186 



equilibriuTn of statical electricity ; but I regard these derangements of 

 equilibrium to be simply the effects of solar heat, and not (as Dr. Lament 

 believes) the results of an electrical force emanating directly from the 

 sun. 



It is well known that the earth and the atm.osphere are, in ordinary 

 circumstances, in opposite electrical states — the electricity of the earth 

 being negative, and that of the atmosphere positive. It is also known 

 that the electricity of the air increases rapidly with the height, a 

 few feet — and in some cases even a few inches — being sufficient to ma- 

 nifest a difference of electrical tension. The rate of this increase is very 

 different at different periods of the day, the difference appearing to be 

 due to the greater or less conductibility of the lower strata of the atmo- 

 sphere, giving rise to a greater or less interchange of the opposite elec- 

 tricities. 



ISTow, 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 temperatures of the neigh- 

 bouring portions of the earth's surface. The earth being, in its normal 

 state, negatively electrical, this 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 these parts from the place of ob- 

 servation. 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 current, at 1^ 30"" 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, ac- 

 cordingly, greatest at those parts of the earth at which the increment of 

 temperature corresponding to a given distance is greatest. 



The secondarj^ 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 recombina- 

 tion in producing horizontal currents in the earth's crust will, of course, 

 be differential only, and will depend on the excess of the positive elec- 

 tricity thus transported at the places on the same meridian which are 

 nearer to the equator. In confirmation 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 directions at Brussels, the morning maximum of atmospheric elec- 

 tricity, 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 there- 

 fore 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. 



