448 Sir G. C. Haiighton on the Common Nature of 



to 45°, the time occupied was 5h seconds ; while its return 

 from 45° to 0°, after being again left to itself, took 6fj^ seconds. 



It might be supposed, a priori, that the north end of the 

 bar-magnet when brought to the south and left to itself would 

 be repelled with violence; but so far from this, as seems to be 

 the case when small needles are observed, its movement is 

 extremely slow, and thei'e is a loss of a second or two before 

 it acquires the gradually augmenting momentum that is to 

 carry it back: a momentum indeed which increases so rapidly, 

 that though the bar consumed ll^^ seconds to carry it back 

 from 180° to 135°, it only required 15^ seconds to carry it 

 from 165° to 0"^, and of this time 13^ seconds were occupied 

 in reaching 45°, after which the remaining 45° were completed 

 in 2 seconds; yet the bar required 6^ seconds to return from 

 45^ to 0°, when it had not the benefit acquired by the pre- 

 ceding momentum. As a further proof how much the slow- 

 ness of the movement of the bar increases near 0°, when not 

 assisted by previous momentum, 6| seconds was the space of 

 time requisite to carry the bar from 5° to 0°, yet when it had 

 the advantage of the momentum acquired between 10 and 5°, 

 it completed the whole in ^ second less time, that is the space 

 from 10° to 0° in 6^ seconds, while it has just been seen that 

 it required only 2 seconds to carry it from 45° to 0°, when it 

 had the advantage of the previous momentum acquired by 

 setting off from 165°. 



There was a peculiarity in the return movement of the 

 magnetic bar which is worthy of attention. When the north 

 end moved from 180° to 135°, it required, as has just been 

 seen, llg- seconds to complete the amplitude, while it only 

 took 6^ seconds to traverse over the parallel amplitude from 

 45° to 0°. The reason of this ditference is, that in the first 

 case its momentum is in an increasing ratio, and the force that 

 is to move it has gradually to augment, while from 45° it sets 

 off with a strong momentum which it retains till it reaches 0°. 

 What the nature is of this force which goes on increasing on 

 the return of the bar up to 90°, and from that [.oint continues 

 decreasing down to 0°, is a question of much importance in 

 the theory of terrestrial magnetism. It is evidently one at 

 right angles to the natural position of the needle, and seems 

 to be caused by a principle identical with that of the con- 

 junctive wire of the pile, and would strongly tend to confirm 

 Ampere's theory of electric currents from east to west, if the 

 supposition of the efficiency of these were not opposed by 

 considerations to which I may return on a future occasion. 



While upon this point it will be interesting to see the rela- 

 tive times occupied by the needle when drawn back to 175°, 



