aU hiown Substances to the Magnetic Injluence, S^c. 329 



But the advantage of surface over mass was rendered pecu- 

 liarly striking, by presenting the bars in combination (laid up- 

 on each other, a situation of much disadvantage), at the same 

 distance from the compass as before, according to the following 

 arrangements : — 



Hence, comparing the tangents of the angles of deviation, — 

 the measure of the relative directive forces as established by 

 subsequent results*, — we find that the bars d and e in combina- 

 tion (being 0.22 inch in thickness), had a greater power than 

 the single bar c (of nearly the same thickness), in the propor- 

 tion of about 8 to 5. But if we take into account the injury 

 done by the combination to the power of the bars, that propor- 

 tion will be nearly as 2 to 1. For after the experiments were 

 completed, the directive force of c was found to have diminished 

 from 29° to 26°; d, from 29° to 27 J°; and e, from 27 J° to 



23J°.t 



h. The comparative directive force of bars of different lengths 



— ^increasing also proportionably in their other dimensions — was 

 the next object of inquiry. 



The design of this investigation, as also, indeed, that of most 

 of the experiments in this section, was not so much to attain 

 perfect results, di% practical results, applicable to the method pro- 

 posed for measuring otherwise indeterminable distances. 



• Were the magnet presented to the compass at right angles to its deviated 

 position (or the position the needle actually assumes under the influence of 

 the magnet), then the force exerted in drawing the needle from its meridional 

 position would be represented by the sines of the angles of deviation. But 

 when the magnet is placed always at right angles to the meridional position of 

 the needle, the force is correctly represented by the tangents of deviation. 



f The greater loss sustained by bar e than the rest, confirmed me in an 



opinion previously formed, that its temper was less perfect than the rest. 



o 



