150 Analyses of Books. [Feb. 



4. Kupffer has shewn, in regard to the effect of terrestrial mag- 

 netism upon bars not thoroughly magnetized, that a vertical magnetic 

 bar has more force when the north pole is directed downward than 

 in the opposite direction. 



5. Quetelet has found that a single friction is sufficient to reverse 

 the poles of a magnet, and place the bar in a contrary magnetic 

 state, and that the influence which tends to bring the poles to their 

 primary state is the most powerful, although, after a certain number 

 of reversions this tendency diminishes. 



6. Magnetic needles which are not too short possess directing 

 powers with an equal diameter proportional to their lengths, provided 

 their transverse section be always the same. It results from Cou- 

 lomb's experiments, that ceteris paribus needles should possess no 

 greater thickness than is necessary to prevent them from bending. 



7. Mr. Barlow has succeeded in determining the law which 

 regulates the action of iron upon magnetic needles as on board of 

 ship, viz. : the tangents of deviation are proportional to the cubes of 

 the diameters, or as the power | of the surfaces, whatever be the 

 solid contents. The magnetic force being as the surface, and the 

 tangent of deviation | of the surface, it follows that the square of the 

 tangent of deviation varies directly as the cube of the force, or the 

 tangent of deviation varies directly as the power £ of the force. 



8. The experiments of Haldat tend to shew that in untwisting an 

 iron wire which has been magnetized by torsion, its polarity is 

 removed ; and Nobili has almost proved that magnetism increases 

 more in proportion to the degree of tempering than to the mass of 

 the magnetic body. 



9. Newton, in his optics, has stated that iron raised to a red heat 

 is not magnetic, while Kircher has made an exactly opposite remark. 



Barlow found the action of iron raised to a blood red heat very 

 " intense, but extinct at a white heat ; between a red and white heat 

 he observed that the action increases in proportion as the bar is raised 

 above the needle, while at a low temperature the action of a bar of 

 iron in the same circumstances goes on always diminishing. He 

 heated bars of copper to an intense temperature, and on approachiug 

 the needle could detect no action. Hence, the heat does not act 

 independent of the iron. He supposes that during the cooling of 

 the bars the extremities where the cooling is most rapid become 

 magnetic before the rest of the metal, giving rise to a complex action, 

 but admits that this explanation is not sufficiently satisfactory. 

 Coulomb obtained similar results ; and Kupffer has demonstrated 

 that the diurnal variations do not contribute to diminish the magnetic 

 influence of the needle, but that the magnetic force of soft iron 

 increases with the heat. Mr. Christie found that between the 

 limits of 112" and 212" an increase of temperature produced an 

 increase of force in the magnetic power of iron, which seems to 

 argue against the idea that the action of the iron upon the needle 

 proceeds from the polarity which is communicated to it by the earth. 

 10. Coulomb was the first who pointed out the nicest method of 

 ascertaining the presence of magnetism in all bodies in nature, 



