248 Observations on Magnetism. 
It is certain that the sun having thus drawn the dews and 
moisture into the leaves, their hairs absorb and decompose 
it, and the sun thus reduces it to vapour. It is the light which 
produces this phenomenon much more than the caloric. J have 
often seen the water decomposed through the thin cuticle, just 
as it is done by the galvanic trough in the machine: but the 
light must be pretty strong. 
LIV. Observations on Magnetism: extracted from a Letter to 
Mr. C. Ruxxer, of Hamburgh, from Professor HANSTEEN, 
of Christiana *. 
Wirn the little oscillatory instrument, which you saw at my 
residence in London (consisting of a magnetized steel cylinder, 
suspended by a very fine silk thread, and inclosed in a glazed 
case), I observed here, at Christiana, in the months of Novem- 
ber and December 1819, and in March, April, and May 1820, 
seven or eight times every day, the time of 300 oscillations, by 
which I have found : 
First ; that the magnetic intensity of the earth is subject to a 
diurnal variation, so that it decreases, from the first hours of 
morning till about ten or eleven, when it arrives at its minimum ; 
from that time it goes on increasing till four in the afternoon, 
and, in the latter months, till six or seven in the evening. This 
force afterwards decreases anew during the night, and about 
three in the morning reaches its maximum ; whence it again 
returns, by little and little, to its mznzmum about ten or eleven 
in the morning, and so on continually. 
Second; that whenever the moon passes the equator, the 
magnetic intensity is considerably weaker in the two or three 
following days. 
Third ; that the magnetic intensity is still more reduced, du- 
ring the "appearance of an aurora borealis, and is so much the 
weaker as this meteor is extensive and powerful. ‘The common 
intensity returns only by degrees, and 24 hours afterwards. 
Fourth; that the magnetic intensity appears to have a very 
considerable annual variation, being stronger in the winter 
months than in the summer months. 
When the magnetic cylinder makes 300 oscillations in 813° 6 
seconds of time, I assume the corresponding intensity = 1-0000, 
and, as the intensities are in the inverse ratio of the squares of 
the time of the oscillations, we can always express, in these sup- 
posed parts, every intensity answering to the times of the oscil- 
lations. It is according to this supposition that I have observed 
and found the results which I offer here in the following tables. 
* From Zach’s Correspondance Astronomique, Geographique, Hydrogra- 
phique et Statistique. 
