THE REIGN OF UNIVERSAL WINTER. 405 



calculations of the mathematicians would indicate. It 

 seems inevitable, therefore, that the earth should have ex- 

 pended sufficient heat in 2500 years to effect a sensible re- 

 duction in the length of the day. 



Thanks to the mathematicians, they have again come to 

 our aid. The tide- wave is a protuberance of the ocean- 

 waters raised by the moon, and following the moon around 

 the earth from east to west. This motion is contrary to 

 the earth's diurnal rotation, and the friction of the tidal 

 waters against the shore and the standing waters must 

 necessarily tend to retard the rotary motion of the earth. 

 Now it has been calculated that this retardation must have 

 amounted to one sixteenth of a second in 2500 years. If, 

 therefore, no counteracting tendency has been experienced, 

 the sidereal day is one sixteenth of a second longer than it 

 was in the time of Hipparchus. But Laplace has shown 

 that the sidereal day has not varied in length. It follows, 

 therefore, that the shrinkage of the earth from loss of heat 

 has tended to accelerate its rotation to the extent of one 

 sixteenth of a second in twenty-five centuries. Such an 

 acceleration corresponds to a shortening of the diameter 

 about sixty feet, and a reduction of the temperature of the 

 whole mass of the earth one fourteenth of a degree. 



When the earth was in its youth, just emerging from a 

 molten state, the loss of heat and consequent contraction 

 must necessarily have been rapid. During this period the 

 sidereal day underwent a much more rapid shortening than 

 at present. In the distant future, on the contrary, the loss 

 of heat will become diminished to an extreme extent, and, 

 as a consequence, the retardation caused by the tide-wave 

 will gain the ascendency, and the day will eventually be 

 lengthened to such an extent that the earth will always 

 turn the same side toward the sun, as the moon always 

 turns the same side toward the earth. The historic period 



