Proceedings of the Polytechnic Association. 919 



i. <?., a stratum nine inches thick per month. This, when evaporated, 

 would give a stratum of steam at 212'' Fahrenheit, and under ordinary 

 atmospheric pressure forty-tliree feet in thickness per day, to be formed 

 and condensed. 



This, if I have figured correctly (supposing the sea to cover 150,- 

 000,000 square miles, and a cubic foot of water to weigh sixty-two 

 and a half pounds), amounts to 2,157,463,500 of tons to be evapor- 

 ated per minute. This vast amount is diffused daily through the 

 atmosphere. 



Now, if the smallest evolution of vapor from salt water gives an 

 effect, appreciable by the electrometer, is it not surprising that in the 

 formation and condensation of the immense amount shown by these 

 figures, we do not encounter infinitely more of visible phenomena 

 than we do ? 



We have now seen that the atmosphere is constantly charged with 

 positive electricity, furnished by the vapors that arise from the sea, 

 and that the earth is negatively electrized. 



Tropical Curkents. 



In the tropical regions, where the water is most salt, and evaporation 

 most abundant, there is an upward current which carries the vapor 

 to a great height, and then, setting out both north and south, consti- 

 tuting tropical currents, which descend in proportion as they reach 

 the higher latitudes. On reaching a region sufficiently cold, precipi- 

 tation of snow, or of ice in some other form, takes place, in which 

 electrical phenomena are almost sure to appear. 



Under the tropical currents are boveal and austral currents, on 

 their way to take the place of the ascending lieated air of the tropi- 

 cal region. 



The enormous amount of electrified vapors, rising high in the 

 torrid zone, and men descending as they approach the poles, come 

 in contact with many conflicting conditions in the atmosphere strata 

 which underlie them, so that the neutralization of their positive elec- 

 tricity is allowed to occur very difl^erently at different times and 

 places, and under different circumstances. 



All these are comparatively local, and are variable according as 

 they are over the ocean, the land or fresh water lakes, and according 

 to variations of tefnperature and the different electrical conditions 

 met with. 



In many regions of the temperate zones vapors arise which are 



