NOTES. XXXV 



with Bache, Observations at the Magn. and Met. Observatory at the Girard Col- 

 lege, Philadelphia, made in the years 18401845 (3 vols., containing 3212 

 pages), vol. i. p. 709, vol. ii. p. 1285, vol. iii. p. 2167 and 2702. Notwith- 

 standing the vicinity of the two places (Philadelphia is only 1 4' north, and 

 Qh jm 335 eas fc O f Washington), I find a difference in the times of the westerly 

 secondary maximum and secondary minimum ; the former being an hour and a 

 half, and the latter two hours and a quarter, earlier in Philadelphia than in 

 Washington. 



( 155 ) p. 129. I find instances of such small differences of time given by 

 Lieutenant Gilliss, in his Magnetic Observations at Washington, p. 328. In the 

 more northern latitude of Makerstoun in Scotland (lat. 55 35'), there are fluc- 

 tuations in the secondary minimum, which in the four last and three first months 

 of the year takes place at 21 h , but in the other five months (April to August) 

 at 19 h ; contrary, therefore, to Berlin and Greenwich. (Broun, Obs. made at 

 Makerstoun, p. 225.) The nocturnal movements of the needle, the secondary 

 maximum and secondary minimum, speak most clearly against the regular 

 diurnal variation of the declination being due to thermal variations; (the morning 

 and principal minimum does take place not far from the minimum of tempera- 

 ture, and the principal maximum near the maximum of temperature). " There 

 are two maxima and two minima of declination in the twenty-four hours, but only 

 one maximum and one minimum of temperature in the same period." (Relshuber, 

 in Poggend. Annalen der Physik und Chemie, Bd. 85, 1852, S. 416.) On 

 the normal march of the magnetic needle in North Germany, see an excellent 

 and faithful description in a Memoir of Dove's (Poggend. Ann. Bd. xix. S. 364 

 374). 



( 156 ) p. 129. Voyage en Islande et au Greenland, exe'cute" en 1835 et en 

 1836, sarla corvette " la Recherche;" Physique (1838), p. 214 225 and 

 358367. 



( !57 ) p. 129. Sabine, Account of Pendulum Experiments, 1825, p. 500. 



( 153 ) p. 130. See Barlow's Account of the Observations at Port Bowen, in 

 the Edinb. New. Phil. Journal, vol. ii. 1827, p. 347. 



( 159 ) p. 130. Professor Orlebar, at Oxford, at one time superintendent of the 

 Magnetic Observatory established at the expense of the East India Company 

 on the Island of Colaba, has tried to make out the complicated laws of the va- 

 _riation of the declination in the sub-periods (" Observations made at the Magn. 

 and Met. Observatory at Bombay, in 1845;" Results, p. 2 7). I have been 

 struck, in the first period, from April to August, by the agreement with the 

 march of the needle in Middle Europe (W.min. 19^ h , max. h , min. 5 h , mas. 

 7 h ). The month of October is a transition period, for in November and Decem- 



c 2 



