PRESIDENTIAL ADDRESSES. 511 



law of magnetic force at a distance, that the 

 magnetic force due to the sun at the earth's dis- 

 tance from it, in any direction, would be only a 

 twelve-millionth of the actual force of terrestrial 

 magnetisation at any point of the earth's surface, 

 in a corresponding position relatively to the mag- 

 netic axis. Hence the sun must be a magnet x 

 of not much short of 12,000 times the average 

 intensity of the terrestrial magnet (a not absolutely 

 inconceivable supposition, as we shall presently 

 see) to produce, by direct action simply as a 

 magnet, any disturbance of terrestrial magnetic 

 force sensible to the instruments of our magnetic 

 observatories. 



Considering probabilities and possibilities as 

 to the history of the earth from its beginning 

 to. the present time, I find it unimaginable but 

 that terrestrial magnetism is due to the greatness 

 and the rotation of the earth. If it is true that 



1 The moon's apparent diameter being always nearly the same 

 as the sun's, the statements of the last four sentences are applicable 

 to the moon as well as to the sun, and are important in connection 

 with speculation as to the cause of the lunar disturbance of terrestrial 

 magnetism, discovered nearly fifty years ago by Kreil and Sabine. 



