208 ANNUAL KEPOKT SMITHSONIAN INSTITUTION, 1913. 



There is at present another matter of no little interest with regard 

 to England which should be pointed out here. It will be seen from 

 the London curve that the dip of the needle below the horizon 

 reached its maximum amount of 74.4° in about 1688. At this time 

 the compass changed its direction the maximum amount of 13' per 

 year. The curve would seem to indicate that the time of a minimum 

 dip is now approaching; this phase has already occurred at Pawlowsk 

 and seems to be now taking place at Potsdam and is traveling west- 

 ward. Whether it will reach London and when can not be answered 

 definitely. However, it is a matter of no little interest, in this 

 connection, to observe that the annual amount of change in the 

 compass direction has in recent years received a remarkable accelera- 

 tion in this part of the earth. Thus, as is shown by the magnetic 

 observatory records, it has almost steadily risen from 4' per year in 

 1902 to about 9' per year in 1912. Whether this portends an early 

 approach of the phase of minimum dip at London is one of the many 

 interesting questions continually arising respecting the perplexing 

 phenomena of the earth's magnetism. The course of the needle 

 since 1890 has been about as shown by the arrow; thus in 1910 the 

 magnetic declination was approximately 15.9° W. and the dip was 

 66.9°. 



One thing more. Note that for each of the three curves as far 

 as drawn, the motion of the freely suspended magnetic needle has 

 been clockwise, i. e., the same as the motion of the hands of a watch. 

 This fact, as shown by the curves in other parts of the world, con- 

 structed with the aid of the available observations, appears to hold 

 generally in both the Northern and Southern Hemispheres, except 

 for certain retrograrle motions which thus far have not been of the 

 same extent as the direct one, although, of course, it is not affirmed 

 that they may not become so later. Such retrograde motions are 

 at present being experienced in certain parts of the United States. 

 Thus, for example, the compass pointed in 1910 6.25° W. at Balti- 

 more and 13.35° W. at Boston, and in the same year the magnetic 

 dip was 70.9° at Baltimore and 73.1° at Boston. If we plot these 

 values on the diagram, we shall find that the curves for Boston and 

 Baltimore, instead of progressing in the direction of the arrows, 

 passed through a secondary crest about 1895 and then bent over to 

 the left ; how long this will continue can not be foretold. 



The question as to the cause of the remarkable changes from time 

 to time in the earth's magnetic condition, as indicated by these curves, 

 has been a fruitful source of speculation since 1634, when Gellibrand 

 definitely proved the fact that the compass direction varies from 

 year to year. Some of the best minds have been engaged with the 

 discovery of the cause, but the riddle is still unsolved. Hence as 

 regards the actual motions of the earth's magnetic poles and the 



