MAGNETIC DISTURBANCES. 131 



at the Cape for six years, from May to September, according 

 to which, almost precisely as at St. Helena, the needle moves 

 westward till 1 1 h. 30 m. A.M. from its extreme eastern posi- 

 tion (7h. 30m. A.M.), while from October to March it moves 

 eastward from 8h. 30m. A.M. to Ih. 30m. and 2 P.M. The 

 discovery of this well-attested, but still unexplained and 

 obscure phenomenon, has more especially proved the import- 

 ance of observations continued uninterruptedly from hour to 

 hour for many years. Disturbances which, as we shall soon 

 have occasion to show, have the power of diverting the 

 needle either to the eastward or westward for a length of 

 time, would render the isolated observations of travellers 

 uncertain. 



By means of extended navigation and the application of 

 the compass to geodetic surveys, it was very early noticed 

 that at certain times the magnetic needle exhibited an ex- 

 traordinary disturbance in its direction, which was frequently 

 connected with a vibratory, trembling and fluctuating mo- 

 tion. It became customary to ascribe this phenomenon to 

 some special condition of the needle itself, and this was 

 characteristically designated by French sailors Taffolemeni de 

 V aiguille, and it was recommended that une aiguille affolee 

 should be again more strongly magnetised. Halley was cer- 

 tainly the first who inferred that polar light was a magnetic 

 phenomenon a statement CT which he made on the occasion 



principal ground of objection against de la Rive's thermal explanation 

 (Annales de Chimie et de Physique, t. xxv, 1849, p. 310) of the pheno- 

 menon of St. Helena here referred to, which, although it seems at first 

 sight apparently abnormal, is nevertheless entirely in accordance with 

 established law, and is found to occur at other points." See Sabine, in 

 the Proceedings of the Royal Society, 1849, p. 821. 



6 ' Halley, Account of the late surprising appearance of Lights in the 

 Air, in the Phil. Transact, vol. xxix, 1714 1716, No. 347, pp. 422 

 428. Halley's explanation of the Aurora boi-ealis is unfortunately con- 

 nected with the fantastic hypothesis which had been enounced by him 

 twenty-five years earlier, in the Phil. Transact, for 1693, vol. xvii, 

 No. 195, p. 563, according to which there was a luminous fluid in th^ 

 hollow terrestrial sphere lying between the outer shell which we inhabit 

 and the inner denser micleus, which is also inhabited by human beings. 

 These are his words : "In order to make that inner globe capable of 

 being inhabited, there might not improbably be contained some lumi- 

 nous medium between the balls, so as to make a perpetual day below." 

 Since the outer shell of the earth's crust is far less thick in the region 

 of the poles of rotation (owing to the compression produced at tho.-j 



K 2 



