192 



Prof. E. Bouty's Studies on Magnetism. 



portionality of their distance from the ends of the needle to the 

 first power of the diameter. 



Comparison of the results furnished by observation on needles 

 of different diameters is attended with great practical difficulties. 

 The multiplicity of comparisons of apparatus required by these 

 experiments, the considerable influence of the slightest errors in 

 the estimation of the minute diameters upon the ratios to be de- 

 termined, and, above all, the difficulty of giving an identical 

 temper to needles of different diameters are grave obstacles 

 which it is not easy entirely to surmount. It is right, however, 

 to remark that the difficulty in regard to the tempering is less 

 for needles that are tempered very hard, such as we have always 

 employed, because in this case the coercive power varies little for 

 rather large variations in the temperature at which the steeping 

 was effected. 



The following Table relates to the law of the polar distances. 

 The first column gives the diameters of the needles, the second 



the abscissa D, the third the value of the ratio -7 (which should 



be constant); and the last column gives the differences of the 

 numbers in the third column from their mean. 



Table IV. 



d. 



D. 



D 



d' 



8. 



millim. 



millim. 







0175 



177 



10113 



+0-718 



0-282 



2-45 



8-688 



-0-707 



0-368 



3-60 



9-783 



+0-388 



0-551 



5-25 



9528 



J-0133 



1036 



9-90 



9556 



+0-161 



1-290 



12-40 



9612 



+0-307 



1-988 



1680 



8-451 



-0-944 



These numbers verify the law, if we take into consideration 

 the above-mentioned multiplicity of the causes of error. 



The law relating to the power of the poles is equally well 

 verified, as will be seen in Table V., the third and fourth columns 

 of which give the absolute and relative values of the difference 



Q 



between -^ and the corresponding mean. 



