330 ATMOSPHEKIC ELECTKICITY. 



not subsequently observe as mucli regularity in tlie electrical periods 

 on account of the cloudy weather. 



The observations of M. Arago indicate a first maximum toward nine 

 o'clock in the morning ; but as all the observations of the succeeding 

 hours have not been collected, we cannot determine the hour of the 

 second maximum, nor the hours of the two minima; still we can always 

 establish their existence. 



Schiibler* found that the electrical maxima and minima do not 

 always take place at the same hour of the day. They vary, particularly 

 with the periods of the rising and setting of the sun ; in the longest 

 days of summer the first maximum appears sooner, and in the shortest 

 days it happens later, and approaches the hour of noon ; the second 

 maximum, on the contrary, is retarded in the first instance, and hap- 

 pens sooner in the second. It follows that the greatest distance 

 between the first maximum and the second fall in the middle of the 

 summer, and the least in the middle of winter, when the two maxima, 

 especially in a cold and cloudy day, appear blended together. This 

 result does not at all accord with that which De Saussuref deduced 

 from his observations. " Generally in summer," he says, "when the 

 earth is dry, on account of the drought of the preceding weather, and 

 when a day occurs which is clear, dry, and warm, the electricity of the 

 air goes on increasing from the rising of the sun, when it is nearly 

 imperceptible until towards three or four o'clock in the afternoon, 

 when it acquires its greatest force. It then gradually decreases till 

 the moment of the fall of the dew, when it increases again, then 

 diminishes, and finally ceases entirely in the night." 



We ought, however, to remark that SchiiblerJ: sometimes observed 

 that the maximum happened later than ordinary in the morning, and 

 sooner in the evening, when the temperature of the air was colder 

 than usual at the same season. He observed this phenomenon also 

 during a severely cold winter, and in some cool days of summer. 

 However this may be, we give the mean periods of the two maxima 

 and of the first minimum for the different months of the year, such as 

 follow from the observations of this philosopher. As for the second 

 minimum, which happens in the afternoon, Schiibler found in all his 

 observations the same period of two o'clock, because atmospheric elec- 

 tricity at that hour is very near its minimum in all seasons, and in 

 winter it coincides with it. 



'■= Journal de Schweigger, tome VIII, page 25. 



t Voyages dans les Alpes, tome II, § 803, page 225. 



X Journal de Itchweigger, tome VIII, page 2G. 



