ON COMPAEING AND EEDUCING MAGNETIC OBSERVATIONS. 81 



the atmosphere which circulate round the earth in the direction of 

 parallels of latitude, such currents will not be subject to any considerable 

 solar-diurnal variation. For, while the conductivity 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, or 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 incon 

 stanoy of current— in other words, no great solar-diurnal variation. 



12. 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 

 toward the poles. Whether these currents reach the poles or come down 

 in some intermediate region may be left an open question. 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 

 intermediate regions where they descend. 



13. Again, for the purpose of this argument we may, without sensible 

 error, regard the magnetic globe in the way already mentioned ; that is to 

 say, as represented by a small globe, wrapped round with wires, conveying 

 currents that go round it from east to west. Now, if an external in- 

 sulated circuit of wire a trifle larger than the diameter of this globe be 

 supposed to travel from the equator to either of the poles, it will leave 

 behind it more convolutions of the primary globe current than it 

 approaches, and will therefore be traversed by an induced current in the 

 same direction as that of the primary ; and the continuous travelling of 

 such an external system might be supposed to increase the magnetic 

 power of the globe. Applying the same sort of reasoning to the earth 

 and to the convection currents under consideration, these may be imagined 

 to be traversed by equatorial electric currents, the tendency of which in 

 both electric hemispheres would be to increase the general magnetism of 

 the globe. For the reason already given such currents would have littlc- 

 solar-diurnal variation, but yet they would be dependent upon the state of 

 the sun, and would vary with it. For imagine a change to take place in the 

 radiation of our luminary, producing an excess of such rays as are greedily 

 absorbed by the upper atmospheric regions, there would be (as already 

 remarked) a sensible increase in the conductivity of these regions, eveii 

 if the electromotive force remained unaltered ; and hence there would be 

 an increase in the supposed equatorial current. In other words, such 

 currents, while presenting no great diurnal variation due to the carriage 

 of a constant sun round the earth, would yet be eminently susceptible to 

 any inconstancy in the sun itself. 



14. Now here it will be asked. Have we any such phenomenon con- 

 nected with terrestrial magnetism. ? The reply to this question will be an 

 afi&rmatiye one. The late John Allan Broun has shown that we have 

 changes in the mean daily value of the horizontal force, which are aimnl- 

 taneous and in the same direction at places on the earth's surface very 

 far removed from each other; and the author of these remarks has 

 endeavoured to show that the changes of this nature as recorded by Mr. 

 Broun depend, as far as we can judge from somewhat imperfect records^ 



1886. r, 



