1 14 TERRESTRIAL MAGNETISM. 



the total force is a double progression, whilst that of the incli- 

 nation is a double progression only from October to March, 

 and a single progression from April to September. ( l35 ) At 

 the Cape of Good Hope the inclination has only a single 

 maximum and single minimum, whilst the total force has a 

 double progression, the principal minimum occurring at the 

 same hour as the minimum of inclination. 



We must also distinguish between results obtained by a 

 series of observations with a dipping-needle at certain hours 

 of the forenoon compared with a similar series at certain 

 hours in the afternoon (which can at most only give the 

 difference in the amount of the inclination at those two 

 periods of the twenty-four hours), and results obtained by 

 hourly observations of the horizontal and vertical force 

 magnetometers, which give the horary variations of the incli- 

 nation and total force for every hour. Amongst the horary 

 variations of the inclination obtained by either of these 

 methods may be cited the following : - 



I. In the Northern Hemisphere. 



Greenwich. Prom observations with a dipping-needle 

 three hours before and three hours after noon, the north dip 

 was found to be greater at 9 A.M. than at 3 P.M. The 

 difference in 184? was 0''7. In four years out of five, the 

 dip was higher at 9 A.M. than at 3 P.M., but in one year 

 (1845) the reverse appeared, the dip being greater by 

 T-3 at 3 P.M. than at 9 A.M. 



Paris. Erom observations with a dipping-needle at 9 A.M. 

 and 6 P.M., the mean north dip appeared to be greatest at 



9 A.M. 



