

/ 
ON ELECTRICAL OBSERVATIONS AT KEW. 121 
rises above 100 div. of Volta No. 1, except at particular hours; this will 
form a subject of discussion further on; in the meantime it enables us to 
_ gain some insight into the reason of the diurnal period during the summer 
months in each year being more in accordance with itself than that of the 
entire year. ‘The defect of the early morning hours is not so great as the 
- excess at 10 p.M.; consequently the mean line cuts the entire curve more 
equably, exhibiting the two maxima above, and the two minima below it. 
This doubtless arises from the very few tensions above 50 div. that occur 
during the summer nights, as well as from the observations at 6 A.M., which 
are generally low. We have therefore a period that differs but little, if any, 
from the natural progression of the electrical tension: 2 A.M. is the epoch of 
the principal minimum; the tension gradually rises from this hour until 10 a.M., 
the forenoon maximum ; the succeeding minimum occurs at noon, the de- 
cline in the two hours being 13°3 div. ; the rise is then very slow and gradual 
until 4 p.M., only 1°8 div.; at 6 p.m. the tension increases and mounts rapidly 
until 10 p.m., the principal maximum ; the decline is then very considerable 
from 10 p.m. to midnight. 
TABLE IX. 
Comparison of the excess or defect from the mean of the diurnal periods de- 
duced from all the observations, and from those made during the summer 
months. 





Season. | Mid.|2 a.m.|4 a.m.|6 a.m.'8'a.m.|10 a.m.|Noon.|2 p.m.{4 p-m.|6 p.m.|8 p.m.|10 p.m.|Mean. 
diy. | div. died div. | div. | div. ae: an div. div. diy. diy. div, 
—|—-}—-|/—-|/4+} 4+ ]}/4+]}4+}+])4+]4.] 4 
Year . . | 44-3) 46-8) 46-4] 32°7| 1:3 | 21:2] 85 | 46 | 2:2 | 17-9] 35°5| 37-1 | 66°9 
—~{|-—|—|—-—| 4+] + —|/+)+ |] + 
) Summer.| 15-9} 19-4] 18-3} 4:0| 6:4 95 | 38) 271| 2:0 1°7| 13:6} 26:2 | 37:2 

The above table places the diurnal period of the summer months in con- 
trast with that of the entire year. 
The annexed curves (fig. 3) exhibit the diurnal march of the tension 
during the summer months. The same similarity of movement is noticed as in 
the yearly curves; it is however worthy of remark, that the depression in or 
about the afternoon does not differ very essentially from that of the entire 
year, with the exception of the minimum occurring at noon. During the 
summer the evening maximum is 16°7 div. above the forenoon maximum, 
and during the entire year it is 159 div. The afternoon minimum is de- 
pressed below the evening maximum during the year 34°9 div., during the 
summer it is 30°0 div. This is in decided contrast with the lower branches 
of the curves, which exhibit a much greater difference. The difference of 
range in the two series of curves has not been exhibited, from the considera- 
tion that the nocturnal minimum of the entire year is probably too low. 
Diurnal period. Winter—The months constituting the winter half-year 
are, October, November, December, January, February and March. In the 
tables that follow, the means are not of consecutive months, but of January, 
February and March at the commencement, and October, November and 
December at the end of each year. 

