M. Kreil's Magnetic Observations. 243 



greatest near tlie time of the summer solstice and least in 

 December. 



4. The intensity of the horizontal force increases from Ja- 

 nuary to June, and decreases from July to December. 



5. The declination begins to increase at 8 in the morning, 

 and increases rapidly until 1 or 2 in the afternoon, when it 

 attains its highest amount; it then decreases more slowly until 

 it reaches its lowest value : in the winter months the declina- 

 tion is usually less at 1 1 in the evening than at 8 in the morn- 

 ing, but only occasionally so at other seasons of the year. 



6. The mean value of the difference between the greatest 

 and the least declination is 12' 2". The difference is greatest 

 in the month which follows the vernal equinox, and least in 

 December. 



7. This diversity is the consequence of the annual change 

 in the declination, which at different hours in the day follows 

 an opposite course ; in the forenoon, it decreases in spring 

 and increases in autumn ; in the afternoon, it increases in 

 spring and decreases in autumn ; hence it follows that there 

 must be some hour in the day which is free from the annual 

 change in the declination and which is therefore most suitable 

 for insulated experiments. At Milan and at the present epoch, 

 this hour is between 1 and 1 1 a.m. 



8. The total force attains its least intensity at 8 in the 

 morning or earlier, and its greatest intensity between 1 and 4 

 p.m. We are still in want of a sufficiently extended series of 

 observations to furnish with equal certainty other facts re- 

 lating to this element. 



9. The inclination increases in the morning till towards 

 10 o'clock, when it decreases, but not uninterruptedly, for it 

 increases again in the afternoon, and at 4 attains a second 

 maximum, after which it decreases without interruption till 

 near midnight This alteration in the inclination retards the 

 time of the maximum and minimum of the horizontal force 

 by two or three hours ; and its second maximum explains the 

 anomaly mentioned in No. 1. 



10. The amount of the alteration of the inclination seems 

 to be also dependent on the season of the year. A greater 

 alteration (above a minute) was observed in summer, and a 

 less (about half a minute) in winter. 



11. The times of the absolute maximum and minimum are 

 very variable, but even in this variability a law is plainly ma- 

 nifest. In January and February the maximum was observed 

 at 4. 30' p.m., in March and April at 10. 30' a.m., from May 

 to August at 8 a.m., in September and October at 10. 30' 

 a.m., in November at 1 p.m., and in December at 4. 30' p.m. 



R 2 



