334 DR. C. CHREE: ATMOSPHERIC ELECTRIC POTENTIAL RESULTS AT KEW. 
The annual variation of c 3 is regular and strongly developed, but is of a very unusual 
character. The winter and summer values of a 3 in Table XVIII. are nearly opposite 
in phase. There is also a considerable difference between the winter and summer 
values of a 3 in Table V,, but the difference is twice as big for B as for P and is exactly 
in the opposite direction. 
In the case of the 6-hour term the phase angles in Band Pvary somewhat similarly 
with the season, and the difference in the case of the mean diurnal inequalities for 
the year represents a difference of only some 10 minutes in time. The annual 
variation of c 4 is somewhat irregular in both elements, and a long series of year’s data 
would be necessary for a satisfactory comparison. 
§ 32. When comparing the Kew data with those from the Bureau Central and the 
Eiffel Tower, we found that the 12-hour term in the diurnal inequality of potential 
was the only one which presented under all conditions closely similar features as 
regards times of maxima and minima, and as regards amplitude near the ground. 
Similarly, it is well known that the 12-hour term in the diurnal inequality of 
barometric pressure presents at the ground level phenomena of a much more regular 
character than does the 24-hour term. The latter term seems largely dependent on 
local conditions, whereas the 12-hour term is usually pretty much the same at places 
of nearly the same latitude. These phenomena are shown in a striking fashion by 
comparing the data of Table XVIII. with corresponding data for Jersey* based on 
observations by Dechevrens. 
In view of these facts, it is highly significant that the general resemblance that 
has been noticed between the diurnal inequalities of potential gradient and barometric 
pressure is now found to proceed almost entirely from the parts that have a 12-hour 
period. At Kew, at least, so far as the 24-hour terms are concerned, it is a case not 
of resemblance, but of marked dissimilarity. 
During the seven years whose data have been considered, the charge of the 
electrograph and the taking of the absolute observations have devolved upon 
Mr. E. G. Constable, the Senior Assistant engaged in the meteorological work of 
the Observatory Department, and his Junior Assistant, Mr. E. Boxall. The 
selected days were chosen by Mr. Constable in consultation with myself. 
The chief burden of the preliminary work was borne by Mr. Constable, who drew 
the free-hand pencil curves and took all the measurements. A good deal of the 
arithmetical work was done by other members of the staff, especially by Messrs. 
Badderly and Francis. All the reductions and calculations have been carefully 
checked, mostly by myself, and notwithstanding the multiplicity of details I hope no 
serious errors remain undetected. It remains to express my thanks to the Meteoro¬ 
logical Office—to whom the Electrograph belongs—and to the Director, Dr. Shaw, 
for permission to publish the results. 
* ‘ Meteorologische Zeitschrift,’ August, 1905, p. 384. 
7 JULMI6 
