330 Rev. Mr Scoresby on the Uniform Permeability of 



Hence, instead of procuring a set of magnets expressly con- 

 structed for the inquiry, whose quality and temper should be 

 the same, and whose lateral dimensions should be in proportion 

 to their lengths, I merely employed such magnets as I happened 

 to have by me, with a view of ascertaining principally this ques- 

 tion, Whether the deviations produced by magnets of different 

 lengths are similar, when their distances, in measures of their 

 own lengths, are proportional ? 



The following is a tabular view of the comparative devia- 

 tions (being the sum of the actions of the two poles) produced 

 by different bar-magnets, at proportional distances from the 

 centre of the compass, in measures of their own lengths : — 



These results are sufficient to shew that the directive force of 

 bars of different lengths — the other dimensions and magnetic 

 energy being proportional, and their quality similar, is very 

 nearly in the relation of their lengths. In each series of experi- 

 ments, it will be perceived that the advantage is somewhat in fa- 

 vour of the shorter bars ; but in no case has the short bar, at 

 six lengths'* distance, the influence of the longest bar at five 

 lengths"* distance ; and although, at the first length, the differ- 

 ences are very great, yet part of the excess in favour of the 

 shorter manjnet, is to be attributed, as will hereafter be shewn, to 

 the greater proportional nearness of the attracted pole of the 

 compass, than when this approximation of the pole bears a less 

 proportion to the distance of the magnet. 



(3.) The last inquiry belonging to this part of the subject is 

 the effect of the number of magnets on the deviations produced. 



If the tangent of the angle of deviation be the measure of the 



