332 Proceedings of the Royal Society of Edinburgh. [Sess. 
year at Kew and Edinburgh. It will be noticed that the agreement be- 
tween Kew and Edinburgh is very good (again with the exception of a 4 in 
winter) and is closer than that between Kew and Bureau Central, Paris. 
§ 7. The comparison between the values of the potential gradient and 
its diurnal and seasonal variations obtained at Edinburgh and other 
stations should give an idea of the effect of town air on these phenomena, 
since the Physical Laboratory is situated in the centre of Edinburgh and 
is surrounded by buildings. The above observations seem to show that 
this effect is smaller than one might expect, as was also indicated by a 
comparison of electrograph records obtained at the Royal Observatory, 
Blackford Hill, with those obtained at the Physical Laboratory.* A few 
isolated observations of the electrical conductivity of the air in the 
vicinity of the laboratory have shown that this quantity is of the order 
5 x 10" 5 E.S.U. 
The conductivity was measured by means of a Gerdien apparatus, and 
its value agrees with values obtained by means of a Wilson Universal 
electrometer at the Physical Laboratory in 1909 (Carse and MacOwan, 
loc. cit.), while Wilson and Gerdien obtained values of the order 5 x 10~ 4 
E.S.U. for country air. 
In view of the fact that a high conductivity is generally associated 
with a low value of the potential gradient and vice versa, the compara- 
tively low value of the conductivity at Edinburgh would naturally be 
accompanied by a high potential gradient. The mean value of the 
potential gradient for the year (167 volts per metre) is somewhat higher 
than that obtained at many stations in Germany. 
The apparatus used in this investigation was supplied by the Carnegie 
Trust and the Tait Memorial Fund, and we are indebted to the late 
Professor J. G. MacGregor, F.R.S., for the kindly interest he took in 
the work. 
* Carse, Shearer, Jameson, Proc. Boy. Soc. Edin ., vol. xxxiii. pt. ii. p. 194, 1913. 
Physical Laboratory, 
University of Edinburgh, 
June 1913. 
{Issued separately December 8, 1913.) 
