198 Professors J. Perry andW. E. Ayrton on 



explanation of these phenomena which does not take into 

 account the fact that the earth and other members of the solar 

 system are the electrified coatings of condensers, since, although 

 the mutual coefficients of induction between the different 

 members of the solar system may be very small compared 

 with the capacities of long telegraph-cables, still the differ- 

 ences of potential between the sun and the planets may 

 be extremely great, so that the charges in the condensers ill 

 question may be so large that a slight change in the capacities, 

 produced by rotations or changes in the positions of the pla- 

 nets, may set up in the bodies themselves electric currents of 

 considerable magnitude. 



We are at present engaged in the solution of the problem to 

 determine mathematically what is the strength of the currents 

 produced in the earth as it rotates under the inductive action 

 of the sun ; and we may mention that the moderate conduc- 

 tivity of the earth, combined with the probability of the exist- 

 ence of an iron core, will enable us very shortly to present the 

 results drawn from our theory in a numerical form. In the 

 meantime, however, we desire to show how it follows from 

 this theory that movements in the atmospheres of the earth 

 and sun, and especially the motions of rain-clouds, or clouds 

 of vapour, are connected with the phenomena of atmospheric 

 electricity and earth-currents. The connexion of these latter 

 phenomena with earthquakes, which we dwelt on in our recent 

 paper on observations of atmospheric electricity, will more 

 suitably be taken up again in our next paper. 



When a dielectric is composed of air at different pressures, 

 or of a mixture of gases, our experiments (recently communi- 

 cated • to the Asiatic Society of Japan in a paper on the 

 Specific Inductive Capacity of Gases) showed that K, the 

 specific inductive capacity of the medium, is different at differ- 

 ent parts, so that (see C. Maxwell's 'Electricity') the fun- 

 damental equation connecting the potential Y and p the charge 

 of electricity per unit volume of the medium at different points, 

 is 



Consequently, if there is no real charge in the medium, we 

 have 



l(K-S+|(K-fH(K-f)-»- ■ • » 



Now, if we imagine the non-homogeneous dielectric to be all 

 replaced by dry air at 7 GO millimetres pressure and at 0° C. 

 temperature, and if at any point where the potential is V there 



