J. R. Mayer on Celestial Dynamics. 413 
: during the last 2500 years is far too great to have been without 
sensible effect on the velocity of the earth’s rotation. The rea- 
son why, in spite of this accelerating cause, the length of a day 
has nevertheless remained constant since the most ancient times, 
must be attributed to an opposite retarding action, This con- 
sists in the attraction of the sun and moon on the liquid parts 
of the earth’s surface, as explained in the last chapter. 
According to the calculations of the last chapter, the retard- 
ing pressure of the tides against the earth’s rotation would cause, 
during the lapse of 2500 years, a sidereal day to be lengthened 
to the extent of ;,th of a second; as the length of a day, how- 
ever, has remained constant, the cooling effect of the earth 
during the same period of time must have shortened the day 
ysth of a second. A diminution of the earth’s radius to the 
amount of 44 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 twenty-five 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 v pro- 
Se ete PU ea a ee ee eee 
eee aie 4 Le ee 
Pe adders aes 
. Cesses, in considering these phenomena a necessary consequence 
a of the continual cooling of an earth which is still in a molten 
: State in its interior. 
cooling of its then very hot mass. This accelerating cause - 
ually diminished, and as the retarding pressure of the tidal wave 
il appear to be 
ts shortest 
