September 24, 1903] 



NA TURE 



503 



Frcm among numerous examples of disturbance of the de- 

 clination on land, two may be quoted. In the Rapakivi dis- 

 trict, near Wiborg, a Russian surveying officer in the year 

 1S90 observed a disturbance of i8o°. or, in other words, the 

 north point of his compass pointed due south. At Inver- 

 cargill, 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 observations in one 

 position often convey but a partial truth ; they should be 

 supplemented by as many more as possible in the neighbour- 

 hood or district. Erroneous values of the secular change 

 have also been published from the various observers not hav- 

 ing occupied exactly the same spot, and even varied heights 

 of the instrument from the ground may make a serious dif- 

 ference, 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 3J feet above 

 the ground 56° 18' \. at the same spot. 



All mountainous districts are specially open to suspicion of 

 magnetic disturbance, 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 inven<:ed." I 

 can hardly say this idea is yet exploded, but from what has 

 already been said about local magnetic disturbance on land, 

 it is not a matter of surprise that similar sources of dis- 

 turbance should exist in the land under the sea, for it has 

 been found that in certain localities, in depths of wa'.er suf- 

 ficient 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.W. Australia, H.M.S. Penguin, 

 a surveying-ship provided with the necessary magnetic in- 

 -truments, was sent by the Admiralty in 189 1 to make a com- 

 i'lete magnetic survey of the locality, with a view to ascer- 

 tain the facts and place them on a scientific basis. An 

 area of disturbance 3-5 miles long by 2 miles 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 principal disturbance 

 — the dip on board was increased by 29°, and this at a dis- 

 tance of more than 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 first sight hardly ap- 

 pears to be applicable. We have, however, become familiar 

 with the terms " ridge line, valley line, peak, and col," as 

 applied to areas of magnetic disturbance on land ; there- 

 fore I think we may conveniently designate areas of mag- 

 netic disturbance in land under the sea " Magnetic Shoals." 



This year H.M. surveying-ship Research has examined 

 and placed a magnetic shoal in East Loch Rcag (Island of 

 Lewis), but as all our surveying-ships are practically iron 

 ships, it was impossible from observations on board to obtain 

 the exact values of the disturbing 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 in- 

 duction takes place, and we can no longer determine to what 

 extent the observed disturbances are due to the ship's newly 

 developed magnetism, or to what extent the shoal alone pro- 

 duces them. 



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

 place and show the dimensions of a magnetic shoal and the 

 direction in which a ship's compass will be deflected in any 

 part of it by compass observations only. Is it not, therefore, 

 the duty of any ship meeting with such shoals to stop and fix 

 their position? 



NO. 1769, VOL. 68] 



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 towards the focus of 

 greatest dip ; in the southern hemisphere it is repelled. The 

 results at East Loch Roag proved an exception, the north 

 point of the compass being repelled. 



Terrestrial Magnetism and Geology. 



I have already referred to the question of local magnetic 

 disturbance as one of great importance in magnetic surveys. 

 The causes of these disturbances were at one time a matter 

 of opinion, but the evidence of the elaborate magnetic sur- 

 veys I have alluded to, when compared with the geological 

 maps of the same countries, points clearly to magnetic rocks 

 as their chief origin. 



Magnetic rocks may be present, but from their peculiar 

 position fail to disturb the needle ; but, on the other hand, 

 as Rucker writes in his summary of the results of the great 

 magnetic survey of the British Isles conducted by Thorpe 

 and himself, " the magnet would be capable of detecting 

 large masses of magnetic rock at a depth of several miles," 

 a distance not yet attained by the science of the geologist. 



Again, Dr. Rijckevorsel, in his survey of Holland for the 

 epoch 1891, was convinced that " in some cases, in many 

 perhaps, there must be a direct relation between geology and 

 terrestrial magnetism, and that many of the magnetic fea- 

 tures must be in some way determined by the geological 

 structure of the under-ground." 



During the years 1897-99 a magnetic survey was made of 

 the Kaiser-stuhl, a mountainous district in the neighbour- 

 hood of Freiburg, in Baden, by Dr. G. Meyer. Exact topo- 

 graphical and geological surveys had been previously made, 

 and the object of the magnetic survey was to show how far 

 the magnetic disturbances of the needle were connected with 

 geological conformations. Here, again, it was found that 

 the magnetic and geological features of the district showed 

 considerable agreement, basaltic rocks being the origin of 

 the disturbance. This was not all, for in the level country 

 adjacent to the Rhine and near Breisach unsuspected masses 

 of basalt were found by the agency of the magnetic needle. 



More recently we find our naval officers in H.M.S. Pen- 

 guin, with a complete outfit of magnetic instruments, mak- 

 ing a magnetic survey of Funafuti atoll and assisting the 

 geologist by pointing out, by means of the observed dis- 

 turbance of the needle, the probable positions in the lagoon 

 in which rock would be most accessible to their boring 

 apparatus. 



Leaving the geologist and the magnetician to work in 

 harmony for their common weal, let us turn to some other 

 aspects of the good work already accomplished and to be 

 accomplished by magnetic observers. 



Magnetic Charts. 



Of the valuable work of the several fixed magnetic observ- 

 atories of the world, I may remark that they are constantly 

 recording the never-ceasing movements of the needle, the 

 key to many mysteries to science existing in the world and 

 external to it, but of which we have not yet learnt the use. 

 Unfortunately many of these once fixed observatories have 

 become travellers to positions where the earth can carry on 

 its work on the needle undisturbed by electric trams and rail- 

 ways which have sprung up near them, and it is to be hoped 

 they will find rest there for many years to come. 



Of the forty-two observatories which publish the values 

 of the magnetic elements obtained there, thirty-two are situ- 

 ated northward of the parallel of 30° N., and only four in 

 south latitude ; and it is a grief to magneticians that so im- 

 portant a position as Capetown or its neighbourhood does 

 not make an additional fixed magnetic observatory of the first 

 order. 



Thus, so far as our present question of magnetic charts 

 and their compilation is concerned, the observatories do not 

 contribute largely, but we should be very grateful to them 

 for the accurate observations of the secular change they pro- 

 vide which are so difficult to obtain elsewhere. 



Of the value of magnetic charts for different epochs I have 

 much to say, as they are required for purely scientific inquiry 

 as well as for practical uses. It is only by their means that 

 we can really compare the enormous changes which take 

 place in the magnetism of the globe as a whole ; they are use- 

 ful to the miner, but considerably more so to the seaman. 



