Mr. J. A. Broun on Terrestrial Magnetism, 89 



In both hemispheres the diurnal range varies with the sun's 

 declination, being greatest in the northern hemisphere, on the 

 whole, when the sun is furthest north, at which time it is least 

 in the southern hemisphere ; and least in the northern hemi- 

 sphere when tlie sun is furthest south, the diurnal range being 

 then greatest in the southern hemisphere. 



The conclusion that M. Arago^drew fx-om these facts was, that 

 there must be a line on the earth's surface where in the morning 

 the needle moves neither to the east nor to the west, that is to 

 say, remains stationary. This conclusion is evidently too general, 

 and so far inaccurate. The legitimate conclusion would have 

 been, that if there was a period of the year for which the two 

 movements were equal in amount in two hemispheres, then there 

 should be a Hue of separation where for that period the needle 

 would rest stationary. Even this conclusion assumes not only 

 that the amounts were equal, but that the law of the movements 

 was exactly the same with reference to the two poles. It is not a 

 little curious that M. Arago has cited* the "dit avoir observer^' 

 of M. d'Abbadie, "que la variation diurne de declinaison a 

 completement change a Feruambuco du moment oil le soleil a 

 passe d'un cote du zenith a I'autre," as a confirmation of his 

 conclusion, while General Sabine has pointed out the same result 

 from the St. Helena observations as proof of its inaccuracy. 

 There can be no doubt that the statement of M. d'Abbadie gives 

 no evidence of a line of no movement. The facts seem to have 

 disproved M. Arago's supposition. The movement, it is con- 

 cluded, is inverted; but it does not pass from one form to the 

 other through a period of no movement, but rather by a sliding 

 transfer of the epochs about the period of the equinoxes. 



I think that the only conclusion that could have been deduced 

 from the facts long known is the true one ; namely, that for a 

 given position of the sun there should be a line of no movement, 

 if the movements in the two hemispheres follow the law of oppo- 

 site directions, and equal amounts for simdar positions of the 

 needle and the sun. This proviso is not strictly true; and 

 therefore, instead of a line of no movement for a given position 

 of the sun, we should substitute a line of minimum movement. 

 St. Helena is too far from both the terrestrial and the magnetic 

 equators (especially from the latter) to show this result clearly ; 

 but the observations here (in 8^° N. lat. 2° south dip) will, 1 

 think, jjrovc this, and render it probable that there are at least 

 points of no diurnal movement for one position of the sun (pro- 

 bably near the nodes of the magnetic equator). The following 

 are my conclusions as to the range of the diurnal oscillation. 

 The mean diurnal range of magnetic declination is a minimum 

 * Notices Scientifiqucs, vol. i. p. 491 (1854). 



