Magnetic Intensity of the Earth. 221 



log. C = 5.45064 ; in Christiania, in the open air, by the mean 

 of the year, T is nearly constant = 814/'76. By equation 

 No. I., I then find, 



For 1820, F' = 1.4306 



1825, F' = 1.4093 



Difference in five years, F' — F = — 0.0213 



Yearly dF = — 0.00426. 



By putting di = — 3.24, dT = O, in equation No.' II., you will find 



dF 



-=r= — 0.003, and dF = — F. 0.003 = — 0.00426, as before. 

 r 



By comparing my own observations of 300 vibrations of 

 the magnetic cylinder at London and Paris in 1819, with those 

 of Captain Kater and Mr Arago in 1823, I find the follow- 

 ing differences: 



London. Paris. 



1819, Hansteen, ?77"79 Hansteen, 756"19 



1823, Kater, 775"34 Arago, 753"03 



Difference = — 2"45 Difference = — 3"1 6. 



I must confess that my observations were not made in the 

 open air, as those in 1823, but in the middle of great rooms, 

 whene no iron was visible. Accordingly, they are not quite 

 unexceptionable. But as they agree in making the time of 

 300 vibrations in both places nearly 3" longer than in 1823, 

 I will assume the annual decrease of T for London dT = 

 — 0"61, for Paris = ■— 0"79- Further, supposing in 1821, 



At Paris, i = 68°23', di = — 3'84, T = 754"61 , d T = — 0"79, 

 At London, *' = 70°3', di — — 3'22, T = 776"56, dT = — 0"61, 



by these suppositions, Formula II. gives dF at Paris = — 0.00098, and 



at London = —0.00138. 



By comparing my own observations at Berlin with those of 

 Mr Humboldt, I have found, that the intensity there is also 

 flecreasing ; namely, 



Christiania, dF = — 0.00426 

 Berlin, =—0.00193 



London, = — 0.00138 



Paris, = — 0.00098. 



Hence it seems evident, that the decrease of the intensity i<t 

 greater in the northerly and easterly parts of Europe than in 

 the southern and westerly. The cause of this change is evi- 



