MAGNETIC VARIATION. 



is not changed in our northern zone by the earlier or later 

 time of sunrise. At the two solstitial periods and the three 

 equinoxes, at which, conjointly with Oltrnanns, I watched 

 the horary variations for five to six consecutive days and 

 nights, I found that the eastern turning point remained fixed 

 between 7h. 45m. A.M. and 8h. 15m. A.M. both in summer 

 and in winter, and was only very slightly anticipated by the 

 earlier period at which the sun rose.* 



In the high northern latitudes near the Arctic circle, and 

 between the latter and the pole of the earth's rotation, the 

 regularity of the horary declination has not yet been very 

 clearly recognized, although there has been no deficiency in 

 the number of very carefully-conducted observations regard- 

 ing this point. The local action of the rocks and the fre- 

 quency of the disturbing action of the polar light, either in 

 the immediate vicinity or at a distance, made Lottin hesi- 

 tate in drawing definite conclusions in reference to these 

 turning hours, from his own great and careful labors, which 

 were carried on during the French scientific expedition of 

 Lilloise in 1836, or from the earlier results that had been 

 obtained with much care and accuracy by Lowenorn in 

 1786. It would appear that at Beikjavik, in Iceland, 64 

 S / lat., as well as at Godthaab, on the coast of Greenland, 

 according to observations made by the missionary Genge, 

 the minimum of the western variation fell almost as in the 

 middle latitudes at about 9 or 10 A.M., while the maximum 

 did not appear to occur before 9 or 10 P.M.f Farther to 



* Examples of the slightly earlier occurrence of the turning hours 

 arc given by Lieutenant Gilliss, in his Magn. Observ. of Washington, 

 p. 328. At Makerston, in Scotland (55 35' N. lat.), variations %rc 

 observed in the secondary minimum, which occurs about 9 A.M. in 

 the first three and the last four months of the year, and about 7A.M. 

 in the remaining five months (from April till August), the reverse be- 

 ing the case at Berlin and Greenwich (Allan Broun, Observ. made at 

 Makerston, p. 225). The idea of heat exerting an influence on the 

 regular changes of the horary variation, whose minimum falls in the 

 morning near the time of the minimum of the temperature, as the 

 maximum very nearly coincides with maximum heat, is most distinct- 

 ly contradicted 1 by the nocturnal motions of the needle, constituting 

 the secondary minimum arid secondary maximum. "There are two 

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

 one minimum and one maximum of temperature" (Relshuber, in Pog- 

 gend., Annalen der Physik und Chemlc, bd. 85, 1852, s. 416). On the 

 normal motion of the magnetic needle in Northern Germany, see Dove, 

 Poggend., Annalen, bd. xix., s. 304-374. 



f Voy. en Islande et en Greenland, execute on 1835 et 1836, sur la 

 Corr. la Recherche; Physique (1838), p. 214-225, 358-3G7. 



VOL. V. F 



