-.18 POPULAR LECTURES AND ADDRESSES. 



centre of gravity, magnetised it, and discovered 

 the dip. When the dip was first so discovered by 

 Robert Norman it was greater than it is now. 

 The dip has gone on decreasing, and is still 

 decreasing. In these great changes of terrestrial 

 magnetism, or in the mere existence of terrestrial 

 magnetism, we have always before us one of the 

 greatest mysteries of science a mystery which I 

 might almost say is to myself a subject of daily 

 contemplation. What can be the cause of this 

 magnetism in the interior of the earth ? Rigid 

 magnetization, like that of steel or the loadstone, 

 has no quality in itself in virtue of which we can 

 conceive it to migrate round in the magnetised 

 bar. Electric currents afford the more favoured 

 hypothesis ; they are more mobile. If we can 

 conceive electric currents at all, we may conceive 

 them flitting about. But what sustains the electric 

 currents ? People sometimes say, heedlessly or 

 ignorantly, that thermo-electricity does it. We 

 have none of the elements of the problem of 

 thermo-electricity in the state of underground 

 temperature which could possibly explain, in 



