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SYDNEY CHAPMAN, ESQ., M.A., D.SC, ON 



motions to the two magnetic axes. Even such a modification, 

 it is now known, is insufficient to account for the irregularities of 

 the actual distribution and changes of the earth's magnetic field. 



The desirability of further observations was, however, brought 

 into prominence by Halley 's treatise of 1683, and in consequence 

 the Government of King William III was induced to provide 

 a ship on which, under Halley 's command, observations of 

 declination were to be made wherever possible in the north and 

 south Atlantic ocean. On his return, after two years' continual 

 voyaging (1698-1700), Halley embodied his and other observa- 

 tions on the first chart of lines of equal magnetic declination. 

 This chart embraced mainly the Atlantic Ocean and the regions 

 bordering upon it ; about a year later, in 1702, he prepared a 

 further chart covering the whole world, as far as the available 

 observations allowed. These charts gained a wide popularity, 

 and were frequently republished, and, later on, revised, as the 

 secular change made this necessary. 



The principle underlying their construction was very simple. 

 Points at which the declination has a given value, say 5° west, 

 are joined by a continuous line, different lines referring to different 

 values of the declination, at intervals of 1 °. At points between 

 these lines the declination is gauged approximately by inter- 

 polation. It is not necessary, of course, to have observations 

 of the compass " error " at all points of the lines of equal declina- 

 tion, these lines being drawn by a consideration of whatever 

 observations are actually available. 



The same principle can evidently be applied in the representa- 

 tion of the distribution of magnetic dip and intensity of magnetic 

 force ; this is, in fact, now done as part of the regular work of 

 the hydrographic departments of various national governments. 

 Since Halley "s time, and especially during the nineteenth and 

 present centuries, the magnetic field at the earth's surface, 

 both by sea and land, has been the subject of much attention. 

 The British Association initiated the first general magnetic 

 survev of a whole country when, in 1836, Sabine, Ross, and others 

 of its members combined to survey Great Britain from this 

 aspect. The United Kingdom has been re-surveyed three or 

 four times since then, and there are few, if any, civilized countries 

 where at least one magnetic survey has not been made. At 

 sea naval and other vessels frequently make magnetic observa- 

 tions, and many polar expeditions have been specially equipped 

 for the investigation of the earth's magnetism in arctic and 



