196 REPORT—1849. 
Taste XCVII. 
Comparison of the negative readings at Kew previous to 1845, with those 
also at Kew from January 1845 to July 1848 inclusive, and both with the 
mean amount of cloud at Greenwich from 1841 to 1846 inclusive, at hourly 
and two-hourly intervals. rt 
A.M. P.M. 
M.}1)2)3/4/5/6)]7)8)9j1O/11)IN.}1] 2]3)4)5/6]7/8]9 {10/11 


Neg....] ... Jese[e-sJeeefeee| 1} 5] 9)11]10/11)22/21/21)17/23/22)14}12/13)10) 7] 2)... 
Neg....| 8 |...|12).../12}...|18}...|34]...|56).../46]...|52).../55).../60).../38).../33]... 
Cloud .} ... }65].../67|...}69)...|70)...|71]...171)...]71].../69}...|56]...]62).../60}.../61 
The numbers in these tables agree, so far as the general fact is concerned, 
in exhibiting a greater quantity of negative readings during a portion of the 
day which is distinguished by a greater prevalence of cloud. Dividing the 
day into two periods, viz. from 8 A.M. to 8 p.m. and from 8 P.M. to 8 A.M., we 
find that the occurrence of negative electricity is very considerable in the 
day as compared with the night. Inthe three years 1845 to 1847 (including 
also the first seven months of 1848), which furnish a comparable scale of num- 
bers with regard to the cloudiness, the proportion of night to day negative 
readings is as 2 to 5 very nearly. The same portion of the day, viz. from 
8 a.m. to 8 P.M., gives,as compared with the remaining twelve hours, the great- 
est prevalence of cloud, the mean amount being about 68 hundredths of the 
whole sky: during the night the mean amount is 65 hundredths, or about 
three hundredths less. In connexion with this, it may be remarked that the 
greater prevalence of cloud is rather in advance of the development of ne- 
gative electricity: the period from 7 A.M. to 7 p.M., and vice versd, gives 
double the difference between the day and night cloudiness; the mean 
amount in this case for the day being very nearly 7 tenths, while that for the 
night is 64 hundredths, or about 6 hundredths less. The proportion of the 
negative readings is the same. From Table XCII. it may be inferred that 
on most occasions when negative electricity occurred, the sky was entirely 
covered with clouds; and this might suggest that it is not so much the general 
existence of cloudiness in the atmosphere that may be connected with ne- ~ 
gative electricity, as the presence of certain clouds—cumulo-stratus for 
instance, or more probably cirro-stratus, from its almost constant occurrence 
with negative electricity. ‘The remarkable changes that frequently occur 
from one kind of electricity to the other, often very suddenly, and at the 
same time very considerable in intensity, clearly show that at the time dis- 
turbances of no ordinary character prevail, and it may readily be conceived 
(in addition to the suggestion already offered) that different strata of cloud 
in different electrical states, operating on each other and on the earth, may 
very violently disturb the ordinary march either of the electricity of serene 
weather or of tle aqueous vapour ; and although these disturbances (taking 
them singly and considering the great uncertainty of their occurrence) may 
be regarded as purely accidental and obeying no recognized law of periodicity, 
yet should they result from causes which in themselves are not subject to 
mere accidental manifestations, but are the results of forces operating on the 
earth’s atmosphere in a definite manner—producing for instance a greater 
accumulation of cloud at one period of the day rather than at another, and 
giving rise to a well-defined march in the manifestation of the cloudiness of 
the atmosphere, within small limits it is true, but yet sufficient, from six years’ 
careful observation, to characterize the curve as that of a single progression 
