249 
M. Kreil’s Magnetic Observations, 
at hand to be used in quieting the needle ; but he was antici- 
pated, for the arc of vibration suddenly diminished to less 
than a minute, so that the needle appeared quite stationary; 
at the same time the declination increased so rapidly, that its 
alteration in the course of a minute of time amounted to 
6' of arc, a quantity which at this season of the year is hardly 
traversed by the needle in the course of an entire day. 
The influence of the disturbing forces usually affects all the 
elements at the same time, but it also happens not unfre- 
quently, particularly in minor disturbances, that their effect 
only reaches the most sensitive of the three elements, i. e. 
the declination ; and some cases, though rare ones, have oc- 
curred in which the time of vibration of the horizontal needle 
underwent considerable alteration without the declination be- 
ing affected at the same time; an example of this occurred on 
the 15th of November 1837 between 6 and 10 a.m. 
20. The greatest change of declination during a perturba- 
tion yet observed in Milan was on the 14th of November 
1837. It amounted to 1° 11', which is nearly ten times the 
mean daily alteration in this month : by reason of its great 
variability this element returns to its usual value sooner than 
the other elements. More examination is still required to 
manifest, whether the general tendency of the perturbations 
is to increase or diminish the average amount of declination, 
and whether their occurrence is connected with the hour of 
the day. 
21. The time of vibration of the horizontal needle is always 
increased by a disturbance, i. e. the force is lessened ; but even 
in this respect there are such fluctuations, especially soon 
after the beginning of a disturbance, that sometimes very 
small times of vibration occur, though this is only of very short 
continuance. The greatest change of this kind was observed 
on the 17th of January 1838, when it amounted to nearly 
0"*3, the time of one vibration being 22"*3. In the greater 
disturbances it is sometimes the second or third day before 
the time of vibration returns to its previous value. 
22. During a disturbance the inclination always seems to 
be greater, but it is subject to as great fluctuations as the 
other elements. The greatest alteration which we observed 
was on the 21st of February 1838, when it amounted to 
8' 45"; whereas the mean diurnal alteration in this month is 
only 1' 6"'6. The dipping-needle usually returns to its or- 
dinary direction on the following day, but sometimes not for 
a few days. 
23. The disturbances likewise increase the times of vibra- 
tion of the dipping-needle, showing that their influence ex- 
