TRANSACTIONS OF THB SECTIONS. 29 



needles by the deviations of a compass attached to the instrument, 

 under perfectly analogous circumstances, as to the distance and re- 

 lative position of the bars and the compass. 



By means of this instrument, the extreme susceptibility of soft 

 iron for the magnetic condition, in the small measure of permanency 

 belonging to that substance, was exhibited in the case of a cylin- 

 drical bar of almost six inches in length and a quarter of an inch in 

 diameter. This bar being laid in a groove of the moveable limb of 

 the magnetometer, and adjusted in the plane of the magnetic equator, 

 was shovni to be entirely devoid of action upon the compass needle, 

 only 1^ inch distance; the bar was then cautiously removed, and, 

 whilst held in a vertical position, was merely rubbed down two or 

 three times by the naked hand, and then replaced as before on the 

 instrument, when it was now found, very much in accordance with 

 former experiments, to have acquired magnetism to the extent of 

 producing a deviation of 5° on the needle of the contiguous com- 

 pass. 



The object of this experiment was to point out the extreme cau- 

 tion which is requisite to be observed in the mere moving or handling 

 of the substances made use of in delicate magnetical investigations, 

 such as the needles employed in experiments on the magnetizing in- 

 fluence of the solar rays, since, as was now shown, the slightest 

 concussion, or even the friction of the fingers on a bar of iron or 

 soft steel favourably situated, may be productive of such striking 

 effects. 



Mr. Scoresby's new magnetical instrument, a compound compass 

 needle or bar, was then exhibited to the Section, and its construc- 

 tion, adjustments, and capabilities, as far as had hitherto been as- 

 certained, were described. The bar, which was sixteen inches in 

 length, consisted of six equal and similar plates or ribs of tempered 

 steel, placed parallel to each other, but not in contact ; which ribs, 

 in this case, were composed of the ordinary steel busks of the shops. 

 It was suspended on a point of steel, and its weight partly borne, 

 in any required proportion of the whole weight, by a single horse- 

 hair (the torsion of which within the limited range of the vibrations 

 of the bar was insensible), suspended from a spring fixed on a cross 

 bar, supported on pillars, and adjusted in an exact vertical position 

 above the centre of suspension. I'he magnetic position was indi- 

 cated by a graduated arch in the toj) of the instrument, with a ver- 

 nier attached to each end of the bar. The principle, from which 

 this bar was considered to have its superiority over a single bar of 

 the same weight and magnitude, was stated to be, that several thin 

 bars of tempered steel (tempered throughout the massj are found to 

 have a greater capacity for permanent magnetism than what is af- 

 forded by the mere proportional of their mass similarly tempered, 

 so that the six tempered bars were capable of receiving a degree of 

 magnetic energy considerablj' greater than it was believed could be 

 permanently induced in any single bar of equivalent mass, whatever 

 might be its condition as to temper. 



