5io POPULAR LECTURES AND ADDRESSES. 



British Association of 1881 (p. 469), "On Magnetic 

 Disturbances and Earth Currents " : " Thus we 

 see that the magnetic changes which take place at 

 various points of the earth's surface at the same 

 instant are so large as to be quite comparable with 

 the earth's total magnetic force ; and in order that 

 any cause may be a true and sufficient one, it 

 must be capable of producing these changes 

 rapidly." 



The primary difficulty, in fact, is to imagine the 

 sun a variable magnet or electro-magnet, powerful 

 enough to produce at the earth's distance 

 changes of magnetic force amounting, in ex- 

 treme cases, to as much as Jy- or ..', and 

 frequently, in ordinary magnetic storms, to as 

 much as ,,', of the undisturbed terrestrial 

 magnetic force. 



The earth's distance from the sun is 228 times the 

 sun's radius, and the cube of this number is about 

 12,000,000. Hence, if the sun were, as Gilbert 

 found the earth to be, a globular magnet, and if it 

 were of the same average intensity of magnetisa- 

 tion as the earth, we sec, according to the known 



