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VI. Experiments on the influence of the Aurora Borealis on the magnetic needle. 
By the Reverend James Farquharson, F.R.S. Minister of Alford , Aber- 
deenshire. In letters addressed to Captain Edward Sabine, Secretary of 
the Royal Society. 
Read January 28, March 4, and April 1, 1830. 
Alford, December 15 th, 1829. —The apparatus, belonging to the Royal So- 
ciety, with which these experiments were made, consists of a horizontal brass 
circle, about one foot in diameter, graduated to divisions of 10 minutes, and 
capable of adjustment to a perfect level by means of spirit levels and screwed 
feet. Concentrically within this divided circle moves a circular horizontal brass 
plate, its edge touching the divisions, and having at opposite points two verniers, 
which, by means of attached microscopes, indicate the movements which it makes 
to 60th parts of 10 minutes, or 10". The movement of the plate within the circle 
is effected by means of a screw. A circular brass needle-box is attached to the 
surface of the inner plate, and a vertical pointed steel wire for supporting the 
needle forms the centre. At opposite points in the needle-box are fixed two 
micrometers with cross wires in the foci, for adjusting the needle to a level, and 
observing any change in its direction. The top of the needle-box is a circular 
plate of ground glass in a brass ring, made to slip easily off and on, and having 
screwed into its centre a vertical brass tube about 8 inches long, for the purpose 
of suspending the needle with fibres of silk, for measuring the time of its oscil- 
lations. A horizontal brass pin, with a minute perforation for the silk near its 
middle, passes through the vertical tube near its top, and being contrived with 
several motions, serves to adjust the suspended needle, and bring it correctly 
over the steel point, where its levelling can be completely ascertained. 
The magnetic needlec itself is a rectangular plate about 5 inches long, half 
an inch broad, and V 0 th of an inch thick. An agate cup set in brass ad- 
mits of being screwed in either at the narrow or flat side of the needle ; and a 
little fixt ring of brass, with a minute perforation in its top, rising over the cup, 
MDCCCXXX. 
O 
