[ 199 ] 
XI. On the Variable Intensity of Terrestrial Magnetism, and the Influence of 
the Aurora Borealis upon it. By Robert Were Fox. Communicated by 
Davies Gilbert, Esq. M.P. V.P.R.S. 
Read March 17, 1831. 
In the annexed Table are given the results of a series of observations on the 
vibrations of the magnetic needle, which I undertook last summer, for the pur- 
pose of ascertaining whether its intensity is or is not affected by the changes in 
the earth’s distance from the sun, or by its declination with respect to the plane 
of his equator ; for, if we refer the nodes of the planetary orbits to this plane, 
there appears to be so considerable a degree of coincidence in most of them, 
as would seem to imply the existence of a more definite law than we are ac- 
customed to attach to the abstract principle of gravitation*. 
I am not at present prepared to say much respecting this part of my inves- 
tigation ; but I have obtained results, which appear to be interesting, relative to 
the variable force of the magnetic attraction, and the action of the aurora 
borealis on the direction and intensity of the needle. 
I have used two needles, one possessing the north, and the other the south 
polarity in excess. To effect this, I have employed, in each case, a slip of sea- 
soned oak, split with the grain, and suspended near the centre by unspun silk 
several inches long. This slip of wood serves as a support for a magnetic bar 
or needle, which is firmly riveted to it near one of its poles — say, for example, 
the north pole ; whilst the south pole is at liberty to yield longitudinally to any 
* I had been making numerous experiments with the same object in view some time before I com- 
menced the series now given ; but the results were unsatisfactory, from my not having employed a sta- 
tionary magnetic apparatus, which should always be done in cases in which great accuracy is required. 
If the needle ha- been touched with the hand for one or two seconds only, its action is disturbed for 
some little time; indeed, it requires not a little piactice to appreciate all the precautions that are 
necessary in vibrating the magnetic needle. 
