POTENTIAL AND EARTH CURRENTS. 179 



trie condition of the earth's surface, its electricity is 

 doubtless affected, directly, by conditions similar to 

 those which affect the earth's electricity. 



In its combination of dry air arid watery vapor ; the 

 one, an insulator, and the other, a conductor ; separate 

 parts heated and cooled alternately, twice in twerrty- 

 four hours, we have thermo-electric conditions similar to 

 those already noticed in the earth's surface ; though 

 the resulting electric disturbance is, perhaps, less in- 

 tense, as the composition of the atmosphere is nearly 

 uniform, while that of the earth's surface presents great 

 diversity. 



DIFFERENCE OF POTENTIAL BETWEEN ATMOS- 

 PHERIC STRATA. Another cause of atmospheric elec- 

 tric disturbance is found in the great difference of 

 electric resistance between the upper and lower atmos- 

 pheric strata; caused by the density below and rarity 

 above. This resistance makes the dense lower stratum, 

 where most of our observations are made, an excellent 

 insulator ; while the rarity of higher strata facilitates 

 electric transmission ; a constant decrease of resistance 

 taking place, from the lower to the higher, till a point 

 is reached, where it is reduced to that of the ordinary 

 Geissler tube ; while, in still higher strata, the resist- 

 ance increases, on account of the extreme rarity of the 

 air : which equals, and finally exceeds, that of the best 

 vacuum tubes. 



The existence of a corresponding difference of elec- 

 tric potential has been proved by numerous experi- 

 ments ; among which may be noted the following : 



From an elevated position, a metal - pointed arrow 

 was shot upward to a vertical height of 250 feet: a con- 

 ducting cord, connected with it, and properly insulated, 



