
ON ELECTRICAL OBSERVATIONS AT KEW. 163 
Tasie LXXI. 
Monthly differences between the annual periods at sunrise and sunset. 

Jan,| Feb.|Mar.| April. 
52:9] 47°7| 34°6| 28:8 | 23:8] 23°6| 18-9] 19-0) 22:2 | 27-3) 27°8) 55°8| 31-6 
May.| June. | July. |Aug.| Sept. | Oct.|Nov./Dec.|Mean. 





It will be remarked that these differences gradually decrease from De- 
cember to July and gradually increase from July to December, but it should 
not be forgotten that the variability of the epochs of sunrise and sunset has 
a great tendency to produce a difference between the mean monthly values 
of the tensions of the two periods in the contrary direction, ¢.e. a greater 
difference in June and a less difference in December. In the summer, when 
we have the least difference in consequence of the general low tension of the 
season, the epochs of observation are the furthest removed from each other, 
that of sunrise nearly coinciding with or being but little removed from the 
epoch of the principal minimum, and that of sunset being brought within 
two hours of the epoch of the principal maximum: under these circumstances, 
and leaving out of consideration the effects of other movements, we ought to 
have the greatest difference between the tensions. On the other hand, in 
winter the epoch of sunrise occurs within two hours of the forenoon maximum, 
and that of sunset nearly coincides with the afternoon minimum ; it is conse- 
quently manifest that the differences existing under these circumstances should 
be the least, and the entire series of monthly differences ought to enter as 
corrections in deducing the true annual period from observations at sunrise and 
sunset. It is however clear, from the series of differences before us, that this _ 
object cannot be attained by a mode of discussion which regards them only, the 
two opposite series of differences being mingled together in those presented 
to our notice ; but if we compute the ratios of the sunrise to the sunset mean 
tensions, we shall probably discover the effects of the recess and approach of 
the epochs of observation according to the season of the year, the further 
they are removed from each other the lower the ratio—coincidence of value 
being considered as unity—and on the contrary the nearer they approach 
each other the higher the ratio, the proportional difference being less. The 
following numbers clearly exhibit these proportional differences. 
Tasie LXXII. 
Ratio of the mean electrical tension at sunrise to that at sunset, for each 
month of the year. 

Jan.| Feb./Mar.|April.|May.| June.| July. | Aug.| Sept. | Oct. |Nov.|Dec.|Mean. 

[-————$ | —$— | ——$—. | —__—— — | | —— — | — | ————— 
*691| -618| -596| °578 |°536| -440| 570|-517| +502 |-501) 680/562) 596 
eer iene bese OND eee KIN, Ore iS ee ee ee 
In these ratios the effect of the variability of the epochs of observation is 
very apparent, the difference between June and January being as much as 
‘251, which is more than one-third of the ratio in January. In June the 
mean tension at sunrise is less than one-half of the mean tension at sunset ; 
‘in January it is considerably more than in June, being very nearly seven- 
tenths of the sunset mean tension. ‘There are two or three anomalies in the 
M2 
