GRAVITY-PULL WITHIN THE EARTH 223 



be small, all the curves dependent on gravity are involved and the effects 

 are felt throughout the whole interior of the earth. Prom the gravity 

 maximum, at or near the surface, the acceleration value declines both 

 outward and inward, outward because the lines of pull are spread over 

 greater space, inward because the lines of pull counterbalance one an- 

 other. The best basis for forming a picture of the inward decline is found 

 in the principle of the hollow sphere. To illustrate this, the line 1/3 V. 

 of Diagram IV is drawn at 816 miles below the surface, which is the 

 depth that divides the outer from the inner half of the earth, measured, 

 of course, by volume, not by mass. This line may be taken as well as any 

 other to define the inner side of a hollow sphere. Xow, if a body is per- 

 fectly spherical, and if its layers are perfectly homogeneous in density in 

 themselves (even though they differ from one another in density, "as they 

 usually do), the gravity of any symmetrical shell concentric to the center 

 counterbalances or neutralizes itself as felt at any point within it. If the 

 sphere is not perfectly spherical and homogeneous, corrections must be 

 made according to the amount of deviation from sphericity and homo- 

 geneity. In the light of this, it is easy to see that as a body descends 

 toward the center, the accelerating power of the earth on it falls off in 

 proportion to the mass of the shell of matter left behind. The speed of 

 an ideal body falling uninterruptedly to the center would, however, con- 

 tinue to increase all the way, but at a declining rate that would vanish at 

 the center. 



The gravitative Pressure avithin the Earth 



' The rise of gravitative pressure within the earth is one of the most 

 familiar of geologic themes and needs only a passing recognition here. 

 It is dependent on the inherent weight of the matter and on its degree of 

 compression. In a rough way, it ranges from one atmosphere, or mega- 

 dyne, at the surface to about 3,000,000 atmospheres, or megadynes, at 

 the center; but the precise distribution of the pressure is not yet deter- 

 mined. Geophysicists, mathematicians, and others have endeavored to 

 derive as close an approximation to the curve of internal pressure as ex- 

 isting data will permit,^ but the results are to be held as tentative until 

 the qualifying conditions are better determined. The tenor of accumu- 

 lating evidence implies that the conditions which affected the progressive 

 self-compression of the body were more important than those which 

 affected the original assortment of the material. 



* Chamberlin and SaUsbury : Geology, vol. i, p. 540. 



