Page:Popular Science Monthly Volume 59.djvu/357

Rh by building two brick walls parallel to each other and eight inches apart and filling in the space with scrap iron. A delicate magnetometer showed that this structure allowed no leakage of lines of force through it but offered an impenetrable barrier to the magnetic influence of the working dynamos.

Gilbert's greatest discovery is that the earth itself is a vast globular magnet. 'Magnus magnes ipse est globus terrestris' are his own words. It has its poles, its axis and equator just as the lodestone or terrella. The pole in our hemisphere he variously calls north, boreal, arctic, whilst that in the other hemisphere he calls south, austral, antarctic. He is quite aware that his theory is a grand generalization; and admits that it is 'a new and till now unheard-of view,' and so confident is he in its worth that he is not afraid to say that 'it will stand as firm as aught that ever was produced in philosophy, backed by ingenious argumentation or buttressed by mathematical demonstration.' Three hundred years have passed away, and Gilbert's theory is accepted by every man of science and is taught in every school of physics. Moreover, save the correction of a few errors of observation, no change of any importance has been made in it.

Gilbert sought to explain the magnetic condition of our globe by the presence, especially in its innermost parts, of what he calls true terrene matter, homogeneous in structure and endowed with magnetic properties, so that every separate fragment of the earth exhibits the whole force of magnetic matter. We attribute terrestrial magnetism to the vast masses of magnetic material which lie near the surface, for at a depth of ten or twelve miles the temperature of the ferruginous masses would deprive them of all magnetic properties. The magnetic condition of the earth is also attributed to the action of electric currents continually flowing through the crust of the earth. Both these theories, as Professor Rücker, of London, said in 1891, are beset with difficulties; at present we must content ourselves with accumulating facts in the hope that a clue to an explanation may hereafter be found.

Gilbert's discovery enabled him to offer a philosophical explanation of the behavior of both compass and dipping needles, as well as of a great many other phenomena. The declination was known before Gilbert's time. Columbus noticed this want of coincidence between the geographical and magnetic meridians in his first voyage to the New World. It was on September 13, 1492, when 200 leagues west of Teneriffe, that his attentive eye observed that the magnet pointed slightly west of north, and that this angular deviation increased during the following days. For a time he kept the secret in his own mind; the pilots, however, soon perceived the variations and grew alarmed, deserted, as