198 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
rain, and it is perhaps to be expected that the curves would agree best 
under these circumstances ; but when the wild and rapid variations, often 
ranging from +500 volts/metre to —500 volts/metre, are considered, it is 
remarkable that the two stations, so distant from one another and in such 
different surroundings, should give curves which agree down to small 
details. 
As the change from positive to negative is often a very sudden one, 
it has been found possible to get some numerical data with regard to the 
times at which corresponding changes took place. 
The following table gives the results of the examination of some curves, 
and it will be seen that the points which from the curve obviously 
correspond to one another occur at very nearly the same time at the two 
stations : — 
Table II. 
Date, 
Times of 
Corresponding Points. 
Type of 
Potential 
Gradient. 
Wind. 
Remarks. 
1912. 
Physical 
Laboratory. 
Royal 
Observatory. 
Jan. 
6 
6.20 p.m. 
6.18 p.m. 
Neg. 
E. 
Calm, snow-shower in 
evening. 
55 
6- 
8.38 p.m. 
8.30 p.m. 
Neg. 
E. 
Fresh breeze. 
55 
6 
9.11 p.m. 
9.11 p.m. 
Neg. 
N.E. 
Fresh breeze. 
55 
7 
8.30 a.m. 
8.25 a.m. 
Pos. 
N. 
Breeze, frosty, fine. 
55 
11 
12.08 a.m. 
12.06 a.m. 
Neg. 
E.S.E. 
Gale, dull. 
55 
11 
12.28 a.m. 
12.30 a.m. 
Neg. 
55 
11 
1.14 a.m. 
1.13 a.m. 
Neg. f Ne 
g. during 1 
3 mins. 
55 
11 
1.27 a.m. 
1.26 a.m. 
Neg.J 
at both sta 
tions. 
55 
11 
4.09 a.m. 
4.11 a.m. 
Neg. 
S.S.E. 
55 
11 
6.42 a.m. 
6.40 a.m. 
Neg. 
55 
11 
8.06 a.m. 
8.12 a.m. 
Neg. 
S.S.W. 
Moderate breeze. 
55 
11 
8.29 a.m. 
8.34 a.m. 
Neg. 
55 
13 
2.58 a.m. 
3.06 a.m. 
Neg. 
s' 
Fresh breeze, fine. 
55 
13 
3.40 a.m. 
3.45 a m. 
Neg. 
55 
55 
13 
3.54 a.m. 
3.57 a.m. 
Neg. 
55 
55 
13 
4.06 a.m. 
4.06 a.m. 
Neg. 
55 
55 
14 
3.15 p.m. 
3.12 p.m. 
Neg. 
s'.' 
Moderate breeze, rain. 
55 
14 
4.44 p.m. 
4.46 p.m. 
Neg. 
S.S.W. 
55 
55 
14 
5.12 p.m. 
5.21 p.m. 
Neg. 
S.S.W. 
Light breeze. 
55 
15 
1.11 p.m. 
1.11 p.m. 
Neg. 
S.E. 
Breeze, damp, overcast. 
55 
16 
12.21 a.m. 
12.20 a.m. 
Neg. 
S.S.E. 
Gale, showery. 
55 
16 
2.23 a.m. 
2.26 a.m. 
Neg. 
S.S.E. 
55 
55 
16 
3.04 a.m. 
3.00 a.m. 
Neg. 
S.S.E. 
55 # 
55 
16 
7.50 a.m. 
7.54 a.m. 
Neg. 
S.S.E. 
Gale, rain. 
The times of apparently corresponding points have been measured to 
within two minutes, but too much stress must not be laid on the time 
differences on account of the difficulty, already mentioned, of picking out 
