TRANSACTIONS OF SECTION E. 707 



Local Magnetic Disturbance. 



In Map No. 2 normal lines of equal value of the declination are recorded, and 

 as far as the greater part of the globe covered by water is concerned, vs'e may 

 accept them as undisturbed values, for we have yet to learn that there are any 

 local magnetic disturbances of the needle in depths beyond 100 fathoms. _ 



When, however, we come to the land, there is an increasinp- difficulty in finding 

 districts of only a few miles in extent where the observed values of the magnetic 

 elements at different stations therein do not differ more widely than they 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 tliey form extraordinary loops and curves differing largely 

 from the normal curves of calculation. 



From amono- numerous examples of disturbance of the declination on land, two 

 may be quoted. In the Rapakivi district, near Wiborg, a Russian surveying officer 

 in the year 1890 observed a disturbance of 180°, or, in other words, the north 

 point of his compass pointed due south. At Invercargill, in New Zealand, within 

 a circle of 30 feet radius, a difference of 56° was found. Even on board ships in 

 the same harbour different results are sometimes observed, as our training squadron 

 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 largely subject 

 to like disturbance, but I do so in order to warn travellers and surveyors that ob- 

 servations in one position often convey but a partial truth ; they should be supple- 

 mented by as many more as possible in the neighbourhood or district. Erroneous 

 values of the secular change have also been published from the various observers 

 not having occupied exactly the same spot, and even varied heights of the instru- 

 ment from the ground may make a serious difference, as at Rapakivi before 

 mentioned, and at Madeira, where the officers of the ' Challenger ' expedition found 

 the dip at a foot above the ground to be 48° 46' N. ; at 2>\ feet above the ground 

 56° 18' N, at the same spot. 



All mountainous districts are specially open to suspicion of magnetic disturb- 

 ance, and we know from comparison with normal observations 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 attraction of the 

 neighbouring dry land, causing those ships to be unsuspectingly diverted from their 

 correct course, was long a favourite theory of those who discussed the causes of 

 shipwreck, but it was ' a fond thing vainly invented.' I can hardly say this idea 

 is yet exploded, but from what has ah-eady been .said about local magnetic dis- 

 turbance on laud, it is not a matter of sui-prise that similar sources of disturbance 

 should exist in the land under the sea, for it has been found that in certain 

 localities, in depths of water sufficient to float the largest ironclad, considerable 

 disturbances are caused in the compasses of ships. 



An area of remarkable disturbance having been reported as existing off 

 Cossack, N.VV. Australia, H.M.S. 'Penguin,' a svirveying-ship provided with 

 the necessary magnetic instruments, was sent by the Admiralty in 1891 to make 

 a complete magnetic survey of the locality, with a view to ascertaining the facts 

 and placing them on a scientific basis. An area of disturbance .3'6 miles long by 

 2 mile.s broad, with not less than 8 fathoms of water over it, was found lying 

 in a N.E, by E. and S.W. by W. direction. At one position the disturbing force 

 was sufficient to deflect the ' Penguin's' compass 56° ; in another — the focus of prin- 

 cipal disturbance — ^the dip on board was increased by 29°, 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. 



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