magnetick horizon of, [80].
proportion of the forces in, [81], [82].
experiment with iron sphere, [85].
small iron sphere and rod, [94], [102].
centre of magnetick virtue in, [95].
irregular terrella to exhibit variation, [155], [157].
to illustrate the dip of the needle, [190], [192].
analogy of, with the earth, [41], [78], [119], [211].
testing loadstones, methods of, [108].