118 COSMOS. 



Ih. 45m. P.M., the magnetic needle continues to retrograde 

 toward 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 toward the west, until the minimum 

 or eastern position is reached at 8h. 15m. A.M. This noc- 

 turnal period, which was formerly entirely overlooked, since 

 a gradual and uninterrupted retrogression toward the east 

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

 had already been carefully studied by me at Home, when I 

 was engaged with Gay-Lussac in observing the horary 

 changes of variation with one of Prony's magnetic tele- 

 scopes. As the needle is generally unsteady as long as the 

 sun is below the horizon, the small nocturnal motion west- 

 ward 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 differently from the manner in 

 which it moves during the occurrence of the phenomenon 

 which I have named a magnetic storm. It is very remark- 

 able that when the needle changes its continuous western 

 motion into an eastern movement, 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 mid- 

 night and toward the early morning. On the other hand, it 

 lias been observed at Berlin, and during the subterranean 

 observations at Freiberg, as well as at Greenwich, Makers- 



of the Pacific (all these being places with an eastern variation). I 

 would here observe that the mean declination was 2 15' 42" west at 

 Pekin (Dec., 1831) (Poggend., Annalen, bd. xxxiv., s. 54); 4 T 44" 

 west at Ncrtschinsk (Sept., 1832) (Poggend., Op. cit., s. 61); 1 .33' 

 west at Toronto (November, 1847) (see Observ. at the Magnetical and 

 Meteorological Observatory at Toronto, vol. i., p. 11 ; and Sabine, in the 

 Phil. Transact, for 1851, pt. iL, p. 636), 2 21' east at Kasan (August, 

 1828) (Kupffer, Simonoff, and Erman, Reise urn die Erde, bd. ii., s. 

 532); 28 16' east at Sitka (November, 1829) (Erman, Op. cit., s. 

 546); 6 33' east at Marmato (August, 1828) (Humboldt, in Poggend., 

 Annalen, bd. xv/, s. 331) ; 8 56' east at Payta (August, 1823) (Du- 

 perrey, in the Connaissance des Temps pour 1828, p. 252). At Tiflis 

 the declination was westerly from 7 A.M. till 2 P.M. (Parrot, Rcise- 

 zum Ararat, 1834, th. ii., s. 58). 



