304 Anniversary Meeting. [Nov. 30, 



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 magnetic axis. Hence the 

 sun must be a magnet* 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 oar 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 rota- 

 tion of the earth. If it is true that terrestrial magnetism is a neces- 

 sary consequence of the magnitude and the rotation of the earth, 

 other bodies comparable in these qualities with the earth, and com- 

 parable also with the earth in respect to material and temperature, 

 such as Venus and Mars, must be magnets comparable in strength with 

 the terrestrial magnet, and they must have poles similar to the earth's 

 north and south poles on the north and south sides of their equators, 

 because their directions of rotation, as seen from the north side of the 

 ecliptic, are the same as that of the earth. It seems probable, also, 

 that the sun, because of its great mass and its rotation in the same 

 direction as the earth's rotation, is a magnet with polarities on the 

 north and south sides of its equator, similar to the terrestrial northern 

 and southern magnetic polarities. As the sun's equatorial surface- 

 velocity is nearly four and a half times the earth's, it seems probable 

 that the average solar magnetic moment exceeds the terrestrial con- 

 siderably more than according to the proportion of bulk. Absolutely 

 ignorant as we are regarding the effect of cold solid rotating bodies 

 such as the earth, or Mars, or Venus, or of hot fluid rotating bodies 

 such as the sun, in straining the circumambient ether, we cannot 

 say that the sun might not be 1000, or 10,000, or 100,000 times as 

 intense a magnet as the earth. It is, therefore, a perfectly proper 

 object for investigation to find whether there is, or is not, any dis- 

 turbance of terrestrial magnetism, such as might be produced by a 

 constant magnet in the sun's place with its magnetic axis coincident 

 with the sun's axis of rotation. Neglecting for the present the seven 

 degrees of obliquity of the sun's equator, and supposing the axis to 

 be exactly perpendicular to the ecliptic, we have an exceedingly simple 

 case of magnetic action to be considered : a magnetic force perpen- 

 dicular to the ecliptic at every part of the earth's orbit and varying 



* 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. 



