1-We know that a considerable part of earth’s mass (volume) is consisted of molten and doughy materials .
2-We also know that the gravitational force acts between the earth and objects,and attracts them toward the center of the earth ,and that in a free fall under vacuum,it causes a gravitational acceleration “ g “ of about 9.8 meters per second per second .
3- Besides slight differences of “ g “ in different points on the earth’s surface of different latitudes, which is due to the centrifugal force produced at a given perpendicular distance from the axis of rotation of the earth, and distance from the earth’s center of mass (which varies from 9.78 to 9.83 – and these variations don’t effect the solution), there is another factor that may change the value of “g“ on the earth’s surface. In the following paragraphs I am going to give the explanation that would be the major guide to the final solution.
4-The molten and doughy materials inside the earth are not homogeneous, they are consisted of distinct and huge bulks with different densities which generally vary from 2.8gr/cubic cm to 10.6 gr/cubic cm (In fact, in addition to the factors mentioned above, the differences of the values of “g “ are mainly due to the differences of those densities).
5- The presence of very high and different temperatures in various parts of molten and doughy materials inside the earth, large differences of densities of various molten and doughy bulks , orogeny phenomenon ( that generally happened long time ago ) , appearance and reality of volcanoes , the occurrences of many earthquakes on the earth ;all prove that the molten and doughy materials inside the earth are not calm and motionless, but they have a kind of very slow rotational movement .The regime of intermittent fluctuation of the Caspian sea level shows that the period of this movement is about 100 years.The proof of this rotational motion and the determination of its period are possible through measuring the values of “ g “ in different proper points on the earth’s surface and its renewals every 6 months or annually for the same points and locations, and by observing the variations of “ g “ and its displacement direction, and also by measuring the amount of displacement resulted from the very movement of these light and heavy masses of the earth's inside.The linear diffusing speed of this rotational movement on the earth’s surface is estimated to be about 45 m/h.