118 M. Hauy on Magnetism. [August ; 



oxidated, or it is combined with some principle which deprives 

 it of its magnetic power, as, for example, in arsenuretted or 

 sulphuretted iron ; it is then necessary to heat a small fragment 

 of the mineral in the flame of a taper, in order to render it mag- 

 netic ; sometimes the action of the blow-pipe is required, but it 

 may generally be dispensed with. To perform these experiments 

 with success, we must procure a highly magnetized needle, 

 furnished with a cap of agate or rock crystal, and supported on 

 a very delicate point. 



In order to understand the method which M. Haiiy adops in ex- 

 hibiting small quantities of magnetism, the principle is explained 

 by which the needle is preserved in the magnetic meridian. Sup- 

 pose the needle to be in our hemisphere, where it is the nearest to 

 the north pole of the globe. The fluid which resides in this pole 

 acts by its attraction for the south pole of the needle, and its 

 repulsion for its north pole ; the reverse takes place with respect 

 to the south pole of the globe ; but because it is more remote, 

 we may consider the needle as influenced solely by the force of 

 the north pole of the globe, in proportion to the excess of thi«3 

 force over that of the other pole. If we conceive that the needle 

 is removed a little from the plain of its magnetic meridian, its 

 directing force will immediately act in order to restore it ; it has 

 been proved by M. Coulomb, that this directing force is propor- 

 tional to the sine of the angle which the needle makes when 

 removed from its natural direction with the natural direction, 

 that is with the magnetic meridian. Before any body which is 

 presented to the needle can act upon it, it will have to over- 

 come this directing force as well as the little degree of resistance 

 which the needle must experience at the point of suspension ; 

 and we may easily conceive that the quantity which is presented 

 to the magnet may be too small to overcome the sum of both 

 these forces, and that the needle will, therefore, remain immov- 

 able. 



From reflecting on these circumstances, the author conceived 

 the idea of diminishing the force opposed to the action of the 

 needle, so that it would detect portions of iron too small to be 

 sensible by the ordinary means. To accomplish this object, he 

 places a magnetic bar at a certain distance from the needle on 

 the same level, and in the direction of its axis, and with its poles 

 situated contrary to those of the needle. If we suppose the 

 magnetic bar to be placed to the south of the needle, the south 

 poles of the magnet and the needle will be opposed to each 

 other ; and if we cause the magnet to approach the needle, the 

 needle will move on its centre, towards one side or the other, 

 until an equilibrium is produced between the action of the 

 magnet and the needle on each other, and that which exists 

 between the needle and the earth. It follows from the law 

 which was laid down by Coulomb, that in proportion as the 





