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ee en ee eee ee ee ann 
ON ELECTRICAL OBSERVATIONS AT KEW. 175 
In this table the depression of the summer readings below and elevation 
of the winter readings above the mean line at each epoch are very apparent. 
The differences however between the annual periods at each epoch and their 
approach to, or departure from, the mean is rendered more perceptible to the 
eye by the annexed curves (fig. 18). 



Curves exhibiting the annual periods of the electrical tension (low) at the four observation- 
epochs for the year 1844, 
It will be apparent from these curves that the forenoon and afternoon 
movements upon the annual period are strikingly different. There is con- 
siderable agreement between the curves of sunrise and 9 A.M., and also 
between those of 3 p.m. and sunset; the four curves forming two pairs, each 
pair presenting many features in common. The greatest difference between 
the curves is noticed at 9 a.M.and3 p.m. There is more accordance between 
the sunrise and sunset curves, the principal difference occurring in the opposite 
movements in September. The sunrise curve is evidently modified by the 
movements at 9 A.m., as is that of sunset by the movements at 3 p.m. From 
this we may probably infer that the movements in the middle of the day, at 
least between 9 a.m. and 3 P.M., are much more irregular even in the low 
tensions than at any other period. The range (see Table XCI.), which is 
lowest at sunrise, increases rapidly between 9 a.m. and 3 p.m. the maximum ; 
and this circumstance, taken in connexion with the cloudiness of the atmo- 
Sphere (a subject to which we shall have occasion particularly to refer when 
treating on negative electricity), strongly indicates that the irregularity of 
movement in the middle of the day is more or less connected with the dis- 
