146 TERRESTRIAL MAGNETISM. 



of the line of no inclination for a definite epoch, have 

 all the desired importance, until ships charged with 

 the sole duty of tracing these lines uninterruptedly 

 throughout their course shall be sent contempora- 

 neously to different seas. I here renew a solicitation 

 to which I have repeatedly permitted myself to give 

 free expression. ( 186 ) For systematic investigations in 

 terrestrial magnetism it is of primary importance that 

 the determinations should be contemporaneous. 



According to our present general knowledge of the 

 " lines of no declination," instead of four such lines 

 running as meridians from pole to pole, as was sup- 

 posed at the end of the 16th century, ( 187 ) there appear 

 to be three very differently shaped systems, using the 

 word " system " in this case to denote a group of 

 isogonic lines which include a line of no declination 

 unconnected, so far as we know, with any other such 

 line. Of these three systems, which I shall presently 

 describe separately, the Atlantic one consists simply of 

 a line of no decimation directed from SSE. to NNW., 

 and recognised as extending from the 65th degree of 

 south to the 67th degree of north latitude. The second 

 system is situated fully 150 degrees to the eastward of 

 the first (looking in both cases only at the points of 

 intersection of the lines of no declination with the 

 geographical equator) ; it is the broadest and most 

 complex system, and occupies the whole of Asia and 

 Australia. It has one summit directed to the north, 

 and one to the south: in the north-eastern part the 

 line of no declination takes an oval shape, and returns 

 into itself, surrounding lines of successively and rapidly 

 increasing declination. The east and west sides of this 



