of Professor Hanst£:ex. ^99 
when the sun is in the tropics, or, perhaps more correctly, when 
the earth is in its perihelion or aphelion. It is a minimum near 
the equinoxes, or when the earth is at its mean distance from 
the sun. The greatest daily variation is least in the winter, 
and greatest in the summer. The greatest difference of the an- 
nual variation is 0,0359. 
Suppose I call the horizontal part of the magnetic force 
= H, the whole force ” F, the magnetic inclination n ; then is 
F = H X cos n. Hence it follows, that H may be variable, 
although F be constant, allowing only the inclination to be va- 
riable. From a series of observations which I made with an 
inclinatorium (a dipping-needle) of Dollond’s, I found, that the 
inclination during the summer is about 15' greater than what it 
is in the winter, and in the forenoon about 4' or 5' greater than 
what it is in the afternoon ; which agrees perfectly well with the 
former observations. ' 
I next made an experiment like that of October 1. continued 
without interruption from noon till noon, during an aurora bo- 
realis, which clearly proves the influence of this phenomenon of 
weakening the magnetic force, and consequently shews the rela- 
tion between magnetism and electricity.” Similar experiments 
were made by the Professor, which prove that the magnetic in- 
tensity is always weakened when the moon crosses the equator* 
In the course of last year. Professor Hansteen spent a few 
days in Copenhagen, and resided in the Round Tower. Here 
he repeated his experiments, and found, to his surprise, that the 
duration of 300 vibrations was not less than 836", 57, although 
he found it in a garden below only 719. Now, from Bugge’s 
Ohservationes Jstronomic^e, it is known, that the walls of this 
tower, which is 126 feet high, are 4 feet 4 inches thick ; and an- 
other hollow cylinder of 4 feet 6 inches inner diameter is contained 
within it, round which a walk of seven turnings leads to the top. 
After several experiments below, which all gave 787" for the 
mean duration, Hansteen returned to the tower, and found in the 
highest part.of the tower the duration of 300 oscillations =:842",37 
1 turning below, - « - 836,57 
2 turnings lower, got - - - 837,30 
1 1 turning lower, - . _ 834,43 
2 turnings lower, - - , 804,07 
Down below, within doors, - - 813,00 
