MAGNETIC VARIATION. 121 



towards the east throughout the whole of the afternoon and 

 a portion of the night till midnight, or 1 A.M., while it often 

 makes a short pause about 6 P.M. In the night there is again 

 a slight movement towards the west, until the minimum or 

 eastern position is reached at 8h. 15m. A.M. This nocturnal 

 period which was formerly entirely overlooked, since a gradual 

 and uninterrupted retrogression towards the east between 

 Ih. 45m. P.M. and 8h. 15m. A.M. was assumed, had already 

 been carefully studied by me at Rome, when I was engaged 

 with Gay-Lussac in observing the horary changes of variation 

 with one of Prony's magnetic telescopes. As the needle is 

 generally unsteady as long as the sun is below the horizon, 

 the small nocturnal motion westward is more seldom and 

 less distinctly manifested. At those occasions when this 

 motion was clearly discernible, I never saw it accompanied 

 by any restlessness of the needle. The needle, during this 

 small western period, passes quietly from point to point of 

 the dial, exactly in the same manner as in the reliable diurnal 

 period, between 8h. 15m. A.M. and Ih. 45m. P.M., and very dif- 

 ferently from the manner in which it moves during the 

 occurrence of the phenomenon which I have named a mag- 

 netic storm. It is very remarkable that when the needle 

 changes its continuous western motion into an eastern move- 

 ment, or conversely, it does not continue unchanged for any 

 length of time, but it turns round almost suddenly, more 

 especially by day, at the above-named periods, 8h. 15m. A.M. 

 and Ih. 45m. P.M. The slight motion westward does not 

 commonly occur until after midnight and towards the early 

 morning. On the other hand, it has been observed at Berlin, 

 and during the subterranean observations at Freiberg, as well 

 as at Greenwich, Makerstoun in Scotland, Washington and 

 Toronto, soon after 10 or 11 P.M. 



The four movements of the needle, which I recognised in 

 1805, 53 have been represented in the admirable collection of 

 observations made at Greenwich in the years 1845, 1846, and 



53 See extracts from a letter, which I addressed to Karsten, from 

 Eome, June the 22ud, 1805, " On four motions of the magnetic needle, 

 constituting, as it were, four periods of magnetic ebbing and flowing, 

 analogous to the barometrical periods." This communication was 

 printed in Hansteen's Magnetismus der Erde, 1819, s. 459. On the long 

 disregarded nocturnal alterations of variation, see Faraday, OntheNiyht 

 Episode, . 30123024. 



