391 



markable confirmation in the doubly-refracting structure which the 

 author discovered in chabasie ; and they also enable us to under- 

 stand the nature of that influence which heat produces on doubly- 

 refracting crystals, as discovered by Prof. Mitscherlich. The optical 

 phenomena exhibited by fluids under the influence of heat and pres- 

 sure, and by crystals exposed to compressing or dilating forces, are 

 also in perfect conformity with the above views, and would in them- 

 selves have been sufficient to establish the principle that the forces 

 of double refraction are not resident in the molecules themselves, 

 but are the immediate result of those mechanical forces by which 

 these molecules constitute solid bodies. 



Experiments on the Influence of the Aurora Borealis on the Magnetic 



Needle. By the Rev. James Farquharson, F.R.S. Minister of Al- 



ford, Aberdeenshire. In Letters addressed to Captain Edward Sa- 



bine, Sec.R.S. Read January 28, March 4, and April 1, 1830. 



[Phil. Trans. 1830, ;?. 97.] 



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 ba- 

 lance, is instantaneous in its operation. The watch is again released 

 from the stop at the commencement of a new observation ; thus com- 

 pensating, on the principle of the repeating circle, for any inaccuracy 

 in the reading off, or any inequality in the divisions of the dial- 

 plate. 



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. 



His experiments on the oscillations of the needle have not yet en- 

 abled him to determine satisfactorily, whether any change of mag- 

 netic intensity accompanied these changes of direction. 



In a second letter, dated December 26, he gives the results of later 

 observations. From a comparison of his own with the observations 

 of the Rev. James Paull, minister of Tullynessle, he infers that the 

 height of the particular aurora which was seen by them on the 20th, did 



