133 
we shall find that the curve representing the daily variations 
ot declination of the magnetic needle has well marked points 
of similarity to that of the daily rainfall. Taking for data 
the mean results of ten years’ observations of magnetic 
declination made at the Royal Observatory, Greenwich, 
( Greenwich Observations for 1859,^ we have the following 
comparison of the times of maxima and minima of the two 
phenomena, the westerly deviations of the magnetic needle 
from the mean position, being taken with a negative sign. 
Daily Rainfall. 
Daily Oscillations of 
Magnetic Needle. 
Principal maximum at 
8*h. 
a.m. 
lOh. a.m. 
Secondary „ „ 
8h. 
p.m. 
7|h. p.m. 
Principal minimum „ 
2h. 
>> 
lh. „ 
Secondary ,, „ 
1th. 
a.m. 
3h. a.m. 
Owing to the shortness of the period over which Mr. 
Bates’ observations extend, it is probable that the results 
derived from them may be open to considerable correction, 
when a more extended series becomes available. I find, 
however, that when this short series is divided into tu o or 
even three groups, the main features of the daily variation 
are still preserved in the results of each group, and I do not, 
therefore, anticipate that any corrections which may hereafter 
be found to be necessary, will be of such a character as to 
affect materially the probability of a close connection between 
the daily variations of rainfall, and the diurnal oscillations of 
the magnetic needle, and I hope to be able in a future 
communication to bring forward other facts and results 
bearing upon this interesting and important question. 
Whatever views may be taken as to the cause of the unequal 
distribution of the rainfall in the different hours of the day, it 
could, at all events, hardly have been expected that the 
.greatest amount of rain would fall at that period of the day 
when the rate of increase of the difference between the tem- 
perature of the dew point and that of the air, is greatest ; and 
/?.£- 
