158 ANNUAL OF SCIENTIFIC DISCOVERY. 



of beat, 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 transformations of the earth's 

 surface by earthquakes and volcanic eruptions ; 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, velocity of rotation must have in- 

 creased 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 there- 

 fore continually decrease. Between these two states we have a period of 

 equilibrium, a period when the influence of the cooling and that of the ti- 

 dal pressure counterbalance each other ; the whole life of the earth there- 

 fore may be divided into three periods, youth with increasing, mid- 

 dle 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 mo- 

 ment the pressure of the tides must prevail. In a physical sense, 

 however, when measured by human standards, the influence of the 

 cooling, and still more so that of the tidal wave, may for ages be con- 

 sidered constant, and there must consequently exist a period of many 

 thousand years 1 duration 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 existence. 



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

 tion, to be placed in this period. Whether we are at the present mo- 

 ment 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 generations. 



The continual cooling of the earth cannot be without an influence 

 on the temperature of its surface, and consequently on the climate ; 

 scientific men, led by Buffon, in fact, have advanced the supposition 

 that the loss of heat sustained by our globe must at some time render 

 it an unfit habitation for organic life. Such an apprehension has evi- 

 dently no foundation, for the warmth of the earth's surface is even 

 now much more dependent on the rays of the sun than on the heat 

 which reaches us from the interior. According to Pouillet's measure- 

 ments, the earth receives 8,000 cubic miles of heat a day from the sun, 

 whereas the heat which reaches the surface from the earth's interior 

 may be estimated at two hundred cubic miles per diem. The heat 

 therefore obtained from the latter source every day is but small in 

 comparison to the diurnal heat received from the sun. 



Jl we imagine the solar radiation to be constant, and the heat we 

 receive; from the store in the interior of the earth to be cut off, we 

 should have as a consequence various changes in the physical con- 



