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of iron, about the sixteenth of an inch thick, effectually intercepts 

 the action of a revolving magnet on a disc of copper, the same re- 

 sult is not obtained when the disc acted upon is also of iron, instead 

 of being of copper ; unless the mass of iron interposed be very con- 

 siderable. The screening influence he found to depend on the mass 

 of iron that is interposed, and not on the surface merely. He was 

 led to suspect that a similar effect might be obtained by employing 

 substances not of a ferruginous nature, provided they were inter- 

 posed in considerable masses, and the result of his trials justified 

 his conjecture. An account is given of several experiments made 

 with large masses of silver, copper, or zinc, of about four inches in 

 thickness, which being interposed between a revolving magnetic 

 plate and a delicately suspended disc of tinned iron, completely in- 

 tercepted the action of the magnet on the iron. 



The author considers this interceptive property to be more or 

 less common to every class of substance ; and that in order to render 

 it sensible, it is only necessary to employ the bodies in masses, bear- 

 ing some direct ratio to their respective magnetic energies. Thus 

 lead, having a weaker magnetic energy than copper, must be em- 

 ployed in a larger mass in order to produce an equal effect ; and to 

 render the screening power of ice sensible would require it to be 

 above thirty feet in thickness. If, instead of interposing the screen 

 of iron immediately between the revolving magnet and the sus- 

 pended disc of copper, the iron be brought very near the under 

 surface of the magnet, a similar neutralizing influence is exerted. 



In the second paper, the investigation of this subject is resumed, 

 and the neutralizing power of a mass of iron investigated under 

 different circumstances. From the experiments detailed by the au- 

 thor, he is led to infer that substances highly susceptible of receiving 

 transient magnetism, are the most efficient in their operation as 

 screens; this operation being referable to their neutralizing power. 

 It is, however, very difficult to render this power sensible in the case 

 of non-ferruginous bodies, unless they be actually placed between 

 the magnet and the substance acted upon, so as to neutralize effec- 

 tually the actions of those points which are nearest to each other. 

 The attractive force exerted between a magnet and a mass of iron 

 he finds to be always in the direct ratio of this controlling or screen- 

 ing power of the iron, or, in other words, to its neutralizing power 

 in similar circumstances. 



The author suggests that a temporary magnetic state may be 

 conceived to be induced in a substance in two ways: either by the 

 immediate action of the magnet upon each individual particle of the 

 given substance, or else by the action of each particle of that sub- 

 stance on the next in succession, producing a propagation of mag- 

 netism from the one to the other. It may also, however, take place 

 in both these ways at the same time. But these different modes of 

 action appear to be in some inverse ratio of each other: for when 

 the retentive or absorbing power of the substance is considerable, 

 the power of the magnet becomes soon controlled; because the par- 

 ticles of the substance first acted upon, begin to operate as screens 



