NOTES. XXXlll 



tory at Toronto, vol. i. p. xi.; and Sabine, in the Phil. Trans, for 1851, Ft. II. 

 p. 636) ; Kasan (Aug. 1828), 2 21' E. (Kupffer, Simonoff, and Erman, Reise 

 urn die Erde, Bd. ii. S. 532) ; Sitka (Nov. 1829), 28 16' E. (Erman, Reise 

 urn die Erde, Bd. ii. S. 546) ; Marmato (Aug. 1828), 6 33' E. (Humboldt, in 

 Poggend. Ann. Bd. xv. S. 331) ; Payta (Aug. 1823), 8 56' E. (Duperrey, 

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

 westward from 19 h to 2 h (Parrot, Reise zum Ararat, 1834, Th. II. S. 58). 



( 153 ) p. 128. Extracts from a letter written by me to Karsten, from Rome, 

 June 22, 1805, printed in Hansteen's Magnetismus der Erde, 1819, S. 459, on 

 the subject of " four movements of the magnetic needle, as it were four magnetic 

 ebbs and flows analogous to the barometric periods." On the subject of the so 

 long neglected nocturnal variations of the declination, compare Faraday, " On 

 the Night Episode," 30123024. 



( 1M ) p. 128. Airy, Magn. and Met. Observations made at Greenwich, 1845 

 (Results), p. 6 ; 1846, p. 94 ; 1847, p. 236. I quote from a letter, dated the 

 llth October 1836, from my friend, the distinguished director of the Berlin 

 Observatory, Encke, to show how well the earliest assigned turning hours, both 

 of the day and of the night, derived from corresponding observations at Berlin 

 and Breslau, accord with those which have since been obtained from the fuller 

 system of observation at the Greenwich and Toronto observatories: " I do not 

 think that there can be any doubt, in general, as to the existence of the nocturnal 

 maximum, or inflection of the curve of the diurnal variation of the declination , 

 Dove also inferred it from the Freiberg observations in 1830 (Poggend. Ann. 

 Bd. xix. S. 373). In judging of the phenomena, graphical representations are 

 much more advantageous than tables of numbers. In the former, great irregu- 

 larities are easily seen to be such, and admit of a mean line being drawn ; while 

 in the latter the eye is frequently deceived, and takes a single striking irregu- 

 larity for a real maximum or minimum. The period appears to be determined 

 by the four turning hours : 



" Greatest east declination ... 20 h - - principal max. east. 



west - - - 1 - - principal min. east. 



Secondary east . _ _ 10 - - minor max. east. 



,, west - - - ] 6 - - minor min. east. 



" The secondary minimum, or nocturnal elongation towards the west, falls, pro- 

 perly speaking, between 15 h and 17 h , being sometimes nearer to the one, and 

 sometimes to the other of those hours." It is scarcely necessary to remark, 

 that what Encke and I called minima to the east (at l h and 16 h ) are the 

 same which, at the British and American observatories founded in 1840, are 

 VOL. IV. c 



