66 AN INTRODUCTION TO SCIENCE 



the sun keep, as it were, a hold upon them. The heaping 

 up of the tide is not merely a rising of the water towards 

 the moon, but the wave is drawn backwards with regard 

 to the movement of the earth. Now, wherever there is 

 movement of matter upon matter, whether the substances 

 rubbed against one another be solid, liquid or gaseous, 

 energy is liberated. This energy takes the form of heat, 

 which is dissipated into space, unless there be some coun- 

 tervailing conditions which restore the heat to the body 

 losing it. The friction of the tidal wave upon the surface 

 of the earth diminishes the energy of the earth's rotation in 

 exactly the same manner as a brake diminishes the energy 

 of rotation of a wheel. In point of fact, the moon puts a 

 continuous brake upon the earth. 



There is, therefore, no reason to call in question the ac- 

 curacy of the records of eclipses of the moon. They supply 

 historical evidence that the earth rotated more rapidly in 

 former times than it does now. If we had no such records 

 we should still be able to prove that the friction of the 

 tides must have produced a slowing effect. 



The evidence as to the actual rate of rotation can be put 

 to a most important use. We know that once this earth 

 was so hot that it was molten. Now, when a fluid sphere 

 is made to rotate the centrifugal force at its equator increases, 

 while the polar diameter is diminished, and the extent to 

 which it assumes the form of a disc with a rounded edge 

 (an oblate spheroid) depends upon the amount of the cen- 

 trifugal force, i.e., upon the velocity of its rotation. The 

 earth, as we know it, is almost perfectly rigid. We still 

 speak of the "crust of the earth " as if its surface only were 

 solid and its contents molten, but this theory has been 

 abandoned. Physicists hold now that the earth is solid to 

 its core, except for patches of molten lava. At any rate, it 

 is so nearly rigid that any deformation by centrifugal force 

 or return towards sphericity owing to the diminution of cen- 

 trifugal force, is out of the question. The earth has the 



