MAGNETIC VARIATION. 127 



wore made at Singapore every two hours during the years 

 18-11 and 1842, Sabine again finds the St. Helena types in 

 the motion of the needle from May to August, and from 

 November to February ; the same occurs at the Cape of 

 Good Hope, which is 34° distant from the geographical and 

 still more remote from the magnetic equator, and where the 

 inclination is 53° south and the sun never reaches the ze- 

 nith.* We possess the published horary observations made 

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

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

 westward till llh. 30m. A.M. from its extreme eastern po- 

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

 moves eastward from 8h. 30m. A.M. to lh. 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 nee- 

 dle either to the eastward or westward for a length of time, 

 would render the isolated observations of travelers uncer- 

 tain. 



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- 



* Phil. Transact, for 1847, pt. i., p. 52, 57; and Sabine, Observa- 

 tions made at the Magn. and Meteor. Observatory at the Cape of Good 

 Hope, 1841-1810, vol. i., p. xii.-xxiii., pi. hi. See also Faraday's in- 

 genious views regarding the causes of those phenomena, which depend 

 upon the alternations of the seasons, in his Experiments on Atmospheric 

 Magnetism, § 3027-3068, and on the analogies with St. Petersburg, § 

 3017. It would appear that the singular type of magnetic declination, 

 varying with the seasons, which prevails at the Cape of Good Hope, 

 St. Helena, and Singapore, has been noticed on the southern shores 

 of the Red Sea by the careful observer D' Abbadie (Airy, On the Present 

 State of the Science of Terrestrial Magnetism, 1850, p. 2). " It results 

 from the present position of the four points of maximum of intensity 

 at the surface of the earth," observes Sabine, "that the important 

 curve of the relatively, but not absolutely, weakest intensity in the 

 Southern Atlantic Ocean should incline away from the vicinity of St. 

 Helena, in the direction of the southern extremity of Africa. The as- 

 tronomico-geographical position of this southern extremity, where tbe 

 sun remains throughout the whole year north of the zenith, affords a 

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

 {Annates de CJdmie et de Physique, t. xxv., 1849, p. 310) of the phenom- 

 enon 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. 



