xl NOTES. 



tude of the arc of oscillation is by no means generally proportioned to the 

 magnitude of the alteration of the declination; that the oscillations may be very 

 great, while the alteration of the declination is quite inconsiderable; and that, 

 on the other hand, the progression of the needle in east or west declination may 

 be rapid and considerable, without any vibratory movement; and also that these 

 manifestations of magnetic activity may assume at different places a peculiar and 

 different character. 



( Irl ) p. 139. Unusual Disturbances, vol. i. Pt. I. p. 69 and 101. 



( 172 ) p. 139. This was at the end of September 1806. The fact was 

 published in Poggendorffs Annalen der Physik, Bd. xv. (April 1829) S. 330. 

 It is there said " My older hourly observations, made in conjunction with 

 Oltmanns, had the advantage that at that time (1806 and 1807) no similar 

 ones had been made either in France or in England. They gave the nocturnal 

 maxima and minima [of the diurnal variation, Ed.] ; and they made known the 

 remarkable magnetic storms, which, by the strength of the oscillatory movement 

 of the needle, often make observation impossible, and which return for several 

 successive nights about the same time, without its having yet been possible to 

 discern the operation of any meteorological relations." It was not, therefore, in 

 1839 that a certain periodicity [i. e. a disposition to return at the same hours 

 on successive nights, Ed.] in the extraordinary disturbances was first recognised. 

 (Report of the Fifteenth Meeting of the British Association at Cambridge, 1845, 

 Pt. II. p. 12.) 



( 173 ) p. 139. Kupffer, Voyage au Mont Elbruz dans le Caucase, 1829, 

 p. 108: " Les deviations irre'gulieres se re'petent sou vent a la m&ne heure et 

 pendant plusieurs jours conse'cutifs." 



( m ) p. 140. Sabine, Unusual Disturb., vol. i. Pt. I. p. xxi., and Young- 

 husband, On periodical Laws in the larger Magnetic Disturbances, in the Phil. 

 Trans, for 1853, Pt. I. p. 173. 



( 17S ) p. 140. Sabine, in the Phil. Trans, for 1851, Pt. I. p. 125 to 127: 

 " The diurnal variation observed is, in fact, constituted by two variations super- 

 posed upon each other, having different laws, and bearing different proportions to 

 each other in different parts of the globe. At tropical stations the influence of 

 what have been hitherto called the irregular disturbances (magnetic storms) is 

 comparatively feeble; but it is otherwise at stations situated as are Toronto 

 (Canada) and Hobarton (Van Diemen Island), where their influence is both 

 really and proportionally greater, and amounts to a clearly recognisable part of 

 the whole diurnal variation." There takes place here, in the complex result of 

 different causes acting at the same time, that which Poisson has so well described 

 in the theory of waves (Annales de Chimie et de Physique, t. vii. 1817, p. 293): 



