Solar-Diurnal Variations of Terrestrial Magnetism. 439 



Earth's magnetism, as observed at Trevandrum, is greater 

 during the day than during the night, and greater perhaps, 

 too, at times of maximum than at times of minimum sun-spot 

 frequency. If the arguments now advanced be generally 

 assented to, we may presume that, while it is unlikely that the 

 cause of the daily variation exists anywhere else than in the 

 upper regions of the Earth's atmosphere, there is no objection 

 to our imagining that it resides there in the form of a set of 

 electric currents. Such currents will of course affect the 

 needle directly, but they will also affect the magnetic earth, 

 and any affection of the magnetic earth will again influence 

 the needle. The question thus arises, To what extent do 

 these hypothetical currents act directly upon the needle, and 

 to what extent do they act upon it indirectly by influencing 

 the magnetism of the Earth ? That this question is not un- 

 important will be manifest by considering what takes place 

 when a powerful electromagnet influences a needle. In this 

 case probably only a small portion of the influence is due to 

 the direct action of the current, while a large portion is due 

 to the iron of the electromagnet that has been magnetized by 

 the current. In order to study this point it will perhaps be 

 desirable that we limit ourselves to the solar diurnal varia- 

 tions of declination, as these are of a simple definite type in 

 each hemisphere, the type in the one hemisphere being the 

 reverse of that in the other. Let us therefore consider the 

 peculiarities of this variation, particularly in the regions on 

 either side of the magnetic equator. 



Here we find, speaking generally, that north of the equator 

 the extreme westerly deflection of the marked pole of the 

 needle is reached at 2 p.m., while south of the equator this 

 pole reaches its extreme easterly deflection at the same hour. 

 Now, if we attempt to picture to ourselves any general 

 change in the magnetic system of the Earth that will produce 

 these results, it will be one of a very artificial character. Od 

 the other hand, these motions of the needle, which cannot well 

 be explained by any easily conceivable temporary change of 

 the Earth's magnetic system, can very easily be explained by 

 a system of atmospheric currents. In the northern hemi- 

 sphere we should have a set of positive currents flowing from 

 the equator northwards, while in the southern we should 

 have in like manner a similar set of positive currents flowing 

 southwards. And, therefore, since positive currents flow out 

 from the equator on both sides of it, we should have to con- 

 template a vertical stream of positive electricity at the equator 

 which there splits itself up into two portions — one for each 

 hemisphere. 



