GILBERT OF COLCHESTER. 347 



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 

 itj 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. ']\Iagnus 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, ])oreal, 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 argumenta- 

 tion or buttressed by mathematical demonstration.' Three hundred 

 years have jDassed aw^ay, 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 condi- 

 tion of the earth is also attributed to the action of electric currents con- 

 tinually flowing through the crust of the earth. Both these theories, 

 as Professor Eiicker, 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 Gil- 

 bert'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 Ten- 

 eriffe, that his attentive eye observed that the magnet pointed slightly 

 west of north, and that this angular deviation increased during the fol- 

 lowing days.* For a time he kept the secret in his own mind ; the pilots, 

 however, soon perceived the variations and grew alarmed, deserted, as 



* Columbus was thus the first to observe that the declination or 'variation 

 of the compass,' as it is called, changes with place. 



