TERRESTRIAL MAGNETISM.] METEOROLOGY 175 Station. Effect on Declinution. Effect on Horizontal Force. Effect on Dip. At Equinoxes compared to Solstices. At June Solstice compared to December Solstice. At Equinoxes compared to Solstices. At June Solstice compared to December Solstice. At Equinoxes compared to Solstices. At June Solstice compared to December Solstice. Makerstcmn or Kew Increase. Increase. Increase. Decrease. Decrease. Undecided. Undecided. Decrease. Decrease. Decrease. Decrease. Increase. Increase. Decrease. Decrease. Decrease. Decrease. Decrease. Inappreciable. Increase. Increase. Decrease. Increase. Inappreciable. Decrease. Decrease. Decrease. Decrease. St Helena 78. In discussing the results of this table we shall assume that the sun acts, ana in all probability acts indirectly, upon the magnetic system of the earth. This point will afterwards be further examined. Meanwhile, assuming this indirect action of the sun, and assuming, to fix our thoughts, that it is in close alliance with the convection system of the earth s atmosphere, we can readily imagine that such solar action would act most strongly on the earth s magnetic poles at the solstices, and that in the June solstice the pole or poles in the northern hemisphere and in the December solstice those in the southern hemisphere would be most affected. Now a strong action of this kind upon either magnetic pole may well be presumed to increase the general magnetism of the earth, or at least that portion of it which is most readily affected by external action, that is to say, the induction system. Again, if the solar magnetic influence is connected with the convection currents of the earth, we can readily imagine that the influence in the northern hemisphere where there is much land should exceed that in the southern hemisphere where there is much water. If these views be reasonable we might expect two things to follow: (1) the earth s induction system should be stronger at the solstices than at the equinoxes, and (2) it should be more especially strong at the June solstice, when the sun acts in the northern hemi sphere. AVe must bear in mind, however, that so vast is the earth that a stimulus applied to its particles most susceptible of magnetism may not be instantaneously propagated throughout its mass, but that time may enter as an element of the question, in which case, inasnvuch as the action of the sun at the June solstice is in the northern hemisphere, a station near the south pole may not fully partake of the magnetic effects of this action. 79. An hypothesis of this nature would appear to be consistent with the results of Table XIX. In the first place, if the earth should become stronger as a magnet in one or in both of its magnetic systems this would show itself by an increase of horizontal force at least in all such stations as those at which absolute observations are made. An influence which increases the horizontal force at these various stations is therefore naturally regarded, and was regarded by Broun, as one increasing the strength of one or both of the magnetic systems of the earth whether of one or of both will presently appear. We may therefore assume from our observations that one or both of the earth s mag netic systems are strongest at the solstices. In the next place we may imagine that the changes of declination and dip which the table exhibits as occurring at the solstices are the very changes which would be wrought in these elements by an increase of power in the earth. For we see very well that an increase of horizontal force at the various stations may be regarded as denoting an increase of the earth s magnetic power. We cannot, however, see with equal facility what changes would be produced in the declination and dip by an increase in power of one or both of the magnetic systems; but we may well imagine that such changes of these elements as are found to accompany an increase of horizon tal force are those that denote an increase of the earth s power. We have thus ascertained the probable nature of those changes of the three elements which denote an increase of power. Now it will be noticed from the table that the effect at the June as compared with that at the December solstice is of the opposite nature to the effect at the equinoxes as compared with the solstices, that is to say, the earth is more powerfully affected in June than in December, the only well-established exception to this being Hobart Town in the far south. But, assuming that time is an element in the develop ment of this preponderating influence acting in the north, it is easy to see why Hobart Town should not exhibit its full