262 Rev. Mr Scoresby oti of some of the Laws 



magnetic direction, where, being in the plane of the magnetic 

 equator, it was free from any disturbing influence from terres- 

 trial magnetism. The magnets were then placed at various 

 successive distances in the line of the bar, according to the ar- 

 rangement represented in PL II. Fig. 3. The distance of the 

 bar from the compass, which throughout the series of experi- 

 ments was always precisely the same, was 5^ inches, and the 

 distance of the magnets from the bar was first 12 inches, and 

 then at equal successive intervals, as far as four feet. The ob- 

 ject of fixing upon intervals of 12 inches was, because that dis- 

 tance was very nearly the Jbcal length of the magnets, or the 

 length betwixt the two foci or centres of attraction in either half 

 of the magnets, to which the whole of the varying magnetic in- 

 fluences, in such segments, might be referred. The employ- 

 ment of intervals, therefore, of focal lengths of the magnet and 

 iron bar, was calculated to simplify the investigation, and to af- 

 ford more satisfactory results. Though I had a motive for fix- 

 ing upon the position given to the iron bar, the distance of its 

 focal point from the centre of the compass being about one-half 

 of its focal length ; yet, whilst its position was, on every trial, 

 always the same, the particular distance was of little importance. 

 In this position of the bar, the first situation of the magnets 

 was (5| -H ]3f -f- 12) = 31 inches from the centre of the com- 

 pass ; the second 43 ; the third 55 ; and the fourth 67 inches. 

 Placing now the magnets, in the general line, with their south 

 poles directed towards the east, at the shortest interval, the bar not 

 being yet placed intermediately, the power of the magnets alone, 

 as determined by their action on the needle of the compass, was 

 first observed, and found to be 27° 42'. The iron-bar, the mag- 

 netism of which had been carefully neutralized, was then inter- 

 posed in its assigned position (according to Fig. 3.), each end 

 of the bar being alternately presented towards the compass, 

 when the mean deviation, by a double set of careful observations, 

 proved to be 38° 3^'. The unassisted action of the magnets 

 was then verified by another trial after the bar was removed, 

 and found to be unaltered ; consequently the increase of devia- 

 tion of (38° 3|/ — 27° 42') = 10° 21 J', indicated the proportion 

 of action due to the magnetism induced, under the particular 

 circumstances, into the bar of iron ; or rather, (being directly in- 



