, if at the same time, the velocity of the 

 moon has not clianged. Tlie month will appear longer, if Ihe 

 length of the day has diminished; and on the contrary, shorter, 



Vol. VIII. I 



