OF THE EARTH-CURRENTS. 301 



Now, we have in this machinery, as it appears to me, means 

 fully adequate to the production of the observed effects. If it be 

 assumed that the sun produces these changes by its calorific action, 

 the effects at any given place will depend upon the relative tempe- 

 ratures of the neighbouring portions of the earth's surface. The 

 earth being, in its normal state, negatively electrical, the negative 

 electricity will be greatest or the positive electricity least, at 

 the parts most heated ; and there will, consequently, be a flow of 

 electricity to those parts from the place of observation. Thus the 

 varying azimuth of the current, which is directed towards the most 

 heated parts of the earth's surface, is explained. The maximum 

 intensity of the current, at l h 30 m P.M. is also accounted for, that 

 being the period of the day when the solar calorific action is 

 most intense. It should be noted, however, that the magnitude 

 of the effect will depend, not on the absolute temperature, but on 

 its relative increase. It is, accordingly, greatest at those parts of 

 the earth at which the increment of temperature corresponding to 

 a given distance is greatest. 



The secondary maxima are probably due to the recombination 

 of the atmospheric and terrestrial electricities, through the medium 

 of vapour in the lower regions of the atmosphere. The effects of 

 this recombination in producing horizontal currents in the earth's 

 crust will, of course, be differential only, and will depend on the 

 excess of the positive electricity thus transported at the places 

 on the same meridian which are nearer to the equator. In confir- 

 mation of this view it may be observed, that the epochs correspond 

 with those of the maxima of atmospheric electricity, as deduced 

 by Quetelet from the observations made under his direction at 

 Brussels, the morning maximum of atmospheric electricity in 

 summer occurring at 8 A.M., and the evening maximum at 9 P.M. 



The phenomena hitherto described are such as would take place 

 if all the parts of the earth's crust were similarly constituted, and 

 therefore similarly acted on by the solar rays. In order to be able 

 to explain the diversity which exists in the magnetic phenomena 

 at different places, we must know something more of the nature 

 of the solar action, and of the mode in which electricity is developed 

 by it. 



The speculations respecting the origin of atmospheric and ter- 

 restrial electricity are various. Thus De Saussure believed that 

 this electricity was developed by evaporation, the vapour taking 



