NOTES. XXXIX 



TSr 1806. At 14 h 46 m , a great storm, but of short duration ; then en- 

 tire quiet ; and at 16 h 30 m another equally great storm. 



The great storm of the f| th Sept. had been preceded by a still stronger one, 

 lasting from 7 b 8 m to 9 h ll m . During the winter which followed, the number of 

 disturbances was very small, and at no time to be compared with the autumnal 

 disturbances. I give the name of " great storm" to a state in which the needle 

 makes oscillations of from 20 to 38 minutes of arc, or passes altogether out of the 

 scale, or moves so rapidly as to make any observation impracticable. In " small 

 storms" the oscillations are irregular, and of from 5 to 8 minutes in amount. 



( l:o ) p. 138. During ten years of diligent observation at Paris, up to 1829, 

 Arago never saw oscillations without their being accompanied by a " change of 

 declination." He wrote to me in the above-named year: "J'ai communique a 

 1'Acade'mie les resultats de nos observations simultane'es. J'ai e^e' surpris des 

 oscillations qu'eprouve parfois 1'aiguille de de'clinaison h, Berlin dans les observa- 

 tions de 1806, 1807, et de 1828 et 1829, lors mme que la de'clinaison moyenne 

 n'est pas alte're'e. Ici (a Paris) nous ne trouvons jamais rien de semblable. Si 

 1'aiguille e'prouve de fortes oscillations, c'est seulement en terns d'aurore boreale, 

 et lorsque sa direction absolue a etc uotablement de'range'e ; et encore le plus 

 souvent les derangements dans la direction ne sont-ils pas accompagnes du 

 mouvement oscillatoire." It is quite otherwise, however, with the phaenoinena 

 observed at Toronto (N. lat. 43 39'), which agree entirely with those at Berlin. 

 The observers at Toronto were very attentive to this kind of motion, and noted 

 " strong " and " slight vibrations," shocks, and all degrees of disturbance, 

 according to determinate subdivisions of a definite scale, and according to a 

 determinate and uniform nomenclature. (Sabine, Days of Unusual Magnetic 

 Disturbances, vol. i. Pt. I. p. 46.) In this volume, comprising the observations 

 of 1840 and 1841, an account is given in detail of six groups of successive days 

 (amounting in all to 146 days in those two years), in which the oscillations were 

 often very considerable (" with strong shocks "), without sensible change, from 

 the normal declination appropriate to the hour. Such groups are designated by 

 the heading, " Times of Observation at Toronto, at which the Magnetometers 

 were disturbed, but the mean readings were not materially changed." (See pp. 

 47, 54, 74, 88, 95 and 101.) The alterations in the declination during the 

 frequently occurring displays of aurora borealis were almost always accompanied 

 at Toronto by strong oscillation ; often such as even to make readings impossible. 

 We learn, then, by these phaenomena (the still further examination of which can- 

 not be too earnestly recommended), that although great momentary disquiet is 

 often followed by considerable definitive variation in the declination (Young- 

 husband, Unusual Disturbances, Pt. II. p. x.), yet that, on the whole, the magni- 



