by William Gilbert, On the Magnet — Book II. Chap. 12… (1900)
“ We maintain, however, that the magnetick horizon is a plane level throughout touching the earth or a terrella in the place of some one region, with which plane the semi-diameter, whether of the earth or of the terrella, produced to the place of the region, makes right angles on every side. Such a plane is to be considered in the earth itself and also in the terrella, for magnetick proofs and demonstrations. For we consider the bodies themselves only, not the general appearances of the world. ”
