84 THE POPULAR SCIENCE MONTHLY. 



ors crossing lines of magnetic force ; we may therefore expect them 

 to be the vehicle of electric currents. Such electric currents will of 

 course react on the magnetism of the earth. Now, since the velocity 

 of these upper-currents has a daily variation, their influence, as exhib- 

 ited at any place upon the magnetism of the earth, may be expected 

 to have a daily variation also. 



The question thus arises. Have we possibly here a cause which may 

 account for the well-known daily magnetic variation ? Are the pecu- 

 liarities of this variation such as to correspond to those which might 

 be expected to belong to such electric currents ? I think it may be 

 said that, as far as we can judge, there is a likeness of this kind between 

 the peculiarities of these two things, but a more pi'olonged scrutiny 

 will of course be essential before we can be absolutely certain that 

 such currents ai'e fitted to produce the daily variation of the earth's 

 magnetism. 



Besides the daily and yearly periodic changes in these upper con- 

 vection-cul*rents we should also expect occasional and abrupt changes 

 forming the counterparts of those disturbances in the lower strata 

 with which we are familiar. And these may be expected in like man- 

 ner to produce non-periodic occasional disturbances of the magnetism 

 of the earth. Now, it is well known that such disturbances do occur; 

 and, further, that they are most frequent in those years when cyclones 

 are most frequent ; that is to say, in years of maximum sun-spots. In 

 one word, it appears to be a tenable hypothesis to attribute at least 

 the most prominent magnetic changes to atmospheric motions taking 

 place in the upper regions of the atmosphere where each moving stra- 

 tum of air becomes a conductor moving across lines of magnetic force ; 

 and it was Sir William Thomson, I believe, who first suggested that 

 the motion of conductors across the lines of the earth's magnetic force 

 must be taken into account in any attempted explanation of terrestrial 

 magnetism. 



It thus seems possible tliat the excessive magnetic disturbances 

 which take place in years of maximum sun-spots may not be directly 

 caused by any solar action, but may rather be due to the excessive 

 meteorological disturbances which are likewise characteristic of such 

 years. On the other hand, that magnetic and meteorological influence 

 which Mr. Broun has found to be connected with the sun's rotation 

 points to some unknown direct efiect produced by our luminary, even 

 if we imagine that the magnetic part of it is caused by the meteoro- 

 logical. Mr. Broun is of opinion that this efi'ect of the sun does not 

 depend upon the amount of spots on his surface. 



In the next place, that influence of the sun, in virtue of which we 

 have most cyclones and greater meteorological disturbance in the 

 years of maximum spots, cannot, I think (as far as we know at pres- 

 ent), be attributed to a change in the heating power of the sun. We 

 have, no doubt, traces of a temperature effect which appears to depend 



