10 TERRESTRIAL MAGNETISM in Paris, which lies 2 20' E. of London, th< needle points! lue N. in 1663. Its subsequen us have closely resembled those of the London needle ; but the Paris needle ceased to move westward as early as 1817, and attainec a maximum declination of only 22$. Now il unltiiie these facts with the changes ol .dination, we see at once that they poin to a movement of the northern magnetic pole from a position between London and the N. pole in the middle of the 17th century to its present position in the extreme north of the lean continent (or rather in the archi- pelago which lies beyond those parts north- ward i. Fur in the middle of the 17th century the needle pointed northward, while afterward it pi MM! iMl i-tvard. Then the magnetic pole lay at that time either directly beyond the N. pole of the earth, or somewhere on (or near) the arc joining London and the K pole. But if the magnetic pole had lain beyond the true pole, the inclination would have been much less than that corresponding to a magnetic pole at the true pole of the earth, that is, less than 61$. Instead of this, however, the inclina- tion was much greater. Moreover, the incli- nation, which would then have been at a mini- mum had the magnetic pole been beyond the true pole, appears to have then been at a maxi- mum. For though exact observations of the inclination have not been made during so many years as observations of the declination, we find that in 1720 the inclination was 74 42' in London ; in 1800, 70 85' ; in 1865, 68 9' ; in 1870, 67 55'; and in 1873, 67 45'. The northern magnetic pole was therefore between London and the N. pole of the earth in the middle of the 17th century, and has since trav- elled westward, or in a direction from E. to W. around the true pole. If we assume the motion to be uniform (which is probably not the case), and that the needle at Greenwich responds uniformly to such motion (which is certainly not the case), we may calculate the period of polar revolution. Thus, taking the magnetic pole as due N. in 1657, and in 1833, according t i:,,Ws observations, as 95 W. of Greenwich, we have for the period of revolu- tion ^(18831857) years = 667 years about. < ming Ross's estimate with the Paris epoch, we get a period of ^(1833^-1663) years = 644 years about. We may take 650 rears as a not improbable period of revolution I may be added, as confirming the above, that m Ku- M^netic inclination has now iched a minimum, while in Peking it is in- M cau-i- of this change is at pres- ely unknown ; it has no analogy with tier class of physical phenomena with which we are acquainted. By a rough com- n of the isothermal lines and the lines of equal magnetic intensity, a general similarity has been observed, and hence the two have >een considered as referable to the same cause- but it will be perceived that this analogy does not hold, since the magnetic lines are in con- stant motion, while the isothermal lines retain very nearly a fixed position, or at least change in comparison with the other lines with ex- treme slowness. The second system of changes has evident relation to the annual position of the earth in its orbit round the sun, and its revolution on its axis. These were at first ascribed to the influence of the heat of the sun on different parts of the earth ; but they have the remarkable characteristic of exhibiting notably the same amount in the southern hemisphere as in the northern, and in the tropical as in the temperate zones. The mag- netic force is found to be greater in the months of December, January, and February, when the sun is nearest to the earth, than in those of May, June, and July, when it is most dis- tant from it ; whereas, were the effect due to temperature, the two hemispheres would be oppositely instead of similarly affected in each of these two periods. We must therefore ascribe the effect to the direct magnetism of the sun itself, and consider it established that this luminary like the earth possesses attract- ing and repelling poles, and that the effects on the needle result from the different positions of the earth in regard to these centres of ac- tion. The pole of the needle which is least distant from the sun makes a deuble diurnal movement in the following manner. It arrives it its greatest western excursion four or five hours before the sun passes the meridian of the place, as if it were repelled ; it then turns east- ward with increasing celerity, and reaches the limit of its eastern excursion one or two hours after that passage. As the sun passes the in- ferior meridian, there is repeated in the night the same variation as that which took place in the day. To illustrate the action, let us sup- pose two globes, a larger and a smaller, placed upon the same plane, with their axes of revo- lution not precisely parallel to each other, as n the case of the earth and the sun ; and let as further suppose that one globe is made to revolve round the other, the axis of the former being constantly parallel to itself. It is evi- dent that in one half of the orbit of the mov- ng globe the northern poles will be inclined toward each other, while in the other half of "he orbit the southern poles will be similarly nclined; and if we further suppose that the magnetic axis of the sun, as in the case of the earth, does not differ very much from the axis of rotation, we shall have an explanation of the effects observed in the records of the diur- nal motions of the needle. The K end of the needle, which is attracted by the N. pole of
- he earth, will be repelled by the N. pole of
iie sun, provided it has dissimilar magnetism to that of the earth, and consequently will de- Jlme from the sun ; and as, on account of the revolution of the earth on its axis, this lumi- ary appears on the E. of every place in the rthern hemisphere in the morning and on.
