35^ Rev. Mr Scoresby on the Uniform Permeahiliiy of 



(2.) The comparative directive powers of magnets of different 

 dimensions was more carefully and fully investigated. 



a. As to the thickness of bars in other respects the same. — 

 A set of straight bars, kindly furnished me by Mr Abraham of 

 Sheffield, made out of the same mass of steel, and tempered in 

 a similar manner, being of the same length and breadth, but 

 differing as to thickness, afforded a satisfactory apparatus for 

 the determination of this point. These bars were five in num- 

 ber, each twelve inches long and one inch broad, and of the se- 

 veral thicknesses of 0.55, 0.28, 0.20, 0.14, and 0.08 inch. 



Each bar being successively placed between the different poles 

 of a pair of good two feet bar-magnets, was magnetized, first, 

 after the manner of ^pinus, and then, after their directive force 

 had been tried, by MichePs method, — a single stroke of a power- 

 ful horse-shoe magnet being made from end to end of the series, 

 —when the following results as to their action on a compass at 

 the distance of eighteen inches, and in the direction of the west 

 point of the compass, were obtained : — 



Comparing the last three columns, it would appear that the 

 bars being magnetized beyond saturation, were not able to re- 

 tain the power that was given to them ; hence, a diminution of 

 intensity was discovered on the second trial, though, in the mean 

 time, they were kept separate, and not allowed to touch any 

 magnetic substance, lest their power should be altered. 



These results were sufficient to verify the previous deductions 

 of different magneticians, that little, if any, increase of power is 

 gained by increase of thickness in magnets beyond a given mi- 

 nimum ; for the intensity of directive force of the thickest bar 

 above that of the others, is perhaps not greater than what is 

 simply due to its greater extent of surface. 



