424 Prof. E. Edlund on Atmospheric Electricity 



account of them would require too much space ; and it is be- 

 sides the less necessary as we already possess a statement, 

 followed by a very well executed critical discussion, of those 

 observations*. Some physicists have essayed to explain the 

 results obtained by observation by assuming that the air itself 

 is electropositive in its normal state ; while others have thought 

 that the surface of the earth is electronegative, and that it pro- 

 duces by induction the electrical phenomena observed in the 

 atmosphere. Opinions have been much divided upon the true 

 cause of the electric state of the air or of that of the earth's 

 surface. Some have assumed that evaporation from the sea, 

 lakes, and the wet surface of the ground renders the atmo- 

 sphere electropositive and the earth electronegative ; others, 

 on the contrary, that the distribution of electricity has its effi- 

 cient cause in vegetation, in the putrefaction of organic mat- 

 ters at the surface of the earth, in the friction of the air against 

 the ground or the water, or in the condensation and rarefac- 

 tion of the air, &c. More exact researches having shown that 

 none of these opinions can be correct, the phenomenon has 

 been attributed to a purely cosmic cause, having its seat in the 

 empty space surrounding the earth, or in other celestial bodies, 

 chiefly the sun. In a word, up to this hour no one has suc- 

 ceeded, either in consequence of observations, or by theoretic 

 considerations, in assigning a valid and trustworthy cause for 

 the electric phenomena of the terrestrial atmosphere. From 

 the above-mentioned observations, however, it appears that the 

 atmosphere, under normal conditions, can be regarded as elec- 

 tropositive in its lower strata (accessible to observation), that 

 the amount of electricity increases w 7 ith the height above the 

 earth's surface, and that the amount is subject to a diurnal 

 and an annual variation. 



From the preceding exposition of the cause of unipolar in- 

 duction it follows that the atmosphere must be electropositive, 

 and the earth electronegative. The electric molecules at and 

 beneath the earth's surface are directed by the force of terres- 

 trial unipolar induction into the atmosphere, where they accu- 

 mulate until they attain a certain density, dependent on the 

 greater or less conductivity of the strata of air. It is only 

 successively that, impelled by the same force, they arrive in 

 the upper strata of the atmosphere, endowed with a high de- 

 gree of conductivity, but certainly inferior to that of the metals. 

 At the same time that the electric molecules rise above the sur- 

 face of the earth, they are carried, in both hemispheres, from 

 the lower into the higher latitudes, where the electric density 



* Om den .?. k. lufteleklricite! en (Sur VeJectricite atmospherique), par 

 H.-E. Hamherpr. Upeala, 1872. 



