334 NEWTON S PRINCIPIA. 



Mr. Hopkins has performed some very laborious calcula 

 tions with a view of determining this question. He 

 begins with some general remarks. There are two ways 

 in which a body may cool, by conduction or convection. 

 The earth being at first fluid, would begin to cool by 

 convection. Now the temperature and pressure will both 

 be greater at the centre than near the circumference. 

 Because the temperature is greater at the centre, the body 

 will solidify first at the outer parts, and the earth would 

 become a crust containing a heterogeneous fluid. But 

 because the pressure is greatest at the centre, the body 

 will tend to solidify first at the centre, and thus on cooling 

 it would become solid throughout. We cannot tell which 

 is the predominating cause, and the investigation of the 

 earth s refrigeration leaves the point uncertain. But 

 there may be other tests whereby we can determine this 

 question. 



The precession of the equinoxes is caused by the attrac 

 tion of the heavenly bodies on the ring of matter surrounding 

 the earth s equator. One consequence of this attraction 

 we have already seen to be the recession of the nodes in 

 which it cuts the ecliptic. But this ring is fastened to the 

 earth : its nodes cannot, therefore, recede as fast as they 

 would do if the ring were left to itself. The earth is a 

 heavy load which it has to pull round with it. The less 

 this load the greater would be the precession. If the 

 interior surface of the solid crust be spherical, then neg 

 lecting the friction between it and the interior fluid, the 

 ring of matter surrounding the equator will only have to 

 pull round with it the solid crust ; the fluid will not turn 

 with it. Hence the precession will be greater than if the 

 earth were solid throughout. 



But the interior surface of the crust cannot be supposed 

 spherical ; it is most probably spheroidal. Supposing it 

 so, there will be pressures between its interior surface and 

 the contained fluid, caused partly by the motion of the 

 spheroid and partly by the tidal actions in the fluid caused 



