328 DR. C. CHREE: ATMOSPHERIC ELECTRIC POTENTIAL RESULTS AT KEW 
For the ratio of (potential decrease)/(temperature increase) we have from— 
Winter and equinox. 
Winter and summer. 
Equinox and summer. 
Ratio. 
7-2 
5-3 
3-5 
Mean of temperatures . . . 
44°-0 
49°-2 
54°-0 
The values thus diminish markedly as the mean temperature alters. Further, for 
nearly equal increments in the mean temperatures we have nearly equal decrements 
in the ratio. Also, if the ratio were to diminish at the same rate as the mean 
temperature rose further, we should have it vanishing altogether below 64°, i.e., 
below the mean temperature of the selected days in July. 
§ 23. The only other meteorological element which I have investigated in the same 
way as pressure and temperature is the duration of sunshine. Several authorities, 
e.g., Elster and Geitel at one time, have regarded ultra-violet radiation from the 
sun as probably a most important agent in the phenomena of atmospheric electricity. 
If this were the case, one would expect to find a conspicuous difference in summer 
between days of much and of little sunshine. The grouping of the days according to 
sunshine in the way described above led to the following results for the excesses in 
the groups of days of most sunshine :— 
Table XVI. 
Sunshine, 
hours per day. 
Mean potential. 
Non-cyclic effect. 
Potential 
range. 
Algebraic. 
Numerical. 
June. 
7'2 
- 9 
- 4 -27 
+ 2 
July. 
6-7 
+ 23 
+ 28 1 +9 
+ 2 
August .... 
6*0 
+ 7 
- 8 +14 
-4 
If the July data alone existed, then the conclusion one would naturally draw from 
the above figures—-just as from the corresponding figures in Tables XIII. and XIV.— 
is, that with long duration of sunshine there is a very appreciable rise both in the 
mean potential and in its daily range. If June and August however are combined, 
we get for practically the same excess in hours of sunshine a wholly negligible effect 
on the value of the mean potential ; whilst the apparent effect on the range, such as 
it is, is opposite in sign to that observed in July. 
§ 24. The effects of temperature, barometric pressure, wind and sunshine on the 
simultaneous values of the potential gradient at Kew were also considered in my 
