Dr. J. R. Mayer on Celestial Dynamics. 427 



a day, however, has remained constant, the cooling effect of 

 the earth during the same period of time must have shortened 

 the day yg-th °f a second. A diminution of the earth's radius 

 to the amount of 4J metres in 2500 years, and a daily loss of 

 200 cubic miles of heat, correspond to this effect. Hence, in 

 the course of the last 25 centuries, the temperature of the 

 whole mass of the earth must have decreased ^°. 



The not inconsiderable contraction of the earth resulting 

 from such a loss of heat, agrees with the continual transforma- 

 tions of the earth's surface by earthquakes and volcanic erup- 

 tions ; and we agree with Cordier, the industrious observer of 

 volcanic processes, in considering these phenomena a necessary 

 consequence of the continual cooling of an earth which is still 

 in a molten state in its interior. 



When our earth was in its youth, its velocity of rotation 

 must have increased to a very sensible degree, on account of 

 the rapid cooling of its then very hot mass. This accelerating 

 cause gradually diminished, and as the retarding pressure of 

 the tidal wave remains nearly constant, the latter must finally 

 preponderate, and the velocity of rotation therefore continually 

 decrease. Between these two states we have a period of equi- 

 librium, a period when the influence of the cooling and that of 

 the tidal pressure counterbalance each other ; the whole life of 

 the earth therefore may be divided into three periods — youth 

 with increasing, middle age with uniform, and old age with 

 decreasing velocity of rotation. 



The time during which the two opposed influences on the 

 rotation of the earth are in equilibrium can, strictly speaking, 

 only be very short, inasmuch as in one moment the cooling, 

 and in the next moment the pressure of the tides must prevail. 

 In a physical sense, however, when measured by human stand- 

 ards, the influence of the cooling, and still more so that of the 

 tidal wave, may for ages be considered constant, and there 

 must consequently exist a period of many thousand years' dura- 

 tion during which these counteracting influences will appear to 

 be equal. Within this period a sidereal day attains its shortest 

 length, and the velocity of the earth's rotation its maximum — 

 circumstances which, according to mathematical analysis, would 

 tend to lengthen the duration of this period of the earth's ex- 

 istence. 



The historical times of mankind are, according to Laplace's 

 calculation, to be placed in this period. Whether we are at the 

 present moment still near its commencement, its middle, or 

 are approaching its conclusion, is a question which cannot be 

 solved by our present data, and must be left to future ge- 

 nerations. 



