444 Solar-Diurnal Variations of Terrestrial Magnetism. 



Again, for the purpose of this argument we may, without 

 sensible error, look upon the magnetic Earth as similar to the 

 class model that is sometimes used to represent it ; that is 

 to say, as a globe wrapped round continuously with insulated 

 wires in the same direction which convey an electric current, 

 the circles of such wires being small near the poles, and of 

 course large at the equator. Now if an external insulated 

 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 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 

 insulated 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 currents, 

 the tendency of which in both hemispheres would be to in- 

 crease the general magnetism of the globe. From the reason 

 already given such currents would have no 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 I have before re- 

 marked) a sensible increase in the conductivity of these 

 regions even if the electromotive force remained altered, 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. Now have we any such pheno- 

 menon connected with terrestrial magnetism ? I think we have. 

 The late John Allan Broun has, I believe, proved that we 

 have changes in the mean daily value of the horizontal force 

 which are simultaneous, and in the same direction at places 

 on the Earth's surface very far removed from each other ; and 

 I have endeavoured to show that the changes of this nature 

 recorded by him depend, as far as we can judge from some- 

 what imperfect records, upon the state of the Sun's surface, 

 an increased area of spotted surface coinciding apparently 

 with increased values of the daily means of horizontal force 

 all over the Earth. But my chief object in giving prominence 

 to this part of the subject is with the view of advocating that 

 the Gaussian method of analysis should not be applied merely 

 to the solar diurnal variations of the three magnetic elements, 



