146 Mr Christie on the Diurnal Variation 
was observed by Mr Barlow and myself, arose from the cause 
which I have assigned for it in my former paper ; namely, the 
difference in the changes of temperature in the magnets when 
in-doors and when in the open air. 
66 The diurnal changes in the terrestrial magnetic intensity 
have been determined by Professor Hansteen, by means of the 
vibrations of a needle delicately suspended. From these obser- 
vations it appears, that, in general, the time of minimum inten- 
sity was between 10 and 11 o’clock in the morning ; that the 
maximum happened between 4 and 7 for the month of May 
1820, and about 7 o’clock in the evening for the month of June. 
The intensity which, in these observations, is taken as unity, is 
that deduced from an observation made during an aurora bo- 
realis ; but, for the purpose of comparison, I have, for the 
months of May and June, taken the intensity deduced from his 
observations at 10 h 30™ in the morning as unity, reduced the 
intensities, which he gives for other times in the day, to this 
standard, and placed them in the following table, with the cor- 
responding intensities deduced from my own observations. 
Intensity deduced from Hansteen' s 
Observations in 1820. 
Intensity deduced from the preceding 
Observations in 1823. 
Time. 
May. 
J une. 
Time. 
May. 
June. 
8 h 00 m A. M. 
1.00034 
1.00010 
7 h 30 m A. M. 
1.00114 
1.00061 
10 30 
1.00000 
1.00000 
10 30 
1.00000 
1.00000 
4 00 p. m. 
1.00299 
1.00251 
4 30 p. m. 
1.00175 
1.00223 
7 00 
1.00294 
1.00302 
7 30 
1.00220 
1.00239 
10 30 
1.00191 
1.00267 
9 30 
1.00231 
1.00209 
“ The principal difference to be observed in the nature of the 
changes of intensity during the day, in the two cases, is, that, 
from my observations, the intensity appears to decrease more 
rapidly in the morning, and increase more slowly in the after- 
noon, than it does from those of Professor Hansteen ; but the 
general character of these changes is as nearly the same as we 
can expect from methods so different, at different times, and at 
places where both the variation and dip of the needle are dif- 
ferent. My object, however, was to point out what might be 
deduced from a series of such observations as I have detailed, 
rather than to compare the results deduced from them with 
