MOVEMENTS AXD DEFORMATIONS OF THE EARTH'S BODY. 553 



How much gravitational energy- can possibly be transformed into 

 heat and other forms of energy- in the future, can only be computed by 

 maldng assumptions as to the possible extent of further contraction, 

 and that involves hypotheses as to the atomic and sub-atomic constitu- 

 tion of the earth's matter, and its behavior imder the prodigious pressures 

 of the earth's interior. All slirmkage develops added gravitational 

 force and further tendency to shrinkage, which follows when the heat 

 generated by the shrinkage is lost; and where the process may end, 

 in a body of the dimensions of the earth, is beyond present determina- 

 tion. If there were no limit to the density that might be attained, it 

 would be impossible to assign any limit to the energy- that might be 

 transformed. It has usually been assmned that contraction could not 

 go on indefinitely because the atoms would come into actual con- 

 tact, and prevent further increase of density. This conception rests on 

 the recently prevalent hypothesis of the atomic constitution of matter; 

 but the more recent hypotheses that substitute multitudes of revolving 

 corpuscles or electrons for irreducible atoms, do not carry the same 

 presumption of a rigorous limit to condensation. It is not therefore 

 prudent to try to set such a limit, or to make it a feature in the dynamical 

 doctrines of the earth. It is even less prudent to try to measure the 

 limit of future conversion of the graAdtational energy of the sun into 

 heat, and so to set a limit to the habitability of the earth. 



The force of gravity may be defined as the effort of gravitational 

 energy to change mto other forms of energy. It is most familiarly 

 expressed in terms of weight, which is the resultant of the gravitational 

 force of the whole earth upon a given portion. Weight is determined 

 by the distances and directions of the given portion from all parts of 

 the attractmg mass, the amount of the attraction being directly as the 

 mass and inversely as the square of the distance, modified by the direc- 

 tion. It is greatest about 610 miles below the surface, where it is 

 1.0392 times that at the surface. Below this point it declines, and at 

 the center it is zero. The sum total of the earth's gravitative force at 

 the present time is equivalent to about 6X10^^ tons. This gives rise to 

 a pressure of about 3.000.000 atmospheres at the center of the earth. 



Gravitational force is also expressed in terms of the earth's ability 

 to accelerate the velocity of falling bodies at its surface, which is now 

 approximately 32 feet per second. For certain purposes, the force of 

 gravity may be better pictured by means of the velocity required to 



