Silvanus P. Thompson

“ Gilbert further pointed out that the loadstone is surrounded by a sort of atmosphere or "orbe of virtue" within which the magnetical effects can be observed. Book 3, on the directive force of the magnet, is full of most instructive experiments, in which the terrella figures largely, relating to the question how one magnet influences another and tends to make it point toward it. All this was leading up to the theory of terrestrial magnetism; for we find him naming the parts of his loadstone globes with poles, equator and meridians. ”

“ At the time when steering by the compass was introduced into navigation, the compass pointed in Middle Europe so nearly truly to the north that with the rough instrumental appliances at hand its deviation from the true north was seldom noticed, or if noticed ascribed to some error in the setting of the needle. Later the compass-makers began to set their needles slightly askew beneath the card, according to the variation in the place of origin. ”

“ Gilbert seized with avidity on Norman's discovery of this effect, and devised an improved form of dipping-needle. He experimented on the dip of compass-needles placed at different points over his terrella, and evolved a theory on the proportion which he conceived to exist between the latitude and the dip. Arguing from all too imperfect data, he propounded the view that the dip was the same13 in any given latitude; and proposed that seamen should ascertain their latitudes by simply observing the dip. He was aware that local irregularities might occur, as they do in the declination ”

“ William Gilbert. Nurtured, as we have seen, in the Cambridge which had so recently been the home of Linacre and of Kaye--the Kaye who founded Caius College--Gilbert had, during his subsequent sojourn in Italy, conversed with all the learned men of his time. He had experimented on the magnet with Fra Paolo Sarpi: he had, there is reason to think, met Giordano Bruno: he was the friend and correspondent of Giovanni Francesco Sagredo. Being a man of means and a bachelor, he spent money freely upon books, maps, instruments, minerals, and magnets. ”

“ For twenty years he experimented ceaselessly, and read, and wrote and speculated, and tested his speculations by new experiments. For eighteen years he kept beside him the manuscript of his treatise, which in the8 year 1600 saw the light under the title of De Magnete, to which was added the sub-title: magneticisque corporibus, et de magno magnete tellure, physiologia nova. ”