28 Experimental Philosophy. [Lecture 3. 



same place at different times. In some parts of 

 the North American continent, the needle now 

 points north and south ; at others, it deviates or 

 varies from this position, the variation or de- 

 clination, as it is technically called, being in some 

 places westerly, in others easterly. At London, 

 the declination of the needle in the year 1580, 

 was 11 15' towards the east. From that time 

 the declination, easterly, gradually diminished 

 until the year 1658, when the position of the 

 horizontal needle at London was precisely north 

 and south. From that period to the present 

 the north end of the needle has deviated more 

 and more from the true north towards the west, 

 until now (in the autumn of 1819), the declination 

 at London is 24 19' W. In like manner at 

 Dublin, Edinburgh, Paris, Copenhagen, and 

 other places, where the declination has been long 

 observed, it is found to increase westerly: though 

 in none of those places is the declination the same 

 at it is at London. In all of them, however, it 

 has increased but ittle during the last ten or 

 fifteen years. In 1800, the declination at London 

 was 24 3' ; hence, during the last nineteen years, 

 the declination has not, on the average, varied a 

 minute in a year : and, it is exceedingly probable, 

 that it has nearly, if not quite, attained its greatest 

 western limit in England. 



Besides this constant variation in the decima- 

 tion, as referred from year to year, there are 

 minor variations in different parts of the year, 



