178 ELEMENTS OF STATIC ELECTRICITY. 



January, and the minimum in May and June. They 

 are doubtless due, in part, to the different conduct- 

 ivity of the earth's surface in summer and winter, 

 as already mentioned ; and also to the dry winter 

 atmosphere, when atmospheric insulation is high, as 

 compared with the damp atmosphere of spring and 

 early summer, when it is low ; the greatest minimum 

 occurring in the months when our atmosphere, in the 

 north temperate zone, is most heavily laden with vapor. 

 At this season the earth is covered with green, suc- 

 culent herbage ; wet with frequent showers, and laden 

 at night with heavy dews ; forming a conducting sur- 

 face, which offers but slight resistance to electric trans- 

 mission. 



Towards the close of summer, the grain ripens, the 

 showers become less frequent, the dews lighter, and a 

 vast expanse of dry straw and stubble, with a parched 

 soil beneath it, offering high electric resistance, takes 

 the place of the former conducting surface. As fall ad- 

 vances, and the grass becomes dry and withered, and 

 the trees shed their leaves, there is a constant increase 

 of this surface resistance, and a corresponding increase 

 of electric potential, till the winter maximum is reached. 



While this difference of conductivity in the land 

 surface is taking place, the conductivity of the water 

 surface remains practically constant : hence the period 

 of minimum potential corresponds to that in which the 

 difference of conductivity, between the land and water 

 surfaces, is least; while the period of maximum poten- 

 tial corresponds to that in which it is greatest ; point- 

 ing clearly to this difference as a probable cause. 



In addition to the changes of electric potential, in- 

 duced in the atmosphere by these changes in the elec- 



