26(3 THE OCEAN. [Book X. 



at all points within a distance from the earth's 

 surface at least as great as the limits of the atmo- 

 sphere. 



Because objects within at least those limits are 

 drawn to the earth's surface by the force of gravita- 

 tion ; therefore the force drawing towards the earth 

 is greater than that drawing from it : 



But the force drawing from the earth is the 

 normal force which opposes the direct force of the 

 earth's gravitation increased by that which opposes 

 the motion caused by the revolving force : 



Therefore these two are together less than the 

 direct force. 



But the force drawing from the earth in conse- 

 quence of the action of the revolving force is equal 

 to the revolving force : 



Therefore the sun's revolving force and the 

 normal force of astral gravitation are, within those 

 limits, less than the direct force of the earth's gravi- 

 tation. 



PEOPOSITION XXXII. 



Theorem. — At some certain distance from the earth 

 the direct force of its gravitation must be brought 

 into a state of equilibrium as regards the combined 



