206 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1913. 



circles, but more or less devious lines. Could these magnetic merid- 

 ians be followed into the Arctic regions, they would be found to 

 intersect at the north magnetic pole. 



Owing to the irregular distribution of the earth's magnetism, the 

 points of greatest intensity of the total magnetic force depart widely 

 in their locations from the magnetic poles. Thus there are in the 

 Northern Hemisphere two distinct maxima of total magnetic force, 

 one in the northeast of Siberia and the other in Canada to the south- 

 west, approximately, of Hudson Bay. A magnetic survey of the 

 latter region is being nuule this summer by an expedition sent out 

 by the Department of Terrestrial Magnetism. 



DO THE MAGNETIC POLES MOVE? 



Possibly the most frequent question asked of thooe engaged in 

 magnetic work is: "Do the magnetic poles move with the lapse of 

 years, and if so, why ? " Unfortunately, as has already been shown, 

 there are no direct observations as yet on which to base a definite 

 statement. But it would be singular, indeed, if these points remained 

 fixed and were not affected by fluctuations such as are now known 

 from three centuries of observations to exist in every one of the 

 earth's magnetic phenomena. It is quite possible, in fact, that the 

 magnetic poles pass through certain motions even in the course of 

 a day or suffer displacements during magnetic storms. 



The diagram (pi. 6, fig. 1) shows the changes in the direction of the 

 compass (magnetic declination), as well as in the direction of the dij) 

 needle (magnetic inclination), as far as known, for London, Balti- 

 more, and Boston. Imagine yourself, if you will, standing at the 

 center of a great magnetized needle so suspended as to be free to 

 assume the direction actually taken hj the lines of magnetic force at 

 the place of observation, and let us suppose j^ou are looking toward 

 the north-pointing end of the needle. Could you gaze long enough, 

 you would see a curve described in space by the observed end of the 

 needle. This curve would lie on a sphere whose radius is the half- 

 length of the suspended needle and for graphical representation we 

 may take a central projection of it on a plane tangent to the sphere 

 at about the middle point of the curve. The curves here given were 

 constructed by me wnth the aid of the accumulated observations up to 

 about 1895; the course followed by the needle since 1895 will be dis- 

 cussed later. (PI. 6, fig. 1.) 



A number of intefesting and instructive facts follow from these 

 curves; time will permit us to give our attention only to the chief 

 ones. It is seen that at London, for example, the compass reached 

 its maximum easterly direction of about 11° in the year 1580, hence 

 during the middle of Queen Elizabeth's reign; thereafter the easterlv 



