THE EARTH S MAGNETISM BAUEE. 209 



precise cause or causes, we may still say with Halley that these are 

 '' Secrets as yet utterly unknown to Mankind, and are reserv'd for 

 the Industry of future Ages." 



A mathematical analysis of the accumulated material shows that, 

 in order to find an adequate explanation of the secular variation of 

 the earth's magnetism, we must reckon with systems of magnetic 

 or electric forces having their seats both below and above the earth's 

 crust. There would also appear to be some evidence that in addi- 

 tion to a motion of the magnetic poles or magnetic axes of the earth, 

 we may also have to take into account a possible diminution in the 

 earth's magnetic moment or intensity of magnetization. 



THE ORIGIN OF THE EARTH'S MAGNETISM. 



Before concluding this lecture, we ought, perhaps, in the few min- 

 utes remaining, to say something regarding the status of the ever- 

 recurring question as to the origin of the earth's magnetism. Assum- 

 ing that the magnetism of our planet is uniformly distributed 

 throughout its mass, it is found that the average intensity of mag- 

 netization is only about one ten-thousandth of very highly magnet- 

 ized hard steel. Prof. Fleming, in his very suggestive popular 

 lecture on the " Earth, a great magnet," given at the meeting in 189G 

 of the British Association for the Advancement of Science, made 

 this statement: 



Taken as a whole, the earth is a feeble magnet. If onr globe were wholly 

 made of steel and magnetized as highly as an ordinary steel-bar magnet, the 

 magnetic forces at its surface would be at least 100 times as great as they are 

 now. That might be an advantage or a very great disadvantage. 



If, however, we could penetrate the earth's crust we would find 

 at a distance of only about 12 miles a temperature so great that, 

 according to present laboratory facts, all magnetization would neces- 

 sarily cease. Hence, if the earth's magnetic field arises from an ac- 

 tual magnetization of the substances composing the earth, these sub- 

 stances must be confined within a comparatively thin shell. But 

 the question immediately arises: Is this argument correct? May it 

 not be that just as the point of liquefaction is raised by increased 

 pressure, so is also the critical temperature of magnetization. It 

 may thus occur that the effect due to increase of pressure with depth 

 of penetration more than balances that due to increased temperature. 

 There are at present no wholly decisive experiments which may be 

 drawn upon to answer this query. 



The hypothesis that the earth may be an electromagnet also meets 

 with difficulties when xve attempt to account for the origin, direction, 

 and maintenance of the required currents. In spite of the accumu- 

 lated facts of over three centuries, we are still unable to snj definitelj'' 

 to what the earth's magnetic field is really due. Perhaps w^e may 

 44863°— SM 1913 14 



