188 Messrs, Babbage and Herschel on the [Sept. 



proposition, it is no less evident, is independent of the axis of 

 suspension, which may pass through a part of the system any 

 how situated with respect to the magnets, in virtue of the 

 property of a magnet wnose force to turn a system, of which it 

 makes a part, round a fixed centre, is the same whenever it is 

 placed in the system, and the same as if it were in the centre. 



Hence it follows, that if two equal and similar magnets be laid 

 parallel to each other, but in a reversed position, on a horizontal 

 glass plate, freely suspended by a thread, the system will be 

 devoid of any polar tendency, (which we shall express by calling 

 such a system neutral.) It is difficult however to procure two 

 magnets exactly equal, and of equal force. But fortunately this 

 is of no consequence, as a shght deviation from perfect neutrality 

 may be corrected by inclining the stronger needle a little more 

 or less to the plane of the plate. In fact the proposition is 

 general; and by a proper adjustment of the positions of two 

 magnets however unequal, with respect to the axis and to each 

 other, they may be made to neutralize each other. 



As this adjustment however is nice, and as magnets influence 

 each other, and our object moreover called for the utmost deli- 

 cacy, we adopted a more refined application of the principle just 

 detailed. A circular glass disc was prepared, eight inches in 

 diameter, and suspended by three silk threads from a filament of 

 silk, descending along the axis of a copper tube about five feet 

 long, passing with stiff friction through collars in the cieling of 

 the apartment, and serving nicely by means of an index to regu- 

 late the height of the glass disc. 



At the opposite extremities of two diameters at right angles 

 to each other, four equal small bar magnets were fixed in a 

 vertical position, having alternately their north and south poles 

 downwards. This position promised to present two material 

 advantages ; first, tnat in neutralizing the system we have not 

 the whole polarity of the magnets to contend with, but only the 

 small remains of directive tendency which arises from the 

 magnetic axis in each not being precisely coincident with its 

 axis of figure, since it is evident that an infinitely thin magnetic 

 cyUnder placed perpendicularly to the horizon, would from that 

 cause alone be indifferent as to situation ; secondly, that in this 

 situation their poles interfere with each other's action on the 

 plate revolving below them, less than in any other. Instead of 

 lour we might (and as will be seen occasionally did) place a 



freater number of magnets round the circle, or within its area, 

 ut for the experiments now in view four were enough. 

 The system so constructed was found to require no after 

 adjustment, being to all appearance perfectly neutral, so that 

 this part of our purpose was completely accompUshed, and the 

 earth's action eliminated from the inquiry. The irregular torsion 



