Solar Diurnal Variations of Terrestrial Magnetism, 443 



the supposition that the greater portion of the disturbing- 

 currents lies outside the Earth's surface. 



I now come to consider another aspect of the subject. It 

 seems quite possible to conceive a set of curr ents to exist in 

 the Earth's atmosphere without exhibiting a considerable 

 diurnal variation. Let us take, for instance, an ordinary 

 electric circuit, say of a circular shape and horizontal, and 

 heat it by causing some source of heat, such as a lamp, to 

 travel slowly around it with a definite rate of progress. It 

 will be evident that we shall have (assuming the current to 

 be otherwise constant) no variation in flow due to this heating 

 effect. In like manner, if there be electric currents in the 

 atmosphere which circulate round the Earth in the direction 

 of parallels of latitude, such currents will not be subject to a 

 considerable solar diurnal variation. For while the conduc- 

 tivity of a given region would vary according to the position 

 of the Sun with regard to it, yet the whole circuit round 

 the Earth, which would always embrace a region affected by 

 the Sun, would not have its total resistance altered, at least 

 not greatly altered ; and as there would be no cause for much 

 alteration of the total electromotive force, there would be no 

 great reason for inconstancy of current — in other words, no 

 great solar diurnal variation. 



It may, however, be asked if there are any grounds for 

 imagining the existence of such a set of currents. To this I 

 would reply that, assuming the existence of currents in the 

 upper regions of the atmosphere, their disposition would cer- 

 tainly be to some extent subject to magneto-electric conside- 

 rations. If we bear in mind the distinct connection between 

 auroral beams and lines of magnetic force, we must certainly, 

 I think, be prepared to admit the possibility of perhaps even 

 a marked reference to magneto-electric considerations in the 

 disposition of the currents in question. 



For the purpose of the following argument we may consider 

 the Earth to be at rest (i. e. devoid of rotation), and imagine 

 that the Sun circulates round the equator in twenty-four 

 hours. As a consequence of solar influence, we shall have 

 convection-currents in the upper regions of the atmosphere 

 flowing from the equator northwards and southwards towards 

 the poles. Whether these currents reach the poles or come 

 down in some intermediate region may be left an open ques- 

 tion. Now such currents will not only be conductors, but 

 they will form a movable system of conductors, which we may 

 suppose to be created at the equator when they rise into the 

 upper regions, and destroyed at the poles or those interme- 

 diate regions where they descend. 



