118 TERRESTRIAL MAGNETISM. 



exact instruments, but in a shorter interval of time, found for 

 Dublin 2''38 annual decrease. ( 137 ) In Paris the inclination 

 is also diminishing, and more rapidly than in London. The 

 very ingenious methods of determining the dip devised by 

 Coulomb were not practically successful in the hands of 

 their inventor, for he was led by them to erroneous 

 results. The first observation with a good instrument of 

 Lenoir^s was made in 1798 at the Observatory of Paris, 

 when, conjointly with Chevalier Borda, I found by several 

 repetitions 69 51'*0 ; in 1810, I found, conjointly with 

 Arago, 68 50''2; and in 1826, with Mathieu, 67 56''7. 

 In 1841 Arago found 679''0; and in 1851 Laugier and 

 Mauvais found 66 35' : all these determinations were 

 made by the same method, and with similar instruments. 

 The entire period, exceeding half a century, (1798 to 

 1851) gives a mean annual decrease of inclination at Paris 

 of 3 /f 69, being for the several intervals : 



i from 1798 1810 at the rate of 5'-08 

 1810 1826 3'-37 



1826 1841 3'-] 3 



1841 1851 3'-40 



The rate of decrease underwent a remarkable retardation 

 between the first and second interval, but the change was a 

 gradual one, for a very careful observation of Gay-Lussac's, 

 made in 1806 on his return from Berlin, where he had 

 accompanied me after our return from our Italian journey, 

 gave the inclination 69 12', corresponding to a rate of 

 decrease of 4*87. The nearer the node of the magnetic 

 equator, in its secular movement from east to west, ap- 



