and Remarks on the TJieory of Magnetism. 29 



if we find that the south pole always receives the magnetic fluid, 

 and the north gives it out, it follows that it must necessarily 

 point north and south, according to the direction of the current 

 that moves it. 



Upon this principle also, we would readily conceive why the 

 needle dips when rendered magnetic, as it will be disposed to 

 dip to the inclination, which the stream of the subtle bodies, al- 

 ready alluded to, must assume in passing continually from the 

 sun and central parallel line of the earth, to the north and south 

 poles. 



The sun, indeed, in a clear atmosphere, has a visible effect on 

 the dip of the needle. And if we take a thin bar of steel, about 

 the thickness of the main-spring of a watch, and two feet in 

 length, and render it magnetic, and balance it on a fine pivot, 

 we can observe a slight variation occasionally in the dip of the 

 needle, by a graduated scale, placed opposite one of its extre- 

 mities, corresponding to the clearness of the atmosphere and time 

 of the day. 



Upon the same principle also, the annual variation of the 

 magnetic needle may be partly accounted for, by the radiating 

 heat produced by the sun, and the other fluids already mention- 

 ed, being conducted in greater proportions for a course of years 

 towards the western part of our hemisphere, from a combina- 

 tion ot causes no doubt similar to those which sometimes pro- 

 duce a series of warm seasons to the west, and sometimes to the 

 east of the world ; ' and perhaps this may be affected by the 

 comparative progress of cultivation in the diff'erent nations of 

 the earth. And this seems to coincide with the accounts of our 

 late navigators, who have found the ice more melted toward 

 the west than toward the east of the North Pole. 



I should conceive it to be but a very imperfect method of 

 determining the magnetic intensity at any place, to subject the 

 needle to vibration or torsion, as the state of the atmosphere, 

 the influence of the sun at different periods of the year, and at 

 different times of the day, local attraction, the attraction of gra- 

 vitation, and the law of the vibration of the pendulum, must all 

 have their share in the calculation ; and all these may be modi- 

 fied by circumstances not readily perceived. 



Brighton Crescent, Portobello, 

 \st October 1827. 



