l8') French Katlona! hiftilutf. 



dies formed of fuch b;irs will give, in regard to a bundle 

 double in ihicknefs, the hmc refult. 



Tlieorv indicaies jilfo, that when a bar i:'> compared wiih 

 ilfclf in ditferent ftates, whatever n)ay be its dimenlions, if 

 ii be magnetized to faturation, its ilirettlve force ought to 

 he augmented with the coercive force wliich keeps t!ie niac;- 

 neiicflnids fenarated. Hut as this coercive force isconftant mr 

 equal Itates of the ftccl, it thence rcfulls, that whatever may 

 be the dimcniionsof a bar it will give a conUant ratio when 

 its diici^iive force is compared in two cl>flVrent liates. Here, 

 alfo, experience is aifreeable to thcorv. ]?ars made from the 

 fimic piece of Knglilh fteel-plate, whatever their number and 

 dinicnfions, when hardened at a cherry red, gave directive 

 forces greater than when brought down from a higher to that 

 temper in the ratio of 24 to 10, whicli announces that the 

 coercive forces are in the fame ratio. 



The fame bars brought to a blue temper gave, in regard to 

 thofc brought down to a white temper, the ratio of 164 to 

 100; but in this there is a little variety, becaufe it is almott 

 impoffible to enfure the fame degree of icmpcring. 



In fteel of inferior (juality there is Itill more advantage in 

 giving it a greater decree of temper. The author found bars 

 of German licel the coercive force of which, when they were 

 tempered at a cherry red, was to the fame force, when brought 

 from a higher to the fiime temper, as 34 to ro; but, on the 

 other hand, this ftcel l)rought to a white temper has a di- 

 rective force much lefs than ileel of the Englilh plates. 



C. Coulomb has read to the clafs only this lirit part of his 

 memoir. 



The fecond part is deRined to determine the curve of the 

 maonetic denfities and its extent in bars of a white temper, 

 in liiofe brought back to the ftate of fpring, and in thofe 

 brought down to white temper. 'J'he object of this refearch 

 is very interelling, either for conftrudting artificial magnets, 

 or for fixing the itate and dimenfions of needles molt advan- 

 tageous for thofe employed in the mariner's compafs. The 

 author found, for example, that when the bars are very long 

 in regard to their other dimenfions, it is of advantage, after 

 thev^are tempered almoft to a white, to bring them back to 

 blue: for the excellent cylindric wires of Heel ufed in com- 

 merce, when magnetized with the ftrongeil magnets, have the 

 fane direftive force whether they are tempered to a white or 

 are brought back to the ftate of blue fpring when their length 

 is thirty-three times their diameter; but if they are much 

 lonaer than thirty-three times their diameter there is a great 

 idvautage iu bringing them to the llate of fpring ; and on 



6 the 



