] 26 TERRESTRIAL MAGNETISM. 



declination. ( 15 ) The horary variations or changes of decli- 

 nation were recognised as facts by Hellibrand and Tachard, 

 at Louvo, in Siam ; they were first observed, circumstan- 

 tially and almost satisfactorily, by Graham in 1722. Celsius 

 was the first who arranged concerted simultaneous measure- 

 ments of these variations by different observers at two 

 distant points. ( l51 ) 



Passing now to the phsenomena presented by the declina- 

 tion of the magnetic needle, we will consider them, first, in 

 reference to their variations according to the hours of the 

 day or night, the seasons of the year, and their mean state 

 in different years ; next in regard to the influence exercised 

 on those variations by the extraordinary, and yet periodic, 

 disturbances, and by the position of the places of obser- 

 vation to the north or south of the magnetic equator ; and 

 lastly, we will consider them according to the linear relations 

 of places on the earth's surface having equal, or (it may be) 

 no, declination. These linear relations are indeed that part 

 of the acquired knowledge which is the most important in 

 its direct and practical application to navigation ; but all 

 the general magnetic phsenomena (among which the extra- 

 ordinary disturbances, or magnetic storms, acting often 

 simultaneously at such great distances, are among the most 

 mysterious), are so intimately connected together, that, with 

 a view to the gradual completion of the mathematical 

 theory of terrestrial magnetism, we should neglect none of 

 them. 



In the middle latitudes of the northern magnetic hemi- 

 sphere (dividing the earth at the magnetic equator), the 

 north end of the needle, '. e. the end which points in those 

 latitudes towards the north, points more to the east at 