effect. It remains to determine from the observations themselves which of the magnetic systems it is that exhibits these oscillations. Analogy would of course point to the induction system, but it is desirable to determine this from the observations themselves. In 54, when discussing the disturbance-diurnal variation of declination, it was found that Toronto and Kew may be regarded as on one side of the Siberian pole, while Peking, Nertchinsk, and Bombay are on the other. Now, if it be this pole that is influenced by the oscillations under discussion, we might expect that the influ ence on declination at Toronto and Kew should be the opposite of that at Trevandrum and Bombay. We find by the table that this is the case, and we are thus inclined to attribute these changes to the Siberian instead of the American pole. It would thus appear that the observations of Table XIX. bear out the provisional working hypothesis which we have ventured to introduce. It is quite pos sible that these remarks may not stand the test of more complete inquiry, but they are here introduced rather as denoting a method of looking at the subject which ought we think to be pursued than as embodying conclusions of a final nature. 80. Effect of the State of the Sun s Surface upon the Absolute Magnetism of the Earth. We have now to consider whether the state of the sun s surface permanently influences the magnetism of the earth. It will at once be seen that any such action will appar ently manifest itself as an oscillation in the secular change. We must, however, carefully guard ourselves against prematurely concluding that it implies a variation in the amount of true secular change. There may be two distinct things true secular change due to one cause, and action depending on sun-spots due to another. These, from the nature of the case, are necessarily mixed up together in the yearly changes which we examine; it does not, however, follow that there is any real identity between them. We shall now give one example of the method to be pursued in the attempt to detect a solar influence of this nature. Let us turn to Table II I., and take the decli nation yearly values at Toronto from 1856 to 1871. Subtracting the value for 1856 from that for 1871, we find that the westerly declina tion had increased in fifteen years 51 6, that is to say, at the mean rate of 3 "44 per annum. Again, the average declination for the sixteen years 1856-71 is 2 20 8 corresponding to the epoch at the commencement of the year 1864. Taking the average value and epoch, and also the average yearly increase above given, we are able to construct the following table (XX.), in which calculated and observed values at Toronto are compared together : Observed. Calculated. Difference. 1856 / 1 56-30 1 55-00 / +1-80 1857 2 0-50 1 58-44 + 2-06 1858 2 4-50 2 1-88 + 2-62 185!) 2 7-40 2 5-32 + 2-08 1860 2 10-60 2 8-76 + 1-84 1861 2 14-40 2 12-20 + 2-20 1862 2 15-70 2 15-64 + 0-06 1863 2 19-10 2 19-08 +0-02 1864 2 21-90 2 22-52 -0-62 1865 2 24-80 2 25-96 1-18 186 2 27-60 2 29-40 1-80 1867 2 29-80 2 32-84 -3-04 1868 2 33-20 2 36-28 3-08 1869 2 37-10 2 39-72 2-62 1870 2 41-90 2 43-16 -1-26 1871 2 47-90 2 46-60 + 1-30 It may be gathered from this table that the years which corre spond to minimum sun-spots have in the last column a greater negative or lower positive sign than those which correspond to maximum sun-spots, and hence we may conclude that at Toronto the tendency of many sun-spots is to increase the westerly declination. 81. Performing a similar operation for all those cases in which we have a sufficiently extensive series of observations to work upon, we obtain the following table : TABLE XXI. Effect of Numerous Sun-Sj>ots mi the Values of Magnetic Elements. Station. Declination. Horizontal Force Dip. Kew Increase. Inappreciable. Increase. Increase. Increase. Increase. Increase (?). Uncertain. Increase. Cape of Good Hope Decrease. Increase. 82. We have good grounds for supposing that the sun is most powerful when there are numerous spots on his surface, and therefore the above table represents a state of things which we may imagine to be caused in one way or another by increased solar power. Now the most natural hypothesis is to imagine that an increase of spots acts in producing an increase of disturbances, and that for those stations at which the disturbances tend on the whole to affect the elements in a definite direction there will be left behind a permanent effect in this direction. A comparison of Table XXI. with Table XII. will, however, show that this explanation is not valid. For
instance, at Toronto and Kew disturbances tend rather to diminish