Natural-Philosophical Collections. 69 



bear no ratio, in their combination with other sensations, to the intensity of these 

 colours ; their maximum of influence resides in a certain pale tint, on either side 

 of which this influence diminishes : thus, the blue tint, which has this maximum 

 with respect to red and yellow, is that of the sky in its most highly -coloured 

 parts. — Dissert, sur qvtelques proprietts, Sj^c. ; par M. J. Plateau. Qvo. Litge. 

 1029. 



On the Influence of the Aurora Borealis on the Magjietic Needle. — Extract 

 from a Letter from the Rev. Mr. Farquharson. F.li.S. to Dr. Hooker, 

 UthFeb. 1030. 



" You may, perhaps, feel some interest in learning that the council of the 

 Royal Society, finding still much diffference of opinion regarding the eftect of the 

 Aurora Borealis on the magnetic needle, sent me a properly constructed needle for 

 experiment ; and that, having had many very favourable opportunities since Sep- 

 tember last, I have obtained such results as appear to reconcile the discrepancies of 

 former observations. I have seen the needle always disturbed when the fringes 

 (arches) of streamers pass into the plane of the dip, but not otherwise ; and this 

 agrees with Captain Franklins' observations at Fort Enterprise, who found the 

 needle distiurbed there when the arches passed the zenith, the dip being then 86° 

 59'." 



The needle, furnished to Mr. Farquharson by the Royal Society, was so deli- 

 cately suspended as to render very sensible changes in the declination as small as 

 lO". In his experiments on the magnetic intensity, the intervals of time occu- 

 pied in the needles' 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 

 pemmencement 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 dial plate. 



. The observations made on an Aurora Borealis, which appeared on the night 

 of the 14th of December, have been particularly detailed by Mr. Farquharson 

 "to the Royal Society. On that occasion, the disturbance of the magnetic de- 

 clination 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 

 jthis influence. The needle, however, was affected at those times only when 

 the fringes of the aurora were in a position such as to inchide the needle in 

 their planes. It appeared to him also, that the side towards which the needle 

 declined was the greater where the aurora gave out the most vivid light. 

 ; His experiments on the oscillations of the needle have not yet enabled him to 

 determine satisfactorily whether any change of magnetic intensity accompanied 

 these changes of direction. 



From a comparison of his own with the observations of the Rev. James Paull, 

 minister of Tullynestle, he infers that the height of an aurora, which was seen 

 by them on the 20th, did not, at its upper extremities, exceed 4000 feet above 

 the ground ; and is led to the conclusion, that the aurora borealis is situated in 

 the region immediately above the clouds, and therefore varies much in height, 

 according to the diiferent states of the atmosphere. He believes it to be an effect 

 of the developement of electricity from the condensation of vapour. The position 

 of the fringes, which are constantly at right angles to the magnetic meridian, 

 their progressive movements from the north magnetic pole, and their influence on 

 the needle whenever they come into the plane of the dip, are all of thorn circum- 

 . stances which establish the relation of this phenomenon to magnetism ; while 

 they at the same time illustrate the intimate connexion subsisting between mag- 

 netism and electricity. 



