536 LECTURE LV. 



position ; and a still more remarkable change occurs continually in the 

 diurnal variation. In these climates the north pole of the needle moves 

 slowly westwards from about 8 in the morning till 2, and in the 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 magnetic 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 con- 

 stant for the same place, or even if it varied according to any discoverable 

 law ; since it would afford a ready mode of determining 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 east or 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 have moved more uni- 

 formly 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 5 near the island of 

 Barbadoes, from 1700 to 1756. (Plate XLI. Fig. 574 . . 576. Plate 

 XLII. XLIII.) 



The dip of the north pole of the needle in the neighbourhood of London 

 is 72.t Hence the lower end of a bar standing upright, as a poker, or a 

 lamp iron, becomes always a north pole, and the temporary south pole of 

 a piece of soft iron being uppermost, it is somewhat more strongly attracted 

 by the north pole of a magnet placed over it, than by its south pole ; the 

 distribution of the fluid in the magnet itself being also a little more favour- 

 able to the attraction, while its north pole is downwards. It is obvious 

 that the magnetism of the northern magnetic pole of the earth must 

 resemble that of the south pole of a magnet, since it attracts the north pole ; 



* Graham, Obs. made in 1722. Ph.Tr. xxxiii. 96, 383. The daily variation has 

 been more accurately observed by Christie, Ph. Tr. 1823-5-7 ; and at Gottinge^ by 

 Goldschmidt and others, Res. des Mag. Vereins, v. y. 



t It is now about 69. 



