THE term average radial gravity \[gmacr](r) may be defined as the arithmetic mean of all the gravity values lying on the circumference of a circle of radius r. A similar definition holds good in the magnetic case also. This can be represented as a series expansion in terms of the even vertical derivatives at the centre as: where a2, a4, a6,... are closely connected with the even vertical derivatives at the centre by the relations: and a0 approximates the gravity value at the centre.