ON ELECTRICAL OBSERVATIONS AT KEW. 197 
having an ascending and descending branch, the maximum occurring about 
40 minutes before noon, and the minimum between 9 and 10 at night—then 
they must necessarily exhibit somewhat of the same subjection to the laws 
of periodicity which is characteristic of the causes themselves. That the 
diurnal occurrence of negative electricity is of a periodical character, the ob- 
servations of five years, viz. from August 1843 to August 1848, testify in a 
very unequivocal manner; and although its connexion with the general. 
cloudiness of the atmosphere may not be satisfactorily made out, yet it by no 
means follows that it may not be more immediately connected with certain 
classes of cloud ; for as we have determined a diurnal period in the cloudiness 
generally, it is not unlikely that certain clouds, the cirro-stratus for instance, 
may likewise exhibit a diurnal period, being much more frequent in its oc- 
currence at one portion of the day rather than at another. Upon the whole, 
the negative readings are obvious indications of considerable disturbances, 
and their occurrence in much greater frequency at a particular period of the 
day renders it highly probable that the disturbances themselves are of a sy- 
stematic character and subject to well-defined laws of diurnal periodicity. 
Negative readings from January 1845 to July 1848 inclusive —During 
this period 424 negative charges of the conductor were observed. Their 
distribution among the twelve observation-hours is seen in the following 
table, which also includes the mean value of the negative tension at each 
observation-hour, and the excess or defect of such mean as compared with 
the mean of the whole. 
Taste XCVIII. 
Number of readings, mean tension, and excess or defect above or below 
the mean of all the negative observations from January 1845 to July 1848, 
as referred to the twelve observation-hours. 



Sums 
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.| and 
Means. 
8 | 12 12 |} 18 | 34 56 46 | 52 | 55 | 60 | 38 33 424 
div. | div. | div. | div. | div. | div. | div. | div. | div. | div. | div. | div. | div. 
36°0 | 24°8 |109-4 |316-3 1938-6 | 566:2 871-7 [891-3 |907°6 |729°9 |721°9 | 870°2| 725°3 
—~/-}-|—/+]/-{4+])4])4]/4])-] 4 
689°3 |700°5 |615°9 |409-0 |213:3 | 159°1 146-4 |166-0 |182°3| 4:6} 3:4| 144:9] 725:°3 




_ We have already alluded to the greater frequency of the occurrence of nega- 
tive electricity in the middle of the day, and have remarked that the period 
under consideration agrees with the previous seventeen months in this parti- 
cular. The line of mean tensions in the above table, in addition to the greater 
frequency of occurrence in the middle of the day, exhibits upon the whole 
period a corresponding increase of tension, particularly from 8 a.m. to 4 P.M., 
a portion of the day characterized by the greater prevalence of cloud (see 
Table XCVI.). The maximum occurs at 8 A.M., but. from the close approxi- 
mation in the values of the mean tensions at noon, 2 and 4 p.m., it can hardly 
be considered as the true inaximum of the diurnal period: it is to be remarked 
that only 34 observations contribute to its determination, and until a more 
extended series can be obtained, it must remain a matter of question. The 
mean tensions at noon, 2 and 4: p.M., taken in connexion with those at 10 A.M. 
and 6 P.M., present a well-rounded and very regular portion of a curve, which 
in the absence of further observations may probably be considered as repre- 
senting at least approximately the portion of the diurnal period of negative 
/ 
