400 TERRESTRIAL MAGNETISM. 



LOCAL MAGNETIC DISTURBAXCK. 



Ill Map No. 2 iiorinal lines of equal value of the declination are 

 reoorded, and a.s far as the greater part of the globe covered bj' water 

 is concerned we may accept them as undisturbed values, for we have 

 yet to learn that there are any local mag-netic disturbances of the needle 

 in depths be3"ond 100 fathoms. 



When, however, we come' to the land, there is an increasing diffi- 

 culty in finding districts of only a few miles in extent wdiere the 

 observed values of the magnetic elements at different stations therein 

 do not differ more widely than the\" should if we considered only their 

 relative position on the earth as a magnet. Take Riicker and Thorpe's 

 maps of the British Isles and those of the United States, for example, 

 where the lines of equal value are drawn in accordance with the 

 observations, with the result that they form extraordinary loops and 

 curves differing largely from the normal curves of calculation. 



From among numerous examples of disturbance of the declination 

 on land, two may be quoted. In the Rapakivi district, near Wiborg, 

 a Kussian surveying officer in the vear 1890 observed a disturbance of 

 180 degrees, or. in other words, the north point of his compass pointed 

 due south. At Invercarg.ill, in New Zealand, within a circle of 80 feet 

 radius a difference of 50 degrees was found. Even on board ships in 

 the same har})()r different results are sometimes observed, as our train- 

 ing xjuadron found at Reikiavik, in Iceland, and notably in our ships 

 at Bermuda. 



It is hardly necessary to add that the dip and force are often largelj' 

 subject to like disturbance, but I do so in order to warn trav^elers and 

 surveyors that observations in one position often convev but a partial 

 truth; thev should be supplemented by as mam' more as possit)le in the 

 neighborhood or district. Erroneous values of the secular change have 

 also been pul)lished from the various observers not having occupied 

 exactly the same spot, and even varied heights of the instrument from 

 the ground may make a serious difference, as at Rapakivi, before men- 

 tioned, and at Madeira, where the officers of the Challenger expedition 

 found the dip at a foot above the ground to be -18- 46' north: at 8i 

 feet above the ground, 56° 18' north, at the .same spot. 



All mountainous districts are specially open to suspicion of magnetic 

 distui-t)ance. and we know from comparison with normal ol)servations 

 at sea that those mountains standing out of the deep sea, which we call 

 islands, are considerably so affected. 



MAGNETIC SHOALS. 



The idea that the compasses of ships could be affected by the attrac- 

 tion of the neighboring diT land, causing those ships to be unsus- 

 pectingly diverted from their correct course, was long a favorite 

 theory of those who discussed the causes of shipwreck, but it was '"a 



