154 



ATMOSPHERIC ELECTRICITY. 



mum. In like manner A and A' represent the places where the electricity is 

 at the morning and evening maximum. The curve of electric intensity has, 

 therefore, the form of an oval ; the longer axis, B B', being inclined at a small 

 angle to the direction of the sun, and the lesser axis, a a', being at right angles 

 to it. As the position of the sun is gradually changed by its apparent motion 

 from east to west, these axes of the oval follow it, always keeping the same 

 relative position with respect to it in the absence of disturbing causes. 



The first philosopher who presented a complete and connected series of ob- 

 servations on the electricity of the air was Schiibler, who observed at Stutt- 

 gard, and published his observations, taken at various hours, daily, from May, 

 1811, to June, 1812. As an example of the actual succession of changes ex- 

 hibited in a single day, the following table of the observations taken on the 

 11th of May, 1811, will serve: — 



Perfectly clear. After a short 

 time the heavens became vapor- 

 ous, and dews began to fall. 



The heavens clear to the hori- 

 zon ; the tint of the firmament a 

 pure blue. 



Vapors begin t© be formed, and 

 dew falls. 



Heavens perfectly clear. 



ANNUAL VARIATION OF THE ELECTRICITY. 



As the diurnal change in the position of the sun, relatively to a given place, 

 produces a periodical variation in the electric state of the air, the change of its 

 declination from month to month may be expected to be followed by some cor- 

 responding periodical effect on the mean amount of the maxima and minima 

 values of the electricity. On comparing the mean values from month to month, 

 it is accordingly found that the values of the two daily maxima and minima 

 undergo a progressive decrease from January to July, and a progressive in- 

 crease from July to January. It is found, also, that during the winter the elec- 

 tricity of the air increases as the thermometer falls. 



On comparing the mean values of the maxima and minima throughout the 

 year, it is found that the morning values of each are a little less than the eve- 

 ning values. 



The hours at which the electricity attains its maxima and minima values are 

 likewise subject to variation from month to month. The hour of the morning 

 minimum and maximum continually advances toward noon from winter to sum- 

 mer, and undergoes the contrary change in the latter part of the year. 



The observations of Schiibler indicate that the hour of the evening minimum 

 is invariable. From June, 1811, to June, 1812, it took place at Stuttgard al- 



