738 COSMICAL PHYSICS 



Through rainfall, negative electricity is communicated to the 

 earth, while, in the higher layers of the atmosphere, where the clouds 

 originate, an excess of positive electricity remains behind. In this 

 simple way, according to J. J. Thomson, atmospheric electricity is 

 explained. Part of the negative charge of the earth's surface goes 

 back into the air by conduction. This phenomenon is much more 

 marked in summer than in winter, and hence, the reason for the 

 smaller negative charge of the earth's surface in summer is estab- 

 lished. 



Observations on polar lights indicate that in the highest layers of 

 the atmosphere there is again a negative charge present. To explain 

 this, it is assumed that small globules, which are formed in the neigh- 

 borhood of the sun through condensation on negative electrons, are 

 driven away from the sun by radiation pressure and later entangled, 

 to a certain extent, by the highest layers of the atmospheric envelope 

 of the planets. Thus the relation, discovered by Busch, between the 

 dust content of the highest atmospheric layers and the eleven-year 

 sun-spot period is quite intelligible. These globules carry their nega- 

 tive charge with them, and consequently there originate the electrical 

 discharges which give rise to the polar lights. In this way is explained 

 the coincidence, found by Schwabe, of the sun-spot periods with the 

 polar lights. Furthermore, the ions of the air, produced by the dis- 

 charges, give rise to the condensation of water- vapor; and in this way 

 the remarkable frequency, noticed by T}^cho Brahe, and prominently 

 mentioned by Ad. Paulsen, of the occurrence of higher clouds in polar 

 light years, is explained. 



We have now reached a very interesting part of our discussion, in 

 which the facts observed by astronomers and meteorological obser- 

 vations stand in very close connection. Sir Norman Lockyer has 

 treated this subject very comprehensively in a report to the Interna- 

 tional Solar Committee in Southport in 1903, and I can therefore 

 refer you to this report. 



Among the most puzzling phenomena in connection with meteor- 

 ological data and known facts concerning the sun, the half-yearly 

 variation of barometric pressure is to be mentioned. This variation 

 shows a decided parallelism with the polar lights; so there is no doubt 

 of the existence of a common cause for both. 



The small charged particles in the highest layers of the atmosphere 

 are carried along by air-currents, and so give rise to magnetic phe- 

 nomena. Thus, the periodic daily variation of the earth's magnetic 

 field, and the cause for this variation being much greater (about 

 double) in years when sun-spots are prevalent, are explained. An 

 exhaustive study of this variation would perhaps furnish us with 

 a knowledge of the currents in the very highest layers of the atmo- 

 sphere. Since this knowledge is of the greatest importance in the inter- 



