Experiments on the influence of the aurora borealis on the magnetic needle. By the Rev. James Farquharson, F. R. S. Minister of Al­ford, Aberdeenshire. In letters addressed to Captain Edward Sa­bine, Sec. R. S

Abstract

In the first letter, dated from Alford, Dec. 15, 1829, the author gives a description of the instrument which was furnished to him by the Royal Society for measuring the variation of the magnetic needle, and also the magnetic intensity; and of his mode of using it. The needle was so delicately suspended as to render changes in the declination as small as 10'´ very sensible. In his experiments on the magnetic intensity, the intervals of time occupied in the needle’s performing 50 oscillations, commencing with an arc of 12°, were noted by a stop-watch, in which the stop, being applied on the ba­lance, is instantaneous in its operation. The watch is again released from the stop at the commencement of a new observation; thus com­pensating, on the principle of the repeating circle, for any inaccuracy in the reading off, or any inequality in the divisions of the dialplate. The observations made on an Aurora borealis which appeared on the night of the 14th of December, are particularly detailed. On that occasion, the disturbance of the magnetic declination was so great, and so frequently changing from east to west, and the reverse, as to leave no doubt in the mind of the author of the reality of this influence. The needle, however, was affected at those times only when the fringes of the aurora were in such a position as to include the needle in their planes. It appeared to him, also, that the side towards which the needle declined, was the quarter where the aurora gave out the most vivid light.