200 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



If we follow a line passing through the points of maximum change 

 in the x^tlantic Ocean, we find for the following points : 



Values of the magnetic declination in 1100 and 1910. 



We see, accordingly, that the compass direction, in the course of 

 time, suffers large changes; for the region and time interval con- 

 sidered the changes \&.rj from about 3° off New York to 28° in the 

 Atlantic Ocean about midway between Buenos Aires and Cape Town. 

 Even these amounts may not represent the total or maximum change 

 during the period in question. 



Equally to be noted with these large changes with time is the 

 important fact that the amount of change is as dependent upon 

 locality as is the prevailing compass direction itself, which for over 

 four centuries has been known to be anything but " true to the pole." 



We have thus had impressed upon us this important fact: Two 

 sailing vessels cruising in the Atlantic Ocean from port to port — 

 the one in 1700 and the other in 1910 — were forced by the prevailing 

 winds to follow very closely identical courses. If, however, these 

 two vessels had been directed to follow certain defi.nite magnetic 

 courses, and if we may suppose that they had such motive power 

 as to render them independent of the winds, then their respective 

 paths would have diverged considerably. For example, if the 

 Carnegie had set out from St. Johns, Newfoundland, to follow the 

 same magnetic courses as those of the Paramour Pink, instead of 

 coming to anchor in Falmouth Harbor (pi. 3), she would have made a 

 landfall somewhere on the northwest coast of Scotland. In brief, 

 while the sailing directions as governed by the winds over the 

 Atlantic Ocean are the same now as they were during Halley's time, 

 the magnetic directions or bearings of the compass that a vessel 

 must follow to reach a given port have greatly altered. To quote 

 from the suggestive essay on terrestrial magnetism by John F. W. 

 Herschel : ^ 



The configuration of our globe — the distribution of temperature in its in- 

 terior, the tides and ciirrents of the ocean, the general course of winds and the 

 affections of climate — whatever slow changes may be induced in them by those 

 revolutions which geology traces — yet remain for thousands of years appreciably 



1 Essays from the Edinburgh and Quarterly Reviews, with addresses and other pieces, 

 by Sir .rohn F. W. Herschel, London. 1857, pp. (^9-70. 



