322 Proceedings of the Boyal Society of Edinburgh. [Sess. 
All curves were measured except those which had to be rejected on 
account of the freezing of the water-spout or breakdowns of the insulation 
or of the mechanism of the apparatus. In deducing mean values all the 
measurements made were used. 
§ 4. Table I. gives the mean value of the potential gradient in volts 
per metre for each hour of the day, for the various months of the year. 
The last three columns give the diurnal variation for the whole year and 
for the winter and summer months. By winter is meant the six months 
October to March, while April to September are included under summer- — 
a division of the year which will be discussed more fully later. 
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan. 
Fig. 2. — Annual Variation. 
In the last row of Table I. will be found the mean monthly values of 
the potential gradient, and fig. 2 shows graphically the variation of the 
mean monthly value during the year. It will be noticed that there is a 
well-marked maximum in January, and a minimum during the summer 
months. Of course it is not to be expected that such a curve deduced 
from one year’s observations would be a smooth one ; nevertheless it 
is clear that the mean value undergoes a periodic change during the 
year with a minimum in the summer and a maximum in mid-winter. 
The results obtained above are in agreement with the annual varia- 
tion got by other observers, e.g. at Kew,* Karasjok,-]- Potsdam, J and 
Kremsmunster,§ etc. 
Table II. gives the diurnal inequality for the various months and seasons 
* Chree, Phil. Trans., 206, 299, 1907. 
t Simpson, Phil. Trans., 205, 61, 1906. 
X Liideling, Meteor. Zeit., 23, 114, 1906. 
§ Blumenschein, Wien. Ber., 121, 25, 1912. 
