Dr. Thompson, in his ‘Annals of Philosophy,’ vol. iv. p. 431 (1814), mentions as an authenticated fact that during the prevalence of the Aurora the magnetic needle was frequently observed to become unsteady, and (p. 432) concludes that cylinders of Aurora cannot be doubted to be magnets. The only three bodies capable of assuming magnetic properties are iron, nickel, and cobalt. When meteors are considered, it is not altogether extravagant to conjecture that bodies similar in their nature to some of the solid bodies which constitute our globe may exist in some unknown state in the atmosphere.
During the Aurora of 13th May, 1869, the declination at Greenwich varied 1° 25´, while the vertical force experienced four successive maxima, and the greatest oscillation amounted to 0·04 of the total mean value. The horizontal force varied only 0·014 of its mean value.
During the Aurora of 15th April, 1869, the declination at Stonyhurst varied 2° 23´ 14″ in nine minutes.
Plate IX.
Auroræ, Magnetic Disturbances, and Sun-spots.
Auroræ, magnetic disturbances, and sun-spots in Italy.
Auroræ were frequent in Italy in April 1871. On the 10th a remarkable one was seen, with declinometer deflected towards the east, and 63 sun-spots were counted. On the morning of the 10th the deflection continued, and at midday 97 sun-spots were counted.
On the 18th a brilliant Aurora lasted to 10 o’clock at night. From this time till the 23rd the Aurora appeared constantly, giving a reddish tinge in the north and north-west. A brilliant display took place on the evening of the 23rd. On the evenings when the Aurora appeared the magnetometers were disturbed throughout Italy, and ended by a violent agitation during the whole of the 24th. Sun-spots were observed at Rome, Palermo, and Moncalieri, but the greater number on the days of the Auroræ. A brilliant display at Moncalieri on June 18 was accompanied by very violent magnetic disturbance.
Proctor’s sun-spots and Aurora.