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2. On a method of rendering Magnetical Instruments Self- 

 Registering. By J. A. Broun, Esq. Communicated by 

 Sir T. M. Brisbane, Bart. 



A sheet of paper being stretched tightly across a flat frame, if 

 fine iron filings be dusted uniformly over it, and a fine pointed 

 sewing needle, magnetised to saturation, held vertically, be moved 

 below it, the point being kept at a short distance from the paper, 

 the motion of the needle-point will be indicated by a fine line of 

 filings which, attracted by the magnet, separate themselves from 

 the homogeneous dust around. The fineness of this line will de- 

 pend on the sharpness of the needle point, and the smallness of the 

 particles of iron ; the distance of the point from the paper will de- 

 pend on the strength of the magnet, and the smoothness of the 

 surface on which the particles lie. In some experiments already 

 made, lines have been obtained as fine as could be produced by the 

 sharpest pointed pencil, and this with a portion only of a small 

 sewing needle. It will require a more extensive series of experi- 

 ments to determine the best size for the magnet, and the best kind 

 of paper for the filings. The particles of iron having a certain 

 weight, are not easily shaken from their position if moderate care 

 be taken. 



If a fine needle be placed vertically at one extremity of a decli- 

 nation magnet, and a sheet of paper dusted with iron filings be 

 moved by clock-work, in the direction of the magnetic meridian, the 

 oscillations of the magnet towards the E. or W. will be indicated in 

 the iron filings by the corresponding motions of the vertical needle. 



One scale division of the declinometer used in the Makerstoun 

 Observatory is equivalent to about 40", and a tenth of a division or 

 4" can be estimated : with the addition of a light and rigid rod to 

 the declination magnet, by means of which the vertical needle could 

 be placed at a distance of about three feet from the suspension 

 thread, a change of declination to the amount of 6" would be equiva- 

 lent to a motion of -001 of an inch at the needle, a quantity which 

 may be estimated with the aid of a scale and vernier. 



Mr Brown proposes to convert a vertical into a horizontal motion, 

 by employing the influence of a magnet moved vertically upon 

 another suspended horizontally. In this way he intends to measure 

 the movements of the vertical force magnetometer, and of the com- 

 mon baromoter, by means of a floating magnet. 



