On the Figure of the Earth. 375 



Toid, is equal to the product of that depth, by the elevation of 

 the temperature of the surface of the earth, above the state of 

 which I have just spoken, and l)y a factor independent of the di- 

 mensions 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 in- 

 crease of heat would be very sensible at depths to which we have 

 penetrated, and where nevertheless it has not been found. 



Note added hy the Editor of Annales de Chimie, &c. 



Our readers, we think, will not be displeased to find here 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 revo- 

 lution of the earth on its own axis, increased by the mean appa- 

 rent motion df 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 

 onlv 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, for example, its conjunction, 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 suffering 

 any alteration. From hence results a very simple method of dis- 

 covering if the duration of the solar day has changed. 



Suppose the duration of a lunar month to be now ascertained 

 by direct observation ; that is, how m.any days, and fractions of 

 days, the moon occupies in returning to its conjunction with the 

 sun. It is evident that on repeating this observation at another 

 time, a different result will be found, if the length of the day has 

 changed, if at the same time the velocity of the moon has not 

 changed. The month will appear longer, if the length of the 

 day has diminished ; and on the contrary, shorter, if the day has 

 increased. The constancy of the lunar month will indicate the 

 invariability of the length of the day. 



All observations combine to prove that from the time of the 

 Chaldeans, to our own days, the duration of the lunar month has 

 been gradually diminishing. It follows, therefore, from what has 

 been stated, cither that the velocity of the moon has increased, 

 or that the solar day has lengthened. But M. de Laplace has 

 discovered by theory, that there is in t!ie motion of the moon, 

 an inc(juality known by the name of secular ecjuatlon, which de- 

 pends on the variation of the excentricity of the earth's orbit, 

 and of which the value in each century may be deduced from 

 the change of this cxccntricitv. By the assistance of this eciua- 

 Aa4 tion. 



