258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



as yet is known about the rocks of the remaining three-fourths, 

 although sometliing may be inferred from the geology of the ocean 

 bottom by observations on oceanic islands. Direct sampling of the 

 ocean floor has been limited to a few inches in depth until quite re- 

 cently, the depth having now been extended to several feet.^ 



Supplementing the facts of geology, the phenomenon of volcanism 

 yields important information concerning what we may call the near 

 interior. The existence of volcanoes and the outpouring of vast 

 quantities of hot gases and molten lava furnish direct and striking 

 evidence of conditions many miles below the surface. Additional 

 information of value has been supplied by measuring the temperature 

 of lava lakes, of lava flows and of the gases emerging from fumaroles. 

 Further evidence of a hot interior is derived from the measurement of 

 temperature in deep mines and boreholes. The temperature in- 

 creases steadily with depth, but for reasons yet unknown the tem- 

 perature gradient varies within wide limits from place to place in 

 the earth. It may increase as much as 1° C. for each 18 meters or 

 as little as 1° C. for over 100 meters. According to Van Orstrand,* 

 the greatest depth attained in any boring is 12,800 feet (in Upton 

 County, Tex.); the highest temperature that has been measured is 

 118° C. in Cahforaia at a depth of 9,000 feet. 



Astronomy yields data of high precision concerning the motion of 

 the earth, from which can be calculated its moment of inertia. The 

 precession of the equinox was discovered by the Greek astronomer 

 Hipparchus in 134 B. C. From the accuratelj^ known value of the 

 constant of precession, it follows that the moment of inertia of the 

 earth about the polar axis is 8.06X10"g cm^. This is a quantity 

 that depends on the distribution of density within the earth. For a 

 given mass, and for a given mean density, the moment of inertia 

 depends on the distribution of light and heavy substances; if there is 

 heavy material at the center and light material at the surface, the 

 moment of inertia would be considerably less than if the central 

 density were smaller than that of the surface. The moment of inertia 

 of a body may be described quahtatively as its tendency to continue 

 spinning when once it has been set in motion. It is obvious that a 

 fly-wheel loaded at the center will spin less persistently than if the 

 same load were fastened to the rim. Similarly, the known moment 

 of inertia of the earth allows us to make important deductions con- 

 cerning the mass or density at various positions from surface to center. 

 By far the most direct and definite evidence concerning the interior 

 of the earth is supplied by earthquake waves, especially in combina- 

 tion with laboratory measurements on various rocks and minerals. 

 The story has been told before, but it will bear brief repetition. When 

 an earthquake occurs, elastic vibrations of various kinds are generated. 



> Piggot, C. S., Bull. Geol. Soc. Amer., vol. 47, pp. 675-684, 1936. 

 « Van Orstrand, C. E., personal communication. 



