198 REPORT—1849. 
electricity from 10 A.M. to 6 P.M. At 8 p.m. the diminution is so exceedingly 
slight as almost to indicate a tendency to rise at that hour, and at 10 p.m. we 
have a decided increase: but in connexion with this, it should be borne in 
mind, that at one of the 33 observations contributing to its determination, the 
Henley’s electrometer read 70°; and it is easily seen that this high tension very 
materially influences the result, for if we abstract it, the mean tension is 
lower than that at 8 p.m. With regard to the mean tensions at midnight 
and 2 a.., the same remarks apply which we offered relative to the positive 
tensions at these hours (see pages 118, 119) ; they are for the same reason pro- 
bably lower than the truth, and indeed more particularly so in the case of nega- 
tive electricity ; for it is likely that when such electricity has been indicated by 
the conductor on other occasions than the eight and twelve recorded, it has 
exhibited much higher tensions than 50 div. of Volta No. 1. The remarkable 
difference between the values of the mean of all the positive observations for 
three years (66:9 div.) and of all the negative during 43 months (725°3 div.) 
is exceedingly interesting, as indicating at once the character of the moye- 
ments giving rise to the negative exhibitions, viz. disturbances. 
Fig. 19. 
Negative 
Electricity. 
Cloudiness. 

Diurnal Curves of Negative Electricity and Cloudiness. 
The annexed curves (fig. 19) exhibit to the eye the principal diurnal 
phznomena of negative electricity and cloudiness: 1000 divisions of Volta’s 
electrometer No. 1 are considered equal to two vertical divisions of the scale 
on which the negative tensions are projected ; eight hundredths of the scale of 
cloudiness being also considered equal to two of the same divisions. The 
points of the curve of cloudiness are placed about one-third of each horizontal 
division from the vertical or hour lines, the determination being at even hours 
of Gottingen mean time. The greater prevalence of cloud being in advance 
of the exhibition of negative electricity, which we noticed when treating of 
the frequency of its occurrence in the middle of the day, is very striking 
in the curves before us, which show that the same phenomena obtain in the 
comparison of the two, whether we regard the occurrence or the value of the 
tension of negative electricity. There is also another feature which ought 
not by any means to be overlooked ; it is the similarity in this respect that 
exists between the curves of negative electricity and cloudiness, and those of 
the annual period of positive electricity and humidity (see page 153). In. 
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