at a slow rate. This increase will continue, becoming more rapid for some 

 hours after sunrise, when it will attain a maximum ; after which it will again 

 decrease, at first slowly, and afterward more rapidly. This gradual decrease 

 will continue for some time after the sun passes the meridian, when it will 

 cease, the electrical intensity again attaining a minimum. It will then begin 

 to increase, at first slowly, and afterward more rapidly, until it attains another 

 maximum sometime after sunset. It will then begin to decrease, and continue 

 to decrease until midnight. 



If the line M N M, fig. 1, be imagined to represent the interval of time be- 

 tween midnight and midnight, its middle point, N, representing the intermedi- 



Fig. 1. 

 b' 



B' 



ate noon, and the other points the various hours before and after noon, and if 

 from each point, such as P, a perpendicular be drawn, representing the inten- 

 sity of the atmospheric electricity at the hour corresponding to P, a curve 

 would be formed, the distances of which from the line M N M would represent 

 the electric state of the atmosphere. 



The undulating line X b B b' B' X then represents, in its general character, 

 the diurnal variation of the electricity of the atmosphere when the weather is 

 clear and no extraordinary disturbing influence intervenes to modify the com- 

 mon effects. The points a and a' represent the times of the morning and eve- 

 ning minima, and the perpendiculars a b and a' b' the values of these minima ; 

 and the points A and A' represent the morning and evening maxima, and the 

 perpendiculars A B and A! B' the values of these maxima. 



If, throughout the same parallel of latitude, no disturbing cause be supposed 

 to be in operation, and the production of electricity in the same position of the 

 sun be everywhere the same, the state of the electricity of the air around the 

 parallel may be represented in a similar way. Let E N W M, fig. 2, repre- 



Fig. 2. 



sent the parallel ; E N W the enlightened, and E M W the dark part ; C S 

 the direction of the meridian passing through the sun. 



At the point N the time will be noon, and at M it will be midnight ; at E it 

 will be sunset, and at W sunrise. The point a represents the place where the 

 electricity is at the morning minimum, and a' where it is at the evening mini- 



