The Airy theory, announced by G. B. Airy in 1855, proposed that continents and

islands are resting hydrostatically on highly plastic or liquid material, with roots or

projections penetrating the inner material of the earth just as icebergs extend

downward into the water (Figure 1-13). The greater the elevation of mountain masses

above the earth, the deeper the penetration of the roots. It has been called the Roots

of Mountain Theory, and has the support of some geologists.

j. The theoretical value of gravity at a point on the ellipsoid depends on the size and shape of the

ellipsoid and the observed value of gravity at the equator (978 gals). It also varies with the latitude of

the observation point, assuming that the earth is a surface without mountains and oceans, having no

variations in rock densities or in the thickness of the crust. The theoretical value of gravity represents

the force of the earth's attraction due to gravitation, minus the centrifugal force due to the rotation of the

earth.

k. Since the earth is not an ellipsoid and there are variations in both the crust material and the

terrain, the observed gravity of the geoid varies from point to point. Gravity observations must be made

so that the distance between the geoid and the