436 Prof. Balfour Stewart on the Cause of the 



doubt whether it will account for the diurnal oscillation as we 

 know it. It will form a cause, but yet it will not be the chief 

 cause. It is obviously one prominent peculiarity of Faraday's 

 hypothesis, that in it the action of the Sun must, to ensure its 

 probability, be associated with the great mass of the Earth's 

 atmosphere, that is to say with the lower strata as well as 

 with the upper ; or rather with the lower strata in preference 

 to the upper, bearing in mind the superior mass of the former 

 and their greater nearness to the magnetic Earth. Now we 

 know both from observations of the declination and horizontal 

 force (Proc. Roy. Soc. March 22, 1877, and Phil. Trans. 

 1880, p. 541) that the action of the Sun in producing diurnal 

 variations of these elements is one and a half times as 

 powerful at epochs of maximum as it is at epochs of minimum 

 sun-spot frequency. 



But it is very difficult, if not impossible, to imagine that the 

 great mass of the atmosphere can be heated by the Sun one 

 and a half times as much at epochs of maximum sun-spot 

 frequency as it is at epochs of minimum sun-spot frequency; 

 and yet we must imagine this if we suppose that Faraday's 

 hypothesis accounts for the diurnal variation. Certainly 

 the facts of meteorology are strongly against any such 

 belief. Again, during our summer the heating influence of 

 the Sun is considerably greater in the Northern than in the 

 Southern Hemisphere, while during our winter the reverse 

 will be the case. Totally apart from the diurnal variation we 

 ought therefore, if Faraday's hypothesis be true, to find a very 

 perceptible difference in the mean magnetic condition of the 

 Northern Hemisphere between summer and winter, and a like 

 difference too in the Southern Hemisphere, the one being at 

 the same moment differently affected from the other. But 

 there are no traces of such a phenomenon, the annual and 

 semi-annual variations being quite of a different nature and 

 none of them very large. 



(3) Let me now discuss the hypothesis that these variations 

 are caused by earth-currents. These currents were first 

 detected by W. H. Barlow, and were at a later period 

 observed and studied by C. V. Walker. They are now 

 continuously recorded by photography at the Royal Obser- 

 vatory, Greenwich, and at the Central Russian Observatory. 



With respect to these currents, I have always maintained 

 that in type they bear all the marks of being secondary 

 currents generated in the moist conducting crust of the 

 Earth, and caused by small but abrupt changes in the mag- 

 netism of the Earth. Whether this be the true explanation 

 or not, it is at any rate, I venture to think, consistent with what 



