Prof. E. Rutherford, the well-known authority on radium, suggests that possibly radium is a source of heat from within the earth. Traces of radium have been detected in many rocks and soils, and even in sea water. Calculation shows that the total amount distributed through the earth’s crust is enormously large, although relatively small “compared with the annual output of coal for the world.” The amount of radium necessary to compensate for the present loss of heat from the earth “corresponds to only five parts in one hundred million millions per unit mass,” and the “observations of Elster and Gertel show that the radio-activity observed in soils corresponds to the presence of about this proportion of radium.”[62]
The earth has 12 different motions. These are as follows:—
1. Rotation on its axis, having a period of 24 hours.
2. Revolution round the sun; period 365¼ days.
3. Precession; period of about 25,765 years.
4. Semi-lunar gravitation; period 28 days.
5. Nutation; period 18½ years.
6. Variation in obliquity of the ecliptic; about 47″ in 100 years.
7. Variation of eccentricity of orbit.
8. Change of line of apsides; period about 21,000 years.