308 DR. C. CHREE: ATMOSPHERIC ELECTRIC POTENTIAL RESULTS AT KEW 
the forces to which the diurnal inequality is due as is the size of the 24 hourly 
differences from the mean. Both these quantities, as recorded in Table III., agree in 
representing a maximum of activity in February and a minimum in November, 
though the latter month differs hut little from June, July, August and September. 
The annual variation of the sum of the 24 differences is represented by the heavy 
broken line in fig. 1, unity representing the mean of the 12 monthly values. 
§ 7. In view of theories as to the cause of the diurnal variation, interest attaches 
to the comparative size of the potential changes during the day and the night. The 
forenoon maximum and afternoon minimum occur at Kew when the sun is above the 
horizon, the afternoon maximum and the morning minimum when the sun is below 
the horizon. I have thus taken the difference between the first-mentioned maximum 
and minimum as a measure of the change of potential during the day, and the 
difference between the other maximum and minimum as a measure of the change 
of potential during the night, terming the first the “ Day fall, the latter the 
“ Night fall.” 
Table IV. 
Absolute values, volts 
per metre— 
Ratios borne to corresponding mean 
monthly values of potential 
gradient by— 
Day fall. 
Night fall. 
Day fall. 
Night fall. 
Range. 
January . 
35-7 
7U0 
0-178 
0-353 
0-363 
February . 
61-1 
87-0 
0-273 
0-389 
0-420 
March. 
63-6 
79-5 
0 • 354 
0-442 
0-462 
April. 
74-1 
52-7 
0-535 
0-381 
0-535 
May. 
54-9 
47-7 
0-447 
0-388 
0-575 
June. 
46-8 
43-7 
0-423 
0-395 
0-513 
July. 
54-3 
47-6 
0-556 
0-488 
0-567 
August. 
48-3 
59-6 
0-422 
0-520 
0-520 
September. 
40-0 
54-0 
0-330 
0-446 
0-446 
October. 
62-6 
58-0 
0-409 
0-379 
0-409 
November. 
28-6 
51-7 
0-143 
0-259 
0-259 
December. 
33-7 
82-1 
0-139 
0-338 
0-338 
Arithmetic means for— 
Year. 
50-3 
61-2 
0-351 
0-398 
0-451 
Winter. 
39-8 
73-0 
0-183 
0-335 
0-345 
Equinox. 
60-1 
61-0 
0-407 
0-412 
0-463 
Summer. 
51-1 
49-7 
0-462 
0-448 
0-544 
Table IV. shows the day and night falls from the diurnal inequalities of the 
several months, and also the ratio borne by these two quantities and by the daily 
range to the mean monthly value of P. Considered absolutely, the day fall has a 
