242 M. Kreil's Magnetic Observations. 



If these two needles are employed for determining the de- 

 clination precisely in the same manner as described in p. 146 

 of the 1° Supplemento alle Ephemeridi di Milatio, 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 Br era, 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 phsenomenon, unless it is 

 caused by temporary magnetism. Flowever 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 phenomena ; 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 ph£E- 

 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 



