ON MAGNETISM. 69I 



6 or 7 degrees to the westward of its usual position; and a still more remark- 

 able change occurs continually in the diurnal variation. In these climates 

 the north pole of the needle moves slowly westwards from about 8 in the morn- 

 ing till 2, and in tiie evening returns again; a change which has with great 

 probability been attributed to the temporary elevation of the temperature of 

 the earth, eastwards of the place of observation, where the sun's action takes 

 place at an earlier hour in the morning, and to the diminution of the mag- 

 netic attraction in consequence of the heat thus communicated. In winter this 

 variation amounts to about 7 minutes, in summer to 13 or 14. 



Important as the use of the compass is at present to navigation, it would be 

 still more valuable if its declination from the true meridian were constant for 

 the same place, or even if it varied according to any discoverable law; since 

 it would alford a ready mode of deternrniing the longitude of a place by a 

 comparison of an astronomical observation of its latitude with another of the 

 magnitude of the declination. And in some cases it may even now be applied 

 to this purpose, where we have a collection of late and numerous observations. 

 Such observations have from time to time been arranged in charts, furnished 

 with lines indicating the magnitude of the declination or variation at the 

 places through which they pass, beginning from the line of no variation, 

 and proceeding on the opposite sides of this line to show the magnitude of the 

 variation castor west. It is obvious that the intersection of a given parallel 

 of latitude, with the line showing the magnitude of the variation, will indicate 

 the precise situation of the place at which the observations have been made. 



The line of no variation passed in 1657 through London, and in 1666 through 

 Paris: its northern extremity appears to have moved continually eastwards, 

 and its southern parts westwards ; and it now passes through the middle of 

 Asia. The opposite portion seems to liave moved more uniformly westwards; 

 it now runs from North America to the middle of the South Atlantic. On 

 the European side of these lines, the declination is westerly; on the South 

 American side, it is easterly. The variation in London has been for several 

 years a little more than 24 °. -In the West Indies it changes but slowly ; for 

 instance it was 6" near the island of Barbadoes, from. 1700 to 17^6. (Plate 

 XLI. Fig. 574 . . 576. Plate XLIL XLIII.) 



