Let and be the horizontal and vertical components of earth’s magnetic field Since is the angle of dip

or (i)

Suppose planes 1 and 2 are two mutually perpendicular planes and respectively make angles and with the magnetic meridian. The vertical components of earth’s magnetic field remain same in the two planes but the effective horizontal components in the planes will be and

The angles of dip and in the two planes are given by

or

Similarly, (iii)

From eqns. (ii) and (iii)

[From eqn. (i)]