THE earth's magnetism BAUER. 207 



direction began to diminish until about 1658, the year of Cromwell's 

 death, when the needle bore due north and then swung over to the 

 west, continuing to do so until it reached a maxinuim westerly direc- 

 tion of somewhat over 24° in about 1812. Hence in the interval of 

 about 232 years (1580-1812) the compass direction changed at Lon- 

 don from 11° E. to 24° W., or 35°. At the present time it points 

 about 15|° W., or nearly 9° less than in 1812, and a most interesting 

 question doubtless immediately occurs to all of us: Will the freelj^ 

 suspended magnetic needle ever return precisely to a direction taken 

 at some previous time, or is there any definite cycle of changes which 

 will repeat itself from time to time? 



Here again no wholly definite answer can be giA^en, primarily 

 because of the fact, as will be seen from the diagram, that, if there 

 be such a cycle, it embraces many more years than are covered thus 

 far by the interval of observation. For some European stations, 

 e. g., Paris and Rome, the observation interval is somewhat longer 

 than at London, but still not long enough for definite prediction as 

 to the future course of the magnetic needle. 



The diagram shows also that in the United States the changes 

 in the compass direction, as far back as they are known, have not 

 been as great as those during the same time at London. Thus, at 

 Baltimore, for example, the compass appears to have reached a 

 maximum westerly amount of about 6.1° near 1670, and a minimum 

 of 1° in 1802, after which, instead of passing through a zero value as 

 at London in 1658, and swinging to the eastward, it turned back and 

 began to increase its westerly direction until at the present time the 

 amount is about 6^°. Thus, at this station the compass direction 

 passed from a maximum to a minimum in about 132 years and the 

 total change was but 5^°, or only one-sixth to one-seventh of that 

 at London. 



In brief, the facts revealed by the known compass changes in 

 my country can not be brought in harmony with those witnessed in 

 your country, unless we assume that the length of the cycle of 

 complete change is many times longer than merely twice the period 

 between a maximum and a minimum bearing of the compass.. There 

 are evidences futhermore, into which we can not go here, to indicate 

 that the cycle of change at one station is not of the type which 

 would result were we to close the apparently nearly completed 

 curve at London by uniting the two ends in some simple manner. 

 On the contrary, the evidences point to cycles within cycles and to 

 the probability that the secular variation curve, instead of being a 

 single closed curve, may consist of smaller loops within a larger one, 

 etc. ; it is even questionable whether there ever will be exact closure 

 of the curve. 



