327 
1912-13.] Atmospheric Electric Potential Results. 
hours of the day, while irregularities appear later, so that it would seem as 
if the effect of soot, etc., was by no means the sole or even the most important 
cause of the irregularities. 
The daily and yearly variation of the potential gradient at Edinburgh 
is also shown graphically by fig. 5, in which the isopleths corresponding to 
potential gradients of 300, 250, 200, 170, 150, 130 volts per metre have been 
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan. 
Fig. 5. 
drawn. After the examples of Lutz * and Blumenschein, f curves showing 
the times of sunrise and sunset have been inserted in this diagram ; they 
are indicated by the dotted lines. 
§ 5. The curves for the various months and seasons of the year have 
been analysed into Fourier series of the form — 
y = a 0 + a 1 sin ( t + aj) + a 2 sin (2k+ a 2 ) + ... 
* Munich. Ber ., p. 305, 1911. 
t Loc. cit. 
