December 23, 1897] 



NA TURE 



i8i 



An interesting method of depicting these changes has lately 

 been suggested by Dr. Bauer. He imagines the observer to be 

 stationed at the central point of a magnet suspended so that 

 it can turn freely in any direction. To a person thus situated, 

 and capable of continuing his observations through periods far 

 exceeding the range of human life, the north pole of the magnet 

 would appear to describe a curve. It would move sometimes 

 to the right, sometimes to the left, and would simultaneously 

 rise or fall. The facts which have been observed during the 

 last 1 50 years, and, in some places, for longer periods enable 

 us to draw these curves. At the great majority of the places first 



LONDON 



23 30 18 B 14 12 K) S 



OECLINATIOH 



Fig. 3. 



investigated the pole of the magnet would always appear to the 

 observer to describe its orbit in the same direction as the hands of a 

 watch, so that on the whole when the needle was to the east of its 

 mean position it would generally be falling, and when to the west 

 rising. Unfortunately, however, this statement is not of universal 

 application, as there are a certain number of stations on the west 

 coast of America where the ordinary movement appears to be re- 

 versed. If figures representing a number of these magnetic orbits 

 are arranged, in order from north to south, as in Fig. 3, and if those 

 selected are appropriate to places the longitude of which is not very 



NO. 1469, VOL. 57] 



far distant from that of Greenwich, it is found that in passing from 

 north to south the area enclosed by the figure becomes larger, 

 and that it again diminishes when the equator is passed. In 

 other words', for these stations the rule holds good that the orbital 

 sweep of the needle is greatest near to the equator. There is an- 

 another curious peculiarity, namely that for stations of approxi- 

 mately the same latitude in the northern and southern hemi- 

 spheres, respectively, the curves are larger in the Southern 

 Hemisphere. 



If, however, we turn to the American continent, the curves 

 are diflFerent, the range of declination is less, and the figures, 

 instead of being of an approximately circular form, become 

 elliptical. 



In no case have the phenomena been registered for a suffi- 

 ciently long time to enable us to complete any one figure. For 

 London we have fairly accurate data for about three centuries, 

 and the earlier observations enable us to extend the curve still 

 further backwards with some probability of accuracy. In the case 

 of our own country we have probably followed the movement of 

 the needle from an extreme easterly position attained in the time 

 of Elizabeth to an extreme westerly position which was reached 

 about 1824, but we have no right to assume that the return 

 journey from west to east will occupy the same time as that 

 which has been watched from east to west. 



The first conclusion, then, which I wish to draw from these 

 figures is that they and others like them display what is practi- 

 cally the whole of our knowledge as to the time during which 

 magnetic phenomena go through a complete cycle of change. 

 Calculations have been given by high authorities, which lead to 

 the conclusion that the magnetic pole would perform an orbit 

 round the pole of the globe in about 960 years ; but a glance at 

 the curves is sufficient to show how uncertain are the data upon 

 which such estimates are based. 



The speed with which the secular orbit is described does not 

 appear to be constant. It may be more or less rapid in the 

 future than in the past. The curves, though so far smooth and 

 continuous, may in the future develop loops or irregularities of 

 various kinds. We may doubt whether all the orbits are de- 

 scribed in the same time. It is quite possible that the paths may 

 not return into themselves, or be repeated in successive cycles. 



These difficulties may be illustrated by another suggestive 

 device of Dr. Bauer's. Let us suppose for a moment that the 

 principal part of the magnetic system of the earth consists of 

 forces due to currents or magnetic matter which are unchangeable 

 and are arranged symmetrically with respect to the geographical 

 axis. Let there be superimposed upon this another magnetic 

 system describing within the earth and around the geographical 

 axis an orderly orbit completed in an unknown term of years. 

 Let us further suppose that this second system is itself unalter- 

 able except in position, so that, as it revolves, the magnitude of 

 the forces is unchanged though the position of the points from 

 which they emanate varies. Under these circumstances we 

 should be able to foretell from the present state of the earth 

 what the future cycle of change would be. If the magnetic 

 needle were placed at some fixed point on the surface of the 

 earth, it might take hundreds of years for the revolving magnetic 

 system to make one complete revolution so as to take up every 

 possible position with regard to the needle. But if the needle 

 were suspended like Mahomet's coffin near to the surface of the 

 earth, and were held there while the globe rotated beneath it, 

 then in a single day the relative positions of the revolving mag- 

 netic system and the suspended needle would undergo every 

 possible change, and the pole of the needle would describe in 

 twenty-four hours the path which may in reality be accomplished 

 in a millenium. 



Of course we cannot thus suspend a magnet in space, but the 

 same end would be attained by carrying it round the globe along 

 a parallel of latitude, and we have sufficient knowledge of the 

 magnetic conditions of the surface of the earth to determine the 

 curve which the pole of the needle would describe during such 

 a journey. 



Three of these curves have been drawn by Dr. Bauer for the 

 equator and for latitudes 40° N. and 40° S. , and a glance at the 

 diagram on which they are depicted shows that though they are 

 larger, they bear a general resemblance to the secular curves 

 which portray the movements of the needle at various stations 

 near to the prime meridian. In particular the curious difference 

 in the size of the orbits for equal latitudes north and south of 

 the equator respectively is reproduced. Dr. Bauer has drawn 

 curves for three different dates, of which only those correspond- 



