108 COSMOS. 



British magnetic stations this opposition and the periodicity 

 of the horary variation in the dip have been firmly estab- 

 lished by several thousand regularly prosecuted observations, 

 which have all been submitted to a careful discussion since 

 1840. The present would seem the most fitting place to 

 notice the facts that have been obtained as materials on 

 which to base a general theory of terrestrial magnetism. 

 It must, however, first be observed, that if we consider the 

 periodical variations which are recognised in the three ele- 

 ments of terrestrial magnetism, we must, with Sabine, dis- 

 tinguish in the turning hours at which the maxima or 

 minima occur, two greater, and therefore more important, ex- 

 tremes, and other slight variations, which seem to be inter- 

 calated amongst the others, as it were, and which are for the 

 most part of an irregular character. The recurring move- 

 ments of the horizontal and dipping needles, as well as the 

 variation in the intensity of the total force, consequently 

 present principal and secondary maxima or minima, and 

 generally some of either type, which therefore constitutes a 

 double progression with four turning hours (the ordinary 

 case), and a simple progression with two turning hours, that 

 is to say, with a single maximum and a single minimum. 

 Thus, for instance, in Van Diemen's Land, the intensity or 

 total force exhibits a simple progression, combined with a 



was greater in the evening than towards morning. At Greenwich the 

 principal maximum of the horizontal force was about 6 P.M., the prin- 

 cipal minimum about 10 A.M. or at noon; at Schulzendorf, near Berlin, 

 tile maximum falls at 8 P.M., the minimum at 9 A.M.; at St. Petersburg 

 the max. falls at 8 P.M., the min. at llh. 20m. A.M. ; at Toronto the 

 max. falls at 4 P.M., the min. at 11 A.M. The time is always rec- 

 koned according to the true time of the respective places (Airy, Magn. 

 Observ. at Greenwich for 1845, p. 13 ; for 1846, p. 102 ; for 1847, p. 241; 

 Riess and Moser, in Poggend. Annalen. Ed. xix, 1830, s. 175 ; Kupffer, 

 Compterendu Annuel de V Observatoire Centrale Magn. de St. Petersb. 

 1852, p. 28 ; and Sabine, Magn. Observ. at Toronto, vol. i, 1840 1842, 

 p. xlii). The turning hours at the Cape of Good Hope and at St. Helena, 

 where the horizontal force is the weakest in the evening, seem to be 

 singularly at variance, and almost the very opposite of one another 

 (Sabine, Magn. Obs. at the Cape of Good Hope, p. xl, at St. Helena, 

 p. 40). Such, however, is not the case further eastward, in other parts 

 of the great, southern hemisphere. " The principal feature in the 

 diurnal change of the horizontal force at Hobarton is the decrease of 

 force in the forenoon and its subsequent increase in the afternoon" 

 (Sabine, Magn. Obs. at Hobarton, vol. i, p. liv. vol. ii, p. xliii). 



