M. Kreil's Mametic Observations. 249 



b 



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 rapidlj-, 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 w-as 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 v/ith 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 



o en 



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 tlie 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 l' 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- 



