PRESIDENTIAL ADDRESS. SECTION A. 23 



been found it is possible, by substituting in it the geographical 

 co-ordinates of an actual station, to calculate the value a given 

 element — say, the declination — ought to have. These calcu- 

 lated values represent the magnetism due to the most likely 

 mean field, as derived from the observations; if such values 

 are then subtracted from the actually observed values we have 

 a residual effect, which may reasonably be attributed to the 

 special magnetic features of the country. I need scarcely point 

 out to you how cautiously one must proceed in any deductions 

 of this nature. We have only to consider how many correc- 

 tions have to be applied to the observational results, and how 

 inexact the knowledge of these is, even in countries where the 

 magnetic elements have been the subject of study ever since 

 Gauss's time. The method of districts has also been used by 

 Mathias.* He has taken the mean results of the nine districts 

 derived by Thorpe and Riicker for the British Isles, and has 

 by the method of least squares obtained formulae which repro- 

 duce the results at these mean places with a degree of accuracy 

 as great as that given by the general formulas of the original 

 workers; his residual field is also practically identical with 

 theirs. 



The systematic use of the forms of isomagnetic lines in 

 larger surveys has been of very recent introduction; in the 

 case of smaller surveys, however, the value of the lines for the 

 discovery and subsequent survey of ironstone masses has been 

 long recognised in Sweden and in America. Even in cases 

 where the surface gives no indication of the existence of iron 

 ore, deposits have been discovered by the use of the magnet. 

 In Sweden it is almost universally the custom to make a mag- 

 netic map of every iron mine, and by help of this to show 

 where shafts ought to be sunk, levels driven, and develop- 

 ment pushed forward. An example of this is a plan of the 

 great ore deposits in Swedish Lapland, showing not only the 

 outcrops, but the non-exposed extensions of the ore-mass, whose 

 position had been determined by magnetic surveying. f The 

 magnet has also been used as an iron-ore finder in America: in 

 some parts over 50 per cent, of the new iron mines have been 

 discovered in this way. Curiously enough, the idea seems at 

 one time to have been prevalent that magnetic methods for 

 finding iron ore could not be used in lower latitudes. Professor 

 Nordenstrom+ called attention to this belief, and disposed of 

 it by himself applying the method in Spain, and showing that 

 it was quite as efficacious there as in Sweden itself. The results 

 obtained with the magnet as an ore-finder are to the more 

 ignorant so astonishing that many of the marvels attributed to 

 the divining-rod have been transferred to the compass needle, 

 and the attempts at explaining the result by the ordinarv pro- 

 spector are quite as fanciful as anything a water-finder can 



* Annales de I'Observatoire Astronomique, Magnetique et Meteorologique 

 de'^Tonlouse, T. 7. Recherches sur le Magnetisme terrestre par E. Mathias. 

 Paris, 1907. 



t Terrestrial 3.1ag., Vol. IV., p. 276. Baltimore, 1899. 



t Journal of the Iron and Steel Institute, II., page 5:;, 1898. 



