708 REPORT— 1903. 



which at first sight hardly appears to be applicable. V/e have, however, become 

 familiar with the terms ' ridge line, valley line, peak, and col,' as applied to areas 

 of magnetic disturbance on land ; therefore I think we may conveniently designate 

 areas of magnetic disturbance in land under the sea ' Magnetic Shoals.' 



This year II. M. surveying-ship ' Research ' has e.\amined and placed a magnetic 

 shoal in East Loch Roag (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 

 i.s 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 induction 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 ? 



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 lloag 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 tlie 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 Riicker 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. 



Ao'ain, 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 

 features 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 neighbourhood of Freiburg in Baden, by Dr. G. Meyer, 

 "^xact topographical 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 consider- 

 able 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. ' Penguin,' with a complete 

 outfit of magnetic instruments, making a magnetic survey of Funafuti atoll and 

 assisting the geologist by pointing out, by means of the observed disturbance 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 accom* 

 plished and to be accomplished by magnetic observers. 



