28 EVOLUTION OF THE EARTH 



earth, since the different planetoids were traveling in the same 

 general direction, but nevertheless a state of incandescence and 

 liquidity would be likely to result from the sizes of the masses 

 involved. If in addition the infall of masses was sufficiently 

 rapid to bury the heat of previous infalls before it could be 

 dissipated by conduction to the surface, a general heating and 

 liquefaction of the earth would tend to take place, both from 

 the increased compression of the deeper nucleus and the effects 

 of impact at higher levels. 



The fact that the planets have cleared up the zones about 

 them, whereas the planetoids have remained permanently in 

 a scattered state, is an argument for holding that the existence 

 of dominating nuclei determined the growth of the planets. 

 It is likely that the nuclei were of various sizes, were clustered 

 to various degrees, and many of them united by their impact. 

 A somewhat limited number and considerable size of the units 

 as well as their grouping would be in accord with the lack of 

 relation of the amount of eccentricity and inclination of orbit 

 to the masses of the several planets. 



Indications of primordial tidal retardation. The moon 

 keeps the same face turned always toward the earth. Con- 

 sequently, from a point in outer space, it would be seen to 

 rotate on its axis in exactly the same time that it completed its 

 orbital revolution. Mercury is known also to keep the same 

 face turned always toward the sun and the same relation is 

 probably true of Venus. The other planets revolve many 

 times on their axes during the period of revolution, the earth, 

 for example, 366 times. The exact correspondence in the 

 moon, Venus, and Mercury between their times of axial rota- 

 tion and orbital revolution points to some causal relation be- 

 tween the two periods. That relation is one of tidal forces. 



The moon distorts slightly the earth's figure, but as the tidal 

 forces due to the moon are weak and the earth is very rigid, 

 this distortion in figure is expressed mostly by the rise and 



