TERRESTRIAL MAGNETISM. 401 



fond rhino- vainly invented/"' 1 can hardly say this idea is yet exploded, 

 but from what has already l>een said about local magnetic disturl)ance 

 on land, it is not a matter of surprise that similar sources of disturb- 

 ance should exist in the laud under the sea, for it has been found that 

 in certain localities, in depths of water suthcient to float the largest 

 ironclad, considerable disturbances are caused in the compasses of 

 ships. 



An area of remarkable disturl)ance having been reported as existing- 

 ofl[' Cossack, northwestern Australia, II. M. S. Penguin^ a surveying ship 

 provided with the necessary magnetic instruments, was sent by the 

 Admiralty in 18*.*1 to make a complete magnetic survey of the localit}', 

 W'ith a view^ to ascertaining the facts and placing them on a scientilic 

 basis.' An area of disturbance 3.5 miles long by 2 miles broad, with 

 not less than S fathoms of AViiter over it, was found h'ing in a north- 

 east b}" east and southwest by west direction. At one position the dis- 

 turbing force was sufficient to deflect t\\Q, Penguin'' s compass .56 degrees; 

 in another — the focus of i)rincipal disturbance — the dip on board was 

 increased by 29 degrees, and this at a distance of over 2 miles from 

 the nearest visible land, upon which only a small disturbance of the 

 dip was found. 



This remarkable area of disturbance was then called a "magnetic 

 shoal."" a term which at rirst sight hai'dly appears to be applicable. We 

 have, however, become familiar with the terms '' ridge line, \alley line, 

 peak, and col," as applied to areas of magnetic disturl)ance on land; 

 therefore I think we may con^'eniently designate areas of magnetic 

 disturbance in land luider the sea "magnetic shoals."* 



This year His Majesty's surveying ship Re^eni'ch has examined and 

 placed a magnetic shoal in East Loch Roag (island of Lewis), l)ut as 

 all our surveying .ships are practically iron ships, it was impossible 

 from observations on board to oV)tain the exact values of the disturb- 

 ing forces prevailing in this shoal. The reason for this is that, 

 although we may accurately measure the disturbing forces of the iron 

 of the ship in deep water, directly she is placed over the shoal induc- 

 tion takes place, and wt^ can no longei- determine to what extent the 

 observed disturl)ances are due to the ship's newly developed magnet- 

 ism, or to whiit extent the shoal alone produces them. 



We can, nevertheless, even in an iron ship, accurately place and 

 show the dimensions of a magni^tic shoal and the direction in which a 

 ship's compass will ])e detlected in any part of it by compass observa- 

 tions only. Is it not, therefore, the duty of any ship meeting with 

 such shoals to stop and flx their position^ 



The general law governing the distribution of magnetism on these 

 magnetic shoals is that in the Northern Hemisphere the north point of 

 the compass is drawn toward the focus of greatest dip: in the Southern 

 Hemis[)here it is repelled. The results at East Loch Koag proved an 

 exception, the north point ot" the compass being repelled. 



