242 M. Kreil’s Magnetic Obsermtions. 
If these two needles are employed for determining the de- 
clination precisely in the same manner as described in p. 146 
of the 1° Supjplemento alle Ephemeridi di Milano^ a constant 
difference of nearly 8 minutes is found between their results, 
the weaker needle (No. XVII.) giving the greatest declina- 
tion. With needles of still weaker magnetism, some of which 
have been made here, this difference amounts even to half a 
degree, not only when the observations are made within the 
influence of the iron of Brera, but also in the open meadow. 
All the observations of declination hitherto published have 
been made with the stronger needle (No. 4.). I have not been 
able to discover any reason for this phaenomenon, unless it is 
caused by temporary magnetism. However it showed itself 
in the first year too plainly to be doubted ; and I should have 
mentioned it before, if I had not been desirous of obtaining 
the greatest certainty which repeated observations could give, 
and of waiting to see whether the same circumstance might 
not have been remarked by other observers. In respect to 
the horizontal intensity I have not as yet been able to per- 
ceive any difference between the determinations made with 
these two needles. 
The observations for the periodical changes were made six 
times a day ; they included in the first year observations of de- 
clination and of the time of vibration of the horizontal needle ; 
in the second year the inclination was observed in addition 
to the above phsenomena ; and in the third year the time of 
oscillation of the dipping-needle was also included. The 
result has shown that no clear view of these complicated phae- 
nomena can be obtained unless they are all observed. The 
results derived from our observations are the following: 
1. In Milan, and at the present epoch, the horizontal por- 
tion of the magnetic force reaches its least daily intensity 
between 8 and 10. 30 a.m., it then immediately begins to in- 
crease rapidly and attains its greatest intensity between 4. 30 
and 7. 30 p.m., after which it decreases. An irregularity 
shows itself in the increase of the force, which is still rapid 
between 1 and 2 o’clock, becomes almost imperceptible be- 
tween 2 and 4, is again more rapid between 4 and 6, and is 
then changed into a decrease; we shall see presently the ex- 
planation of this. 
2. These epochs of the maximum and minimum of the 
horizontal intensity, which as they are obtained from all the 
observations may be termed mean epochs, are not constant. 
In the summer months the minimum is earlier and the maxi- 
mum is later than in the winter. 
3. The difference between the maximum and minimum is 
